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Peetie the Pilot

Active Member
:confused: Up until recently, I had been flying my RV-9A without the wheel pants & gearleg fairings and was getting speeds in the 130 - 145 knot range. I have now installed these parts and getting speeds of 150 - 158 knots. My problem now is I am starting to feel elevator shimmy in the control stick at these higher speeds. It is a very light flutter. I have checked all the linkages and all is good. Is there anyone else out there experiencing these "flutters" and how did you fix it? I thank all in advance who reply.

Peter Marshall
RV-9A
Flying
 
Stop

If you are truly feeling flutter you should stop flying until you can ID the source.

IT could be something else but you should proceed with caution. Please call van's aircraft. I don't know what the Vne is on the RV-9, but top speed is listed at 170kts and guess Vne is much higher. You speed is well below either of these speeds.

What you did was good. I assume you have 100% balanced elevators. I think the RV-9 uses an extrusion for the trailing edge of the control surfaces. The control surfaces have bent trailing edges on RV-4, -6, -7 etc. If the radius is too large, it can cause "snatch" or hunting (vibration not flutter).

I can guess, but this is serious and you should not fly or fly at lower speeds until you can identify the reason for the control vibrations.

Is it possible it is from the ailerons? It could be nothing like airflow off the wing causing the vibration in the elevators and thus the stick.

Regards George
 
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Peter

Do you know about what the frequency is? I get a low frequency vibration in the stick at cruise speeds, maybe 1 or 2 Hz. The amplitude is very low. It feels like a vibration running through the structure not coming from the elevators, eventhough the vibration is felt through the stick. One day when I get the time I'll instrument the plane and find out where it's coming from, until then I don't feel it's a big deal. My guess is that the vibration I feel is just some order of the prop speed. Plus I figure it just comes with having control surfaces actually attached to the stick.

I believe (you aero engineers correct me here) flutter is a much higher frquency. I understand it's more of a buzz feeling.
 
I'm not sure about a buzz. I got the nose pointed down in my 172 once and built up a little (OK a Lot) extra speed and when I pulled up to level the plane it shook like a dog with a snake in its mouth. The yoke was moving maybe two inches in and out. It scared the living !?!? out of me. I assumed I experienced elevator flutter. I bought a new airspeed indicator. I also know someone who wound an RV-9 up so tight that he broke the airspeed indicator. I'll ask him if he experienced any vibration or flutter next time I see him.
Jim Wright 90919 RV-9A wings.
 
Do Not Fly

Flutter is nothing to mess with. Do not fly until you have an A&P check out everything.

Did the event occur as a result of a power setting or decent airspeed? As someone noted it could be power related.

The other thought that comes to mind is that something you attached (fairings, pants etc..) is not secured well and could be moving and setting up a flutter in those parts.

Good luck, keep everyone posted.

Darwin N. Barrie
P19
 
Peetie the Pilot said:
............... My problem now is I am starting to feel elevator shimmy in the control stick at these higher speeds. It is a very light flutter. I have checked all the linkages..........................
Peter Marshall
RV-9A
Flying
Click to expand...
Peter,
You say you have checked all the linkages. Okay, but what did you really find? What control feel do you consider acceptable? A recent thread discussed the phenomena of "stick slap." Some builders consider minor play in the linkages as acceptable. Perhaps in some situations it is a non-issue and such aircraft can go on to fly for thousands of hours. Others may not be so lucky. Is it even remotely possible that "stick slap" built into your control assemby and coupled with the specific aerodynamic issues unique to your airframe rigging (not the least of which is the specific bend radius of your elevators' trailing edges) can work in concert against you to set up the (forgive my analogy ) "perfect storm" conditions you describe? There are a lot of pivot points between your joystick and the elevator horns. I'd give those linkages a very critical look once again and if the slightest "stick slap" or play is felt in that axis, I'd address the condition first. Let us know what you find. Best regards.
Rick Galati RV-6A "Darla" 68 hours
RV-8A empennage complete
 
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RV7Guy said:
Flutter is nothing to mess with. Do not fly until you have an A&P check out everything.

Did the event occur as a result of a power setting or decent airspeed? As someone noted it could be power related.

The other thought that comes to mind is that something you attached (fairings, pants etc..) is not secured well and could be moving and setting up a flutter in those parts.

Good luck, keep everyone posted.

Darwin N. Barrie
P19
Click to expand...

Hmm, I agree it should be checked out, but A&P's aren't going to help much unless they're already a RV expert. For instance, I'm an A&P and I wouldn't know where to start except checking control play and rigging.
 
N916K said:
Peter

Do you know about what the frequency is? I get a low frequency vibration in the stick at cruise speeds, maybe 1 or 2 Hz. The amplitude is very low. It feels like a vibration running through the structure not coming from the elevators, eventhough the vibration is felt through the stick. One day when I get the time I'll instrument the plane and find out where it's coming from, until then I don't feel it's a big deal. My guess is that the vibration I feel is just some order of the prop speed. Plus I figure it just comes with having control surfaces actually attached to the stick.

I believe (you aero engineers correct me here) flutter is a much higher frquency. I understand it's more of a buzz feeling.
Click to expand...

So Cam, I think you're on to something here.

On my 9A (400 hours of good flying fun) I have a certian speed and set of conditions that I can't replicate but has happened more than once...

