Rotor Revs
 

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

tip speed how many sonic booms would you like :}

Not revs, speed!
 

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

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.

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:

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.

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

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

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).

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

Yes, its called a "fixed wing"
D

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

Hughes XH-17 - Wikipedia, the free encyclopedia

RJ

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:

"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

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.

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.

Crab:
It's called 'advance ratio'.

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.

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.

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?