R/C Lee

My Feedback: (3)

Yesterday, on the third flight of my MAN plans-built Sukhoi I managed to lose both stab/elevator halves in flight. Needless to say, it was exciting. I had finally balanced and trimmed the airplane and it was flying beautifully. I did a split S, followed by a long pullout, built up some speed and the left stab just went POP! I thought I had it somewhat in control, then the right stab did the same thing. Having been a long-time builder, I know that it was built right, having used carefully selected 1/4" framing covered with quality 3/32 sheet. Since the plans didn't mention stab bracing, I didn't add any. Big mistake.

It was a large airplane and I had an OS 120 4-stroke up front. After looking over the remains, I noticed that both JR elevator servos had stripped gears. Since the fuselage was intact, I'm sure that the servo gears stripped in mid-air. It reminds me of a time when I was flying a friend's Goldberg Sukhoi and under the same conditions it developed so much flutter that the elevator servo gears stripped resulting in a crash. My thoughts are that there is a lot of turbulence built up over the tail of the Sukhoi and the airplane needs bracing to avoid the problem. So, if you know anyone who is building/flying a large Sukhoi, tell them to add some tail bracing if they haven't already done it. It may save their airplane.

Lee

jrf

My Feedback: (551)

Based on the servo gears stripping, it sounds like what you experienced was elevator flutter, not stab flutter. Elevator flutter will tear the stab off in the blink of an eye. Tail bracing isn't going to solve an elevator flutter problem. You need to look at your linkage setup and geometry. Was there any slop in the linkages or the servos? Did you have the pushrods connected as far out on the control horns as possible? And as far in on the servo arms as possible? Was your ATV at 100% or higher? Did you seal the hinge gaps?

Jim

R/C Lee

My Feedback: (3)

Jim, thanks for your comments and it could very well be that it was elevator flutter. I didn't hear any flutter, but maybe it all happened in that microsecond when the horizontal stab sides came apart. There was a delay between the left side separating and the right side separating of about 4 seconds during which I chopped the throttle and the airplane slowed down some. I understand linkage and geometry, there was no slop, pushrod/horn attach points proper, Sullivan steel horns, ATV at 100%, gaps all sealed. As I mentioned, since this also happened on a friend's Goldberg Sukhoi under the same conditions, a different design but still a Sukhoi, I believe that there is a lot of turbulence flowing across the stab/el. In any event, I won't be building another Sukhoi [>:]. Thanks again, Lee

mnrcaerobat

My Feedback: (197)

I had the same thing happen on my old super sportster. I ended up putting some diagonal .078 music wire on it and never had the problem again. It wasn't pretty, but worked well.

I would look at:

http://www2.towerhobbies.com/cgi-bin...&I=LXFV30&P=ML

The Sig somethin Extra also came with tail bracing. Take a look:

http://www2.towerhobbies.com/cgi-bin...?&I=LXFG18&P=7

Another thing I did to stop the flutter in my elevators was to mount the servos in the tail and used a really short linkage, thus eliminating any slop in my connection. I'll see if I can dig up a picture of it.

mnrcaerobat

My Feedback: (197)

Here is a pic. As you can see it wasn't a pretty plane, but flew great. It looks like on the the flying wires is missing in the pic and you can also see the rear mounted servo if you look close. I also had to rebuild both horiz stabs as they sheared off near the fuse. I was able to use one of the original elevators. It is hard to believe, but I landed the plane with both elevators flopping.

dbacque

My Feedback: (3)

Actually the stab on the Sukhoi is considerably higher than the wing putting it in clean air. It probably was not turbulance.

I lost a Goldberg Extra the same way. On mine I'm sure it was the Goldberg split pushrod that was the weak point. Never again. I'll always use separate tail mounted servos for each surface from now on.

MinnFlyer

My Feedback: (4)

It could also be that the servo gears got stripped when the elevators got ripped from the plane. Not before

jrf

My Feedback: (551)

This also happened to me on a Phoenix 1.20 size Suchoi, but in my case it was because the pin pulled out of a nylon clevis. Fortunately, I was able to land with only one stab/elevator. I replaced the nylon clevises with metal and had no further problems.

Jim