Trim Tab & Boom Bounce

[hharney]

Does the trim tab play cause boom bounce? Some have indicated that it does. I have experienced this phenomena in the years past and really wasn't sure I was really feeling the boom bounce back then. I remember at least 5 or 6 years ago that I had these symptoms and could actually look back and see the boom moving. Then I saw others talk and post messages about it. So it's a real thing? We talked about it in Dayton and several other members said that they had experienced it. Loose cable tension, elevator play, trim tab play and other theories were circulated around the room. Some thought that the adjustments they did worked and others were still skeptical. Here is my dilemma.

I had not felt or seen boom bounce for 6 or so years. Then just 2 weeks ago it was rearing it's ugly head on a flight at 9,000 ft, cloudy, wet day in the air and relativity smooth air except in the clouds. I found that it was more pronounced than I remember. I could look back at the tail and see the booms moving up and down. I know what you are thinking, yes I have that DANGEROUSLY MODIFIED AIRCRAFT. Well, this boom bounce was around before I made the last modifications that caused the Cessna Company to write the SID's, or some think that this is the case. I could actually make it stop by moving the control yoke fore and aft. I could also change the RPM on the rear engine and cause it to change or stop.

Anyway, back to the point, we looked at the trim tab and it does indeed have some play. Maybe around 1/4 to 3/8 inch. Too much in my estimation. But the other attention getter was the play in the elevator. It was again like 1/4 to 3/8 inch of play when one would move the elevator without allowing the cables to move. We found the play in the rod end fork form the cable that attached to the bearing bracket on the elevator. The bolt was slightly worn and semi loose on the bearing. The book says to torque 10 to 40 which leaves a lot of area for question. We tightened the bolt on the bearing to the higher book value and the play was gone. All of it, no play at all. The next two flights, one 8 hours in the air and the other 5.5 hours in the air resulted in no boom bounce or elevator flutter. But this has happened before, as I said above it was a time of like 6 years between a noticeable incident. Well the trim tab still has the play and I thought it would be good to fix one area first to see if that stopped the problem or not but I don't really want to wait 6 years before I fix the trim tab. So maybe a few more flights with a good sense of detection and then fix the trim tab.

I thought I would just do some investigating on the position of the trim tab while in flight. I marked the trim wheel so that I know where the tab is perfect in line with the elevator. Interestingly enough the tab does not stay even with the elevator. If you already know this sorry to bore you with my findings. The photos below indicate the position of the trim tab when the aircraft is in perfect level flight with one and two aboard in the front seats. Some luggage in the rear and in the middle seats but no big load. I just find it interesting because I had never done this before and thought you might like to see this. It would be good to have others try this too and see if it is different with different models. I know that Cessna changed the size of the tab later in the G and H models.

[billsheila]

Herb;
First let me say I don't think your "Dangerously Modified Aircraft" has anything to do with this, it seems far too common a problem. You will have to wait to find a Herb unique problem to blame on your lovely looking mods.

My mechanic believes the root cause of this is both in the elevator trim tab play and in the elevator bell-crank play. Both have a series of hardware (bushings, bolts etc) that are relatively cheap and easy to replace and he claims keep these fresh and you shouldn't have any problems. In my case the trim tab hardware was all replaced by the prior owner and my mechanic replaced all the elevator bell-crank stuff a couple of annuals ago. Again back to his theory, he claims that the elevator is subjected to an inordinate amount of "prop pulse" given how close the rear prop is to the elevator. As I recall it, the hardware in the bell crank was indeed all pretty cheap except for one long bushing. I also recall that the tolerance for play in the bell crank was really tight...their was a series of thin washers that you buy from Cessna as well and then install as few or as many as necessary to get within spec on the bellcrank play. The expensive bushing could have been fabricated I guess but I ended up shelling out to Cessna for it...like $170 as I recall for the one key long bushing.

