Soku39
Topic Author
Posts: 1731
Joined: Fri Nov 03, 2000 7:16 am

### Positive, Nuetral, Negative Stability

Just a few questions regarding the stability of airplanes. Most trainers are positively stable such as the 152/72s I'm learning on. I'm assuming engineers can purposely design planes with one of the three in mind, I just have no clue on how you go about designing a plan to be one of the three. I would guess that it has something to do with the location of the center of gravity/wing/elevator placement, but these are total guesses. I was also wondering about the general rules (if there are any) of stability tendancies for small GA, King Air sized turbo props, on up to 757 sized airliners.

Thanks.
The Ohio Player

FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

### RE: Positive, Nuetral, Negative Stability

First off, you have to specify which axis you are talking about when talking stability. It is very common not to do so though, and this usually means talking pitch stability.

Almost all aircraft are designed with positive static stability. The exceptions are, by and large, fly-by-wire jet fighters. For positive static stability, you have to have the center of gravity ahead of the aerodynamic center of the aircraft. Think of a dart. The center of gravity forward and the flights in the rear end, and it will always align itself with the direction of flight. Throw it backwards and you have negative static stability. An FBW aircraft with negative static stability use computers to balance the aircraft and effectively keep the dart flying backwards.

As for tendencies, you want to look at the stability margin, which is a measure of just how stable or unstable they are.

JAS-39 Gripen - statically unstable.
EF-2000 - statically unstable.
F-16 Fighting Falcon - approximately neutral static stability
Wright Flyer I - slightly statically unstable

The last one is interesting. The Wright brothers believed an aircraft had to be unstable to be manoeuverable - and it definitely did not have computers or FBW. However, the instability at the slow speeds of flight the Flyer achieved is slight enough for a pilot to be able to manually balance the aircraft.

Cheers,
/Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.

EridanMan
Posts: 113
Joined: Sat Dec 31, 2005 12:49 pm

### RE: Positive, Nuetral, Negative Stability

To Expand on what fredP said-

Aircraft stability is directly tied to to relationship between three key points - aircraft center of mass, aircraft center of lift, and aircraft center of drag.

Generally when you're talking 'statically stable' (not specifying an axis), essentially what you are saying is that the airplane, if left too its own accord, will keep the pointy end pointed into the wind. Thus you are primarily talking about pitch and yaw.

The (slightly oversimplified) overview is that the aircraft's stability can be directly defined by the relative locations of the center of Drag and center of mass... if the center of mass is in front of the center of drag, the plane will be statically stable (like and arrow), if they're in the same place, it will have neutral stability, and if the center of drag is in front of the center of mass, the plane will be statically unstable.

Roll is another animal entirely, forget it for a moment.

The situation is a bit more complex than that - first off, while center of mass is (relatively) stable in the aircraft, the center of drag will change based on the aircraft's attitude with the oncoming air stream... To be 'truly' statically stable, the aircraft's center of drag must ALWAYS remain behind the center of mass regardless of the plane's attitude.

Furthermore, The above simplification applies almost directly to yaw, pitch stability is a slightly different matter, because- as a rule, pitch stability is a simple matter that the center of lift of the aircraft MUST be aligned precisely with the center of mass... if the center of lift moves behind the center of mass, the plane will nose down. If it moves in front of, it will nose up. This need for very precise alignment of these two points is why aircraft have trim mechanisms.

So to answer your question - want a plane with a little more yaw stability? increase the size of the vertical stabalizer... This will move the center of drag far backwards whenever the plane is off axis horizontally, and keep the fuselage in line with the airstream. Conversely, want a faster (less drag) more manuverable aircraft? shrink the vertical stabalizer, and the center of drag will be less effected by off-axis body movements, so there will be less of a tendency for the plane to want to 'right itself' back into the wind.

