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Beginning a digest of the elementary and basic functions of a tail whether it is positioned at the front or rear or even hiding somewhere disguised as something else. Parti

Figure 1-1 — Equilibrium

By George B. Collinge (EAA 67, Lifetime) 5037 Marlin Way Oxnard, CA 93030 Illustrations by the Author

IT IS ANTICIPATED that even the

series, they may not be the only ones around but these are in plain English

wing lift would change and the

and will mesh readily with observations of other related aerodynamic

be disarranged.

phenomena without revamping theories midstream. The business of a tail is in large part, concerned with longitudinal stability which conventionally is ex-

center of lift (cp) begins to move forward or backward (more on this later). So unless somehow constrained, a runaway cp could completely topple the airplane. A rear

amined in itself, separate from both

tail prevents this from happening and

lateral and directional stability.

does it in an extremely simple and elegant fashion, which might help explain why it has been with us for so

many different shapes of airplanes

To start, if an airplane's four principal forces were hypothetically balanced through a single point at a steady speed, there would be no need for a tailplane (Figure 1-1). Of course,

are now coming onto the aeronautical

this condition, if ever achieved, could

scene.

not remain long. For if the speed should change just a little bit, the

most knowledgeable will give assent

to a review of some fundamental aspects of why there are tails, how they work and don't work, especially as so

As with all explanations in this

exactly counterpoised vectors would If the angle of attack alters, the

long.

An airplane is conventionally made stable by the use of "aerodynamic decalage" (Ref. 1) or "Longitudinal dihedral" (Ref. 2) shown exaggerated in Figure 1-2. On certain machines the

Figure 1-2 — Most Airplanes SPORT AVIATION 27

The airplane is again balanced, but in a glide. To climb, the throttle is opened and the slipstream and downwash increase the stab down-force to cause a nose-up condition. Thus, longitudinal dihedral makes an airplane seek to fly at an initially programmed angle of attack. If opening the throttle causes too much or too abrupt a noseup movement, a possible alleviating remedy is a slight down-thrust built into the motor mount (Figure 1-3). It is distressing to a few that this conventional airplane has to carry an induced tail-down-load, however small. Yet, a look at the whole picture shows that the commonly used "unstable-type" airfoil generates quite high values of lift and can very easily sacrifice a tiny percentage to the tail in return for this most convenient and highly practical system of stabilization. In a nutshell, the degree of pitch stability is governed by the CG location, the stab area, the angle at which it is set, its aspect ratio and the distance from the CG. Tails also function in other ways. The tailplane along with the fin and rudder serve like feathers on an arrow, to quickly point

Figure 1-3 — Pietenpol Air Camper 1932

the airplane in the direction it is actually going, after a large disturbance

such as a lomcevak or tail slide (Figure 1-4). The tail's chord is often large, to benefit from as high a Reynolds number as possible. Additionally, low-aspect ratio helps the tail to resist separation (stalling) and to remain effective at high angles, especially after the main wing is stalled or malpositioned. Tail crosssections are of symmetric proportions if they are to operate equally well, negatively or positively. On most airplanes, more up than down elevator is provided, to be able to raise the nose adequately during a flare for landing when the CG is at

Figure 1-4 — Arrow Stability Going To

Work

the full-forward limit and the speed is

Figure 1-5 — A Pitts it ain't.

incidence setting of the tailplane will

be positive, with no geometric longitudinal dihedral. However, there

should still be effective dihedral because the tail is operating in downwash and this can cause the actual angle of attack to be less than the angle of incidence. A rear tail functions in the follow-

ing manner. At a selected speed, a

deliberate nose-down couple, comprised of the cp and weight (CG) is equalized by a small download on the stabilizer. This arrangement performs automatically. If the nose low28 APRIL 1984

ers (tor some reason) the speed increases. This creates a stronger down-

low. Elevator down-travel is based on safe stall recovery when at full-aft CG limit. If "normal" airplanes were made to do aerobatics and inverted flight, their elevators might not have enough power to hold the nose "up" (Figure 1-5). This lack of control

force on the stabilizer, which consequently brings the nose back up,

would be particularly evident if the wing airfoil was highly cambered, as

value. If the nose should be displaced

when inverted. It doesn't lift well

stab down-force lessens sufficiently to

high angle of attack to support the

reducing the speed to the preset

upward, the airplane slows and the allow the CG to lower the nose.

If the engine stops, the airplane slows and the stab down-force diminishes as before. But this time the nose lowers and stays down. Speed increases until countered by the originally established stab down-force.

this type becomes inordinately stable

either, upside down, necessitating a weight of the airplane. Therefore,

greater elevator power may be needed

just when there might not be any

more.

On the other hand, inverted flight would be no problem if the airplane was expressly designed, with a sym-

reduction correspondingly demands a pull force. And this elevator displacement can again result in part of the tail pushing down and part pushing up. For example: during an approach, the speed is decreased by applying a gradually stronger pull, deflecting the elevator up. Initially, the upelevator combines with the longitudinal dihedral to intensify the dwindling tail-down force. At low speeds, the aft end is down so far that the normal stab down-load diminishes to zero. In fact, the stab can start lifting, trying to move the nose downward, to dutifully recover the original angle of attack and airspeed. In opposing the stab with up-elevator the pilot once more sets up contrary reactions over the tail (Figure 1-8). A high-positioned tailplane cannot benefit greatly from downwash. Therefore, a one-piece design may be appreciably more efficient in this case, presenting a single, uninterrupted surface and a single reaction. Next month . . . more on pitch stability.

Figure 1-6 — Much better.

References:

1. Aerodynamic Decalage, Aerodynamics of the Airplane by Clark B. Milikan, John Wiley & Sons, Inc., New York,

Figure 1-7 — Tail plane at cross purposes ... In a dive.

1941, page 145.

2. Longitudinal Dihedral, Mechanics of Flight by A. C. Kermode, Sir Isaac Pitman & Sons, Ltd., London, 1942, page 152. ABOUT THE AUTHOR

Figure 1-8 — On an approach.

metrical main-wing airfoil and with elevators of ample travel, both up and down. Wing and tail would be set at zero degrees incidence. Longitudinal dihedral would pertain no matter which was was up, due to downwash from the front wing flowing over the tail (Figure 1-6). And there should be little trim change, if any, when switching from upright to inverted and vice versa. Because a pilot must change an aircraft's attitude in order to climb, zoom, descent or dive, he will, understandably at these times, override the basic stabilizing function of the tail. For example, in a dive, the speed and longitudinal dihedral act to increase the tail down-load forcing the rear end downward bringing the nose back up and the airplane out of the dive.

Accordingly, if a pilot wishes to dive he must provide an opposite force and push into it and continue pushing to hold it in (Figure 1-7). Stick force should naturally increase with speed. If he releases the pressure, the nose immediately rises. Normal use of trimming devices in no way alters the inherent stabilizing mechanism. However, its employment is avoided in this review in order to hold explanations to a minimum. Be that as it may, when a trimmer is operated, the sense should be the same as the primary controls; that is, forward to relieve a stick push-force, and backward to ease a pull-force. As already described, to dive or to increase speed should necessiate a push on the stick. A climb or speed

George Collinge (EAA 67) is one of the earliest EAA members . . . early enough that he was the designer of the EAA logo. A native of Canada, he enlisted in the RCAF in 1940, learned to fly in the system, then became an instructor and eventually attained one of the top military flight instructional (a1) certificates. Most types of aircraft in the inventory were flown regularly, from Tiger Moths to Lancasters. Also during World War II, he lectured on aerodynamics, engine handling and range/endurance at CFS Trenton and ECFS Hullavington. From 1947 to 1951 George was a jet fighter pilot with the Canadian 400 Squadron, after which he and his family emigrated to the U.S. where he has subsequently worked for the computer and aircraft equipment industries in Southern California. He was a Charter member of both the San Fernando and Santa Paula EAA Chapters. Throughout EAA's existence, George has been a frequent contributor of both articles and artwork for SPORT AVIATION. SPORT AVIATION 29

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Part 2 By George B. Collinge (EAA 67, Lifetime) 5037 Marlin Way Oxnard, CA 93030 Illustrations by the Author

