I have been writing about the logical and practical need for stabilizer incidence for about 45 years now. It explained the need for drooping stab trim in the elevators of the Sea Fury and Mustunts in 1972. I use a bit more than some because my semi-scale airplanes typically had stabilizers mounted a bit higher than the wing/center gravity.. For most conventional stunters, I agree that a degree should be significant. On the other hand as long as we circle counter clockwise and use clockwise turning props (viewed from the rear) all off our airplanes will have a nose up pitching moment and the incidence is needed all of the time in all maneuvers as long as we have these rotation directions.
The following excerpt was lifted verbatim from an article of mine published in October 2007 and January 2008 Control Line World as a two part article because of its size. This topic from Control Line World and book 1 of my video contained 25,047 words and 61 photos. The rest of book 1 was a 1977 NATs/Walker Trophy winning semi-scale Mustang construction feature. That portion of book 1 took an additional 53,662 words and 534 photos. Total words and photos on the 3 book sale disk is 187,426 words and 1601 photos.
Mustang V Incidence
The Mustang V was the first stunt ship designed with intentional incidence in
the tailplane. The Sea Furys and Mustunts all had tail surfaces installed
without incidence. At the time, nobody had tried using incidence or was
aware of any reason why it might be useful. There are two forces affecting
the pitch trim of our airplanes. The first of these forces is aerodynamic. It
affects stunt ships with the horizontal tails mounted above the wing center
line. When maneuvering inside corners, the flaps are down which directs the
wash, or turbulence, of the wing under the horizontal tail surfaces. This
allows the elevators to operate in undisturbed air with great efficiency. On
outside corners, the flaps are up directing the wash of the wing toward the
horizontal tail surfaces, noticeably reducing their effectiveness. Fortunately,
this disparity of elevator efficiency can be corrected by applying more down
elevator than up to maneuver. This apparently can be done without causing
secondary trim problems. When designing the Mustang, I was aware of this
disparity. My Sea Furys and Mustunts trimmed out with approximately
3/16" of down elevator. When designing the Mustang V, I "faired" the
horizontal tail by designing the stab with 3/32" of incidence and the elevators
drooped 3/32". This amounts to about 2 degrees of incidence in the stab.
Admittedly, my tail planes were much higher than commonly used in
conventional stunt ships, but the theory applies, if to a smaller degree.
Conventional stunt ships can offset the aerodynamic wake effect by using less
incidence than I needed.
The second force operating with a nose up moment is gyroscopic precession.
When we fly in a clockwise direction, the very fact that our airplanes are
tethered means that the propeller blade on the outside of the circle is being
slightly pushed while the inside blade is slightly pulled. Since gyroscopic
precession operates 90 degrees later in the plane of rotation, this results in a
slight push on the bottom of the propeller disk and s slight pull on the top.
This is a nose up moment. By itself, it is probably worth about a half degree
of incidence to compensate. Using about 1/32" incidence for each inch of stab root,
works well for semiscale stunt ships with stabs located relatively higher than usual. This is a little
less than two degrees. For stunt ships of conventional configuration, a single
degree of incidence, or about 1/64" for each inch of root chord of the stab
would likely result in a better flying airplane, easier to trim for equal
turning. With an appropriate amount of stab incidence, elevator position
should trim out with the elevators nearly faired (aligned with the stab at
neutral). Beginning with the 1973 Mustang V, all of my airplanes have two
degrees of incidence. The stabs and elevators are faired in jigged assembly
without adjustments.
Topic: Stabiliser incidence (Read 1166 times)
