Is negative pitch used in autorotation?

[alltorque]

Is it necessary to present negative collective pitch to the airflow to drive the rotor in autorotation or does the descent cause the angle of attack to become one which drives the rotor at zero or positive collective pitch - ie how to determine the angle that drives the rotor at a stable descent rate vs one which imparts no acceleration to the rotor but maintains head speed vs one which starts to rob the disc of energy that will be needed to arrest the descent rate?

TIA

[Vfrpilotpb]

I will hope this may give you some answer to your question, I take it from your question that you are not really familiar with the ways and controls of Helis, however as from my small knowlege of this, it the following,

If you enter Autorotation, it is because of two possibilities,

1. everything has gone quiet, it then is an imperitive to enter Autorotation.

2. You need to get down quickly for whatever reason, so you would enter under your control( however I dont know many who would do this)

The control inside the craft that imparts or controls rotor pitch is the collective, (the cyclic controls the disk) this not only controls rotor pitch but also blade and power attitude Ie course(high power) or fine pitch(low power) of rotor blades, the angle of attack is the finess of presentation to the atmosphere of the leading edge of the rotor blade, and this is also a task controlled by the collective.

If things go quiet( engine failure for whatever reason) it varies from type to type but your immediate re-action is to close or pushdown on the collective, this puts the rotor into its most fine setting, to give a slightly easier example, if a blade at rest = 0,or fine pitch(low power),.. course pitch would be 10(high power), the collective as such has a bottom and a top limit set by the manufacture, whilst it is quite possible to over pitch ie=10, open the face of the rotor so much that you are demanding more than is possible from that rotor/engine or turbines it is not possible to present a negative pitch for this would have dire consequence's on rotor blade/performance, and could lead to blade intrustion, this would spoil your day, nor is it possible to push the collective past the manufacturers preset closed position, IE =0

So if you are unfortunate enough to have to select autorotation, you as the pilot holding the collective, must then control the descent of your heli by judicial use of the collective, and craft attitude with the cyclic, with great care the pilot will stop what is called an overspeed of the rotor which in itself can be structurally dire and cause problems.
A helicopter flys because the engine linked to the rotor produces enough energy in the form of downwards air flow to allow the craft to overcome gravity, in the hover and in forward flight there is always some element of downward thrust from the engine through the rotor(s), remove the energy from the engine and you then have to rely rotor inertia and the upcomming atmosphere or airflow to give you the rotor authority to hit terra firma under controlled conditions.

As I am waiting for my wife to do something for me, I have had time on my hands, this I fear is my longest and possibly most boring answer ever to any thread, I hope it makes sense to you, I am not an English master so forgive incorrect spelling and grammar :8

[Arm out the window]

In short, the blade has a low but positive pitch angle with respect to it's plane of rotation, with the collective fully down (although I believe the Lynx can apply negative pitch - correct me if I'm wrong here).
In autorotation, the rate of descent means that there is quite a deal of airflow coming from below the rotor. Add to this the 'oncoming' airflow caused by the blade's rotational speed and you get the resultant relative airflow that the blade 'feels'.
Lift is produced, perpendicular to the relative airflow, and also drag, parallel to it.
Because a lot of the relative airflow is coming from below, the aerodynamic force produced by the blade can be thought of as being tilted quite a way forward (thinking of the advancing blade makes this easier to picture).
Resolving this force in both the plane and axis of blade rotation, you get lift (keeps the machine flying rather than falling) and, surprisingly, a component that acts to keep the blade turning like a windmill.
So you don't need negative pitch, it's the rate of descent that does the magic.

[MightyGem]

Yes, you can apply negative pitch in a Lynx, but only on the ground. It goes down quite fast enough in auto as it is! :eek: It is done during the engine setup after an engine change and, I believe, the Navy use it to assist in staying on the deck.

Additionally, I never was sure whether it was actually negative, ie less than 0, or just 0 degrees.

[Lu Zuckerman]

Making several assumptions that a helicopter blade has a seven degree negative twist when the collective is fully down with the helicopter static on the ground the root of the blade is at seven-degrees and the tip of the blade is at zero-degrees. In flight when the helicopter is in flight and has to enter autorotation for any reason the same conditions apply. However when the pilot enters autoratation he moves the cyclic stick forward the blade can enter into a negative pitch at the tips but the aerodynamic aspects (which are beyond me) do not reflect a negative pitch due to the inflow of air.