Ok, all disclaimers done with... Where the elevator will ocilate back and forth just enough that you can feel it in the stick. It's not flutter, but simply like it's hunting a bit. It comes from a bit of play in stick linkage setup (I think). If you hold a bit of up or down it goes away, but if perfectly trimed and stick neutral (between the very small play range) it will do this. Seems to only stay for less than 60 seconds or so and then the conditions are not the same and I can't replicate it.

In looking for the free play the only thing I found was that the pilot's stick pivot point was a bit too loose on the bronze bushing that it turns on. So you can get just a bit of play in moving it up and down. I have replaced the bushing, but it seems that the over zelous builder (me) reamed the steel tube on the stick pivot point a bit too much. I'm not talking about much play here, just barely enough to feel.

I tested Van's 129RV and it was solid and smooth in the elevator movement.

Good luck finding the source on your setup!
 
:confused: Thanks for all the input! The frequency of this "flutter" is VERY low and could be best described as wollowing of the control stick in pitch. The control of the airframe is fine with no nasty characteristics. Like Andy says " It last for a short time and I can't replicate this problem" A 50- 100 RPM change in the power and it disappears and it does not appear under 150KTS. I will go over the control linkages again but I did not detect any play that seemed to be abnormal. I will keep the group posted on further developments.

Peter
 
How did this story end?

I'm just finishing the elevators and getting ready to drill the control horns. The first (or last) link in the elevator control chain. Did you ever find the cause of this flutter? Something to look out for inthe build?
 
Maybe not Elevator

When I fitted my Wheel Fairings, on the next flight I experienced a low frequency vibration throught the airframe.
Having experience the same flying heavy metal, I GENTLY applied the brakes and the vibration stopped.
The tyre sticking out of the fairings into the airstream was spinning up thge wheel which was obviously slightly out of balance.
It has only occured twice since.

Pete.
 
Peetie the Pilot said:
:confused: Up until recently, I had been flying my RV-9A without the wheel pants & gearleg fairings and was getting speeds in the 130 - 145 knot range. I have now installed these parts and getting speeds of 150 - 158 knots. My problem now is I am starting to feel elevator shimmy in the control stick at these higher speeds. It is a very light flutter. I have checked all the linkages and all is good. Is there anyone else out there experiencing these "flutters" and how did you fix it? I thank all in advance who reply.

Peter Marshall
RV-9A
Flying
Click to expand...

Peter, where is your CG?
If you are flying with an aft CG, the elevator will get mighty sensitive in pitch at high speeds. The elevators could be near neutral at 158 knots.

dd
 
Peetie the Pilot said:
:confused: Thanks for all the input! The frequency of this "flutter" is VERY low and could be best described as wollowing of the control stick in pitch. The control of the airframe is fine with no nasty characteristics. Like Andy says " It last for a short time and I can't replicate this problem" A 50- 100 RPM change in the power and it disappears and it does not appear under 150KTS. I will go over the control linkages again but I did not detect any play that seemed to be abnormal. I will keep the group posted on further developments.

Peter
Click to expand...

Did not see this post before previous message.....I'm betting it is a CG issue.

dd
 
Sorry if dogpiling here, but any concern about your control system needs to be resolved before flying again. I have 10-15 hrs in -9A and the elevator control should have no hysteresis, slop, vibration or any other odd characteristic. Get it checked out by some knowledgable RV folks and maybe even talk with the factory.
 
Could it be the trailing edge

Vans always sez that the finish on the trailing edge is very important to the control feel on an RV. Could this be the problem here?
 
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This hunting sounds a lot like what the plastic airplane guys describe when their trailing edges are too sharp. So, they file a flat (small, maybe 1/16" wide) to fix it. Also makes the control forces lighter. Look at most plastic airplanes and the trailing edges on the control surfaces are not dead sharp.
 
Make sure the locknuts are secure on the rod end bearings of the elevators and rudder. If they work slightly loose it will allow unwanted vertical deflections of the spar of the elevator or rudder relative to the rear spar of the horizontal or vertical.
 
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Nuisance said:
This hunting sounds a lot like what the plastic airplane guys describe when their trailing edges are too sharp. So, they file a flat (small, maybe 1/16" wide) to fix it. Also makes the control forces lighter. Look at most plastic airplanes and the trailing edges on the control surfaces are not dead sharp.
Click to expand...

There is more to it than that. The canard elevator also had to be absolutely flat on the bottom, but you are correct about the trailing edge. The templates show it just as you describe.

My theory on pitch instability comes from canard experience. When the CG was at the aft limit, the elevator became VERY sensitive and did present an interesting control challenge, not unlike what Peter reports, but of course the RV is a totally different creature so I may well be all wet on this one.

I agree, the control system must be checked along with elevator balance. Elevators must be balanced correctly or they are a disaster waiting to happen. Van's does not make a big deal of elevator ballance because the if the parts specified are used, and directions followed, it should be OK. Van's says the elevator should balance "in trail" (after paint) and this is accomplished by drilling holes in the counter weight if need be. In the canard world there was a counter weight limit plus the training edge had to trail down a precise number of degrees. If it did not, you had to build a new elevator with less epoxy to get it right. :(

dd
 
The problem may be elevator "hunting" due to an improper HS incidence angle.

When flying at cruise speed, with the CG well within tolerance, look back at the elevator and note the position of the counterbalance leading edge. It should be about 1/4" - 3/8" UP (slight down elevator). If it is close to nuetral the elevator will "hunt" at low frequency.

Increase or decrease the angle of incidence by adding or removing shims from the front of the elevator.

Ben Beaird
Plymouth, WI
-6A
 
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