There are CO[sub]2[/sub]-powered motors for “microlight” model airplanes. Instead of using combustion like most engines, these little motors use pressurized gas.
Can a motor be made that runs on a “non-greenhouse” gas? Can such a motor and fuel tank be built that is large enough and light enough to power a human-carrying aircraft? Of course I know that it would never be practical. I’d just like to know if it’s possible.
Certainly it’s possible.
To make a motorized flying device, all you need is a lift surface (wings) and a motorized propelling device (propeller).
You can run a motor on just about anything… you specify using compressed gas.
Let’s use nitrogen… it’s almost completely harmless, far as I know. Except that we’ve got it EXTREMELY compressed in some kinda metal tank.
The mechanical force of the gas escaping the tank can be used to drive the propellor… easy as cake.
Now, making the whole thing big enough to fly with a person in it, light enough to fly with a big heavy tank of nitrogen in it, and able to hold enough nitrogen to be worth taking off, is a whole other story. But you knew that. q;}
Right. The Wrights flew with only 12 horsepower. Of course there are models that will fly with a CO[sub]2[/sub]-powered motor, but models tend to be very light. Combustion-powered models seem to have a much higher power-to-weight ratio than manned aircraft. Flying is a balancing act. In the case of an air-powered aircraft, you can build a motor that has enough power to make a full-sized aircraft fly – but the motor and associated fuel tank and its contents might be too heavy for it to lift the aircraft and the motor system.
As to whether it is possible to make it “worth” it, it obviously isn’t practical. But then, neither are human-powered aircraft. It would, though, be very interesting to see it done.
To get anywhere with it you’ll need a very light - but very strong - tank. You’ll probably have to go with fiber reinforced composites to do the job. I’ve seen a high pressure air tank that was used on a german submarine during WWII. That tank would probably hold enough pressure to power a small plane, but I don’t think it could carry its own weight. The walls were nearly two inch thick and made of steel. That was one heavy puppy, let me tell you.
Tanks designed for use with SCBA (as opposed to SCUBA) are now being made with fiberglas composites, making them very light. I have worn tanks from Scott and MSA that dramatically increase mobility. But, why are we limited ourselves to compressed gas in a cylinder? Why not a small, electrically driven air compressor with a “reserve cylinder” for take-off and emergencies. On the ground, the compressor could be run by normal electrical service; but in the air, it could be powered by something like an EPU - a little propellor that is in the air stream and connected to a small generator. The EPU would provide just enough power to drive the small compressor (and to fill our “reserve cylinder”). Of course, this sounds good and may even look good on paper, but IANAE and I do not play one on T.V. BTW, the best solution may be a “hybrid aircraft” that uses the best ideas from several power technologies; like maybe JATO (really a rocket) to get airborne and compressed air to stay aloft, or some such.
I was thinking of something powered entirely by compressed gas, rather than a hybrid. Just trying to keep it simple.
I don’t think this would work. It sounds like you’re getting something for nothing. That is, your cylinder provides compressed gas to run the motor that turns the propeller which pulls the aircraft through the air which turns a small propeller which turns a generator that powers a compressor to fill the tank that powers the motor that powers the prop.
The problem with using multiple technologies for aircraft propulsion is that they all add dead weight when they aren’t working. Arguably JATO rockets are jettisoned but they need a strong attachment point and that alone adds weight to what must be an extremely light aircraft.
Unfortunately storing energy as a mechanical force (compressed air, rubber bands) is not weight efficient compared to stored chemical energy.
How about a blimp with compressed gas thrusters?
And when you say, “can it fly?”, I say “for how long?” Would a 12 second flight like the first at Kitty Hawk count?
Usually the power source of an airplane accounts for 1/4 of it´s weight; if a compressed gas engine and storage tank system can be made that light, it will fly. The endurance is another thing thou… perhaps using cryogenic nitrogen. Anyway, you won´t go far, unless you install a very light power system on a glider, use it to bust the plane to some altitude and then glide taking advantage of air currents; that , in fact, could work quite well.
Currently there are tanks rated to 5000 PSI and 10000 PSI coming soon, meant to be used to store high pressure hidrogen for cars, those thanks are quite lightweight being made of carbon fibre.
Yes. I’m curious if a compressed-gas powered aircraft could be made that can take off under its own power (no JATO – just the air-engine) and fly, preferably out-of-ground effect.
I´m pretty sure such a thing can take off; if electric planes have flown, why not this kind? Certainly battery packs should weight more than a compressed gas cylinder, although the issue of the endurance is still a concern.
As for the CO2 engines in the OP link… they are so sweet, expensive but sweet; I´ll keep powering my model airplanes with rubber for now. 
I must confess that I only skimmed that page. I was looking for a link so that people would know what I was talking about when I mentioned CO[sub]2[/sub] engines. I didn’t see any prices. But I did see the radial engine. Very sweet, indeed!
Good grief! - quite aside from Padeye’s point about not getting something for nothing, you’ve got multiple-stage energy conversions going on; each stage losing some energy through inefficiency - Air flow powering a generator charging a battery powering a compressor powering the prop - sounds like a system that might turn out to be a more efficient heater than anything else.
Here is a helpful link.
Any engineers in the house?
Someone must know how much horsepower is possible per cubic inch/cc or displacement. Someone else might know what materials can be used for the tank and the motor, and know how much they weigh. With that, we can pretty-much figure out the power-to-weight ratio. If the numbers balance, it just might work.
Of course, I’m too heavy to act as the pilot; and I really have no interest in building one. (If I were to build something on a lark, it would be a replica M2-F1 lifting body.) But it would be an interesting thing to see.
I’m reading Sock Munkey’s link. It says:
Three minutes? When I was diving I remember how hot the tanks got as they were being refilled – in a lot more than three minutes.
Actually, you would probably be better off basing the design on a sailplane body as some types carry little engines for taking off without a towplane.
I think a sailplane (as I think of them) might be too heavy. One interesting thing from your site: They mention a nitrogen-powered engine that makes 15 hp. The Wrights flew on 12. If the nitrogen engine and tank were light enough, it looks like it might work.
I was actually thinking of the Gossamer Albatross. IIRC, a human athlete in top condition puts out about 5 hp. The airframe, being made of kevlar(?) and mylar was very light. It did have the long skinny wings like a sailplane.
In what way are you thinking of sailplanes that would make them too heavy?
hmmm… okay, if you say it won’t work, then I’ll accept that. I was just thinking about different technologies I have worked with, like a small air-driven turbine (smaller than a pack of cigarettes) that was powered by an air stream from quarter-inch tubing that was run from an industrial air compressor (on the order of 100 psi) that provided electricity to run 24v devices at remote sites. And the EPU I was thinking about was the one I saw on F-4 aircraft - it was an Emergency Power Unit that provided electric power for the “essential buss” after a catastrophic failure. But, as you all have pointed out, the added weight would be a limiting factor. What if the added equipment could be made of some radical new material? Like polymers or titanium? And what if we added solar panels to augment the small generator? BTW, I think we can dispense with the battery and just close-couple the generator to the compressor. Again, if this doesn’t sound feasible, I’ll stop already.
