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RV8R999

Well Known Member
Since I started flying my -8 last August after the inspection I always thought the low speed trim stability was a little week. So after reading so many posts about the radius on all those parts we had to bend ourselves I decided to check my elevators and trim tabs. Yep - they had excessive "bow". Even though roll control was very light and stable throughout the entire envelope I discovered even my ailerons were excessively bowed as well.

Soooo....I used my hand seamer with a piece of felt and squeezed all the radius surfaces.

Pitch trim effectiveness was certainly improved through the entire envelope and the airplane actually flew a bit faster (a few MPH). However, roll control forces were much greater - at least double beyond about 2" of stick deflection. The aileron radius after I squeezed is right on target and the airplane flies level stick free (no heavy wing). At slow speed the plane roll forces are light and nimble but they we this way at higher speeds prior to the squeeze event.

Not sure the remedy here...looking for others advice. Sometimes leaving things alone is the better choice...grrrrrr

Thanks,

Ken
 
Remedy?

Doesnt sound like you meed one. Theyre supposed to stiifen up at higher speeds. Id try to fly with someone else before you try to "fix" it.
 
I've flown a bunch of airplanes with reversible flight control systems and can tell you the control forces at even moderate speeds (>150) are way too high. More importantly I seem to have lost the control harmony. Roll force is decidedly non-linear and more likely even exponential now. I'm amazed because the ailerons were not bowed very much at all so the delta is very small.

Has anyone with an -8 actually measured roll control forces as a function of airspeed?
 
CAFE

I'm pretty sure the CAFE guys flew and reported on the Vans demonstrator. I'll bet you can google up the report. Its sure to have roll force plots. Good Luck.
 
stick force

some of the discussions on squeezing an aileron trailing edge to assist with a heavy wing have mentioned that one result is a substantial increase in stick force -see for example http://www.vansairforce.com/community/printthread.php?t=30979
I wonder what Van's would suggest? Is a radius squeezed to take out all the bow over-squeezed? I have read that some people have tried to "unsqueeze" the aileron by using a hammer and a piece of wood against the trailing edge - doesn't sound likely to do much, though.
Bill brooks
Ottawa, Canada
RV-6A finishing kit
 
tape

i seem to remember a post by mike stewart a while back that might help. If i remember correctly he put a layer of tape down the trailing edge of the aileron and it lightened the forces a bunch. Maybe Mike will see this and comment.

You might be able to experiment with this (a little at a time).

I would have a bit of concern with the tape possibly coming loose though.
 
A... Unique Fix idea

I fly Extra's quite often and have the new 330LX model on order. Every Extra I have flown has slightly different roll characteristics due to the variability in composite construction (and the TREMENDOUS roll authority in the Extras, small variations can be VERY noticeable). What we do to change them is add a piece of weather stripping to the lower trailing edge of the aileron. I have seen some airplanes with 2 or 3 feet of WX strip and some with almost none. With this technique, you can balance the ailerons exactly how you like. It takes about 5 flights of trial and error to get the ailerons in balance the way you like it. Each person is different in the Extra and like them balanced differently based on their flying technique. Note that when I talk about control balance, control force is directly correlated. Let me know if it works, on the RV-8 if my Aerodynamic thinking is correct, it should work quite well!
 
Thanks Forster - can you elaborate on what you define as Wx strip? Many sizes and shapes exist in the aviation aisle of the Home Depot.
 
RV8R999 said:
Thanks Forster - can you elaborate on what you define as Wx strip? Many sizes and shapes exist in the aviation aisle of the Home Depot.
Click to expand...

There is a specific cross section everyone seems to use, I have a roll of it out in the hangar. I will check the specifics on it tomorrow morning and update as necessary!
 
Ken, I don't recall exact details (racing around the hangar one afternoon months ago), but I'm pretty sure I used a wood block and a 2 lb hammer to bump one of my trailing edges toward fatness, ie reverse a little too much squeeze. The block was a strip of 3/4" MDO board about a foot long and 2" wide with a few stacked strips of masking tape on one face as a cushion for the paint.

Like Foster said, the "best" feel probably varies with pilot preference. I squeezed until my ailerons are not twitchy near neutral, yet every pilot-passenger comments on how wonderfully light the forces are at normal cruise speeds. They do stiffen up by 200 knots and I think they should for two reasons.....all pilot input should be more deliberate at VNE+, and more hand pressure is a built-in flutter damper.
 
Weather Stripping

The weather stripping I was talking about is D profile, 23/64 " width and 5/16" height. Works great and should accomplish what you are looking for without any modification to the trailing edge. Please let me know if it works!
 
sledge hammer

Tried the wood block and hammer technique this weekend. It does work but will be an iterative process as I'm not inclined to BANG to hard. Was thinking of using my rivet gun against my block of wood set very low to provide consistency. I have a couple of similar sample I'll try first for practice.

I don't want to add anything to the trailing edge if I can help it to prevent moving the aileron CG aft resulting in less flutter margin.
 
Problem solved

revisiting my over-squeezing issue again...

I didn't like the idea of using a hammer and block of wood to bump out my over squeezed ailerons which resulted in significantly increased lateral stick force gradient - didn't like it at all.

at this point my ailerons were very flat, no bow, no convex.

I placed 6 layers of 100mph tape on the flat side of my flush rivet head so as to prevent marring the paint and turned the rivet gun air down to a very low, low setting.

I held the head perpendicular to the trailing edge and slid along the aileron while gently pounding some shape back in. I made two quick passes per aileron and used a straight edge to measure the effect. I now have a uniform bow across the span of both ailerons that measures barely over 1/32" from the over-squeezed state.

Flew the plane. It flies balanced and the forces are nice and light just how I want them :) Truly amazing how such a small change can have such a noticeable effect in handling qualities. Please keep this in mind when making aerodynamic modifications of any kind...

I left the elevator and rudder trailing edges squeezed as I like the increased longitudinal stick force gradient and the increased maneuver stability.
 
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