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.

 

[rb1957]

why not chuck a little gas (petrol) into the compressed air ?

 

[prost]

continuing with rb1957's line of thought..

..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?

 

[btrueblood]

Mmm. Close prost, but he wants to "replace the propellor" with a compressor (one made of unobtanium, since the weight is "a non-issue"). Maybe an axial-flow, multi-stage compressor, to keep the efficiency high.

 

[Kwan]

Now you can use the rotating machinery to drive the axial compressor. Then you have something!!!!!

 

[lengould]

So do I take it that eg. 1000 shaft kw into an air compressor operating at eg. 50 psi, resulting airstream directed into a venturi volume multiplier will produce propulsive effect just as efficiently as 1000 shaft kw driving a propellor?

No objection?

Pechez les vaches.

 

[MiketheEngineer]

Check out the kids toys made by Air-Hog (I think). They use compressed air to drive a small piston motor that drives the propeller. Works pretty good!! Watch your fingers!!

 

[rb1957]

no, i believe you've missed the sarcasm included with the responses

 

[lengould]

Actually i haven't missed the sarcasm, or the absence of creativity. Thanks anyway.

Pechez les vaches.

 

[rb1957]

yes, the bright spark of creativity is always challenged by the fire extinguisher of experience

 

[lengould]

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

Pechez les vaches.

 

[prost]

You might want to work backwards--what kind of thrust do you need? How big of a tank of compressed air would you need to deliver this amount of thrust for say a minute? And how high must be the pressure?

When you say efficiency, what is the reference source? The power to pump the tank up to a designated pressure?

 

[rb1957]

i thought the OP's idea was more along the lines of a water jet, that he wasn't storing the compressed air, just inhaling atmospheric air and expelling a high pressure jet (mixed with low pressure by-pass 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.

 

[KENAT]

I assume you're talking about compressing the air in a completely separate operation on the ground and then just filling a tank on the aircraft with the compressed air which then exhausts through a nozzle and produces thrust.

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.

 

[lengould]

The application is actually a (relatively) low-speed (max. 150 kmhr) air-propelled ground-effect vehicle using low-pressure industial compressor powerplant, primarily because of other requirements for the compressed air aside from propulsion. My question relates to "eg. if I already have 33% of required propulsion kw onboard as a highly efficient air compressor, can I reduce my installed standard propulsion unit kw and employ the compressors for additional propulsion, with any hope of efficiency?

Pechez les vaches.

 

[KENAT]

Isn't compressed air on modern A/C usually bled off of the gas turbine?

If you're talking about carrying around a completely separate system I'd suspect there'd be mass penalties.

 

[rb1957]

i suspect that the power/weight ratio of an air compressor is probably very low, and the presumably large inlet will cause alot of drag, further reducing the performance. although i suspect that having a bigger inlet and creating a by-pass flow would improve things ... maybe someone else can help with the propulsive efficiency of a by-pass jet. I wouldn't confuse the power required to run the compressor as the power output; i think the compressor creates a high pressure eflux (of a certain area, a certain mass/volume flow rate). you need to calculate the (propulsive) power of this ... you know the inlet conditions (area, volume flow rate, pressure, density); you know some of the outlet conditions (pressure, area) and you can calculate the others (density, flow rate) and (eventually) determine the thrust available form the pure compressed air jet. then you could add the by-pss flow on top of this ... this is 4th year aero. ... something i did too long ago !!

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

 

[MikeHalloran]

Ken, Google "ekranoplan"



Mike Halloran
Pembroke Pines, FL, USA

 

[KENAT]

I've seen the 'Caspian Sea Monster' before, in fact one of the guys at my last employer was trying to build a model WIGE vehicle.

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.

 

[WKTaylor]

Guys...

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