Possibility of air drag reduction by total-surface-based propulsive mechanisms?

[jjalexand]

If a vehicle could continuously cool the air in front of it and heat the air behind it, the air in front should contract and the air behind expand, reducing the pressure in front of the vehilce and increasing it behind the vehicle. This should pull the vehicle into the cooler air at the front, and push the vehicle away from the hot air at the back, propelling the vehicle through the atmosphere without moving parts or any additional propulsion. (Let's assume for the moment a helium balloon type object, that just floats at a certain height, for simplicity).

Another way: if a similar blimp-like vehicle could somehow draw air over its surface from front to back, using some inherent powered attributes of the (obviously special) surface, it should also be pulled through the air without any additional propulsion.

It seems to me that these mechanisms might tend to overcome wind resistance, by using the propulsive energy (that might otherwise be used to drive say a front propellor) to directly move the air over the surface of the vehicle instead, applying the propulsive force at the very point where resistance develops, in an attempt to overcome or eliminate it.

For example, let's say for example, that a film of oil could be moved over the surface continuously from front to back by some applied energy, then returned to the front of the device by being pumped through a tube from back to front. Or perhaps moving belts could more or less cover the surface of the vehicle (all traveling backwards, and somehow return in a loop to the front internally. Any friction between the external air and the oil or belts would tend to flow the air backwards and thus draw the vehicle forwards, apparently with reduced air drag over a more conventional method of propulsion.

Any comments, thoughts, etc on whether the same amount of energy applied in this manner would result in much reduced frictional air drag or not?

Since air drag is proportional to v squared for macroscopic objects at normal speeds, it maybe could save large amounts of energy if we could overcome this problem. Alternatively, we might be able to go faster for the same energy consumption :) This would apply to cars, planes, etc, not just blimps.

 

[russ_watters]

Sounds good in theory, but the actual application could be difficult and may not be beneficial. Off the top of my head, you'd probably want a refrigerant loop, that way you can heat one side and cool the other without having any impact on the lifting ability of the envelope. The only energy expended is in circulating the fluid. The catch of course is this system adds a lot of weight.

A slightly different concept is jet engines like on the http://www.fas.org/irp/mystery/nasp.htm The fuselage is shaped to act as both an intake and exhaust, using the pressure drag of the airframe to compress the air for the intake and generate thrust from the exhaust. Coolant (fuel) circulated through the forward body of the aircraft would increase the compression efficiency (not much), reduce skin temperatures as so not to melt the airframe (a lot), and increase the combustion efficiency of the fuel (a little).

 

[jjalexand]

Reply to Russ Watters

Hi Russ,

The example you mention is certainly relevant.

I'm sorry if my post was not all that clear. I'm not really proposing the cool/hot system as a practical device, merely using it in an aid to establishing the possible validity of the general concept, although it may not have been that relevant.

What I'm saying is that any method that either implements or simulates the effect of moving the vehicle skin backwards would pull the vehicle forwards due to friction, i.e. in a sense using drag as a form of propulsion rather than as a nuisance. (if you can't beat em, join em!)

Total elimination of drag of course requires that the velocities of skin movement in the reverse direction exactly match the natural airflow velocities over the surface at various points that would be present if the vehicle was propelled by more conventional means at the same speed. (ie match the velocity vectors over the surface)

Of course, it is hard to see how to achieve this effect in reality. How can we have a vehicle skin that constantly moves backwards? Isn't that impossible or at least totally infeasible?

But there may be methods.

For example imagine a flexible skin with moving waves generated over it via some sub-surface transducers.

By synthesizing chosen sine waves at various points on the surface using appropriate transducers, desired moving waves may be able to be generated via a process similar to Fourier analysis (where any wave can be represented as the sum of a set of sine waves) but in the opposite direction (the desired wave is created from the sum of a calculated set of sine waves).

These moving waves could perhaps be shaped to maximize the pull of air along with them, causing the air (or other ambient fluid, eg water) to be pulled over the surface at the required velocities.

The speed, shape and direction of the waves could be dynamically adjusted over the surface by intelligent homeostatic mechanisms in real-time as vehicle speed increased.

Braking could perhaps also be achieved by reversing the direction of the waves.

 

[wimms]

Energy spent on mechanical action of "moving skin" to compensate for drag will most probably exceed energy spent on overcoming drag oldfasioned way. And it too will have squared relationship with v.

Of radical methods to reduce drag, I recall reading of ideas that destroy air in close proximity to body. If you can create vacuum tunnel that opens before you enter and closes after you pass, you avoid drag. Then only energy needed is to create the "space slot" sufficiently fast.

Then, they've got mach-1 torpedos now.

But idea of using drag in propulsion is nice.
Term vortex generator comes to mind. http://www.amasci.com/amateur/vortgen.html

 

[jjalexand]

Reply to Wimms

Thanks for the vortex gun link, it's really cool, and might offer an new and interesting form of propulsion, especially for helium balloons.

Also, Russ Watters link to the X30 scramjet was interesting, they had an idea related to the general concept I was suggesting.

They wanted to use the heat from drag to preheat the hydrogen fuel, to provide extra energy. In theory this might have enabled them to achieve much higher mach numbers. The brilliant thing about the idea was that the faster they went, the more heat energy they could feed back into the fuel, so to some extent it offered a way around v-squared non-linear relationship of drag to velocity.

 

[drag]

Greetings !

Originally posted by jjalexand
If a vehicle could continuously cool the air in front of it and heat the air behind it, the air in front should contract and the air behind expand, reducing the pressure in front of the vehilce and increasing it behind the vehicle. This should pull the vehicle into the cooler air at the front, and push the vehicle away from the hot air at the back, propelling the vehicle through the atmosphere without moving parts or any additional propulsion. (Let's assume for the moment a helium balloon type object, that just floats at a certain height, for simplicity).

Such an aircraft would not be able to achive high
velocities and would use up lots of energy.
Basicly, a hot air baloon or an airship do the
same thing, but the airship requires no power source
and the hot air baloon is more efficient than something
like the above is likely to be.

As for the skin idea, I really don't have a clue about anything
like that. I do know that there were and are experiments that
attempt the use of a pimpled - kin'na like a golf ball's, surfaces
that, supposedly, reduce drag. I'm not sure though weather its
applicable for all velocities - subsonic and supersonic and how
long it'll take for us to see flying golf balls, but it
seems promising.

Live long and prosper.