Low g
Thread Starter
Join Date: Jul 2010
Location: france
Age: 51
Posts: 255
Likes: 0
Received 0 Likes
on
0 Posts
Low g
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
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
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.
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.
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.
The physics are as That lights normal has stated.
Join Date: Sep 2007
Location: On the Rump of Pendle Hill Lancashi
Posts: 614
Likes: 0
Received 0 Likes
on
0 Posts
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
Peter R-B
Lancashire
UK
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
Peter R-B
Lancashire
UK
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.
Join Date: Feb 2008
Location: UK
Posts: 392
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Last edited by FLY 7; 27th Apr 2013 at 08:02.
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
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
Last edited by mickjoebill; 27th Apr 2013 at 10:55.
Anyone care to elaborate and quantify the differences between turbulence and control input induced negative G?
skadi
Last edited by skadi; 27th Apr 2013 at 11:18.
Join Date: Mar 2009
Location: Planet Earth
Age: 57
Posts: 75
Likes: 0
Received 0 Likes
on
0 Posts
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!
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!
Low g
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!!!!
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!!!!
(in a Robbie, of course)
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.
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.
Last edited by ericferret; 27th Apr 2013 at 22:31.
Join Date: Feb 2005
Location: Australia
Posts: 1,957
Likes: 0
Received 0 Likes
on
0 Posts
Anyone care to elaborate and quantify the differences between turbulence and control input induced negative G?
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
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...
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...
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.
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.
KJ,
I suspect that the answers about mast bumping etc are not what he wanted:
He doesn't understand what creates the low g situation in the first place
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,
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,
Thread Starter
Join Date: Jul 2010
Location: france
Age: 51
Posts: 255
Likes: 0
Received 0 Likes
on
0 Posts
Low g
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
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
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.
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.
Thread Starter
Join Date: Jul 2010
Location: france
Age: 51
Posts: 255
Likes: 0
Received 0 Likes
on
0 Posts
Low g
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
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