Re: Just Another Thought on Wheel Camber

David J. Gall


Good question!

Michel Royer was the first to take my alignment advice, but he ran into difficulty getting the wheels angled within the wheel pant as much as I had suggested.
He only achieved half of the correction, yet he reported great improvement in ground handling. I think he "split the diffference": As you noted, the
aerodynamic lift on the canard alleviates some of the landing-gear-caused flex at speed. Also, the critical speed (onset of instability) is a function of how much
negative wheel camber is present. So, it is plausible that some negative camber is permissible at rest, allowing for the camber to approach neutral or to go
positive as speed (aerodynamic lift) increases in such a fashion that the critical speed gets pushed ever higher as you accelerate, and starts at some value higher
than touchdown speed on landing. Thus, you would "always" have some margin between the current rolling speed and the (higher) critical speed.

Then the question becomes: How large is this margin and what factors can erode this margin? The magnitude of the margin would be hard to determine.
Extensive, controlled testing would be required. But two facts are known: Negative camber is destabilizing and critical speed is a ground rolling speed, not an
airspeed. Thus to be certain of having everything in our favor, I recommended that the camber be "never negative," no matter what the aerodynamic lift
conditions or the load conditions. So, at gross weight the camber should be zero. It can become more positive through a host of mechanisms and that's okay.

In my opinion, there are too many ways to manage to cancel out the contribution of aerodynamic lift to the camber angle. I'd rather know that the camber is
always positive than guess at the circumstance that may put me into the unstable regime. However, there are those who've done otherwise. You decide.
Remember that anything you do to increase the flex of the canard at speed will make the airplane more prone to instability. Wheel landings, high gross weight,
no-wind or downwind operations, and high density altitude are some things that may make the airplane more directionally unstable due to more-negative
camber than anticipated. Even the amount of crown built into the runway surface can affect the operational camber.

Bear in mind also that setting the camber neutral or even positive still won't make a taildragger directionally stable all by itself. There is a contribution required
from the tailwheel and/or vertical stab and rudder that just may not be sufficient on the Q-birds (or most taildraggers). Without using a locking tailwheel, the
pilot's technique on the rudder pedals and the amount of friction in the tailwheel control circuit play a major part in determining the airplane's ground directional
stability. If you can lock your ankles firmly to simulate a locking tailwheel, the flexibility of the tailspring and the stretchiness of the control cables become
factors. There's just all sorts of things that make life a little more uncertain than theory -- thus my recommendation to set the resting camber at zero and
"suffer" the increased camber (and increased stability) as the canard starts to lift the load from the wheels.

Finally, whether you choose to change your alignment or not, be sure to incorporate all elements of the Jim-Bob six-pack into your airplane. There is just no
excuse for having a tailwheel that isn't coordinated to the rest of the airplane, or for not having quality independent disc brakes that work correctly and reliably.

Good luck with your project,

David J. Gall

----- Original Message -----
From: "larry severson" <larry2@...>
To: Q-LIST@...
Subject: Re: [Q-LIST] Re: Just Another Thought on Wheel Camber
Date: Sun, 26 Jun 2005 20:11:20 -0700

Negative camber (on the main gear of a taildragger) is destabilizing.
Positive camber is stabilizing. Positive camber
will not "transfer the controllability problem to a higher speed regime."
I stand corrected. How much positive camber should the wheels have at rest?

Larry Severson
Fountain Valley, CA 92708
(714) 968-9852

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