What Ref By Bob Whittier, EAA 1235 should be used on certain airplanes. I'm sure that the planes just mentioned have down-thrust for good reasons, and it would be interesting and informative to learn why. After all,

one goal of homebuilding is supposed to be education!

Drawings by L. Pozmany

T

HE PURPOSE of this article is to stir up a bit of thinking on some points of the design of small aircraft. In working out the details of my own ship I've made much use of NACA reports, and as many design books as I can obtain. And, by

In the case of tapered wings, such as on Cessna and Bonanza airplanes, I can see quite readily that the small-

For instance, every book I've laid eyes on says quite emphatically that wing root fillets are of very great importance in obtaining good climb,

elliptical tip is reduced. Also it seems

in low-wing airplanes. The reasons they give make sense, and they go into detail on the best forms for fillet designs. It's all so reasonable and convincing that I'm willing to buy it. But—when I go to the airport or look in the magazines, what do I see? Row after row of low-wing airplanes — American, European and homebuilt — all with either no fillets at all or very rudimentary ones. Will some of the experts kindly explain wha' hoppen, how come, and what gives?

Similarly, all the standard books I've got make it very clear that a nicely rounded wing tip is much to be desired, especially on a rectangular wing. We see the result of this when we look at practically any airplane of the middle and late 1930's . . . which is when the clean, effi-

an adjustable stabilizer mechanism. On drawings of the old Stinson SM-8

wings!

golly, it seems to me that the professors have got some explaining to do!

speed, buffet and stall characteristics

I've been hunting high and low for some published reference on how one can calculate, figure or guess how much up- or down-travel to build into

coupe planes, has blunt, even starkly square, tips. Even Champion has gone over to blunt tips on rectangular

in December, 1958 "Model Airplane News", I note that the stabilizer has a travel of plus three deg. and minus

er the tip chord becomes, the less of a wing tip vortice will be formed and hence the importance of a semilogical that when such a wing has flaps inboard and large ailerons outboard, it's more work than is worthwhile to go to the effort needed to round off the outboard ends of the ailerons to blend them into a rounded tip. But to get back to the rectangular

wing which many homebuilders prefer, I'm sure I have not seen anything in print that would explain

why the designers have done a complete about-face and adopted blunt tips whole-hog! On looking at plans for such a

wide variety of airplanes as the Cur-

seven deg. The designer obviously didn't pull these limits out of a hat! It would be most helpful to read an explanation of how one should go about arriving at suitable limits. There's been quite a lot in print

about how important it is to make the leading portion of a wing as smooth and true as possible. Some

even go to the extent of making up a special micrometer to measure surface waviness. To listen to the experts, you'd think a glass-smooth, micrometer-true forward portion was as important as avoiding rusty chromium on the radiator of a brand new Rolls Royce.

tiss Jenny, the Pietenpol, the Er-

Yet, I have before me a copy of

coupe and the Piper Pacer, it is readily apparent that they all have a

NACA Technical Note No. 428, "Characteristics of an Airfoil as Affected by Fabric Sag". While it is dated 1932, we amateurs would be impertinent to assume that the facts which i( reports are just so much baloney,

noticeable amount of downthrust built into their engines. Not one of the innumerable engineering texts

I've looked at has a single word of explanation of w h y down-thrust

for obviously the test was carried

cient small airplane as we know it began to show up. The way the books

explain it, the aerodynamic reasons for rounding the tips off are quite reasonable. Yet today just about everything coming out of the factories,

except the Piper and Forney Air12

Both the books and practical point; that spanwise ridges radii of wing airfoils are far than is chordwise fabric sag

experience agree on at least one and breaks in the leading edge more deleterious to performance between ribs. MARCH 1959

ence Material? out by competent men under controlled laboratory conditions. In this report are given the results of tests made with carefully-shaped model wings. One had a dead smooth surface; no ribs, no humps, no roughness, no ridges of any kind. A second had its top surface carefully filed and polished down from the front spar position aft to the trailing edge, to reproduce the fabric sag as meas ured off a real airplane having the same airfoil. A third had the fabric sag carried all the way to the leading edge, to represent a wood-and-fabric wing having no metal or plywood nosing ahead of the front spar. We've been told how terribly important it is to cover leading edges with aluminum or plywood to preserve the airfoil shape. Well, the controlled tests referred to reveal that the differences in lift and drag coefficients between the three wings was quite negligible. You have to use a magnifying glass to pick out the variation in lift and drag curves at all normal flight angles! The wing with the worst case of fabric sag showed the best maximum high angle of attack lift coefficient!

of attack. Now you can really see the humps and sags caused by lack of leading edge aluminum. It looks awful. Yet, unquestionably the effect of having highs and lows is to create an average; when such a wing is working at high angle of attack the actual effect of fabric sag is to automatically give the wing a thinner leading edge. It's a proven wind-tunnel fact that on normally thick airfoils, a decrease in leading edge thickness at high angles will increase their lift. This explains why the wing with sagging

nose fabric had the highest coefficient of lift in the above tests.

