BCP's, gotta question: I put the 11x29"s on the 206. The Gar Aero STC says 8 to 12 psi. I put in 10. Everything is good with them except, they bounce in flight. I'm not talking the post take off bounce that is fixed by a brake tap, it's a non-stop, annoying, aerodynamic shimmy, both sides. It's not bad, or a big deal, just a little annoying and, well, weird. Has anybody else experienced this with big tires? Any way to stop it? It seems to definitely be more pronounced at higher speeds. I guess, for now, I'll just fly slow. Thanks!
Chris

Sounds like they need a mammogram.

I see metal fatigue in the future.
Try higher pressure, maybe the tyre is depressing in front though not likely.
C180/185 get same problem with 31" i've been told.

I'd venture to guess that it has nothing to do with inflation pressure. You've effectively hung heavier weights at the end of your gear legs, preloading them more than your old small tires. Any little fluctuation in wing loading causes a nice well-balanced oscillation.

I think the only solution is stiffer gear legs, but I'm not sure what your options are for a 206.

Are they spinning or just bouncing? My guess is something is causing uneven airflow, whether it is a slightly out of round tires or they are mounted not quite square to the airflow it sounds like a low level harmonic imbalance. It may just be annoying now but I agree that it could cause long term damage, likely to the wheel bearings, axle or even the gear itself. For now try different tire pressures.

Good ideas, also check the gear leg attachment to fuse is tight, a loose bolt won't be noticeable on the ground when the weight is on the wheels.

I ran a pair of 26 inch bushwheels on my 180 for a bit.... They did bounce around like you mention quite a bit.It seemed to come in waves ....somedays they seemed to bounce more than others , it was pretty irritating.

I experienced bouncy bouncy thing in a kitfox with the grove spring gear. My own kitfox has the same tires as that kitfox but the stock tube gear and does not do the bouncy bouncy. I think Zane is on the right track as to the cause.

flyingzebra wrote: BCP's, gotta question: I put the 11x29"s on the 206. The Gar Aero STC says 8 to 12 psi. I put in 10. Everything is good with them except, they bounce in flight.....

The 850's on my C150/150TD bounce a bit in flight, the 800's don't do that. Annoying but I doubt it's enough to cause metal fatigue, even in the long run.
BTW Chris, was that your C206 at Jefferson County yesterday? I saw you gassing it up & noticed the 29x10 set-up. Nice.

The oscillation/vibration also occurs on wide gear Maules with 29 or 31" but not on the oleo gear versions.
Different speeds change the frequency and at the correct speed the shake goes away.
Aerodynamics apparently. Annoying and leaves a question.

maules.com wrote:The oscillation/vibration also occurs on wide gear Maules with 29 or 31" but not on the oleo gear versions.

I would expect it to be a characteristic of spring gear only, the Oleos are damped to avoid sharp rebound, which helps with small displacement oscillation apparently too. You're maules.com, I'm not tell you anything you don't know, just agreeing.

maules.com wrote:Aerodynamics apparently. Annoying and leaves a question.

Perhaps at the right airspeed, there's enough lift force generated by the tire or the gear leg itself to cancel out the extra weight of the big tire, reducing the preloading of the gear leg and breaking the oscillation.

I have a theory. The oscillation is normal like a flag waving in the breeze or hanging a balloon on a string out the car window. The gear is stiff enough to prevent this with a normal size tire. With the larger tire the oscillation is a greater force and the gear bounces. When it is deflected one time by the wind the gear springs back because is perfectly balanced and free to travel in each direction unlike an oleo gear that is dampened. The solution may be to make it unbalanced in one direction by flying either a little more nose up or down at cruise to tip the gear one way or the other, away from the perfectly balanced angle of flight you have in trimmed cruise. That would cause one direction of the oscillation to become stronger and dominate, overcoming the rebound. It might settle down. Maybe that would explain why the change of speed calms it, because it also changes the pitch trim of the aircraft and the angle of approaching air to the gear/tire assembly favoring the pressure against the spring in one direction over the other.

Bueller, still awake?

