Why do aircrafts support less G´s in the negative side?
Joined: Oct 2012
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From: SF Bay area, CA USA
Maybe because you are on 1 G during level flight. And turns are generally done with positive G's. And the dirt stays on the floor with positive G's.
Last edited by jack11111; 4th December 2016 at 20:41.
Thread Starter
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From: lisboa
Ok, I got it, however, I don´t see any difference in terms of design, in other words, can anyone give one example of structure able to support +5 G´s and not -3 G´s? If I´m not wrong I guess it is just the same force with different sign so I don´t see why a stabilator -example- can just tolerates high forces in the "possitive side".
Cheers.
Cheers.
Educated Hillbilly
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From: From the Hills
Just like to add the following to the clarify the above;
In tension the failure of the material is limited by rupture occurring when the Ultimate Tensile Allowable for the material (ftu) is exceeded; but note Aluminium is ductile so exhibits a reasonable amount of elastic deformation before failure/rupture; infact the failure does not occur until permanent deformation occurs, so for single event Ultimate cases allowing for the permanent deformation (plastic correction) can allow you to exceed the "elastic" allowable. So in tension the material property is the determining factor.
In compression the failure mode is a factor of the Fcy (Allowable Compressive yield stress of the material) and the geometry (normally the aspect ratio / thickness width ratio part as PDR stated). For thin / high aspect ratio sections a material will initially become locally unstable and buckle, however a degree of elastic buckling may be permitted (ie: the point at which it will spring back to the unloaded shape). Beyond the local instability you get the crippling phase where the material loses any capability to carry load.
The geometric t/width ratio correction means the local instability and / or allowable crippling stress can be significantly below the actual material compressive yield allowable. The allowable compressive stress of a most aviation grade aluminiums (2000 or 7000 series) is in the region of 70 to 80% of the Ultimate Tension Allowable.
In turn the geometric correction means the instability / crippling compression allowable is only a proportion of the compressive allowable.
So the tension capability of a "thin geometric" shape is significantly greater than the compression capability.
Just trying to think PDR, while the normal level flight 1 g case load case is upper compression, lower tension it isn't a critical case; the negative gust cases could easily see the load reversal meaning upper and lower booms probably need to have mirrored dimensions for the ultimate cases. The 1 g level flight case is more an issue for fatigue; the zero to tension is more damaging than zero to compression.
As the human being can take more positive g (6 to 9) than negative g (-3g); for an aerobatic aircraft pulling higher positive sustained g manoeuvres then sustained negative, it would means you would generally need the upper boom to be made a lot stronger than the lower boom.
In tension the failure of the material is limited by rupture occurring when the Ultimate Tensile Allowable for the material (ftu) is exceeded; but note Aluminium is ductile so exhibits a reasonable amount of elastic deformation before failure/rupture; infact the failure does not occur until permanent deformation occurs, so for single event Ultimate cases allowing for the permanent deformation (plastic correction) can allow you to exceed the "elastic" allowable. So in tension the material property is the determining factor.
In compression the failure mode is a factor of the Fcy (Allowable Compressive yield stress of the material) and the geometry (normally the aspect ratio / thickness width ratio part as PDR stated). For thin / high aspect ratio sections a material will initially become locally unstable and buckle, however a degree of elastic buckling may be permitted (ie: the point at which it will spring back to the unloaded shape). Beyond the local instability you get the crippling phase where the material loses any capability to carry load.
The geometric t/width ratio correction means the local instability and / or allowable crippling stress can be significantly below the actual material compressive yield allowable. The allowable compressive stress of a most aviation grade aluminiums (2000 or 7000 series) is in the region of 70 to 80% of the Ultimate Tension Allowable.
In turn the geometric correction means the instability / crippling compression allowable is only a proportion of the compressive allowable.
So the tension capability of a "thin geometric" shape is significantly greater than the compression capability.
Just trying to think PDR, while the normal level flight 1 g case load case is upper compression, lower tension it isn't a critical case; the negative gust cases could easily see the load reversal meaning upper and lower booms probably need to have mirrored dimensions for the ultimate cases. The 1 g level flight case is more an issue for fatigue; the zero to tension is more damaging than zero to compression.
As the human being can take more positive g (6 to 9) than negative g (-3g); for an aerobatic aircraft pulling higher positive sustained g manoeuvres then sustained negative, it would means you would generally need the upper boom to be made a lot stronger than the lower boom.
Educated Hillbilly
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From: From the Hills
Sorry PDR, what I meant to say was, you stated everything correctly in your previous posts, but think you got muddled between a symmetric aerofoil and symmetric load case. The su26 has a symmetric aerofoil to enable easy inverted flight; but the load cases are still mainly positive sustained g.
Joined: Mar 2001
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From: I wouldn't know.
Well, certified to +12g/-10g, so the loadcase isn't symmetrical apparently. But mainly positive?
One could always use the Extra 300 and it's derivatives as an example, they are certified to +-10g, so have a symmetrical load case, although not all have a symmetrical airfoil.
One could always use the Extra 300 and it's derivatives as an example, they are certified to +-10g, so have a symmetrical load case, although not all have a symmetrical airfoil.
Educated Hillbilly
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From: From the Hills
Denti,
Symmetric aerofoils and symmetric load cases are not directly related and vice versa.
The outside profile of an aerofoil determines the wing / lift characteristics. The dcl/dalpha curve is a factor in wing loading but the internal structure of that aerofoil can be tuned / skewed however you need it to match the load cases. So I can tune (which is what PDR stated earlier) the internal structure within a given external profile to match whatever you want (within the laws of physics).
Symmetric aerofoils and symmetric load cases are not directly related and vice versa.
The outside profile of an aerofoil determines the wing / lift characteristics. The dcl/dalpha curve is a factor in wing loading but the internal structure of that aerofoil can be tuned / skewed however you need it to match the load cases. So I can tune (which is what PDR stated earlier) the internal structure within a given external profile to match whatever you want (within the laws of physics).
Educated Hillbilly
Joined: Dec 2004
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From: From the Hills
Load cases are a mix of "mission" cases (ie: what you want the aircraft to do, aerobatics, fighter aircraft) and the regulatory cases (CS23 and CS25 will specify, gust cases, rolling cases etc). It may the case that designing for one of the regulatory cases can result in a structure that is capable of reacting more negative g than you necessarily need for the "mission profile".
Joined: Jan 2007
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From: Lincolnshire
When Negative G can be useful
Ok, I got it, however, I don´t see any difference in terms of design, in other words, can anyone give one example of structure able to support +5 G´s and not -3 G´s? If I´m not wrong I guess it is just the same force with different sign so I don´t see why a stabilator -example- can just tolerates high forces in the "possitive side".
Cheers.
Cheers.
This link might be of interest and is reported by the late pilot concerned.
https://historic.aerobatics.org.uk/r...ng_failure.htm
Joined: Jan 2007
Posts: 102
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From: Lincolnshire
Upside Down with Only Smiths on the Clock
Not to mention :-
Not an appropriate moment to consider reference to FRCs. 
I was just beginning to think that I might make it after all when the engine died. I checked the fuel pressure - zero. A check around the cockpit revealed the fact that the main fuel cock had been knocked off. This could possibly have been the result of the jolt which accompanied the initial failure. I think I was probably thrown around in the cockpit and may well have accidentally knocked the cock then. I selected reserve fuel and almost immediately realised that this position would take fuel from the bottom of the gravity tank, which was of course now upside down. I therefore re-selected main tank and after a few coughs the engine started and ran at full power.