1JONGITUDINAL STABILITY CONCERNS the action of an airplane, where, after a pitch disturbance and without pilot interference it either returns to or moves farther away from the original state (see Figure 2-1). It is usually studied in two main categories - Static and Dynamic. Static stability refers only to the beginning phase; that is, whether or not an airplane initially begins to return. If it simply starts, but doesn't complete the movement, it is still classed as "statically stable". Dynamic stability pertains to the subsequent and remaining motion, whatever that might turn out to be. Most airplanes have "unstable" airfoils. These have positively-cambered median lines and generate high liftcoefficients. They push a lot of air downward. Unstable airfoils are saddled with a cp that moves forward when the angle of attack is increased, tending to further increase the angle. With low angles (high speed) the cp moves rearward causing a nose-down reaction. Instability of this kind is one of the chief reasons for a tail in the first place. In contrast, a "stable" airfoil is one where the median line is flattened or

Fig. 2-1 What will it do?

employed on blades and tailless airplanes. As the CG is positioned farther forward, an airplane becomes more and more stable, balancing out at a higher speed. Stick forces become heavier because there is a greater effective weight to maneuver. A loss of "noseup" elevator power would be evident in the low-speed range, requiring a tail of greater influence. Therefore, the forward CG limit is determined chiefly by control rather than stability. A stability standard, the compliance of which has resulted in the general good-handling of modern airplanes, mandates that a stable airplane requires a large stick-movement and an increase of pressure to start a speed change. Also it will require a progressively stronger stickforce to increase the rate of change. In other words, to pull faster means to pull harder at the same time. As the CG of an airplane is made to move aft, a point will be reached where stability is neutral. It will tend to stay in whatever attitude it is put.

or almost. Some of these airfoils incorporate a cp that even moves aft with angle increase and forward with a decrease, so helping to restore the original angle of attack and airspeed. "Tailless" aircraft generally have to

it can produce, hence the concept of increased efficiency in an all-wing airplane by eliminating the tail, is flawed by the very nature of its wing's low lifting-power. The terms "stable" and "unstable" as applied to airfoils, while undoubtedly fixtures as regards accepted nomenclature, may not be altogether appropriate, especially when ascertaining the stability characteristics of the total airplane. Airplanes with unstable airfoils are, of course, easily made stable and some aircraft with stable airfoils can at times be anything but stable. Although academic, beyond the stall the cp of all airfoils migrates toward a mid-chord position. At a 90 degree angle of attack, all will exhibit the flat-plate predisposition of a 50 percent chord cp location. The airfoils in Figure 2-2 show a range of types, the most cambered median-line (top example) indicative of the highest lifting power (CD of the group. For comparison, the various median lines are all wrapped with the same streamline-function of about 15 percent thickness. Airfoil No. 1 is very unstable, has high lift and is for lower-speed aircraft. No. 2 is less unstable, provides moderate lift for a wide speed-range. No. 3 is stable and has lower lift. It requires minimal trim alterations with variations of speed, also used on rotary-wing blades and tailless

favor these airfoils. Unfortunately,

airplanes. No. 4 has a reflex to

the more stable an airfoil, the less lift

stabilize its cambered entry. It too is

loop. Even forward stick might not stop it.

perhaps reflexed. The cp is stationary,

Past this point and the airplane begins to change pitch too easily, especially in rough air. It will be work for

the pilot to keep it from getting worse. If disturbed, the airplane will tend to diverge. Example: if the airplane's nose goes up due to a disturbance, the wing will lift into an ever-tighter

SPORT AVIATION 49

To recap - as it is moved aft, the ability of the CG to lower the nose is reduced. Stability thereby decreases. Stick forces become lighter as the lever-arm length between the CG and the tail gets shorter, although control may seem adequate if only because there is less nose weight to overcome. With continued aft CG travel, the airplane will eventually run out of elevator nose-down power so that stall and spin recovery will become more difficult if not impossible. In a dive, this reduced leverage has the same effect as too small a tail or too short a fuselage, the airplane would continue to accelerate though the stick would be fully back! It might seem a paradox, but moving the CG forward (making the nose heavier) allows an easier dive recovery. A more forward CG increases the power (leverage) of the tail. Normal allowable CG travel for an average airfoil on an average airplane is seldom more than 20 percent of the mean aerodynamic chord (see Figure 2-3). The rear CG position can be roughly determined as that which allows hands-off flying and the forward position that for good control on landing. An aft CG condition can tighten up a turn or pull out, such as occurred in the early Spitfires with full fuel load. A push force was needed to prevent too much G. Instability can show up

Fig. 2-2 Airfoil types.

-^•^ /«g

Fig. 2-3 CG limits.

as any or all of the following partial

list of symptoms: to recover from a dive, instead of releasing a forward push, the airplane needs a strong pull, if an airplane will increase speed easily with only a small push but while continuing to dive the stick comes back to its original position, if an airplane demands a push force just before a three-point touchdown. Standardization of terminology was early decided. Delineation of the motions that decide the pitch-stability classifications is shown in Figure 2-4. Conditions 1, 2, 3 and 6 are statically stable because they tend to return to level flight, even though 3 and 6 never achieve it. An average airplane can take 20 to 60 seconds between each oscillation, the usual damping factor results in two or three phugoids before ending. It is possible that an airplane is so stable and so sluggish in response that after an upset only one cycle or perhaps a half cycle is performed, though up to 60 seconds in length. Another aircraft is classed less stable because it requires five cycles notwithstanding that it completes them all in less time. And it is possible that oscillations in an unstable sense can be so slow that it might not be considered dangerous or difficult to control. 50 MAY 1984

DOWH

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Fig. 2-4 The Gamut.

Fig. 2-5 Weight lifter.

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cruise. But once immersed in the highly-angled downwash flowing off large-area flaps it generates the necessary increased authority. Sometimes inverted slats are essential to prevent tails from stalling, particularly near the roots where the airstream is degraded by the fuselage. Aircraft across the full spectrum of size utilize inverted tails, from the Beechcraft Musketeer to the McDonnell F-4 to the multi-engined Breguet 941. These examples should not be confused with what was purely an early application of longitudinal dihedral, exhibited by the WW I Pfalz III (see Figure 2-6). It was superseded on the Pfalz Ilia by a symmetrical section with greater chord and area (Reference 1, 2, 3). A number of high-wing monoplanes can tolerate a more aft CG and still be quite stable because during a pitch-up for instance, the CG effectively moves forward of even an unstable cp, helping to lower the nose (see Figure 2-7). Contrariwise, with its CG above the low wing, an airplane can become progressively more unstable as the nose goes higher (see Figure 2-8). Low-wing monoplanes as a consequences are happier with a more forward CG and/or a larger tail. A more stable airfoil also helps. While not considered in these elementary notes, the effect of the fuselage, propeller(s) and engine nacelles are all additional factors

Fig. 2-6 Pfalz III 1917.

which, with the wing cp, combine to

form an overall airplane cp. Their sometimes strong influence can help explain abnormal deviations of aircraft behavior from those prognosticated by basic airfoil action alone. As the CG is so important, it is possible on large transports to adjust its location during flight. The French Airbus 310-300 was planned to have the capability of pumping fuel into its horizontal tail while cruising, to decrease the negative load. The aft shift in CG unloads the main wing, reducing its wing loading and induced drag (Reference 4). Next month . . . Lifting tails and servos.

Fig. 2-7 Pendulum effect.

Fig. 2-8 Upsetting.

The use of camber-increasing flaps

to obtain extremely high CL from thin highspeed airfoils is eminently possi-

ble as long as there is a tail out back. While lowered flaps always cause the cp to move aft normally creating a nose-down attitude, on certain aircraft the increased downwash over the tail is so strong as to initially cause a nose-up tendency. Leading-edge slats or Kreuger flaps

are mandatory in conjunction with

really big flaps because so much more air at extraordinary velocity goes over

the nose which would otherwise invite separation. In some cases the tail is modified to produce an extra-large down-load to handle the stability decrease with

flaps (see Figure 2-5). An inverted

tail is set to produce only enough down-load for normal stability during

References 1. Cross & Cockade, Vol. 1, No. 1, Winter, Santa Ana USA, 1960. Pages 36, 37 and 47. 2. Jane's All the World's Aircraft,

Sampson Low Marston UK, 1919. Pages 339a through 343a. 3. Pfalz Dili, Profile Publications

Ltd., Hills & Lacy Ltd., No. 43, London UK, 1965. Page 5. 4. Airbus A310-300 Definition Completed by Jeffrey M. Lenorovitz, Aviation Week, Aug. 29, 1982. Page 31. SPORT AVIATION 51

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Lifting Tails, One-Piece Tails and Servos Parts By George B. Collinge (EAA 67, Lifetime) 5037 Marlin Way Oxnard, CA 93030 Illustrations by the Author

