Fastest theoretical cruise speed?

[sling load]

Nick,
Could you tell us the highest cruise speed that a helicopter could possibly get to, given the blades reach compressibility at the tips, what do you think is the fastest we would expect to get to, and if in fact this high speed brings in other problems. would like your opinion, thanks.

[Nick Lappos]

Nick,
Could you tell us the highest cruise speed that a helicopter could possibly get to, given the blades reach compressibility at the tips, what do you think is the fastest we would expect to get to, and if in fact this high speed brings in other problems. would like your opinion, thanks

slingload,
The real culpret is retreating blade stall, not the advancing blade compressibility. If it were Mach effects, we'd just slow the rotor down with higher speed, but in reality, this would simply help the retreating blade stall.

Most rotors get into stall when they must provide lift and also thrust to pull the machine forward thru the air. Any means to reduce lift (like wings) and reduce thrust (like a pusher prop) will reduce the stall effect, and increase the speed. The max speed a pure helicopter has ever demonstrated was done with the Lynx at about 249MPH (they used some excess engine power to provide jet thrust, but stayed below the max allowable to still qualify as a pure helicopter).

The max speed of a compound helicopter (extra thrust provided by a jet or prop)is the Sikorsky ABC, which achieved 299 MPH in level flight in the mid 1970's. Here is a site that describes it:
http://www.russian.ee/~star/vertigo/sik_s-69-r.html

Previosly, a winged compound Huey went about 245 MPH
http://www.russian.ee/~star/vertigo/bell_533-r.html

Neither aircraft were practical in their test configurations, but both showed that speeds up to 300 MPH are achievable in rotorcraft.

Beacuse a tilt rotor offloads the rotor and uses the wing to do its lifting in cruise, its speeds are quite a bit higher, while it also has good cruise efficiency, so it seems to be a pretty good high speed solution, if you are willing to face the payload penalty it has in a hover when compared to helicopters (about 40 to 50% less payload for the same power and empty weight).

[sling load]

Thanks Nick,
The blade tips on the S-92, are they for lift or noise purposes? What are their main benefits?


Cheers

S.L

[Nick Lappos]

Sling load asked:
The blade tips on the S-92, are they for lift or noise purposes? What are their main benefits?

Nick sez:

The swept, tapered and drooped (anhedral) tips are there to enhance performance and also reduce noise. The sweep and taper reduce thickness at the high speed tip, and so are helpful in reducing noise. They also cut power needs at high speed cruise, so they allow us to get more range and/or speed.

The droop helps control the vortex shed from the blade so it does not hit the next blade in line while hovering. This reduces the power needed to hover, and therefore raises the hover performance by a substantial amount (the anhedral tip alone is worth about 800 pounds of lift on an S-92 in a hover). The droop costs a bit of cruise performance, due to higher drag, but this does not outweigh the benifit of the hover performance increase.

[The Nr Fairy]

Nick :

Those tips, are they derived from the BERP ( British Experimental Rotor Program ) blades, late 80's, early 90's if I remember right ? Of course they didn't have droop ( I don't recall EH101 blades having anhedral ).

[sling load]

Thanks Nick

[Nick Lappos]

the Nr Fairey asked:
Those tips, are they derived from the BERP (British Experimental Rotor Program) blades, late 80's, early 90's if I remember right ? Of course they didn't have droop (I don't recall EH101 blades having anhedral).

Nick sez:
This is an example of the power of good press agents! The BERP blade is a very efficient one, now in its 3rd or 4th evolution, but it is not so greatly out of line with the typical high technology advanced blade. However, it has an acronym, which makes it clearly differentiable!

The swept, tapered anhedral tip of the S-92 and the Black Hawk Growth Rotor Blade (the same blade, S-92 uses an extender cuff to make a larger radius)is a purely Sikorsky design, and is approximately equal to the BERP, I believe.

The Japanese have a blade that is about 5% better than the BERP, and Sikorsky has one that is even better yet under development.

Let the games begin!

[vorticey]

Speed over a closed circuit of 500 km without payload : 345.74 km/h

Date of flight: 08/02/82
Pilot: Thomas F. DOYLE Jr. (USA)
Course/place: West Palm Beach, FL (USA)

Rotorcraft:
Sikorsky S-76 A (2 Allison 250-C30, 650 shp each)
Registered 'N5445J'


Speed over a straight 15/25 km course : 400.87 km/h

Date of flight: 11/08/86
Pilot: John Trevor EGGINTON (UK)
Crew: Derek James CLEWS
Course/place: Glastonbury, Somerset (UK)

Rotorcraft:
Westland Lynx (2 Rolls Royce GEM Mk. 531, 1 177 kW each)
Registered 'G-LYNX'


world records heli (http://records.fai.org/rotorcraft/current.asp?id1=112&id2=1&id3=2)

[Grey Area]

BERP tips do have some anhedral - one aim being to "throw" the tip vortices down and clear of subsequent blade paths (unsuccessfully in the case of the Lynx and manifesting itself in a very distinct and uncomfortable 1R vibration at low forward speed [15-18 kts]). Haven't flown 101 yet, but am looking forward to it.

GA