Airscraft Propulsion by Compressed Air

[lengould]

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?

Pechez les vaches.

 

[KENAT]

I've been led to believe that the performance for the air car may have been exagerated. Even if not, given the limited performance in the car how much more limited would the performance be in an aircraft, where mass and volume are much bigger considerations.

 

[MintJulep]

Am I missing any law of physics which makes this a lot less efficient per unit input power than a propellor?

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.

 

[MikeHalloran]

Riiiight. The vegetable oil- lubricated expander/ compressor/ motor/ generator/ allthingstoallpeoplemechanism compresses air to store in the aerospacetechnology fiber- wound pressure bottle for later expansion.


Bonus question: Why are scuba compressors lubricated with soap, not oil?



Mike Halloran
Pembroke Pines, FL, USA

 

[MintJulep]

That would be because the lube may get hot enough to partially combust, resulting in CO, which is a bad thing to breath.

 

[MikeHalloran]

.... close, but no cigar for you today.

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

 

[btrueblood]

Hm. Much better idea: use liquid nitrogen fed thru an atmospheric-temp. heat exchanger, then feed the boiled LN2 vapor into an expansion engine.

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.

http://www.aa.washington.edu/AERP/CRYOCAR/CryoCar.htm

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?)