## air requirements for a large subsonic ramjet

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### air requirements for a large subsonic ramjet

Something that has been rattling around in the back of my mind is a possible method of creating massive amounts of "high speed" air. The problem with most fans is that they're either they're too small to generate large volumes of air or they're large enough to generate the volume but not the higher speeds.

What we need is an airplane. It can generate large volume of air due to the size of the propeller and also high speed air because most airplane engines turn at between 4000 rpm and 8000 rpm (some higher, but not many lower) The problem is that an airplane is rather expensive. We need an airplane-like setup. Looking at www.aircraftspruce.com I found I can get a wooden two blade propeller for fairly cheap (less than \$200) Ok, but how to turn it? High torque is needed to get it spinning and to keep it spinning. Most aircraft engines are several thousand dollars and so that is out. Then it hit me, I already have the perfect setup! I have a good engine but even better than that, I have a great transmission that can produce tons of torque in the low gears and can keep it there in the high-gears. My SUV (I have an older Toyota 4Runner) Now, how to connect the propeller to the drive train of my 4Runner? I could jack it up, place it on jack stands and use the rear wheel. The wheel is the direct output from the drive train. If I take the actual wheel off, I have the lug bolts that I can bolt a sprocket to that can turn a reduction gear so that the rpms of the rear wheel is increased to what is needed at the propeller. Now, for a bit of quick maths:

The diameter of my rear wheel is 32 inches which gives me a circumference of 100.5 inches. (2*r*Pi or 32*3.1415) If we take the wheel traveling at 65 mph and convert that to inches per minute we get 68640 ipm (inches per minute)((mph*63360)/60 or (65*63360)/60) divide that by the circumfernce of the wheel to get rpm. (68640/100.5 = 683 rpm) 683rpm doesn't turn the propeller very fast and so while we have the volume needed, the speed isn't there. If we were to introduce a 5:1 reduction gear(in reverse) between the wheel and the propeller we would get 3415 rpm. This is now near the low end of our desired propeller speed of between 4000 and 8000 rpm It is possible to use a second reduction gear to increase the propeller speed or a gear with a high reduction but for the sake of argument lets leave it as is for now.

Now we have a 48in diameter, two blade propeller spinning at near 3500 rpm generating lots of air movement at a decent speed but not quite the speed we need. If we were to build a duct to bring the air down from 48 inch diameter to something smaller, say 24 inch diameter, what would that do to the air speed and air pressure?

So for under \$500(plywood for the wind tunnel ducting, prop, reduction gears, bearrings and steel pipe) we could make a halfway decent wind tunnel that should produce more than enough air volume and acceptable air speeds.

Questions, comments, concerns, expressions of outrage?

Zippiot
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You sir are crazy, I like it!

I have wanted to make a small wind tunnel, like 6 inches diameter. A big centrifugal compressor with a lawnmower engine (I got an electric mower engine sitting around here), would make testing ramjets waaay easier, the bigger the windtunnel the cooler the jet you could make!

DO IT AND TAKE PICS
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larry cottrill
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This is an EXCELLENT Red Green type project! Leaving the mechanics of spinning the prop entirely to you, I will offer the following:

An 'open air' prop will have a lot of losses around the tips when you try to duct it. HOWEVER, I would NOT recommend shortening it to square off the ends unless you really know what you're doing! This is because introducing unseen flaws in the wood structure can cause disastrous disassembly at high speed.

There will be a lot of 'spin' in the ducted air, which will continue downstream unabated for several meters. Because of this, you need a 'flow straightener' section. This could be as simple as a whole bunch of cardboard tubes epoxied together side-to-side, or you could build up a honeycomb structure of thin plywood or something. The whole idea is to divide up the large spinning flow into a bunch of small-area flows and then blend them all back together again. The length of such a gimmick should be about the same as the overall flow diameter, or at least not much shorter than that.

Your 'choke cone' should be long and tapering, like a ramjet diffuser, not short and blunt. If you try to make the area ratio too high, you will create a LOT of "back pressure" that will degrade performance of the prop and greatly increase the power demand. You MIGHT get away with an area ratio of four, which will give you roughly four times the flow speed that comes off the prop. I don't know how to calculate the exact back pressure that will give you, but it will be PLENTY, and you can expect a lot of backflow leaking out around the tips of your prop, even if the fit to the duct is really close.

Make sure you're providing adequate HP for the prop you get. If your prop is too big, you'll never get it up to efficient RPM if you gear it down enough for the engine to drive it. (But, you knew that ;-) It will probably be VERY advantageous to have an 'intake flare' just forward of the prop "disk" to ease air going in (especially if you have some gearworks up close, partially in the way).

Just some things to consider. I don't know the half of it, of course.

Man, this will be something to see if you can really pull it off!

