Horizontal Stab Flutter?

[k2bords]

Hey Guys - I'm teaching a guy in his Champ with an O-200, 69x48 McCauley on it I believe. The airplane looks/runs/flies great. However - on the first flight I noticed as we were climbing out an intermittent vibration. At first I thought it was the engine running rich (field elevation is ~5500' msl). After flying around a while I looked back and noticed the leading edge of the horizontal stab vibrating intermittently. After getting back on the ground we looked over the stab and all the flying wires are tensioned well and the attach points to the fuselage are all solid. Has anyone here ever experienced/heard of this? I checked out the trim tab and I think the two screws that attach the horn to the tab might be able to be tightened up a bit, but could a very slightly sloppy trim tab cause the whole tail to vibrate?

Matt

[Nathan K. Hammond]

could a very slightly sloppy trim tab cause the whole tail to vibrate?

It can and will. Does it change with a fore/aft CG?

nkh

[Paul Agaliotis]

Matt,
The stab should be loaded down all the time. If it's shaking something's loose. I would look real good and find the problem before flying it again. The Cub's will shake due to the trim jack-screw setup but the Champ should stay pretty still.
Make sure you don't have a loose fairing or something that's inducing an airframe vibration. If you have streamline tail wires angle the leading edge down some. While you're there make sure the tail is rigged. Not just square to itself but square to the long/lat and vertical axis of the airframe. Most aren't.
Paul

[Jerry Jackson]

I experienced vibration in the left horizontal stabilizer on my 65TC. The vibration only occurred while climbing with full engine power and only the left stabilizer vibrated. The vibration appeared to be torsional, with the largest deflection occurring at the front of the tip. No deflection was evident at the back of the stabilizer, along the trailing edge. Varying propeller speed during cruise (higher airspeed & lower AOA) did not excite the vibration. I tried increasing and decreasing the tension in the flying wires but the vibration never went away. Now I wished that I had tried to measure the effect of tension on vibration amplitude and engine speed at resonance.

I formed the opinion that the vibration was excited by the pulsating slipstream from the prop. The slipstream will have the strongest pulsation at low speed and with high power. The vertical location of the slipstream measured at the stabilizer is affected by the airplane's angle of attack and it is plausible that the AOA during climb caused the stabilizer to be located in the region of strongest pulsations. I admit that reasoning is based on in-expert, subjective opinion and it would be much safer to have a quantitative understanding of causes and steps for mitigating the vibration. But I don't have that knowledge. What I can do is to measure and compare the torsional stiffness of the left/right stabilizers. If there is a pronounced difference then one would suspect the weakest one might have structural damage such a cracks. I will make those measurements in the next several days and report the results.

I definitely do not believe the vibration I experience was due to classical aerodynamic flutter for two reasons. First, as I understand it, aerodynamic flutter occurs when a critical airspeed is exceeded and it does not disappear with further increases of airspeed. That is not the pattern I experienced. Second, classical aerodynamic flutter generally leads to catastrophic structural failure---but I'm still here!

Jerry Jackson

[k2bords]

Jerry - I would tend to agree with you, it seems if I remember correctly to mostly be a problem in the climb and I didn't necessarily notice any flutter in the elevator/stick. I'm going to fly again today, I'll tighten up the trim tab horn and see if that makes any difference.

Paul - It's got regular round flying wires. To check all those dimensions would I measure stab tip to some fixed point forward of the tail on the top center of the fuselage and then stab tip to rudder tip maybe?

Matt

[Paul Agaliotis]

Matt,
Level the plane and level the horizontal stab. Once fixed make sure the vertical stab is square to the horizontal. This way tail is flying in the same direction as the wings. You can have the tail square to itself but crooked to the plane, this will give you a heavy wing. I've seen strut adjustments at a maximum position, even trim tabs on the aileron. And the tail rig was the culprit.
It's not as easy as it sounds. To move the vertical you have to loosen one side and tighten the other. Plus don't over tighten the lower wires.
The rear mount for the horizontal stab is only supported by those sheet metal brackets that the cover is screwed to. If the shaking continues it will crack those brackets, T/W shimmy will do the same thing. I would suggest that you take a good look back there for damage or something loose. Nathan pointed out the tab, and that would be a good place to start.
Paul

[Jerry Jackson]

Previously I recounted my experience with visible deflection of the leading edge of the left horizontal stabilizer while under high power, low airspeed climb. Only the left stabilizer showed vibration so it is natural to wonder what is unique about it? Did it have different torsional stiffness than the right? Or is the left exposed to a stronger periodic force?

I tested the torsional stiffness of the left and right horizontal stabilizer of a 65TC. I repeated the test in both torque directions to cover the possibility that crack closure-opening would exhibit itself by asymmetric stiffness values. The four stiffness values were virtually identical---the peak to peak spread of the values was slightly less than 1%. This result intensifies my belief that the vibration is caused by difference in the prop slipstream measured at the left and right stabilizers. I don't know what that difference is and I don't know how to reduce the vibration other than to avoid operating at high power and low airspeed climb whenever possible.

[bsafran]

This vibration worries me. I hope Bill Pancake pops in and gives his thoughts on the matter because he has seen or heard just about everything about these planes. Airplane parts aren't supposed to shake unless they are on old radial engine biplanes or helicopters. On those things, everything shakes! But to have a Champ tail vibrate, even on a higher horsepower conversion, is concerning. It seems like the previous posters have looked into almost everything that could cause this problem and you seem to have ruled out their suggestions. I don't believe it to be flutter since that would most likely affect the trim and elevator before it would a stailizer. If the stab is in good airworthy condition, I would like you to look in another direction at a problem Mr. Pancake pointed out to me in our fuselage repair work. He wants everyone to inspect the cluster of tubes that make up the forward attach point for the horizontal stabilizer. Tube number 7 runs the length of the top of the rear fuselage and butts into the tubing that the stabilizer slips into. He warns us of bad corrosion concentrating there and I have found a few fuselages with tubing rusted completely through. Could a weakened tube in this area allow the stabilizer to move just a little bit under the right circumstances? The picture does not show the corrosion I am writing about, only where to look to find it if it does exist.

[Jerry Jackson]

Those tubes on my 65TC were in good condition.

[joea]

Agree with Brian.

Excessive vibration is not good and the source needs to be found...

My first place to look would be a prop that is out of balance...

[gusc]

I go along with the prop wash theory, and this is only my opinion.

The prop wash sort of screws itself around the airframe and hits one side of the elevator differently than the other side.

I'm not saying there is not another cause also but I've experienced this in other aircraft, the C-150 for one in which I've had many hours as a CFI.

I just never paid much attention to it, maybe I should have!!

I don't recall this ever happening in my Champ but I could have just ignored it if it did.

Dennis,

Is your Camp finished yet? Mine is out of action because of a weak cyl.

[Dennis]

Hi Gus,

Yes, it has been flying for over a year now. See post under N83147

Dennis

[gusc]

Looks great, I'll have to fly over one of these days and have a look.

21 years, wow! Makes my two years seen pretty small!