Tail Vibration

[KOZ]

I was wondering if anyone has had any problems or noticed how much the horizontal stab moves on the 180Hp PA-18? I have 3 180Hp New Experimental Supercubs here and at a high angle of attack and low airspeed, It is frightening to watch the tail! Each planes tail is an exact copy of the Piper design and built very well. My 150Hp Pa-12 and a friends 150hp Pa-18 move also but not like the big motor/prop combo. Has there ever been an accident due to tail failure on a 18?

 

[Steve Pierce]

Charlie Centers has the solution. Have 180 hp PA12 banner tow plane in the hanger. We just rebuilt the tail and I think he wants to do the Crosswind STOL mod.

http://www.crosswindsstol.com/_borders/Tail_Strut.gif

 

[StewartB]

The tail mod is one of Charly's favorite subjects. I believe it came into being as a result of a tail failure in one of his dad's planes (Cal Center).

SB

 

[cubscout]

I quit lookin' back at 180 Supercub tailfeathers a long time ago: I got scared.

But preflight the tail REAL hard, looking especially at the bends on the flying wire fittings, I've found some cracks there. Also check for split-pins ("cotter" pins) on the rudder pins. There are only two hinge-pins on a Supercub, and the loss of one could spoil your whole day. Yup, found that one on preflights, too.

At 100 hour/annual inspections, the IA needs to look REAL hard at this stuff.

Just my limited experience.

Cubscout

 

[KOZ]

I like the look of Charlies solution. I think I'll try it on our next one.

Koz

 

[Henry Baker]

Hello guys
I,m at the tail section now and wonder if anyone has the plans or at least some pitures of how to do that?

 

[Steve Pierce]

Call Charlie.

http://www.crosswindsstol.com/prod05.htm

 

[Bill Rusk]

Koz

In the Hatz community (plans built 2 place experimental biplane) we saw similar results when the engine was upgraded from the design, Cont 0-200 to the Lyc 0-320. The stronger air pulses from the larger engine and prop combo cause significant vibration especially at high power high AOA settings. Most have gone to double brace wires to solve the problem. No tail failures but like the Piper Tramahawk it scares you to look back and see that thing flopping around.

Bill

 

[don d]

I have a 200hp on my experimental cub. Prop wash was flogging the devil outh of the horizontal stab on takeoff. Developed cracks where the last rib attached to the stab spar. Went to a Husky type trim system and added a strut to the front of the stab. Not as efficient trim system, but the tail is staying on.

Don

 

[gcgilpin]

Does anyone see vibration on the left lift struts when at high AOA / high RPM?

I'm thinking this is due to the prop wash drumming on the underside of the left wing.

How about loose fabric / loose drag wire?

 

[bob turner]

What happens on a Cub when a tail brace wire fails? Do we die?

 

[smitty172]

Hey Koz

Congrats to our newest A&P...

nice to see you on this site again..

 

[jimboflying]

Bob,

We had a local Cub break an upper tail wire in flight. He was able to fly home carefully and landed with no problem.

 

[bob turner]

Thanks, Jimbo - that is about the fourth time I asked, and I believe I had one other response. I suppose if it were deadly, there would be an AD - the main strut, I think, has only let go about four times. That is usually catastrophic.

 

[N929DW]

What happens on a Cub when a tail brace wire fails? Do we die?

It is an attention getter. I lost a lower brace wire once while operating in deep snow with and icy crust and bone jarring turbulence. The clip failed during or shortly after take off. The horizontal stabilizer will jump around impressively but gingerly getting back on the ground worked OK. One test flight is hardly definitive. It will forever change your pre-flight habits and ground ops in deep snow.
Bryan

 

[mvivion]

One of the reasons for the "strake" in front of the tail on the BLR Vortex Generator kit is to help smooth the airflow over the horizontal tail. The vibration is particularly noticeable when flaps are deployed. The strake helps, for sure.

