Low g

[baobab72]

Hi folks
I am a bit puzzled about why when you push the nose over of a helicopter you enter a low g condition.
I do understand the effects and the dangers that a push-over entails but i was more interested in the physics behind this low g phenomenon, where does it stem from? And why to recover the cyclic must be moved back?

Many thanks

Baobab

[That lights normal!]

Assuming this isn't a wind up.

In level flight, the application of forward cyclic will accelerate the AC towards the ground. Low G.
If sufficient cyclic is applied, this acceleration will exceeded 9.8m/s2. Negative G.

Aft cyclic has the opposite effect.

[[email protected]]

Baobab - usually the low g is experienced with a combination of lowering the collective and pushing the cyclic forward - although if you are sufficiently aggressive with the cyclic you can experience it with that input alone.

The physics are as That lights normal has stated.

[Peter-RB]

Baobab,

It would be totally stupid of you if you persue any form of test on Low G, it will bite your French backside far faster than you think and could lead to many unrecoverable flight situations.

The Physics are simple, enter Neg G, loose your tail or rotor, and life..!

You need to be a very experianced pilot to want to examine the physical effects, and it is even then still a risk not acceptable!

Dont go there :eek::=

Peter R-B
Lancashire
UK

[212man]

As above, plus the caveat (for the OP) that it is only during the rotation itself - when the acceleration is being applied - that the variance in perceived g loading occurs. Once in the steady state descent, or climb, the 1g condition returns.

[FLY 7]

I'm guessing you are flying an R22/44 with a teetering rotor head.

In the RFM it says:

"Never push the cyclic forward to decend or terminate a pull up. This may initiate a low-G condition which may result in the main rotor blade striking the cabin . Always use the collective to initiate a decent".

This is why many pilots prefer a helicopter with a fixed or fully articulated rotorhead.

[mickjoebill]

Presumably in severe turbulence there is a negative G, albeit for a short duration.

The most severe turbulence I have encountered, in the back seat, was flying around The Cape, short sharp thumps, really pulling me up against the straps.
Local pilot was unperturbed of course!

Anyone care to elaborate and quantify the differences between turbulence and control input induced negative G?



Mickjoebill

[skadi]

Anyone care to elaborate and quantify the differences between turbulence and control input induced negative G?The latter is more fun ( in a BO105 ) :E

skadi

[Harry the Hun]

It would be totally stupid of you if you persue any form of test on Low G, it will bite your French backside far faster than you think and could lead to many unrecoverable flight situations.

The Physics are simple, enter Neg G, loose your tail or rotor, and life..!

You need to be a very experianced pilot to want to examine the physical effects, and it is even then still a risk not acceptable!

Dont go there



Not true for all helicopters!

EC-665 Flight Envelope -1 G for 5 seconds.
Depends really on your rotorsystem, with a rigid rotorsystem no worries!

[Flying Bull]

Harry the Hun is right.
I'm happy to fly sophisticated helicopters, where low or Even negative G is possible.
With enough height and proper landing area in reach (just in case…) its a real fun to excecute…
But never ever do it with a Robby!!!!

[Captino]

...or never even, dare try it!!! :} (in a Robbie, of course)

[ericferret]

We had been suffering from a number of unexplained and expensive main rotor star cracks on our Dauphins. For those not familiar witht he 365 the star is the main load carrying structure in the main rotor head.

In conversation with a hoist operator he began to talk about pilot X's party trick.

Apparently X would make a pen magically rise into the air from the centre console.

Pilot X was taken to task and we suffered no more star cracks.

[Ascend Charlie]

I would hope he is now an X-pilot?

[topendtorque]

Anyone care to elaborate and quantify the differences between turbulence and control input induced negative G?I think we have on the thread of the accident in the french mountains of the R22 driven by two very revered pilots.

Turbulence negative is usually / hopefully an avoidable and forecast option. Either by Area Forecast or pilot observance.

Control input negative is very avoidable by adhering to the AFM

There is no difference with the catastrophe in the R22 when a vertical ascent issuddenly changed to a vertical descent thus causing floating pens and floating blades in non rigid heads or floating ab initio astronauts in a big F/W doing
weightless demonstrations.
tet

[Arm out the window]

Yep, teetering head type fuselage is fundamentally a box hanging pendulum-style off the rotor hub.

With positive g, tilt the disc and in a short while drag will make the box tilt the same way, keeping the rotor hub and mast aligned in a healthy way.

Apply negative g (as an extreme example, imagine turning the whole thing upside down), means the box can and will flop around uncontrollably with respect to the rotor disc, perhaps chopping the head off due to rotating bits of it smashing into the mast, or blades chopping the tail boom off, or more.

So! Use cyclic for airspeed control and fly up and down the hills with collective...

[krypton_john]

Not sure if the other posters have quite explained what you were after so I will attempt a different perspective:

In low g, there is no loading on the main rotor disc.

Because the thrust vector of the tail rotor , as viewed in the fore-aft plane, is higher than the lateral center of resistance to sideways movement, the airframe will want to roll over.

If the roll is not corrected, a dangerous attitude will ensue.

Because there is no loading in the main rotor disc, applying cyclic against this roll will not have any correcting effect.