D,s

ISTINCT FROM SYMMETRICSECTION tails that are set at a small positive angle of incidence but in reality are operating at a negative angle of attack in downwash and in direct contrast to negatively-arched stabilizers, there have been such things as "lifting" tails. Rarely employed, except in the very early days, their use has been for purposes other than basic longitudinal stability. Example: the prototype of a popular biplane of the 30's exhibited a tendency to go flat during a spin (Ref. 1). Rather than redesign and adjust the CG to a more forward position and/or enlarge the existing tail area, the stabilizer was given the quick fix of a positivelycambered airfoil section. A degree of longitudinal dihedral remained which gave acceptable stability, but spin behavior was improved. The cambered stabilizer helped in lifting the aft end and forcing the nose down. Spin recovery was further encouraged by the restriction — "Solo from front seat only". When this airplane was produced in quantity early in WW-II and put into elementary military-training, it was naturally subjected to an extremely wide-range of aerodynamic situations not normally experienced in civilian use. As the main wings employed a highly-cambered airfoil section, they became super stable when inverted (see Fig. 3-1). The 38 JUNE 1984

airplane would stiffen on the top of a loop and attempts to roll off or execute an Immelman (lots of rudder on this aircraft due to low-response ailerons) frequently resulted in a flat but this time inverted spin, a condition assisted by the now-inverted tail and by the addition of a large squarish winter-canopy. Recovery was sometimes difficult, especially with a pair of green student pilots aboard doing "mutual practice" and suffering acute disorientation. A number of these airplanes spun into the ground, keel upwards. For a designer to arrange the four forces (lift, weight, thrust and drag) through a single point is an ideal only, but the airplane type that seems to most closely approach that state of perfection in numbers has to be the so-called midget racer. In this category, mid-wings predominate in company with minimum cp-travel airfoils. Fuel location close to the CG

Fig. 3-2 Look alikes.

ensures little trim drag throughout the flight envelope and only smallish tails are needed (see Fig. 3-2). In contrast, one of the most difficult aircraft to give reasonable longitudinal stability and handling is the high thrust-line boat. A typical early example, a British Bradford-built twin, incorporated a tail with an inverted RAF 15 section, mounted at a moderate angle to provide nominal "engines-off longitudinal dihedral but which was greatly augmented by the increased tail down load in the "engines-on" slipstream. The large nose-down couple created by power application was thereby neatly counterbalanced (see Fig. 3-3). Some early designers were obviously concerned about the simultaneous up and down loads that occurred with the two-piece tailplane. On the other hand, the one-piece, all-moving tail was always smooth and flat and required only a small deflection to be

effective. The French Morane was a pioneer in the utilization of this kind of tail, as were its many copies, including Fokker and Pfalz (see Fig. 3-4). However, unless the control column was held firmly by the pilot (or by

other means) longitudinal stability was nil. This because a plain freefloating surface tends to trail, thus providing no resistance to pitch

changes. Steady flight would have required constant pilot attention (Ref. 2). Early airplanes with an all-moving

tailplane generally had the highlycambered main-wing airfoil. A nosedown disturbance would move the cp

Fig. 3-3 P-5 Cork 1 1918.

well aft. Unless checked immediately,

an ever-increasing angle of dive would result because of the aforementioned absence of an automatically imposed down-load on the tail (see

Fig. 3-5). During a nose-up displacement, the cp forward-travel would pull the nose up farther, requiring a forward push-force to stop it (see Fig. 3-6). This pitch instability, accompanied

by the heavy feel of wing warping, no fixed fin plus the handling quirks of

a rotary engine produced airplanes

Fig. 3-4 M5K Fokker 1914.

difficult to fly, particularly as gun platforms, a requirement that suddenly became very important in those

years. As speeds approached 100 mph this type of tail became increasingly

difficult to handle, falling into disuse.

In the latter part of WW-I and into

the post-war years, airplanes went faster and controls became much

heavier to move. Some got very heavy. This situation would probably

have remained for a lot longer than it

did except for the fact that many aircraft were multi-engined and non-

feathering propellers exacerbated the difficulty of engine-out flying. Pilots

Fig. 3-5 Pull against dive.

just did not have the leg strength to

push on and hold on adequate rudder to fully compensate for the asymmetrical thrust of the live engine combined with the drag of a windmilling propeller. Hence, servos first became popular on rudders.

Anton Flettner had invented the

servo in Germany during WW-I. Subsequent royalty fees financed his later extensive helicopter work (Ref. 3). The servo was for years referred to as the "Flettner tab" and has been

used in various roles on a great many

of the world's airplanes. Its original basic function was to provide a light-to-operate control

which in turn aerodynamically moved

Fig. 3-6 Push against dive.

SPORT AVIATION 39

a large main-surface. Early adaptions were sometimes given the added leverage of an extended boom (Fig.

3-7). A variation, the spring-centered servo, gained wide usage during and after WW-II although it could result in sluggish or soggy response on the

initial portion of take-offs and landing runs. With other servo types, the pilot directly commanded the main surface which incorporated a geared servo. In this case the servo always moved a fixed percentage of the angular travel of the boosted main-surface.

In concert with these developments,

servos doubled as trimmers on all main control-surfaces; in the pitching

Fig. 3-7 Bolton-Paul 1934.

plane on many aircraft, it displaced the adjustable stabilizer. Servos or boosters employed on ailerons lighten

the feel and in effect, increase the

rate of roll (Fig. 3-8). Servos are used

extensively, right up to the realm of irreversible,

hydraulically-actuated

surfaces with artificial feel. The elevens of an English (general aircraft) experimental tailless-glider of 1940 each utilized a tab that

functioned as a servo to lighten the aileron action. With elevator movement the same tab operated only as a trim tab. This ingenious adaption kept the control feel properly harmonized, that of the elevators heavy and that of the ailerons light (Ref. 4). Widespread activity with tabs was bound to lead to their application as "anti" servos, to revitalize the long disused one-piece or slab tail. While the slab's undesirable floating or trailing characteristics could be minimized by centering it with a bungee, springs per se tend to veil true feel. A better system is a geared servo,

Fig. 3-8 Harvard mkll 1940.

reversed in its action so as to oppose

the main surface angular travel. Powerful aerodynamic centering action results, but with a high degree of feel, the magnitude of which can be readily tailored to suit a specific airplane-design (Fig. 3-9). It can double at the same time as a bias (trimmer). This

Fig. 3-9 Glider anti-servo tab.

concept, applied for in July 1945, was

granted a U.S. patent in August 1951

to John W. Thorp, assignor to Lockheed Aircraft Corporation. The "anti-servo tab", as it came to be called, has moreover been designed into elevators of two-piece tails to reduce trail and/or restore feel when masked by springs or weights in the system. Next month — Pitch stability

further examined.

40 JUNE 1984

References:

1. Aerobatic and Amateur Built

Aircraft by Robert Whittier, SPORT

AVIATION, April 1972, pages 13-15.

2. Soaring, August 1982, page 2. Harland Ross describes flight in his homemade 1937 RS-1 Zanonia

sailplane which had a plain "flying

stab/elevator" (later changed). It was tiring because of "oscillating flight". 3. German Rotocraft Pioneer Comes Back by David A. Anderton, Aviation Week, Nov. 29, 1954, pages 26-28.

4. Towed Tailless, Flight, Sept. 26, 1946, pages 328-329. ,

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Pitch Stability Further Examined Part 4 By George B. Collinge (EAA 67, Lifetime) 5037 Marlin Way Oxnard, CA 93030 Illustrations by the Author

PILOT NEEDS to feel stick forces to help judge speed and how much load to apply to the airframe. If the control forces, from a trimmed state, increase with any attitude change, then the airplane is, in all likelihood, stable. However, there is a little more to add. So far in this review, longitudinal stability has been broadly covered under a two-part classification. Terms, static and dynamic were used to sequence the

reaction to an upset.

There is yet a third category. When an airplane is purposely disturbed or given a gross displacement for observational intent and allowed to fly by itself, a difference is noted if the stick is then held firmly or if it is left free. Stick Fixed - For this mode, at a trimmed steady speed, the stick is pushed about one half of its forward travel until the nose is well down. The stick is then immediately returned to its original position and held or clamped immobile (Figure 4-

1), while the aircraft's movements are watched. The test is repeated except that the stick is pulled backward, then centralized. Usually, in each case, the nose should come back to the beginning position, go past (due to inertia) then reverse and so damp out in a few cycles. If it does not subside, the CG is too far aft and/or the stabilizer is too small or at the wrong angle. The distance between the stabilizer and the

CG could be too short as well.

Fig. 4-1 Fixed.

SPORT AVIATION 39

Fig. 4-2 Free.

