MODEL SOLAR CARS AND AERODYNAMIC LIFT
Over the years I have often had the suggestion made to me that significant lift or down
force can be generated aerodynamically. In order to clarify this proposal I decided to take
“a quick look” at this topic and publish the results.
Firstly down force, why would I want it? Yes it will help hold the car down onto the
track around corners and over the hill but. Down force will increase the wheel loadings
and consequently raise rolling friction slowing your car. As very fast cars without any
deliberately generated aerodynamic down force have managed to win races for years,
why do you need it. Design and build your car properly. Remember too that there is
always a drag force generated in conjunction with lift, or down force (down force is just
lift turned upside down) this additional drag will only assist in slowing your car down,
not a particularly good idea.
Secondly lift can reduce wheel loads thus reducing rolling resistance, as well as the most
undesirable effect of increasing the tendency to take off over the hill and disengage
guides when cornering. Sounds like a great idea, gain a slight reduction in rolling
resistance and crash out in the corners or over the hill.
There have even been suggestions that cars could become airborne due to unintentionally
generated lift. So how much lift can we reasonably expect to be generated? I will assume
insanity runs in your family and you have produced a car with a body in the shape of a
wing designed to generate lift instead of the more sensible non lift low drag strut shape. I
will though, credit you with sufficient intelligence to orient your wing for zero angle of
attack. Well how much lift can we expect?
Using data from Marks Standard Handbook for Mechanical Engineers Eighth Edition
section 11 AERONAUTICS with characteristics for a NACA 4415 wing section at a
velocity of 8 m/sec. (ie. a 16 second lap) and a wing of dimensions 300mm by 400mm.
Calculated lift generated at zero angle of attack for this wing.
Lift = Lift coeff x 0.5 x air density(slugs/ft cubed) x velocity squared (ft/sec) x area (ft
sq)
= 0.2 x0.5 x 0.002378 x 25 x 25 x 1.3
= 0.1932 lb. (british units used in calculations as text is from USA)
= 87 gm lift
Re calculated for angle of attack which gives maximum lift coefficient ( ie. 20 deg.) and I
assume only a total lunatic would build a car with a high lift wing section set at a 20 deg
angle of attack if lift force was not intended.
= 1.54 lb lift
= 700 gm lift, still a lot short of taking off as the minimum reasonable
weight of a car powered by a 6 watt panel is 900 gm and for a 12 watt panel is 2100 gm.
This much lift however, is more than enough to cause instability problems
Yes I can hear the cries of: but you ignored Reynolds number and ground effect. Yes I
did I also ignored the fact that any wing you produced would most probably have such a
rough surface and imperfect shape due to the solar cells that performance anywhere near
to that of a standard wing is all but impossible. While we are about it you must also
consider that a 20 deg angle of attack is possible within the 180 mm allowed car height