flyenforfun

flyenforfun

Well Known Member
I just did the 3rd flight of my RV-8 and my dad did the 4th. We both experienced something very unnerving. At about 135kts the plane started a slow up and down vibration or oscillation. I would say the pace of the oscillation was motion up and down in 1 second. IT was pretty slight but definitely noticeable. At first I thought it was turbulence but it happened to both my dad and I on two separate flights, and it was too steady and constant to be turbulence. For me when I pulled the power back and slowed down it stopped. It also stopped when I got up above Id say 145 kts. No oscillation at 160 kts. And I couldn't feel it in the stick, just the whole plane kind of bouncing up and down. Is this flutter? Is something loose? This really killed my phase 1 buzz and now I am worried we have a major problem. Any suggestions would be great. I can't image it would be flutter at that speed, everything is tight and the gaps in the control surfaces are correct as far as I know. Really upset about this one..
 
Could the tires be spinning? Gear leg fairings causing gear oscillation? The best way to describe how it feels is say the bouncing you would feel if your gear legs started bouncing up and down, but it was happening level at 3000 feet..

Gear let bolts loose after 3.2 hours of flying? I JUST DONT KNOW!!
 
Safety

I am no expert but I would NOT fly that plane until every bit of linkage, cable, trim, hardware for control surfaces is gone over by you AND at least one set of fresh eyes and something is found and fixed.

That just doesnt sound right at all, and if its happened a couple times and two pilots agree.... PARK IT - FIX IT.

Fly safe.
 
Not flutter which happens very fast. It sounds like a damped short period phugoid mode oscillation. Did it ever feel like the oscillations were diverging (getting bigger)?
 
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It surely can't be flutter at those low speeds. Did you try yawing the plane left and right with the rudder to see if that aggravated or diminished the phenomenon?

Back in 2010 when we were first flying the RV-8 in my avatar, it had a funky wierd vibration at higher airspeeds that could be canceled out or made worse by yawing the plane... turned out to be the canopy vibrating against the rear turtledeck. Adding some extra foam weatherstripping under the canopy skirt fixed that, but that doesn't sound like what you're experiencing at all.
 
Neal@F14 said:
It surely can't be flutter at those low speeds. Did you try yawing the plane left and right with the rudder to see if that aggravated or diminished the phenomenon?

Back in 2010 when we were first flying the RV-8 in my avatar, it had a funky wierd vibration at higher airspeeds that could be canceled out or made worse by yawing the plane... turned out to be the canopy vibrating against the rear turtledeck. Adding some extra foam weatherstripping under the canopy skirt fixed that, but that doesn't sound like what you're experiencing at all.
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I get a vibration in takeoff from what I think is an imbalance in my wheels. Hitting the brakes stop this. Can my wheels be turning in the air and causing the gear to oscillate.
 
Do a search for things like control surface shape and vibration. It sounds sort of like what some other owners have reported as being due to control surface shape or skin flexing. Perhaps oil-canning of the skin, changing shape and therefore hinge moment. Something along those lines.

Dave
 
I have no experience in -8's, but could this be related to the gear leg/wing root interference issues that I have read about? This may be a stall issue, but just wondering if there could be a relationship? I think the fix involved some kind of fence or mod to the gear intersection fairing.
 
flyenforfun said:
Not, It was just a very slight and slow up and down bouncing.
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One difference between the first two flights and this one is that my tire pressures were lower and I know I have at least one tire out of balance. Could it have started rotating in the air? I didn't think to try hitting the brakes.
 
Pat Hatch said:
I have no experience in -8's, but could this be related to the gear leg/wing root interference issues that I have read about? This may be a stall issue, but just wondering if there could be a relationship? I think the fix involved some kind of fence or mod to the gear intersection fairing.
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My upper gear leg fairing is one I had bought from rvbits on here and I couldn't get it to fit very well. Maybe this is a problem.
 
Check your elevator for balance.

I know of a couple of RV10's over here that have had this kind of thing and too much paint I suspect at the back of the elevator meant the weights at the front needed a bit more mass.
 
RV10inOz said:
Check your elevator for balance.

I know of a couple of RV10's over here that have had this kind of thing and too much paint I suspect at the back of the elevator meant the weights at the front needed a bit more mass.
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Our elevators were balanced well. Maybe possibly heavy toward the counterweight. No paint on our plane yet
 
flyenforfun said:
Can my wheels be turning in the air and causing the gear to oscillate.
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Yes, at least they will continue to turn for a while after takeoff.

Next time you get the vibration, try hitting the brakes.

Do you have the wheel pants on yet?
 
tire balance

I had the same thing happen to me several times. Drove me nuts util I hit the brakes and it stopped. Yep tires out of balance. Got one ot those cheap Harbor Freight balancers and no more bounce. You will love the 8 for sure.
 
Matt,
This problem (as you have described it) has nothing to do with elevator balance or upper gear leg fairings, and is probably not control system related either (since you do not feel anything in the stick).

Wheel imbalance is a very good possibility.