Pitch stability is similar but slightly more complicated. Giving the aircraft a good mechanism to finely trim its attitude so that the center of lift stays aligned with the center of mass is the first step. In general though - the horizontal stabalizer is much larger than the vertical stabalizer, so center of drag shifts based on pitch changes will be larger than with the rudder. The key thing to making a 'pitch stable' aircraft is sizing the elevators correctly so that the effect that they have on the center of lift is commessurate with the wing's lifting ability, such that it provides an 'intuitive feel' for the pilot... Or - in plane english - Make the horizontal stabalizer and elevator smaller, and the elevator's deflection will have less of an effect on the aircraft's center of lift, causing slower pitch movements ('lazy, stable in pitch'), make the elevator and horizontal stabalizer huge, and its motions will cause much greater changes in center of lift (because the horizontal eppenepage will exert greater forces relative to the aircraft's wing) and the plane will be very 'light' in pitch.

All of this and I haven't touched roll stability yet

There are two main mechanisms to achieve roll stability... one is the pendulum - I.E, keep the aircraft's center of gravity below the center of lift. The other is positive Dihedral - tilt the wings upward a bit causes the center of lift to shift from one side to the other in a bank (the Wing that is more perpendicular to the axis of gravity is using more of its lift force for lift than turning), hense the center of lift will shift towards the low wing, providing a force to re 'right' the aircraft. Its worth noting that center of lift starts to go a bit haywire in un-coordinated high roll angles - so very few aircraft will be roll stable past ~30 degrees of bank.

So - as an aircraft designer trying to tweak your roll stability - create a high wing plane with positive dihedral and... well, you have pretty severe overkill - but the combination of pendulum action and center of lift shifting will make an aircraft that doesn't like to turn (very positively stable). Create a low wing plane with negative dihedral, and you'll need to fight the whole time to keep the shiney end pointed upwards.

This is all ameteur understanding, based on reading books and discussions with my (PHD in Aeronaughtical engineering) father.. I'm sure there are intracacies that I've missed, but for the most part I'm pretty confident in it.

Thus- want a _really_ roll-stable

KELPkid
Posts: 5247
Joined: Wed Nov 02, 2005 5:33 am

### RE: Positive, Nuetral, Negative Stability

 Quoting FredT (Reply 1):F-16 Fighting Falcon - approximately neutral static stability

Maybe with the fly-by-wire system. I'd heard that the reason it is a fly-by-wire aircraft was because the aircraft's architecture made the design exibit divergent static instability (i.e. natural pilot inputs in response to aircraft movements will always lead to pilot-induced oscillations, or divergent stability). Nothing that the computer can't correct   I'm not an authority on this, however....

Aren't most aerobatic aircraft (fighters included) rigged very close to neutral stability?
Celebrating the birth of KELPkidJR on August 5, 2009 :-)

Starlionblue
Posts: 17952
Joined: Fri Feb 27, 2004 9:54 pm

### RE: Positive, Nuetral, Negative Stability

 Quoting KELPkid (Reply 3):Quoting FredT (Reply 1): F-16 Fighting Falcon - approximately neutral static stability Maybe with the fly-by-wire system. I'd heard that the reason it is a fly-by-wire aircraft was because the aircraft's architecture made the design exibit divergent static instability (i.e. natural pilot inputs in response to aircraft movements will always lead to pilot-induced oscillations, or divergent stability). Nothing that the computer can't correct Wink I'm not an authority on this, however....

The objective of static instability ("relaxed stability") in fighters is to allow them to turn faster. Positive stability makes turning harder, ie more energy is needed to turn the plane.

And yes, the reason FBW systems are de rigeur in relaxed stability aircraft is that pilots cannot compensate fast enough.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

Soku39
Topic Author
Posts: 1731
Joined: Fri Nov 03, 2000 7:16 am

### RE: Positive, Nuetral, Negative Stability

I did indeed mean pitchwise, as in if you bump the yoke forward the plane a. oscillates up and down until returning to level flight b. stays at the pitch angle that the yoke was bumped to c. a bump into it turns into a nose dive. so Fred and Eridan you both helped a little bit. I at least understand why you would want the elevator and stabilizer smaller to go faster (coupled w/ a rear CG), and vice versa for maximum control, but I'll have to digest the rest of it.
The Ohio Player

FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

### RE: Positive, Nuetral, Negative Stability

EridanMan, overall a good reply but you came slightly short of target on a few points.

