"Flat" Ramjets as in Fleissners Patent, feasability?

[Röör]

Hi,

This is my first post. As I saw that this is the forum on the net about PJ's and Ramjets, with the most experienced (non army) people, I thought this is, where I can post my questions...

I think some of you know Fleissner's Patent (US 2,939,648). Its about a rotating Jet aircraft.
[
The principle is just like the Hiller helicopters just that he doesn't only have a few rotor blades and a few ramjets, but a lot of very small ramjets all around the circumference (No 4 in the pic). I think this makes the building a bit easier, as the forces due to the centrifugal acceleration will not be as big, due that smaller (lighter) ramjets are distributed more evenly around. Additionally you have now quite some space for some fuel...

Nevertheless. What amazed me, was the actual shape of the Ramjets he proposed. They are flat, more or less rectangular shaped...
This really interested me. As you are experienced people, what would you think of rectangular shaped PJ's or Ramjets compared to circular shaped ones...Advantages, Disadvantages?

The strange thing is, Fleissner was an expert in fluid dynamics, so this design can't be so wrong as it looks at first hand...Or not?

Some sources (unfortunately not reliable) say, it flew, but used up huge amounts of fuel.
Additionally it is said, it was made to be able to fly on many different fuels, even coal powder...
How can you feed a Ramjet with coal powder???

I'm not interested about a discussion about the saucer shape, or anything like that. I'm more interested in the idea of the rectangular Ramjets. Is this the idea, that you actually take a big turbine and use the room in between the blades as actual ramjet engines?

Thanks for any answer.

[Mark]

Here's a few tidbits on a coal idea.
http://www.fiddlersgreen.net/AC/aircraf ... ppisch.php
http://www.youtube.com/watch?v=MvtxjSrImHw

[Röör]

Thank you, quite odd this cage of coal inside, but Interesting. A flying stove...
So it's good, that Ramjets only start to work at quite some air speed, otherwise all our stoves would fly away

Here's the Ramjet constructional descriptions of the patent:

[...]jet channels are spaced within and around said circumference of said disc, each of said channels extending from the top of said disc in a sloping direction away from the vertical to the bottom of said disc, each of said channels being open to exterior of said disc at the top and bottom thereof, said opening at said top being an elongated slot, said opening at said bottom being an elongated slot of greater length than said first-mentioned slot, and said fuel injectors extending in a substantially horizontal direction from said tanks into said channels, a plurality of openings in said injectors to evenly distribute fuel from said tanks into said channels, electromagnetic means to regulate the quantity of fuel flowing from said tanks to said elements.[...]

Gee, there's "said" a lot...

I get the feeling, that due to the rotation, the air in the Ramjets will have quite some centrifugal force. I think this should be considered. Maybe like that, one could get a much higher compression, even if the air speed is not that fast...
E.g. if one would like to have 500km/h (140m/s) air speed at the Ramjets, and does have a diameter of 10m. This would mean, that the centrifugal force at the Ramjet would be 400 times the force of gravity! I surely think this will have an impact on the working of the Ramjet...What do you think about this point?
On the other hand, as far as I know the Ramjet helicopters made, did not have any special Ramjets which made use of this...So this speaks against this theory...

[Röör]

I saw, in this forum, that there are pictures from two professional ram jet helicopters. What astounded me was, that both of them have flat ramjets!

I personally really wonder why did they all use flat ramjets for this?
When I look at one of these helicopter ramjets, it looks more like two conventional circular ramjets beneath each other merged into one.
Then I recognized, what could be the advantage. In the decker paper is said, that the length of the ramjet is dependant on the diameter. If you now want to have a short ramjet, but still quite some thrust, you have a problem. For if you make the diameter bigger, it will have to be longer. But what if you just take two, or three smaller ramjets beneath each other. They would develop the same thrust, but are shorter. And it seems, you can just merge these little ramjets into one without big problems...Like that you can get a ramjet, with quite some thrust, but still very short (a "flat" ramjet). It looks like the narrower side length of the ramjet opening is the demanding factor for the ramjet length...
And why not just a conventional. but longer ramjet. Well as I said above, maybe due to the centrifugal forces, some very strange effects very probably start to occur the longer you make your ramjet if you use it on a circular mount, as the different parts of your ramjet are then on a different radius and have so different accellerations. This will "push" the air out of both sides from the middle of the ramjet,which is contraproductive for the functioning of the ramjet.
Just my guess!

