A Discussion of the Physics of Certain

DELTA WING PUSHERS

By Teruc Fujii EAA 9981 1200 N. Hale Ave. Fullerton, Calif. 92631

During Take-Off N THE QUEST for an aircraft different from the common store variety, few enthusiasts are turning toward Idelta-wing pushers. Some of the already completed ones have been outstanding. The physics behind certain delta-wing pushers are different enough from conventional aircraft that a little discussion may help especially those contemplating flight testing their own creation with no prior experience in such an aircraft. It is true that a few dozen flights will do much to enlighten the pilot on the different flight characteristics. Unfortunately, the take-off characteristic is different and potentially a killer for the unfamiliar. Several taxi tests and simulated take-off runs can be used to determine lift-off speed. Until better familiarity is generated, the initial lift-off for an actual take-off should not exceed the predetermined lift-off speed by an excessive amount. If the predetermined lift-off speed were 70 mph, one should not try to hold the aircraft down on the ground until a speed of 125 mph is built up, at least for the initial take-off. "Why?" you may ask. "Isn't it safer to build up speed to make certain of avoiding a stall on take-off?" Well, here is the discussion: On a delta-wing aircraft with the engine positioned toward the aft, the main landing gear is also positioned toward the rear. The thrust, -as shown in Fig. 1, develops a moment about the main landing gear wheels. This moment tends to force the nose down. At excessive thrusts developed at very high speeds on the runway, this moment may be great enough to make pulling the nose wheel off the ground difficult and perhaps even impossible. Now, if we pull back the throttle during this condition to abort the take-off attempt, something awful can potentially happen. A propeller with the engine throttle way back acts as a drag device much as a parachute would. This newly created drag now causes a new

moment in the reverse direction as shown in Fig. 2. This reverse moment could very well lift off the nose wheel! Let us presume that the pilot sticks with his decision to abort the take-off attempt and now applies full forward pressure on the stick. Another phenomena may now take place. The propeller is still the drag device and for illustration can just as well be replaced by a solid disk. Needless to say, the turbulent air around the propeller is no longer flowing straight back. Now, if the elevators happen to be next to the propeller with its turbulent air, there may be little or no elevator control, as shown in Fig. 3. If the throttle is still kept back, the aircraft may stall or go into a state of oscillation and porpoise. If this oscillation does not damp out, the final result may also be a complete stall. Since the reverse moment may be quite large, the aircraft could almost instantaneously jump off the ground, afford insufficient elevator control, stall and crash. At fast speeds, the aircraft could attain a dangerous altitude from which to fall in a complete stall. It is best to avoid the above condition, but what can be done if one suddenly finds himself speeding down the runway at 125 mph with the wheels still on the ground? 1. If one wanted to continue with the take-off, he may carefully reduce the throttle setting enough to lower the forward moment to just allow the nose wheel to lift off. Once the aircraft is in the air, full power may be applied. The elevators should operate since there would be adequate air flow over the controls. 2. If one desired to abort the take-off, he should use forward stick pressure and apply wheel brakes first. This applies more forward moment and allows for throttling (Continued on bottom of next page)

FIG. 1. Forward moment is caused by thrust and distance from the thrust line to the pivot point (axle of main wheels).

This article has been especially prepared by Teruo Fujii

for SPORT AVIATION. Mr. Fujii holds a masters degree in engineering and has been a contributor with his first story. "Mountain Flying With Homebuilts" appearing in the May 1961 issue.

FIG. 2. Reverse moment is caused by the propeller drag and the distance from the line of drag to the pivot point (axle of main wheels). SPORT AVIATION

21

FROM THE DESIGNEE FILE . . . BIPLANE RIGGING . . .

B

IPLANE RIGGING. Richard Gleason, EAA Designee 10, of Excelsior, Minnesota has noted (along with EAA Headquarters) an increased interest in biplanes. In light of this, Gleason submitted the following information on rigging the wings of these machines. The first step is to level the fuselage lengthwise and spanwise. Erect center section on cabane struts and install the center section brace wires. Using two plumb bobs, center the center section over the fuselage by dropping the plumb bobs from the front spar fittings and measuring to the fuselage fittings from the plumb bob lines. Centering of the center section is accomplished by adjusting the roll wires until measurements are equal on both sides. Fore and aft alignment is accomplished by ad-

justing the diagonal strut, if adjustable, or the stagger wires if wires are used. This adjustment will also square the center section over the fuselage. Check by measuring from the butt rib trailing edge to the tail post; the dimension should be the same on both sides. This procedure is very important as the final alignment of the entire upper wing is dependent on the center section. The normal method of biplane assembly is such that the lower wings can be hung and supported by the landing wires after the center section is rigged. However, on some of the homebuilt designs, upper wing panel attachments are such that it is necessary to install the upper panels at the same time that the center section is installed, making rigging a little more difficult. After hanging the lower panels, the N struts can be installed and then the upper panels can be installed followed by the flying wires. Final rigging is accomplished by stretching a string tip to tip across the upper wing. Upper wing dihedral is adjusted by use of the landing wires, and reference to the line from tip to tip. Before tightening flying wires, make a final check on the incidence angle of both upper and lower wings. Lower wing incidence is usually set

by the attachment to the fuselage, so cannot very well be changed. The upper wing incidence is set by the cabane struts, and if adjustable ends are used, any changes necessary can readily be made. Lower wing dihedral is set by the length of the N struts, so it is important that the fabrication of them is accurate. After all of the above points have been checked, tightening of the flying wires will complete the rigging. One point of rigging that is often misunderstood is wash-in and wash-out. This pertains to the relationship of the tip rib incidence as compared to the butt rib incidence. Wash4n: the tip rib has a greater angle of incidence than the butt rib. Wash-out: the tip rib has a lesser angle of incidence than the butt rib. By utilizing the adjustment of wash-in or wash-out rather than trim tabs on the ailerons, wing-heavy conditions can be corrected and often cruise speeds can be improved a little at the same time. Invariably when this problem comes up, the first inclination is to wash-in the heavy wing; crank in more lift. This is fine except for one thing: by cranking in more lift, more drag is induced on the heavy wing. Consequently, other problems will then crop up aggravating the trim problem. The logical solution to the problem is to wash-out the light wing, and reduce the lift. By reducing the lift on the light wing, the heavy wing will then come up to a normal position without increasing drag. One item of importance that actually comes long before rigging the biplane, and that is to TRY to insure that you build adjustable end fittings for your N struts. Even though the drawings may not show adjustable end fittings for N or cabane struts, it would be advisable to copy proven design end fittings for the purpose of final rigging adjustments. Without heavy production jigs it is next to impossible to weld solid struts that will fit according to plan, due to expansion and contraction during welding. ®

DELTA WING PUSHERS . . .

(Continued from preceding page)

back carefully to keep the reverse moment from ever exceeding the forward moment. If the reverse moment overcomes the forward moment at any time, immediately apply more wheel braking. This should keep the nose

wheel down as long as the main wheels have not left the ground. If the aircraft should jump off the ground in such a circumstance, the only thing left is the stall recovery: Full power and stick forward. But low altitude stall recovery is tricky and dangerous, and such a situation should be avoided if at all possible. Does this mean that delta-wing pushers are danger-

ous? Not necessarily. Various design factors may be changed and some delta-wing pushers may not even be susceptible to the above condition. Even the aircraft which displays such tendencies is not bad once the pilot realizes what the nature of the problem is and applies this knowledge to advantage. ® 22

MARCH 1*67

FIG. 3. Air turbulence about the propeller in the throttled back state may affect effectiveness of the elevators.