As the CG position is moved aft due

to design or loading (passengers, cargo or fuel) the degree of stick

movement necessary to cause a speed

change will get less and less until, just past the neutral point, the airplane will go either way, up or down, by itself (mentioned in Part 2

of this series).

It just might be satisfactory for an

airplane to have neutral stick-fixed stability provided that the controls feel normal and it still requires a push

to go faster and a pull to go slower, as pressure is more important to the pilot than actual movement or travel of the stick.

Fig. 4-3 Fix.

40 JULY 1984

Stick Free - For this test, again the same stick movements as for "fixed" only it is not brought back to the reference position but completely released (Figure 4-2). When stick free, airplanes take longer to decay after an upset, depending on how readily the elevator floats and at what angles. Only the fixed part, the stabilizer, has any real effect and it is of considerably less area than the entire tail. An airplane will be stable in the stick-free mode as long as the correct stick movements are still necessary to change the airspeed from a trimmed condition. Whereas stickfixed stability is governed by tail size

and CG position and is generally permanent once the airplane has been built, the degree of stick-free stability can be easily enhanced, with only

small modifications to the control system. In other words, it is possible that an airplane with only neutral stick-fixed stability can be persuaded to accept a measure of stick-free stability. This is because one or more automatic contrivances actually move the elevator to cause a nosedown recovery. By so doing, the airplane can then perhaps tolerate an even more aft position of the neutral point. This increase in stick-free stability in the low-speed

Fig. 4-4 More fixes.

mode and provision of a nose-down message through the stick to the pilot is done with a spring tab (Figure 4-3). At low speeds, air pressure diminishes so that the spring pulls the tab up. The tab in turn moves the elevator down, making the airplane nose down to restore speed. At high speeds the tab has no effect as air pressure is too high for the spring strength to overcome. Two other devices to give the pilot better feel and to increase stick-free stability are either a spring or weight bearing directly on the stick or somewhere in the control run (Figure 4-4). One or all of these three fixes can improve stick-free stability better

than the stick-fixed mode. Springs are not effected by acceleration (g) but weights are. Therefore, weights are superior because they give an increasing force with acceleration. From this it is obvious that a tail-trimming system based on springs can alter stability characteristics over the speed range. A fourth item, the geared anti-servo tab, is more effective with speed at increasing stick-free stability and has a further advantage of working both up and down (Figure 4-5). One-piece tails with anti-servos can also be reduced in area, but not too much, because then stick-fixed stability will begin to suffer.

Some designers of high-performance gliders settle for reduced stability in exchange for reduced drag, by following the idea of rather smallarea tailplanes. By choosing the twopiece design and intentionally omitting weighty mass-balance, some stick-free stability is retained. For example, due to inertia, an up-gust causes the elevator to lag. This downelevator helps lower the nose to recover airspeed. One of the trade-offs is that the elevator inertia, feeding back through the stick will tend to increase the normal pull-force that is required in a turn or a pull-out. Next month . . . Tailless.

Fig. 4-5 Anti-servo.

SPORT AVIATION 41

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By George B. Collinge (EAA 67, Lifetime) 5037 Marlin Way Oxnard, CA 93030 Illustrations by the Author

"TAILLESS" AIRPLANES really tailless? By utilizing fixed wings for lift, manually-controlled stable flight is most difficult if not impossible without a tail (ref. 1). Nevertheless the primary inclination of designers of all-wing aircraft has been to increase efficiency by eradicating the fuselage and tail as non-essentail drag and weight-producing appendages. The "all-wing" purists favor the concept that the wing itself should contain everything the airplane has to carry. Although eighty years" 'and millions of airplanes has resulted in the conventional tail-at-the-rear formula, aviation history records many efforts to do it differently. But however it is disguised or camouflaged, the tail or rather the tail-substitute, is always there someplace, in some form or shape. A positively-cambered airfoil when used inverted, or a sufficiently reflexed (bent up) airfoil tend to inhibit cp travel or even reverse its movement so that its effect is to resist any change in angle of attack thus providing a degree of stability. The rear portions of these sections act as negatively-angled tails, the forward parts as conventional wings.

Tailless wings usually have a large amount of twist in order that the

outer areas (routinely swept at least slightly) can act as a tail although like the reflexed airfoils, they function with minimal leverage. And because of their limited power, the range of CG travel that they control must also be severely restricted, if not immovable.

38 AUGUST 1984

Unfortunately, wings of this kind plainly create less CL than "unstable" wings. To carry the same load more wing area is needed. This and the impossibility of effectively using highlift trailing-edge flaps (because of the aft cp shift) defeats much of the original rationale of the all-wing airplane. It quickly becomes apparent that any claimed advantage of the type is sharply compromised. All-wing aircraft have at times been fitted with trailing-edge flaps that go down but at the same time have an equal area go up. This is to average out the median line and maintain balance. About the only result is drag, though quite a useful and on occasion a most desirable force. Within their small operating envelopes, all-wing aircraft can add to their already necessarily stable characteristics by incorporating springs and/or weights in the elevator control-system. As with a conventional airplane, by limiting controlsurface travel, an all-wing airplane can also be made to resist stalling and spinning at low speeds, even when the stick is fully aft, which capability seems to be a penchant of a fair percentage of designers (ref. 2). There will always be dreamers with a favorite or pet formula in spite of the realization that their ideals may harbor innate flaws. It can be surprising to note the tremendous human endeavour that has been expended, in cases, in efforts to bring such ideas to successful fruition. Case in point, the Mel63 (ref. 3 to 10). Beset by inherent tailless problems, the liquid-rocket motor, with all

its then terrible propensities was employed by Alexander Lippisch in his passion to see his brainchild accepted as what else, a superior airplane. But that was not to be, although a num-

ber of Komets did become operational. It had the usual features of a singlewing, moderate aspect-ratio aircraft. The 23.3 degrees of sweep was a concession, not to boost the critical mach number, rather to allow a rearward displacement of the ailerons where they could do double duty providing control in pitch. A large amount of twist was necessary to provide sufficient longitudinal dihedral (Fig. 5-1). The landing flaps, referred to in German literature as brakes or dive flaps, were positioned in a mid-chord location, to minimize the resultant aft cp movement. Other all-wing aircraft have used the same flap location, and also just for drag. The Mel63 flaps could only have been intended for drag because ' they gave a CL increase of barely 0.1 and this is not subtracting the CL loss due to up-travel trimmers. It may be worth noting that the effect in pitch of Komet's flaps was onethird down-nose slightly up. From two-thirds down-nose lowered, requiring "tail-heavy trim". With the positions of the cp and CG pretty well fixed, only leading-edge slats were permissible without problems. They increase the usable angle of attack but do not cause significant cp movement.

Enabled by wartime German decision-system which was essentially controlled by political people, the Mel63 "proved" the concept, sort of. A

total of 360 were built in Germany (ref. 10) and 7 in Japan (ref. 11). Other than this one decreed adaption, there could be no real useful work for the tailless. One good thing about the Mel63 was what it spawned. It is obvious that Lippisch was finally convinced of the all-wing dead end and in May 1943 he left Germany to do new design work in Vienna. When the wings of a tailless design are swept sufficiently to provide really useful longitudinal dihedral plus a means of ensuring an adequate lever-arm for the elevators, closing in of the space between the arms of the wing was natural and inevitable and resulted in the delta as is known today. Besides structural advantages and a large increase of internal volume, it is an unchallenged fact that the narrow-angle delta planform has ideal aerodynamic characteristics for certain high-speed aircraft while at the same time is capable of great lift at low speeds, without the use of flaps which it, like the all-wing cannot use anyway. The large factor of induced drag at the controllable high-angles of attack well substitutes for flaps in

the desirable steepening of the approach, in the lowering of landing speeds and practically eliminating any semblance of unwanted float. When the elevens of the delta are

raised for the flare, the entire

airplane rotates though its inertia

Fig.

5-1 Me 163 1942.

keeps it momentarily going straight ahead. Thus the huge and instantaneous increase in CL, coupled with a tremendous ground effect, put the delta on gently, the aerodynamic drag slowing it automatically until the nose wheel is lowered onto the runway. Upon examining history, it becomes clear that the revolutionary Mel63 was not so revolutionary after all. Because, back in the early days, a man named John William Dunne

had invented the swept and twisted wing, to be flown without a separate tailplane. He had begun his experiments in 1904, through monoplane to triplane, his biplanes being the best known. Dunne, along with Samuel Franklin Cody in 1906, were employees of the Royal Aircraft Establishment, Balloon Factory, at Farnborough. Both these men (among others) were trying to be the first to

Fig. 5-2 Dunne d-5 1910.

