Nicely said. You could extend this to explain "reverse aileron steering,"
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David J. Gall
From: Q-LIST@... [mailto:Q-LIST@...]
On Behalf Of withidl
Sent: Wednesday, June 22, 2005 9:02 PM
Subject: [Q-LIST] Just Another Thought on Wheel Camber
In an effort to understand the physics surrounding the ground
handling problems of the plans built Q-Bird (I'm still building
#2710) I have tried many times to visualize the problem.
Several months ago while I was reviewing an article in a
motorcycle magazine (I have a 1999 Suzuki GSX1300R Hayabusa)
regarding how a motorcycle executes a turn it became clear to
me that the negative camber of the plans built Q-Bird creates
ground control problems basis the same physics as that of a
motorcycle leaned over into a turn.
When a motorcycle at speed is counter steered into a turn the
wheels lean (can't call it +/- camber because there's no left
or right wheel reference), YET ONCE IN THE TURN THE
MOTORCYCLE'S FRONT WHEEL IS STILL POINTED STRAIGHT AHEAD EVEN
THOUGH THE CYCLE IS TURNING, i.e. its angle to the chassis is
such that it is not actually turning the motorcycle as the
front wheels of a car do (NOTE: At very slow speed the front
wheel DOES steer the cycle as per a car).
The motorcycle turns because it's lean sets up a virtual
"cone" from the axles of BOTH wheels to a projected point on
the ground vs. the tires contact patch to that same projected
point on the ground. As you know, if you roll a conical
object on a flat surface it will turn around its pointed end.
This works great for a motorcycle since the two wheels are
acting in concert with one another.
On the other hand the negative camber of the Q-Bird's landing
gear oppose one another, i.e. they lean AGAINST each other,
to turn the plane toward the opposite side of the runway. As long
as each wheel is carrying the same load all is balanced. As
downward pressure on a wheel increases, either due to yaw
inputs by the pilot, runway bumps or crosswinds, two things
happen. The canard flex increases which increases negative
camber (the motorcycle is leaned further, thus more turning
effort is generated) and the wheels tractive authority
increases, both of which exacerbate yaw control problems
because the pilot has no timely way to anticipate or correct
for them. The plans change 2 degree toe out was an effort to
counteract this but just can't totally do so because of the
constantly changing camber (canard flex) and wheel tractive authority.
So all the above being said, correcting the camber at maximum
canard flex toward a neutral or positive position when the
aircraft is at gross is a good thing. Excessive positive
camber would have little adverse affect on the Q-Bird since
it would only evidence when the plane is being turned toward
the inside wheel (with the increasing positive camber) which
would be lifting and thus lessening it's tractive authority
and thereby negating any potential deleterious effect the
positive camber may have.
There have been more technical explanations on the
"negatives" of negative camber on the Q-Bird, but I thought
the above may offer a more elementary perspective for those
of us not into the physics of the matter.
Additionally, not all negative camber is bad, as my highway
vehicle has considerable OEM negative camber on the rear
wheels to enhance stability, just depends on the application.