# Dynamic Stability



## Zipper730 (Dec 2, 2018)

Static stability seems to be based on the relationship in the C/L and CG. Dynamic stability I don't know how one determines that particular factor?


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## whstlngdeath (Dec 2, 2018)

I build and fly model R/C aircraft, and what seems to determine the dynamic stability of a particular model is, number one, how well the model is designed, and how well it is built. Aircraft tend to fly best when built straight and true. I would describe dynamic stability as the time that it takes for an airplane to respond to a disturbance in flight. Wing dihedral, that is, the upward angle of the wing, at the tips, to the centerline of the wing plays a big role in dynamic stability. Wings with large amounts of dihedral, say a Cessna 172, tend to recover from a "_reasonable_" disturbance pretty much hands off. An aircraft with little to no dihedral, like a high performance fighter jet, will usually not recover hands off and will need pilot intervention or the aid of computer control. The purpose of the design of a particular aircraft will determine its inherent stability both static and dynamic. High performance aircraft need to be inherently unstable to perform their function. So, to answer your question, I would say that the dynamic stability of a airplane is designed from the start, by carefully controlling design parameters such as wing dihedral, tail moment, wing washout, sweepback, incidence and decalage. And, oh yes, that CG is also a big factor whether the dynamic stability will be positive, neutral or negative.


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## Zipper730 (Dec 2, 2018)

whstlngdeath said:


> Wing dihedral, that is, the upward angle of the wing, at the tips, to the centerline of the wing plays a big role in dynamic stability. Wings with large amounts of dihedral, say a Cessna 172, tend to recover from a "_reasonable_" disturbance pretty much hands off.


Understood


> I would say that the dynamic stability of a airplane is designed from the start, by carefully controlling design parameters such as wing dihedral, tail moment, wing washout, sweepback, incidence and decalage.


When you say tail-moment, you mean the leverage of the control surfaces? I have no idea what decalage is unless you mean the application of decals...


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## whstlngdeath (Dec 2, 2018)

Tail moment would be like leverage, where the fuselage is the lever, and the CG is the fulcrum. A long tail, (lever), is more powerful and requires less input to cause movement. However, the movement will be slower, or more stable. A short tail, (short coupled), is a less powerful lever, needing more input to cause movement, but that movement will be quicker and less stable. Decalage is the difference in the angle of the wing's chord in relation to the horizontal stab's chord. In a biplane, it's the difference in the incidence of the upper wing to the lower wing. Decalage can be adjusted to trim out bad habits in flight such as a tendency to climb or dive with power changes, and with biplanes, it can help by having the upper wing stall first, dropping the nose, and helping in stall recovery. I adjust decalage in my models mainly to help with the power changes.


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## Zipper730 (Dec 3, 2018)

whstlngdeath said:


> Tail moment would be like leverage, where the fuselage is the lever, and the CG is the fulcrum. A long tail, (lever), is more powerful and requires less input to cause movement. However, the movement will be slower, or more stable. A short tail, (short coupled), is a less powerful lever, needing more input to cause movement, but that movement will be quicker and less stable.


I'd have figured longer would be quicker responding too... that's weird


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## DerAdlerIstGelandet (Dec 3, 2018)

Here read this. An aircraft is designed to have either positive, neutral or negative dynamic stability. Typically training aircraft such as your typical Cessna will have a positive dynamic stability. Fighters which are supposed to be highly maneuverable are designed to have a negative dynamic stability.

The 3 Types Of Static And Dynamic Aircraft Stability

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## gumbyk (Dec 3, 2018)

DerAdlerIstGelandet said:


> Here read this. An aircraft is designed to have either positive, neutral or negative dynamic stability. Typically training aircraft such as your typical Cessna will have a positive dynamic stability. Fighters which are supposed to be highly maneuverable are designed to have a negative dynamic stability.
> 
> The 3 Types Of Static And Dynamic Aircraft Stability


Great, simple description. And don't forget that you have all of these considerations around each axis too.


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## DerAdlerIstGelandet (Dec 3, 2018)

gumbyk said:


> Great, simple description. And don't forget that you have all of these considerations around each axis too.



Absolutely.

The link is from the Bold Method. Great source of information for pilots. When I was learning to fly, I was having a lil trouble with landing. I would flare too high. My instructor gave me a link to the Bold Method which taught you about how to use sight picture while landing. Fixed my landings almost immediately.


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## XBe02Drvr (Dec 6, 2018)

DerAdlerIstGelandet said:


> My instructor gave me a link to the Bold Method which taught you about how to use sight picture while landing. Fixed my landings a


When you see the runway "flatten out" in front of you it's time to flare. Pull till it stops coming up at you then hold, and when it resumes coming up, pull a little more, repeat til you feel and hear the thump and rumble, and don't stop flying the plane til the prop ticks to a stop at your parking spot. Landing in a nutshell.
Cheers,
Wes

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## DerAdlerIstGelandet (Dec 6, 2018)

XBe02Drvr said:


> When you see the runway "flatten out" in front of you it's time to flare. Pull till it stops coming up at you then hold, and when it resumes coming up, pull a little more, repeat til you feel and hear the thump and rumble, and don't stop flying the plane til the prop ticks to a stop at your parking spot. Landing in a nutshell.
> Cheers,
> Wes



Yeah it took me a bit to get that. Got it now though...


