QuickTalk 4 - COMPOSITE NEWS
- Category: Q-Talk Articles
- Published: Wednesday, 30 June 1982 07:11
- Written by Jim Masal
- Hits: 1787
The MAY/JUN issue of QUICKTALK referenced an article in the April edition of . In that article, designer David Thurston commented that, "...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..."
Well, Burt Rutan took exception to that and sent us a copy of his (unpublished) reply to AOPA. Below is a transcript of that letter:
"I read with interest the article by Dave Thurston on light aircraft design in the April '82 issue. I was disappointed though, to see his comments published on stall of tandem-wing aircraft. We have seen other experienced designers fail to grasp the basic concept of the natural stall-limiting possible with tandem wings. Because of our role in applying this capability to several designs, I feel obliged to offer the following comments to correct the conjectures Dave has written. I should first clarify the distinction between the "loaded" canard approach of the modern tandem aircraft and the unloaded "floating" canard of the WW-II Curtis Ascender, since their longitudinal stability criteria and high angle-of-attack characteristics are as opposite as it is possible to be. Perhaps it is the Ascender experience that causes the old-timers to speculate or to conjecture.
A proper tandem wing aircraft does not limit angle-of-attack by restricting surface deflection. Stops are set to attain the highest lift possible for forward-cg low-speed performance. At aft cg the canard can be stalled with a small control deflection, just as in a single-wing aircraft where the wing can be stalled with a small control deflection. The tandem aircraft limits itself to a safe angle-of-attack regardless of cg by initial stall of the canard. After this occurs, all the remaining elevator deflection produces only a minor increase in angle-of-attack.
Canard stall does result in tumble or spin. When the canard stalls, very strong restoring moments force the aircraft to maintain the natural-limit angle-of-attack, a condition that allows control instead of departure and climb instead of mush. The characteristics at full-aft-stick is a mild bobble as the aircraft seeks the designed angle. Extensive flight tests conducted by NASA on the Long-EZ show that any combination of sharp, repeated three-axis control inputs or even tailslides do not produce divergence or departure from controlled flight. It is indeed a welcome feature to know that you can make a sharp turn to final, even slapping in full aft stick, full rudder and crossed aileron and not have the ground rise up to smite you!"
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