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There has been a rash of articles appearing in the popular aviation press recently regarding composites. The following are highly interesting and recommended reading: April AOPA Pilot titled "Materials of a Different Mold", April Flying, "Pour on the Plastic", and May , "Lear Fantastic! Reno's Big Pusher".

The article is especially worthwhile as it contains information that is comforting, to say the least, to all composite builders. Fox example, it states that when aluminum parts fail, they fail catastrophically, all at once. "Because of the many fibers in a composite, loads are shared more uniformly. Composite parts do not propagate cracks nearly as easily as aluminum ones do, and tolerate flexing and fatigue better. When excessive loads are applied to a composite structure, failures occur gradually."


MaterialWeight (lb/cu. in.)Compression Strength (psi)Tensile Strength (psi)
Carbon Fiber.055165,000185,000
Kevlar 49.05040,000200,000
7075-T6 AL.10670,42083,000


Bob Beebe of Indianapolis saw the following article by David Thurston in the same April issue of AOPA Pilot. He says it would be interesting to hear comments on the same subject.

"...A less conventional approach to the development of a stall-proof airplane is the canard design, such as Burt Rutan's family of canard aircraft...The configuration is attractive because the forward, or canard, surface contributes to overall lift when balancing the airplane in flight. Conversely, the conventional airplane's tail must carry a down load in flight to balance the wing section moment...which reduces cruising speed and slightly increases landing speed.

The lift produced by the canard surface can be limited by restricting trim angle or elevator surface deflection, or both, to keep airplane longitudinal trim angles below the main wing angle-of-attack stalling angle. Theoretically, this feature should prevent wing stall and so inhibit spinning. Despite the success of the Rutan designs, it may not be possible always to prevent stall of either the canard surface or the main wing under certain gust or accelerated maneuver conditions. A sharp vertical gust could stall the canard while the main wing is still developing lift. Since the center of gravity (CG) is ahead of the main wing, the canard airplane could tumble about the CG and possibly break up in flight. A sharp vertical gust could create an equally dangerous situation by stalling the main wing and not the canard surface, which could result in a sharp pitch-up followed by structural failure or an inverted spin."

Before any starts making a mad dash to sell their project, be sure you read the above article closely. Although Mr. Thurston is an acknowledged writer of aircraft design, he has punctuated the last paragraph with phrases of "could stall", "may not be possible", "theoretically", etc. To our knowledge, there has not been an aircraft incident, which has been traced to this problem. In any event, Mr. Thurston's article makes an interesting hypothesis until more testing can be accomplished.

You can order a PDF or printed copy of QuickTalk #3 by using the Q-talk Back Issue Order Page.