RamFan Engine?

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Hawking
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RamFan Engine?

Post by Hawking » Sun Oct 16, 2005 12:43 am

There's been much discussion in regards to ramjet powered helicopters. The biggest drawbacks being noise and fuel economy. However, the simplicity of the design has always intrigued me. I'm new to jet building and I am curious if anyone has ever tried to depart from the standard 'stick a ramjet on the wingtip' approach.

I fly a small hovercraft powered by a 2 cylinder, 2 stroke engine that generates about 50 bhp. While a fun and useful craft, it lacks performance. I've recently been thinking of upgrading the powerplant to something a bit more exotic.

OK, On to the point.

I'd like to try the ramjet/tipjet method. My idea differs from most in that I would use the entire length of the blade as a centrifugal compressor of sorts. A channel along the leading edge leading to the cumbustion chamber at the tip, with the exhaust along the trailing edge (about 60% out from the hub). Effectively, the entire blade becomes the engine, as oppsed to just the tip.

Just an idea, and I have no idea if it will work correctly. I have already stated that I am new to the field. I do have a little experience with carbon and Kevlar fibers as well as the appropriate machining tools to work with if the idea is workable.

As an aside, I did already effectively deal with the astronimcal fuel pressure problem due to centrifugal force, as well as starting and ingiting the jet (at least in theory...)

I welcome anyone's opinion, criticism, bashing, or laughter.

Thanks
Don
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steve
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re: RamFan Engine?

Post by steve » Sun Oct 16, 2005 4:10 am

There is a valveless engine in the same layout you described

I'm sure someone will post the picture.

this also reminds me of the hughes xh-17, although that used gas turbines to force gas through the rotor before it was burned at the tips.
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re: RamFan Engine?

Post by steve » Sun Oct 16, 2005 4:14 am

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Hawking
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re: RamFan Engine?

Post by Hawking » Mon Oct 17, 2005 1:06 am

Another little detail maybe someone can offer some knowledge. I'm not sure how much thrust I should be aiming for. In the helicopter example, if there are 2 engines, each producing 35lbs of thrust, how does one calculate the total available thrust produced from the rotating engine/blade combination? Obviously, it's more than the 70 lbs. I'm not looking for an exact number, but an estimate would be useful in determining the engine size required.

Thinking out loud:

The blades will be spinning within a duct. Magnets are embedded in the inner circumference of the duct wall. A small coil at each blade/engine tip serves to power the spark plug in that engine. As the blades rotate past the magnets AC current is produced constantly firing the plug. I have heard that this causes engines to not run properly, although I think that pulsejets are where the timing is critical. Ramjets burn constantly anyway (hopefully).

Same theory in reverse. Magnets in each blade tip and coils mounted in the duct wall function as a starter/generator. Powering each succesive coil in turn allows the blades/engines to be accellerated to operating speed without the use of a shaft connected starter motor. Once running, the output of the duct coils is rectified/regulated to charge the starting battery.

The rotational speed can always be constant. Collectively varying the pitch of the blades can vary net thrust output of the fan. The fuel system can then be optimized to run at one specific pressure, rather than varying pressures due to changes in cetrifugal force.

I would like to use kerosene for fuel as it is easy to handle, and requires very little ancillary equipment. Perhaps a small low pressure pump to bring fuel to the fan hub, with cetrifugal force doing the rest. Rotating shaft seals should allow the fuel to be stored in a standard fuel tank (not mounted to the hub), with the fuel flowing through the blade root. Throttling the contraption may be as easy as an inline flow regulator. Turning off fuel flow completely would be an effective means of shutting it off, although I suppose increasing blade pitch beyond the stall angle would work as well.

Ideally, the fuel system would be self controlling, automatically varying the flow rate to maintain a preset fan RPM. The collective pitch control would be used to vary the thrust output.

Reverse thrust would be possible by applying negative collective pitch

As for construction materials, stainless steel engine components, and vacuum formed carbon fiber blade sections seem to the the most durable available to me. The interior engine shape will be asymmetrical (in order to further incorporate the engine into the shape of the airfoil), and will have to be machined perhaps in two halves and joined together.

I can only assume that due to the large airflow though the system, cooling should not be a problem, although only testing will tell. The ducted design also allows the bypass air (accellerated by the fan blade) to capture much of the radiated heat from the engines as well as the exhaust heat, further increasing the accellerated air speed. I don't know if this will increase the net available thrust, but I doubt it will do any harm.

Seeing this all in type has brought up a few other design ideas. My mediocre AutoCad skills are about to be pushed to their limits...


