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My Rudder Pedals Buzz

petersb

Well Known Member
Getting uncomfortable vibration through rudder pedals side walls etc. in cruise

Aircraft details:

Engine o-360
Prop Hartzell Constant Speed Blended Airfoil
3/4 inch sound proof foam on floor
Prop tracks perfectly
Dynamically balanced to .06 IPS

Don't notice vibration on climb out -- Full throttle 2400 to 2600 rpm range

Seems to develop once leveled out 2300 to 2400 rpm.

My thoughts are, on climb out the prop is on the fine stops and this makes both blades of equal pitch. When the rpm is dialled back the blades come off the stops and may allow uneven pitch, I understand this can cause vibration


Any thought

Peter
 
When dynamic balancing, we were taught to make sure the prop is pulled back at least enough to get the blades off the stops. It is possibly a harmonic vibration. What happens if you pull it back to 2,200rpm in cruise? Basically, I would recommend that you find out the actual range of rpm of the vibration then take it back to your balancer and rebalance it at your desired cruise MAP and RPM.
 
When dynamic balancing, we were taught to make sure the prop is pulled back at least enough to get the blades off the stops. It is possibly a harmonic vibration. What happens if you pull it back to 2,200rpm in cruise? Basically, I would recommend that you find out the actual range of rpm of the vibration then take it back to your balancer and rebalance it at your desired cruise MAP and RPM.

The vibration is at all rpm's. The amplitude does increase at higher throttle settings ( manifold pressure ) at all rpm's.

How can the blade pitch be checked without removing from aircraft ?

It was balanced at 2400 rpm.

Peter
 
In that case, I would think that it is something in the airframe causing the vibration at higher airspeeds. Does it decrease as you slow down? Pull the power and slow down to your cruise IAS and see if it decreases or goes away.

Just a thought. Does tapping your brakes make a difference?
 
In that case, I would think that it is something in the airframe causing the vibration at higher airspeeds. Does it decrease as you slow down? Pull the power and slow down to your cruise IAS and see if it decreases or goes away.

Just a thought. Does tapping your brakes make a difference?

No, it is rpm dependant. change of airspeed has no effect

Peter
 
Do you have carpet or something on the floor? If there is nothing there for the skins, it will vibrate.
 
Do you have carpet or something on the floor? If there is nothing there for the skins, it will vibrate.

Yes, 3/4 inch sound deadening foam and carpet.

Think I might have to check each blade pitch, however this is annoying on a constant speed prop due to captured bolts
 
When dynamic balancing, we were taught to make sure the prop is pulled back at least enough to get the blades off the stops. It is possibly a harmonic vibration. What happens if you pull it back to 2,200rpm in cruise? Basically, I would recommend that you find out the actual range of rpm of the vibration then take it back to your balancer and rebalance it at your desired cruise MAP and RPM.

If you have an aerodynamic imbalance due to blade track or pitch problems you will not be able to dynamic balance the assembly. I've watched it both ways and it makes no difference balancing a prop either on or off the stops. If the balance changes when the prop is moved off the stops then the prop has a problem that cannot be corrected by dynamic balancing. Aerodynamic issues also change as RPM changes so its usually impossible to balance a prop with blade issues as the vibration level is constantly changing magnitude and polar location with RPM changes.

I would recommend the OP get a vibration spectrum analysis done, this can often help to diagnose vibration problems and to check to see if the balance changes with the prop off the stops.
 
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I don't think it's the prop.

My pedals "buzz", too, and I feel it throughout the airframe. Take-off roll, full power (375 + Hartzell), things are smooth, until passing through 100 IAS, then it's as though a switch were thrown and the buzz kicks in. Sudden like. Gets your attention. And so it remains throughout the flight. Haven't thoroughly messed around with slower airspeeds, figuring it was the prop, but I will now, because...

I just installed a Whirl Wind 74RV and the symptom remains. I suspect "oil canning" somewhere. I've felt this sort of vibration in other RVs, but never bothered determining the source since the magnitude was less and no sudden onset.

John Siebold
 
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My pedals also buzz. It showed up after I balanced the prop and changed to a Matco front axle. Unfortunately I did both at the same time. I had the prop re-balanced and the numbers came out good. I thought maybe the front wheel was spinning in flight with the new free turning axle and the front wheel was not balanced. I tighten the bearings and tried to check the wheel. So far the buzz remains.
 
My pedals also buzz. It showed up after I balanced the prop and changed to a Matco front axle. Unfortunately I did both at the same time. I had the prop re-balanced and the numbers came out good. I thought maybe the front wheel was spinning in flight with the new free turning axle and the front wheel was not balanced. I tighten the bearings and tried to check the wheel. So far the buzz remains.

Flew to Smith Falls breakfast this morning and played around during the flight, this is what I discovered.

The buzz is coming through the floor, with my heels on the floor and toes on the rudder pedals you would swear the vibration was coming through the pedals, however, when you slide your feet up on the pedals and heals off the floor the vibration almost goes away.

I also noticed it was more noticeable above 125 mph, although the frequency still seemed to be rpm dependant.

I also have the Matco axle, so it seems it might be aerodynamic buffeting which is related to the propeller and the airspeed. That is probably why it felt smooth on climb out because the AS is usually around 120 mph.
 
Sounds like it could be exhaust pulses on the floorboards/belly skin. There are many threads on this issue here on VAF.

Some have reported success by extending and turning the exhaust pipes downward a bit. Worth a search.
 
Make sure your exhaust isnt touching the cowl. Ive had my hangars slip and it feels like something is coming apart. Just a thought. Terry
 
Make sure your exhaust isnt touching the cowl. Ive had my hangars slip and it feels like something is coming apart. Just a thought. Terry

No contact in the cowl from exhaust etc.

Currently installing the circular fairing at the bottom of the firewall to smooth exit air flow, will see if this stops airflow turbulence on the floor. May even give some extra speed!
 
I had the same thing happen on a long XC flight about 150 kts.... I thought I had a major engine issue since it "came from nowhere", ...

it turned out to be my gear leg fairings, they were moving a fraction of an inch back and forth in the slipstream at high speed...I fixed this by installing a couple of new washers on the fairing set pin; the hole through the fairing had wallowed out enough to allow a bit of movement.

its worth a look...

I fly at T-18 so your results my vary
 
My guess a missing .............

A miss match of washers somewhere near the motor mount
 
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