Just joined the Fat Avid Club

[1avidflyer 2025]

That is a fine looking airplane.  Glad you are having fun with it.  JImChuk

[ChrisBolkan 151]

Thanks Jim. I have accumulated around 800 hours in it. A LOT more than I ever planned to when I bought it just to fly while I fixed my Magnum! But that's life.

[Av8r3400 1188]

Happy day!

I flew my converted Fat Avid yesterday for the first time and I ham happy to say she is tame and well behaved taking off and landing. It was a lot less short couple feeling than I expected, although my landings were all on grass yesterday.

The trike to TW conversion took longer than I thought but turned out real well. I don't have $1K total into the conversion so I am very happy with that. I plan to fly it for a while on the pillow softs before converting to the cartoon tires.  It took a bit to get the brake geometry right using the Matco master cylinders with the Cleveland double puck brakes. Here is a pic of it while I was waiting for someone to move a vehicle out of the way so I could put it back in the hangar when I was done flying.

My friction lock for the manual flap handle is a complete bust. There is way too much torque for it to hold. I am going to have to come up with some sort of indexing/locking mechanism to hold the flaps in position. I would appreciate any ideas on that.

This was my solution for a "gated" flap control lever.  Very simple and used the original mechanism.
http://avidfoxflyers.com/index.php?/topic/709-classic-iv-restoration/&do=findComment&comment=10911

[saskavid 48]

Hey Chris looks nice and will be interesting to see the cruise speed difference between the pictured tires and the "big boots" still to come. An external caliper cleveland bottom end with dual piston caliper is about as good a performance condition as you can put on a plane like this. Looking like in some of the pictures the matco masters are the super short mc-4g? One thing with the avid rudder bar configuration I'd be watching for is that with brake activation master cylinder travel is that they can't travel backwards,towards the rudder pedal when you step on the brakes. With rudder bar flexing there is then potential to get an over center lockup condition. I went with the mc-4d. Longer,always moving away when stepping on the brakes.

[ChrisBolkan 151]

Thanks! I will look at the geometry for that. I did have to lengthen the existing "adaptors" that screw on to the top of the cylinder and attach to the pedal. I could not get the brakes to activate very well with full rudder deflection. I will definitely look to see if it is possible for the brake to over center as you describe with my setup.

[ChrisBolkan 151]

I've been having a ton of fun with my TW conversion. However, I've encountered the same TW shimmy with my Bearhawk TW assembly that I had on the Magnum. It took me a long time to come to the realization that really the ONLY thing that will absolutely eliminate TW shimmy is the proper amount of friction or resistive damping in the back and forth movement of the TW. Most tailwheel assemblies have some provision for adjusting the amount of friction and it ends up being a delicate balance of enough friction to eliminate shimmy while not having too much friction to prevent the chains and springs from being able to steer the plane on the ground. When tailwheels get big and heavy enough, so much damping is required that the tailwheel can no longer be steered with chains and springs attached to the rudder.

The Bearhawk TW is an ingenious, relatively lightweight and simple tailwheel assembly, but its fatal flaw is that it has no provision for resistive damping, which is why I could never kill shimmy. I experimented with smaller and bigger tires, tire profiles, kingpin caster angle and so forth until I came to the realization that the ONLY cure was friction. Since the TW was never going to work as it was, I decided to modify it. I threaded the kingpin, removed the steering and release mechanism, and installed 2 belleville washers and a castle nut to allow the introduction of friction into the system. The new setup has no steering chains and springs. It is full castering and is steered by differential braking at low speeds and by air across the rudder at higher speeds or with blasts of power sending wind across the rudder at low speeds. It works great! It's different, but not better or worse than chains and springs and ALL shimmy is gone! I couldn't be happier.

Another thing I am trying is a light weight tailwheel rim and tire. So far it is working great on everything I have landed on, but I have not really hammered it with big rocks or anything extreme. the weight of this wheel and tire is half the weight of the 10 X 3.5 X 4 Aero Classic tire and 3 piece cast aluminum wheel  you see used on big Cessnas and so forth. It's a reinforced nylon wheel made by Azusa and a Cheng Shin 4.10 X 3.50-4 4 ply Go Kart tire. This wheel and tire are so light weight, that much less friction is required to kill shimmy, so it might be possible to use steering chains and springs with it on a tail wheel assembly that has a provision for friction adjustment. This light weigh tire and wheel will significantly improve my baggage capacity too. An added bonus. 

Hopefully it is strong enough and holds up over time. here are a couple pics.

Edited 27 April by ChrisBolkan
typo

[Av8r3400 1188]

The caster angle is causing the shimmy.  The king pin should be perpendicular to the ground or slightly angled back when in 3-point attitude.

[ChrisBolkan 151]

I am aware of only two ways to prevent shimmy.

1) A tailwheel that is light enough relative to the mass and stiffness of the spring and fuselage, that the natural resonant frequency of the system is too high to get excited and break into oscillation.

2) A tailwheel that is heavy enough relative to the mass and stiffness of the spring and fuselage, that the system WILL get excited and break into oscillation, in which case the ONLY absolute cure is to critically dampen the oscillation with resistive damping / friction. 

Caster angle is one component to help reduce the tendency of shimmy to develop and will help in borderline situations. A negative caster angle (bottom of kingpin behind top) will promote shimmy and a positive angle top behind bottom of kingpin will tend to stabilize shimmy. 

However when you get a massive tail wheel in comparison to the mass and stiffness of the airframe, and DO NOT have enough resistive damping/ friction in the back and forth movement of the tailwheel to critically dampen the system, no amount of caster angle will prevent shimmy once it gets excited, or you will need so much caster angle that the tailwheel will not want to steer. 

Resistive damping is the only guaranteed solution. I am 100% convinced of that. If you put a massive enough tailwheel on your plane WITHOUT resistive damping, it WILL shimmy no matter what you do, other than apply adequate resistive damping. 

My tailwheel in the photos has positive caster angle by several degrees. For some reason the photos make it look negative. Here is a better picture. And thanks for pointing this out. I received similar feedback on the Facebook forum. It must be the fisheye effect of my iPhone or the angle I took the pics from 

Edited 29 April by ChrisBolkan
Add photo and explanation