I've seen a good many homebuilts at the Fly-Ins which have had rather rough jobs of bending the aluminum

over their leading edges. Very noticeable discontinuities in leading edge radius, humps and ridges at the top of the front spar, and other irregulari-

ties are observable. I can imagine that it isn't too easy to bend sheets of aluminum to a smooth, accurate curvature. It has been stressed time and again in the engineering books (in this case I can understand what they are driving at) that airfoils are

quite sensitive to spanwise discon-

tinuities such as these; far more so than to fabric sag which runs chordwise. The more I think of it the more it seems to me that the amateur who is building a wood-and-fabric wing

ought to leave off the leading edge aluminum. By doing so he avoids the possibility of throwing deleterious spanwise irregularities into his wing through amateurish workmanship. He saves the time, money and nuisance of fiddling metal nosing onto his

wing. Using a template, he can very easily plane and sandpaper out a wooden leading edge strip that has

the all-important perfect leading edge

Go find some airplane such as a Fleet biplane, a Waco 10, a Pietenpol, or Marion McClure's Wiley Post. These ships have no leading edge aluminum, and the nose ribs stick out like a wart on a beautiful gal's schnozzle. Level up such a ship with

a box under the tail and look at the

wing from dead ahead. You will see that the maximum thickness of the

airfoil at about 30 percent of the chord is still practically the same as it would be were there aluminum

on the nose of the wing. It's the maximum thickness that still largely

governs lift.

Now drop the tail of this ship to

the ground, and squat down 30 ft. or so in front of the wing and look at it as if the line of your sight was

the same as the wind at high angle

SPORT AVIATION

The books tell us that nicely rounded wing tips and generous root fillets, as on the PT-19, are of fundamental importance in good low-wing performance. By contrast, the vast majority of low-wing airplanes built in the last ten years employ blunt tips and little or no filleting. (See Bonanza photo on page 15). Homebuilder's dilemma; what to do on his own design? 13

According to NACA Technical Note No. 428, fabric sag between wing ribs, as on this old SE-5, (above), Wiley Post, (lower left), and the Waco 9, (lower right), has an almost

radius. According to NACA T. N. 428,

he won't lose a mite of efficiency in

normal flight attitudes, and will actually get a bit more lift at high angles

of attack, and presumably a lower landing speed!

It seems to me that the prevalence

of aluminum covered leading edges

on commercial planes is related more

to taking care of air pressure on the leading edge than to aerodynamic efficiency. With the enormously strong

Irish linen we now have available, I

doubt if the average 80 to 100 mph

immeasurably small effect on lift and drag coefficients. This is in conflict with recent stress put on importance of

eliminating all waviness and irregularities in wings.

volving the Reynolds Number. I must have read 50 articles and chapters on it, and I still haven't found out how to apply all the gobbledeygook to a simple matter. From all I've read,

Suppose we finish up the fuselage of a new light ship and decide to static-test it for tail loads. Just exactly where and how are we supposed to support the forward end of

principles of scale effect and Reynolds Number, a larger wing area will carry proportionally more than a smaller area of similar shape. The

sandbags to the tail, the strains of actual flight will be reproduced with

I've finally doped out that due to the

way it looks, if I take a wing chord of four ft. and increase it to five ft., which is a 25 percent increase, the lift will actually be increased more

than 25 percent. There's nothing in

amateur-built plane would have any trouble with unsupported leading edge fabric caving in. We want simplicity and low cost. Think this over!

my books that I am able to recognize as instructions on how to figure this

In working on my design I've bumped into engineering matters in-

ing about aspect ratio and induced dra3 problems.

scale effect into the wing calculations; it all seems to deal with worry-

the fuselage so that when we add

fair accuracy? Mounting it by the engine mount seems wrong because that makes the fuselage a longer

lever than it actually is. Mounting it by the wing fittings seems wrong

because that won't take into account

the down-pull of the engine's weight in a pull-out. I can't find a blinkin'

blankety blank thing in all my books

about running static tests. I mean thorough and meaningful tests.

