Rotor Revs

[VfrpilotPB/2]

Good evening Ppruners

It seems that every spec I read indicates that main rotors nearly all operate in the 400 to 450 RPM, Why? is this a speed range worked out on Strength to weight ratio's or kinetics or what, ... why not higher would a higher rev range give or offer greater safety in EOL situations,.. or is it down to size of main shaft governs highest rev limits, therfore higher revs = heavier kit ?

Peter R-B
Vfr

[CRAZYBROADSWORD]

tip speed how many sonic booms would you like :}

[2beers]

This has been mentioned before in other threads, but it is all about the speed of the blade (-tip), so a smaller diameter rotor will turn faster than a larger. I think the tip speed is usually somewhere around 400-430KTS. With my manuals at home the number stand to be corrected. The rotor tip airspeed will then of course vary with the helicopters speed through the air x 2.

Two problems with the rotor speed: To slow -> retreating bladestall, to fast -> compression problems due to getting close to speed of sound.

Hmm, when I come to think of it, the rotor rpm is almost always the same no matter the size of helicopter: 100-105% :}

/2beers

[ShyTorque]

Tip speed governs max Nr, as already stated.

The aircraft I trained on (Whirlwind 10, three long, spindly blades and a single Gnome turbine) had a normal Nr of 210 rpm (I think, it was three decades ago) and just 190 rpm / 80 kts (more certain of that) for a range auto. At 190 rpm you could see the individual blades passing.

[Ascend Charlie]

2beers,
It's not airspeed x 2, it is + or - the airspeed.

Another factor in RRPM is having to make the hub and grips stronger to cope with the centripetal force. Double the RPM, four times the force.

Huey blades are at 324 RPM, I think the R22 is about 430 or so.:bored:

[ramen noodles]

The reason for rotor rpm is to make the blades have enough airspeed, obviously. The max rpm is selected to make the blade tips (the fastest part) move at about 700 feet per second. This allows them to make lots of lift in a hover, but also avoid transonic speeds at cruise. Most rotors spin so that the blade tips hover at 675 to 725 feet per second.

This is why the rpm is higher for smaller radius helicopters - with less radius, it takes more rpm to make the needed tip speed.

[Pilot DAR]

If you watch the huge Russian MIL helicopters, the rotor turns so slowly, you can actually count the rotations.

[Farmer 1]

2beers,
It's not airspeed x 2, it is + or - the airspeed.

It's x 2, Ascend. The advancing blade speed increases by the airspeed, the retreating blade speed decreases by the airspeed.

[Shawn Coyle]

The MI-26, being the largest helicopter currently in operation, has a rotor RPM of 80 RPM.
I commented to the Russian pilot who flew with me that it should be in RPD (Revolutions per Day).

[slowrotor]

So, in theory, an infinitely large rotor would have an rpm of zero!:}

[whyisitsohard]

Yes, its called a "fixed wing"
D

[RJ Kanary]

THIS gem got close to that.....................

Hughes XH-17 - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Hughes_XH-17)

RJ

[Ascend Charlie]

Ah, farmer, it's in the terminology. The blade has its airflow from rotation, then as the machine moves forward at a certain airspeed, the blade tip then sees rotation+airspeed on the advancing side, and rotation-airspeed on the retreating side. The difference between them is definitely 2 x airspeed, but the blade tip never sees rotation+2xairspeed.:ok:

[rotorfan]

"It seems that every spec I read indicates that main rotors nearly all operate in the 400 to 450 RPM"

They differ quite a bit, actually. The local Guard base operates Chinooks, which I can hear coming well before seeing them. I can follow the blades going roundy-round, because they're relatively slow, being a large disk. The R22 disk turns at 530 RPM (104%), which gives 672 FPS tip-speed, or about 398 knots. That's right in line with ramen's comment "blade tips hover at 675 to 725 feet per second." and 2 Beers "the tip speed is usually somewhere around 400-430KTS".

What I don't understand is how some ships can have such high max airspeeds without getting RBS. Are their rotors turning so fast that the tips are barely subsonic, or what? :ugh:

rf

[ramen noodles]

rotorfan asked:
What I don't understand is how some ships can have such high max airspeeds without getting RBS. Are their rotors turning so fast that the tips are barely subsonic, or what?