In a teetering two bladed rotor, applying more cyclic is not only futile, it will lead to mast bumping and possible separation of the rotor head from the mast.

So the correction is to add load to the main rotor disc by one or both of aft cyclic and raised collective.

Ok, shoot me now.

[212man]

KJ,
I suspect that the answers about mast bumping etc are not what he wanted:
I am a bit puzzled about why when you push the nose over of a helicopter you enter a low g condition.
I do understand the effects and the dangers that a push-over entails but i was more interested in the physics behind this low g phenomenon,

He doesn't understand what creates the low g situation in the first place

[baobab72]

Hi many tks for all your replies.
I haveva toal of a whopping 00:36 hrs in the heli and i do not intend to become a test pilot, EVER!
My question stemmed from the label on the robbie's cyclic that warns you against pulling low g's so i was just trying to research a bit this limitation, only from a theoretical standpoint!!!
It is my understanding that a low g condition occurs when the rotor thrust vector is oriented below the horizontal plane acting in the same direction as gravity and resulting in a perceived acceleration of weightless. Am i correct?

Many thanks

Baobab72

[[email protected]]

No, in level flight the vertical component of rotor thrust equals the weight (mass of the helo x acceleration due to gravity). The rotor head is opposing the pull of the earth by producing lift and the fuselage is hanging below the rotor.

In a steady state descent, the rotor thrust is again balancing the weight and normal g is experienced.

All that is required for a reduction in the g experienced by the aircraft is to reduce that vertical component of rotor thrust quickly ie produce an acceleration - the faster you do that, the lower the g.

A fatal low g situation (in a teetering head helicopter) is usually produced by a rapid lowering of the collective (harsh entry to auto) accompanied by pushing instead of pulling with the cyclic - this will produce the effects described by Krypton John and AOTW.

[baobab72]

So if i get this right, for experiencing a low g condition is sufficient for the weight - mass times gravitational acceleration -, to overcome the vertical component of thrust - lift? By pushing the cyclic forward then the total thrust will be split in a horizontal component and in a vertical one causing the aircraft to accelerate downward by the pull pf weight? If that is the case, why is not the mrs being accelerated downward as well folllowing the box while the two tend to separate?
Trying to figure out a better way of wording my question but i couldn t!! Must be the 9 hr flight i did yesterday!!!!!

Baobab

[Arrrj]

Baobab72,

I am not sure I can answer your question with physics (other chaps have tried), so I will answer it in a practical way. Rule number one flying a two blade helicopter is to keep the disk loaded. i.e. you don't want it acting by itself, keep the pressure on it, and all will be good.

And yes, that does apply to all two blade helis...not just Robbies.:ugh:

Arrrj

[John R81]

I do remember back in my training days, retruning from a Nav Ex in an R44 with my instructor calmly sitting in the LEFT seat not touching the controls, relaxed, looking out of the side window ..... probably plotting some fuses to pull for emergency drill training.

Being a little high to duck under the Gatwick zone to rejoin EGKR, I pushed forward with cyclic and at the same time dropped the collective sharply, but opnly a little. I have never seen anyone react more sharply - it was like something out of the MATRIX film as he grabbed collective to load the disc and took the cyclic. The look on is face told me more about how wrong that manouver was than the detailed, lengthy debief with diagrams and examples from the AAIB publications that followed.

Safe to say, I have not done that since in any teetering head machine. Smooth control movements, and never use cyclic fwd / power down to lose height in a 2-blade.

[Armchairflyer]

Maybe this explanation corresponds somewhat to what you are looking for: NEGATIVE G AND MAST BUMPING | Becker Helicopters (http://www.myaviationschool.com/aviation-articles/becker-helicopters/negative-g-and-mast-bumping.html) .

[Soave_Pilot]

A fatal low g situation (in a teetering head helicopter) is usually produced by a rapid lowering of the collective (harsh entry to auto) accompanied by pushing instead of pulling with the cyclic - this will produce the effects described by Krypton John and AOTW.

Not quite true I think mate...

I believe harsh entry to auto is never a risk to mast bumping, as it is referred on Robinson's safety notice 11 " the low G which occurs during a rapid autorotation entry is not a problem because lowering the collective reduces both rotor lift and rotor torque at the same time"

[FYR]

This is OLD, but the physics havent changed, and the tech of a robbie 22/44/66? rotorhead is basically the same as the old viet-era birds...

nm8iV_uiBsI

[[email protected]]

Not quite true I think mate...

I believe harsh entry to auto is never a risk to mast bumping, as it is referred on Robinson's safety notice 11 " the low G which occurs during a rapid autorotation entry is not a problem because lowering the collective reduces both rotor lift and rotor torque at the same time"
29th Apr 2013 20:49

Except that lowering the lever will also cause the nose to drop.

Robinson are correct if they mean that a normal entry to auto will involve bringing the cyclic back to reduce speed and increase Nr - that will help keep the rotor loaded.

I suspect their notice is worded the way it is to prevent pilots being nervous about rapid entry to auto which IS needed in the event of an engine failure to preserve the Nr due to the low inertia head.

Negative g is negative g regardless of any reduction in rotor torque.