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My Rudder Pedals Buzz

P

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.
 
Jesse said:
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.
Click to expand...

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?
 
Jesse said:
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?
Click to expand...

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.
 
TroyBranch said:
Do you have carpet or something on the floor? If there is nothing there for the skins, it will vibrate.
Click to expand...

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
 
Jesse said:
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.
Click to expand...

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.
 
dougmattson said:
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.
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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
 
zlinman said:
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
Click to expand...

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
 
I know this is an old thread, but what did the resolution turn out to be? I'm in Phase 1 right now and have basically the same symptoms. The floors are bare and have a buzz that you can feel through your heels, which I know is common. I've been running my new engine hard for break in and other than the floorboards it's very smooth and vibration free at 25/25.

Yesterday was the first time I played around with anything less than 25/25 cruise power settings. Regardless of manifold pressure and seemingly not tied to airspeed, when I pull the blue knob back to 2300-2400 rpm, I get an low magnitude but extremely high frequency "buzz" type vibration that I can feel pretty much everywhere. When I say everywhere I mean I can feel it in the stick, on the armrests etc.

Any ideas?
 
I get the similar buzz with my fixed pitch prop when I cruise on autopilot at 2620 rpm or higher. Nothing significant but when cruising on autopilot, the workload went down and I could feel it at my rudder pedals through my heels. But when I was maneuvering with the RPM fluctuating while pitching up or pitching down, then I didn't notice it as much or not noticing it at all. Also there is a definite harmonic vibration when slowing down to pattern altitude with RPM in the range of 1500-1600 to maintain 90kts. The vibration is definitely at the lower frequency comparing to the cruizing buzz.

I have not had my prop balanced yet.
 
I call that “the rumble” — smooth as glass at 2700RPM down to 2450, but as I continue to pull the prop through ~2400, I feel the rumble - it resolves at around 2200 RPM

RV’s == snare drum + bass drum…

Fixes:

1. Dynamically balance the prop at desired cruise RPM - 2400RPM might be better than 2700…
2. Turn down the exhaust pipes - the 4-2 Vetterman’s already have this feature.
3. Use 3/4” foam and carpet the floor in between the firewall and front spar carry through.
4. Carpet baggage area.
5. Change prop to composite type (wood, fiber glass-wood, C/F/foam core, etc.)
6. If equipped with dynamic spark advance (LSE,PMAG,SureFly) consider switching to fixed timing — 25° or 20°.
 
My configuration is:

-IO390 counterbalanced engine
-Whirlwind 300-72 3 bladed prop
-gear strut fairings on
-intersection fairings & wheel pants not yet installed (but the aluminum brackets are there)
-vertterman 4 into 2 exhaust with the downturned tips pointed in toward each other slightly
-pmags with the jumper in
-no rudder trim tab

just to be clear, the floor drumming is always there and is most pronounced in the two outboard bays. Basically where the pilots left foot or the copilots right foot resides. The higher frequency and lower magnitude buzz shows up between 2300-2400 rpm
 
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