Automotive Aerodynamics can be a booger! As mentioned below, the
effects of the ground are profound. When using a wind tunnel, if the
tunnel doesnot have a moving ground plane that mimics the speed of the
air then the data willbe only trend data. That's because the air under a
vehicle is a major contributor to the overall aerodynamics of the
vehicle. And the wheels and tires need to be turning on the ground plane
at the same speed to effectively produce the effects of wind shears and
flows over the wheels in and around the wheel wells. With a stream
linere this is not as important. The lateral stability of the car also
has another major contributor that hardly anyone seems to mention:
ground vehicles have tire contact patches which primarily govern the yaw
control, at least up to the point where the turning moment forces
overcome the frinction forces between the tire and surface. The rear
wheels, in a nominal rear wheel driven vehicle provide little in the way
of lateral stability until the moment of upset and then they provide a
significant amout of yaw control. They simply want to roll straight
without hinderance and the skid forces can be great. Multiply those
forces by the moment arm to the CG and the resisting moment is fairly
lagre. Up until the friction is broken. Then the forces drop
drastically. The front wheels are turnable and provide a greater amount
of yaw control than the rear. Increasing the caster angle also helps the
wheel to keep pointed in the forward direction. I experienced this last
October when I went for a pendoulous slide for what seemed like hours.
During that wild ride, and since I was just p[retty much a passenger, I
watched what was going on. The steering wheel was really trying very
hard to make the car go in the later axis direction, I have
approximately 16 degrees of caster in the front of my car. it is really
a bear to push around the shop but on the straight line surface, it
works for me! A lot has been said about the polar moment with regards to
the vehicle. A high polar moment of inertia resists the yawing
effectbecause of the inertia, lol. Conversely when the yaw is out of
control, it is also hard to stoip the spin or yaw until other forces
come into play. A small polar moment of inertia makes teh car responsive
to yaw changes such as steerin around a road course. MAkes it easy to
change directions and makes the road grip needed to change those
directions less.
I suspect thare are a numbe rof you on the list who are fully capable of
making a dynamics case study of the yaw of a typical vehicle when all of
the forces are involved. If you would do so, I would be happy to post
the general solution(s) on my web site with full attribution tot he
author or authors. I think it woul dbe great to have some benchmarks
which provide guidance to the desing of a vehicle for the salt given all
the factors involved. Unfortunately, while I think I coul dmake some
preliminary analyses, I have 3 cars going at themoment, a large
presentation to be made to the Sunbeam folk, a large family reunion to
go to in Tex, and some major dental activity all in the n ext few
months. So I am booked up righ tnow. But I will take a peek at the
problem and see if I can develop the equations of motion.
One of the tools which coul dhelp is the use of CFD. However, CFD is
pretty expensive and each vehicle case would require a different model.
That's probably all right for the unlimited speed folk who have lots of
backers and can pay, but not for the rest of us, I fear.
In any cae, I am off to get another cuppa and to fully open my eyes...
great topic for LSR!
mayf
Greg Meyers wrote:
>Actually, I believe that the Wright brothers were making small models and
>affixing them to the end of beams which were jutting forward from moving
>vehicles (bicycles?...). One of the problems though, will be the the fact
>that with ground based vehicles, there is always the effect of the ground on
>one side and sky on the other. The MPG group has been discussing this
>thread....
>Greg
>http://salt2salt.com
>
>
>>I wonder if some of the scale model body shapes available like to the RC
>>model builders are big enough to test in a small wind tunnel. I even wonder
>>if this could be a sophisticated "garage project?
>>
>>
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