Low G pushover recovery

[krobar]

Question:
Once in a low G pushover, could one not simply pull the collective as high as it'll go, to load the disc?
Woud one still be within engine limits, since the disc isn't loaded?
Would the blades stall?
Kro

[Fun Police]

you might be able to add massive overtorque to your list as the rotor system loads up (depending on the situation, of course) and the collective is in your armpit.

[thecontroller]

more collective = more torque = more left pedal = more right roll ???

[vorticey]

any thing that loads the rotor will work; more pitch, flare.

[Johe02]

I know the principle of loading the disc etc.. . . but I must ask. . How is it applied to a real life situation?

The only time I've found myself in or about to enter a low G pushover is when a student is getting it badly wrong and tries to climb like a plank (nose up - high power setting) they suddenly realise (or its pointed out) they are losing airspeed etc. and are about to push the stick forward. . . and you say 'I have control'

Dropped the lever and gently applied forward stick. .

How can you load the disc when you are nearly full power and how can you flare when you are already nose up?

Obviously I have survived these situations but am doing the right thing. . .??

:hmm:

[Disguise Delimit]

Roll on your side and apply backstick to load it.

Nose will drop, speed recovers. Unload disc, roll back to level. Change underwear.

[krobar]

From what I've heard, huey pilots had severe promblems with Low G, by hugging terrain during vietnam.
You don't need to raise the collective to climb, if you have some airspeed to spare, so theoretically you could have some power to spare.
What would your toque guage show if you're in a 'disc unloaded' situation? Would it drop? With the disc unloaded, surely the power required to fly is nil?
Or is it?

[NickLappos]

The best way to think of this is not to think of where your nose is or where the collective is. The condition is brought about by doing things that lower the load factor ("g") and cured by raising the load factor.

I have hit zero g upside down, and usually practice it starting from a big nose up, so that I hit zero g while the nose is still pretty far above the horizon.

It is the RATE of the movement of the nose, and the RATE of movement of the collective that help/hurt you. Nose down rate causes low g, and down collective rate does as well. The two together is a mighty fine way to get low g.

Remember that the problem is mast bumping, which you might cause when you wiggle the cyclic while at low g. Don't be rash or abrupt, even to recover from low g, because you could cause the bumping that is what you really fear.

Best way to recover from low g: Don't get there. Avoid big nose downward pitch attitude changes. Avoid big fast collective bottomings.

Low g is caused when you put the aircraft into a Ferris Wheel motion, where the aircraft is at the top and rotating (nose down) while speeding along in that downward circle. It is precisely like driving your car off a cliff, where the car can drop at the same vertical rate as gravity desires. When the road disappears from beneath the wheels, the driver feels zero g. In a helicopter, you can cause this, you can "unstick the road" from beneath the wheels, because you have the ability to reduce the thrust with the cyclic and collective. In effect, you unload the rotor with forward stick and down collective. You hit zero g when the "hill" you are scribing in the sky matches the downward acceleration of gravity, when you force the machine to drop as fast as it would if it simply fell.

It is unbelievably rare to imagine an atmospheric circumstance where the aircraft in 1 g flight can be made to hit zero while the innocent pilot holds the sticks steady.

How to recover from low g - stop the forward stick rate, and stop the down collective rate. This will cause the g to rise, because YOU were making it reduce when you made the aircraft maneuver. The aircraft cannot go to low g by itself, you have to push it over, so stop! Do not do anything abruptly. Gently pull the stick rearward, while maintaining collective or raising it a bit. The rotor will "bite" and the g will rise.

If the aircraft starts to roll off in either direction while at low low g, do not put an armful of cyclic in to correct it. Let it roll a bit, wait until you feel the g build back to near normal before you make big roll cyclic inputs.

All this caution applies to teetering rotors, where the hinge is at the mast. For articulated rotors, where the flapping hinge is offset from the mast, there is a long way below zero g before you lose control. In a typical articulated system (Black Hawk, Apache, S-76, and virtually all the new EC and MD's) you can get to and hold zero g long enough to make the engines worry about oil supply, and you can, at zero g, roll the wings sharply with all the control you have left over.

Why? Because the myth that in all helicopters it is rotor lift that creates cyclic control (you tilt the lift, right?) is WRONG, but that is another story.

[gentleben]

thecontroller;

Excuse my ignorance, but if it is the antitorque effect of the tail rotor which causes the aircraft to yaw and then roll when the main rotor is unloaded (i.e. great reduction in main rotor torque), wouldn't stomping in right pedal (in counterclockwise rotating systems) conteract the effect and keep you straight until the disc is reloaded? Not that I'll be trying this in the near future...

