Why do aircrafts support less G´s in the negative side?

[smthngdffrnt]

Thank you! :ok:

[jack11111]

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.

[paco]

In brief, because the aerofoil has a greater positive camber.

[Chesty Morgan]

Your head would pop at -9g that's why.

[PDR1]

They don't - not inherently, at any rate. It's a design choice. If your aeroplane requires equal positive andf negative G capability this can be desigtned into the chosen structural and aerodynamic design solution. Very few aircraft (or more to the point very few missions usually undertaken with aircraft) need this capability, so an "asymmetric" solution is designed which is both lighter and has lower drag.

People can be made to tolerate far more positive G than negative G (G-suits only work for positive G), so for agile combat aircraft the prefered solution is one that give a large postive G capability (and also a large dG/dt) coupled to a rapid roll rate so that the aeroplane can go where needed by rapidly rotating the G-vector intro the desired direction and pulling the G. Interestingly - the same solution is used in nature by small, agile birds.

[smthngdffrnt]

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.

[PDR1]

Compression and tension stresses are not the same, and need different structures to carry them.

A typical spar has upper and lower booms and a shear web (or other shear structure). Under positive G the upper boom is loaded in compression while the lower boom is loaded in tension, and the web is loaded in shear.

To react the tension load the lower spar only needs tensile strength. The application of a tensile load inherently stabilises it.

To react the shear loads the web just needs to be joined to the upper and lower booms (either continuously or at short intervals) and it has to have sufficient shear strength.

To react the compression loads the upper boom has to have both sufficient compressive strength AND sufficient stiffness to ensure that it dows not buckle under the applied load, because compressive loads are inherently [i]de[/e]stabilising.

So whilst upper and lower booms will be handling almost identical maginitudes of stress, the difference between the compressive and tensile stress means that the upper boom needs to be much STIFFER than the lower one. This can be achieved with additional material, additional bracing or the same material formed into a more buckle-resistant shape. But the point is that the upper and lower booms will be different if you are looking for the lightest structure solution for an asymmetric loading requirement.

Obviuously if you're looking to design a wing for an SU-26 or similar with *symmetrical* load cases then you would design it with the required stiffness in both upper and lower spar elements - which would be heavier, of course.

[portsharbourflyer]

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.

[portsharbourflyer]

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.

[Denti]

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.

[portsharbourflyer]

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).

[portsharbourflyer]

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".

[PDR1]

Apologies - it wasn't confusion it was an assumption. I assumed that any aeroplane in that class would be designed for equal positive and negative cases. The point is that if you need it you design for it - if you don't then NOT designing for it will reduce weight and drag, and thus give greater performance for the intended mission. So the asymmetry in the G clearance is just a design choice.

[PDR1]

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.


Whilst true, if you want to add gust cases (I was keeping it simple) you have to add them to both upright and inverted cases so they have the same effect throughout the envelope. Suggesting fatigue is the dominant case makes certain assumptions about the way the structure reacts the stresses, and also that the structure is made from a material which suffers fatigue failure modes.

[portsharbourflyer]

Apologies PDR forgot you did have symmetric in asterisks and you did say design as required: so no you didn't get the terms muddled.

[PDR1]

No worries. I think between us we have answered the OP's question!

[smthngdffrnt]

Totally clear. Now I have enough information to keep working.

[cyclic35]

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.

Somewhere from the distant past, my memory recalled a specific 1970 event.
This link might be of interest and is reported by the late pilot concerned.


https://historic.aerobatics.org.uk/repeats/zlin_wing_failure.htm

[PDR1]

Indeed - he talks about this incident in his book. I love Neil's trademark understatement

Also the supply of adrenalin was getting rather low by this time.

Althopugh a write-off, the aeroplane came out with remarkably little damage considering the drastic nature of the failure:

http://i30.photobucket.com/albums/c309/india42/Mobile%20Uploads/870CA35E-8D8E-4535-B4AC-05BEA48F056D_zpsv5s231r7.gif

[cyclic35]

Not to mention :-

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.

Not an appropriate moment to consider reference to FRCs. :=