On the Cheyenne the helicopter which is a compound helicopter flies with the blades at low pitch and the disc is used for control while the wings support the helicopter. In this case when the pilot introduced cyclic the blades entered negative pitch and diverged from the disc plane with catastrophic results.


:E

[Straight Up Again]

I believe the navy does use negative pitch, to hold them on the deck. The EH 101 has up to -10 deg negative pitch (if memory is correct).

Wouldn't using negative pitch overspeed the rotor? I thought the collective was used to hold Nr around the 105% -110% Nr in auto (dependant on aircraft type), as this sort of speed gives enough stored energy for the flare/landing. Even during an auto, I didn't think the collective was fully down, except maybe initially. If you were caught by suprise (no warning of engine trouble, and you had to put your magazine down before grabbing the controls) and the Nr decayed a bit too much, I suppose the negative pitch scenario may help to give you bit more Nr, but then once established I don't think you would stay in that position, but you would return the collective to a positive pitch position to control Nr.

As ever, I await corrections. I may have experienced quite a few autos (all eng on), but not being a pilot have never controlled one.

[SEL]

'Arm out the window' has got it right as far as the aerodynamics go. It also true that with negative blade twist, in forward flight, with cyclic pushed forward, the overall angle of attack can be negative.

If the blade pitch itself was negative, with respect to the horizontal, then just considering it as an aerofoil, with only a horizontal airflow approaching it, it would produce lift downwards. If you then add the upflow from the descent, as 'Arm out the window' said, the resultant airflow to the rotor approaches from an angle below it. The combined result of the relative flow from below and the negative blade angle is a negative angle of attack which produces a negative lift.

This is also true of forward flight, if the angle of attack is negative, so is the lift. With certain twist values and cyclic pitch, outboard sections of the blade will produce negative lift.

This is putting it all simply as the blade angle also affects the rotor speed which affects the angle of attack which affects the rate of descent which affects which affects the angle of attack.....helicopters, dont you just love 'em.

So, nope, you dont want a negative blade angle (set with the collective). As has been said, autorotations are fast enough with some postive lift, with negative...well, you'd only do it once.

[alltorque]

Thanks all. I am just trying to understand how the disc continues to be driven without without negative collective pitch. And please excuse my ignorance of the physics going on here. I have limited hours in an R22 but concerned at the moment with the mathematical properties of rotors. I have read about the driven and driving sections of the blades but can't quite understand how the energy is imparted to the in-plane axis; rather than the lift axis which, given positive pitch, (therefore a lifting angle of incidence to the airstream) will control the rate of descent.

The weight of the helicopter will pull it down through the airstream and this flow through the disc will impart energy to the blades according to their pitch - and if positive or "fine" will no doubt generate enough lift to control the rate of descent.

I am trying to see it in terms of a propeller pulled through a fluid. A positive pitch (in this instance) will not spin faster (in a given direction) but rather slow and reverse, but a negative pitch will "spin-up" accelerating in the direction the propeller is already turning in.

[SEL]

Alltorque,

There's another thread about autorotation that may help:

http://www.pprune.org/forums/showthread.php?s=&threadid=104671

Steve

[MightyGem]

With the Lynx, with the collective down fully, you're still in the normal pitch range. It requires quite a hefty push to get it into negative pitch, so it's not something that you're likely to do by mistake in auto.

[Flingwing207]

Hi alltorque,

Instead of thinking of a propeller in fluid, think of an airplane wing. What keeps an airplane gliding forward when the engine stops? It is quite apparent that an airplane maintains a positive AOA in a glide, yet it doesn't slow down then start going backward.

Remember that what causes lift is a pressure difference - think of it as a string pulling at 90 degrees to the relative airflow. With a gliding wing (or driving section of helicopter rotor), that pressure difference ends up pulling both up and forward. The change in the direction of relative airflow is very dramatic when you go from powered flight to autorotation, which can make the visualization somewhat more difficult.

[RDRickster]

Flingwing... best explanation yet! :ok:

Still, SEL and Dave's comments on the related thread listed above will bring things together nicely.

[alltorque]

That is starting to make much more sense - thanks guys.