Confession of a Homebuilder
Rick & Linda Henry <RickH@...>
OK, I screwed up.
It took me five years and 500 landings to finally look into why my tailwheel Mustang II didn't handle very well in the 30 to 50 knot range on takeoffs and landings. Most other builders said their airplanes handled just fine...
There was a consistent "squirrliness" - a little on takeoff, but especially noticeable on landing just after the tail went down - that always required fast, fancy footwork to keep the airplane somewhere near the centerline after landing. Even in no-wind conditions. I eventually adapted to the situation by braking fairly heavily to decelerate through the "sporty" speed range in a minimum time.
Here's what was wrong:
In 1984, when the center section was still in the vertical jig, I aligned the gear torque tube components for welding [way before this was available as an option from Mustang Aero.], allowing a 4° positive camber. (Bottom of the tires being closer together than the tops of the tires - when viewed from the front). Here's an illustration of camber:
This 4° camber would be reduced when the airplane would be on the ground - hopefully to a very slightly positive amount. (Top of tires tilting out). Mind you, this was aligned and welded in the days of bubble levels and plastic protractors - long before digital levels were even thought of by a starving commuter pilot.
Did I measure this after the airplane was built, loaded, ready to fly [now in the day of digital levels..] ?
Ummmmm... Next question please....
Actual measurements five years hence:
Left gear had 1° negative camber, right gear had 1° positive camber. Not good.
(Well they were both going in the same direction -- to the edge of the runway !! )
Toe is the difference in distance between the front and rear of the tires. Here's an illustration:
Yes, I measured the toe when aligning the gear. I compared the angle of the axles while up on jacks, and they both indicated 1.25° toe in.
Now, what's wrong with that sentence?
Clue: "...up on jacks."
Actual measurements, on the ground, airplane loaded:
Both gear spread out to the point of having 1/2° toe OUT. Not good.
As our friends at Whiteline ( http://www.whiteline.com.au/ ) say:
"Excessive toe-out makes the car nervous and lacking in directional stability."
I can relate to that. Additionally, the more loaded the gear is, the more it will tend to toe out. So, my analysis is that when a swerve occurs, the load on that side increases, increasing the toe out which points the tire more toward the edge of the runway! That's followed by a larger opposite rudder input and a not-so-impressive rollout.
1) Put down two metal plates under each gear with grease or oil smeared between them. Make the bottom plate large enough to allow the upper plate to slide outward as the weight of the plane spreads the gear out.
2) Clamp four foot long levels (or other rectangular straight-edges) to the brake disks. I used two "Pony" clamps on each. Center the straight-edges below the same spot on the axles. These will then extend two feet forward, and two feet aft of the wheels. (Some use a framing square.)
3) Find the center line of the plane and draw that on the floor under the center section. I used wide masking tape on the floor and marked the centerline with a "Sharpie" marker.
4) You can load the airplane to your "average" operating weight. I found though, that the camber and toe changed very little between measurements of light and heavy weights. Less than 2/10ths of a degree with a 500 pound change. "Your results may vary..."
5) Calibrate your digital level. Use the "Super Set" option if you have a "Smart Level".
6) Either try to level the airplane, or note the angle that it sits for all of the following lateral measurements. For example, all of my measurements were corrected for a .2° "right wing low" situation of my hangar floor.
7) Clamp the digital level in a near-vertical position to the side of the four foot level. This will show the camber.
8) Correct for either "wing low". (This can be a bit confusing - I drew a rear view to see which way to "rotate" the corrections.)
9) Measure the distance from the corners of the four foot straight edges to the centerline. I used a small square to mark those spots on the floor (on wide making tape), to make the measuring easier and more precise.
10) The distances between the aft end spots should be slightly greater than those across the front to get a toe in situation.
Some notes on shims: The axle bolt pattern and thus the taper of the shim are not square (or perpendicular) to the level ground. I measured my bolt pattern to be about 30° up from horizontal, which is a convenient number for the trig... This paragraph could get quite complicated, so let me just say that a 1° shim doesn't change the toe or camber by 1°, but by either about .5° or .86°, depending on how you orient it. Again, we're talking the tailwheel version (tail down) here. Also, the shims I came across came as 1/2°, 1° and 2°. For 5 inch axles, my shims were square, so they could be oriented any way. For six inch axles which have a rectangular [not square] bolt pattern, they come tapered - either 'this way' or 'that way'. (A picture would have helped here...)
The square shims needed to be ground to clear the brake assembly. Make sure that they clear and won't bind the brake caliper. As a reminder, the bottom bolts are high strength NAS bolts, not regular 1/4" AN bolts as in the "top" two positions.
After trying different shims, I came up with the following dimensions:
Front: 73 1/2" total: (36 7/8" left, 36 5/8" right of 'centerline')
Back: 74 5/8" total: (37 7/16 left, 37 3/16 right of 'centerline')
Doing the math, I came up with: 9/16" divided by 48 inches=.0117 which is the sin (or also the tan in this case) of .67°.
So, the final toe in is 2/3° for each wheel.
The final camber showed 89.6° on the level, or .4° positive camber for each wheel.
And the flight test....
Was great! No zig-zag at all. Tracks as straight as a BMW on the Autobahn.
I have a "New" airplane!
I'll be anxious to do some more flight testing with seeing how the tracking behaves under crosswinds, different gross weights, etc. But for now, it has been a tremendous improvement !
Bottom line: I recommend to spend the time to get the gear accurately aligned in the beginning, and the Mustang II will be everything you hoped for in an airplane!
(Don't wait five years like I did...)