Grant
Member Since : 2007
Posts(687)
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How low is low G ?

03-25-2011 01:20:31

Hi all,

I'm new to this forum.

It's very interesting, full of informations, and people speaks freely of good and bad things.

I'm a low time helicopter CFI ( 600 hrs ) and relatively high time  ( 9 000 hrs ) fixed wing CFI.
So I know I'm at risk with low G mast bumping.

I've read about it. But I wonder how "low" is low G ?
Do you need zero G to get into the right rolling moment ?
I suppose the lower the G the faster the rolling, but I wonder where is the limit.
I've experienced low G feeling in turbulences, but I wonder then how far I am from the low G / mast bumping situation.

Any thoughts on this ?

Thanks.

Replies

Rossco
Member Since : 2010
Posts(41)
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03-24-2011 04:37:32
I have experienced negative G in severe turbulence where the seat belt is holding me down to the seat. I got no rolling motion to the right but i was instinctively pulling aft cyclic.. I think is has to do with how high the tail rotor obove the rest of the aircraft is when in nose down attitude when you get a right roll together with forward cyclic...  ?? I have 200 hours maybe someone with more experience can comment
rotordude
Member Since : 2015
Posts(39)
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03-24-2011 06:31:44
I am not an expert on Low-G, but the way I think of it is that the main rotor needs to be unloaded for several seconds for it to teeter and flap its way into coming into contact with the tailcone.  Yes, you can be "light in the seat" during turbulence, but the unloading of the rotor is for only a fraction of a second.  Compare this to pushing the cyclic forward and holding it there for several seconds, keeping the rotor unloaded for several seconds.  I had one flight instructor that would demonstrate "low-G" as part of the SFAR training.  Obviously, it was only "low-G lite", since it was enough that we could easily recover.  My current insurance checkride instructor will not demonstrate the effect.  I guess a man has to know his own limitations....

So can turbulence cause Low-G?  Yes, but it is harmless.  Similar to you driving your car over a speed bump really fast and feeling "light in the seat" for a split second.  Compare this to the Thelma and Louise car trick where driving off a cliff caused a longer lasting  "light in the seat" experience. 
melvynheli
Member Since : 2007
Posts(105)
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03-25-2011 02:52:20
The key issue here is if the tail rotor thrust line is above the center of gravity and then we unload the main rotor for some time, not a fraction of a second, but a little bit more, a right turn will develop. There is a long tail rotor leverage.

This is more porne to happen if the speed is high as the nose will be low and the tail high.

If the negative G situation is kept long and hard enough, it is possible to strike the tail cone. The main blades will for sure flex enough.
A hard landing, for instance, this is with a high descending speed will possibly make the blades to strike the tailcone as they unload.

The next problem is when we unload the main rotor at high speed and we experience the right turn is to try to counteract the turn by just moving the cyclic to the left. The unloaded rotor will allow the rotor head to move freely without "pulling" the helicopter, remember, no thrust situation,  and the rotor head (actually the blade roots),  will hit the main shaft. If it is hard enough, they will shear the shaft...

So it is a must to first load the main rotor so it can "pull" de helicopter, and then correct the turn with the cyclic.

The R22 has a very, very quick and noticeable right turn tendency at high speed, real quick but at low speed, say 40 knots, it is very mild a and slow.

The R44 is much more stable and the right turn tendency is much less at high speed.


I hope this helps.
Melvyn
xaviermarchant
Member Since : 2015
Posts(4)
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03-25-2011 10:55:00
Thanks all for theses answers.

I've found this article, which is quite good at explaining what is happening

http://www.myaviationschool.com/aviation-articles/36-becker-helicopters/52-negative-g-and-mast-bumping

An other question :

The author says to raise the collective to increase the load on the rotor. Sound like a good idea, but what about the torque and thus the increased tail rotor thrust needed to conteract the torque ?

Following the same idea, would right pedal help reduce the rolling moment by reducing tail rotor thrust ?



melvynheli
Member Since : 2007
Posts(105)
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03-26-2011 01:17:43
If we are in a low G, the main rotor is unloaded. regardless of the collective position.

We can reaload it with aft cyclic and up collective.

Mast bumping is only possible because the rotor head is the teetering type. Fully articulated or rigid ones cannot.

Of course, hitting the tail cone is still possible with all.

Melvyn
semik
Member Since : 2015
Posts(28)
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03-26-2011 09:34:34
Some simple video: