Glenn, John, Sparky:
In landspeed, the stability we are looking for is in the direction that the
airplane guys call "yaw". That is-- if we imagine a VERTICAL axis through
the approximate center of the vehicle-- does the vehicle have a tendency to
point forward, or to turn about the axis?
I agree with you about the idea that increasing the mass-moment-of-inertia
adds to stability/ resists any tendency to turn about a given axis; it's
sophomore engineering
dynamics stuff. I don't think there is legitimate argument about that
particular point.
The REAL problem is when we try to apply that to the other compromises we
have to make to race landspeed-- and you and John seem to be conflicting
over one of those "compromise" problems.
If you get enough horsepower, you start needing to "load" the drivewheels in
order to achieve sufficient traction. Most landspeed vehicles are
rear-wheel drive. If load the wheels by adding mass to the rear-end, you
begin to move the center-of-mass aft of the center of pressure-- which seems
to be the biggest "no-no" in landspeed stability. The vehicle wants to
turn-around, and drive tail-forward. The larger the rear mass-moment (the
longer the rear moment-arm, for example) the worse the problem becomes
(which supports John's reasoning for not putting ballast behind the rear
axle).
If the vehicle has enough weight in the front to move the center of mass
well-ahead of the center of pressure (say-- for example-- the vehicle has a
60/40 front/rear weight bias, and a big fin in back)-- then I think the idea
of having a large percentage of the masses OUTSIDE the axles (both front and
rear) is much more interesting, from a stability standpoint. The front
weight bias w/ rear fin would remove the tendency of the vehicle to want to
turn-around and run backward, and the large polar moments would resist any
unstabilizing effect from cross-winds or one-sided ruts or bumps.
(I think the argument that it would be harder to "catch" a high mass-moment
vehicle has merit, but I think not getting into the spin in the first place
is more important. For you fliers-- you want to be able to "catch" a high
roll-rate airplane like a P-51-- but you probably won't roll a B-29 unless
you loose a wing, so why worry about "catching" it--???)
Unfortunately, a 60/40 weight bias design might put an lsr vehicle into
traction problems, if we're talking about more than 400hp (or so) per rear
wheel. You'd have to keep adding weight to each end-- maybe to the point
where you didn't have sufficient acceleration (a la Al Teague).
One solution would be to divide the power between all 4 wheels, so that ALL
the vehicle's mass would contribute to traction (like White, Burkland,
etc.).
Another would be to go maybe 75/25 front/rear bias (REALLY stable, like a
dart), and use FWD. Of course, FWD has the benefit of natural understeer--
adding to straight-line stability.
Somebody should do that! (without the big frontal area that a cross-wise
engine requires).
Russ, #1226B
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