Bungee system help
 
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Bungee system help

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Dennis Crenshaw
Reputable Member

@moparmolnargmail-com  Dave, with the tailwheel sitting on the ground with weight on the wheel, there should be a little slack in the chain/spring linkage.  It should not be tight.  I have a dash 3 so the geometry is different than for the small tail airplanes.  On my airplane, I have the spring and 5 chain links.  On small tail airplanes, I believe it's more like 3 chain links.  At any rate, there should be a little slack back there.  But I digress, this doesn't really address your yaw issue.

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Posted : 19/05/2021 1:37 am
Dave Molnar
Eminent Member

Some more diagnosis.  Today I flew it for an hour through all phases of flight and here’s what I found.  Straight and level it appears that the ball is almost full deflection to the right, which would indicate excessive left rudder input.  Using the bungee trim did nothing to correct this.  After landing, I had the mechanic loosen the cables that connect the trim system (turnbuckle connection)  to see if taking that out of the equation would help. It did nothing.  Also, before loosening it we discovered that the cable on the left side of the plane was extremely loose and the one on huge right was way tight. He tightened both, back in the air we go, abs that didn’t do squat except render the trim basically inoperative too.  The bendable trim tab was slightly bent to the right if looking at the tail from behind the plane.  This would make sense since aerodynamically it would push the rudder to the left resulting in the ball going right.  Holding constant pressure on the right rudder corrected the ball issue and it would stay there as long as I held constant right rudder in.  Also, I had to use right aileron too. A neutral yoke would roll the plane left. This was not a fuel balance problem and even adding the correct rudder still resulted in a right roll needed. 

Stalling the plane resulted in a nice gentle nose drop, rather mushy, but straight ahead with no wing drop.  So with these indications, what do you guys suggest?  

So what’s next?  Next week I’ll head up again with rudder the rudder trim tab bent to the Left which should correct the extreme left rudder input since it should force it to the right when airborne.  After that I’ll have the mechanic tighten the trim cables to see about effectiveness of the system after the rudder sway issue is fixed.

I think I’m on the right track.  Frustrating, but I want to fix it. 

cheers

dave

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Topic starter Posted : 26/05/2021 10:33 pm
lawheelock
Prominent Member

Dave, this is a stupid question, but do you feel the instability?  Or, is the ball just wandering?  Could it be a ball that does not have the proper damping fluid in its circular tube?  Like I said, a stupid question.

But rest assured that Stinsons are not supposed to fly like that!  I have been flying mind for 53 years now and I have never flown or ridden in a more stable airplane including a 747.  I have flown 2 or 3 108-1 and a 108-3 and they all fly the same.

I am sure that by now you have the proper steering springs, P/N 3239.  They should be installed snug, but not tight.  If they go real slack in flight, that may mean your tail spring itself is a possible problem.  How is ground handling?  Does it try to wander? 

I just cannot think of anything that has not already been mentioned.

 

Larry Wheelock

Larry Wheelock, A&P/IAStinson 108 N584LW 180 LycTexas in Winter; Indiana in Summer

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Posted : 27/05/2021 12:56 am
Dave Molnar
Eminent Member

Hi Larry,

You know there’s no stupid question!  There’s no wandering on the ground. Yes, there’s definite feel to this and it’s not just an instrument issue. But I think that the rudder tab may be at fault here with excessive deflection of the rudder left. If you recall earlier, the tail wheel steering springs were the first thing I replaced and it was done after seeing your post to a reply in the thread “Trying to catch the ball” since his symptoms were similar to mine. That helped ground handling, takeoffs, and landings immensely. This thing was akin to trying to push a wet piece of spaghetti across the table as far as keeping it straight on takeoff.  

Thinking of this last night some more and factoring in aerodynamics, p-factor, torque, I’m thinking SOME of what I’m seeing is acceptable since it’s a tailwheel aircraft and not a Cessna/Piper that I’m more familiar with, but think all of what I’m experiencing is exasperated due to the rudder tab being out of whack.  Straight and level, smooth air, holding right rudder in corrects the problem it seems.  Then as turbulence hits, the plane would normally center itself but not in my case since the interrupted airflow causes the vertical stabilizer to wander and that reaction increases with carrying the foot pressure as I’m bouncing around.  So instead of it being stable and returning to a neutral flight attitude, my tail wags since I’m trying to hold that rudder in and varying pressures just make the problem oscillate.  

I don’t know if any of this is true, but I think I’m getting closer to the remedy as it is settling down and new rudder return springs, tailwheel steering springs, and rudder bungee trim Springs all have helped but not eliminated this problem.

more next week when I take her up with the tab offset the other way.

cheers

Dave 

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Topic starter Posted : 27/05/2021 11:01 am
Robert Picard
(@bob-picard)
Trusted Member

@moparmolnargmail-com

I know you said that there is no such thing as a stupid question so I'll test your theory. Why does an airplane having a rudder trim system need a trim tab on the rudder? It seems redundant.

Bob Picard

Bob PicardN6346M Stinson 108-3 Floats/Skis/Wheels
N48923 Taylorcraft L-2B skis/WheelsAnchor Point, Alaska

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Posted : 27/05/2021 8:02 pm
Tim
 Tim
Trusted Member

@bob-picard

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Posted : 28/05/2021 1:32 am
Tim
 Tim
Trusted Member

The original 108 rudder has a bendable trim tab. Stinson just used the same rudder for the straight  108, -1 & -2. The bungee system was added to the -2 because of the increase to a 165hp engine. The bungee system and 165ph engine are the only differences  between the -1 & -2.

