The Case for Tandem Landing Gear - Size

dynamic and structural concerns. From the aerodynamic ... directly into the fuselage structure without the large bending moments ... simple frame with point-to-point mounting automotive- type shock ... The tandem landing gear has several advantages in ... disadvantage is the requirement for accessory outrigger wheels.
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The Case For Tandem Landing Gear (Technical art by Bill Blake) By Edwin R. More, EAA 18475 8 Wescott Rd., Simsbury, Conn. INTRODUCTION

This article is an exposition on the subject of tandem landing gear for use on homebuilt aircraft. It is not a comprehensive survey of landing gear design nor a rainbarrel-tight logical argument, but merely the musings of a

you, the ground handling characteristics of an airplane with three wheels is not the world's most stable wheeled vehicle and is a caution whether the third wheel is forward or aft. With this sense of proportion about the current state of aircraft landing gear layout, we pass on to some relatively modern developments. Apparently some bright ancestor of today's EAA types who lived about 270 years ago came across the notion that two wheels could be placed in tandem rather than side-by-side. That arrangement did not improve the static stability much but it was quickly noted that the bicycle possessed a characteristic dynamic stability. That is, when in motion the bicycle could easily be balanced by shifting one's weight and by proper steering. A revo-

new aircraft is primarily beneficial in that it allows one

lutionary concept was born that has since persisted and still remains — a bicycle. Strangely enough, the one place I can think of where the bicycle wheel concept has been used outside of its original purpose is on the airplane! Now there is a contradiction. First I claim the design of aircraft is hobbled to the three-wheel poorly located landing gear, and then I turn around and state that it has

to thoroughly examine the real requirements and actual

made use of a relatively new arrangement — the tandem-

job of the aircraft's various parts. The following are all of the "pro" arguments that have come to mind and some partial rebuttal is reserved for the conclusions. Read on,

wheel arrangement. Well, the truth of the matter is simply that the tandem landing gear is not popular and that, after all, is the purpose of this article in that it desires to

and don't be afraid to make comment on what appears next.

make it more popular. If we read Jane's All the World's Aircraft and check aviation libraries for forgotten de-

HISTORICAL RAMBUINGS

gear. In our use of the term "tandem landing gear" we take into account both the "bicycle" arrangement of two similar size wheels in tandem, and the large single wheel

fellow EAA member on the occasion of his first original

design project. With this in mind, it is claimed that the author's mind is relatively fresh and that the bias is still mostly in the plywood spar webs. Designing a completely

The dynamics of an aircraft in the difficult seconds between full flight and an easy taxi have long struck fear into the hearts of student pilots and provided interesting problems for the aircraft designer. The many compromises that are met in the design of an aircraft usually favor the performance and flight handling characteristics requirements. Witness to this are the many aircraft that require a minor degree of mountain climbing skill stepping over

a "No Step-Flap" onto a high, smooth sloped surface with precious little hand or foot holds. This is generally the

top-loading bird that places your muddy brogans on the seat cushions just prior to plunking down in that $90.00 suit. The high-wing job that has awkwardly placed, oilcovered struts which are invariably wiped by pants legs is not appreciated either. By the same token, the landing gear on the airplane is generally an "orphan" in the de-

sign family — an orphan that is adopted as a singular appendage with little thought given as to how the rest of the aircraft gets along with it except that it does not interfere with the other flight functional parts. The very first aircraft landing gear was simply the pilot's feet. This was a fine idea until aircraft flight speeds exceeded pilot foot speeds. Next a sleigh runner was tried, but was found to be fragile and quite difficult in ground handling. Next on the list for early experimenters was the float and the wheel. This story is about the use of the wheel, so we'll drop the "wet wing" right now.

When man first conceived the wheel, he probably started with A wheel — that is, just one, and soon found it to be very unstable in that when he climbed aboard, it fell over in any direction. The first attempt to cure this "development" problem, very likely, was to put a stick

signs, we find two rather frequent uses of the tandem

forward and small tailwheel aft.

