Re: Just Another Thought on Wheel Camber
David J. Gall
Larry,toggle quoted messageShow quoted text
Thrust is destabilizing. Camber change due to canard "bowing" under
aerodynamic lift at 1 gee is approximately 40% of camber change due to canard
bowing due to airplane weight fully supported by the wheels alone.
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."
The problem with negative camber is that it responds to small roll
perturbations by pushing the airplane to turn in a direction opposite to the
roll perturbation. This "feeds back" with an amplification of the initial roll
perturbation (due to the height of the CG above the ground), making the
turning force increase, making even more roll, making the turn harder, etc.
etc. etc. This is static and dynamic instability: divergence.
Without going into further detail, this situation first manifests itself at some "critical
speed" and then gets worse the faster you go. You can rudder dance your way through it to liftoff speed, or, perhaps, you can't. You might be able to rudder dance your way through the deceleration after landing, too. Or, perhaps, not.
Positive camber responds to a small roll perturbation with a turn in the same
direction as the perturbation. This "feeds back" as an attenuation of the
initial roll perturbation, thereby lessening the severity of the turn force.
This is static stability. It's not perfect, but it makes the rudder dance a little less exciting.
If you want perfect directional stability, use a locking tailwheel and a laterally stiff tailspring. Or lock your ankles stiffly and use finger brakes. Or build a tri-gear.
With static stability, you have a much better chance of retaining
controllability. Please note that controllability is a completely separate topic. Yes, stability influences controllability. A more unstable craft will respond more aggressively to smaller control inputs. Or, instability can masquerade as a lack of control authority by responding opposite to control inputs. This is what happens with the Q2/200 tailwheel (in my opinion). Yes, control (as in active control, the stuff of brain matter and advanced guidance computers) can "overcome" instability by brute force and quick reflexes, as in the Jim-Bob six pack without the "Gall alignment." Or you can sacrifice some controllability and use a locking tailwheel for directional stability during takeoff and landing.
Do whatever you want. I have it on good authority that the alignment works. The Jim-Bob six pack also works. They work well together. If you're building a Quickie or a Q2 or a Q200, I would consider it MANDATORY that you incorporate all of the six-pack mods, except perhaps the reflexor might not be needed on the Quickie. I would also argue that the alignment should be adjusted, but some people have found success without doing that. Others have found success doing ONLY that. You decide. You could build a larger rudder if you want to enter that debate.
But whatever you do, please know that:
1) somebody else has been there and done that and found out the hard way so you don't have to (and published the easy way for your benefit).
2) Your analysis is erroneous.
David J. Gall
(Now, back to working on the airplane....)
----- Original Message -----
From: "larry severson" <larry2@...>
Subject: Re: [Q-LIST] Re: Just Another Thought on Wheel Camber
Date: Sun, 26 Jun 2005 09:48:54 -0700