Why no negative pitch?

[DroneDog]

My understand is that helicopters can not select negative pitch, in the late 90's when I used to fly very large RC models we had this. The mechanics of our large two stroke models matched the mechanics of full size machines.

When on the ground with a high rotor rpm that was decaying dropping the throttle/collective to lower limit introduced a degree of negative pitch as a safety measure as a decent gust of wind could get you airborne again whilst you were distracted. Negative pitch ensured you stuck to the ground.

But also in auto rotations, again my understanding of helicopters if the engine quits you quickly drop the pitch to maintain rotor rpm relying on the inertia of the rotor system to provide the flare as you land, if you could introduce a degree of negative pitch you could build rotor rpm as you descended.

[Fareastdriver]

IIRC the Westland Wasp, designed to operate off small navy ships, had negative pitch to make sure it stuck to the deck when it landed despite whatever pitch and roll the ship could throw at it.

The Lynx, it's replacement, had a harpoon system the hooked onto a grid so as to do the same thing.

I would only work with a fairly rigid rotor. Big wavy blades on a larger machine would probably strike the boom.

You can always ask yourself why they fit droop stops to rotor heads.

[HeliComparator]

Originally Posted by DroneDog

But also in auto rotations, again my understanding of helicopters if the engine quits you quickly drop the pitch to maintain rotor rpm relying on the inertia of the rotor system to provide the flare as you land, if you could introduce a degree of negative pitch you could build rotor rpm as you descended.

Helicopters autorotate with positive pitch. Normally if you lower the collective fully the rotor rpm will exceed the limit (blades fly off etc!). A stabilised autorotation will be with the collective low but probably not fully down. Therefore there is no need for negative pitch in this context.

[Democritus]

Originally Posted by Fareastdriver

......The Lynx, it's replacement, had a harpoon system the hooked onto a grid so as to do the same thing.....

The old ideas are sometimes the best..... concept demo of a Harpoon system on a RN Dragonfly at RNAS Ford in 1955. Photo courtesy of the son of Lt/Cdr John Sproule (the inventor).

[GipsyMagpie]

Merlin

The Merlin has negative pitch and a harpoon for deck ops. My understanding is that the harpoon is normal option for small ships (frigate/destroyer) but it needs right infrastructure on the deck. If you push through the minimum collective pitch stop in flight, the rate of rotor rpm is very high. The pitch is adjusted to give the right rpm at light weight at a high density altitude (which I think is worst case). So at higher weight or lower density altitude you won't need to bottom part of the range anyway (except for a brief period on entry or when bunting from low speed)

[Sloppy Link]

Negative pitch in any phase of flight is suicidal madness and not what it is designed for. Sticking to decks and engine performance ground runs only. I'm sure there is a pruner who, with the clever use of vector diagrams and colour could go into depth on the subject but from an operators point of view.....NO!

[MightyGem]

Hmm...I'm sure I've posted on this recently. The Lynx, and I suspect all the others that have so called "negative" pitch for sticking to ship decks, don't actually have negative, ie below zero degrees, pitch. They can go to sub minimum(sub min) which is a setting below the normal minimum pitch on the ground. With the Lynx, it's about +2 degrees.

The Lynx also uses this sub min setting for certain ground runs.

[Ascend Charlie]

Most helos hang under the rotor like a bucket hangs from its handle. You can swing the bucket around under positive g and even take it upside down without spilling the contents.

Now try to make the bucket STAY upside down - just doesn't happen, was never designed to do so. Same with helicopters.

Even on the ground, negative pitch will make the blades cone down, and that would be disastrous for a lot of tail booms and people under the disc - an S76 is low enough at the front as it is.

[Kinger]

I may be mistaken, ten years since I touched a Lynx, but I believe blades go into negative pitch at certain selected cyclic movements.
The max sub min previously mentioned would only relate to a cyclic in datum position. Stick full left would see the fwd blade below 0, etc.

[dangermouse]

Negative pitch

Use of submin in flight is not 'suicidal madness', it depends on the type and the flight condition you are in. Rapid entry to autorotation can use collective settings below MPOG to get a good split off, however you need to be aware of the rate of NR rise and adjust accordingly.

Both the Lynx and Merlin have collective settings below 'minimum pitch' that produce negative thrust to stick them on the deck.

As both are fitted with BERP blades the term negative pitch is slightly misleading as the blades have significant twist along their span and the pitch is measured at a nominal radius (some part of the blade is at positive AOA and some at negative AOA whilst at MPOG, however the resultant total thrust vector is zero).

As you drop the collective the torque will drop until minimum at MPOG then increase again as the bottom of the collective range is reached.

Given the stiffness of the BERP blade and the design of the MRH the lower flapping on these helicopters isn't a problem if the cyclic is positioned correctly.

DM

[SansAnhedral]

negative pitch is slightly misleading as the blades have significant twist along their span and the pitch is measured at a nominal radius (some part of the blade is at positive AOA and some at negative AOA whilst at MPOG, however the resultant total thrust vector is zero).

3/4R is typical

[GS-Alpha]

DroneDog

The principles of flight of RC helicopters and full size helicopters are indeed the same. However, the differences in size, weight, rotor RPM etc., result in those same required angles of attack for successful autorotation etc, occurring at different physical blade pitch angles than on RC models. This is why the models require negative pitch to maintain a suitable RPM in autorotation, whereas lower but still positive pitch is suitable for full size.

[ericferret]

I seem to recall that the Westland Wasp had negative pitch to glue it to the deck.
That would be with old tech blades with limited twist.

[MajorLemond]

I fly RC helis. The blades are super stiff (carbon fibre/fibreglass)

They are generally designed with a pitch range of +12 to - 12 degrees.
This is for doing stunts (called 3D, flying upside down etc) but also an angle of -3 degrees will provide you with a nice zero airspeed autorotation, generally speaking. Probably need a lot less if you've got some forward airspeed or are flying into a steady breeze...

This video should explain the concept I'm talking about, it's well worth the 4 mins of your time

https://m.youtube.com/watch?v=y7XKWYdMT78

[SilsoeSid]

Same guy; it's ok though, you don't need to see more than a minute of this vid



You be the judge of what's a better "worth minutes of your time"
(I always liked Plastic Bertrand)

[salomonteneighty]

As far as I understand negative pitch should help regain some RRPM during an autorotation let's say for example in the R22. This way you would have more time to react than the two seconds before you completely stall the rotor.
Mechanically if you would just add a small amount of negative pitch it should not change that much?

[ShyTorque]

The teetering head on a R-22 (and similar designs) means that in negative G, control of the rotor is lost. Negative pitch? No thanks!

[salomonteneighty]

Correct me if I am wrong, but if we look at the case when the helicopter R22 has an engine failure and is falling like a brick since the RRPM felt below 80%, if you then apply negative pitch this does not mean that you get negative g. But changing the pitch of the blades to negative could eventually recover the RRPM above 80% and you could continue your autorotation instead of falling like a brick to the ground..

[helicopter-redeye]

Correct me if I am wrong...

With the “more forgiving” R44 ~91% is as low as you would want to go in rotor droop. Mid to Low 80%. Might recover (maybe). 80% and below. Tricky.

R22. Less forgiving.

Some flare may help transitory recovery.

[rotarywise]

The Wasp does not have and never has had negative pitch. It has the facility to select what is described in the Pilot's Notes as the "superfine collective pitch range' which was useful on a moving deck but ineffective beyond about 15 degrees of heel as I once learned to my cost. The superfine range could be disabled by the insertion of a pip-pin at the base of the collective, although I don't recall it ever being fitted.