Don
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Mike Everman
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re: RamFan Engine?

Post by Mike Everman » Mon Oct 17, 2005 3:03 am

Hi Don, welcome.
Just some random thoughts...

-the worst thing about tip jets on a helicopter to my mind was the drag and high inertia making autorotation nearly impossible. Lose power, they fall like a rock.

-worry about spark last, you don't want it continuously, and it's easily delivered a number of siimple ways on a rotating member

-motive coils at the tips will be a heavy proposition, with a good deal of inertia and resulting precession forces. blade lightness should be key, IMHO.

-pulsejets down the length of the blades is of course desirable, but you'll be battling the need for low chord thickness and need for large-ish motor duct diameter, plus insulation thickness. the latter could be ignored if you could do carbon/carbon composite, as epoxy matrices will not stand the heat.
Mike
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re: RamFan Engine?

Post by Hawking » Mon Oct 17, 2005 3:55 am

Thank you for the input Mike. I agree with you in terms of minimizing rotating mass. The spark theory was born mostly out of a need for a starter motor. Cobalt or other high gauss magnetic material could be added to each blade tip without incurring too much weight gain and could simpify the starting system considerably. I'll leave the coils out for the time being.

I'm doing my best to reduce the drag incurred by the engines by intimately integrating them into the blade shape. Ideally, only a smoothly formed bulge at the blade tip to house the combustion chamber, although the tailpipe length will require it to protrude from the trailing edge of the blade.

I have no experience with carbon/carbon construction techniques. Prepeg is my material of choice. I will research the other options though.

Thanks again for your input.

Don
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re: RamFan Engine?

Post by serverlan » Mon Oct 17, 2005 5:31 am

Hi Don,
Don here.

I don't know how to calculate the thrust,
but this is a rouch rule of thumb for pulsejets -
I believe it goes like
50 lbs thrust is the same as that produced by an engine of 40 hp
spinning a prop.

Maybe you can then scale to appropriate helicopter load/HP ratios.


While we are dreaming - how about desiging the cross-section of a jet, and build it into the profile of the rotor. Sort of a distributed jet.
With block-off walls periodically to stop swirl effects along the length of the blade?

Don

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Re: re: RamFan Engine?

Post by Mark » Mon Oct 17, 2005 1:36 pm

[quote="Mike Everman"]Hi Don, welcome.
Just some random thoughts...

-the worst thing about tip jets on a helicopter to my mind was the drag and high inertia making autorotation nearly impossible. Lose power, they fall like a rock.

I wonder how tricky it would be to design a quick release mechanism that would separate the engines from the tips of the blades if you needed to autorotate?
My brother just flew in from Chicago to Pensacola yesterday, by helicopter. I think they flew at around 1500 feet.
Mark
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Hawking
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re: RamFan Engine?

Post by Hawking » Mon Oct 17, 2005 10:05 pm

Explosive bolts attaching the jets to the rotor? I don't know if I would be comfortable with explosives mounted that close to a ram or pulse jet. Interesting idea though.

@ severlan: Thanks for the power estimate.
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re: RamFan Engine?

Post by El-Kablooey » Mon Oct 17, 2005 11:03 pm

I think the thrust rule of thumb comes out to more like a 30hp engine with an effecient prop puts out about 50lbs thrust. or something like that.
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re: RamFan Engine?

Post by Hawking » Tue Oct 18, 2005 4:10 am

I think the numbers might be higher for a ducted fan arraingement, which is what I'm attempting.

I have data showing about 100 BHO producing something on the order of 200 pounds of thrust in a collegues hovercraft. The real issue here though, is to calculate how much thrust my ducted fan will produce using X amount of ramjet thrust on the blades, not necessarily how much thrust is produced per HP; unless I can convert the ramjet's thrust value in pounds to an equivelent HP number

Strange thing thrust is. Unlike a turbojet or turbofan engine, ducted fans driven by internal combustion engines do not benefit from increased inlet pressure (due to foward velocity). Also, exit velocity should be a low as possible, while at the same time moving the most amount of air. It's simple action/reaction; the more mass moved backwards, the more foward thrust is created. Higher exit velocity does nothing to help mass flow.

In my case, I'm looking to create an engine that should benefit from higher inlet pressure, yet the duct in a ducted fan is actually designed to reduce the inlet pressure.

I suppose the point is moot until I build it a prove that I can overcome the design challenges associated with running a ramjet in a 36 inch circle at 3000 RPM. Still, I'm having fun with the challenge

Don
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