Rough running question
in 912 / 914 Technical Questions
10 years ago
Hi, I have a 912 ULS in my Europa. Warp Drive ground adjustable prop. Most of the time it runs beautifully but I have noticed an issue. When the "airflow is driving the propeller" for want of a better expression, ie. in a high speed, low power descent, a bad high frequency resonating vibration can suddenly start. I've noticed it many times when at cruising speed and throttling back for descent it can suddenly start. A binding throttle cable I've thought in the past, although I have worked on the cables many times and am happy with them. However, experimenting in the air at height I can make it do it by increasing airspeed alone without touching the throttle. If I set the plane up at low speed, (70 kts) and low to mid power, say 4000rpm then start to dive to increase airspeed, a point can be suddenly reached, usually 100 kts + when this vibration suddenly starts - it's more like a very noticeable change in the engine note and is so sudden it's like turning on a switch! It's teeth chattering and the slip ball jiggles about like a pea on a drum! Raising the nose to slow down again and the vibration will suddenly stop again at exactly the same airspeed trigger point as it started and it's back to normal, smooth again.
Take off, climb and cruise it never does it. Only as I say at higher speeds and low ish rpm it can suddenly start. The only way I can guarantee not to experience it on a flight is to make a lower speed descent than normal, ie. my normal cruise is 120 kts at 5,300 Rpm. If I raise the nose to slow down to less than 100 kts THEN throttle back to descend it doesn't do it and I can then descend as required without getting the vibration.
It's a problem that I know about and have been living with by trying to avoid it but just wondered if anyone might be able to shed some light on it.
Cheers!
Ps. Very happy with the carb synch (following much advice on here! Prop set up is also good and no known suspected problem with engine mounts (3 years old).
Take off, climb and cruise it never does it. Only as I say at higher speeds and low ish rpm it can suddenly start. The only way I can guarantee not to experience it on a flight is to make a lower speed descent than normal, ie. my normal cruise is 120 kts at 5,300 Rpm. If I raise the nose to slow down to less than 100 kts THEN throttle back to descend it doesn't do it and I can then descend as required without getting the vibration.
It's a problem that I know about and have been living with by trying to avoid it but just wondered if anyone might be able to shed some light on it.
Cheers!
Ps. Very happy with the carb synch (following much advice on here! Prop set up is also good and no known suspected problem with engine mounts (3 years old).
