What determines aircraft stability in turbulence?

Somedudeintn

Somedudeintn

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somedudeintn
Random question... What causes an airplane to be less impacted (I was going to say affected, or is it effected....impacted is easier) by turbulence? I'm assuming it is mostly a function of weight, wing loading, and speed. The wife isn't a big fan of the bumps (most recently flying in the 172) and I'm just curious what characteristics help to smooth out the ride if I was in the plane shopping market. I know to slowing down helps (and climbing often helps) but I want to go fast and have a smooth ride?

Perhaps a heavy light twin like an Aztec would be better at escaping low level bumps quickly and smooth in them out. Really just curious here. Thanks!
 
You're guesses are about right. I'm not an aerodynamicist but I don't think you are going to get a whole lot of variance in Cl on GA planes unless you can shop around across a large spectrum of performance.
 
A few years ago I flew home in my Cessna 180 on a very bumpy day. I wondered hiw my Cub would compare so I immediately went right back out in the Cub to find out. I assumed the lighter plane would get tossed about at the mercy of the wind and would have a rougher ride. That wasn't the case. While the Cub did get moved around by the turbulence it was less abrupt. Bumps were more like gentle pushes up and down where the Cessna had much sharper jolts. That wasn't expected. I compare it to boats in choppy water. Slower and lighter goes with the flow. Heavier and faster resists directional change and pounds into the waves.
 
Higher speed can make the bumps worse, they feel harder.

There are some aircraft design factors, too:
  • Lower wing loading makes for more bumps, as the plane is easier to displace.
  • Dihedral affects lateral stability, generally more dihedral is more stable.
  • Longitudinal stability is less easily to discuss genericly, but different aircraft are more or less stable in pitch due to design factors like distance from center of lift to the tail, airfoil design of the tail, etc.
  • Generally, longer, heavier planes with shorter wings are more stable. But not always . . . Read reviews of the different models, see what others think, go to ride on bumpy days and see how they feel.
Something that will climb quickly above the bumps is nice (go turbo!), but eventually you will need to descend to land. Have you flown far enough to be out if the bumpy, turbulent area? Hey, range is now something to think about. How much weight was burned off in the flight? (Heavier planes are more stable because it takes more outside force to displace them, so now low fuel burn helps.) Hmmm . . .

Stability is defined as the plane's ability when displaced to return to it's original state (smooth, level cruise). Planes that are stable will do this; planes that are neutrally stable will maintain the displacement; planes that are unstable will continue to diverge.

Read up and test fly!
 
I've always thought that light airplanes with wider wingspans (like a DA42) will be rougher in turbulence than something like an Aztec that's heavier with a smaller wingspan.

Not sure how true that is?
 
deonb said:
I've always thought that light airplanes with wider wingspans (like a DA42) will be rougher in turbulence than something like an Aztec that's heavier with a smaller wingspan.

Not sure how true that is?
Click to expand...

Not quite. It's not the absolute weight or wingspan of the aeroplane, it's the wing loading. Higher wing loading (divide the loaded aircraft weight by wing area) is less likely to be tossed around.

At max gross of 5200 lbs my Aztec (37'-4" wingspan) has a wing loading of 25.1 lbs per sq foot. The DA-42 at max gross of 3935 lbs (44' wingspan) has a wing loading that is slightly lower at 22 lbs per sq foot. A Baron 58 with a 5500 lb gross (37'-10" wingspan) has a wing loading of 27.6 lbs per sq foot.

It is also because of this that for any given aeroplane the rough air manoeuvring speeds in the POH will be higher when the aeroplane is carrying a heavier load (closer to max gross). At 5200 lbs the Aztec rough air manoeuvring speed is 131 KIAS, at 3600 lbs it falls to 112 KIAS.

Somedudeintn said:
Random question... What causes an airplane to be less impacted (I was going to say affected, or is it effected....impacted is easier) by turbulence? I'm assuming it is mostly a function of weight, wing loading, and speed. The wife isn't a big fan of the bumps (most recently flying in the 172) and I'm just curious what characteristics help to smooth out the ride if I was in the plane shopping market. I know to slowing down helps (and climbing often helps) but I want to go fast and have a smooth ride?

Perhaps a heavy light twin like an Aztec would be better at escaping low level bumps quickly and smooth in them out. Really just curious here. Thanks!
Click to expand...

Lancair IV-P. 32.7 lbs per sq foot wing loading. Will be smoother riding than most other airplanes. But watch out if that engine quits (there's another recent thread about exactly that). A high wing loading airplane will generally have a pretty "impressive" approach speed.
 
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You dont think wingspan could be a factor? The wing is a lever in this situation and a bump 10 feet from the fuselage will be much more noticed, at least in roll, than a bump 1 foot from the fuselage.
 
Seanaldinho said:
You dont think wingspan could be a factor? The wing is a lever in this situation and a bump 10 feet from the fuselage will be much more noticed, at least in roll, than a bump 1 foot from the fuselage.
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Remember these are air currents and not actual bumps. What situation are you envisioning where you would have a distinctly different airflow separated by a few feet?
 
Could someone address this and I'm not trying to start a which is better

How would a high wing vs a similarly weighted and wing loaded low wing compare? Surely the mass being above vs. below the wing and lift must have something to do with it as well.
 
