Stalling Negative G???
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Stalling Negative G???
So I was wondering the other night...
In PPL training, we are all taught stalling. Power off stalls, approach configuration stalls (I was even shown full power stalls) all of which point the nose to the horizon until the airflow separates from the wing.
In aerobatic aircraft, you can both stall erect, and inverted, but all point the nose to the sky until the airflow separates from the wing.
The question then… Is it possible to ‘shove’ the nose towards the ground in such a way to stall the wing?
Obviously this could not be done in the average club Cessna or Piper.
Something tells me that in theory it is doable as you can flick roll on the downward path, or have I got the wrong end of the stick?
In PPL training, we are all taught stalling. Power off stalls, approach configuration stalls (I was even shown full power stalls) all of which point the nose to the horizon until the airflow separates from the wing.
In aerobatic aircraft, you can both stall erect, and inverted, but all point the nose to the sky until the airflow separates from the wing.
The question then… Is it possible to ‘shove’ the nose towards the ground in such a way to stall the wing?
Obviously this could not be done in the average club Cessna or Piper.
Something tells me that in theory it is doable as you can flick roll on the downward path, or have I got the wrong end of the stick?
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General rule: You can stall any aircraft in any attitude and at any speed*. Stalling has no direct relationship to the horizon, only to the relative airflow.
A non-symmetrical (cambered) aerofoil (as found on most wings) will stall at a lesser negative angle of attack than it will at a positive AoA. Any wing can be stalled negatively as well as positively, however due to perfomance, limitations and normal usage of light aircraft, negative G stalls are unlikely to be encountered unless you are doing something very unusual.
The fact that in your example an aircraft flick rolls with its nose below the horizon does not automatically equal a negative stall - that depends on wether from that attitude the pilot pulls up or pushes forward to induce an accelerated stall and flick roll. Competition aerobatic aircraft will often have symmetrical aerofoils and large control authority.
* Small print: Zero airspeed or zero G are special situations, and at high speed structural limits may be exceeded before an aerodynamic stall occurs. Available control authority my restrict the ability to induce a stall in some circumstances.
A non-symmetrical (cambered) aerofoil (as found on most wings) will stall at a lesser negative angle of attack than it will at a positive AoA. Any wing can be stalled negatively as well as positively, however due to perfomance, limitations and normal usage of light aircraft, negative G stalls are unlikely to be encountered unless you are doing something very unusual.
The fact that in your example an aircraft flick rolls with its nose below the horizon does not automatically equal a negative stall - that depends on wether from that attitude the pilot pulls up or pushes forward to induce an accelerated stall and flick roll. Competition aerobatic aircraft will often have symmetrical aerofoils and large control authority.
* Small print: Zero airspeed or zero G are special situations, and at high speed structural limits may be exceeded before an aerodynamic stall occurs. Available control authority my restrict the ability to induce a stall in some circumstances.
Last edited by FRFO; 6th Aug 2012 at 13:33.
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but all point the nose to the sky until the airflow separates from the wing.
This would be much the same as you shoving the nose down to do a negative stall, though I would suggest to achieve this you start by pulling the nose up so the aircraft is decelerating, then start a gentle push to give a slight negative G (in this way you can get the speed well below the level stall speed), a little before the nose reaches the horizon again push hard and you might just get your negative stall with the nose down, if you try and just do it from level flight the increase in speed will probably defeat the stall.
This Does of course need to be done in a suitable aircraft!
Last edited by foxmoth; 6th Aug 2012 at 14:26.
There's a manouver called a Zwiebelturm which, if done correctly, results in an inverted spin whilst travelling upwards with the aircraft upright. However it's so disorientating that I honestly can't say whether I've ever achieved this or not.
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FRFO, an astounding reply
Foxmoth, had not thought about that. But it does explain something that happened while flying with a friend on an aero's sortie some time back.
He was trying to fly an Avalanche (half loop - flick roll at the top - finish off the loop). He went beyond the top of loop before the flick, snatched at the controls
. We paused inverted (stall warner screaming) though pointing down, before we entered an incipient (if I recall) spin. Quite uncomfortable 
Foxmoth, had not thought about that. But it does explain something that happened while flying with a friend on an aero's sortie some time back.
He was trying to fly an Avalanche (half loop - flick roll at the top - finish off the loop). He went beyond the top of loop before the flick, snatched at the controls
. We paused inverted (stall warner screaming) though pointing down, before we entered an incipient (if I recall) spin. Quite uncomfortable
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Sounds like that is what happens, also worth noting is that although you were inverted the stall and subsequent spin are both POSITIVE (the spin will be erect not inverted), not negative and easy to recover the spin at the incipient stage just by unloading the G.
Last edited by foxmoth; 6th Aug 2012 at 15:18.
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IMHO the important point to grasp is that stalling (in either the positive or negative sense) has no relation to 'G' as in gravity. It is entirely related to the angle of attack of the wing, which is (roughly) down to a combination of the acceleration on the airframe (producing loading of the wing), and the speed at which the air is passing the wing.
In 'conventional' flying, (g)ravity is a handy way of providing that acceleration so as to allow non-dramatic stalling - we simply slow the airflow until gravity provides enough acceleration for a stall.
More dynamically, as has been said, you can generate that stall in either direction in any attitude with sufficient provocation. The avalanche is a positive manouver, and (should) start about 45deg before the fully inverted position - and yes, flicks are a (fairly) violent experience.