[delta3]

It is true that when the MR looses control when it is no longer "pulling" the heli, the TR destabilizes the heli and pushes it into a roll.

Mind that it is not only the TR, also the tail fin in R22-R44 creates an important roll torque, because the centre of gravity of the aero forces is even higher than the centre of the TR axis.

When trying to compensate this rolling moment as you suggest, you will create a massive yaw moment. When the heli starts yawing, this will create an important roll moment, because of the sideways motions and the resulting aero dynamic forces. This will imho opion create a mast bumping situation that might even be worse, as long as the MR has no authority...

d3

[gentleben]

That makes a lot of sense. I thought I had something missing from my equation. So it is not just the left yaw which causes the right roll, even if it is kept straight there can be a rolling tendancy brought on by the stabilisers (which have now become destabilisers!).

Cheer

[delta3]

Right on.

Look at the simulation I posted earlier (somewhere in the R44 track)

It includes also the control actions, showing as Nick stated that it is not the side ways cyclic you should use, because it will make things worse. Just reverse gently the previous forward cyclic by a gentle backward action, possibly increasing gently the collective. Don't do anything abrupt that makes the heli roll/pitch/yaw because this will make the heli hit the rotor disk.

Nick : I may not agree with you that it is only pilot induced. Flying too low over mountain ridges may create powerfull sudden down drafts that put the heli in a 0.5 g or less position.

d3

[Gerhardt]

Nick, Delta:

THANKS!!! It's incredible the amount I learn here.

[NickLappos]

Delta3,
I could possibly believe that severe turbulance might cause a condition of +0.5g but that is possibly the limit, due mostly to the high blade loading of all helicopters, but I am quite sure that the mast bumping accident requires much lower than 0.5g to initiate the accident sequence.

Therefore, I am quite convinced that NO Mast bumping can be caused to an innocent pilot by the guilty aircraft. The FAA thinks so, as well, otherwise they would not certify teetering rotors.

gentleban,

Remember that the torque of the main rotor is countered by the force of the tail rotor thrusting sideward, as well as the force of the vertical fin, also thrusting sideward.

The tail rotor thrust and the tail fin thrust are both fairly high, quite above the roll centroid of the helo, so they produce a rolling motion to the right. This rolling moment is countered by a slight cyclic trim to the left. But at zero g, there is no cyclic control. With no cyclic control, nothing opposes this tail rotor roll. If the pilot uses cyclic to try and stop it, he can actually cause the mast bumping by putting in excessive cyclic.
I would avoid any thoughts of pulling or stomping anything if you get near zero g. It is my belief that most mast bumpings are probably the result of panicy pilots abusing the controls at zero g, and the excessive control positions they achieve that CAUSE mast bumping.

[bladewashout]

Any ideas about the timescales within which it all starts to go wrong once low g is induced (probably by the pilot)?

I had read that the roll can be incredibly fast, and presumably this can cause an instinctive reaction in the pilot which would be disastrous.

I know that from a personal point of view if I feel the G's dropping away, it has my immediate attention, usually just turbulence!

BW

[paco]

Nick - at the risk of hijacking the thread - why is the myth that in all helicopters it is rotor lift that creates cyclic control (you tilt the lift, right?) WRONG?

Wouldn't be anything to do with Coanda would it?

Why not PM me about publishing a book?

phil

[Ewan Whosearmy]

Didn't an Army Cobra pilot go up and demonstrate mast bumping in his AH-1?

Can't remember the specifics, but pretty sure that he went up very high and pushed the stick forward, but only after sensibly donning a parachute. Entered 0g, rotor seperated and then he bailed out, but his CPG might not have been so lucky.

ISTR is was something to do with proving that mast bumpping was at the root of so many UH-1s augering in in Vietnam.

[NickLappos]

Ewan,

That story is essentially correct, as I understand it, it was a Navy Pax River Cobra flown by an instructor and TP student, doing an envelope demonstration as part of the training syllabus in early 1983 (I was in a test program there in June of 83 and it was a fresh topic among the instructors.) The flight was not a test flight, it was not specifically to get bumping, it was to show the safe envelope limits to a budding TP.
The rotor sliced thru the cabin and killed the front seater, the back seater managed to bail out. Not Pax's finest moment, frankly.

I do think that we must teach those who fly teetering helos to recognize what they do to make this happen. If we try to tell them that "it happens" and not "you make it happen" we will create a bunch of folks who are scared that their machine will leap up and try to get them. In truth, with mast bumping the pilot gets the machine.

To prevent mast bumping, do not push over rapidly, do not dump the collective at high rate while doing so. If you "accidently" maneuver that way, recover by slowing down the forward stick, which will build the g back.