Cooter143 said:
Remember these are air currents and not actual bumps. What situation are you envisioning where you would have a distinctly different airflow separated by a few feet?
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Thermals can be pretty small. A lot of turbulence people experience is thermals and then the associated downdrafts. When flying the glider its easy to feel the thermal lift one wing far more than the other.
 
Seanaldinho said:
Thermals can be pretty small. A lot of turbulence people experience is thermals and then the associated downdrafts. When flying the glider its easy to feel the thermal lift one wing far more than the other.
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I would suggest that what you are referring to is turbulent air within a thermal providing slightly more lift on one wing, momentarily. But he's asking about GA aircraft, the wingspan isn't going to vary significantly. Wing loading will be the largest factor, but even then, there won't be a lot of difference unless he starts stepping up to higher performance aircraft.
 
JustinD said:
Could someone address this and I'm not trying to start a which is better

How would a high wing vs a similarly weighted and wing loaded low wing compare? Surely the mass being above vs. below the wing and lift must have something to do with it as well.
Click to expand...
As far as gust response? I can't imagine that it would be much difference at all between the two, and I have never noticed any difference. Wing design will have the largest impact.
 
Seanaldinho said:
You dont think wingspan could be a factor? The wing is a lever in this situation and a bump 10 feet from the fuselage will be much more noticed, at least in roll, than a bump 1 foot from the fuselage.
Click to expand...

Turbulence, even thermals, are not point loads on the wing.

Differential lift, such as from thermals, may lift one side vs the other, but the overall difference in wingspan for powered GA aircraft won't make any noticeable difference to the occupants. What will make a difference is wing loading. Subject to the same thermal a higher wing loading aeroplane won't bounce around as much as a lower wing loading aeroplane.
 
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JustinD said:
Could someone address this and I'm not trying to start a which is better

How would a high wing vs a similarly weighted and wing loaded low wing compare? Surely the mass being above vs. below the wing and lift must have something to do with it as well.
Click to expand...

Should not make any noticeable difference if the low wing aeroplane has sufficient dihedral.
 
Hank S said:
Higher speed can make the bumps worse, they feel harder.

There are some aircraft design factors, too:
  • Lower wing loading makes for more bumps, as the plane is easier to displace.
  • Dihedral affects lateral stability, generally more dihedral is more stable.
  • Longitudinal stability is less easily to discuss genericly, but different aircraft are more or less stable in pitch due to design factors like distance from center of lift to the tail, airfoil design of the tail, etc.
  • Generally, longer, heavier planes with shorter wings are more stable. But not always . . . Read reviews of the different models, see what others think, go to ride on bumpy days and see how they feel.
Something that will climb quickly above the bumps is nice (go turbo!), but eventually you will need to descend to land. Have you flown far enough to be out if the bumpy, turbulent area? Hey, range is now something to think about. How much weight was burned off in the flight? (Heavier planes are more stable because it takes more outside force to displace them, so now low fuel burn helps.) Hmmm . . .

Stability is defined as the plane's ability when displaced to return to it's original state (smooth, level cruise). Planes that are stable will do this; planes that are neutrally stable will maintain the displacement; planes that are unstable will continue to diverge.

Read up and test fly!
Click to expand...


This has always been my understanding as well; but like with Stewart, I once had the opportunity to do multiple flights on a single day and was surprised at the outcome ===>
The most jolting and uncomfortable ride was in the Arrow, with dihedral, stubby wings, and (relatively) high(er) wing loading.
The -172 was definitely more comfortable, and I'm sure that the fact that the aircraft was 30 years younger with better seat padding had nothing to do with it.
Most surprising was that the most comfort was found in a dinky RV-12 --- like swinging in a gentle breeze and without fillings being knocked out as in the Arrow.
So, who knows .....
 
I'll add that while the Cub was more comfortable in cruise turbulence (comfort is more important than "stability" to me), that is not the case for landing in mechanical turbulence and swirling winds. In that case I enjoy the heavier and faster airplane. Pull power back and it's coming down with more determination than the lighter, slower airplane. But that's perception based. The slower plane provides more time for correction but the slower plane also provides more exposure time to turbulence. Our preferences are usually determined by what we're used to and discomfort is based on deviation from it. Don't expect any concensus on which plane rides better!
 
Stewartb said:
A few years ago I flew home in my Cessna 180 on a very bumpy day. I wondered hiw my Cub would compare so I immediately went right back out in the Cub to find out. I assumed the lighter plane would get tossed about at the mercy of the wind and would have a rougher ride. That wasn't the case. While the Cub did get moved around by the turbulence it was less abrupt. Bumps were more like gentle pushes up and down where the Cessna had much sharper jolts. That wasn't expected. I compare it to boats in choppy water. Slower and lighter goes with the flow. Heavier and faster resists directional change and pounds into the waves.
Click to expand...

I've noticed the same thing with Cubs. The interesting thing is that Cubs are the only light airplane I've noticed this in. It must be just enough slower than the other planes I've flown to make a difference.

Cubs are interesting in the way they handle bumps. They seem very smooth to me, to the point you don't even notice them. You'll notice it primarily by seeing a change in altitude more than getting bumped and tossed about while you're flying around.
 
One thing to watch for is that your perception and the passengers are going to be quite different.

You're looking for stability. How much control input is needed to keep course and altitude? Bumps aren't going to help with that, but there are many factors at play.

Passengers? They're looking for smoothness. A bump is a bump is a bump, whether the pilot needed to make a reaction to it or not. That's all about wing loading.
 
What are some single-engine planes that handle turbulence well? Examples?
 
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