R22 low G pushover and autos
 

Quick question....we all know that low G pushovers are prohibited in Frank's helicopters...does an autorotation not present a low G condition?

Regards,
Mark

No, the blades are loaded all the time.

Mark,

you pobably refer to the initial entry phase of the auto, where you sometimes get the "low" feeling.

This is kind of a low g, but rather induced by poor technique and generally recovered right away.
(Not holding the stick with a little backpressure while entering the Auto and/or lowering the collective too fast - yes that is possible!)You really never get to the point where the ship starts rolling right, though.

Low G by itself is not an issue either.
It is a missconcept that Low G is what kills you, it is not!
It is the WRONG reaction of the pilot to the situation that eventually kills you!
NOT proposing to do Low G's for the fun of it now!!

It needs training and skill to react correctly and is not a maneuver a initial student (even past the CFII) would be comfortable with.
Prudent flying and common sense will avoid the need for recovery - basically fly as taught, and you will never get in the situation.

As there is no need and no practical application for low g, Robinson did well by prohibiting the maneuver outright.

Also, it is NOT a Robinson only "problem". ANY helicopter can experience it.

3top:cool:

It's relative G, of the blades vs the mass suspended beneath the mast.

Actually, I think that you'll find that it is a problem that is somewhat unique to helicopters with teetering rotorheads, and other head designs with a small effective hinge offset.

The key issue is that in a heli with a teetering head or small hinge offset, the torque to change the orientation of the helicopter fuselage is created by tilting the lift vector. When there is little or no lift (low or zero g situation), you can tilt the disk, but it isn't going to cause the fuselage to tilt in response.

On helicopters with a larger hinge offset, you get a "hub moment". Basically, whenever the rotor disk is not perpendicular to the mast, the disk will exert a torque on the fuselage in addition to that caused by tilting the lift vector. If the hinge offset is sufficiently large, then the helicopter has no control issueas at low/zero G. An example would be the bo105.

If you do a pushover instead of a flare on entry to autorotation that's going to end your flight real quick anyway . . .

Vital,

I think you mix something up here:

Low G ( and/or negativ G), are inertia issues - at the moment it happens airflow doesn't matter.

Pushover:
You just where going forward and up, and inertia wants to keep it that way, no matter where you push the rotor-disc or with how much power.
Do keep things positiv you have to let gravity do its thing - once inertia bled off enough your rotor will cause some effect.

Auto:
You where going straight and level and inertia wants to keep it that way.
You enter the auto with collective down too fast and/or not enough backpressure on the stick, the nose will drop, but the helo wants to keep going straight - until gravity does its thing again....

In both occasions it does not matter what airflow does at the time as the rotor can't make the whole helicopter react.
Obviously if you reduce the collective and the pedals, the tendency for the helo to roll right (Robinson) is reduced or eliminated.
The right roll is caused by all the pedal you are carrying to react to the torque you carry.
When the T/R-disc goes above the CG it causes the helo to roll to the right, which you can't stop with the cyclic as the rotor has no effect on the helo, 'cause inertia is at work.
Give it enough time and gravity will take care of it.
However if you don't know what is happening you probably will panic at the "No"-reaction of your initial cyclic input. You give it more left - once you are at the stop the head will bump the mast - depends how hard the initial contact is, you might break the mast, rip it out of the trans, etc.
If that doesn't happen but you still banged it hard enough, the blades may flex low enough to start to chew through your tailboom - it's academic - either one will do you in just fine...

I might have been one of the last to had this demonstrated at the safety course, and it was very interesting to see how easy you can handle this - however it is esential to get it demonstrated by a competent pilot.

I do NOTdemo it nor do I try to practise this!!

Its a little like retreading blade stall - no big deal, if you know what it is and that is coming. Wrong reaction for lack of knowledge and you're dead just like with the low G.

3top

Keep it simple – on a teetering head helicopter the fuselage dangles underneath the rotor head and where the rotor head goes, the fuselage should follow. You move the cyclic to the right, the rotor disc tilts to the right and pulls the weight of the fuselage along with it until the rotor and fuselage are in their normal positions relative to each other except in a right turn.

Now remove the weight of the fuselage – ie zero g, it still has mass but no force acting on it – the rotor will still respond to cyclic inputs but cannot change the attitude of the fuselage – think of the rotor mast as a piece of string that needs to be pulled tight.

On a helicopter with any hinge offset ie not on the axis of rotation, the rotor hub has some leverage so it can make the fuselage follow it even in zero g.

So in a push over / low g situation in a teetering head helicopter, the only rotor capable of affecting the fuselage attitude is the tail rotor because it is fixed to it. Unfortunately this is pushing to the right producing anti torque thrust which will tend to yaw the fuselage and roll it (secondary effect of yaw) aided by the TR thrust creating a moment about the fuselage C of G.

