Adding a pusher prop

[Head Turner]

A technical question;-

In the conventinal main rotor/tail rotor configuration all forward speed is achieved by tilting the rotor and limited by retreating blade stall and other factors.

Why would it not be possible to redesign the tail rotor gearbox to provide the normal 90 degree drive shaft for the tail rotor and an additional rotating shaft protruding rearwards onto which a pusher prop (pp) be attached. The pitch control of this pp linked to the forward cyclic control mechanisum. This would provide a push relative to the forward position of the cyclic and off load the main rotor which in turn could have a reduction in RRPM for exrta flight speed and efficiency.
The effects of weight and CG being solved by design.

[CRAN]

Unless you want to go far faster than is sensible for helicopters.

By virtue of there high disk-loading propellers are far less efficient at producing thrust than is a rotor, even up to quite high speeds. Ray Prouty explained the in's and out's of this in one of his Vertiflite articles. Therefore, unless the object of the exersize is setting a speed record the weight gain due to extra power required, extra fuel and extra prop and drive system, will marginalise the payload too much to be practical.

In addition many studies have been carried out by all of the major helicopter companies into thrust-compounding and lift compounding and found that it isn't really worth the hassle. The usual outcome is that the compound machine will flight a-little faster or be able to manoeuvre at speed a-little more aggressively but will have had it's payload eaten away to the point that it is simply not an economical machine to operate.

Hope this helps.

CRAN
:O

Mind-you, they still went ahead with the Tilt-rotor! :rolleyes:

[Lu Zuckerman]

To: CRAN

Firstly, by virtue of its high disk-loading propellers are far less efficient at producing thrust than is a rotor, even up to quite high speeds. Ray Prouty explained the in's and out's of this in one of his Vertiflite articles.

I find this a bit strange, as Ray Prouty was the chief aerodynamicist on the design of the Cheyenne. Although the Cheyenne had some major problems the propeller was not one of them. It was capable of generating sufficient forward thrust to allow the rotor to be unloaded and fly at very high speed. The propeller could also be reversed in flight to allow the helicopter to be placed in tilted attitudes in order to get into better firing positions for its’ weapons.

:E :E

[Shawn Coyle]

Lu- nice reply!!

Adding a pusher prop is complex, but as the Cheyenne showed, not impossible.
Props are not good at producing thrust at low airspeed, but pretty good at higher airspeeds, hence the attraction for cruise flight.
I understand Piasecki is working on this for a UH-60 as a demonstrator, but I don't know the progress they've made.
The complexities would probably prevent it from being a civil development - not sure how the FAA would certify it to begin with.
But there might be something to this.

[CRAN]

Yes Gentlemen,

I'm well aware that it's possible, the point I was making is that all previous attempts have concluded that it either isn't practical or the advantages are too small to warrant the additional, cost, difficulty, weight and complexity.

With regards Prouty's article, I can't give you the exact Reference of the top of my head, but it was published in the last 18mths in the AHS magazine Vertiflite and carried out a comparitive study on a Robinson R44, with (a) a normal rotor and (b) A Thrust compounded version with a propeller at the back. As I recalled the article concluded that the compounded version would require more power to do the same job. [I'll dig the article out and give you the specifics.]

CRAN

[Dave_Jackson]

Earlier compound helicopters, such as the Bell UH-1, the Lockheed XH-51 and the Sikorsky XH-59A ABC, used jet engines for horizontal thrust. Unfortunately, the jet engine is extremely inefficient for the proposed speeds, of up to 300 knots. This eliminated the jet engine.
_________________________

The proposal in Vertiflite consists of converting a Sikorsky Black Hawk to a compound helicopter by adding a large propeller and a wing. No change was to be made to its rotor geometry.

At 200 knots, the wing was to support 70% of the weight of the aircraft. One can assume that at the proposed maximum speed of 320 knots, the wing would be supporting 100% of the aircraft. This means that the craft is now an airplane. As mentioned by CRAN, this 'airplane' must have enough power to overcome the parasitic drag and the parasitic weight of the main rotor and the tail rotor. This will probably eliminate the compound helicopter as a future contender.
_________________________

A newer idea is starting to arise, and it will eliminate the 'compound' problem. The new concept consists of slowing down the rotor (or 2 counterrotating main rotors), while at the same time increasing the solidity ratio of the disk(s). This eliminates the high compressibility effects on the advancing side and the stall on the retreating side. The rotor(s), which must be a lot stronger, then act as the wings at high forward speeds.

In addition, the stronger rotors provide other advantages such as improved control response etc.

[C4]

Could always just add a Robbie to the back of the UH60 as a propellor:D :D :D

[Specnut727]

Why not stop the main rotor from rotating so it works like a proper wing, feed the power to the prop, and let it do all of the work ? Would be easier if the main only had 2 blades !!!!!

[Lu Zuckerman]

To: Specnut727


There are many reasons why you can't do what you suggested.

1) Assuming you could stop the rotation of the blades across the lateral axis you will have hit the ground due to loss of lift.

2) One blade will be backwards in respect to the relative wind.

3) The blades depend on "centrifugal force" to maintain structural integrity. Without this invisible force the blades would bend upward and fail.

Anyone else want to chime in?

:E :E

[Jcooper]

I think he had a similar idea to what that new boeing UAV is. Knowing you Lu youve had to have heard of that.

[Dave_Jackson]

Lu.

Jcooper has mentioned a good example. It is intended that the Boeing Dragonfly stop its rotor during forward flight. Incidentally, the craft had its first flight last month and there is a video of it at the bottom the web page Canard Rotor/Wing (CRW) (http://www.boeing.com/phantom/crw.html) .

Others such as Stepniewski's Low Tip Speed Design Concept (http://www.unicopter.com/1093.html) and Sikorsky's Reverse Velocity Rotorcraft Concept (http://www.unicopter.com/1281.html) are intended to fly, using strong slow turning rotors to support the craft.

[Specnut727]

Thanks all,

Honestly, I had not seen the links referred to by Dave when I made the comment. I thought it was a bit silly, following on from the previous Robbo comment.

Goes to show that apparently silly ideas can sometimes be developed into something usefull. By the look of the effort Boeing and Sikorsky are putting in, it's only a matter of time until it bears fruit. Who knows what rotorcraft will look like a few years from now !

[Helipolarbear]

Lu......The Cheyanne was a marvellous machine, just like the original Blackhawk with the H-53 head and 20' wings either side.
Fort Campbell, Kentucky had a Cheyanne at the main gates.
Very big two-seater....I believe theres one at the Army Aviation Museam, Fort Rucker, Alabama. It's amazing how such a machine
ends up as a footnote on a thread like this!;)

Lu... The 'X' wing heli concept was based on symetrical airfoils that would produce a percentage of lift when stopped with enough forward speed that would allow smaller wings to be the main lift support......................Theory..........I don't know if they succeeded in actually achieving this. Then along came the Tilt rotor in a big way. Back in 1982, I was lucky enough to get a 5 minute hop in the X-15......second only to a ride in an F16B!
:p