steve's TLAR ramjet

[steve]

I built this on a whim after seeing the engines on hiller's ramjet helicopter (and also because I love the shape of maggie mugs and wanted something that at least resembled it a little). It is modeled loosely after that design (hiller), but greatly simplified.

I attempted a run today on both propane and alcohol, and propane shows the most promise, but I will have to modify the engine so that the fuel is injected behind the flameholder rather then in front of it. I an using a leaf blower as my air source and should be able to simulate speeds up to 200mph.

a should have a sucessful run in a few more days

[larry cottrill]

Steve -

That looks great! I suggest that you do this for thrust measurement [maybe you've thought about this method already]:

Make a rolling stand to which you can mount BOTH the engine and the leaf blower. First, measure the thrust of the blower, without the engine mounted - should be a few ounces. Next, mount the engine but don't try to fuel it, while measuring the thrust with the blower running. This should be significantly smaller. If you've got it set up so you have plenty of air flowing around the engine as well as through it, the difference will be the total drag. Of course, this 'cold' value won't be perfectly accurate for when the engine runs, but should be somewhere close. Then, fire your engine with the blower and measure thrust. Subtracting the blower thrust alone should give you the net thrust at whatever that airspeed is. Adding back in the cold engine drag would give the approximate actual thrust at speed.

I can hardly wait to see what you come up with - that's a really nice model, for sure!

L Cottrill

[steve]

What I'd really like to do with it is mount it on a test rotor and see how fast it will spin. To do that, I first have to figure out how to convert it to liquid fuel. Then I will be able to mount the tank on top of the rotor and let centrapetal force pump the fuel at high pressure into the engine. then the only problem will be igniting it once it is spinning, but I'm sure I could figure something out. (picture me running behind the spinning engine with a blowtorch in one hand)

[Bruno Ogorelec]

What I'd really like to do with it is mount it on a test rotor and see how fast it will spin. To do that, I first have to figure out how to convert it to liquid fuel. Then I will be able to mount the tank on top of the rotor and let centrapetal force pump the fuel at high pressure into the engine. then the only problem will be igniting it once it is spinning, but I'm sure I could figure something out. (picture me running behind the spinning engine with a blowtorch in one hand)

Believe it or not, but it's not as farfetched as you make it sound. The accounts I have read of the testing of the Hiller tipjet helicopter are funnier than any sitcom you can see on TV. (BTW. It's centrifugal force, not centripetal.)

[steve]

but how can it be centrafugal if centrafugal force dosn't technicly exist? hehe

I just put that there to see if anyone would catch it :-)

[Bruno Ogorelec]

It doesn't, of course, but it nevertheless works! :o)

[larry cottrill]

It doesn't, of course, but it nevertheless works! :o)

Nonsense!

As I have railed on before, there are no single forces, only forces in opposed pairs. When I used to fly my Dynajet "buzz bomb" trainer, what I felt was 25 lb of centrifugal force, while what the airplane felt was 25 lb of centripetal force holding it in the circle [i.e. providing the turning acceleration].

Textbooks show a single force acting on an object in space - that's what doesn't exist. It is impossible to have a force applied to an object without a corresponding opposite force applied to something else somewhere else. Forces exist between masses - period! Even Einstein believed that the "inertia" that "opposes" you accelerating some mass was really the net gravitation of the universe as a whole acting on that mass.

You cannot lift a pebble without pushing the earth down slightly under your feet. You cannot drop the pebble without the earth jumping up slightly to catch it.

The moon is the largest known satellite in terms of the planet that holds it "captive". The force of the moon on the earth is so great that the monthly "wobble" of the earth is almost the earth's full diameter!!! This is the action that creates most of the tides. The 'balance point' between the earth and the moon is only a few hundred miles below the earth's surface, and passes beneath our feet every day as the moon appears to cross the meridian where we live [due to the daily rotation of the earth]. This would be at roughly midnight on the day of the full moon. You are thousands of miles closer to the sun at midnight at full moon than at midnight at new moon because of this monthly wobble.

L Cottrill

[Bruno Ogorelec]

Nonsense!

Of course! I fully agree. I remember leading fiery debates with Bruce Simpson years ago over Newton, action & reaction. I've always resented the fact that everyone is taught at school about 'action and reaction' which is pure poppycock. There ain't no such thing.

[larry cottrill]

What I'd really like to do with it is mount it on a test rotor and see how fast it will spin.