If the stall warner was going off, it was a positive stall. Stick position (generally), and whether you're being shoved into, or out of the seat are good clues
In 'conventional' flying, (g)ravity is a handy way of providing that acceleration so as to allow non-dramatic stalling - we simply slow the airflow until gravity provides enough acceleration for a stall.
More dynamically, as has been said, you can generate that stall in either direction in any attitude with sufficient provocation. The avalanche is a positive manouver, and (should) start about 45deg before the fully inverted position - and yes, flicks are a (fairly) violent experience.
If the stall warner was going off, it was a positive stall. Stick position (generally), and whether you're being shoved into, or out of the seat are good clues
Back when I was flying serious competition aerobatics there were some common figures relevant to this discussion.
(been a long time so please forgive any errors in the following details)
Enter an inverted spin from upright. Set it up initially as you would for an upright spin and earlier than normal - full forward stick, full rudder and full opposite aileron. Centralize the aileron once it was spinning. Pretty uncomfortable if aileron is not used initially to get some roll into it.
On occasion we'd also get negative flick rolls from horizontal upright flight.
(been a long time so please forgive any errors in the following details)
Enter an inverted spin from upright. Set it up initially as you would for an upright spin and earlier than normal - full forward stick, full rudder and full opposite aileron. Centralize the aileron once it was spinning. Pretty uncomfortable if aileron is not used initially to get some roll into it.
On occasion we'd also get negative flick rolls from horizontal upright flight.
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The wing has no idea where the ground is (below the aeroplane, above it, in front of it, behind it, or to either side). All it understands is angle of attack (and structural loads, of course). Exceed the critical angle, and it will stall.
Many wings are optimised for erect flight so have a more cambered upper surface than lower, so the critcal angle for stalling the lower surface will be less than for the upper surface. A symmetrical wing will stall just the same either way.
Many wings are optimised for erect flight so have a more cambered upper surface than lower, so the critcal angle for stalling the lower surface will be less than for the upper surface. A symmetrical wing will stall just the same either way.
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IMHO the important point to grasp is that stalling (in either the positive or negative sense) has no relation to 'G' as in gravity
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I think I know what Mark is getting at, but would certainly not say it is the most important thing to grasp and most people will just get confused by it, IMHO one of those things that sounds clever but really just confuses.
Last edited by foxmoth; 7th Aug 2012 at 10:02.
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Tigger Too
This was probably part of my original thinking, 'wing cannot stall if unloaded - 0g'.
I guess then, as negative g increases (and also negative angle of attack), so does the possibility of a negative g stall.
It would be interesting to watch a wing under positive, and negative load with the coloured smoke indicating the airflow, just to see what happens.
Even more fun to be strapped into an Extra (other unlimited aerobatic machines are available - and are just as much fun) to try it out for real.
This was probably part of my original thinking, 'wing cannot stall if unloaded - 0g'.
I guess then, as negative g increases (and also negative angle of attack), so does the possibility of a negative g stall.
It would be interesting to watch a wing under positive, and negative load with the coloured smoke indicating the airflow, just to see what happens.
Even more fun to be strapped into an Extra (other unlimited aerobatic machines are available - and are just as much fun) to try it out for real.
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Apologies - wasn't supposed to be confusing, or to make me look clever I felt that because the unit of acceleration is G (as in 1 gravity), and we were talking about orientation relative to 'earth' that might the source of confusion.
I failed to be explicit that the acceleration of the airframe is the the 'G' you feel in the cockpit, and that *is* what matters. It's not 'gravity', it's acceleration measured in G's. If you're at zero 'G' there's no acceleration, therefore you don't stall (most conviniently / clasically in the vertical, but..).
It's all about wing loading, however that's generated - the result of wing loading is the acceleration (G) you feel. I'm not sure if that's useful, or any more clear..
I suppose to me it's important because it explains that, no matter what is going on, you just have to 'fly the wing' and it will all work, no matter where the green/blue bits are.
Edit just to add to the OP: You're spot on with your last post, and you control the load on the wing with the elevator. Push 'gently' to zero and it can't stall, shove hard, it will.
I felt that because the unit of acceleration is G (as in 1 gravity), and we were talking about orientation relative to 'earth' that might the source of confusion.I failed to be explicit that the acceleration of the airframe is the the 'G' you feel in the cockpit, and that *is* what matters. It's not 'gravity', it's acceleration measured in G's. If you're at zero 'G' there's no acceleration, therefore you don't stall (most conviniently / clasically in the vertical, but..).
It's all about wing loading, however that's generated - the result of wing loading is the acceleration (G) you feel. I'm not sure if that's useful, or any more clear..
I suppose to me it's important because it explains that, no matter what is going on, you just have to 'fly the wing' and it will all work, no matter where the green/blue bits are.
Edit just to add to the OP: You're spot on with your last post, and you control the load on the wing with the elevator. Push 'gently' to zero and it can't stall, shove hard, it will.
Last edited by Mark1234; 7th Aug 2012 at 11:03.
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Quote:
IMHO the important point to grasp is that stalling (in either the positive or negative sense) has no relation to 'G' as in gravity
Well it does. You cannot be stalled if you are completely unloaded - zero 'G'
IMHO the important point to grasp is that stalling (in either the positive or negative sense) has no relation to 'G' as in gravity
Well it does. You cannot be stalled if you are completely unloaded - zero 'G'
Push 'gently' to zero and it can't stall, shove hard, it will.
Of course we don't have an AoA indicator in our SEPs, which for aerobatic types at least is, in my book, a major omission.
