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  #31  
Old 04-21-2011, 03:13 PM
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From a guy who has experienced, and lived through in-flight wing flutter, I'm very comfortable with my 18 year old RV manual trim.
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  #32  
Old 04-21-2011, 08:58 PM
Gerald Loyd Gerald Loyd is offline
 
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Location: Dumas, Arkansas
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Default Frightening Trim Flutter Test

I have watched this thread with great interest and can't stand it anymore. I have to tell my story. The FAA Inspector that signed off my RV-4 in 1991 was concerned ablout the amount of play in the elevator trim tab. He concluded that there was nothing I could do to tighten it up, so he signed it off, but asked me to test it for flutter when I get comfortable with the plane. He said to take it up and dive it to 10% above the red line in 5 mph incerments. About 10 hours into the test flight period, I decided it was time. I took it up to 8,500 ft and pushed over into a dive, throttling back just enough to keep the RPM under 2,700. I pushed it over to 205, then 210, and 215 indicated. I wanted to dive it to 225, but as I pushed over some more it wouldn't go past 215. I pushed harder and got very steep with a lot of wind noise, but I couldn't get more than 215. Then all of a sudden the needle seemed to come unstuck and jumped somewhere over 250. The heart rate went off the scale at this point. I closed the throttle and released the forward pressure instantly and held my breath. The nose came up pretty quick and the RV slowed down without any problem. I have no idea what my true airspeed was, but I feel very sure you guys don't have to worry about trim tab flutter. Thinking back, I believe the inspector should have realized that there were a lot of RVs flying around with the same trim system with no problems and he shouldn't have asked me to test it for flutter.
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  #33  
Old 04-22-2011, 06:38 AM
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KevinL KevinL is offline
 
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Location: Lee's Summit, MO
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A year ago I was instructing in a composite LSA when the trim tab linkage broke. The trim tab flopped from stop to stop, even when I slowed to 40 kts. Was able to get it on the ground before the elevator separated completely (it was hanging on by one attach point).

Way different plane than my RV, but the trim tab looks exactly the same. Every time I preflight the RV I look at the tab and wish is was not installed. In that sense I agree with the inspector.
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  #34  
Old 04-22-2011, 06:44 AM
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Testing for flutter is up to the individual.
Having experienced flutter in my Moni Motorglider that partially "un-skinned" the right wing and tore ribs loose from the rear spar, I did test my -6 up to 226 mph IAS.
I found absolutely no indication of flutter.
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  #35  
Old 04-22-2011, 08:39 AM
Maxrate Maxrate is offline
 
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Location: League city, TX
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All the Cessna singles that I used to instruct in had a small amount of play in the trim tabs as I recall. Never had a problem with flutter even in the 150 aerobat in diving maneuvers at high airspeeds.
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  #36  
Old 04-25-2011, 08:08 PM
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vfrazier vfrazier is offline
 
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Just a bit of data here, draw your own conclusions: there are a large number (hundreds) of Rockets flying with RV tails on them. Many have modified trim tabs on them, some considerably larger than stock tabs.

It's safe to say that virtually all of the Rockets are flown out to speeds that few RVs can reach.

There may be Rockets with RV tails out there that have fluttered. I've never heard of one.

My Rocket has a mostly stock RV-4 tail and it's been out to 250+ many times and past redline (275mph) more then once or twice.

Does this prove anything? No, but it does make me feel pretty good that the system works well as long as nothing breaks.

YMMV
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  #37  
Old 05-04-2012, 07:20 PM
jdiehl jdiehl is offline
 
Join Date: Jan 2011
Location: Williamsport, Pa
Posts: 175
Default elevator trim wiggle

I've read every elevator 'trim tab' wiggle thread in the search section. Guys, my FSDO inspector is adament that the 1/4" of wiggle is unacceptable. I placed a small rubber washer on the nipple that drops into the recess (at the control knob), hoping that it may take up some slack. This had no material impact on the wiggle. I have the cable attached to an adel clamp on the flat tail panel (where the cable exits the aft fuselage and goes into the horiz stab). I was thinking tht I could severely tighten this adel clamp and put some pressure on the cable, thus placing some friction on the inside push-pull cable. At this point, I'm not looking to bash the inspector, but rather find a solution to minimize the wiggle. Somewhere in the past, Smokey posted a thread stating that he set some adel clamps, etc., and ended up with nearly no wiggle. That's what I'm looking for.

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  #38  
Old 05-04-2012, 07:53 PM
alpinelakespilot2000 alpinelakespilot2000 is offline
 
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I tried Smokey's method, Jim, but no dice. Mattered not a bit. I have an email from Van's saying it is normal, impossible to get out, and a non-issue. You might try getting the same from Van's and give a copy to your inspector. Good luck.
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  #39  
Old 05-04-2012, 08:16 PM
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rvbuilder2002 rvbuilder2002 is offline
 
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Jim, the majority of the play in the trim tab is the result of clearance between the cable and the cable housing.
If you stretched out the control cable in a straight line with no bends or curves, there would be just about zero play.
As installed in side by side RV's, the cable makes about a 150 degree reversal in direction at the knob end. This causes the clearance between the cable and the housing to become a longitudinal play in the cable that can bee seen at the trim tab. Play at the trim tab was not nearly as evident in the original RV-3 and 4 installations because the cable was installed in a straight line (no curved section for the cable to move around in).

It may look bad to someone who is unfamiliar, but in thousands of flying RV's it has proven to be trouble free. Years ago, when a GVT analysis was completed on the original RV-8A prototype (manual trim with a reversal in the direction of the cable), the test technicians had some seriously raised eyebrows during their initial inspection of the airplane when they saw the amount of play in the elevator trim tab. At the completion of the testing they were surprised that the test data showed that the airplane was more than satisfactory for the design VNE.

Unfortunately you are stuck satisfying your inspector at this point (unless you want to jump through the hoops of having a denial and then reapplying with someone else).

One thing a few builders have done (It is not necessary but it makes some people <inspectors included> feel better), is to install a biasing spring between the trim tab and the elevator. The spring will keep the tab always pulled to one side of the play range. It is important to use a spring strong enough that air loads will not be likely to ever move it against the spring pressure or you could actually be inducing a flutter potential that wasn't there to start with. I have seen springs attached directly to the elevator and trim tab, and I have seen springs installed over the push/pull rod of the control cable so that the compression of the spring was acting between the horn on the trim tab and the anchor point of the end of the cable.

Hopefully this might be of some help, but as I already mentioned, use caution caution in choosing a spring if you go this route. Make the force too light and you could induce a problem, make it too heavy and over time it could cause damage to the trim tab.
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  #40  
Old 05-05-2012, 07:04 AM
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Walt Walt is offline
 
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Using FAA AC 43.13 for limits:

"If the tab span does not exceed 35 percent of the span of the supporting control surface, the total free play at the tab trailing edge should not exceed 2 percent of the tab chord."

For an RV with approximately (from memory) a 5" tab, that would give you 0.100" free play. An 1/8 is .125"
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