Tail Vibrations

L

LEN

New member
I experience some low frequency vibratioins in my tailplane when I pull G's (Decathlon CS 1977 model). I feels much the same as buffeting before stall, however this is not buffeting (3 g at 140 mph). When I look at the tail I can see the leading edge moving up and down about one inch.
Any ideas of the reason and what to do about it?
LEN
 
Len,
Assuming that your ASI isn't lieing to you it doesn't sound safe at all. Sounds like onset of flutter. Time to take a really good look at all the attachments, the bracing wires and check the integrity of the framework under the covering. Also check that the hinges to the moving surfaces are in good shape.

This sort of thing gets out of hand very quickly and you could loose your tail feathers in a blink of an eye.
Nicco
 
Sounds a lot like the downwash 7/8GCBC can get.....
but then, you dont have flaps.

So this happens only in semetrical manuvers?
Is it repeatable in different senarios? Is it only when pulling out of a loop or something?
Need more info, please.



BTW, its not flutter
 
Jerry,
Could you explain the downwash your talking about on the GCBC. I have in the past noticed when using full flaps a vibration, and no it wasn't close to stall. It really felt like a tail vibration. I was going to replace the bushings in my elevator while I was doing my annual, but the bushings I ordered from ACA were different, smaller inside dia.,appear to be close to the same outside dia but , the old ones are tight in the hole and Mandy didn't tell me the hinge pin used a castle nut so I elected to just wait, it only had a small amount of slack on the left side, anyway I figured the slack was causing the vibration or the vibration had caused the slack.


Woodie
N29763
 
If I recall right, the most interesting downwash comes at about 80 MPH and 3 notches. Its been a while. Anyway, the stick basically moves itself back and forth a few inches.

Anyway, downwash is an adjusted angle of flow down off the back of a lifting body. Downstream is straightens out. The larger the lift coefficent, the bigger the angle of the flow. This changes the angle of attack of the tail feathers if they are too close to the wing (i.e. 7/8GCBC). Anyway, what was happening at 80 was that the airflow was separating at the leading edge of the horizontal stab and then reattaching itself in the elevator area. When it was doing this buffet, it was alternating between two quasi-static states trying to balance itself (the airplane).

It was more of an annoyance than anything. However, there are only 2 cures. Either make the tail into an airfoil (harder than it sounds and might not change anything) or move the tail back further (much too hard but garunteed change).
 
I wonder what vortex generators would do to this. Your probably right on the speeds, I can't really recall, it would only be for a brief moment that you would feel the vibration and it would be over. The best I can remember I would happen after I would turn final and really be slowing down.I passed it off as being caused by a higher than normal nose high attitude,no power and the air hitting the bottom of the elevator. It never gave the sinsation that anything was stalled or about to stall. I never notice it anymore, probably because I've learned the aircraft a lot better and approach a lot slower . Thanks I was just curious!!!



Woodie
N29763
 
Vortex Generators on the tail would likely help a little. The hard part is getting their full potential on a flat plate, like a Champion tail feather.

Votex Generators on the wing would likely just change the speed and flap setting that it happens. Perhaps this would kick it into (or out of) your operating range. *shrug*
 
I have checked internal structure - OK. Bracing wires are tight. Hinges are OK. Elevator cables have tension. Tail plane feels stiff when mooviing up and down. When elevator is down wires for trim is loose, when elevator us up wires for trim is tight. Vibrations cannot be felt in stick. It is possible to induce the vibrations at lower g-s f.ex. in a level turn. Tried higher rpm 2500 instead of 2400, improoved a little. Same vibrations happend last year and I found that the trib tab had loose rives. these were replaced and the vibration stoppped. rivets are tigh now. Bolts connecting taiplane to fuselage replaced, old where shiny. Maybe replacing flat bracing wires wiht round will be my next step. Could this be related to the rimtab og maybe balancing of the elevator - if related to balancing, where do I put wheight?
 
That's odd. Perhaps someone else knows better but the tension on the trim cables should remain approximately constant and should be set between 15 and 25 pounds. The elevator cables are supposed to be between 45 and 55 pounds.

I wonder if you would see or feel any looseness in the tab if you got someone to put the stick into extreme forward and aft positions and move the trim control full up and down.

The elevetor is weighted by the factory in the leading edge outboard tube forward of the hinge line. Shouldn't need playing with and shouldn't be an issue unless some heavy patching has been done on the elevator.
 
Len,
There should not be any stiffness when the elevator is moved. Check the pullys for stiff bearings and the cables for condition. I had one case where the battery had broken it,s mount and was sitting on the elevator cables.
Because the trim pulleys are below the hinge line of the elevator, the trim cable tension will vary with the elevator position. The service manual will give you the proper proceedure for adjustment.
Paul
 
Streamline dragwires are funny that way. It might work....or just give them a little twist to change their angle of attack.

Weight wont help you at low speeds. Its purely for flutter prevention......which only happens at very high speeds.
Well, high speed for a Scout anyway. hehe.
 
LEN said:
Maybe replacing flat bracing wires wiht round will be my next step.
Click to expand...
Another comment on streamlined flying wires: they are prone to fatigue cracks and subsequent failures as a result of vibrating.
In addition to orientation, you said that they are "tight" - the right tightness?
 
I have now checked the tail in all detail. Internal structure with crack detection, replaced bracing wires with round, replaced trim tab with new, checked balancing of elevator and rudder, replaced all bearings, checked pretension of all cables. Everything is OK. Some improvement, i.e. less vibrations can be felt, however still vibrations. One theory now is that a vortex generated from the intersection between gear and fuselage and/or the intersection between the wingstruts and the fuselage (no speed fairings) may hit the tail at high angels of attack. To check this I will tuff the side of the airplane and fly with a chase airplane to see and take pictures. Speed fairings are planned, but no reply yet from ACA
Theory no. 2 is to see if this is a vibration generated by the engine, hitting a resonance in the tail section
So I will measure balancing of engine/propeller and after that try to measure stresses in the aft part of the fuselage with strain guages. Is it possible to put engine mounts with different stiffness?
Comments please
 
len;
I am probably way out on a limb here but, when I bought my Scout it was far from the way I wanted it rigged. In the many hours spent playing with washout, flap position, etc etc. I went through several areas that caused the tail to buffet at various angles of attack. I think you may be very surprised at how little you can change things like the strut length for a large effect on where the air flows.
 
LEN said:
To check this I will tuff the side of the airplane and fly with a chase airplane to see and take pictures. ..
Comments please
Click to expand...
Another good flow vis technique - tape a few paper streamers to trailing edge of wing and strut. They will show exactly where the wake goes to. Have the streamers quite long, just short of the tail. You'll see the results from inside the aeroplane. Only takes a minute to put the streamers on, lot less effort than tufting.

Edited to add: use thin crepe paper streamers about 1/2" wide.
 
You must log in or register to reply here.
Back
Top