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## gumbyk (Dec 7, 2018)

XBe02Drvr said:


> When you see the runway "flatten out" in front of you it's time to flare. Pull till it stops coming up at you then hold, and when it resumes coming up, pull a little more, repeat til you feel and hear the thump and rumble, and don't stop flying the plane til the prop ticks to a stop at your parking spot. Landing in a nutshell.
> Cheers,
> Wes



Unless you're flying a tail-wheel aircraft and doing a wheel-landing...


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## XBe02Drvr (Dec 7, 2018)

gumbyk said:


> Unless you're flying a tail-wheel aircraft and doing a wheel-landing...


Point taken. But who's lucky enough to learn from scratch in a taildragger these days? Around here insurance for taildraggers is prohibitively expensive and flight training not covered at any price.
Cheers,
Wes


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## DerAdlerIstGelandet (Dec 7, 2018)

XBe02Drvr said:


> Point taken. But who's lucky enough to learn from scratch in a taildragger these days? Around here insurance for taildraggers is prohibitively expensive and flight training not covered at any price.
> Cheers,
> Wes



A good friend of mine did. A fellow A&P. Bought a Citabria and learned to fly in it.


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## XBe02Drvr (Dec 8, 2018)

DerAdlerIstGelandet said:


> A good friend of mine did. A fellow A&P. Bought a Citabria and learned to fly in it.


Self-insured for 100M/300M? I wouldn't offer to train him without that. My CFI insurance wouldn't have covered training in a taildragger.
Cheers,
Wes


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## DerAdlerIstGelandet (Dec 8, 2018)

XBe02Drvr said:


> Self-insured for 100M/300M? I wouldn't offer to train him without that. My CFI insurance wouldn't have covered training in a taildragger.
> Cheers,
> Wes



I believe much of his training was in a cessna as well.


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## pbehn (Dec 8, 2018)

There is another aspect to stability, cant remember the name, but it is regarding changes of trim with speed.

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## Zipper730 (Dec 8, 2018)

XBe02Drvr said:


> Self-insured for 100M/300M?


You need 300 million bucks of insurance to fly a tail-dragger?


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## XBe02Drvr (Dec 9, 2018)

pbehn said:


> There is another aspect to stability, cant remember the name, but it is regarding changes of trim with speed.


That's part of dynamic stability. The function of pitch trim is to seek a given speed, and a plane disturbed from equilibrium will dampen its (phugoid) oscillations until it stabilizes at that speed. The power setting determines whether that equilibrium speed results in level flight, a climb or a descent.
Same applies to rudder and aileron trim. For every setting of these there is only one speed that will result in yaw-free, wings-level flight. So if you're flying a plane through a phugoid oscillation, there's going to be a slight yaw and roll aspect to your flight path.
The goal in all of this is to find the trim and power settings that will result in your desired flight path and performance.
Cheers,
Wes

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## XBe02Drvr (Dec 9, 2018)

Zipper730 said:


> You need 300 million bucks of insurance to fly a tail-dragger?


Suppose your student coming in to land collides with a business jet carrying the top executives of Apple or Microsoft, which then crashes into a corporate headquarters under the traffic pattern at a place like HPN (Westchester County, NY), killing a slew of high priced people and depriving their heirs of nearly a billion dollars of potential future earnings. (It has happened!) $300 million looks pretty skimpy here.
This is how insurance underwriters think. Add to that the fact that slick tort lawyers have succeeded in creating a precedent in the public eye by repeatedly selling juries on the idea that taildraggers are a relic from the bad old days, and as a flagrant menace to public safety represent criminal negligence every time they are guilty of committing flight.
"And ladies and gentlemen of the jury, the defendant in this case not only taught his student to fly in one of these deathtraps, he actually allowed said student, an UNLICENSED pilot, to wander about, all alone, unsupervised and unmonitored in said deathtrap, resulting in the carnage you have just seen so graphically presented."
Cheers,
Wes

PS: I used to shake in my shoes every time I flew into Westchester County or Hanscom Field outside Boston. Scary mix of airline, corporate, and flight training with high density and slow/fast traffic mixing it up. Add to that, most of the students were wealthy foreigners with arrogant attitudes, BAD English, and zero comprehension, and you've got an accident looking for a chance to happen.


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## gumbyk (Dec 9, 2018)

XBe02Drvr said:


> Suppose your student coming in to land collides with a business jet carrying the top executives of Apple or Microsoft, which then crashes into a corporate headquarters under the traffic pattern at a place like HPN (Westchester County, NY), killing a slew of high priced people and depriving their heirs of nearly a billion dollars of potential future earnings. (It has happened!) $300 million looks pretty skimpy here.
> This is how insurance underwriters think. Add to that the fact that slick tort lawyers have succeeded in creating a precedent in the public eye by repeatedly selling juries on the idea that taildraggers are a relic from the bad old days, and as a flagrant menace to public safety represent criminal negligence every time they are guilty of committing flight.
> "And ladies and gentlemen of the jury, the defendant in this case not only taught his student to fly in one of these deathtraps, he actually allowed said student, an UNLICENSED pilot, to wander about, all alone, unsupervised and unmonitored in said deathtrap, resulting in the carnage you have just seen so graphically presented."
> Cheers,
> Wes



And that's why I'm glad to live where I do. We've got $1M liability insurance, which is about average. The only time I've been asked about it was when flying into an Air Force Base, and they were happy with the coverage.
Our local aero club is offering training from ab initio in a Piper Pacer.


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