A complicated question

[h1ghf1ve]

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.

 

[Compositepro]

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

 

[h1ghf1ve]

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

 

[KENAT]

Fairy rotodyne may be worth a look too.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at

http://eng-tips.com/market.cfm?

 

[h1ghf1ve]

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?

 

[rb1957]

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)

 

[thruthefence]

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

 

[btrueblood]

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.

 

[KENAT]

You'll also have losses along all that ducting. There's a reason early jets had the shortest jet pipe possible.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at

http://eng-tips.com/market.cfm?

 

[thruthefence]

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.....

 

[h1ghf1ve]

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.

 

[MikeHalloran]

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



Mike Halloran
Pembroke Pines, FL, USA

 

[rb1957]

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

 

[h1ghf1ve]

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?

 

[Dan320]

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

 

[rb1957]

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)

 

[h1ghf1ve]

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

link-

http://files.engineering.com/getfil...b4-4944-affd-795f7fdebfa7&file=Ducted_jet.bmp

 

[KENAT]

Some Hawker designs have bifurcated nozzles, see maybe the Sea Hawk or Harrier.

You may be able to find some information on any problems this causes.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[btrueblood]

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.

 

[h1ghf1ve]

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.