31" Bush Wheels

[runon]

Re: 31" Bush Wheels

Fill them big tires with helium ...

[swoeric]

Re: 31" Bush Wheels

you can improve your cruise and takeoff a bit more with the bigger tires by resetting your cg back to where it was with the small tires or even a bit more back... the larger tires are adding drag below the cg and causing a moment change to give you an artificial forward cg at speed

[Hank Jarrett]

Re: 31" Bush Wheels

NO, NO! CG movement to the rear will change the static margin and the dynamic and static stability of the plane, NOT correct for an artificial forward CG equivalent. What the drag from the wheels do is to slightly increase the nose down pitching moment with velocity, which is actually a DE-stabilizing moment itself. Moving the CG aft will correct for the STATIC stability issue, but will aggravate the dynamic stability derivative and make the plane LESS stable. I doubt it would be critical, but if you exceeded the aft CG limit in combination with diving at high speed lots of bad things could happen. You didn't say to go aft of the aft limit for CG, but if someone did, the plane could become marginally statically stable and in combination with the loss of dynamic stability which could cause a loss of control.

If you have very large tires, you should actually move the aft CG limit slightly FORWARD to compensate for the loss of dynamic stability. There is some safety margin, but you shouldn't knowingly move into it, certainly not where you are also reducing dynamic stability.

Hank

[astjp2]

Re: 31" Bush Wheels

At a +2 for datum, not worth the hassle on big tires to change the CG and not much drag up to a point. Install a scott 3200 and it will be just fine. Tim

you can improve your cruise and takeoff a bit more with the bigger tires by resetting your cg back to where it was with the small tires or even a bit more back... the larger tires are adding drag below the cg and causing a moment change to give you an artificial forward cg at speed

[sibbeston]

Re: 31" Bush Wheels

Nice to see the info on tires. I am rebuilding a bc12-d right now and will add hyd brakes to take a bigger wheel. any insights into the best way to do this? Im unrestricted in mods as it will be uncertified.
Good to see folks up in alaska using the plane in the bush contributing info. Im in yukon and am trying to build for this purpose.
Cheers.

[Hank Jarrett]

Re: 31" Bush Wheels

There are a couple of designs out there with paperwork. There is at least one STC and Chuck Avon has a nice one. Chuck has even worked out some improvements to the way he did it. DON'T make your brakes TOO good. Boot cowls, props and crankshafts are expensive!
Hank