Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

  • Congratulations waross on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

A complicated question

Status
Not open for further replies.

h1ghf1ve

Mechanical
Oct 4, 2008
11
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.
 
Replies continue below

Recommended for you

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

D
 
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 :)

 
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

FAQ731-376
 
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?
 
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.
 
I am missing something here.

How can you lift 300 lbs with 100 lbs of thrust?
Surely you need 300 lbs of thrust!
 
crysta1c1ear

Think about that long and hard.

How much thrust does say a 747 have. How heavy is it.

KENAT,

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

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:
 
Status
Not open for further replies.

Part and Inventory Search

Sponsor