Trim tab and Flutter

RickWoodall · #1 04-17-2011, 07:32 AM

Anyone know of any issues, accidents caused by trim tab flutter with any rv's? The reason I ask is that my inspector seemed to have an issue with it, and kept bringing up the fact that NO homebuilts should have trim tabs. He made it sound like there have been lots of failures. Never heard of such a thing.

Also..i have manual trim and there is about a 1/4 inch of slop in the cable. Vans say that is normal build on and that with a mechanical cable with ball and socket at each end there is no way to make it 100% tight, all is fine. Again, it seemed a sticking point with the inspector. He passed it, but...just let me know he doesnt like the design, slop or fact there is even a trim tab. This is beyond me, just wondered if those with more experience have any info on issues, failures etc. I have never heard of a single event with rvs in regards to trim issues. SEE ATTACHED VIDEO OF ISSUE.

http://www.youtube.com/watch?v=tCcQAwdmcV4

drycreek · #2 04-17-2011, 07:57 AM

Yeah, I see RV's crashing all the time because of the trim tab.

Personally I think there would probably be more accidence if they didn't have trim tabs. But then again, I'm no expert.

Ironflight · #3 04-17-2011, 08:08 AM

It sounds to me like you got an old fashioned inspector who has a completely closed mind and is unaware that aviation has progressed since the Wright's first beach flyer. We all run in to those types innovation now and again - they can't point you to any data, but sure have lots of opinions!

As long as he signed the plane off, you're done with him. RV's are a well proven design - put your mind at ease.

Paul

jcaplins · #4 04-17-2011, 08:45 AM

Quote:

Originally Posted by RickWoodall

NO homebuilts should have trim tabs.

What makes a homebuilt trim tab so different than a certified trim tab?

Congrats on the signoff.

Sid Lambert · #5 04-17-2011, 10:07 AM

I had a run-a-way down electric elevator trim last weekend. I can say that if the RV-4 didn't have trim I would never get in it. I think I still have arm pump from holding the stick back. With both hands.

RVG8tor · #6 04-17-2011, 11:18 AM

Curious what caused your runaway trim. Is it something internal to the trim motor that causes this.

I have not flown but I have the Safety Trim module, it simplified the wiring (no relays) and it has the added features of the ability to reverse the runaway trim to get it where you want it then cut power to the servo. Also you get slower trim motor at higher speeds. Oh and it has a 3 second cut out feature, it will not let a motor run for more that that time. I think it would be an easy retro fit. Oh I don't work for them just think they make a nice product

Quote:

Originally Posted by Sid Lambert

I had a run-a-way down electric elevator trim last weekend. I can say that if the RV-4 didn't have trim I would never get in it. I think I still have arm pump from holding the stick back. With both hands.

Sid Lambert · #7 04-17-2011, 11:53 AM

The 13 year old, 900 hour Ray Allen switch went bad. I managed to get it back to a basically neutral position before I attempted to land. The new switch from them is deeper but seems to be better built. (I had to disassemble & rewire one for use with flaps)

All that run-a-way trim protection stuff seems cool but I don't think I'll retrofit my -4. I would say that you need to fly the airplane during phase 1 with the trim at the stops for up and down. See if you can maintain alt at around 80 MPH.

Flyfalcons · #8 04-17-2011, 12:04 PM

My plane with electric trim will get readily identifiable (most likely colored), pullable circuit breakers within sight and reach of the pilot.

JoeLofton · #9 04-17-2011, 12:32 PM

I'll share an experience with my RV-3 that may or may not be related to the trim tab. Cruising in smooth air at 9500ft, WOT turning about 2600rpm I throttled back slightly and pointed the nose down a bit to start a descent for fuel. Shortly afterward I felt a short, distinct, high-frequency ?buzz? through the airframe. Only lasted a second or less, but sure got my attention. Eased the nose up and throttled back, then finished the descent at considerably lower airspeed. On the ground, no obvious signs of damage, so I refueled and finished the trip.

A data dump from the 496 shows a max ground speed of 220mph at the "buzz" point (two consecutive points with a 15-sec sample rate). VNE is 210mph TAS, and I had an estimated tailwind of 20-25mph. Could be that I exceeded the envelope in that 15-sec interval. (Note: the plane was flutter-tested in smooth air at 10,000ft up to 230mph IAS when built back in 1982).

I have a retrofit electric elevator trim with the servo mounted on the deck below the vertical stabilizer. While checking the airframe I noticed about 3/32" play in the elevator trim tab, measured at the outboard trailing edge. Also noticed some flexing of the deck that contributed to the play. On prior flights in smooth air I had experienced at random times an uncommanded nose-up bump in trim. Speculating that the flex was contributing to the trim bump, and that the trim tab play may have contributed to the "buzz", I stiffened the servo mount with some angle. Now the tab play is about 1/32".

Was this a trim tab flutter issue? Don't know, and I'm not inclined to go back to the envelope edge to test it. In the succeeding 15hrs of x/c flight in smooth air the "bump" hasn't returned. I don't normally cruise at WOT, but on that day I was trading fuel for time so I could land in the daylight. Be cautious at the upper end of the speed envelope ? these RVs pick up speed quickly when pointed downhill. And my understanding is that flutter is a function of TAS, not IAS.

RickWoodall · #10 04-17-2011, 12:33 PM

To be clear I dont believe there is any issue, just with the inspector bringing it up, made me think a bit and wondered if there is a second chapter to the story I need to know??
I do know a local rv9a that tried to take off one time with trim set at limit and he really had to fight it to get airborn and it was a scare. He could not believe the impact that little panel has. Anyway, the comments about trim on homebuilts was aimed at the weakness of the linkages etc I believe. He commented that if the trim had springs on it, kind of like the manual aileron trim on rv's (springs in both directions) it would be ok, but he wasnt impressed with the engineering as is. Anyway, no issue just figured if there was something to this that I have missed over 4 years of reading the forums daily, I would like to know.