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Posted : 28/05/2021 1:39 am
Dave Molnar
Eminent Member

I agree and I’ll add to that stupid question list with why would 15 extra HP warrant a trim system?

On top of that, why the bigger tail on the -3?  Not theory, not why you “think”, show me a document WHY.  There has got to be a reason they did that improvement for better stability.  Probably because of the problem I’m having?  Who knows.  Would be nice to know!

cheers

dave

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Topic starter Posted : 28/05/2021 2:35 am
Robert Picard
(@bob-picard)
Trusted Member
Posted by: @moparmolnargmail-com

I agree and I’ll add to that stupid question list with why would 15 extra HP warrant a trim system?

On top of that, why the bigger tail on the -3?  Not theory, not why you “think”, show me a document WHY.  There has got to be a reason they did that improvement for better stability.  Probably because of the problem I’m having?  Who knows.  Would be nice to know!

cheers

dave

Dave,

I think that a document as to why the -3 is by now long lost to history. I don't think that there is a secret memo floating around in the 1940's version of a thumb drive that is yet to be discovered. There have been loads of articles and books and general knowledge over the years that have given very plausible reasons for the development of the -3. Since you demand proof that I do not have, or believe anyone else has, I will withhold what I have learned about my -3 over the last 33 years of ownership.

As for the reason for the trim system with the F-165 that is probably lost to history as well. I'm sure that Stinson did not want to spend extra money and complicating their fine design unnecessarily unless some CAA bozo thought it was needed. That information should be in the bozos files.

Bob Picard

 

Bob PicardN6346M Stinson 108-3 Floats/Skis/Wheels
N48923 Taylorcraft L-2B skis/WheelsAnchor Point, Alaska

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Posted : 28/05/2021 3:00 am
Robert Picard
(@bob-picard)
Trusted Member
Posted by: @tim

The original 108 rudder has a bendable trim tab. Stinson just used the same rudder for the straight  108, -1 & -2. The bungee system was added to the -2 because of the increase to a 165hp engine. The bungee system and 165ph engine are the only differences  between the -1 & -2.

Tim,

I understand all of that, I'm just curious as to what possible use would a trim tab have on an airplane equipped with cockpit controllable rudder trim. What could you possibly do with both. I guess you could use the manually adjustable tab to give it even more trim should you  run out of cockpit trim. In that case, I think you have other problems.

Bob

 

Bob PicardN6346M Stinson 108-3 Floats/Skis/Wheels
N48923 Taylorcraft L-2B skis/WheelsAnchor Point, Alaska

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Posted : 28/05/2021 3:11 am
Dave Molnar
Eminent Member

Hi Bob,

I was stating to demand proof since there’s a lot of conjecture and assumptions about as to why the -3 had a bigger tail, yet no definitive answer.  I wasn’t sure if that information was still floating around hence my direct question. And if the internet has told me anything, it’s people don’t read a post before replying so I was looking to keep away from the “Well, I think that….” conversations, that’s all.  I have my thoughts on it, but that’s just my thoughts so I was curious if there was something somewhere in the history of the aircraft that explained the design change.  And understood on the trim system too.

I’ll take her up next week for another flight to see how the new tab position works.  In other news, I did get a note from a fellow Stinson owner a few hangars down from me who’s restoring a ‘46.  Nice to have another Stinson owner at the ‘port!!!!

Cheers

Dave 

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Topic starter Posted : 28/05/2021 10:08 am
James
(@paxflyergmail-com)
Active Member

An In-flight video would help. I'd like the shot of the panel w ball and nose, then tell us if feet are on the floor.  Finish with a set up of tailwheel and springs.

Lastly, being that the -3 was specifically built for seaplane flying with much larger engine options, a rudder trim may be either required or practical for high hp ops.

Cheers, 

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Posted : 29/05/2021 9:59 pm
Dennis Crenshaw
Reputable Member

Dave, when you straighten out your fixed rudder trim tab, don't just grab it with your hands and try to straighten it.  I'm trying to picture the small tail trim tab in my mind but I don't have a picture in front of me.  I would clamp a couple of thin boards on both sides of the rudder just in front of the tab to prevent distortion; and likewise clamp a couple of thin boards on both sides of the tab when you bend it to straighten it out.  Keep us posted.

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Posted : 31/05/2021 1:50 am
Jason Tepool
Estimable Member

You know you might have figured it out.  I've had two -2s, neither had a rudder trim tab.  Unless you mean that long rounded flat piece at the bottom of the rudder is being used as one....I don't know that it is meant to be a trim tab though...if so, mine have always been just flat and straight inline with the rest of the rudder.  If you mean there is actually an added on rudder trim tab, that's not supposed to be there.

Jason "Tadpole" Tepool
N247C 108-2 w/220hp

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Posted : 01/06/2021 3:05 pm
Robert Picard
(@bob-picard)
Trusted Member
Posted by: @tadpole

You know you might have figured it out.  I've had two -2s, neither had a rudder trim tab.  Unless you mean that long rounded flat piece at the bottom of the rudder is being used as one....I don't know that it is meant to be a trim tab though...if so, mine have always been just flat and straight inline with the rest of the rudder.  If you mean there is actually an added on rudder trim tab, that's not supposed to be there.

That was my point exactly. Why have two rudder trims that could fight each other? You should have only one.

Bob

This post was modified 1 year ago 2 times by Robert Picard

Bob PicardN6346M Stinson 108-3 Floats/Skis/Wheels
N48923 Taylorcraft L-2B skis/WheelsAnchor Point, Alaska

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Posted : 01/06/2021 5:43 pm
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