Bob Whittier The low-wing type of air- In the case of low wings, few plane has for many years pilots and mechanics have a

have a racy look; there are

had a strong appeal to pilots,

clear-cut realization that such

mechanics and designers, both actual and would-be. The use

sound, practical reasons for their choice. Going back over

airplanes are "a different breed

the years in search of examples,

of cat" and require certain of low wings on a great many knowledge to understand, deof the best-known commercial sign and fly intelligently. Thus, and military airplanes is respon- for the sake of safety in the sible for this. We all like to homebuilt field and to further feel kinship with the biggest the general engineering knowledge, best and the most famous! In- the following facts are presentsofar as homebuilt airplanes are ed. No attempt can be made to concerned it sometimes happens tell how to solve the actual dethat builders choose the low- sign problems, but at least a wing configuration for their pro- realization of their existence jects not for tangible engineer- will help would-be designers to ing reasons but simply because evaluate low wings realistically they like the looks of low-wings and wisely.

and want one themselves.

Early Low Wings

Professionally - designed airplanes get the benefit of a vast Professionals have not favored amount of specialized engineer- low wings just because they ing work in the fields of performance, stability and structural strength. In many cases, wind-tunnel tests are made to investigate matters such as stall and spin characteristics before a big plane is built. In the files of many large factories are complete blueprints of airplanes which were designed but never built because mathematical forecasts or wind tunnel tests showed they had practical shortcomings. But the amateur goes ahead and draws up something that "looks good" to him and goes ahead to build and fly a machine of more or less unknown qualities.

let us first consider the Boeing P-26 fighter of the early 1930's. The next time you see a photo of this airplane notice how little dihedral it has and how high above the wing the general bulk of the fuselage is. Fighters in those days were expected to engage in close-range dogfighting of World War I style and deliberately had minimum

stability worked into their designs so they would whip over and around quickly. The machine guns of biplane fighters had always been placed either in the fuselage or on the lower wing. They were in the fuse-

lage at first because biplane wings were too thin to house them. As airfoils became thick-

er and the number of guns increased, wing mounting became logical. When biplanes first gave way to monoplanes, the low wing was the only practical choice for designers because

of the simple matter of gun and ammunition rack accessibility. As military engines grew larger, propeller torque became worse.

There is a classic story

about a Schneider Trophy racing seaplane which capsized in the water from torque when its huge engine was revved up. In landplane fighters, it was necessary to have a wider land-

ing gear tread to resist torque on takeoffs. Of course you can readily see how much lighter

and easier it is to rig a wider gear out on the wing of a lowwing than it would be to build a wide landing gear with struts

angling in to the fuselage. When cantilever wings came along, again the low wing was a natural. If we take a fuselage and stick a landing gear under it

and a cantilever wing over it, the fuselage structure has to be

made heavier to take the downward load of the wing's weight (which may include heavy fuel tanks) while landing and taxiing. Otherwise the fuselage might be compressed on a bad landing! Look at the Fokker D-8 fighter and note how the wing struts, aside from bracing

the wing in flight, were ideally suited to taking landing loads off the fuselage itself.

If the spars of a cantilever wing have to pass through the

fuselage in a small airplane, the pilot can sit on them as he does in the Culver Cadet and Dart, and no inconvenience results. But consider the Cessna C-37, a small four-place of the late 1930's which had a cantilever wing. The main spar went through the top of the cabin and came down into it so far the occupants of the front seats couldn't get into their seats by

climbing over the seat backs. One seat slid back as far as the

door and the pilot crawled over 11

it and into his own seat. Then the passenger got into the

drawn-back seat and slid it and

himself forward. Once the plane was loaded the occupants of the forward seat were more or less trapped there in the

event of a fire or nose-over. The

plane sold well because it was

admittedly very fast and effici-

ent, but if you should chance to see one somewhere, get into the front reats and see what claustrophobia is like! Retractable Landing Gears

And, of course, with the ad-

vent of retractable landing gears and multi-engined planes, the low wing was ideal for it would enclose a wide-tread landing gear easily when retracted and

put engines and fuel tanks at a reasonable level for servic-

ing.

A landing gear fitted to

a low wing is shorter, lighter

and stronger than a long one

reaching up to a high wing. From the military standpoint,

low-wings always offered good

visibility above and behind, where the enemy usually lurk-

fatal results. When we look at a low wing and ask "Is it stable?" we usually are thinking of lateral sta-

bility, or tendency to roll off

to right or left. That is natural, because the center of gravity

seems to be above the wing and

you'd expect that to be unstable. This is true, and we fix it by using plenty of dihedral. But that is not the end of it by any means. Using more dihedral brings on other troubles. First, visibility to the side is often im-

paired especially if the span is great.

Second,

we

naturally

lose lift as the wings are angled up more and more. Third, directional stability is affected. Few pilots realize that dihedral has as much, if not more, influ-

ence on directional stability as on lateral stability.