SPORT AVIATION 39

Fig. 5-3 Swallow 1946.

fly in the UK. Cody succeeded in sustained flight on Oct. 16, 1908; Dunne's No. 5 machine flew in the spring of 1910. The Wrights were still using a forward elevator and their airplanes were not stable. In fact, at this time they stated that inherent stability was undesirable. Their system was for "hand-controlled equilibrium". Dunne had made his own calculations based on the Zanonia seed well before stability and control had been "clarified" by such as Lanchester (ref. 12). The Wright type front-elevator design greatly influenced beginning aviation, including Cody. Not so Dunne. He felt that airplanes should be definitely stable. Not that they should be able to fly without a pilot, but instead not require hectic, constant attention to pitch control as did the Wright types. He tried forward control-surfaces on one of his early gliders, for more nose-up control during landings. It was discarded, larger rear elevators worked better (ref. 13). A typical Dunne airplane, his first really successful flyer, the D.5 (Fig.

5-2) was a single-engined, twin-propeller biplane with a 52 degree sweep

and a pronounced twist. This powerful longitudinal dihedral gave the pilot a firm and steady airplane. His "horizontal rudders", as they were called then, were flaps at the wing tips which on his D.4 were actuated by a "modern" wheel on a column, the wheel for roll and push/pull for pitch, the original elevons! (Ref. 14) Dunne aircraft had fixed fins but no rudders. At least one of his machines was built under franchise by Nieuport in France (ref. 15) and several by W. Starling Burgess (ref. 16) in the U.S.A., who was trying to avoid the Wright patent on ailerons. In 1909 the British Committee of Imperial Defence, in all their wisdom, decided that the Aeroplane was a lost

40 AUGUST 1984

cause, so (for three more years) they concentrated on kites, balloons and airships (ref. 17). Dunne and Cody

were fired, though both continued experiments on their own. This seemingly congenital urge of English politicians to terminate the airplane by fiat made one of its appearances in 1965 with the scuttling of a number of projects, notably the TsR2. An all-missile or remotely-controlled air force was to be the wave of the future (ref. 18) and this, sadly, from a country that has contributed a good share of the world's finest flying machines. Back to WW-II. A group headed by the Horten brothers, Reimar and Walter, convinced the German Air Ministry that they, too, should be supported for their own particular brand of tailless. As the Hortens could not provide sufficient production facilities, the Ministry -later gave their design to Fig. 5-4 IA-38 1960.

Gotha (ref. 19). Not surprisingly, lit-

tle was accomplished. In 1945, the British Ministry of Supply sponsored a program of tailless research (ref. 20) which resulted in a number of experiments. On March 15, 1946 the first flight took place of a single-seat DeHavilland 108 Swallow (Fig. 5-3), one of three to be built (ref. 21). A few months later, DeHavilland announced the Swallow as an "... experimental basis for later types" (ref. 22) and the upcoming Brabazon IV transport (Comet) in particular. A 1946 full-page ad in Flight showed an artist's rendering of a proposed all-wing airliner (ref. 23). On Sept. 27, the number two DH108 disintegrated in the air with no escape for the test pilot Geoffrey Raoul DeHavilland, (ref. 24). There were no more Swallows built and no all-wing airliner. John Knudsen Northrop was an

References:

1. Aerodynamics of the Airplane by Clark B. Millikan, General Publishing Co. Limited, Toronto 1941, page 142. 2. Homebuilt Aircraft, Werner and

Werner Corporation, Santa Monica, 198,

Fig. 5-5 Fauvel AV-45 1960.

American "tailless" zealot and was responsible for a bevy of aircraft of this kind (ref. 25) including the B-35 and YB-49. He, too, could never make his wings thick enough to get everything inside, there were bumps, pods, blisters and engines all over. Up to the last, the problem of trying to devise

suitable high-lift devices eluded Northrop although he always claimed they were under study (ref. 26).

His Flying-Wing bomber prototype was being flown over the Air Force flight test center at Muroc, California on June 5, 1948. During a stall series the airplane began such violent somersaulting that the resultant strain separated some of the structure and it crashed and exploded with the entire crew onboard. The contract for thirty YRB-49 aircraft was immediately cancelled and the remaining Flying Wings were scrapped. The Air Force went with the B-36. Muroc was renamed in honor of the dead pilot, Captain Glen Walter Edwards (ref. 27). An Institute Aerotechnico type 38

cargo transport was designed by Dr. Reimar Horten and constructed in Argentina (ref. 28). It suffered from cooling problems with its four semiburied, extended-shaft air-cooled 450 hp El Gaucho radial engines. Its relatively high-drag fuselage, which was Fig. 5-6 DC9 Super 80 1982.

found necessary to carry anything the least bulky, again calls attention to the obvious futility of trying to make the all-wing concept a practical carrier (ref. 29). Not helping in his regard was a massive retractible nosewheel, pivoted under the pilot's cockpit. It took up much of the fuselage interior space. The IA-38 made a flight on Dec. 9, 1960 (Fig.5-4). For the last few decades, no large aircraft concern has been really serious about the classic all-wing aircraft. However, individuals have produced some examples (Fig. 5-5). These have been and are mostly single placers with restricted activity and little tolerance for variation of CG position. All have some accommodation to accepted flying techniques. The goal of putting everything inside a wing has proved illusive, nullifying the touted advantage of the tailless concept. In fact, modern transports evidence just the opposite end, where relatively tiny and thin wings support ever-larger fuselages. The sketch of the DC-9 (Fig. 5-6) has been traced exactly from one of a number of well-publicized photographs and so contains no biased distortions which might otherwise serve to enhance this

premise. Next month — Pitch and roll retrospect.

page 8. 3. Rocket Fighter by John T. Dodson, Flying, Jan. 1950. 4. Developing a Rocket Fighter by Rudolph Opitz and Robert Randell Air International, UK, Vol. 21, 1965. 5. Wings of the Luftwaffe by Capt. Eric Brown, Doubleday & Co., Inc., USA 1978, pages 167 to 176. 6. The Komet by William Green, RAF Flying Review, UK, Vol. 18, No. 8, April 1963. 7. Raketjager Mel63 by Mano Ziegler, Motor Press Verlag, Stuttgart, WG, 1961. 8. Ein Dreick Fliegt, The Delta Wing by Alexander Lippisch, Motorbuch Verlag, Stuttgart, WG, 1976. 9. Das Buch der Duetschen Luftfarttechnik by Bruno Lage, Verlag Dieter Hoffman, Mainz, WG, 1970, pages 552,553. 10. The Aeroplane Spotter, March 6, 1948, page 58. 11. The Aeroplane Spotter, April 3, 1948, page 82. 12. Early Aviation (at Farnborough) by Percy Walker, MacDonald & Co., Ltd., London 1974, pages 169, 171. 13. Early Aviation (at Farnborough) by Percy Walker, MacDonald & Co., Ltd. London 1974, page 231. 14. Earl Aviation (at Farnborough) by Perry Walker, MacDonald & Co., Ltd., London 1974, page 185. 15. Aviation Magazine, Paris, June 1959, page 28. 16. Flight, Jan. 3,1930, page 41 and Dec. 11, 1953, page 755; Contact! by Henry S. Villard, Crown Publishers, Inc., NY 1968, pages 166, 167, 238. 17. Early Aviation (at Farnborough) by Percey Walker, MacDonald & Co., Ltd., London 1974, page 327. 18. Project Cancelled by Dereck Wood, The Bobbs-Merrill Company, Ind., Indianapolis 1965. 19. Nazi Jet-Bats Which Never Took Wing by Erwin J. Bulban, Aviation USA, Oct 1945. 20. Aviation Week, Sept. 15, 1952, page 21. 21. Inter-Avia, Vol. 4, Oct. 1949, page 610. 22. Flight, June 6, 1946, page 562. 23. Flight, Nov. 7, 1946, page v. 24. Flight, Oct. 3, 1946, pages 364, 365; Aeroplane, Oct. 4, 1946, pages 380, 395. 25. Northrop Activities, Flight, May 9, 1946, pages 469, 470. 26. The Northrop "All-Wing" Airplane by John K. Northrop, Aviation USA, Dec. 1941; All-Wing Aircraft by John K. Northrop, Flight, June 55 and 12, 1947. 27. Model Airplane News, June 1963. 28. Air Pictorial, Air League of the British Empire, London, July 1961, page 193; Jane's Encyclopedia of Aviation, Vol. 3, Grolier Educational Corporation, Danbury USA 1980, page 478. 29. Tailless Problems by G. H. Lee, RAes paper, Nov. 14, 1946, Flight, Nov. 28, 1946; Stalling Phenomena and the Tailless Aeroplane by A. R. Weyl, series, Flight 1947.___________________