Hope that helps.
Bill

[edasmus]

Hi Herb,

I have made this comment before somewhere in an earlier post. When I first purchased my G model, I had the boom bounce on a fairly regular basis. Getting the cable tensions to spec seems to have solved the problem for me. Have never had it again. (Tomorrow, I'll have it, thanks! ;o) )

Anyway, I would tend to agree with last post as well. I would think that anything that can allow elevator movement beyond what is commanded by the pilot, would cause the issue. I did have my trim tab hardware replaced last annual because it seemed to me it had excessive play. I was not experiencing any problem but my IA agreed the play was too much. I cannot recall at the moment what was all replaced but my records would tell the story.

Anyway, I would agree that anything that keeps flight controls in spec would go a long way to getting factory intended flying qualities. It would seem to me that flight controls out of spec in any manner could cause the aerodynamic gremlins such as the boom bounce many of us have experienced.

Ed

[hharney]

Ed

If you could post what was repaired and/or replaced on the trim tab mechanism that would be a great help.

[edasmus]

Herb,

I apologize but unfortunately I do not have much for you to go on. I checked both the work order and log books and both entries say the same thing. "Trim actuator hardware loose. Secured all trim actuator hardware." Total labor 0.3 hours. The records indicate that nothing was replaced but simply secured.

Sorry, I wish I could be more helpful.

Ed

[hharney]

No worries Ed
I looked into some parts and they are expensive. I will try to rebuild without having to buy new if possible.

[Paul462]

The service manual may be helplul, e.g. the 1965 THRU 1973 SERVICE MANUAL Sect. 8-3 ELEVATOR - TROUBLE SHOOTING table pg. 8-3 has two suggestions to address "SLIGHT UNDULATION OF TAIL DURING FLIGHT".

[travis]

I solved my boom bounce by adding a bit more torque to the elevator hinge bolts. I had to go beyond the recommended value. My IA didn't want the compression on the bearings but in the end the smooth stable flight characteristics were persuasive. Check for even the slightest play and eliminate it. If you can't find anything try a bit more torque on those hinge bolts a bit at a time.

Best of luck

_travis

[SkyMac]

Come on guys this site is about helping us who own skymasters and love to fly, doing just that keeping them flying.

This forum and the constructive helpful advice it provides from many knowledegable owners is invaluable, and I like many other wish to continue to tap into this wealth of information.

But I must say the tone has some-what changed in recent days.


Regards
David

[edasmus]

Thank you SkyMac. I agree with your post.

Look guys, how about we agree to disagree on some points. The bickering is silly. My SkyMaster is as it was when it left the factory. I fly on sunny days and by most standards, people would consider my flying boring. It works for me however. If other people are more creative and adventurous, then so be it. Nothing wrong with that. These airplanes were designed by people, built by people, flown by people, and maintained by people. All the STC's were designed, built, flown, and maintained, by you guessed it, people. We all know what it means when people are involved. Perfection ain't gonna happen.

How about we fly our SkyMasters while using good judgement and common sense, report our experiences so others can benefit. Bickering is counter productive. It is the last thing GA needs.

Fly safe,

Ed

[rick bell]

if my memory serves me correctly about five years ago my door strut cylinder broke of at the attachment tip. i had a machine show make an identical copy of it in stainless steel and it has work perfect ever sense. seems that you asked me to get you one made up as yours broke
(not approved!). i took the time to have the shop make you one and mailed it to you. the
shop charges 125.00 for it, i paid them and ate the postage to you. you never paid me for it.
it may behoove you to be a little more charitable and a little hypocritical and just enjoy the site.
oh if you wish you can send me the cash to my mail box that will forward it to me in mx.
pmb 1401, p.o. box 9011, calexico ca 92232, if not whatever - rick

[Paul462]

SK: The message of your first post appeared to be that, because not Cessna design, no Skymaster STC should be installed. The message of your second post appears to change into: the set of STCs is divided into two subsets: the subset of bad STCs (e.g. windshield center post removal) and the smaller subset of good STCs (e.g. Robertson STOL because well-engineered). In your opinion, who is the decider between the two?

[tropical]

Skyking,

You operate a P337, yet you cannot afford a $125 part????