In Fig. 1A is a monoplane with no dihedral. The sketch

shows the plane yawing towards

us and we can see how the side of the fin will be getting air pressure tending to force the plane back into straight flight.

Note that the angle of attack of

Racers which had to snap

both sides of the wing is the same.

response and because a low wing

same airplane a bit of dihedral in hopes of killing a tendency

ed.

around pylons found it good both because of quick control

would never blank out the view

of the pylon. You may be sure that trained

engineers burned the midnight

oil solving some of the problems

which came along with the advantages of low wings. Stall,

yourself of this business by comparing a J-3 Cub and an Aeronca Champion. The Cub has only a little dihedral and a modest fin. The Champ has more dihedral and a large fin. In level, trimmed flight you don't notice

end up by winding down and slamming into the ground on the nose and one wingtip. We make our fins big enough to secure reasonable directional stability yet not big enough to have too noticeable effect on spiral sta-

are doing their jobs of keeping

have to depend on instruments

much difference because the fins

bility, in contact flight, but we

In Fig.1B we have given the

to drop off to one side. We ac-

complish this . . . . but at the same time we throw a monkey wrench into directional stability. See how the angle of attack of the leading wing has increased? That is going to tend to lift the

plane over into a roll. Partly to prevent the plane from crabAlexander "Bullet" of about bing into this position in the 1930 is a classic example. As I first place, and partly to give the remember it, three or four mod- fin power to overcome the drag ifications were built and each of that wing and pull the plane in turn went into a flat spin and back straight, we make the fin You can convince was wrecked during tests, with bigger. spin and stability characteristics gave them headaches. The

the wings lined up with the relative wind. However, boot

the rudder of the Champ hard

over to deliberately override the fin and the plane will yaw and roll over quickly due to the lift of that "leading" wing. Spiral Stability

to tell us in fog row much cor-

rection to make when we can't

see the nose gradually wandering off and down on the horizon. Coming now to the low wing,

it has even more dihedral and from the above discussion you

can now appreciate that while It goes on from here to be- it may help lateral stability, come more involved. The big- we've got to watch what we are

ger the fin the more tendency it will have to make a plane spirally unstable. Any airplane if left to fly along by itself will sooner or later enter a spiral

doing as regards directional and spiral stability. In gusty air, a

puff of wind coming to the plane from the angle shown in the sketch can give that lead-

dive; this is exactly what hap- ing wing a lot more lift than pens when planes groping its mate has. The plane may through fog without instruments bounce and rock more. Ob-

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viously, too, that wing is riding

is riding at a conventional, nominal distance ahead of the wing's than is its mate; if a gust comes Center of Lift and a download up from ahead and below, that on the stabilizer is keeping the wing may stall out and spin plane flying level. This is typius in. Or if we forward-slip cal of any modern airplane. Now too vigorously while coming in in Fig. 2B this plane has asfor a landing we could just as sumed a greater angle of attack, easily spin out. Sure, these either by nosing up in a steep things don't seem to happen in climb, or by striking a gust. production planes. Somebody That low-down Center of Gravhas experimented with stability ity has swung forward of the arrangements, airfoil stalling Center of Lift a considerable characteristics, wingtip wash- amount and, naturally, is now out and control surface stops working with a lot more leverand has come up with a safe and age as per DI. There is thus a tested plane. strong force tending to bring An aspect of low wing sta- that nose down out of the stall bility which not one pilot in a range. The old Beechcraft bithousand is aware of is that plane, by the way, used a simple longitudinal stability is even trick to gain stability this way. more directly involved with this The lower wing, staggered much closer to the stalling point

type of plane than it is with others. Actually, it might be

safe to say low wings are more sensitive to longitudinal stability than anything else. Consider Fig. 2A in which a

high-wing monoplane is flying level. The Center of Gravity

ing to bring the nose down out

of the dangerous high-angle-of attack condition. Similar Low-Wing

loaded our plane with more than the approved amount of baggage in a rearward baggage compartment, a condition could be reached in which the center of

In Fig. 3A we have a low wing lift would go ahead of the cenof the same general size and ter of gravity. The tail would wing. In level flight, the C.G. be in very serious trouble, stalland C.L. arrangement is the ed completely over in an insame and the two ships handle verted position. This is called more or less identically. How"catastrophic instability" by enever, in Fig. 3B there is a story gineers. In the early days and you should make every effort during World War I, it was not to understand. In the high clearly understood and was the wing, the wing rotated some- undoing of many a valiant airwhat aft as the plane nosed up, man. and the C.G. rotated slightly forward. This gave us the separa- Tail Group Important tion between C.G. and C.L. Aside from the airfoil, the which gave that strong nosing force. But in the low wing the tail group is also important in C. L. far down does not rotate 'securing good stability in low

ahead of the C.G., had a greater

angle of incidence than the upper wing. It would stall out first, leaving the plane hanging on the upper wing. The C.L. of the upper wing being behind

the

plane's

C.G.,

there

was

naturally a powerful force tend-

aft.