SPORT AVIATION 41

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OO ADX/O PITCH AND ROLL RETROSPECT

Part 6 By George B. Collinge (EAA 67, Lifetime) 5037 Marlin Way Oxnard, CA 93030

,

Illustrations by the Author

J.N THE PAST, if an airplane had two wings placed one behind the other and they were of nearly equal area, the term "Tandem" would apply. If the front surface was of considerably smaller area, the airplane was a "Canard". It is fair to assume that both these types are basically different configurations of the same formula. Therefore, in the interests of conformity and with deference to what is already accepted nomenclature, this review regards the entire general class as tandems. The forward wing is the canard regardless of its size. The aircraft designs of Penaud, Caley, Lilienthal and Chanute all em-

braced rear tails that incorporated longitudinal dihedral. While Montgomery and Langley favored the tandem arrangement, they nevertheless added Penaud tails for pitch stability (see Figure 6-1). The Wrights, of course, did not use a rear tail in the beginning. They

were successful with a front elevator. A lot of would-be aeronauts around the world decided to copy. And no one appeared upset, least of all the Wrights, about these obvious imitations of the Wright-style front elevator. What did bother the Wrights no end was what they considered imitations of their lateral control! Over this they had legal exclusivism.

Fig. 6-1 Penaud-tailed Langley 1903

A giant brouhaha was to develop and it got going about the same time that changes were taking place which would help to standardize the method of pitch control also. So as the two

axes of control were intertwined after a fashion, a touch of history on the aileron is included here. The Wrights quite simply regarded direct lateral-control as their property, which others could use. But not for making money. When profit became a factor, which it almost always did, the Wrights wanted in. There were some legitimate technical conflicts naturally, a few of more substance than others, between the Wright's coupled rudder/warp system and that used, for example, by Dunne. He had just two differential-elevators, no rudder surface at all! The original Wright patent application of March 23,1903 (Ref. 1), though denied, resulted in a second request which was ultimately issued as No. 821,393 on May 22, 1906. It carried 16 claims for "improvements in flying machines" but included that most significant award, the one that was to cause all sorts of consternation in the aeronautical world, the one covering manipulation of wing tips in any way or manner in order to achieve lateral control (Ref. 2). Many would try but, in most countries involved, there seemed no way around this legality. Apparently discounted or ignored

by the U. S. Patent Office was early activity regarding lateral handling because at least two existing U. S. patents clearly described such work. One was held by Montgomery, another by Mouillard. Montgomery's manned glider flight in 1885 was made with a glider equipped with

Fig. 6-2 Voisin Goupy No. 1 1908 24 SEPTEMBER 1984

aileron control (Ref. 3). The Mouillard glider, which had been built under the aegis of Chanute, incorporated "annularies", sections of each trailing edge that could be independently lowered by the pilot. While Chanute was

\ financing his patent (No. 582,757, May 18, 1897), Mouillard wrote to him ". . . this device is indispensable . . . it is this which permits going to left and right" (Ref. 4). Additionally, it is on record that Edson Gallaudet had developed a differential wing-lift technique a year before the Wrights (Ref. 5) and that Mathew Boulton of England had patented a small movable wing-tip as far back as 1868 (Ref 6). When Chanute first learned of the proposed Wright coverage, he naturally thought that prior patents would certainly invalidate their claims (Ref 7). But as the world knows, the Wright patent was granted (Ref. 8). Almost no one could fly for money unless a licensing fee of over one thousand dollars a day was paid to the Wrights (Ref 9). That was a lot of cash in those days, so it is not too surprising that after the initial euphoria over the Wright demonstration flights in the USA and abroad, there gradually developed resentment and noncompliance of the patent. Even the granting of another patent, covering midwing "lateral balancing rudders" to Dr. Alexander Graham Bell of the Aerial Experiment Association only served to muddy the legal waters. This patent was issued on Dec. 5, 1911 after a three-year wait. (Ref. 10). Well, as it turned out, the Wrights sued a lot of people and litigations made news up to the full 17 year patent life (Ref 11). Many designers sought to thwart the document that they thought was improperly broad and inclusive. For instance, Burgess and a number of constructors, U. S. and European, used ailerons that only moved downward. A few others, wranked over the controversy, dispensed with lateral control altogether and using side curtains, performed skidding turns with rudder alone (Fig. 6-2). In fact, journalists of the day called attention between flat turns and banked turns, the latter rating a higher accolade. By 1915, Curtiss was making airplanes in Buffalo and shipping them to England without ailerons. Ailerons were manufactured in Toronto and sent abroad separately (Ref. 12). Because it was causing so much havoc, in 1917 with the U. S. almost

a low seventy-five thousand. France contested buying but their courts upheld the Wrights. Germany ignored any fee. They said Chanute, in an early lecture, had talked about wing warping and in Germany that was sufficient to invalidate the entire patent (Ref. 13). Canada obviously had also not recognized the legitimacy of the Wright patents (Ref. 14). Between 1909 and 1913, the Wrights had sold licenses in seven countries, including a syndicate in France and companies in England, Germany and Italy (Ref 15). The Wright patent drawings had been

available to all since disclosure in 1906, and in addition were printed in detail in the 1906 French publication of L'Aerophile (Ref. 16). Because of this, a great many early airplanes around the world, if not actually Wrights, certainly looked like Wrights. The front elevator arrangement was very popular on pushers although as 1910 arrived, different airplane configurations had been tried by many individual designers. Among the best known of the tractors were Bleriot, Esnault-Peltrie, Breguet, Antoinette, Nieuport, Voisin (Goupy) and A. V. Roe.

Fig. 6-3 Voisin 1909

Fig. 6-4 Curtiss D111 1910

at war and needing airplanes, Congress appropriated one million dollars

to acquire the Wright basic patent by condemnation. Thereafter, a crosslicensing or pool of patents within the new Manufacturers Aircraft Association solved most problems. Down from an original two hundred thousand dollar price, beleaguered England was granted rights to the airplane for

Fig. 6-5 Wright with auxiliary elevator

1910 SPORT AVIATION 25

It is well known that the early Wright airplanes were unstable. Wilbur said, "We would arrange the machine so that it would not tend to right itself (Ref. 7). This statement was made well after the fact. And the fact was that their airplanes could not have been much else except unstable, what with highly-cambered main-wings and free-floating, nonloaded front elevators! The skill necessary to aviate satisfactorily must have been of high order. Accordingly, to stabilize airplanes with forward elevators, various constructors employed a long-levered, fixed rear-tail, but still retained the ubiquitous front elevator for pitch control. Curtiss, Farman, Voisin, et al, were originally of this configuration (Feb. 6-3). Later, about when the flying fraternity had grudgingly resolved to go ahead and use warp or ailerons and to pay the toll, elevator action was incorporated into the rear tail (perhaps due to tractor influence) but initially only in conjunction with the existing elevator in front (Fig. 6-4). After this, it was only a relatively short interim before even the diehards gave up their forward elevators. Detailed in the literature are a number of interesting stories of how, for example, the Curtiss front elevators were eventually discarded. One source describes them as being knocked off running into a fence by Lincoln Beachey while barnstorming. The airplane was then hurriedly flown without and what a difference it made! (Ref. 18) Al J. Engel suggested to Curtiss that the front elevator not be installed on new machines at the factory and be removed from all their existing machines in the field. They became "headless" (Ref. 19). In a 1909 London Sphere illustration, Latham is shown in his tractor monoplane over the channel. Among the notations, its fixed horizontal tailplane is labelled "stability fin" (Ref. 20) indicative of its required purpose and the trend of the day. The Wrights had been concerned enough over the difficulty of flying their airplanes to do work on various devices to assist the pilot. At least one patent was granted to them (1909) for an automatic pitch-stabilizer. Actuated by a vane that controlled a compressed-air supply, it was in turn connected to the elevator (Ref. 21). This invention was soon abandoned as unnecessary because after the crash in 1908 in which Selfridge was

killed and Orville seriously hurt, there was a transition period wherein the Wright airplane also flew with an added, fixed tailplane. This was soon modified to act as an elevator, to work together with the front one, much like 26 SEPTEMBER 1984

