Pressure thrust?

[winkosmosis]

Originally Posted by metromizer

Several Bonneville salt flats racers have toyed with the idea of racing a Posche 928 in one of the factory production classes...

but running it with the body on backward!

What would that accomplish?

[Christ]

The rear windshield is a complimentary angle to the roofline, making a smooth entrance to flow, and the "front" (thinking in correct orientation) appears to drop off, in such a manner that appears like the illustration on the webpage.

That's the only thing I can think of... maybe they're inside-joking about the pressure thrust capability?

[theunchosen]

. . .all well and good. . .but where did a lab that postulates a design that can decrease drag. . .who also use it on the front of the vehicle in question. . .get the idea. No way anyone who designs a front end that looks like that developed this idea. If anything they discovered it on a freak accident.

Look at all the blue on the front! Its the same shape.

If they developed it as theory it would look alot more like. . . a teardrop shape. . .because thats exactly what it does. Rounded more Aero nose, lid and sides and I would have believed they developed it.

[Christ]

I don't wish to debate who developed the idea, but it seems like the theory is at least somewhat sound.

It almost seems like a principle of overunity, though, which I believe is hurting the credibility of such an endeavor.

[winkosmosis]

Originally Posted by theunchosen

. . .all well and good. . .but where did a lab that postulates a design that can decrease drag. . .who also use it on the front of the vehicle in question. . .get the idea. No way anyone who designs a front end that looks like that developed this idea. If anything they discovered it on a freak accident.

Look at all the blue on the front! Its the same shape.

If they developed it as theory it would look alot more like. . . a teardrop shape. . .because thats exactly what it does. Rounded more Aero nose, lid and sides and I would have believed they developed it.

It's a parabolic nose

[theunchosen]

Unfortunately I don't want to drive to campus to get access to cad with wind tunnels. . .or I would just test it in about 45 seconds. . .

It does not make alot of sense how material can magically reach higher pressure in a half diffuser without additional BTUs. . . its not like the air cannot travel outside the other imaginary half of the diffuser. . .

[theunchosen]

I can't tell from the drawing. . .I kept clicking trying to rotate it and realized it was a static image. . .not a sketch. . .

lol I know they are probably not the same but if you were using parabolic to describe the front as different I am very quickly going to point out that the rear plotted with the z axis as up is the same graph as y = x^.5. . .or x=y^2. Parabolic.

My point was that the nose could be much more Aero friendly than what they put up, think rain drop.

[donee]

Originally Posted by some_other_dave

My grandpa once told me that a properly-designed full engine cowling on a radial-engined airplane actually pulled the airplane forward a bit. Sounds like the same sort of idea.

(Boy do I miss him!)

-soD

I believe this is because of the heat from the engine is put into the flow as it goes through the engine bay. Similar to what was done with the P51 radiator.

The modern aircraft ideas are also using an engine to feed an energized flow through a gap between the main body and the tail cone.

[Christ]

So your consensus is that with a more aero-friendly nose, the positive pressure generated by the rear slope would have a higher differentialcompared to the initial positive pressure environ that the nose faces, and cause more forward energy to be exhibited?

Or, more simply put, if the face were more aerodynamic, the rear wouldn't have to decelerate the flow as much to exhibit the same principle?

[DaveBirkenstock]

Hi everyone... it's great to see a spirited discussion of this phenomenon, thank you for the interest.

The one thing not clearly identified on that blimp image is the suction inlet, just upstream of the concave tailcone. By adding energy in the form of suction, the mechanism can generate what Fabio Goldschmied called "fuselage self-propulsion."

Goldschmied proved a significant reduction in total power used for streamlined bodies (his baseline body was the rigid blimp, USS Akron) by exploiting pressure thrust.

As you know, separated airflow drag over the rear of a car makes things much worse than a streamlined blimp shape, but you might be interested in the computed drag coefficients from a CFD run I had done:



The vehicle was a generic sedan and the Cd numbers are as shown. The geometry was not best-case though, the intake area was too small and the concave stagnation zone is too big. When the whole car is optimized, the Cd number should go below 0.1