One such use is, of course, the Boeing B-47 and B-52 bombers. The second is the sporting glider and sailplane. Glider designers were extremely quick at recognizing the

aerodynamic efficiency of the tandem gear and its efficacy in ground handling characteristics. As any glider pilot or owner can testify, steering a glider rolling on the runway is a very easy task. When one is working with inertia only for power, easy steering is a must. I know that at this point people will remember the PAR "Special" of 1947 and other sporadic tandem gear applications. To this I would reply that they were also one-of-a-kind types not denoting much popularity. So, having covered the conceptual development of aircraft landing gear and sensing that you are not entirely convinced, I would like to move onto a slightly technical discussion where it is hoped that the physical dynamics of aircraft landing gear will be made clear and where sharp thinking will overpower the reader and convince him that the only way to build his next airplane will be with a tandem landing gear! THE TECHNICAL ARGUMENT

There are only a few criteria for judging the technical competence of a particular design, namely: Functionality, efficiency, durability, and economy. Consideration given each in their more general sense will always serve to decide the technical value of any of man's contrivances. The tandem landing gear has been proven functional

at least by its universal acceptance on glider-type aircraft. However, let us look at its dynamics. Referring to Fig. 1,

through two wheels, generating an axle which was much more stable. Later refinements led to three and four wheels with even more stability and laudable success at providing ground transportation. Several thousand years

let us consider the effect of one conventional wheel touching the ground first, thus creating a large asymmetric wheel run-up drag (strong enough to slip the tire on the pavement leaving a black mark). The force W could also

later the airplane has almost universally chosen the three wheel configuration as the best compromise between desired stability and undesired drag. As any pilot will tell

be a result of a dragging brake or frozen slush or a blown tire. The force W is balanced by part of the thrust T or

inertial forces if the power is off. T, however, acts usually SPORT AVIATION

51

through the center of gravity which leaves a moment effect: Momentw = (W) (wheel tread) 2 This moment can only be resisted by the tail contribution R. Thus: Moment]t = (R) (tail moment arm) = Momentw and: R = (W) (wheel tread) (2) (tail moment arm) It is then apparent that the tail can contribute only its maximum force minus R to the task of steering the aircraft. As some of us who have groundlooped an air-

The relatively short truss involved provides an appreciable weight saving over spring steel types. A much larger wheel can now be used since it will be buried in the fuselage and is even desirable. This means that the

wheel will operate more efficiently over rough ground. The complexity of retractable designs is not needed due to the very low profile drag of tandem fixed gears. The argument for durability can only be satisfied by personal experience because of the wide variety of operating conditions and user treatment of the aircraft. What is durable to one owner may be quite unsatisfactory in the experience of another. However, there are certain more general statements that can be made about potential

for durability. As a generality, it can be said that for a given load requirement a longer and narrower support

structure will always be heavier as a compensation for higher stresses. Weight of spring leaf landing gear struts is one example where structurally weaker wide flat crosssection of the spring leaf requires many additional pounds of steel, directly to provide sufficient strength and stiffness for the strut. This weight penalty is in part made up by choosing lighter and less durable wheels, brakes, bearings, etc. With a shorter length and closer

FORCE "W" ACTS AT WHEEL FORCE "T" IS PROPELLER THRUST FORCE "R" IS LIFT OF RUDDER AND FIN

FIG. 1

plane can testify, one can make a nice left turn with full

right rudder on or vice versa. By contrast, the tandem gear W acts through the CG location as the wheel is centered on the aircraft and R is simply not required, leaving all rudder power to steer. Fig. 2 shows the end view of the same situation and demonstrates that the vertical load at

the wheel, W,,, has to be opposed by additional wing lift on the opposite wing. This can be done with aileron or one can wait until the other gear comes forcibly down and the first rebounds causing the aircraft to skip left to

right. The tandem arrangement does not need this extra wing lift until the airplane starts to tip and the CG swings out over the wheel. Since the eccentricity is small, the

balance can be maintained by the light outriggers shown. Finally, in Fig. 3, it can be seen that if the dimensions between the wheel and the CG are the same there is no difference in balance force required from the elevator and stabilizer. To sum up, the tandem gear provides more steering power and has less tendency to rock with no alteration in porpoising characteristics which means you can still bounce a landing but it is easier to keep the airplane headed straight. The argument about efficiency centers about aerodynamic and structural concerns. From the aerodynamic standpoint it is immediately apparent that a wheel half buried in the fuselage creates far less drag than the entire wheel stuck out on the end of a strut with two or three locations available for generating interference drag. Referring to Fig. 3, it is also apparent that the deeper fuselage will also create more drag. However, the engine and propeller can be moved to take advantage of the tandem gear and reduce fuselage depth. Structurally, the tandem gear is more efficient for several reasons. The most salient is that the landing loads are transmitted directly into the fuselage structure without the large bending moments associated with cantilevered landing gear legs or the extra fittings and drag of a truss-type landing gear leg Fig. 4 shows a representation of what a tandem gear suspension would look like. 52

