Landing Gear Ready For Tacking

11/24/2009

The original engine mount had a spider truss at the the lower half of the mount, this I cut away to get the new gear truss into position. I built an MDF base for the legs to sit on and added weightlifting plates to lock everything into position. This base was positioned with plumb bobs, and then tramelled to the tail post.

All carry thru tubes were added. Some of those coping angles were a b***h, 2 tubes required 3 attempts, 1 other tube took 3/4 of a day, from planning to cutting to screwing up to redux. ugh.

One of the carry thru tubes (see pic) seems to put a weld joint partly in tension, so I am trying to fabricate a strap for that area.

The engine mount is now at the tacking stage. I can't tack with gas, since there will be too much heat being thrown around (I don't want to affect the properties of the steel rod gear inside the tubes), so my buddy will be tacking with a MIG, not ideal, but my TIG buddy who will be finish welding will burn through the MIG tacks. This all has to be done in situ, and the hangar doesn't have enough power for a welder (I have to turn the lights off when using my table saw otherwise the breaker trips!), so I am borrowing a generator for the day.

My 'oops'

When the finished gear truss and legs were in their final position, it became instantly obvious that I had come across my first major 'oops' in this modification: I should have temporarily clamped the legs in the sockets and not drilled them for the upper socket locating bolt. The axles were bolted in a flat plane, this would be fine if it was a Pitts, but the legs rake back (sweep back) 10 inches from station 0. So, because of this rake angle, coupled with the camber of the axle, we actually have a compound angle at the axle. This means if the legs were perpendicular to the longerons (Pitts) I would have neutral-toe, but since I have a rake angle the axles currently sit toe-out (not good). However, maybe i got lucky, (naaaaaa!) once the wheels are on and the tail is dropped, that compound toe-out angle will diminish, and once the weight goes on and the gear 'squats', the toe-out will diminish even more so. Maybe there's a chance I will end up with neutral-toe...

In all likelihood, I am going to have to slightly rotate the legs in the sockets to adjust for an optimum toe angle. This is my proposed plan after the engine mount is all finished welded:

- install the gear, get the wheels on

- put a greased steel plate under each wheel (to simulate a true squat)

- add cement bags to the fuselage to simulate my most common weight configuration. I am thinking solo/full gas-main tank/0 wing tank, so around 1600 pounds. I will distribute the weight in order to obtain an average tail wheel weight.

- once loaded, raise the tailwheel approx' 15 inches. This is the attitude the plane is in when there is no directional control ie, take-off, tail comes up, so no tail wheel control, and not enough wind to control the rudder. I believe this is termed the 'spooky phase'.

- look for positive wheel camber and adjust* for neutral toe. Note: as the plane gets heavier (PAX) the squat will naturally move the wheels to a slight toe-in. Having read around the web regarding the optimum toe angle, it really is a gray area subject. So I decided to simply listen to the experienced pro's, the consensus for these biplanes being positive camber and neutral toe with a hint of toe-in.

A big thank you to Acropilotbret and Kevin Kimball for off-forum tips.

*As afore mentioned, I drilled and bolted already, but I do have a fix for this from the Tailwind guys...elongate the bolt holes in the socket, set the leg, add a thick washer (or steel plate) to each side of the bolt, tighten, tack weld the washer/plate, remove leg and finish weld.