I think most of the various causes are covered by other member's replies: extra weight on the end of the gear changes the vibrational modes of the gear (lowers the frequency), and aerodynamics are providing the oscillatory forces on the gear. The aerodynamics thing is a vortex shedding from the wheel. I've seen this before when we are testing things in the wind tunnel. There are several videos on the web that show this, but this one is kind of cool:

Edit: I had to go read the embed tutorial



Each time the vortex changes sides it provides a little push on the wheel in one direction, and then switches sides. Size and shape will affect that.

I think Zane said it right, stiffer gear, or lighter wheels is the only way to make it work.

I had the same issue with my 180 when I ran 29" Gars at speeds above 125-130 indicated. Below that airspeed it wasn't a problem.

Still I now run 8.50x10 now because they don't have a harmonic at any speed (on my airplane anyway.)

Thanks all for the feedback. I really think I'll just swap them back out for the 8.50's unless I'm planning on gravel bars or other way cool stuff. Besides the annoying bounce, they are eating enough airspeed and useable load that I think I'll need a reason, besides being cool, to run them. Of course, I might just go with nimpocub's thinking and, get a big bra?
Chris

Oh NO! Here they come, the new ABW Wheel Pants!

I think it's a classic damped spring mass system with a forcing function. Soggyc has identified the forcing function. There are a lot of variables involved, some are difficult to control, others are more practical. For instance, it's difficult to change the Reynolds number for your airplane which has an effect on the force generated by the vortex shedding. You could change the stiffness of the gear legs, but come on... do you need to burn that kind of money? If so, maybe I can help. Easiest parameter to change is mass. How? Go back to 8.50's except for gravel bars and other cool stuff.

Metal fatigue is difficult to predict and that's an awful big piece of metal holding up your gear, but, remember the toe bone is connected to the ankle bone.... Your wings ultimately absorb that shaking because they are holding up the fuselage that's holding up the gear. How much force is being applied and at what frequency and how much is damped out before the wing root attachments deal with it and is it a load case that Cessna thought of and examined during design? The British Comet airliner was brought down by metal fatigue due in part to a too-tight radius on the corners of its square windows. Ya never know.

Use the 8.50's. Go fast. Save gas. Sleep well at night.

YB

Yellowbelly wrote:I think it's a classic damped spring mass system with a forcing function. Soggyc has identified the forcing function. There are a lot of variables involved, some are difficult to control, others are more practical. For instance, it's difficult to change the Reynolds number for your airplane which has an effect on the force generated by the vortex shedding. You could change the stiffness of the gear legs, but come on... do you need to burn that kind of money? If so, maybe I can help. Easiest parameter to change is mass. How? Go back to 8.50's except for gravel bars and other cool stuff.

Metal fatigue is difficult to predict and that's an awful big piece of metal holding up your gear, but, remember the toe bone is connected to the ankle bone.... Your wings ultimately absorb that shaking because they are holding up the fuselage that's holding up the gear. How much force is being applied and at what frequency and how much is damped out before the wing root attachments deal with it and is it a load case that Cessna thought of and examined during design? The British Comet airliner was brought down by metal fatigue due in part to a too-tight radius on the corners of its square windows. Ya never know.

Use the 8.50's. Go fast. Save gas. Sleep well at night.

YB

Yeah, I'm glad the science class is over anyway. My head is still hurting. Since the thread originator is sufficiently informed now its time to throw back a few and toss around the rest of the less serious ideas that are going through my head. I actually thought about suggesting VG's on the big wheels to attach the boundary layer and keep it from forming the burbles in the air behind it which is causing the oscillatons and the gear to bounce but then I thought no, they might puncture the tire on touchdown, then I thought about the dimples in golf balls and thought that if the big wheels had dimples in them it would make them pull easier through the air and they wouldn't need the VG's and then I thought, shit my head is really hurting now so I didn't.

Get used to it. Every plane has a vibration frequency, the heavier the tire the slower the tire oscillates and the closer it matches the aircraft harmonic frequency. My experience is that the larger the tire, the more shaking. Small price to pay for the benefits.................

My Maule MX-7-180 has Goodyear 8.50x6 tires, ABW heavy duty gear legs and oleo struts. The Goodyears are somewhat heavier than other 8.50s but there is no tire bouncing. I'm sure it's because of the oleo struts. The spring gear is just...springier...in the air and on the ground.