8GCBC Gear Flutter

M

mr scout

New member
I just installed a set of BushWheels upon reaching 104 knots the gear legs begin to flutter. You can feel it in both seats, looking outside they appear to be moving about 3/8 of an inch and at a rate of about 350 times per minute.
Any cause for alarm? The shake get worse as speed increases..
 
Hey there, Im not sure of your specific situation, but what wheels do you have? Size? Age? I have plain jane 8.50x6 but have flown the 31" and newer AK 33"s and may be able to help alittle if no-one else here has any guidance.
 
There 29s the 8.50s did the same thing only not near as bad. I love the tire but the constant shaking has me wondering. I have spoke to a couple other Scout owners some have the shakes and some dont. I am tempted to put a wood stiffner on the gear leg like is on my RV.
 
I'm not even going to pretend to act like I know what I talking about here. Same thing with the 8.50....? Thats kinda weird. I'd be nervous: just like that Beaver up around Mt. Mckinley couple weeks ago....Flutter can tare an airplane apart.
 
Careful how you guys use the term "Flutter"
IMO, the dangerous aeroelastic version of the term should never have been called that, but too late now.

You have a new enough scout to put on the Aluminum Gear?
Or is this aluminum gear?
What prop do you have?
I assume this is happening only in 'hands-off' flight?
What if you skid around; does it go away?
 
Jerry,
Its a 1999 with 2 blade constant speed prop. It dosent care if its hands off or not, it does it both ways. Like I said in the first post the tire looks like it moves about 3/8 of and inch and at the rate I stated. If I skid it to one hard side it quits..It does it with both sets of tires but much worse with the 29" Bushwheels. In slow flight its fine, does it steady in cruise and get wose in a descent. One thing I have found out is there is a guy in Ca with a newer one that dosent do it but his gear is in poor alignment and wearing down one of his tires. His also has a 3 blade prop. When mine is longitudinally level the toe is Zero. (dont want to be wearing out $3000.00 in tires.
 
jerrymjr said:
Ok, so its worse with speed and requires 'striaght ahead' flying?
Click to expand...

I dont understand your (requires straight ahead flying) comment. If your asking if it only does it in straight ahead flight that would be a yes. It will slow itself to a stop if you put the ball all the way to one side or the other. Or slow down below 90 Knots
 
Yeah Bob won't turn on quotes. :p

Here is a theory...
There is a long term for when you have a blunt body that is perpendicular into airflow, in which the 'tip vortices' alternate between the sides of the body. Jody is looking up the term as I type (He says it begins with F, lol....). So when you are skidding/slipping, you are angling the blunt body such that the separation is constantly off the same side. When you slow down the size of the tip vortex is smaller and likely stops even reaching the gear leg (which is a flat plate that 'could' be influenced by turbulence).

If this theory is correct, things to do about it would either be:
1) Change the flow characteristics about the wheel. Tough to do with Tundra Tires. Wheel Pants or even those trailing edge cone wheel spat things. (read, ugly)
2) Change the characteristics of the gear leg so that its dynamic range is not at all similar to the flow inducing it. Aluminum Gear Legs would be an easy, but somewhat costly way. Perhaps your piece of wood would disrupt the flow on the leg enough to work, hehe.





(Okay Jody found it...look up a Karman Vortex Sheet)
 
This regular pattern of vortices in the wake is called a Karman vortex street. It creates an oscillating flow at a discrete frequency that is correlated to the Reynolds number of the flow. The periodic nature of the vortex shedding phenomenon can sometimes lead to unwanted structural vibrations, especially when the shedding frequency matches one of the resonant frequencies of the structure. One example is the famous Tacoma Narrow bridge incident and this topic has been discussed in great details in the Tacoma bridge link

Jerry,
If the alum. gear will fix the problem then are you thinking this is caused by the gear resonanting? Since it does it with both tires.

Do you suspect I going to have any problem with the airframe attach points or gear or is it just a nuisance. The RV gear legs are prone to vibration and the fix they came up with is the wood stifner on the trailing edge wraped with tape then faired.

( Perhaps your piece of wood would disrupt the flow on the leg enough to work, hehe)

If we just need to disrupt airflow on the gear how about some vortex generators glued to the gear legs? Or maybe I should just strap on some snow shoes.
 
Had a Waco 10 aileron slave strut that shook to beat the band. It would set up a good airframe vibration. Took a piece of 1/8th welding rod and taped it to the strut. The vibration went away. You might try the same thing on your gear.
Paul
 
There is clearly a wealth of technical information here. But, my concern is that of occurence. With this theory, wouldnt more individuals encounter this issue? Every one and thier dog has bushwheels (except me) around here, and this is the first time I've ever heard anything that was described as "flutter". At least that wasnt contributed to a wheel balance issue. I know this was probably your first assumption, but, are they balanced?
 
What would balance have to do with it? Wouldnt the tire have to be rotating in flight for that to be an issue?. Like I said it does it with both sets of tires just much worse with the Bushwheels. It seems there are some that do and some that dont from the owners I have talked to. The ones that dont also tell me they never fly fast anyway...I regulary fly 4-5.5 hr. hops so I really dont want to pull the power back.

Jerry,
I just had another thought, do most guys have gear steps? Would that disrupt the airflow enough to stop this?...
 
Well, the more we talk about it here, its really not the gear leg. The flow is coming from the tire and the pressures are simply pushing on the leg. A landing gear step would likely be too high up to affect it. 95% of the owners don't have them.

An Aluminum Landing Gear would have different frequencies and its also stiffer. Likely, putting it to rest. As much as we would love to sell you a pair, they are just not cheap.

I would try to make some homemade Vortex Generators and duct tape them on and try it out. If it works, You can get something more permanant. If not, well, you tried.

As for damage in current configuration, I would not be overly concerned. Just be diligent and change the through-bolt every 500 hours, as suggested. When you are swapping the thru-bolt just check for elongation in he fuselage attach hole. The U-bolt should not have an issue.
 
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