A Ramjet effect on drone

[oz93666]

TL;DR Summary

Ramjet

I am following someone on Youtube who has built a rocket shaped (4 proprllers) drone for the world speed record ...
He was having problems with internal heat from electical control gear and battery inside the sealed rocket (about 1kw waste heat)
Speed is 500km/hr , I suggested a 4mm hole in the tip of the nose and 8mm hole at rear which should alow sufficient air cooling ..
I said this will increase thrust because the air exiting is hotter .. a bit similar to a ram jet ... was I correct about the aditional thrust?

 

[Lnewqban]

Please, see:

 

[Baluncore]

At 500 kph, the ram air pressure will be about 1.7 psi.
The rocket envelope will need to handle that internal pressure, along the sides and at the rear.

 

[sophiecentaur]

TL;DR Summary: Ramjet

I suggested a 4mm hole in the tip of the nose and 8mm hole at rear which should alow sufficient air cooling ..

I couldn't say whether the idea of a 'ramjet' would apply; it's more complicated than that. Locating two places on the craft which would have high and low pressure would cause internal circulation.

Maybe, if the front and rear orifices were shaped like the basic ramjet design, you could expect ramjet mode but afaik, the ramjet needs to do work, just to cause sufficient air supply. When added, the fuel produces enough for hit and a lot more (propulsion). 500km/h is far short of the supersonic speeds at which ramjets become efficient.

Aerodynamics is not one of those subjects where just intuition works. This may be an example of where it doesn't. All the aerodynamic ducks have to be in a row. (That produced an interesting mental image when I had written it.)

 

[oz93666]

At 500 kph, the ram air pressure will be about 1.7 psi.
The rocket envelope will need to handle that internal pressure, along the sides and at the rear.

Oh ... interesting , 1.7psi... but there is a outlet hole in the rear ...
Trying to figure this out... imagine the rocket shaped plane , 30cm long, with a small entry hole and a larger exit hole , air is heated inside rocket , exit velocity is greater due to expanded heated air ... wouldn't this produce thrust?

 

[jrmichler]

I said this will increase thrust because the air exiting is hotter .. a bit similar to a ram jet ... was I correct about the aditional thrust?

Yes, this is a well known effect. Search terms p51 mustang radiator thrust will bring up several good hits. The Wikipedia article is a good place to start, and suggests that Meredith effect is an even better search term.

 

[Baluncore]

I would expect the efficiency of both cooling and thrust development to be proportional to the temperature difference between the inlet air and the temperature of the electronics.

The length of the internal ducting needs to be minimised to reduce the internal boundary layer drag. That may be a challenge on a long body.

The system will need to be optimised for the altitude and air temperature at which the speed record test will be made. What will that be?

 

[oz93666]

The system will need to be optimised for the altitude and air temperature at which the speed record test will be made. What will that be?

Here's the video for those interested ....

 

[ShadowKraz]

It seems as though the thrust produced would be very minimal, probably not enough to overcome the drag the holes would produce. Or the drag the components needing cooling would produce internally.