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A complicated question

A complicated question

A complicated question

(OP)
If I were to mount a mini-jet engine vertically and direct it's thrust (say 100Lbs) through nozzles at the end of a pair of rotors, via a rotating swivel joint, could I make a mini helicopter capable of lifting someone? The idea being that there would be no torque reaction and only one moving engine part. I guess the question is- would 100Lbs of thrust acting on the ends of the rotors be enough to produce the required lift? (say 300Lbs load) If not, how much thrust would be required?

Anybody care to make an educated guess? I just thought it might be a neat way to make an ultralight flying machine.

RE: A complicated question

It's already been done with ram jets at the rotor tips. Google it.  

RE: A complicated question

(OP)
I have seen those. But they all seem to suffer from very high fuel consumption and problems with accurate throttling. But thanks for reminding me as it seems a Dutch two seater called the 'NHI Kolibri' produced in the 50's had 2 x 20 Kg thrust ramjets. Which is great because maybe one person would then only need 20 Kg's of thrust? Which means an engine as small as this !

http://www.jetlinemodels.co.uk/turbines/Turbines.html

Meh, it could even work smile

RE: A complicated question

(OP)
Thanks for the info. Seems they were creating the 'flame' at the rotor tips by pumping compressed air and fuel to the tips and igniting it? But they were too noisy.  

My idea was to use a micro jet with a long 'split' tailpipe so that the air could be cooled/noise abated before it reached the tips. If I get the time I'll try and make a simple drawing of what I mean. In terms of overall noise it should be possible to make something much quieter than the jet engine itself?

RE: A complicated question

so you duct the jet exhaust all the way along the blades, thru something like two 90deg bends, thru some sort of collector (the jet engine would presumably be fixed in the fuselage, whereas the blades are (naturally) spinning) ... i suspect that the pressure losses would be "huge".  initially i thought you were going to mount small jets at the tips of the blades (which has been proposed previously)

RE: A complicated question

Kind of like that rotating gizmo in the bottom of a dishwasher?

RE: A complicated question

Or a lawn sprinkler.

Remember the jet exhaust may be in the high hundreds of degrees Farenheit, which won't do much to help the structural integrity of the rotor, an already highly stressed device.

RE: A complicated question

you know, that giant heavy lifter that Hughes built back in the '50s used, I believe a couple of gas generator turbines that used bleed air to drive the rotors; still plenty hot tho.....

RE: A complicated question

(OP)

Mmm, kind of. But the path would be more of a gentle 90Deg radius. The rotors would also be slightly curved to help with gas routing. Also I was thinking of using a bypass turbine as more of an 'air pump'? I'm sure there would be some losses, but even if they were 50% (!) , it would only need a 100Lbs or aprox 40Kg thrust to make a feasable single person helicopter. Like I say, I'll draw something up when I get the time.  

RE: A complicated question

Don't gas turbines misbehave rather badly, and expensively, as the back pressure increases?

 

Mike Halloran
Pembroke Pines, FL, USA

RE: A complicated question

yeah, we have to wack them 'round a bit and tell them to "fly straight"

RE: A complicated question

(OP)
So a jet engine will not work with a long tail pipe? Even if the exit pipe is of the same or slightly greater dia than it's normal exhaust nozzle? That's a problem.

However, by using rearward curved blades would centripital force not help 'move' the air mass towards the end of rotor? And thus help reduce back pressure? Or does air not work like that?  

RE: A complicated question


A man named Gluhareff designed and built Propane fuelled jet engines without any moving parts. Really clever designs.
The smaller versions could be tip-mounted on a rotor fitted to the back of the pilot (or victim?).
See if he is still in business.

D  

RE: A complicated question

welll, nooooo ... there's a difference between a long tail pipe and terminating the exhaust against a plate (the bottom of the collector, before it gets deflected into the rotors).

annddd, noooo ... i think you need the pressure in the collector to force the air along the blades.

i have pictured having the fixed engine exhaust into a "tank" that has four exits.  what about blending the four orifices so there is little flat plate surface ... there'd be some sort of roller race, maybe a flange outside of the exhaust ... this would minimise the back pressure, 'cause the flow is being directed out along the blades ... sort of like you might've been thinking about (now that i've written it all out !!) ... the problem could well be maintaining the exhaust area from the jet eflux to the blades (i see the blade area being much smaller than the jet exhaust)

RE: A complicated question

(OP)
I think that can be solved by using a continuously variable cross section rotor so that the volume or cross section of the pipe is always constant? And use a slit type nozzle on the trailing edge of the rotor instead of a round nozzle prehaps?