Any comments about this idea?

[larry cottrill]

Röör -

There is no reason you can't design the engine so the centrifugal effect assists the jetting of the exhaust. The engine could be "bent" so the combustion mass flow is mostly from inboard to outboard, then turned rearward. Some R & D required, but that's always true, anyway. The big problem with tipjets isn't usually engine design per se, but rather, getting fuel flow to work right and be properly controllable for throttling. The centrifugal effect is horrendous for liquid fuel because of the high fluid density. You also have the problem of getting fuel from a stationary location up through the hub into the fuel lines in the rotors, without catastrophic leakage.

In general, the disadvantages of a "flat" engine cross section are lossier flow characteristics and increased thermal losses. Flow efficiencies are especially poor in cases where the duct has tight corners (rectangular or triangular with sharp corners) -- the corner regions become practically dead once air is moving. You'll notice that in modern building construction, there is more and more use of oval ductwork for air moving; it's more efficient (and even quieter running) than the traditional rectangular cross sections.

L Cottrill

[Röör]

Thank you very much for your comment!

The big problem with tipjets isn't usually engine design per se, but rather, getting fuel flow to work right and be properly controllable for throttling. The centrifugal effect is horrendous for liquid fuel because of the high fluid density.

Sure I also think this is quite a problem, as in the example I indicated above, the fuel pressure would be several 100 bars at the tips. This means certainly, that one has to use quite strong conducts for this. But as such high pressure are nothing very special in today's high pressure hydraulic world, I don't think it would be a big problem today to get the corresponding conducts and valves for this...(but for quite a price...) In our machine sector we have hydrostatic bearings which also operate in this pressure range...

You also have the problem of getting fuel from a stationary location up through the hub into the fuel lines in the rotors, without catastrophic leakage.

Certainly also a problem. In the design of the patent he got around it, by placing the tank in the rotating part...but for this he then also needed an aerodynamic "shell" for the rotating part, which is the reason why this "saucer" shape results...
In another patent I saw another interesting idea how to solve this problem: They used a hydrodynamic bearing for the rotor, and just feed the inside part of the bearing stationary from the tanks, and the rotating outside of the bearing delivered the fuel to the rotor tips.
So by using the fuel as liquid for a hydrodynamic bearing they solved this problem...

In general, the disadvantages of a "flat" engine cross section are lossier flow characteristics and increased thermal losses.

I also thought the same. But nevertheless they used them for the helicopter tip ramjets. So there had to be an advantageous reason for this.

[makulit]

Ouch - just lost a long paragraph. my fault. anyway. The US government (NACA) did test stand and flight testing on rectangular ramjets in 1947. The flight article was a short wing section mounted under a P-61. This ramjet was developed for vehicles like the Hermes B winged cruise missile second stage of a V2. Development of the missile continued until the Air Force took over long range missiles and killed the project due to NIH syndrome. So rectangular ramjets certainly work. Advantages are form and computational simplicity. The ramjets were to be the missiles wings. (square in plan, wedge in section).
The multiple blade/airfoil ramjet powered device should work. One of the reasons helicopter rotors are long is what happens at the tip. Air falls off and generates vortices. Long wings have less vortex per wing length than short wings. Another way to solve this with rotating blades would be a duct or a fence so the air can't fall off. Turning Fliessner's jets 90° to the blades could allow their use as fences, making the entire blade useful. Or you could install it in a duct as Fliessner seems to have done in his drawings.
Two main methods were developed to use coal as fuel. One used a rotating cage with pelletized coal. This seems to have been tested by the Germans towards the end of the war. The other used powdered coal in a fluidized bed. This last approach has been used in actual missiles, with powdered metals instead of coal, and fluidized bed coal burners are used for power generation. Fluidized means its carried in a gas under turbulent fully mixed conditions.

