OK, short answer, or opinion, if you prefer. All control
line propeller powered airplanes experience a bit of
gyroscopic precession from the very fact that the airplane is
tethered and in a constant turn as it travels in a circular
flightpath. This is an indisputable physical result of flying
a tethered airplane with a gyroscope mounted on the engine.
This precession, typically nose up on airplanes flying
counter-clockwise, is there all the time while the engine is
running and must be compensated for. You can either attempt
to trim it out using incidence or down thrust, or you will
carry a bit of down in the handle, whether you are aware of it
or not, to make the airplane seem to fly neutrally correct.
The effect is slight, but it is always there as long as a prop
is turning. I am not comfortable with the thought of using
down thrust. Incidence works and will reliably add a bit of
compensating nose down pitching moment (nose up inverted) to
compensate for precession. Dave and Brett have done some test
with variable adjustable stabs and , if I remember correctly,
think that about a half degree of incidence not only counters
precession but adds a bit of longitudinal stability. I forget
the reasoning, but probably has to do with "longitudinal
dihedral".
And then there are asymmetries in airplane configuration.
Most airplanes have the stab located higher than the wing.
When maneuvering, up flaps on outside maneuvers direct wing
wake toward the tailplane reducing their effectiveness. Flaps
down on inside maneuvers direct the wake of the wing away from
the tailplane. This is noticeable in asymmetrically
configured airplanes showing a tendency to turn better inside
(up). There is no perfect trim for this asymmetry but,
practically, we can balance the effect by trimming the
elevator down until the turn rate seems balanced. Or, by
anticipating the need for trim by building in a bit of
incidence to correct not just for precession but also for
configuration asymmetry. Again, I think that just 1/2 to 1
degree of incidence is enough for typical airplanes. My
semi-scale airplanes have a much greater asymmetry of
configuration in that my stabs are usually located much higher
relative to the wings. For this, I need more incidence.
Years ago I found that while trimming a series of competition
airplanes to turn equally that I was typically winding up with
about 3/16" droop of the elevators on most of these
airplanes. The 1972 and 1973 NATs winning Sea Furys both
trimmed out with 3/16" of down elevators. With the
Molded Mustangs, I anticipated the need for asymmetry and
gyroscopic trim by splitting the 3/16" down elevator trim
between the elevators and stab to "fair" (align the
stab and elevators) the surfaces. This resulted in 3/32"
incidence in the stabs and 3/32" of droop in the
elevators which "faired" the assembly with what
turns out to be about 2 degrees of incidence. All of my
airplanes since have 2 degrees of incidence,
"faired" elevators, and track straight in pitch with
loss of line tension.
You pays your money and do what works for you. This is
how and why I use incidence.
Also, I use 2 degrees of engine offset on all of my airplanes,
except those powered by .91s, where line tension is excessive
anyway. As far as I can tell engine offset has no noticeable
effect on pitch trim in maneuvers. Undoubtedly, there are
some negative effects on maneuvers of this configuration, but
I have no idea of what or how much. I think extra line tension
from engine offset is something we have to deal with if we are to
compete in windy conditions. On balance, offset is probably
a bit more positive to the all weather handling characteristics of
our acrobatic models than negative. Whatever. I build it in, trim
the airplanes carefully, and learn, with practice, how my
individual airplanes "like" to handle in each maneuver and windy
conditions.
The use of stab incidence to improve the trim of our stunt ships
for the effects of gyroscopic precession is an original idea, first
used on the molded Mustang V in 1973. It was also used on the
1977 NATs and Walker Trophy winning Snaggletooth Mustang.
Snaggletooth also placed 2nd at the 1978 World Championship in
England. Incidence as a correction for gyroscopic precession was
first published in the Aug 1978 Issue of flying models in an article
titled "Evolution of a Thoroughbred".
Al
Topic: More Incidence (Read 908 times)