Anyhow, I had 10mins so I knocked up a very crude drawing to show what I mean. Gota love MS paint smile

link- http://files.engineering.com/getfile.aspx?folder=d92f8b31-70b4-4944-affd-795f7fdebfa7&;file=Ducted_jet.bmp
 

RE: A complicated question

Oy.  When is your drawing due?

I hate raining on parades.  But I did extensive aerothermodynamic analysis of a device that acted much as your ducted jet rotor did.  The fluid passing down a rotor blade can be imagined to be passing through a stationary coiled tube, whose bending radius gets progressively smaller.  Think of it as "coriolis" forces...but analogies don't work very well.  This causes the flow to swirl axially, incurring very large head losses, which tend to eat up any pressure head generated by centrifugal forces.

What is worse, the fluid in the long tube tends to set up oscillations, the rotor is acting like an amplifier for any perturbations at the rotor inlet.  When this happens, the flow through the rotor essentially stalls repetitively, causing a lot of racket (acoustic noise) and damage to surrounding structures.

NASA did a lot of studies back in the '50's on supersonic compressors, including radial vane compressors.  You should be able to find many of the reports on the NASA servers.  They can be hard to read, but informative.

RE: A complicated question

(OP)
Can the "coriolis" force be countered by gradually morphing from a round to oblong section pipe of the same area?

btw- I did link to a very crude drawing in my last post.

RE: A complicated question

"btw- I did link to a very crude drawing in my last post. "

Yes, you said you had knocked it up...thus I asked when it was due...ba-dum.

The degree of axial rotation of the flow is countered to a slight degree by the aft-ward sweep of the channel, as you've drawn, but it doesn't go away, any more than the centrifugal forces will go away.

RE: A complicated question

Uh, the S- shaped rotor blade has some other problems, above and beyond the gas pressure loss and difficulty of manufacture, even if it's not double- walled.

- It has to be unnecessarily heavy to keep from straigtening out or fracturing from centrifugal force.

- That gets particularly troublesome, as does the thrust vectoring, when you try to translate the bird by cyclic alteration of the AOA.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: A complicated question

Put the nozzles on the ends of the Hiller-bar...

D
 

RE: A complicated question

(OP)
Umm, the intention was to have wider/larger than normal rotors that spin more slowly. Which reduces the centrifugal stress on them, and the "coriolis" force on the gas. I was thinking some sort of temp resistant composite construction might be possible? I know this would limit speed etc, but who'd want to go fast on something like that ?!? It's an idea for a toy, not a commercial aircraft.

"Yes, you said you had knocked it up...thus I asked when it was due...ba-dum."

LOL, very dry..my bad for missing it smile

 

RE: A complicated question

Seems to me that for a system that is limited in power, you're unnecessarily adding weight where it hurts the most.  Your rotors will need to be stronger, more corrosion resistant, survive exhaust pressures, extra plumbing, etc.  These all add weight that must be spun.

If the same amount of power were directed at driving rotors that had half the weight, one would think that you'd get better overall performance.

TTFN

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RE: A complicated question

The centrifugal forces you are trying to eliminate actual strengthen the rotor "disk" by preventing "coning". That is why there is a very narrow rpm range for main rotor speed. For example, "pull pitch" ie raise the collective, from say, the flight idle position,the blade tips will climb up, possibly exceeding the structural design of the blades.And isn't "temp resistant composite construction" sort of an oxymoron?  

RE: A complicated question

(OP)
Thanks for all your input guys. I only wish I had the time/money to experiment with concepts like this. Sadly I have a real life.. and so it will remain nothing more than an idea.

RE: A complicated question

I am missing something here.

How can you lift 300 lbs with 100 lbs of thrust?
Surely you need 300 lbs of thrust!
 

RE: A complicated question

this is a fairly unique installation.  first off, the engines aren't oriented like a 747, so the power/weight ratio will be much higher in this configuration.  2nd (and i thought like crystalclear at first) the fan isn't working like a helicopter either, the thrust is being routed out to the tips and is reacting the drag of the blade (sort of like the ancient greek turbine pix i've seen).  so it May be Possible for a 100 lbs thrust jet to lift a 300 lbs payload, though i doubt you'll get 100% useful thrust out at the blade tip.

RE: A complicated question

I didnt' say it was efficient or that the OP figures would add up but, very few A/C of any configuration have thrust to mass of unity or better..

There have been a couple of A/C with vaguely similar configurations, I doubt they all had unity thrust to weight.

KENAT,

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