Well it was more to start a debate about whether lift is in itself bad. I don't believe it is.
If 10kg of lift is going to upset a car, then driving with very little fuel will have the same effect. The issue there is not lift, it is something else.
But I really want to discuss about the practical possibilities when lift could be used to reduce rolling resistance. Not all the applications where it can't, because I know there are many applications where it can't.
I am assuming road tyres, for a simple sort of "practical" solar car. Road tyres have a rolling resistance of 0.0008-0.0015
For example, a solar car that uses a symmetrical aerofoil (E475) with no angle of attack as the main body/panel carrier.
These aerofoils would have at zero alpha, a drag coefficient of 0.01, however changing this to a lifting aerofoil would produce increase the drag coefficient to 0.016, however the lift coefficient would be 1. This gives 166 times the increase in lift, for the increase in drag.
Aerofoils can and do go into negative lift, the NACA 2415 that I am referencing has neutral lift at 2 degrees, and negative lift at anything below that. A car with trailing arm suspension, as is typical on a solar car could lift the back wheels up and thus decrease the lift generated.
We can calculate the lift/drag, of tyres. For every Newton pushing down on the car tyre, there is between 0.008 and 0.015 Newtons pushing backwards. This gives a lift/drag of between 125 and 66. In the example provided above, we get a lift/additional drag of 166. This strongly suggests that lifting the car would reduce the rolling resistance by more than the extra drag created.
However stability would be an issue and a rudder to move the centre of pressure back would probably be necessary, however proportionally this rudder would have a generous 1/20 frontal area of the wing and because it is a symmetrical aerofoil would have half the Cd, this gives a total additional drag to lift ratio of 162/1.
However, all this overlooks a few assumptions, one is that the rolling resistance is 0.008-0.0015, but any team or company that would put this amount of work into making a car as odd as this would have access to the unobtainable ultra low rolling resistance tyres. It also ignores the ground effect, which may invalidate much of the lift/drag figures.
In response to the technical literature about lift, yes I have read many books and papers, and they all pretty much say that lift on a car is generally a bad thing, and of course I agree. However, as aerohead wrote earlier, imbalance of lift is the issue.
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If the uplift between the front and rear of the car is different, then the slip-angles generated by the front and rear tyres will not be equal; accordingly this will result in an under-or over-steer tendency instead of more neutral-steer characteristics. Thus uncontrolled lift will reduce the vehicle's road holding and may cause steering instability
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But the fact is, that lift itself is not necessarily an issue providing it is balanced between front and rear, such that the load on the tyres does not change balance front/rear, planes do not crash on takeoff, ekranoplans and hovercraft do not crash when lift gets too high. However, on cars, as Julian explains, which are not designed to take advantage of this and have uneven F/R lift coefficients relative to the weight distribution then that is where the problem begins.
Lift on the cars Julian has driven, maybe that is an issue, but I still say that lift itself is not an issue for stability unless the vehicle is not designed to lift. Adding lift and equal distribution of lift is even desirable according to some
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In an ideal development
process, shape changes which reduce drag would also result in desirable overall lift and
distribution of lift, but this rarely occurs in practice and compromises will be required. https://www.mdpi.com/2311-5521/6/1/44
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Again, go read the research. In fact, it as a solar car that I was referring to above that had handling control problems with 14 per cent rear lift. Do you honestly believe that the solar teams have overlooked something like you suggest? Given that many of these cars have aerodynamic lift, it would have been obvious in their testing if total resistance fell with speed - especially if there were no downsides.
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No, you are right, they haven't overlooked what I suggest, in fact they implement it, not all the time but they do.
Ah, yes the honda dream, I have read that paper, the concern was the lift in a crosswind and subsequent instability, and However, this all comes back to the issue and probably real reason why this isn't implemented, is that solar cars have access to ridiculously low lift/drag (Crr) tyres, so there is no way that lifting the body would reduce total drag. At least that is what I thought until I reread the papers
The honda dream was pitched up 0.4 degrees to reduce drag during the race, and the sunswift iv was pitched to generate 10kg lift to decrease drag.
However, I will accept that at high speeds the stability of both of these were an issue, the honda dream had to be pitched down 0.4 degrees to remain completely stable at high speed at the expense of rolling resistance, same with the sunswift iv.
Although it isn't entirely clear whether the downwards pitch increased drag through change in angle of attack or reduced lift. So I may be wrong here.
sources:
https://core.ac.uk/download/pdf/32425613.pdf
https://doi.org/10.1016/S0389-4304(98)00019-8
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