2 engines 2 props?

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HeliDev

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My helo desing is going to require a bit of power, in the order of 200hp. Ive been thinking that perhaps instead of using one 200hp+ engine, using 2 smaller ones. I know that there will be a weight peanalty, but there are some advantages as well, OEI performance for one.
One thing I have been thinking about though is using two props instead of one.
How much would I be able to reduce my prop diameter by? For example if I was using one 50in prop, could I drop to 2 25s? I know the relationship wont be linear, but how closae is it?
Thoughts appreciated
 

orion

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Simply stated, not even close. However without seeing the specifics of what you're trying to do, it is a difficult question to answer.
 

Largeprime

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Feb 18, 2004
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I belive the efficency of a prop is the cube of its diamater, and I think thats the first order variable.

Orion?

But of course the wise one has it. It might be less efficient, but workable according to the requirements.
 

HeliDev

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AHHHHHHH, I love blunt answers.:D
My design is a helo with a pusher prop, kinda like a gyro, but able to hver. There is a price to pay for this, ie weight and complexity, but Im slooooooowly solving them 1 by 1. The thing is I know a bit about single props, but little about twins. In a twin helo the engines power one system, so Im not sure how the balance works for twin props. I have seen fixed wing with two smaller engines, and 2 props, I always assumed that twins were for more power (and to give you one engine to get to the accident), but I was clearly wrong with this assumption.
My thinking was that if the same area was used for 2 props instead of one, then I could use smaller props to help with ground clearance. Rotating isnt such a problem, more the side clearance when doing a slope landing.
Help appreciated.
 

HeliDev

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Simialr, but those guys are a bit more ambitous than me.
Im just looking to make something with a 150knt cruise, those guys are trying to get one that goes faster than a buisness jet.:D
 

pylon500

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Taree Airport Australia
Are you trying to do something like a V-22 Osprey, or like a Focke-Wulf Fw 61, Achgelis? :confused:
Either way, if you build a twin rotor system, you need to couple both rotors mechanically so they both always turn together.
Then you can decide how to apply (and how much) power to the joining shaft, obviously this should be clutched!
Maybe you want to do a COAXIAL system, check out one of my earlier threads;
https://www.homebuiltairplanes.com/forums/showthread.php?threadid=814
:D :gig:
Arthur.
 

HeliDev

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I think you may have miss understood my idea. It relates to the pusher prop on the back, not the rotor system. I am infact using a coax rotor layout, and it will drive through a synchronised gearbox.
Ive got a few books and a fair bit of info on the helicpter side of things, but on the prop side of things I need some more. Hence why I was curious about the two prop configuration. By reducing the prop diameter, I can ruduce the height of the rotor pylon, thereby reducing drag.
Just one of those things, I try to look at every thing before I choose.
 

StRaNgEdAyS

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Ahhh, I get it now...
I was envisioning 2 props, side by side. In that case I think your overall diameter won't be changing that much, the significant benefit of the coaxial layout being that contra rotation will eliminate the need for a tailrotor. Big savings on drag and noise there...:D
 

HeliDev

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Yep couldnt have described it better.
Coax requires a bit more power to hover, but I can reduce my rotor diameter which will make it more compact on the ground, and give me a bit lees drag when going fast. On the other hand the extra drag ontop from the extra rotor is something that will have to be over come.
I looked at a Chyenne layout, but the wings are a pain int hte a55. Restricts hwat you can do "off airport", and takes a lot more power in the hover, as you have to overcome the download on the wings, kinda like standing on your shoelaces.
THe ABC was a fantastic helo except for the jet engines used for thrust compounding, it had about 1/2hr endurance. Sikorsky planned a B model which would have had a ducted fan instead of the jets, increasing endurance.
Most of the compound designs were designed to get upto 400km/hr, that requires a heap of power, reguardless of the layout, my goal is more in the area of 300km/hr. This would give you a quick transit, with VTOL capability, while having a low purchase price (compared with a turbine).
While there are helos already capable of 150knts, most are in the order of 500hp, I hope to get there with 250ish.
While everything is a compromise, the ABC is the best one Ive found, I dont need a wing, and the use of a pusher prop should make better use of the power available in forward flight.
Im mid a bit of a redesign right now, Ive increased the diameter of the fan to 4.5ft, and moved some internals around, I think im probably trying to getit to perfect, but I know Ill have to compromise on some things.
So Im nearly read for my first scale model, hopefully it will be flying before september, depends how much work gets in the way.
 

wsimpso1

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C'mon, you know that disc loading is important from rotor design. Props are just rotors, which are just wings with off velocity distributions,,,

Lift and drag are linear functions of area and angle of attack above the zero lift angle, second order functions of local velocity, and impacted by aspect ratio. Since the local velocities are highest at the prop tips, tip design and aspect ratio plays an even bigger part than in normal foils. Since the tips usually are running large fractions of Mach, they need to thin outside too.

They absorb engine torque with the second power of their speed, and power with the third power of their speed. Shrink the foil but maintain the radius, and you can spin the same diameter prop with a smaller engine. Shrink the radius and enlarge the foil, and watch thrust decrease.

The biggest thing about propellers (parallel to helo rotors) is that they need to be big and turned slowly to get the most out of them.
Short story, no, your blade diameter will not be reduced much with smaller engines and mor of them, but then a small reduction may be all that you need, and so would be an acceptable compromise.

Billski
 
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