Since some time has elapsed from when Mayf first posted his thoughts on
Freiburger's aerodynamic side bar, I thought I would include what Mayf wrote
as a refresher: "Scanning the latest Hot Rod rag, I come across a small side
bar in which David, our boy David Freiburger, attempts to define the
aerodynamic center of pressure. As I read it, he has the Cp ahead of the Cg
and the problem Cp causes is lift on the front end resulting in squirmish
handling."
I read that and thought a line to Ron Ayers, the fellow who designed the
ThrustSSC after retiring from a career in British aerospace might have a few
comments. Sure enough, he did and here is what he said (no snotty comments
about the funny spelling please) -- LandSpeed Louise
FROM RON AYERS
As I do not have the offending article in front of me I will assume that you
are quoting accurately from it. You quote him as using the term 'aerodynamic
centre of pressure'. If he did, then he really is confused, as he is mixing
up two separate terms, namely: aerodynamic centre, and centre of pressure.
These are completely different.
It is genuinely difficult to illustrate these terms in the case of a car as
there are so many non aerodynamic forces involved (and this no doubt
contributes to the confusion of you editor friend). I will use the example
of an aircraft to illustrate the distinction.
Centre of pressure
The centre of pressure is defined as the point at which the sum of all the
pressures on a body is assumed to act. Thus, for an aircraft flying straight
and trimmed in horizontal flight, the centre of pressure coincides with the
centre of gravity, and the magnitude of the aerodynamic force would be equal
and opposite to the force of gravity. This must clearly be true, because if
these two forces did not cancel out the aircraft would not be trimmed.
Aerodynamic centre
The aerodynamic centre is the point through which the resultant lift
increment acts when there is a small change in angle of attack of the
aircraft from its value in steady, straight and level flight. Thus, if the
aircraft is stable, the increment of lift resulting from a small increase in
angle of attack will act behind the centre of gravity, thus encouraging the
angle of attack to return towards its trimmed value. The distance between
the aerodynamic centre and the centre of gravity is called the static
margin. At first sight, you may think that you want as large a static margin
as possible. In practice, too much stability will make maneuvering more
difficult, so a compromise is needed.
The centre of pressure is separated from the aerodynamic centre because the
aircraft is not symmetric about the horizontal plain. Thus, the position of
the centre of pressure is influenced by such things as the wing camber and
the setting of the tailplane angle.
With a car, the picture gets very unclear as vertical forces are reacted by
wheel loads, and horizontal forces are reacted by friction and rolling
resistance. However, if you wish to increase stability or traction by
increasing aerodynamic download, it is probably correct to use the term
centre of pressure when defining the point of application of this download.
For yaw stability, at first sight you want the aerodynamic centre in yaw to
be behind the centre of gravity (i.e. give you a positive yaw static
margin). However, this can only be guaranteed if the coefficient of friction
for all four wheels is the same. In practice the drive wheels may be near
limiting friction due to drive forces, so some more complex stability
criterion would be needed. Despite that, the simple conclusion is still
true, that a nice big fin at the back will give weathercock stability, so do
not get too knotted up with the technical details unless you are trying to
do something really clever. Just to complete the terminological confusion,
since cars (and aeroplanes) are symmetric in the left/right direction, the
yaw centre of pressure and the yaw aerodynamic centre are coincident.
Incidentally, I broke all the rules on ThrustSSC, as the aerodynamic centre
in pitch was a long way in front of the centre of gravity. Indeed, it was as
near as I could get it to the front suspension. This is because the active
suspension worked on the principle that when the rear end was raised, the
resulting increment of aerodynamic download would be most beneficial (in
stopping the nose from lifting) if applied to the front suspension. At first
sight you may think that, to achieve this end, the download should be
applied as far forward as possible. However, if the download was applied in
front of the front wheels, there would have been the unfavourable side
effect of unweighting the rear wheels.
Cars are nasty complex things. That is why I have only designed one.
Coincidentally it happens to look like a very low flying aeroplane.
Feel free to use some or all of this blurb in the magazine. In payment, the
editor can buy me a beer if we ever meet.
Best wishes
Ron
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