Look the airplane over real well, and if everything looks normal, make another flight and try the brakes when the oscillation occurs.
 
flynwest said:
I had the same thing happen to me several times. Drove me nuts util I hit the brakes and it stopped. Yep tires out of balance. Got one ot those cheap Harbor Freight balancers and no more bounce. You will love the 8 for sure.
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Did this only happen to you shortly after takeoff? Because we experienced it well after takeoff in level flight at 3000 feet
 
Twenty years from now....

flyenforfun said:
Really upset about this one..
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These issues are really annoying and can be more so if you don?t tackle it by the numbers as it were.
Make yourself a list of the things you are going to investigate; do one thing at a time and keep good notes.
On the ground do your end to end control integrity check (I?m sure you?ve already done this a couple of times) including a check of all the canopy, cowl, wheel pant and fairing attachments.
Were I investigating this problem I would stabilize the aircraft in unaccelerated, perfectly trimmed flight below the airspeed you are investigating.
No autopilot etc.
Incrementally approach the target airspeed and record everything you see/feel/hear. If you have a tape recorder that is best so as to keep your hands free.
You?ll note if the issue starts with a pitch, roll or yaw or an acoustic warning (such as the canopy vibration issue that scared the bejesus out of me).
Check your TRIM.
Note such things as:
Does retrimming either in or out of the trim condition help or make it worse.
What is the airspeed band.
Can you see anything vibrating (cowl/wing tip).

I know this is ?buzz kill? now but later you and your dad will appreciate the depth of experience these things offer.
 
Spitballing here:

Do you have anything like a progressive rate/airspeed trim system, like what the VP-X will provide? Anything where stray trons could could could be activating the trim tab based on speed? Autopilot? Anything at all that could be influencing elevator/trim tab inputs at all?

Something is changing based on aerodynamic load based on what you're saying, and since there is no rolling moment it's got to be either the horiz stab, elevator or flaps.

Like everyone else says, check all rigging and make sure there it is all correct and within tolerances, that includes the trim tab and flaps.

Do you and your dad wiegh about the same? CG the same? I ask because it sounds like it's correlated with a certain aerodynamic load in the 135-145 range. All the elevator attach hardward is secure? If you push/pull/lift on the HS theres no movement whatsoever? Any noise associated with the pitch moment?

I'd also try installing a video camera, aimed at the tail so you can watch if it happens on your next flight.

Got someone that can fly chase to watch your wheels? I can't see how even gear leg vibration would induce the pitch oscilations you're talking about, but worth investigating if you run out of answers.

Keep us updated, I'm really curious.
 
Keep in mind flutter does not have to occur at high speed only. Several V tail bonanzas were lost on approach to landing and directly after takeoff. All it takes is something to start the "buzz".
 
Matt, you mentioned the fit of the upper gear leg fairings: why not remove them for a flight or two to see what happens. It sounds to me like you have some kind of an airflow separation issue which is being generated at a certain angle of attack, i.e., 135 to 145 kts. I would try to eliminate this as one of the possibilities.
 
Just sayin'...

A QUALIFIED formation partner might be able to see something that you cannot see from the cockpit. A video could be more telling, if the form pilot can carry a pax to film the lead ship...

Be careful with this option! Remember the XB-70 accident.

Carry on!
Mark
 
Where are your elevator counterweights in cruise? Heard of this happening before and a change in HS incidence fixes it.
 
+1 for the suspicion about the fairings. If they don't fit well then the airflow may be doing unexpected things at differing airspeeds. You could remove them and retest or tape over any gaps temporarily and see if this changes the behaviour.
 
I am following this thread because I have experienced the same thing twice in my -9A. I is not a buzz or vibration; it feels like someone is lightly pushing and pulling on the stick about once per second. In my case it only happed at cruise speed and would disappear when I reduced power. I cannot reproduce it. I suspect it is somehow related to speed/CG/fuel load.
 
Maybe

rocketbob said:
Where are your elevator counterweights in cruise? Heard of this happening before and a change in HS incidence fixes it.
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I heard it simply moved the range of the oscillation either up or down in the speed range, if you are referring to a couple of Canadian planes...

Carry on!
Mark
 
check your tires for play, jack up the aircraft and see if you can move the tires back and forth. I had this same problem in my rv4 back in the early 90`s though i had flutter going on. the tire was loose on the axle causing the tire to wobble at about 135mph moving gear legs back and forth causing my rv4 to hop up and down thinking i was having control problems. good luck.
 
Elevator trailing edge may be too thin, or too thick. From the recesses of my aging brain I seem to recall that the TE is critical.

That's why "squeezing" the ailerons is a way to cure a heavy wing. And it is why the TE of some aerobatic planes are thick, and squared off.

Time for one of the aero engineers to pick up the ball here..............
 
Tire

I had a oscillation over 180mph. Turns out my nose wheel would spin up and was out of balance. The fix was retorque the axle bolt.
Rich
 
System drag

Go for a fly and get the airplane to repeat the symptom.Then add enough trim so you have to hand fly it. "load the control system" If the problem goes away, it's elevator related aerodynamic.
 
I had the same problem. The tires were out of balance and were spinning at higher speeds. I could hit my brakes or slow down and it would stop. Balanced the tires and no more problem.
 
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