 Quoting EridanMan (Reply 2):Make the horizontal stabalizer and elevator smaller, and the elevator's deflection will have less of an effect on the aircraft's center of lift, causing slower pitch movements ('lazy, stable in pitch'), make the elevator and horizontal stabalizer huge, and its motions will cause much greater changes in center of lift (because the horizontal eppenepage will exert greater forces relative to the aircraft's wing) and the plane will be very 'light' in pitch.

It is the other way around. If you change nothing else and make the tail surfaces larger, you will end up with a more stable design which takes more effort to turn. You are effectively moving the aerodynamic centre aft. The stabilizer moment arm and size is one of the important considerations in aircraft design.

 Quoting EridanMan (Reply 2):Furthermore, The above simplification applies almost directly to yaw, pitch stability is a slightly different matter,

The aerodynamic centre takes both drag and lift into account, as well as torque. The mechanism is the same for yaw and pitch, no need to keep them apart. Drag is just as much of an issue for pitch stability as it is for yaw stability, i e not very important at all. The lift is what does it, sideways lift in the case of yaw stability. The only difference is that at beta angle 0 (no sideslip), there will ideally be no lateral lift, while most aircraft generate positive vertical (z-axis, aircraft reference system) lift at zero alpha (angle of attack).

 Quoting EridanMan (Reply 2):There are two main mechanisms to achieve roll stability... one is the pendulum - I.E, keep the aircraft's center of gravity below the center of lift.

That is one of the common misconceptions. The pendulum effect isn't. Think about it. The lift is always acting perpendicular to the plane of the wing, regardless of bank angle, and in the middle of the wing span. Thus it will be acting through the centre of gravity of the aircraft and no roll axis force couple will be set up, regardless of the relative vertical positioning of the wing and the centre of gravity.

The stabilizing effect of a high wing design is due to the airflow around the fuselage in a side slip. In a slip, air will stream around the fuselage and hit the windward side going up, increasing the angle of attack and the lift generated. On the leeward side, the air instead hits the wing as it goes down around the fuselage, decreasing the angle of attack and the lift generated. Thus, a stabilizing force couple which works to roll the aircraft away from the direction of slip is set up.

 Quoting KELPkid (Reply 3):Maybe with the fly-by-wire system. I'd heard that the reason it is a fly-by-wire aircraft was because the aircraft's architecture made the design exibit divergent static instability (i.e. natural pilot inputs in response to aircraft movements will always lead to pilot-induced oscillations, or divergent stability). Nothing that the computer can't correct Wink I'm not an authority on this, however....

It is an old design, and as it was pretty unknown territory back then they didn't go all out. It is pretty much neutral. This translates to slightly stable in some cases, slightly unstable in others. There's definitely a need for FBW to keep it flyable, at least with reasonable pilot workload. There is an MPO function, Manual Pitch Override. It is intended for getting out of deep stalls, if I'm not completely misinformed, but I bet a lot of Viper pilots know what it is like to fly with the pitch stability augmentation out of the loop...

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.

EridanMan
Posts: 113
Joined: Sat Dec 31, 2005 12:49 pm

### RE: Positive, Nuetral, Negative Stability

I'm a day late and a dollar short in this reply, but thanks Fred

Do you have any good 'advanced amature' books on aeronaughtical design you'd recommend?

FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

### RE: Positive, Nuetral, Negative Stability

There's one book I always recommend to people getting started, and that is John D. Andersons 'Introduction to Flight".

Other than that, check out See How It Flies.

Rgds,
/Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.

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