This is also why the Husky has that big tail brace strut attached to the leading edge, which is pretty much what Center has done.

MTV

 

[gcgilpin]

Guys:
What about vibration on the main lift struts for the left wing? Anyone see anything in this area?

 

[Dave Calkins]

gilpin, I've never seen vibration on the lift struts.

As to the tail vibration, yep, they vibrate.

I heard the Crosswind Stol tailbrace came about after Cal had a stab problem while ferrying a wreck out that had a broken jackscrew that he discovered after taking flight. Whether it's necessary or not, it firms up the tail.

Losing an elevator hinge pin will not kill you, even if you lose the outboard one. In cruise the elevator will move around alot, but you'll know it, and when you slow down it calms down and stays put.

Mike, those 'THINGS' in front of the stab on a BLR VG-equipped Cub are there for something else.

 

[bob turner]

Handles?

 

[T.J.]

I always thought they were to scrape the moose crap off of your boots before getting in the plane? Was I wrong?

 

[mvivion]

Dave,

According to the original designer of that kit, those "things" are specifically designed to reorganize the airflow over the horizontal tail, particularly when that airflow is disturbed by flap deflection.

MTV

 

[aviationinfo]

Gilpin:

I'm not sure I understand---are you asking should we see vibration similar to the way the tail vibrates, on anything to do with a wing?

I hope to never see that!

Andrew

 

[Frank T]

FWIW...Most vibration "Fixes" just transmit the problem to a different area.

 

[skywagon8a]

According to the original designer of that kit, those "things" are specifically designed to reorganize the airflow over the horizontal tail, particularly when that airflow is disturbed by flap deflection.

MTV

So, if those "things" reduce vibration of the stab when the flaps are deflected, why would they not be a good idea even on Cubs without VGs? Has anyone experimented with this? different shapes? different sizes? Perhaps this tail vibration was the cause of the AD for reinforcing the lower tail wire attachment?

Has anyone done any tuft testing in this area with photographic documentation?

 

[gcgilpin]

FWIW...Most vibration "Fixes" just transmit the problem to a different area.

Just trying to find out if anyone has seen vibration on a lift strut (particularly the left side) that turned out to be due to prop-wash interacting with loose fabric or a structural issue with the wing.

 

[skywagon8a]

gcgilpin

I assume by your question that you are experiencing a vibration in one or both of your left struts? Vibrations are difficult to explain sometimes and I do not claim to be an expert in this topic. The cause of the vibration could be something so totally unrelated to what you might suspect, that you would be very surprised.

Years ago I witnessed some vibration testing on a Twin Comanche at Lock Haven. The airplane was taken to altitude and an attempt to dive it to VD (VNE x 110%) was done. It could not get close to VD or even VNE without showing signs of flutter in the tail. A ground flutter test was then accomplished which also showed a limitation. To make a long story short, It was discovered that this particular Twin Comanche had an oxygen bottle clamped in the aft fuselage which changed the natural vibration frequency of this airplane. They removed the oxygen bottle and the airplane passed the test with flying colors.

The above having been said, your plane could have something unusual that has no relation to the struts. Does it do this when you have an unusual cargo on board? Do you have an unusual engine/propeller combination? Do you have a loose or broken drag/anti-drag wire in the wing? Is it in the other wing? I could come up with a lot of this type of question. You need to ask yourself this type of question and do a through inspection of your plane. If you do find something please report back here so that we know too.

Good luck

 

[gcgilpin]

I suspect two things, fabric that was slightly loose, and a drag wire that was loose.

Both have been corrected. Will post results after the ice goes out.

I'm trying to find out if anyone else has experienced a similar problem / resolution.

 

[bob turner]

This should be so obvious - but I have seen it. Those struts should be straight. A mis-alignment of the jury strut clamps can make them "S" shaped. Might not be a cause of vibration, but something to check on all Cubs.

 

[High Country]

I have seen jury struts installed backwards on a cub before. The owner said it flew fine that way, but I didn't like it.