GLAGAZ
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Passive Stability Of An Aircraft?

Fri Oct 21, 2005 10:55 pm

Anyone like to explain what this is? I've to do a 500 word report on it but can find very little information. I have a basic understanding of what it is but certainly not enough knowledge to write a 500 word report.

Gaz
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corey07850
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RE: Passive Stability Of An Aircraft?

Fri Oct 21, 2005 10:59 pm

Are you sure you aren't looking for "positive" aircraft stability?
 
GLAGAZ
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RE: Passive Stability Of An Aircraft?

Fri Oct 21, 2005 11:04 pm

Quoting Corey07850 (Reply 1):
Are you sure you aren't looking for "positive" aircraft stability?

Sure. It can be on either passive or active stability, even a mix of both if we want. But I am struggling to get info on either. I've ordered a book which should be here on Monday.

Gaz
Neutrality means that u don't really care cos the struggle goes on even when ur not there, blind and unaware
 
DH106
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RE: Passive Stability Of An Aircraft?

Fri Oct 21, 2005 11:25 pm

"Passive" must mean the aircraft's natural aerodynamic stbility, and "active" the computer enhanced stability common these days.
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GLAGAZ
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RE: Passive Stability Of An Aircraft?

Fri Oct 21, 2005 11:29 pm

Quoting Corey07850 (Reply 1):
Are you sure you aren't looking for "positive" aircraft stability?

Found a site which seems to describe positive stability in the same way as what I know passive stability to be. Must be different words for the same thing.

Quoting DH106 (Reply 3):
"Passive" must mean the aircraft's natural aerodynamic stbility, and "active" the computer enhanced stability common these days.

Yes passive its the ability of the aircraft to correct itself without pilot intervension. But I need around 500 words...lol

Gaz
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oly720man
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RE: Passive Stability Of An Aircraft?

Fri Oct 21, 2005 11:37 pm

Aircraft stability is the ability of an aircraft to return to its equilbrium position/attitude after being disturbed in some way (gust, turbulence, whatever).

Imagine an aircraft being hit by an upgust. If it was unstable, the incidence would increase and keep increasing. A stable aircraft would be pitched up then pitch nose down and return to its original flight path.

Stable aircraft require more control input to get them to respond. Combat aircraft are unstable and rely on the FCS to keep them flying.


Covered here in part.
www.owlnet.rice.edu/~mech594/handouts/aircraft_stability_control.pdf
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Starlionblue
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RE: Passive Stability Of An Aircraft?

Fri Oct 21, 2005 11:45 pm

Basically all airliners (most aircraft, in fact) are positively stable. That is, if you let go of the controls they will tend to stop oscillating and fly stably. Note that this can mean they are stably diving to their doom!

This is (very basically) achieved by having the center of gravity forward of the center of lift.

The "opposite" (but there are shades of grey) are aircraft such as Raptor, Gripen, Rafale and Typhoon, where the CoG is behind the center of lift. I have heard them called "relaxed stability aircraft" or "unstable aircraft".

If you think of the aircraft as a lever, with the center of lift as the fulcrum, a force behind the center of lift on a positively stable aircraft will be countered by the natural tendency of the aircraft to stabilize. If the CoG is on your side of the fulcrum, there is not tendency to stabilize, which is why "unstable" aircraft such as Gripen need computers to continually adjust the control surfaces, or they will spin out of control.

I'm pretty sure I bungled it there somewhere  Wink See this Wikipedia article: http://en.wikipedia.org/wiki/Relaxed_stability, which explains it all much better.
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SlamClick
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RE: Passive Stability Of An Aircraft?

Sat Oct 22, 2005 12:24 am

Quoting Oly720man (Reply 5):
A stable aircraft would be pitched up then pitch nose down and return to its original flight path.

Negative stability would continue to depart the flight path. (helicopter)
Neutral stability would stablize in its new flight path. (boat adrift)
Positive stability will return to the original flight path.

I had a great demonstration of this many years ago in a DC-9 simulator.

The instructor had me fly at 10000' at 250 knots, clean, and in trim. Then we noted what fuel flow it took to maintain the 250 knots. Next, he had me pull the thrust levers back to idle and maintain ten thousand feet and not re-trim the airplane as the speed bled off. The speed continued to bleed and the nose got ever heavier and finally we did a straight-ahead, clean stall.

When the stall came he had me put the thrust levers up to the fuel flow that had been used to maintain 250 knots and take my hands and feet OFF the flight controls. The plane, hands-off, lost about 1800 feet in the stall, but recovered straight ahead, climbed back up to about 11600' losing speed, then pitched over, building speed leveled off at about 8800' or so. It kept doing these ever-smaller oscillations until, after a half-dozen or so it stabilized at ten thousand feet and two hundred fifty knots. All hands-off.

Positive dynamic stability.
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Woodreau
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RE: Passive Stability Of An Aircraft?

Sat Oct 22, 2005 12:42 am

Quoting GLAGAZ (Reply 4):
Yes passive its the ability of the aircraft to correct itself without pilot intervension. But I need around 500 words...lol

So when you have to list your references you're going to say airliners.net?

I've never heard of passive or active stability, but there is static and dynamic stability and for each static and dynamic, the stability can be described as negative, positive, or neutral.

So six kinds of stability:
Negative static stability
Positive static stability
Neutral static stability
Negative dynamic stability
Positive dynamic stability
Neutral dynamic stability

Any kind of dynamic stability implies positive static stability. If you have neutral or negative static stability you can't have any dynamic stability. (at least that may be my misguided perception.)

