Helicopter Main Rotor Blade in slow-mo

PFVA63 · Jan 10, 2020

Hi,
That's interesting. Thanks for posting it.
Pat

gumbyk · Jan 10, 2020

Just spinning around, waiting for fatigue to set in...

tyrodtom · Jan 10, 2020

The faster the chopper goes, the more pitch change you've got from when the blade is retreating or advancing.

Helicopters are funny animals, if you go too fast, they can get into what's called "retreating blade stall"

Gnomey · Jan 11, 2020

Good one!

gumbyk · Jan 12, 2020

tyrodtom said:
The faster the chopper goes, the more pitch change you've got from when the blade is retreating or advancing.

Helicopters are funny animals, if you go too fast, they can get into what's called "retreating blade stall"

And this is a rigid rotor, so no lead/lag. A fully articulated rotor will show the blade moving in circles.

VALENGO · Jan 16, 2020

The ¿wind generator? in the background gives a good reference to see that the blade has the same incidence in the same point of the turn.

tyrodtom · Jan 16, 2020

That " wind generator " is the tail rotor.
Or was that humor ?

Sid327 · Jan 17, 2020

tyrodtom said:
The faster the chopper goes, the more pitch change you've got from when the blade is retreating or advancing.
Helicopters are funny animals, if you go too fast, they can get into what's called "retreating blade stall"

Almost correct....
The faster a heli flies ....the advancing blade has less pitch (for equality of lift).
There are three limits to forward speed in conventional rotary/wing aircraft:-
1] Retreating blade stall.
2] Advancing blade tip speed approaching critical mach (hence most modern r/w have swept back tips).
3] Cyclic forward limit stop.

Note also that tail rotors on more modern helis turn rearwards against direction of flight for the same reason as #2

wuzak · Jan 17, 2020

Sid327 said:
Note also that tail rotors on more modern helis turn rearwards against direction of flight for the same reason as #2

Hmmm. Surely one side of the tail rotor will still be heading forward?

tyrodtom · Jan 17, 2020

Sid327 said:
Almost correct....
The faster a heli flies ....the advancing blade has less pitch (for equality of lift).
There are three limits to forward speed in conventional rotary/wing aircraft:-
1] Retreating blade stall.
2] Advancing blade tip speed approaching critical mach (hence most modern r/w have swept back tips).
3] Cyclic forward limit stop.

Note also that tail rotors on more modern helis turn rearwards against direction of flight for the same reason as #2

You left out that not only does the advancing blade have less pitch, but the retreating blade has more pitch, and that's where the stall comes in .

I've been in a chopper that barely tickled the edge of the trait, the helicopter rolls to the side of the retreating blade, and dives. Which if not gotten out of immediately, but with very gentle control inputs, will get into a very quick out of control condition, the helicopter inverts.

One of the students in my WOC class, and the instructor with him died in a TH-55 that possibly experienced this condition.

Sid327 · Jan 17, 2020

wuzak said:
Hmmm. Surely one side of the tail rotor will still be heading forward?

Yes, but that is affected by the downwash from the main rotor.
The Westland Lynx started off with a tail rotor turning forward at the top and it could be heard from miles away. A really loud buzzing sound.
Once it was given opposite rotation the noise level dropped dramatically.

Sid327 · Jan 17, 2020

tyrodtom said:
You left out that not only does the advancing blade have less pitch, but the retreating blade has more pitch, and that's where the stall comes in .
I've been in a chopper that barely tickled the edge of the trait, the helicopter rolls to the side of the retreating blade, and dives. Which if not gotten out of immediately, but with very gentle control inputs, will get into a very quick out of control condition, the helicopter inverts.

I assumed wrongly that it might be obvious why the retreating blade would stall. An aerofoil usually stalls from too high AOA. Okay my bad.
As for actual stall, I agree it will stall to the side that has lost lift (retreating side) though we were taught the aircraft pitches nose-up.
IIRC it did in the training aircraft we used (SA-341 Gazelle) when being demonstrated.

tyrodtom · Jan 17, 2020

I'm commenting on something shown to me 50 years ago.
I remember the roll to the left, but I thought I remembered the nose dropping too.

But I also remembered getting the demonstration on what to do on a engine out.
Kick opposite pedal quick. If you were slow with opposite pedal, the helicopter rolled and the nose fell through.
If you really had a brain fart, and increased pedal the way you held it to counteract torque, the helicopter rolled , dove and inverted.
But that wasn't something the instructor could demonstrate.
Except to barely increase the wrong pedal to show just a little bit of roll and dive.
It's been 50 years since I've flown a helicopter, I can't remember for sure which pedal you held to counteract torque. Right ? I think.

What I've always wonder is how did they flight test the early helicopters like the TH-55.
You couldn't completely explore the entire flight envelope, because in the TH-55, for instance, if you went negative G ( like when inverted) the blades were easily flexible enough to chop off the tail boom, even if you did have a parachute, you'd never get out.
They seemed to set flight parameters that far enough away from the limits so you really had to screw up to go beyond them.
But if you did, you would very likely die.

GrumpyOldCrewChief · Jan 18, 2020

Looks like the camera was mounted on the "Jesus nut"...

VALENGO · Jan 18, 2020

tyrodtom said:
That " wind generator " is the tail rotor.
Or was that humor ?

I have just written the most stupid thing in my entire life, didn' t realize that I was watching the tail rotor. Anyway it gives a nice reference to understand the word "cyclic".

gumbyk · Jan 19, 2020

Sid327 said:
I assumed wrongly that it might be obvious why the retreating blade would stall. An aerofoil usually stalls from too high AOA. Okay my bad.
As for actual stall, I agree it will stall to the side that has lost lift (retreating side) though we were taught the aircraft pitches nose-up.
IIRC it did in the training aircraft we used (SA-341 Gazelle) when being demonstrated.

The pitch is due to gyroscopic forces, so depends on the rotor direction. Could be the reason why you both 'remember' differently...

tyrodtom · Jan 19, 2020

I thought most helicopter main blades rotated counter clockwise, except for some Russian helicopters.

Or at least that was true with the one working Russian helicopter I saw,

Sid327 · Jan 21, 2020

tyrodtom said:
I thought most helicopter main blades rotated counter clockwise, except for some Russian helicopters.

Or at least that was true with the one working Russian helicopter I saw

French helicopters rotate clock-wise (viewed from above).

Zipper730 · Feb 18, 2020

A helicopter blade is essentially in constant windshear (advancing/retreating): I knew a person who talked to a guy who served in the USN in the 1960's (b. 1943), and he supposedly recounted a story about a cam placed on the helicopter rotor that scared the shit out of a lot of to-be helicopter pilots. Some DOR'd after seeing it.

Helicopter Main Rotor Blade in slow-mo

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