Airscraft Propulsion by Compressed Air
Airscraft Propulsion by Compressed Air
(OP)
I'm sure this is a long-sorted issue, but was just wondering about relative efficiency issues. What would be the relative power efficiency of using eg. 3 barg compressed air as propulsion? Assuming of course that
a) The wieght of the compressor machinery would be a non-issue.
b) A venturi eductor system could be applied to convert airstream velocity into volume.
c) Minimal cooling of the compressor output before application to the propulsor jet.
Am I missing any law of physics which makes this a lot less efficient per unit input power than a propellor?
a) The wieght of the compressor machinery would be a non-issue.
b) A venturi eductor system could be applied to convert airstream velocity into volume.
c) Minimal cooling of the compressor output before application to the propulsor jet.
Am I missing any law of physics which makes this a lot less efficient per unit input power than a propellor?
Pechez les vaches.





RE: Airscraft Propulsion by Compressed Air
RE: Airscraft Propulsion by Compressed Air
..and then light the petrol/air mix, put some sort of rotating machinery behind it to drive a propeller...
for my own education, what does
"3 barg compressed air" mean?
RE: Airscraft Propulsion by Compressed Air
RE: Airscraft Propulsion by Compressed Air
RE: Airscraft Propulsion by Compressed Air
No objection?
Pechez les vaches.
RE: Airscraft Propulsion by Compressed Air
RE: Airscraft Propulsion by Compressed Air
RE: Airscraft Propulsion by Compressed Air
Pechez les vaches.
RE: Airscraft Propulsion by Compressed Air
RE: Airscraft Propulsion by Compressed Air
Pechez les vaches.
RE: Airscraft Propulsion by Compressed Air
When you say efficiency, what is the reference source? The power to pump the tank up to a designated pressure?
RE: Airscraft Propulsion by Compressed Air
consider a jet engine. the compressor ahead of the combustor compresses the air alot (i forget the pressure ratio of a typical compressor, maybe 8 ?) but the major source of thrust of a jet engine is the huge increase in pressure due to explosive expansion (of the fuel/air mixture) within the combustor ... we'd rather it wasn't accompanied by a huge temperature increase, but you can't always get what you want. Mixing by-pass air was a later innovation (over the earlier turbo-jet designs) which works (i think) mostly by increasing the mass momentum flow, increasing the diameter of the jet eflux, and so reducing the losses as the jet interacts with the surrounding air.
RE: Airscraft Propulsion by Compressed Air
Think about a SCUBA tank.
How big and relatively heavy is it?
How big would it need to be to get worthwhile thrust & range/endurance in an Aircraft with a reasonable payload?
Even using more advanced materials how heavy would it need to be?
What effect on drag would this size & mass (& hence lift) have?
Given that the performance of a car running on compressed air is limited I doubt we'll see a compressed air aircraft of any size any time soon.
That said there are various ways of using this in toy/model planes. I tried it as a kid with a balloon attached to a little plane hung from a long horizontal string. It kind of worked but once I tried putting it in a better shape A/C it wasn't so successful.
RE: Airscraft Propulsion by Compressed Air
Pechez les vaches.
RE: Airscraft Propulsion by Compressed Air
If you're talking about carrying around a completely separate system I'd suspect there'd be mass penalties.
RE: Airscraft Propulsion by Compressed Air
at the end of this, I'd be very surprised if this was practical, particularly with an "industrial" compressor, maybe (just maybe) with a specialised light weight compressor.
what are the "other requirements for the compressed air aside from propulsion" ? is your design using compressed air for other things ? this'll require a bleed system (rather than a simple exhaust) which will probably induce sizeable losses in the propulsive efficiency
RE: Airscraft Propulsion by Compressed Air
Mike Halloran
Pembroke Pines, FL, USA
RE: Airscraft Propulsion by Compressed Air
If you don't want to bleed the compressed air off of the main propulsion you may want to look at a gas turbine APU. At take off or other times you need thrust this could perhaps be used to add thrust (I recall seeing this proposed for an airliner, can't remember which). When you aren't using it for thrust you could have it providing compressed air instead.
I seem to recall something like this being proposed for aircraft. The various ancillaries that are run off the main propulsion degrade the efficiency and complicate the installation. By leaving the main engines to just generate thrust and having an 'APU' generating the compressed air & power take offs for electricity generation etc you could possibly end up with a more efficient system overall. I'm not sure how far this has been taken in practice.
RE: Airscraft Propulsion by Compressed Air
Suggest You check-out the little european automobile powered by a compressed-air engine at the following website.
http://www.theaircar.com/
I am still a skeptic... however, I can visualize a high pressure fiber-wound air tank to deliver "air-fuel" the engine; and a drive-shaft with a flange for a prop... to "launch" an ultralight or light-weight glider.
Regards, Wil Taylor
RE: Airscraft Propulsion by Compressed Air
RE: Airscraft Propulsion by Compressed Air
Um, yes.
Thrust produced = mass of air X acceleration of the air through the "engine".
The kinetic energy (which is proportional to the required power input) goes up with with the square of the velocity of the air leaving the engine.
Makes more sense to increase thrust by increasing the mass flow than the velocity.
RE: Airscraft Propulsion by Compressed Air
Bonus question: Why are scuba compressors lubricated with soap, not oil?
Mike Halloran
Pembroke Pines, FL, USA
RE: Airscraft Propulsion by Compressed Air
RE: Airscraft Propulsion by Compressed Air
When the machine is working as an air compressor, the hot air entering the tank also carries a fine oil mist from the lube oil. When the discharge pressure gets serious, so does the temperature.
The high pressure part of the system, the one with the fine oil mist and hot air in it, doesn't know that it's not supposed to be a Diesel engine, and every once in a while gets extra confused, and explodes.
Hydraulic accumulators are precharged with nitrogen, not air, for the same reason.
Mike Halloran
Pembroke Pines, FL, USA
RE: Airscraft Propulsion by Compressed Air
Crazy? Well, a little bit. But my old Aero prof's. (Hertzberg and Bruckman) came up with the idea, built a prototype, and wrote the white paper that explained why the concept could actually be economically feasible. Hint: LN2 is a by-product of oxygen production (e.g. for medical uses), and thus LN2 is a cheap commodity.
htt
Apparently U. Texas at Austin has taken up the lance, and continues to joust this particular windmill... (hey, how many metaphors did I just mix? And, how does it taste?)