Some Design Considerations of Delta-Wing Pushers

dence angle relative to the air flow to increase even more. (Fig. 6b). Therefore .... and would be helped tremendously by cards and letters sent to him. They can ...
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Some Design Considerations Of Delta-Wing Pushers By Teruo Fujii (EAA 9981) 1200 N. Hale Avenue Fullerton, California (Artwork by M. Videka)

TRUE DELTA-WING pusher is beautiful. It is a rare combination of art and science merged into an aircraft that is daring and unique. Unfortunately, propeller-driven delta pushers are few and it is difficult to find much authentic information about them. Since jetpowered delta-wing aircraft are relieved of many of the design problems found in propeller-driven deltas, what we can borrow from their approach is limited. Since conventional aircraft are so different from delta-wing pushers, one should not rely completely on their methods either. All in all, brave new approaches must be made to circumvent problem areas of earlier delta designs. As a starter, let us review some of the problems of this type of aircraft: 1. Forward thrust moment about the main wheels, as shown in Fig. 1, may be great enough to prevent take-off under high thrust condition. 2. Reverse drag moment about the main wheels, as shown in Fig. 2, may be great enough to cause the nose wheel to lift off during a take-off abort procedure.

FIG. 3. 3. Air turbulence of the propeller may affect nearby elevators from being effective especially when the engine is throttled back (see Fig. 3). 4. Much control travel and effectiveness is required at low aircraft speeds. 5. The elevator controls become difficult to work at high speeds. 6. The normal stall landing angle for a delta is about 40 degrees, and propeller clearance becomes important. Now let us find methods of treating the above problems. Perhaps the easiest technique of solving these difficulties is to place an elevator on top of the vertical fin, as shown in Fig. 4., or toward the nose, as shown in Fig. 5. But, such solutions take you away from a true delta. What is more, the Canard "solution" with an elevator mounted up front is not without problems either. One bad one is

FIG. I.

that the control is not self dampening. That is, if you lift

Q) ""TMOMENT

NOSE WHEEL TENDS TO LIFT OFF FIG. 2. 40 AUGUST 1972

the nose of the aircraft by increasing the incidence of the elevator (Fig. 6a), the rising nose causes the elevator incidence angle relative to the air flow to increase even more (Fig. 6b). Therefore, let us proceed with the aim of retaining a true delta-wing configuration.

FIG. 7.

FIG. 4.

FIG. 5.

JANGLE

a.

WIND

SPOILERS .

ACTUATED)

FIG. 9

FIG. 6. A Solution Of The Forward-Thrust Moment Problem

The forward-thrust moment problem is basically caused by the fact that the counteracting moment of the elevator control force is relatively small (Fig. 7a). On conventional aircraft, this counteracting moment is relatively large due to the greater distance between the elevator and the axle of the main wheels (Fig. 7b). The solution for the delta pusher, therefore, is to move the main wheels forward to increase the elevator-to-main-wheels moment

arm. Of course, this means that the engine must also be moved forward and coupled to the propeller with a suitable

drive shaft

A Solution To The Reverse-Drag Moment Problem

If a person desires to abort a take-off attempt after already exceeding the optimum lift-off speed by a significant amount, there is a potential hazard in that the deltawing pusher tends to lift the nose wheel off the ground

when the throttle is chopped. This effect results primarily from drag produced by the propeller in the throttled-back state. Another contributory factor is the ground effect produced by the newly created high pressure caused by the air turbulence forward of the unpowered propeller. These effects are depicted in Fig. 8. The initial step to a solution of this problem is to install extremely strong wheel brakes on the main wheels. The forward moment caused by wheel braking should be able to counteract the reverse-drag moment sufficiently to a 1 low take-off abort without the nose wheel lifting off. It should be remembered that the wheel brakes must be applied before the throttle is pulled back. The next step is to install spoilers in the wings as shown in Fig. 9). Such a device can do all sorts of good things

for a delta-wing pusher. Landing speed can be reduced

considerably without coming in with the wings at. a high

angle of attack. Best of all, the spoilers can be actuated along with wheel braking prior to reducing power carefully to abort a take-off attempt. With the spoilers spoiling lift over a large part of the delta wing, the tendency for the aircraft to jump into the air is greatly diminished.

(Continued on Next Page) SPORT AVIATION 41

DESIGN CONSIDERATIONS . . .

A Solution To The Loss Of Elevator Control Problem

(Continued from Preceding Page)

A careful look at the air flow about the delta wings

is rudimentary to understanding the control-loss problem. It is found that the air flow is different when the aircraft speed is slow and when the aircraft speed is fast. The slow speed condition can best be understood if we picture

the aircraft tied down, wheels chocked, and the engine

operating at say 2000 rpm. The air flow would be as

depicted in Fig. 10. Of course, it is easy to picture the relative air flow of a fast moving delta. We must talk of relative air flow since the propeller is now taking an efficient bite and the

