Helicopter top speed

[Ayahuasca]

is limited by the speed of the retreating rotor blade which stalls at speed Vmax.

Would contra-rotating blades solve this as one is always going forward on each side?

[Lefty Two Drams]

I'm no aerodynamicist (where's Shirt when you need him?) But the logic seems sound.

There are plenty of contra-rotating-blade helis around but the Lynx still holds the airspeed record. 250mph-ish IIRC

[Mk2 Jim]

Do we need to factor in conveyor belt speed here too?

[Lefty Two Drams]

[Pothole]

its because the speed of the retreating blade is subtracted from airspeed, and the apeed of advancing blade is added to airspeed. For lift to be equal accross the rotor disk, the rertreating blade has to operate at a higher angle of attack. When angle of attack increases beyond a critical limit, the retrating blade stalls and lift is reduced. Due to 90 degree control lag, this pitches up the rotor and decelerates the helicopter. Not sure how contrarotating rotors would help as there's two lots of retreating blades to counteract any advantage from the two advancing ones...The fastest Helo in the world is still the Lynx, a single rotor job, innit?

ETA to add link to lots of technoguff about this very thing: http://www.helis.com/howflies/maxspeed.php

Edited by Pothole on Saturday 26th December 15:35

[shirt]

Lefty Two Drams said:

I'm no aerodynamicist (where's Shirt when you need him?) But the logic seems sound.

me neither, but with some distant memories of heli lectures i'd also tend to agree.

there are a few variables and limitation that affect both single rotor heli's and contra rotating ones.

the main limiting factor is retreating blade stall as already mentioned, creating dissymmetry of lift across the rotor. this can be compensated for [blade flap] but if you took the case that, at extremely high speed, the a/c was travelling faster than the retreating blade, the blade would stall no matter what the angle of attack.

also, the airspeed of the advancing blade is obviously much higher than the motive speed of the aircraft. if a rotor were to break the speed of sound it would experience wave drag - vastly higher levels of aerodynamic drag associated with the shockwave - and also increased vibration.

these things can be designed out to an extent [articulated rotors, aerofoil design etc] as can problems involving engine power and overall a/c drag. things get more complex when you consider tandem rotor a/c as, whilst you can cancel dissymmetry of lift you are still governed by engine power, overall drag, etc. that may mean you won't get near the Vmax imposed by the dissymmetry.


so to summise, i'm not sure! but if i had to go with one better than the other i'd back coaxial rotors [2 on the same shaft, spinning in opp. directions] as i know they generate more lift for a given engine power as all the drive goes to the rotors and not wasted in powering a torque stabiliser.

[Lefty Two Drams]

Sure I read somewhere that swept rotor blades are being used these days too. The principle being the same as swept fixed wings - less drag for the same wing area.

Or did I make that up?!

[SlipStream77]

Presumably it is the inside of the retreating blade that stalls first, so a partially stalled blade is ok?

[Eric Mc]

Lefty Two Drams said:

Sure I read somewhere that swept rotor blades are being used these days too. The principle being the same as swept fixed wings - less drag for the same wing area.

Or did I make that up?!

I haven't come across any swept back rotor blades yet - but those "paddle" type features that you now see on some rotors (like on the Merlin or later versions of teh Lynx) perform a similar function to sweepback on a normal wing.

[eccles]

Eric Mc said:

Lefty Two Drams said:

Sure I read somewhere that swept rotor blades are being used these days too. The principle being the same as swept fixed wings - less drag for the same wing area.

Or did I make that up?!

I haven't come across any swept back rotor blades yet - but those "paddle" type features that you now see on some rotors (like on the Merlin or later versions of teh Lynx) perform a similar function to sweepback on a normal wing.

The Apache has swept rotor blade tips.

[RDE]

Eric Mc said:

Lefty Two Drams said:

Sure I read somewhere that swept rotor blades are being used these days too. The principle being the same as swept fixed wings - less drag for the same wing area.

Or did I make that up?!

I haven't come across any swept back rotor blades yet - but those "paddle" type features that you now see on some rotors (like on the Merlin or later versions of teh Lynx) perform a similar function to sweepback on a normal wing.

They're called BERP blades (British Experimental Rotor Programme) I believe. I was under the impression that the limiting factor with helicopter forward speed was sonic effects on the advancing blade?

[statts1976uk]

The speed of the blade cannot go any faster than the speed of sound as it would create massive shockwaves for the following blade which is always 90 degrees out of phase. On our civilian Pumas and Sikorsky S92 they have swept tips.

[junglie]

The BERP tips on the Lynx were designed for improved efficency in forward flight but the VMax is actually lower than the old metal blades as is the flight envelope. However the blades a much lighter and easier to 'Tune' when they are replaced.

[dickymint]

Are you thinking of taking one into the jungle?

[rhinochopig]

Top Speed Mach 2.0!!!!

[Mojocvh]

eccles said:

Eric Mc said:

Lefty Two Drams said:

Sure I read somewhere that swept rotor blades are being used these days too. The principle being the same as swept fixed wings - less drag for the same wing area.

Or did I make that up?!

I haven't come across any swept back rotor blades yet - but those "paddle" type features that you now see on some rotors (like on the Merlin or later versions of teh Lynx) perform a similar function to sweepback on a normal wing.

The Apache has swept rotor blade tips.

It's also a slug!

[eccles]

Mojocvh said:

eccles said:

Eric Mc said:

Lefty Two Drams said:

Sure I read somewhere that swept rotor blades are being used these days too. The principle being the same as swept fixed wings - less drag for the same wing area.

Or did I make that up?!

I haven't come across any swept back rotor blades yet - but those "paddle" type features that you now see on some rotors (like on the Merlin or later versions of teh Lynx) perform a similar function to sweepback on a normal wing.

The Apache has swept rotor blade tips.

It's also a slug!

I think it's VMAX is about 180 knots, but in reality they don't often go that fast.