The real factors are tip speed and rotor blade total area. The faster the blades, the more lift at high speed, and the wider the blades, the less angle of attack needed for the retreating blade to do its job. So high speed is gained by having more, wider and faster blades. The number of blades is not terribly important, it is the total blade area as compared to the total disk area that counts.

The problem is that more, faster, wider blades actually reduces hover performance, and the rotor system also weighs more with all that heavy blade area spinning around. For these reasons, high speed is a tradeoff against better hover payload.

[[email protected]]

There is a limiting relationship between tip speed and forward airspeed for helicopters where one is divided by the other and the limit is 0.5. I can't remember the exact name for it as I don't have access to my notes down here but the faster the forward (fuselage speed) the slower the tip speed needs to be.

[Shawn Coyle]

Crab:
It's called 'advance ratio'.

[[email protected]]

Thanks Shawn - it was tucked away in a dusty corner of my brain from a lecture by Simon Newman a good few years ago at Southampton University.

[ShyTorque]

What I don't understand is how some ships can have such high max airspeeds without getting RBS. Are their rotors turning so fast that the tips are barely subsonic, or what?


Modern blades for faster helicopters are becoming by necessity more complicated in shape. Westlands ran the "BERP" project (British Experimental Rotor Programme) in the late 70s / early 80s and came up with an "oblique spade" type of tip, which gave the tip a swept wing effect (good for high speeds) whilst minimising adverse aerodynamic blade twist/flex and without sacrificing too much performance at lower speeds. I don't think anyone these days equips a modern medium or large sized helicopter with a simple "plank" shaped blade design.

[Kelly Hopper]

All modern propeller blades have a twist to compensate for the outer element travelling faster than the inner but I have never seen such a twist in helicopter blades. Is it just a problem of construction and if so what a shame as efficiency would be improved dramatically?

[DOUBLE BOGEY]

Most types of rotor blade have "washout" to even the distribution of lift along the blade, oitherwise the bending forces resulting from higher lift (by virtue of greater speed of the outboard portion of the blade) would subject the blades to undue stress.

[ShyTorque]

In older, metal construction, perhaps not so obvious but more modern types are of composite construction and definitely DO have a "twist", or washout. The aerodynamic profile of these blades sometimes varies greatly along their length. Some blades, such as those on the A109E, even have a trailing edge "flap" on the inner portion.

YouTube - Heli blades (http://uk.youtube.com/watch?v=SjFSt4Msde0&NR=1)

[Oldlae]

ShyTorque.
The Whirlwind Mk 10 and equivalent civil WS55 Series 3 has a Rotor speed of 220-222 rpm.

[ShyTorque]

Thanks, Oldlae.

It's been a long time. :)

Any still flying?

[MightyGem]

The twist is there. It's just not as obvious as with a propellor. Also, some early blades tapered towards the tip to reduce the lift.

[Shawn Coyle]

The 412 has the most awesome twist and taper of any blade I've ever seen.

[ramen noodles]

The washout (reverse twist) for most helicopters falls between -10 degrees and -16 degrees, where the tip has lower angle of attack than the root. This distributes the average lift across the blade, since the tip is going faster, it needs less angle for the same lift.

The angle could be greater, but the structure of the blade is challenged by the twist angle, in that the stiffness in torsion is weakened by the twist.

Variable twist is coming with the advent of on-blade control, in effect, the movable blade tabs will let the flight control computer select the ideal angel for each segment.

[cmacltd]

Shytorque, you may well be right (and yes A109 blades do have significant washout) but as I recall the inboard trailing edge flap was introduced on the early 'C' model. It was a trial initiated by Mario Ceriani and Paolo Ferreri at Agusta Aviation Corporation (to the chagrin of the design aerodynamicists at Cascina Costa!) and was there to kill lift generated by the (then) new profiled blades that had an increased bull-nose on the leading edge at the root. This generated lift and 'patter' at a frequency of 4R over the redesigned & straked aft cowling. The trailing edge slat addessed this generated vibration very well, particularly at high cruise (140+knots)

I offer it as a thought only:8