LOW PRESSURE

FIG

LOW PRESSURE

Fl

aircraft is moving through the air. Such a relative air flow diagram is shown in fig. 11. As we recall, the air passing by an airfoil creates a low pressure area on top of the wing and thereby causes lift. At slow speeds with the delta wing, it can be seen that the low pressure area is formed where the air flow arrows come together in Fig. 10 toward the trailing edge. As the speed of the aircraft increases, this low pressure area moves forward. It should be noted that this change in low pressure area will cause a rapid change in trim. An elevator trim system that is relatively fast should be used on these aircraft. At speeds exceeding the lift off speed with the wheels still contacting the ground, the low pressure area may be quite a distance forward away from the trailing edge. If we suddenly reduced power, the air blocks up in front of the propeller causing a high pressure area at the trailing edge. This high pressure at the trailing edge compounded with the low pressure toward the leading edge causes the leading edge to lift and the trailing edge to lower, and the aircraft may climb out at a surprisingly high angle. This phenomena, of course, is aggravated by the ground effect and the reverse-drag moment mentioned earlier. The area directly in front the propeller is. therefore, not a good place to locate a control surface. That area is susceptible to abrupt changes in pressure and may, in fact, even change from low pressure to high pressure during throttling back of the engine. This fact causes some difficulties since we cannot push the elevators out toward the wing tips without running into the ailerons. Fortunately, the elevators and the ailerons can be combined. Such controls are called elevens and have been used on many "flying wing" type aircraft. The location of the elevens is away from the propeller, as shown in Fig. 12. The elevens depicted are somewhat special. The part toward the wing tips makes use of a technique used in "flying tails" as well as some contional tails such as that on the highly maneuverable Champion "Citabria". Since the pivot point is centered, we have a natural moment arm. At high speeds when controls become difficult to move without power assist,

AIR

FIG.

12. FIG.

42 AUGUST 1972

13.

BOOK REVIEW By Dave Jameson

Wiley Post, His Winnie Mae, and The World's First Pressure Suit, by the Doctors Stanley R. Mohler and Bobby H. Johnson, published by the Smithsonian Press.

THIS WING DROPS

FIG. 14.

the part of the control in front of the pivot point acts as a dynamic counterbalance (see Fig. 13c). As the control is moved, the oncoming air flow assists in further control movement. At slow speeds, the large size of the control surface is an aid to adequate control. The slot is used to keep the control from stalling out as shown in Fig. 13b. When the control is neutralized at high speeds, the slot does not disturb the air flow appreciably as shown in Fig. ISa.In short, the elevon described has two major advantages: 1. Dynamic counterbalance; and 2. More area in clear air. It has one major disadvantage — there is increased chance of aileron tip stall especially in a tight turn at low speeds. It is hoped that the slots will counteract this effect adequately. The purpose of the slot is to prevent aileron stall at high angle control (See Fig. 14). As you can see, there are more than a few problems associated with delta-wing pushers. But with a little time spent investigating the rudiments of a unique aircraft such as a delta pusher, these problems can be overcome. And what are the rewards of designing and constructing such an airplane? Well, the reasons for making a delta-wing pusher may be either a craving for originality, or a desire for clean aerodynamics. Perhaps the incentive is a happy combination of the two. And clean aerodynamics can win you many advantages such as: — Increased speed — Decreased fuel consumption — Increased range — Increased engine life Ultimately, these prizes will be won. And who knows? Perhaps after the first half dozen delta-wing pushers are built and flown, we may find new advantages we hadn't even thought of. And who will build these aircraft? The courageous ones. The ones who do not give up when a few problems come along. When the going gets tough, only the tough get going!

I don't know whether the "His" in this title is there because there are two "Winnie Mac's" — Wiley's original, and mine, a replica — or whether the authors chose this way of making those two inseparable, for they were surely that. Many persons have asked why I copied the "Winnie Mae", and I believe you'll understand once you read this excellent book. During the six years we spent rebuilding our Lockheed "Vega" I came to realize, through research on these planes, that Wiley Post had indeed been a giant in aviation, but that few seemed to remember, or even know, of this greatness. The authors take us from his birth, in 1898, to his first aeroplane ride, through his 99 exhibition parachute jumps, through his learning to fly, and finally to his "Winnie Mae". The book is profusely illustrated. We all know a little of their two flights around the world, but if you want many of the never-before-published details, and if you want to know where those first rungs in our ladder to the stars came from, then I'd heartily recommend one of the best bargains in aviation reading. Send $1.50 to the Superintendent of Documents, Govt. Printing Office, Washington, D. C. 20402, and ask for stock No. 4705-0008.

Communications Made Easy For Pilots, by Dick Doberstein. (Simplified Regulations at 6547 N. 73rd Street in

Milwaukee, Wisconsin 53223, $2.25, soft cover, 38 pp.) One of the "Made Easy" series by Dick Doberstein (others include Regulations Made Easy For Instrument Pilots, . . . Commercial Pilots,... Private Pilots), this book simplifies and clarifies the complicated procedures involved in communication between aircraft and the control towers. It takes the layman through various approaches and patterns on a step-by-step basis and through the communications procedures and jargon the pilot would normally use under those circumstances. Any pilot wishing to refresh his communications procedures or expand his flying into controlled areas would do well to obtain and study this text. It couldn't be made more simpler.

CARDS AND LETTERS APPRECIATED Chuck Hale, who crashed during an air show at Alabaster, Alabama on May 28, is recovering from his injuries and would be helped tremendously by cards and letters sent to him. They can be addressed to him at Birmingham Veterans Hospital, 3rd Floor, in Birmingham, Alabama. SPORT AVIATION 43