Have Wright

Six in-flight simulations were conducted as a part of the standard training curriculum at the Air Force Flight Test Center Test Pilot's School (AFFTC-TPS) at Edwards Air Force Base, California. The rationale behind this is since future test pilots will be eventually tasked to fly and characterize brand new aircraft designs (such as the JSF, on which I am currently working in my day job), they must be given training rating and evaluating aircraft with which they are unfamiliar. This has in the past involved "borrowing" airplanes that the pilots have never flown: I know of pilots who have been sent to Italy to fly MBB-339s, the NTPS in Mojave, CA, owns a complete squadron of 1950's vintage SAAB J-35 Drakens and rents these to AFFTC-TPS, etc. One object of selecting a suitable candidate for a training aircraft is that it should not be docile - it should have some unsuitable or undesirable characteristics that pose a problem to the future test pilots.

As far as the Lear research flights are concerned, the advent of computers (both analog and digital) and feedback control systems has made possible the ability to take one aircraft and make it fly as another would. In practice, this is normally used to take the adverse characteristics out of what is otherwise an acceptable aircraft (for example, the Boeing 707 and all civilian jets since have what is termed a yaw damper - a system which uses feedback to move the rudder such that the airplane behaves as if it had a bigger vertical tail).

Learjets in Flight

What some inventive people were able to do was carry this to extremes - by loading a new flight control system into an airplane specially configured for the job, they could make an F-16 "feel" like a JSF to a pilot, or a Learjet "feel" like a Wright Flyer. Short circuiting a lot of heavy mathematics, airplanes all have numerous dynamic modes - or in layman's terms, distinct ways they respond to such things as pilot commands or disturbances in flight. It is hard to appreciate in today's modern airliners, but if you have been up in a light piston engined aircraft, you know that when the wind hits the airplane from a new direction, it tends to wiggle a bit before settling down. Those wiggles - their magnitude, their frequency, how long it takes to damp out to nothing - all of those characterize a dynamic mode of the airplane.

Since our project is sponsored by the American Institute of Aeronautics and Astronautics (AIAA), the central professional organization for those involved in aviation design and devlopment, our project came to the attention of the Air Force TPS. The offer was made by TPS that if we were to provide the mathematical equations to a subcontractor (the Veridian Corporation, which took over the CALSPAN aeronautic and flight research facilities in Buffalo, NY), they would have those equations programmed into a variable stability Learjet, such that in flight the Learjet would "feel" to the pilot just like a Wright Flyer would, dispite the fact the Lear is travelling at around six to seven times faster than the Wrights did. Obviously, there are limitations to this method - the pilot did not have the same visual cues looking out the cockpit, he used a conventional stick and rudder while seated rather than a hip-cradle while lying prone, etc. - but it is a valid engineering method of analysis. The Air Force was able to accomplish its mission, giving one class of test pilots and flight test engineers a completely new and unknown aircraft to test, characterize, and offer suggestions from a pilot's standpoint how to improve the design, and our project recieved valuable technical input from the test pilot community on how to make our flyable replica safe.

Along those lines, what we are doing with our second replica is akin to what the Wright's did to their airplanes subsequent to the 1903 Flyer. Through a combination of a more forward location of the airplane center of gravity (which decreases the level of instability in the airplane), and subtle changes to the airfoil shape (which allows the airplane to trim without stalling the canard), we are striving for an airplane which normal pilots will be able to fly. The Wrights also moved the center of gravity forward, but increased the size of their canard to improve its ability to trim the airplane instead of de-cambering the wing (which turns out to be counterproductive - bigger canards actually destabilize the airplane while providing more control, however the Wrights were able to find a workable compromise in their 1905 Flyer).

If you wish to read an interesting paper on how the Wrights discovered and solved the problems posed by the highly unstable 1903 Flyer, I would recomend you read "What the Wright Brothers Did and Did Not Understand About Flight Mechanics - In Modern Terms", written by our chief engineer Dr. Fred Culick of Cal Tech, available for download as an Adobe Acrobat (.pdf) file.

- John Latz

Copyright 2001, AIAA Wright Flyer Project, all rights reserved.

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