I posted this a couple of years ago but it needs repeating as the new builders come on line.
Toe in and tow out on aircraft.
There have been a few posts about toe-in and toe-out on aircraft without really explaining the geometry of the spring type landing gear design. Note: This does not apply to oleo strut, trailing link or bogie truck type gears.
With no weight on the wheels, a spring type gear has positive camber by design. This means the top of the tire center line is tilted outward. Negative camber means the top of the tire is tilted inward.
As the aircraft enters a flare the pitch increases. With a pitch increase, the positive camber factors into two components, part positive camber and part toe-in. When the weight of the aircraft settles on the gear, the gear spreads. The spreading decreases the positive camber. The decrease may result in negative camber. This transition is more pronounced at higher weights. As long as the aircraft attitude is nose up with weight on wheels, the now negative camber factors into two components… part negative camber and part toe-out.
This transition from positive camber to negative camber, and toe-in to toe-out occurs on both conventional gear and tricycle gear.
If the aircraft has conventional gear and negative camber with weight on both the main wheels and tailwheel, it will usually have toe-out in the three point attitude.
However, if the aircraft is tricycle, the nose lowers after touchdown. This causes the toe-out effect of negative camber to transition back to near neutral toe-in with negative camber. ( There is a slight decrease in negative camber when some of the weight shifts to the nose gear.)
For tricycle gear aircraft, toe-in is stabilizing. As the aircraft turns, it leans against the outside tire. Because the CG is forward of the tire the aircraft will tend to drift in the direction of the lean. The toe-in counteracts the drifting. Most tricycle gear aircraft are set up with slight toe-in with weight on all three gear.
For conventional gear aircraft, toe-in is unstable. As the aircraft turns it leans against the outside tire which is pointed into the turn. Because the CG is behind the landing gear the aircraft turns sharper and over-steers.
Toe-out on a conventional gear is stabilizing. As the aircraft turns it leans against the outside tire which is pointing away from the turn. The tire angle decreases the turning tendency and directional stability is correctable by the pilot. Most conventional gear aircraft are set up to have a slight negative camber with max weight on wheels. That results in a small toe-out component in three point attitude and neutral toe in two point.
If your aircraft is a bit unstable on landing, you might check the camber and toe angles with the aircraft loaded to the normal operating weight. If the gear is too stiff, or bungees are too tight, the gear may not be settling to the preferred negative camber position.
Notice if you convert your Kitfox from tricycle to tail dragger or back the other way, the caster takes care of the toe-in or toe out. After touchdown, the trike lowers the nose resulting in toe-in and the tail dragger lowers the tail resulting in toe-out, but only if you have the required negative camber with weight on wheels.
In summary, tricycle gear --- neutral or slight toe-in is best. Convention gear --- neutral to slightly toe-out is best. Both types of aircraft will have positive camber with no weight on wheels progressing to slight negative camber with weight on wheels.
Again, the above only applies to aircraft with spring, tubular, or bungee sprung main gear. Trailing link, oleo and bogie trucks have their own set of rules for setting up toe angles.
I hope this clarifies the differences between conventional and tricycle landing gears and how they react with camber, toe-in and toe-out.
John Pitkin
Axle Taper Shims
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