Fig. 6-6 Wright model B 1911

everyone else had been doing (Fig. 6-5). Eventually, the entire front elevator was removed and in 1910, for a more complete reversal of their original policy and to complete the metamorphosis (Ref. 22), the Wrights produced a factory airplane without a forward elevator at all and finally a standard wheeled landing-gear (Ref. 23). This was the "headless Wright" or Model B (Fig. 6-6). It is possible that crashes, in which the heavy mass of the pusher engine let go and thumped the pilot and/or passengers, influenced greatly the ascendancy of the tractor. Its crash worthiness was recognized as superior. Tractor airplanes could also be made smaller, cleaner, faster and with greater regard for pilot comfort. Tractor propellers were, in the long run, more efficient and less hazardous to the pilot in the air. The existing disenchantment with the forward elevator also served to hasten the almost exclusive use of the tractor although there were a few German pushers pressed into service at the beginning of World War 1, with the occasional experimental model produced by the factories as the war continued. However, it was the English who gave the pusher a renewed lease on life, if only a brief one. The problem was non-aerodynamic - that of shooting bullets through a rotating propeller. It may have occurred to contemporary tractor designers to place the guns outboard of the propeller arc and so bypass the complication and weight of synchronizing gears, but the reliability of available machine guns and ammunition required much hand

clearing of jams, necessitating that the breeches be close to the pilot. Until the Allied syncro-system was firmed up, the pusher helped fill in. As years passed, at least up to recently, tandems have been built only occasionally, made pitch-stable

largely by a forward CG and the utilization of a form of longitudinal dihed-

ral, although not without some problems intrinsically associated with the type. Next and concluding part . . . Tandems.

References for Part 6: 1. Solving the Control Riddle by Robert Burkhart, Air Line Pilot, Dec. 1976. 2. American Science and Invention by Mitchell Wilson, Bonanza Books, NY 1960, pages 346-351. 3. Design For Flying by David B. Thurston, McGraw-Hill Book Company, NY 1978, page 2, 3. 4. and 5. A Dream of Wings by Tom D. Crouch, W. W. Norton & Company, NY 1981, page 70, 71; page 307. 6. Dreams and Realities of the Conquest of the Skies by Beril Becker, Atheneum, NY, 1976, page 149. 7. Glenn Curtiss, Pioneer of Flight by C. R. Roseberry, Doubleday and Company, Inc., Garden City, NY, 1972, page 191. 8. One Day At Kitty Hawk by John E. Walsh, Thomas Y. Crowell Company, NY, 1975, pages 20, 175, 216. 9. Curtiss by Louis S. Casey, Crown Publishers, Inc., NY, 1981, pages xi, xii. 10. Homebuilt Aircraft, Werner & Werner Corporation, CA, Sept. 1981, pages 24, 25. 11. Aerial Age, Aerial Age Company, Inc., NY, March 1923, page 139. 12. Glenn Curtiss, Pioneer of Flight by C. R. Roseberry, Doubleday and Company, Inc., Garden City, NY, 1972, page 398. 13. The Story of Flying by Archibold Black, McGraw-Hill Book Company, Inc., NY, 1940, pages 92 to 98. 14. Glenn Curtiss, Pioneer of Flight by C. R. Roseberry, Doubleday and Company, Inc., Garden City, NY, 1972, page 477. 15. Contact! by Henry S. Villard, Bonanza Books, NY, 1968, page 24. 16. Aviation by Christopher Chant, Chartwell Books, Inc., NJ, 1978, page 28. 17. The Flying Machine by Alien Andrews, G. P. Putnam & Sons, NY, 1977, page 92. 18. The First To Fly by Sherwood Harris, Simon and Schuster, NY, 1970, page 226. 19. The Magnificent Old Man and His Flying Machine by William J. Alien, Air Line Pilot, Jan. 1976, page 18. 20. and 21. See Them Flying by Houston Peterson, Richard W. Baron, NY, 1969, page 393; page 44. 22. Early Aviation (At Farnborough) by

Percy B. Walker, MacDonald & Co. Ltd., London, 1974, page 173. 23. The Wright Brothers by C. H. Gibbs-Smith, Science Museum, H. M. Stationery Office, London, 1963, page 26.

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MORE ON TANDEMS Conclusion By George B. Collinge (EAA 67, Lifetime) 5037 Marin Way Oxnard, CA 93030 Illustrations by the Author

u.

' P TO 1910, the Wrights and

are other arguments put forward for

if the aft wing of a tandem pair is

many others flew airplanes with forward elevators. These aircraft cannot be properly classified as tandems.

double wings. But it is important to

mounted at a higher incidence as on

note that when a second wing, of any

the biplane, a very unstable airplane

size, is brought into position near a

would be created (Fig. 7-4). On this

Their primary element consisted of a

first wing, either above it or below as

configuration the airplane is balanced

cambered, biplane wing-cell on which

a biplane or behind it as a tandem, there will obviously be an interference with the pressure distribution around each (Fig. 7-2). Generally, any kind of second-wing placement is going to rob the first of a significant amount of downthrust

the location of the center of gravity was based. To this unit was attached a free-floating "horizontal rudder" actuated by the pilot via a cable or strut. This surface generated no sustained lift and was not intended to share support of the total weight (see Fig. 7-1). It is difficult to imagine a flyable airplane more unstable or more formidable to control in the pitching plane. The Wrights originally utilized a

double or biplane design for purposes of bracing. Later it became part of the concept of twisting or warping in order to alter lift in step with rudder movement. When dual wings are used today, in many designs, it may still be for the purpose of bracing. There

and will at the same time and for the

same reason reduce its own LD ratio (Ref. 1). Acknowledged worldwide is the assumption that a single wing is the most efficient mechanism for obtaining the greatest sub-sonic lift for the least drag. This could nicely explain why one never sees a biplane 707.

Normal decalage and positive stagger (Fig. 7-3) have been routinely employed to reduce performance loss in

the biplane (Ref. 2,3). Unfortunately,

Fig. 7-1 Wright type A 1908. 46 NOVEMBER 1984

at a point equidistant between the cp

of each wing. At a specific speed, the wings could be lifting equally, at the

same angle of attack, even though their incidence angles vary.

With a speed increase, the pilot would normally lower the nose and reduce CL to stay level. The angle of downwash from each wing would then

lessen and the cp of each would move aft causing a nose-down effect. To add to this, the rear wing at its higher angle of incidence and now operating

in less downwash, is lifting a greater proportion of the total weight. Therefore, the overall airplane cp moves considerably rearward. An unstable

nose-down or "tuck" results. The pilot would have to pull on the stick to prevent catastrophic diving,

Fig. 7-2 Potential perturbation.

Fig. 7-3 Lower wing suffers most.

which action is definitely counter to the requirements of a stable airplane. At low speeds, just the opposite: the

cp of both wings at high angles would move forward, pulling the nose up.

The forward wing would lift more due to flying in a greater upwash (higher

angle of attack). Instead of a pilot continuing to pull at the lower speeds, he would have to increasingly push to

keep from stalling. Again, most unstable.

Consequently, it is unusual on tandems to mount the aft wing at a smaller incidence even though it worsens the total L/D ratio (Fig. 7-5). The result of this causes the canard to lift too great a proportion with speed. To offset this problem and still achieve an acceptable equalibriurn at a given speed (usually cruise) the CG is positioned more forward, which necessarily increases the canard wing-loading. The aft wing, at its re-

duced angle, has a much lower cruise CL. But the longitudinal dihedral now provides stability for the tandem. If it goes faster than the selected cruise speed, the canard lifts more, raising the nose to slow the airplane. Less lift at lower speeds depresses the nose, speeding up the airplane. As with a rear-tailed design, the restoring force can be made powerful and pitch stability made quite firm. Negative G, as in real inverted flight, would completely upset the balance of a tandem. The wider the interval between the two wings of a tandem, the greater the physical travel of CG that is possible, while still holding the limits to the same percentage of the separation. This feature has appealed to some designers of cargo aircraft. Directional stability becomes more of a problem though, and large fin area is needed. If multiple engines are not on

or very close to the center line, oneengine-out control would be extremely difficult if not impractical except at high speeds. Lowering a flap increases the CL of

an airfoil but will not appreciably

change the normal stalling angle

(Ref. 4, 5). This characteristic is, of

course, what makes a canard elevator

possible (Fig. 7-6) for it is necessary to be able to depress elevator at low speed, say on the approach, and not be confronted with a sudden stall.

