Quote:
Originally Posted by Old Tele man
Rolling tires have LOTs more friction than an airfoil being pushed through the air.
Trying to push a car on the ground has tire friction to contend with, which an airplane doesn't have as soon as takeoff occurs.
Hang the car in the air by a couple weather ballons filled with helium and THEN maybe the fan idea has possibilities (but economy ain't one of them).
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What?
I think you over estimate the rolling friction, and/or you completely ignore lift drag. Keep in mind as well, a small airplane constantly produces 120-140 HP to stay aloft, unlike the 20-30HP or so required for an economy car steady state. Granted about 1/3rd of the energy sent to the wheels goes to overcoming rolling friction, I think this is still a smaller fraction than the lift drag created by the wing. Discussion on this really has nothing to do with the OPs original idea in the end however.
Anyway, where the OP runs afoul with real world physics is in the fact that it does not matter how the thrust is produced, the energy in vs. energy out equation remains unchanged. What we're all looking for here is a way to convert gasoline into thrust as efficiently as possible, and when the thrust is pushing our vehicles, we want them to go as fast as possible for the given amount of force applied.
His assumption is that somehow a fan is better at converting gasoline into thrust, if this were the case, we'd all have propeller cars. There's no magic in a propeller, a fan would simply not be more efficient at converting gasoline into usable thrust than turning the tires with a drive shaft.
Baseline physics then tells us, if the fan is less efficient, it will not yield an increase in overall fuel economy no matter what fudge factor numbers you want to throw at it, it is dead from the start. No need to get into any details other than that, it would be like having a discussion regarding the orbital dynamics of Pluto as justification for why it would or wouldn't work.