L Cottrill

Jim Berquist
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Just for hoots and grins! The Colibre Helo used ram jets. It had a small auxillary engine to wind up the rotors. How fast did it get them up to on the tips? Those engines looked somewhat too simple and backwards compaired to what we can do now!
WHAT TO FRAP, IT WORKED![url=callto://james.a.berquist][/url]

LINZ
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### Toyota power

A few suggestions about things to watch out for:
Unless your 4 Runner has a lockable differential, you are not going to get any power out of one wheel. As soon as you put any torque load on the wheel you've chosen, the differential will try to send the drive to the other wheel which will then start to spin faster. The result will be that you will only be able to get about 5% of the drivetrain power out of one wheel - not enough to do what you want. If you lock the other wheel so it can't turn, you can get an effective gearing increase of 2 times but you will wear the differential out verry quickly. If it's not in good shape it could even sieze up which could destroy the rear axle and your prop too.
To echo what Larry said, check you pick the right prop for the horsepower you have available. A 48" prop sounds a bit big. If it's meant for a big air boat running a V8, your 4 Runner won't be able to spin it fast enough no matter how you gear it.
Don't try to build something that requires full power from your daily driver to run it. Cars can't make full power while standing still for more than about a minute before overheating. Your cheap wind tunnel will get expensive quickly if you blow your car up running it.
Pick a prop that can achieve full performance on about half the rated power of your 4 Runner - whatever that ends being, size your wind tunnel accordingly. If your 4 Runner has open or limited slip diffs rather than lockable, I respectfully suggest it won't work.

LINZ

pezman
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### Flying 4-runner

A hovercraft prop might be better than a conventional prop. They are designed for ducted operation and an unplanned flight is far less likely.

e.g.: http://www.hoverhawk.com/

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### 4Runner Powered Wind Tunnel

The 4Runner I have has the ability to do 2WD, 4WD limited slip, 4WD locked diff and 4WD low locked diff. I knew that with an open or limited slip diff it would never work (and I knew I'd have to run it in locked diff mode, just a button push away) The 4Runner is over 7 years old and has been paid off for several years and while I wouldn't want to trash it, I have been thinking about getting something new for the last year or so.

I'm not sold on the prop that I picked out. The one I was looking at was designed for ultra-lights. I think the 3.4L V6 (137 kW , 183 HP SAE @ 4,800 rpm; 217 ft lb , 294 Nm @ 3,600 rpm) in the 4Runner wouldn't be able to spin a large 4 blade variable pitch metal prop from a full plane but it should be able to handle a lighter 2 blade wooden prop that is found on most ultra-lights. If there are better props for this, I'm open to suggestions. I'm trying to find a good balance between cost and effectiveness.

I also knew that it is a bad idea to run it at power for any sustained period of time. I was thinking that test runs would be limited to about 1 or 2 minutes total air time with a full cool down between runs. My calculations were for a road speed of 65mph which works out to about 1800rpm which is at the low end of the power curve for the 4Runner. While not quite idle (700 rpm) its not at the top of the power curve either (3200 rpm) and nowhere near redline (6000 rpm)

Looking at the hovercraft fans, it looks like they can produce the volume of air needed (16,200cfm!!!) but I'm not sure of the speed of the air or the back pressure that they can withstand. Looks like they max out at 3600 rpm and while that was about the end speed I was calculating for, I also envisioned a second reduction gear to double the speed if needed (also letting the engine run at a lower speed for longer tests at the same prop speed) With a 22 inch diameter prop and 3600 rpm shaft speed on the prop, the blade tips would be traveling at about 240 mph. Increase the prop size to 36 inches (smallest ultra-light prop I saw) and the prop tip speed is about 380 mph but run a 48 inch diameter prop at 3600 rpm and the prop tip speed is about 500 mph. I know prop tip speed isn't air speed, I'll need to find a way to calculate that.

I've done some model work (haven't we all?) and so creating the ducts for air intake as well as around the prop was in the plan as well as a flow straightener. The flow straightener wouldn't be required (not designing this for true wind tunnel testing) but I had still planed for it. As for the area reduction, I was thinking of reducing it down from a 48 inch diameter to 24 inch diameter over 8 feet (and then going into the flow straightener) I don't know what all it will do to air speed, air pressure and losses due to back pressure but it should increase all three.

-Aaron

Glenn Olson
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Testing ramjets is difficult.

One way, of course, is to stick them on the end of a rotor. This can be a serious problem if it goes too fast or becomes unstable and breaks up at high speed. Rotary Rocket tested theirs in a pit. They can also be tested in a cage. My idea was to wrap several layers of chain-link fence around some large conduit or make a framework and pack sandbags around it. But I never got around to building one.

An airplane propeller sounds good, though it might be overkill. You might be able to find an older un-certified engine and prop laying around an airport for a reasonable price. Some homebuilts, like the Verieze, use volkswagon engines. Some of the smaller, but powerful, Japanese engines turn some good RPMs. Junk yards sell those and used transmissions.

For a smaller test setup you might look into model airplane engines, specifically the weedwacker conversions. You can get 31cc conversions with thrusts on the order of 11-12 lbf (http://www.geocities.com/CapeCanaveral/ ... /ryobi.htm) complete for a couple hundred dollars. In fact, at one time I was considering building and selling such ramjet test stand kits. There's probably a good market for them.

With the right prop and tapered ducting you should be able to test small (3-4 inch diameter) ramjets up to about 200 fps. If not then there are larger engines. If that's not enough then chat it up with some of the airplane owners and see if they'll let you attach your ramjet to the outside of their aircraft. That's how the Germans started.

Zippiot
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Im thinkin some kind of upside down lawnmower with the blade able to freespin!!!

I like the idea of actually seeing the jet spin/move something but fire whirling around at 200+mph has to be illegal in some way...
Sailing Student- How do I know if my life jacket is tight enough?
Me- Can you breathe?
Sailing Student- Yes
Me- Then its too loose!