[makulit]

Regarding rotating ramjets: Assuming a tip speed of 600 mph and a diameter of 6 feet, your rpm would be 2801, which is equivalent to the rotation of a car tire at 200 mph. The bearings could be pretty conventional and still work. Using fuel to cool them would probably be nice, and the rotating blades would essentially be the fuel pump. Pressure at the tip would be nearly 4000 psi. That rectangular ramjet program conducted by NACA determined high pressures and small orifice sizes gave better atomization of fuel which gave better combustion efficiency. Seems like 4000 psi would do this really well. Pressure calculated using the same eqn Decker uses in his paper on Practical Subsonic Ramjets.
P = 29.35(N/60)²R²/144 where P is pressure in psi, N is RPM and R is Diameter(ft)/2
Note that the pressure is dependent on the tip speed regardless of the diameter/rpm used to obtain that tip speed.
I've included my spreadsheet used in these calculations as a zip file. Beware of Microsoft attempting to open the Open Office File with its own software. Unpack first and use open with OpenOffice Calc.

[sockmonkey]

Seems a little like the idea I posted here. viewtopic.php?f=4&t=5105&start=15
One issue I can see is that the angle of the jets is fixed and so would only have maximum efficiency at one specific RPM and forward speed. I can think of a few ways to adjust for this but it would probably get complex.

[Röör]

Didn't look at this thread since quite some time now. But the idea emerged again.

And now I'm quite curious and would like to make a little Flat-Ramjet. Maybe similar like this:

"Professional" flat ramjet

I think one advantage would certainly be, that a flat Ramjet is much much easier to construct.

I think I would build up a test stand, where it rotates with about 2m diameter. and maybe around 3'000RPM.
Similar, as to Makulit's example.

But a few questions emerged:

1) The Decker manual, which is the only one I know of, is for circular ramjets. I would certainly think the input area and output area is the same calculation. But what about the shape itself. Should that also be the same? As the total flow would certainly be different in a 2D version of this shape (flat ramjet )and a circular 3D version (as in the Decker manual). The 2D version for the same shape would have less diffusion, as it could only expand up and down but not sideways, as in a circular version.

Any idea, how to alter the calculations for such a flat (2D shape) design?

2) According to Decker, the nozzle should have about 1.4 times the area of the inlet. Interestingly in Fleissner's patent this is very similar, but he didn't make the slit of the flat ramjet wider, but rather made it longer at the nozzle. What would be the advantage/disadvantage of this???

3) Should there be any asymmetries used due to the rotation? E.g. in one example of a tip jet helicopter, one clearly sees the asymmetric nozzle (see attachment)

Back view of tipjet helicopter ramjet

Or is this idea to make the outer rim side area bigger the same as that of Fleissner which made the slit longer at the rim?

4) How much does the size of a ramjet affect it's performance (E.g. if I make a small model)? I would think the surface is much bigger in relation to the size , and so should also be the drag much bigger.

5) Am I right, that even if one would do something like valves in front of a ramjet it would not work as a pulse-jet as the long exhaust tube is missing?
I would have had the idea, to have a rotating vane in front of the ramjet so that it would open and close, just like the valves in a pulse jet, and when the speed is reached it would stop and just always let the inlet open.
Has actually anybody ever made rotating vane-valves for a pulse-jet? I would think if it is synchronous with the pulse-jet itself it should work and have basically no wear, and still the U-tube construction as in a valveless design wouldn't be needed.
But it would probably be very hard to synchronize, especially for start-up.

[sockmonkey]

I'm reminded of that patent for a tip-ramjet helicopter rotor where the air intake was in the hub with hollow tubes leading to the jets at the tips. The centrifugal force was used to suck the air in and compress it before it got to the jets. Sounds good in theory. Is there any reason it wouldn't work?

[Röör]

I don't think this would make much sense, as the compression due to the centrifugal force is very small.
This is because air has a very low density as e.g. compared to a liquid.
Probably also drag losses would be quite big.

Then you would have to rather do it, like in a typical radial compressor, but then you anyway do not have anymore a "ramjet", but a big radial compression jet engine.

[sockmonkey]

Hmm, what's the equation for calculating centrifugal force at rotor blade tips?

[tufty]

m v^2 / r

[Röör]

Most people make this thinking error, when they think just by rotation you get a high compression. The problem is the "m". Air has a very small mass.

Therefore if you calculate it, the compression will be small.

You have to think of the air in e.g. a rotating drum like a column to the center. And the weight of this column is very small, therefore you will only get a small compression.

Always remember, we have here at earth level 1 bar pressure because there's a huge air column above it which presses downwards.

Thats why you need blades and a proper shaping of the inlet also in a radial compressor. There the blades are responsible for the compression and not the centrifugal force.