MARCH 1970

supported tandem gear, the weight savings may be put into more robust versions of these components. Moreover, automotive components become feasible for possible cost reductions. The more robust components translate into greater durability and better service due to greater functional capabilities such as more effective braking and easier ride over rough ground. So far, durability has been discussed in a serviceability sense. Picking up the dictionary definition as the ability to endure, particularly applied loads, another durability comparison can be drawn. Landing gear struts of conventional design are

A I R C R A F T WEIGHT

f

t

WHEEL LOAD

WHEEL LOAD

CONVENTIONAL

SINGLE-WHEEL NO YAW

FIG. 2

well known for their lack of resistance to side loads. The layout of a tandem gear shown in Fig. 4 has a yoke wheel

retention highly resistant to side loads and fore-aft loads as well. Economy to most homebuilders is measured in dollars and cents culled out of the family budget. There might be 2,000 man-hours in the project but if it has cost only $1,000.00 out of pocket it is displayed as an economical

airplane. Since homebuilding is strictly a hobby, I will go

along with this concept of economy and reflect on the out-of-pocket costs. If the landing gear struts are made out of steel tubing, which is bought by the foot, then the

W , WEIGHT TL = T A I L LIFT WD = WHEEL DRAG

FIG. 3

TANDEM LANDING GEARS . . . (Continued from preceding page)

tandem gear having struts only half as long as conventional designs would cost half as much for materials. But, since the strut is also stiffer because of its shortness, it can use thinner and lighter tubing for even more economy. The second out-of-pocket costs on landing gears is paying for some form of shock absorption. This could be bungees, oleos, rubber doughnuts, or spring steel. In every case it requires extra hardware, ma-

FRONTVIEW

small caster wheel or skid mounted directly to the wing tip undersurface does admirably. The generally shorterwinged powered aircraft presents a more difficult problem. If the aircraft center of gravity is kept low and a

mid or low-wing position is chosen, the outrigger may be located out near the wing tip and consist of a light skid or even a thin tube strut with a caster wheel added to its end. The low wing, low CG, and outboard location of the

outrigger contribute to light applied loads and subsequent simplicity in mounting to the wing.

The scheme found on the Fournier RF-4 points out how simple and light the outrigger can be. As a final summation I can only say that the tandem landing gear is not a valid modification to most of the existing homebuilt designs. However, it does hold great promise in application on original design projects and I sincerely suggest consideration of it on your next original design project. ® EXPERIMENTAL AIRCRAFT ASSOCIATION

SPRING SHOCK

AIR MUSEUM FOUNDATION

Contributions are tax deductible under Internal Revenue Service Code 501 (c)3.

BOTTOM VIEW

TOTAL CONTRIBUTIONS

FIG. 4

$191,551.22 This Month's Contributors

chining, and welding. The tandem gear shown in Fig. 4 is a simple frame with point-to-point mounting automotivetype shock absorbers. With a little exercise at the base of the cranium a thought comes that the wheel could be supported by a torsion arm mounted in rubber blocks. Then there is the obvious economy of having only one

wheel. Then someone says: "What if the tire goes flat?" On a tandem gear you go bouncing along on the rim — straight along. On a conventional gear with one flat tire you are lucky to get away with a mild groundloop. Of course, a tricycle gear layout helps in that event but that also gives three full-size wheels and struts to create drag. THE SUMMATION

We have talked at some length on a rather "narrow" subject of tandem landing gear. It might have been a "broad" subject like wing spans, but it seems that the task of designing an airplane is a large collection of "rather narrow" discussions. When these discussions are linked together properly an entire airplane results. In an identical fashion the aircraft gets built through a large collection of small jobs linked together in proper sequence. It takes a similar form of project organization and total effort to design an airplane, maybe even more so to do a good job. Very little has been said about the effect of tandem landing gear on the design of the rest of the aircraft. Aside from the primary occupation of providing an easy rolling action and shock absorption, the landing gear provides a very necessary separation of the propeller from the ground. Although this is a very common function, it is in no way an immutable arrangement. In original design, the tandem gear can easily be incorporated on all but high-wing types. The high-wing configuration is possible but usually results in a rather steep and uncomfortable "lean" on the ground. The tandem landing gear has several advantages in the categories of functionality, efficiency, durability, and economy, but it also has one serious disadvantage. That disadvantage is the requirement for accessory outrigger

wheels. In the case of the glider with its long wings, a

JAMES BEDE, Cleveland, Ohio

. . . . . . . . . . . . . . . . . . . . . . . . . .$575.80

ANONYMOUS, Emeryville, Calif .......................... 500.00

EAA CHAPTER 101, Addison, III. .......................... 121.18 ROBERT J. LANNEN, Farmington, Mien. ................. 100.00 EAA CHAPTER 243, Merrill, Wis. ........................50.00

EAA CHAPTER 190, Tennessee Valley .................... B. L. TICKTIN, Huntsville, Alabama ......................

