Wah-Wah's
caused by resonant
frequency due to frame flexing
|
Wah-Wah's
caused by resonant |
This idea came from a Raptor pilot in Sweden, Carl-Olof
Sandberg. He has an interesting idea that I have not
heard anyone else mention. Basically he found his problem
was due to a resonance frequency of the frameset. As the
servo tray section and engine mount section flexed, it
changed the distance between the throttle servo and the
carb throttle arm. The resulting oscillation causes the
engine throttle to move back and forth and if at the
right frequency can amplify the problem. On his
helicopter, he found two problems that promoted this
situations, slop in the throttle linkage (about a half
millimeter) and in the servo. Changing the servo and
fixing the links reduced the wah-wah problem a lot but
didn't completely eliminate it. One thing he had noticed
was that it was always at the same engine/rotor rpm where
the wah-wah oscillations started and you could hear a low
frequency change in the engine tone during hover and FF
at that specific engine/rotor rpmīs even if the wah wah
oscillations has not started. By increasing the engine/rotor
rpm by decreasing the pitch, that low frequency sound
disapeared. The frameset flexing is going to happen with plastic frames, but with a tight throttle linkage the carb arm would only be moved back and forth by a small amount, so the flexing would have to be a lot to start up the oscillation. However with the loose link and/or servo you can imagine the carb arm could move enough to change the engine torque which then might relieve the flexing and make the link go the other way, then the whole cycle could begin again. As another note, I always noticed that when my clutch
failed I would get the wah-wahs. With the above in mind,
this makes me think that it was caused by the vibration
from the off-balanced broke clutch setting up the
oscillation. |