You should be

able to

visualize from Fig. 3B that there is considerably less nosing down force in the low wing. This has serious implications. The airfoil for a low wing must

be chosen with a critical eye on

its center of pressure (same thing as C.L.) travel. If the tenter of pressure has the characteristic of moving forward at high angles of attack, obviously it will move forward towards the center of gravity. The restoring force D2, smaller than that in the high wing to begin with, will become smaller and smaller. As a matter of fact,

wings. Area must be large enough to provide positive control at low speeds in high angle of attack conditions, and the stabilizer and fin must both be located with an eye towards the blanketing effect of the low wing. Even wing span can have a vital effect on stability. As you will realize from your study of Figs. 2A, 2B, 3A and 3B, the vertical location of the cen-

ter of gravity and center of lift

are important. In a low 'wing, we would want to get as much v e r t i c a l separation between them as possible, for, as the high wing shows, the greater this

if we had an airfoil which had

vertical separation the greater

its center of pressure move appreciably forward and carelessly

should be as low as possible, pi-

is the restoring arm.

Fuel tanks

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Engineers of Bensen Aircraft Corporation in R a l e i g h , N. C., have assisted customer Ray Umbaugh in developing a cabin enclosure for the B - 7M Gyrocopter designed by Igor

Bensen. The photographs show the "before" and "after" effect brought about by the addition of the cabin enclosure. It will make flights in the Gyrocopter more comfortable, especially in

lot should sit as low as possible, etc. The greater the dihedral, the higher will be the center of lift for, assuming each wing's lift to be concentrated at some point outboard, the C.L. rises as dihedral increases. Fig. 4 illustrates this. The greater the span, the farther out on each wing will be this transverse center of lift. Three degrees of dihedral in a low wing of 30 ft. span have more beneficial effect on longitudinal stability than three degrees on a low wing of 15 ft. span for this reason! Of course, sight must certainly not be lost of the fact that moving the center of gravity forward will also help longitudinal stability. Moving the engine

several inches forward, or adding a lead weight at the firewall, will bring the C.G. forward where it can be safely ahead of the C.L. The resulting noseheavy condition can be corrected by increasing stabilizer download. In any case, when building and test flying a low-

arid and arctic regions. The cabin is built of fibreglas and Plexiglas and affords the pilot sufficient room to operate the Gyrocopter in comfort. The pilot also has a good field of vision, comparable to that of most present day helicopters. The 20 ft. rotor which creates vertical lift of the Gyrocopter is auto-rotating at all times, while the pusher propeller provides the forward thrust. A steel spar

runs the entire length of each blade of laminated plywood construction. A short take-off speed is about 25 mph, but landing speed is only seven mph. In calm air the landing roll is about 20 ft. Gyrocopter construction kits are marketed by the firm to amateur builders at prices ranging from $395 to $895, less engine.

wing take it up high to feel out its stalling and high-angle-of-attack flying qualities. See how it reacts way up high to side slips and forward slips, and let it fly hands off to see what happens in respect to going off into spirals. If it gets jumpy and finicky near the stall and requires

when the wing is mushing. On the side view of a plane, make a pencil point at the wing trailing edge. Extend pencil line vertically from this point and then with a compass measure off 60 degrees. A line drawn from the trailing edge, angling upwards and backwards as in Fig. 5 should pass over the tail group. If the tail group is partly inside this blanketed area you will want to watch out about spinning. Obviously, the greater the wing chord, the larger an area will that wing blanket in a stall. Taking a lightplane of average size, if it is fitted with a low aspect ratio wing of six-foot chord, the tail will be more apt to be blanketed than if the wing has a high aspect ratio with say a four-foot chord. Then, too, for the same given airfoil your sixfoot-chord wing is going to have more center of pressure travel than the four-foot-chord wing. Summed up, if you must build a low-wing then study care fully the best books you can find about design, use proportions in your plane which closely follow those of tested and proven planes, and, unless you are a very skillful pilot indeed, stay strictly away from short span, low-aspect-ratio low-wing monoplanes. An excellent article on the spinning characteristics of lowwings appeared on page 291 of the May, 1935 issue of "Popular Aviation" and it is hop:d we may republish it soon in the "EXPERIMENTER".

a lot of work in gusty weather to keep it level, try moving the C.G. forward a little. Of course, moving the C.G. forward can not be overdone because it will increase the drag of the airplane due to increased negative angle of attack required of the stabilizer to trim the ship, and it will make it hard to get the tail down in a landing. Elevator Control

Because elevator control is obviously going to be depended on for safety when trying out the stall and low speed handling of a low-wing, the tail group must be located so that it will not be adversely blanketed by the wing

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