30.00 25.00

ARTHUR J. MEAGHER, St. Louis, Mo. ....................

25.00

FREDERICK C. COLE, Riverdale, Georgia ................ JOHN M. TOMISHIN, Cleveland, Ohio ...................

25.00 25.00

EAA CHAPTER 10, Bixby, Oklahoma

25.00

.....................

DAVID S. McCLURE, Bloomington, III. ................... 25.00 DIDACUS POLK, Tulsa, Oklahoma ....................... 25.00 WILBUR SMITH, Bloomington, III. ....................... 25.00 WILLIAM H. JOHNSON, Brooklyn, N.Y. ................... 25.00 RALPH P. KLIEGLE, Hampton Falls, N.H. ................ 25.00

V. E. FLANDERS, Bloomington, III ....................... ROBERT J. ZUBACK, Milwaukee, Wis. ................... ELMER T. ROGNE, Minneapolis, Minn. ..................

15.00 12.00 10.00

ALDEN E. ROBINSON, Accord, N.Y. ......................

1000

NELSON E. BAXTER, Coldwell, Ohio ..................... WALTER A. GUENTER (on behalf of St. John's Men's Club) Greenfield, Wis. ............................... CHARLES EATON, Burbank, Calif. ....................... R. E. RANDOLPH, Manitowoc, Wis. ......................

J. L. CONNOLLY, Las Vegas, Nev ....................... CARSON E. THOMPSON, Elmhurst, III.

..................

10.00

10.00 10.00 10.00

8.00

8.00

L. G. SMITH, Redwood City, California .................. J. WESTOVER, Middleburg Heights, Ohio ................ H. LaCOUNTE, Anaheim, California ...................... GEORGE KOST, Nome, Alaska ........................... JOHN LINNERT, Glen Ellyn, III. .......................... RAYMOND J. SENEGAL, Garden Grove, Calif. ............ LeROY HURRY, Minneapolis, Minn. ...................... VERNON L. GREEN, New Paris, Pa. ..................... JOHN F. AMENDOLA, Bellevue, Wash. ....................

8.00 6.80 6.50 5.00 5.00 5.00 5.00 5.00 5.00

MAX ILCEHER, Santa Monica, Calif. .................... JOHN SMELKA, Akron, Ohio ............................ DAVID L. RAMSEY, Oak Ridge, Tenn. .....................

5.00 5.00 5.00

F. W. THOMPSON, Fairbarn, Ohio ......................

5.00

VINCE MANANI, Findley, Ohio

..........................

3.80

LESLIE SZANTO, Montreal, Quebec, Canada .............

2.00

K. W. BYRYNESTAD, Puyallop, Wash. .................... 5.00 JOSEPH R. KURKURA, Genoa, III. ........................ 5.00

THOMAS L. HENK, Bricktown, N.J. ...................... H. LANGLOIS, Costa Miso, Calif. ........................ CHARLES H. ROBINSON, Faucett, Mo. .................. RICHARD P. SELINSKI, Minneapolis, Minn. .............. MICHAEL BRAZO, Vineland, N.J. ........................ GERALD N. TRETTEN, Kaukauna, Wis. .................. C. H. TOWNSON, Marble, N.C. .......................... JOHN BURKETT, Stuttgart, Ark. ........................ EAA CHAPTER 80, Omaha, Nebr. ........................ ROBERT RICHARDSON, Belleville, Kans. ................ WALTER HOSTER, Ocean City, N.J. ...................... EAA CHAPTER 17, Knoxville, Tenn. ...................... FRANK J. SPOLER, Wakeman, Ohio ..................... ALLAN W. MOJZISIK, Richmond Heights, Ohio .......... E. DYKES CHRISTENSEN, JR., Layton, Utah .............. DON GLANDT, Laramie, Wyo. ............................

5.00

4.40 4.40

3.20 3.00 3.00 3.00 3.00 2.28 2.00 2.00 1.84 1.80 1.50 1.40 1.00

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