Most of us remember the wooden propellers of 20 or more years ago.

MARCH 1959

I'm including a picture of such a propeller out of the Air Associates catalog. Why is the blade angled back as it is? There must be some good reason why practically all wooden propeller builders went to the trouble of throwing that extra curve into the blade. None of the books say why. Nobody I've asked knows why. It can't be for variable-pitch effects as on the Wittman "Scimitar" propeller, because the blades of these stock propellers are too thick and rigid to bend as does the "Scimitar".

how they have a sharp entering edge. Compare them to the rounded leading edge on J-3 ailerons. Purely by chance, several years ago I picked up the information that the reason for this change in otherwise very similar wings was to get the desired aileron feel on the Coupe. Presumably the

which is what I'm doing. Yet, the books are silent even on advice about guessing and testing. Teh! Teh! I suppose that's what happens when books are written by professors having no practical experience. Too many textbooks are written by professors whose

balancing pressures than the round edge.

are!

It even extends to simple nuts and bolts problems. None of my books on

motor. It makes the interesting comment that the control column requires the pilot to lean far forward

aircraft mechanics and none of the

catalogs has anything to say about this so simple and obvious a question: why do they make star washers

in both internal and external form, and what is the proper function of each? Routed wing spars are commonplace but, never having worked in a factory, I've never seen the kind of machinery used to perform the routing. If I had to make a routed spar today I'd have no idea at all what kind of machinery, what kind of cutters, what cutter speeds, and what

guiding or templating means to use. The books are all dead silent on the subject.

sharp edge would develop different

In the

October,

1958

issue of

"Science and Mechanics" is a fine

report of a test ride in a Ford Tri-

and back to move the wheel through its full travel. This just happened to be the way things worked out after the designer had put the right combination of leverage into the elevator control system to get the control feel he wanted. I once noted that the rudder horn on an old Waco Taperwing had three bolt holes at each end, obviously so that rudder feel could be altered to suit. This is the sum total of what I've been able to learn about the matter of figuring the leverage ratios in the stick assembly and control horns to get good control feel. Certainly, in

view of the importance of "feel" to the pilot, some useful practical commentary ought to appear in the design books . . . but they are as devoid of such advice as an Eskimo's igloo is of picture windows. NACA has some reports such as "Appreciation

and Prediction of the Flying Qualities of Airplanes" and "Requirements for Satisfactory Flying Qualities of Airplanes", but it's all so abstract and

theoretical that it's next to useless in the practical design of a small airplane.

The books all go into exhaustive

detail about welding techniques but don't say anything useful about such tricks or hints as there may be about the preliminary job of cutting and filing the ends of tubing to fit, quick-

ly and neatly. Oh, I know I can fudge around with the hacksaw and rattail file. Considering all the steel tube fuselages the industry has turned out however, it seems logical to assume

that some good ideas have been devised for cutting and bending tub-

ing handily. I guess the few men who

do know are kept in dungeons by

the factories so that such prime secrets can never be broadcast. Next time you see a Piper J-4 Coupe, look at its ailerons and see SPORT AVIATION

Oh, sure, I realize that it's part science and part art, and that a certain amount of cut-and-try is involved,

main objective is to impress other

professors with how profound they

Look at the canted-forward rudder hinge lines of the Stinson Station Wagon, and Mooney, and at the

skewed ailerons on a Bird biplane or

Waco RNF. Why? The books don't say. Look at the squared-off tips on

many modern propellers. Why? The books don't say. What are the advantages and design tricks of the "flying tail"? The books don't say.

One of the reasons why I no longer subscribe to the "world's most widely read aviation magazine" is because

it just simply fails to keep me well informed on the innovations we see all the time in this game. It isn't enough to tell me that a certain new airplane features the seldom-used

Snakehips 2-U airfoil; I want to know why! It doesn't mean a thing if you don't know why.

Intellectuals bemoan the fact that people today read few books. They tell us, "Books are a golden key to all knowledge and to great accomplishments". I'll tell them, "Yeh? That so? My own impression is that aviation books at least, are passports to frustration and bewilderment!" Okay then, anyone who has anything to say about the subject I've touched upon is welcome to write me his comments. My address is 57 Swift Ave., Osterville, Mass. If I get enough illuminating comments I'll be pleased to edit them all into a good follow-up to this article. Or better yet, how about some articles on these subjects for publication in SPORT AVIATION? Then we can all benefit from them! A

Relatively square tips and filletless design of the high performance Beech Bonanza.