[Land-speed] [BULK]Re: Polar Moment

drmayf drmayf at mayfco.com
Wed Jan 16 09:18:58 MST 2008


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?  
>>    
>>
>_______________________________________________
>Support Team.Net  http://www.team.net/donate.html
>
>Land-speed mailing list
>
>You are subscribed as drmayf at mayfco.com
>
>http://autox.team.net/mailman/listinfo/land-speed


More information about the Land-speed mailing list