Using SlamClick's simulator demo as an example - that was positive dynamic stability.
If it were neutral dynamic stability - the airplane would continue to oscillate between 8800ft and 11600ft indefinitely.
If it were negative dynamic stability - the airplane's oscillation would continue to worsen so that the altitude ranges between the oscillation would get larger and larger, e.g. next low 6000ft next high 13000ft next low 4000ft next high 15000ft, etc...

[Edited 2005-10-21 17:43:13]
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GLAGAZ
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RE: Passive Stability Of An Aircraft?

Sat Oct 22, 2005 3:23 am

Quoting Woodreau (Reply 8):

Perhaps even a direct link to this thread :P

Thanks all for your replies.

Gaz
Neutrality means that u don't really care cos the struggle goes on even when ur not there, blind and unaware
 
prebennorholm
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RE: Passive Stability Of An Aircraft?

Sat Oct 22, 2005 5:09 am

Stallionblue and SlamClick, you did a perfect job explaining a/c stability. And your combined effort only included 499 words!!!

But for some readers I think that one basic thing has to be added: Aircraft pitch stability is not something either/or. It's a gradual scale which at some point passes a neutral point (or zero point).

We can put a lot of adjectives on certain regions of that scale - passive, active, negative, positive, relaxed, neutral etc. The more adjectives, the more confusion.

Airliners always have positive stability, and a rather large positive stability. That is needed because at take-off the center of gravity (CG) is never known exactly. The balance calculation for instance doesn't know if the fat men are sitting in the back or up front. Also the very dirty wing with large multi section flaps and leading edge slats, and inflight use of spoilers, call for a generous, positive stability.

Some mainly long range airliners can reduce the stability inflight while cruising by pumping fuel to a tail tank, and visa versa. During cruise reduced stability improves performance simply because the horizontal tail has to produce less negative lift and the wing has to produce the same amount of less positive lift. Since both wings have to produce less lift, then drag is reduced.

Some of the most advanced combat aircrafts have negative stability - that is the CG is behind the wing center of lift. Then stabilized flight can only be done by constant elevator control inputs at computer speed. It has the advantage that maneuvability is increased since both wings lift in the same direction. Just to ad to the number of adjectives this is also called "artificial" stability. Such planes are always FBW planes.

That doesn't mean that a FBW airliner can have negative pitch stability. Never ever!!! Anyway a combat plane with negative pitch stability is also perfectly safe, but only because when things go really wrong, then you jump. I think that we all noticed that when the numerous SAAB Gripen prototypes were being tested - and crashed any time a video camera was pointing their way.

(And then we haven't even started on lateral stability yet).

Anyway, thanks for the bandwidth! (I don't dare to count the words).
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Starlionblue
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RE: Passive Stability Of An Aircraft?

Sat Oct 22, 2005 5:38 am

Quoting Prebennorholm (Reply 10):
hat doesn't mean that a FBW airliner can have negative pitch stability. Never ever!!! Anyway a combat plane with negative pitch stability is also perfectly safe, but only because when things go really wrong, then you jump. I think that we all noticed that when the numerous SAAB Gripen prototypes were being tested - and crashed any time a video camera was pointing their way.

Prebennorholm. Good input as usual.

I would like to add on the Gripen crashes that both were basically pilot error (just like one Raptor crash). Specifically Pilot Induced Oscillation. Gripen initially had no feedback in the stick, so the pilot didn't feel the aircraft in the stick. The pilot then overcorrected because his reflexes, honed by decades of flying other aircraft, told him that the plane needed still more control inputs. The famous Gripen loss of control over Stockholm during the Water Festival (I was there and saw it crash) was preceded by no less than 3 warnings that the surfaces had reached their maximum deflection.

Point: On relaxed stability aircraft, the pilot can still, in almost all cases, release the controls and count on the control systems to artifically stabilize the aircraft. That is, behavior should be the same as on any other plane.

On the F/A-18E/F, there is an additional aid in case of loss of control. Arrows on the MFDs will appear, pointing to where the pilot should pull the stick to ensure recovery. The MiG-25 had a cruder method, with a line painted on the panel indicating stick direction for spin recovery.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
prebennorholm
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RE: Passive Stability Of An Aircraft?

Sat Oct 22, 2005 6:12 am

Oh my God, Stallionblue, you actually saw the water festival crash!!!

And yes, it was indeed a pilot error.

But on the other hand, when the most experienced company test pilot fails to fly the plane during a show over the capital city because of inferior flight control skill, then there is room for improvement on the plane before it enters ordinary squadron service. That was also what happened.

Quoting Starlionblue (Reply 11):
The MiG-25 had a cruder method, with a line painted on the panel indicating stick direction for spin recovery.

I remember reading about the Korean War. A North Korean MiG-15 pilot deflected to the south with a perfectly good plane for which he was paid something like $100,000.

The famous USAF test pilot Charles E. (Chuck) Yeager and his boss General Boyd was hasily flown out there to evaluate the MiG and find out how to best engage it with the F-86 Sabre.

The North Korean pilot gave Yeager an instruction with hand language only. He "said" - dancing on the tarmac - that the plane had a bad tendensy to enter a spin when pulled too hard in a turn. When that happened, then the stick should - depending on the way it was spinning - be pointed against one of two chalk marks on the instrument panel.

If it didn't recover within ten seconds (showing all ten fingers), then... - the North Korean pilot pointed at the ejection seat handle. Instruction over.
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