A total or abrupt canard stall must

be guarded against at all costs otherwise the plain (flapless) rear-wing, still lifting after the canard stalls, would put the airplane into a dive or somersault, from which it would be awkward to recover and from which a large loss of height would occur. So the canard is made resistant to the low-speed stall. A wide range of modern airfoils offers a choice of lift SPORT AVIATION 47

curves that do not peak sharply. In

it might be needed most, for instance,

yesteryears, the CL profile of conventional sections was flattened by other means. The Focke-Wulf and Rohr canard surfaces (Fig. 7-7) both used similar forward-swept elevators, assisted by low-aspect ratio (Ref. 6, 7,

during a slow flare for landing. On such an occasion, the only "up" pitch control left to the tandem pilot

8,9). As with any flapped wing, there is

would be the addition of engine

power. On a conventional airplane,

the increased slipstream might serve

to hold down the tail and so help to

reduce the landing speed. Adding

a practical limit to down-travel. At

power to a canard-controlled airplane,

depressed to the point of maximum

only increase speed and/or cause it to

low speeds, if the canard elevator is

lift, any additional travel (stick back) would only decrease the lift. This effective reduction of elevator power to

under the same circumstances would

gain height. Hihg-speed stalling, however, is usually possible despite slow-speed

raise the nose, restricts the pilot's

restrictions and can result in some

lar in result to the limited elevatortravel occasionally encountered on conventionally-tailed airplanes and

the past are to be given credence, "safe" limited-control airplanes are for the most part apparently disliked

ability to create a full stall. It is simi-

which is also done to achieve stall/ spin resistance.

Thus, a few airplanes, conventional

pretty wild rides. If sales records of

by trained pilots.

If there is a deterioration of wing

air-flow causing loss of lift, due to

and tandem alike, can be flown slowly

bugs, grass, rain, frost or ice, it could

back, with little risk of an inadvertent loss of lift. Regretably, it is impossible to design for a "soft stall" or "no stall" feature and at the same time retain enough elevator authority to prevent running out of pitch control just when

even to cause a nose-up change. The

(after a fashion) with the stick fully

be felt primarily by the canard, due to its higher wing-loading. Yet, a considerably larger aft wing may loose sufficiently to equalize the effect or

reduction in lift can be much more

noticeable on a tandem as opposed to

the same amount barely detected on

Fig. 7-4 Unstable.

Fig. 48 NOVEMBER 1984

7-5 Stable.

a conventional airplane. This is because the conventional has its CG relatively close to its cp, which relationship remains reasonably constant. The tandem's CG, however, is supported between two widely-spaced lift centers. Any diminution of lift greater on one wing over the other not only causes a height decrease but at the same time destroys the delicatelytuned balance resulting in a situation that is not always fully controllable. Consequently, the delegation of some or all pitch management to the rear wing (Fig. 7-8) or the addition of a horizontal tail (Fig. 7-9) is and has been a practical method of increasing tandem stability and controllability. During the early thirties, the Flea was a tandem employing a most incongruous scheme, that of a fixed rear wing set at high incidence in company with a loaded but semi-floating canard! An aft elevator or trimmer was found desirable by Henri Mignet, its designer, for his own personal Flea (HM18). With the angular travel of the forward wing limited to narrow limits, its envelope of manageability still remained small. Adding a rear elevator negates a lot of the design features and com-

promises originally part of the "appeal" of a tandem. But such a change could be seen as a logical and obvious development much like what took place in 1910, when pitch control moved from the front end to the aft end, on pushers and tractors alike. There are other items with which

the designer of tandems has to cope.

One is a tendency of a canard elevator

to ride up in flight, because of the pressure differential, which causes the nose to lower. To hold the elevator down, the pilot would have to exert a pull force with increased speed while in a dive, which action once again is

reversed to normal practice. To improve stability and generate a more acceptable feel, a spring-loaded trimmer, or better, a simple locked servo

(Fig. 10) is attached to the elevator. It has little effect at low speeds. Fig. 7-6 Flap increases CL.

Another item is flaps. Conventional trailing-edge camber-increasing flaps

would greatly improve airport performance of tandems. Long, flat ap-

proaches could be advantageously steepened, prolonged floating reduced

and touch-on speeds lowered. But this high-lift device causes an extreme nose-down trim change. Without a rear tail to automatically assume an

increased, compensating down-load, the flapped tandem would be required to generate a greatly increased upload on its canard. Trouble is, at approach speeds with front elevator deflected, the ordinary

canard is already very close to its finely adjusted CL max. Therefore, if

the aft wing is to even moderately lower or extend flap, then a different

kind of canard must be used, one that can increase its CL to a high enough

value to counter the CL increase of the aft wing and maintain a balanced

airplane.

Canard flaps of this kind obviously

don't double well as elevator controls, Hg. 7-7 FW19a 1931 & Rohr 1947

that function then residing in the aft wing. Black-box coupling of front and

rear flaps ensures a constantly balanced airplane (Fig. 7-11). Very low aspect ratio suppresses stalling, is suited to the delta configuration and is a much simpler expedient than

slats or Kreuger flaps.

A different approach is one where the canard itself sweeps forward, its increased leverage providing a balancing up-load for a limited amount of trailing-edge flap extension. Complication on the march. While the pure delta layout already provides a steep approach and a slow landing speed, it cannot normally use trailing edge landing flaps either. Machines of this type have been briefly tested with retractable Fig. 7-8 Despretz mk II 1965.

canards or

"moustaches" on the

theory that at high angles, not as SPORT AVIATION 49

the canard has computer sensing and control inputs up to 40 times per second! (Ref. 10) EPILOG

The question, posed by the title of

this series can possibly be answered

"there is no free lunch." If the tail is

discarded, then a substitute must take its place. It may be a reflexed trailing edge, sweep and twist, or a canard that reaches CL max before its companion aft wing. It would seem evident all the same, that since 1910

a simple rear-tail, separate or inte-

gral (delta style) has, through usage, been the dominant, almost exclusive Fig. 7-9 Luton 1936.

form of pitch control for manually

controlled,

fixed-wing

airplanes.

Whether it remains so depends on a

replacement not just different but truly superior.

Fig. 7-10 Fix by fixed servo.

Fig. 7-11 The Viggen interconnection.

much up-elevon would be needed,

leaving more lift on the wing, giving

even lower landing speeds. The practical result of these tests has manifested in the now common

use, on low aspect ratio, high performance airplanes, of small fixed forward surfaces. Called canards, a more

accurate term would be "king-size vortex-generators" which are employed, as is the forward wing of the Vigen, to prolong the main-wing, upper-surface attached-flow to a very steep angle and to improve control by energizing the stream across the fin(s) and rudder(s) when they would otherwise be immersed in an extensive 50 NOVEMBER 1984

dead-air region. It has already been detailed that the Wrights originally flew a decidedly unstable arrangement that

demanded a high degree of pilot concentration. A modern, fly-by-wire, experimental, aft-tailed fighter has been deliberately made likewise unstable, by moving the CG aft of the neutral point. The theory this time is quicker maneuvering although (and the secret of success) angular deviation requires close electronic monitoring to prevent lightning-quick, catastrophic divergence. Similarly, Grumman is working on a tandem design where, to create a flyable airplane,

References: 1. Tandem Arrangements, Airplane Design by Edward P. Warner, McGraw-Hill Book Company, Inc., NY 1936. Pages 275, 277 and 278. "The performance of a tandem combination is always poorer than that of the individual wings, as might be foreseen from the slightest consideration of induced drag theory. Since, if the forward wing is lifting, its downwash rotates the lift and drag axes of the aft wing, effectively increasing the value of the drag component of its lift vector. Unfortunately, the combination of angles of attack offering the greatest aerodynamic efficiency has serious disadvantages on the score of stability." 2. Mechanics of Flight by A. C. Kermode, Sir Issac Pitman and Sons, Ltd., London 1942, page 66. 3. Aerodynamics of the Airplane by Clark B. Millikan, John Wiley and Sons, Inc., London 1941, page 77. 4. Tail First, Aeronautics Oct. 1939, page 36. 5. Report 824, Summary of Airfoil Data, NACA 1945. 6. Midnight Oiler, Skyways, Nov. 1946. 7. Lightweight Canard, Flight, Jan. 16, 1947, pages 55, 56. 8. The Ugly Dickling by Rex King, Aeroplane Monthly, Oct. 1973, pages 275 through 278. 9. Das Buch der Deutschen Luffahrttechnik by Bruno Lange, Verlag Dieter Hoffman, Mainz WB 1970, pages 272, 273, 274. 10. Aviation Week, Dec. 6, 1982, page 67. Grumman Aerospace Corporation ad ". . . without computers sensing and moving these canards 40 times each second, the craft would not be flyable because the pilot couldn't adjust positions fast enough