THE HISTORY OF V.A.R.M.S The Annual Diary 1990 - 2009

34
Adverse aileron yaw is worse at low air speeds. At low airspeeds the wing has a greater
angle of attack and therefore has a high amount of induced drag. The worse case scenario is when
the wing is close to the stall and down-going aileron increases the camber to a point that the wingtip
stalls prematurely. This is because higher camber airfoils stall at a smaller angle of attack and the
down-moving aileron effectively increases the angle of attack of the wing over the span of the
aileron.
Keeping our example of rolling to the right, if the left down-going aileron causes the tip on
that side to stall, not only won't you get the desired right roll, but you will get a sudden flick roll to
the left accompanied by the nose of the aircraft dropping dramatically. I have witnessed this
situation many times over the years and generally radio gear failure is blamed for the outcome. The
pilot is certain that they initiated a right hand turn and a rapid left one into the ground has occurred
instead - must be the radio!!
What to do?
There are two methods to overcome the problem of adverse aileron yaw. The actual hinging
method of the aileron itself can influence and significantly reduce the problem. Two methods I am
familiar with are as follows; 1. Top Hinging, and, 2. Frise Ailerons (Bottom hinging).
Another method of overcoming adverse aileron yaw is the use of differential aileron. This
involves gearing the linkages to the aileron in such a way that the movement of the down-going
aileron is less than the up-going aileron. This method is popular in full size aircraft and a number of
aircraft that I have modelled over the years have employed its use. The Tiger Moth has noticeable
differential aileron and the Pirat workshop manual recommends the following aileron throws: 30º
± 2° up and 16° ± 1° down. The amount of difference in movement can be quite pronounced to
the point that a Taylor Monoplane that I built had virtually no downward movement on the ailerons
at all.
In installations that use separate servos for each aileron, differential can obviously be
obtained using computer radio to reduce the travel in the downward direction. Another method is to
adjust the servo to aileron linkages in such a way that the differential is mechanically built in. This
has the advantage of being able to be used on single servo aileron installations.
The theory of obtaining differential is the same whether it be applied at the servo,
intervening bell crank, control surface itself or torque rod drives. In all cases it is a matter of not
connecting the clevis at the normal 90° to the pivot point. If this is done at the servo, the linear
movement created from the rotational movement of the servo is greater in one direction than the
other. This can also be achieved at an intervening bell crank but will require a bell crank of less
than 90°.
If the application is to be at the control surface horn or torque rod drive, the equal linear
movement of the push rod is converted to unequal angular movement of the control surface, that is,
more up than down.
Using mechanical linkages there is a limit to the amount of effective differential that can be
obtained and a ration of 2 : 1 is a good aim point. On computer radios 100% differential (all up and
no down movement) can be obtained and it is easy to experiment with different amounts and then
test fly to monitor the difference in flight characteristics.
~~~
49th 1995/96 Nationals at Ballarat Vic.:
F3J Thermal 1. Alan Mayhew 2. Rob Benton 3. Rod Watkins
F3B Multi task 1. Phil Bird 2. Scott Lennon 3. Tim Mellor
Thermal Task A 1. Rob Benton 2. Tim Mellor 3. Scott Lennon
7 Cell Elect. 1. Scott Lennon 2. D Whitten 3. Bob Hickman
F3F Slope Pylon Cancelled due to NO wind, a LARGE Wedgie and a very potent
incoming electrical storm on the top of Mt. Hollowback.