The result is mast bumping as the pilot tries in vain to control the fuselage attitude with the cyclic or worse still, the MR and tail boom coming into conflict and the MR usually wins.

Times like this that you'd miss Lu Z.:(

Yup. A healty fight about 72 degres. I found out lately, that the BO105 has that too, that would have been interesting.
I think we had a lot more good technical discussions when he was around. Nick Lappos was allready steaming from the ears when a new post from Lu showed up. I had arguments with him but now I miss him.
:{

3top:

'vital' is correct. The change in relative airflow causes the total rotor thrust to be destroyed (due to increased induced flow), hence the inertia is lost.

The important thing about zero g is that it creates, for teetering rotor heads, zero cyclic control. That is because the only means for a teetering rotor to control the helo is to redirect the rotor thrust. No thrust, no control, no hope. A contributing factor is that when the helo has no control, as the helo is departing level flight the pilot moves the stick all over the place and that makes the head bump the mast. After mast bumping, the weakend mast lets go and then the head removes itself, a very embarassing event. It is often said that low G makes mast bumping. Well that is as correct as saying that teeth give you cavities - correct but not nearly the whole story.

The real problem is that every helo guidebook for neewbies and instructors is fundamentally wrong in how they explain cyclic control, so most helo pilots get confused as to how a teetering helo has this Low G problem when the others (articulated or bearingless/rigid) don't. Helo "technical" guideooks, like the FAA Handbook, all show that the cyclic simply tilts the thrust, and this makes the helo tilt and this gives control.

Articulated and bearingless/rigid rotors derive most of the cyclic control from the fact that they make the helo rotate about the mast just due to the strong moment they produce due to flapping. This strong moment control is available even though there may be no rotor thrust - in other words, most helicopters have excellent rotor control at zero g, except teetering rotors. The measure of the amount of moment control that a rotor head has is made by ratioing the spanwise distance of the flapping hinge to the total radius of the rotor - spoken of in percent. A typical articulated rotor has about 4 to 5% "hinge offset". How does a bearingless rotor get this moment control? It bends as if it had a bearing, and has a "equivilent offset" that is usually even better than an articulated rotor, typically about 7 to 11% hinge offset for a bearingless. This hinge offset is why articulated rotors feel snappier than teetering, and bearingless rotors feel even snappier.

Now is autorotation a zero g event? No, the rotor produces a ton of lift in autorotation, equal to the weight of the helo. In entry, if the pilot slams down the collective, there can be a bit less than 1 G for a fraction of a second, but even that is not zero g. In short, unless the pilot slams down the collective, mast bumping is very unlikely in an auto.

http://webpages.charter.net/nlappos/control.jpg

"I say again": Nick, please write a book!
I'm ashamed I never saw a picture like the "articulated rotor-moment control" one.
Thanks.

Thank you all for the excellent insight and explanations.

Regards,
Mark

To oversimplify somewhat inaccurately for illustration:

In a pushover, the rotor pushes the airframe down and makes you light in the seat.

In a rapidly-entered autorotation, the airframe pulls the rotor down, making you light in the seat until the autorotative airflow is established.

However, as long as you go down-right-aft (or left in those helis), you will not get the "decouple" or the rolling moment due to T/R thrust, so no mast bumping (or droop-stop pounding) is likely.

Fling,

I would not simplify the control issue by showing how to not-die by clever application of un-working controls after you drive your aircraft out of control. I would rather teach not driving out of control to begin with. I would teach how to not get zero g.

Also, in the rapid auto entry zero g event, the rotor is driven down (by the pilot's fast collective drop) so that the rotor descends faster than the fuselage, thus unloading the rotorhead.

Does this assume the ability for negative pitch or is this regardless?

I think we've gotten crossed up, Nick, or I'm missing something. If I'm flying level at 65kt, and I drop the collective while maintaining a 65kt attitude, how would this unload the rotor disk to the point of decoupling? Yes there is a reduction of thrust, but the weight of the airframe is still acting on the rotor system, is it not?

BTW - I'm a firm believer in maintaining positive "G" - I was using these examples for discussion, not practice.

Fling, I think the answer is the inertia of the fuselage - the lift force opposing the weight is suddenly removed and the acceleration of the fuselage towards the ground takes a short while to build. In that time the rotor blades are flapping down and coning down as the collective is bottomed so as Nick says the rotor head is trying to descend faster than the fuselage for a brief period of time.

Whatever the explanation, I know it happens - I have demonstrated rapid entry to auto in a helo with a g meter fitted and you can see the reduction - not to zero g by any means but a good bit less than 1 g which in a teetering head helo starts to reduce your control power.

I agree that you should always try to flare as you enter auto - the main reason being to try and recover the rapidly dwindling Nr (in the case of a real engine failure). And before anyone tells me I know that this technique needs to be modified for the low speed/climbout scenario.

Twiddle, no you don't need negative pitch for this, we are talking about normal everyday helicopters.