Strange resonance frequence at full throttle

When flight-testing my RV4 (O 320, wood prop), this happened: All went fine, but when applying full throttle after 30 minutes of flight the whole aircraft started shaking (amplitude app. 5 times per second) so the gauges where shaking and the canopy shakes as well. reducing power just a litlle bit and the problem disappeared. The follwing was tested to avoid this: Running on 1, 2 or both mags, giving electric fuel pump as well, pulling carb heat, turning the prop by 180 degrees. No success. It happens with full throttle, no matter what RPM, but only from time to time. The aircraft is equipped with an Ellison TBI.
Any idea what could be the reason??
Thanks, Dirk

Prop?

Dirk,

You probably already checked this, but just in case...

1. check torque of prop bolts since these change with the weather on a wood prop

2. check tracking of prop blades. They should be within 1/16 of each other. Move the prop so that the blades so that they are vertical. Place a stool under the low blade and pile up books or whatever you need to get it with in a few mm's of the lower blade. Slip a piece of paper under there. Don't let the paper move. Mark where the blade falls. Keeping the paper in the exact same place, rotate to the other blade and make the same mark in the exact same manner. Are you within 1/16", then you are good on that. If not, you may have to loosen the bolts and try another torque sequence.

The other item that I wanted to mention is that when I am full throttle in my -8 I can get some exhaust system drumming on the bottom of the forward fuselage. Probably not as bad as guys with the -7 experience, but it is noticable.

If the prop checks turn out OK, then I would think about having a prop shop do a dynamic balance. If you wanted to first start the process at home, you can do a static balance check of the prop to see how it does. But to do this, you need to remove the prop and go through a balance procedure to check for a heavy blade.

Probably not much of a help, but somewhere to start...

Viel Glueck mit der Loesung,

Rob

Maybe if I actually READ your post...

Sorry Dirk,

You said that you changed the prob blade rotation, so forget my comment about torque, you would have already done that. Blade tracking would be worthwhile checking though.

Another thought - you may have a mag that is starting to fail under certain conditions. I had this happen in our Acrosport I a while back and we spent a lot of time "chasing demons" until we figued it out. We had the old style Bendix mags and the coil is held in with a "key" that is actually more of a wedge. In fact it is literally inserted like a wedge. We are talking 1940's farm tractor technology here... One of our wedges came loose and partially damaged one of the plastic gears in the mag. However with ours, it ultimately would fail the mag check.

Just some more food for thought.

Engine or baffles or airbox or ? is rubbing the cowl (or engine mount). That's my guess.

Engine or baffles or airbox or ? is rubbing the cowl (or engine mount). That's my guess.

Prop? Tires?

I would attempt to swap props if possible, perhaps to a metal prop. While 300 hz doesn't sound like something directly rpm related, props have been known to do strange things. Is the prop new? Some odd flexing at high torque could be possible. That could cause bigger problems. 300 hz almost sounds like a tire out of balance and rotating, but that shouldn't be related to engine power and would occur only in flight. Hitting the brakes would stop that, of course.

Bob Kelly

I think it would be 5 hertz

videobobk said:

I would attempt to swap props if possible, perhaps to a metal prop. While 300 hz doesn't sound like something directly rpm related, props have been known to do strange things. Is the prop new? Some odd flexing at high torque could be possible. That could cause bigger problems. 300 hz almost sounds like a tire out of balance and rotating, but that shouldn't be related to engine power and would occur only in flight. Hitting the brakes would stop that, of course.

Bob Kelly

Click to expand...

Bob,
I think (IMHO) 5 times a second would really be 5hz not 300hz.

Mark

Mark Burns said:

Bob,
I think (IMHO) 5 times a second would really be 5hz not 300hz.

Mark

Click to expand...

(Blush) I knew that. It must be getting late, yes that's it. Seriously, thanks for correcting me on that. Somehow got the rpm mixed up with...no, think I'll quit while I'm not too far behind.

Bob

Dirk,
5 hertz is too slow to be a first order (once per revolution) balance issue; 2700 RPM is 45 hz. Individual cylinder firing is 22.5 hz, so toss that too. Since you were sharp enough to note the 5 hz frequency, I suspect you already know all that.

Given that the vibration is power-related, I would be concerned about propeller blade flutter. Using a good variable rate strobe light, I've observed a defective maple prop fluttering a single blade; flutter frequency doesn't depend on rotational frequency. Static inspection found no cracks or failed glue joints, although it wouldn't have stayed that way for long if the pilot had ignored the vibration and noise during the initial mag check runup following prop installation.

If you have no access to a strobe light, can you borrow a propeller for a comparision run?