I am concerned when Robbie pilots use terms like "pull the collective as high as it'll go" and "stomping in right pedal" as somehow helpful. This shows two bad things: That the mast bumping happens to them, and that they have to intervene rapidly and forcefully to save themselves from this situation delt them. In fact, mast bumping is caused by pilot aggressive behavior, where the pilot propels his machine to a place where it is out of control.

[Johe02]

I heard a German pilot took 3 of the German aviation authority up in an R44 to demonstrate correct recovery from mast bumping. .

None survived.

[vorticey]

is the bumping you get at full cyclic mast bumping? you get it in a KH4 or a R44.:confused:

[delta3]

What do you mean by bumping ? In what context, or part of flight ?

Using max-controls can also introduce vibrations.


d3

[vorticey]

while on the ground at flight rpm and you push the cyclic full (or close to) forward or taking of from a slope with full cyclic. is this bumping on the bump stops or just vibration from the hooks/ universal joint in the head??

[delta3]

These are vibrations caused by the more pronounced tilting of the rotor disk, not bumping.

d3

[Flingwing207]

Nick, I just want to be sure I'm understanding you right, so let me rephrase: pushing the nose over rapidly can/will induce low-G, lowering the collective as you do this can exacerbate it.

However, if you're going 80 KT, and you maintain an 80-KT attitude as you rapidly lower the collective (which would mean coming aft with the cyclic to keep the advancing blade from flapping down), your butt may go low-G in the seat, but there would be no appreciable low-G condition between the airframe and the rotor. The weight of the airframe is in this case pulling the rotor down, so a couple remains between the airframe and the rotor (this assuming the rotor system is not capable of less-than-zero pitch).

[Gav500D]

Guys bascically to recover from a low g situation where the heli has rolled right eg : R22, R44.
Either aft cylic to reload the disc or the best way is to lower the collective.
The only reason why the heli is rolling is because the main rotor has become unloaded and for the given power setting the tail rotor thrust causes a the heli to roll.
The easiest way to understand this is enter a autorotation quite quickly and you will get weightless eg: low g. The heli will not roll as there is no torque being produced so the tail rotor will not roll the heli.
All the way in a auto and in the R22 below 18" you can and push and pull the cylic and the heli will not roll because the tail rotor is not producing enough thrust to roll the heli.
I find teaching lowering the collective the easiest way to advoid mast bumpig to a student. As in the mountains approaching a pass simulating bad weather you usually have alot of power in use and if they bring their speed back to 20-30kts this is not enough airspeed to relaod the disc if they nose the heli over adburtly so lowering the colective is the safest.

[delta3]

Lowering the collective


I would agree that this can be a preventive measure, nl do some kind of flaring to get the heli to descend.

It is not a corrective action for low G !

Once a low G is provoqued, with the associated rolling, lowering the collective is not going to be very helpfull in loading the disk, I would say quite the contrary.

First action should be to gently reload the disk by moving cyclic backwards.


d3


Flingwing207


To put it in your wording : G\'s are measured at "your butt ", assuming it is still sitting where it should be, using your seat belts.

As long as the rotor is attached to the heli :

G-pilot=G-heli=G-rotor.

(angular accelerations may be neglected in this case).

So if you lower the collective first/faster than pulling back the cyclic, so that you feel low-G, then the heli and the rotor are feeling the same.



d3

[Arm out the window]

No offence to you personally, Gav, but I find it concerning that you would be teaching people that lowering the collective further is the way to go in a low-g situation.
Removing torque won't necessarily fix the rolling situation in itself; it's pedal position, C of G and tail rotor RPM that determine the side force and therefore rolling moment produced by the tail rotor.
Your main priority is to load up the rotor again to avoid mast bumping, so raising collective and flaring if possible are going to do that.
I'd say that in the low-g, banked situation, my response would be to freeze the cyclic, pull collective as much as I felt I could without pulling it over on its back and maybe use some pedal to help get the nose down to build up airspeed and give me some flaring potential.

[NickLappos]

Flingwing207,

If you lower the collective fast enough, you can reach zero g, and it might be possible to create a problem even if you do not make much pitch down, too, but it would take one hell of a down collective motion.

I don't want to rule out a "collective only" zero g event, although it is far less likely than an event that includes significant nose down motion, as well. Remember, if you down collective and do not use enough aft cyclic, you get both, which is the place you want to avoid.

I also think that the roll contributes to the mast bumping problem, but once at zero g, the mast bumping can occur for other reasons. Significant sideslip (badly coordinated pedal) can also make the flapping go toward limits in the 204, 205 and 206, so I imagine it can in a Robbie as well. Thus, extreme dumping of the collective might get you to a mast bumping, as well, especially since it involves a chance to mess up the pedals while getting to zero g.