I'm not sure your jet is quite tiny/light enough, but what I've thought of doing is mounting it on the rim of a full-size bicycle wheel, and screwing one end of the axle into a properly sized threaded socket cast into a concrete construction block. The bike wheel would make it very easy to add the appropriate counterweights, etc., and should be easy to 'spin up' to a reasonable outer velocity. It's also a very strong and fairly lightweight structure itself. The only real problem is that fuel use will affect precise balance, unless you really can locate it at dead center and get centrifugal feed, as you're suggesting.

L Cottrill

[steve]

I was thinking that I could mount a 10ft test rotor on top of a bike wheel, since they are so easy to obtain. the rotor would just be a length of electrical conduit. the fuel tank would be at the center as you said.

I got it to run sucessfully today! It was very impressive and extremely HOT!!. The exaust was much hotter then that of any pulsejet I have run. I could literally heat my entire house with this thing in about 5 mins, and it would probably burn less fuel then the normal oil heater uses.

Another interesting thing was the noise. It ignights with an impressive WHUMP (which I don't have a video of, sorry), and then produces a suprisingly loud rumbing sound that partially drowns out the noise of the leaf blower even at full throttle! you can hear this in the video below:

[Bruno Ogorelec]

Steve, this is a great video! Do you think the thrust could be increased with a gently flaring extension to the exhaust? Looks to me like the exhaust flow could be usefully entrained some more.

[larry cottrill]

Yes, THAT is impressive - exactly what I imagined coming from Maggie Muggs, only you can probably run yours indefinitely without it falling apart! That 'asterisk' flameholder seems wonderfully effective!

Your video should be a great answer to all the "experts" who claim a ramjet won't work without those "textbook" conical flameholders! What a hoot, as Ben would say!

There is no question in my mind that you will have sufficient thrust here to drive the kind of "rotor" you're describing. I'd be wary of trying to use a bicycle hub for an extension like that, though. You should try to get a small motorcycle front end hub or a small car spindle and rotor. Something like that. The bike hub will never hold up at speed under slight imbalance [which will happen sooner or later] if the mass is significant. A bike wheel is a VERY lightweight structure compared to any extended arm you can build.

Another thing to be careful of is to make sure your center of mass is not above or below your bearing, but pulls as straight out from the bearing center as possible. Think of how a car wheel is not protruding out off the end of the spindle, but domed so the spindle is out there in the midst of the mass of the wheel and tire. There is a science to applying weights to the inside vs outside of a wheel when balancing, referred to sometimes as 'dynamic balance' although that seems like sort of a vague term to me.

L Cottrill

[larry cottrill]

Steve -

Do you know any aeromodelers who use big ducted fans for their planes? If you do, see if you can get one to help you rig a large ducted fan on the front end of your ramjet to find out what it would do as a motorjet! Don't laugh; there are some really powerful little fans being produced for propulsion these days - even electrics.

L Cottrill

[larry cottrill]

While interesting and fun, motorjets are hugely inefficient. For flight, you would be much better off adding a second battery to a plane propelled by a ducted fan than adding a ramjet, fueling system, and fuel tank.

Ben, while I understand what you're saying about efficiency, I still have to disagree. Yes, you could boost a fan's output with more power, but the increase in stream momentum from well-contained combustion is HUGE, even at these relatively modest velocities. The Coanda Turbine Plane was a motorjet, not a turbojet, and it got off the ground all right! [If it had been occupied by a pilot instead of an inventor, it would be mentioned in history books today, right behind the Wright Brothers.]

If you were very clever, and could duct it such that the fan could be bypassed at speed, perhaps you could use it to get the plane up to a speed where the ramjet was worthwhile on its own. It would be faster and cheaper to just use a rocket launch, though.

I have little confidence that rocket launching is really practical for model aircraft of significant size and weight. I know very powerful rocket engines are available, but you also need a pretty good duration to get the ramjet up to speed - it seems to me that you're talking about a pretty big, heavy rocket motor to get the job done.

Another advantage of the motorjet approach is that you could be sure your ramjet is running properly for fuel mixture, etc. before launch. A disadvantage is that you waste some fuel ground-running the jet and using it for takeoff where the efficiency is at its poorest. You would also need to ensure that when the fan stops operating, the engine would be immediately starved of fuel so you don't have a hot, flamed-out engine with fuel still pouring in! Of course, with a bypass scheme like you mentioned, this wouldn't be an issue. That's also more appealing from a design standpoint, since you get to use your engine as a true ramjet, as it was designed to be.

L Cottrill

[larry cottrill]

Steve -

An important question I should have asked already, since it bears on the testing of Maggie Muggs:

Obviously, the way you have your air supply located and positioned, you're getting a lot of air flowing around your engine as well as through it. Did you see any indication of red hot metal anywhere? It certainly doesn't show in the pictures and video.

Any comments you'd like to make about heating of your engine would be interesting, I think.

L Cottrill