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Old Jan 22, 2013, 06:41 PM
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United States, IN, Bloomington
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What characteristics minimize the wind's effect?

I realize that all planes are effected by wind. But what are defining characteristics when it comes to wind resilience.

I guess I should start by defining wind resilience.

The plane is not easily diverted from its line of travel by changing wind conditions. Eg. The plane resists changes in yaw, pitch, and roll caused by wind.

Lets start with basic Newtonian physics. The plane is being accelerated in a direction because it has a force applied to it by the wind.

F=MA
Since what we care about here is A lets rearrange.
A=F/M

So it looks like I have a few options here. I can increase the Mass. Thus if the wind provides the same force, the acceleration is diminished. But this isn't very good because yes it makes it harder for the wind to push me around, but it also makes it harder for my control surfaces to push me around.

So this leads me to our other term... Force. If I can decrease the force I can decrease the acceleration. The only way I know to decrease the force is by changing the planes shape. Hence the question. What characteristics of a plane (preferably shape here) make it more or less resistant to wind?
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Old Jan 22, 2013, 07:18 PM
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United States, NE, Kearney
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A higher wing loading (increased weight carried by the area of the wing) will lessen the effect of the winds.

Imagine an indoor plane, flown outdoors on a calm evening, and then the same plane in 12-15 mph with gusts approaching 20mph.
Other planes with higher wing loadings than the indoor planes can be controlled and flown while the indoor planes are all but uncontrollable as they get blown around.

You might think raw power is a solution, but to illustrate... imagine having triple the normal amount of power on the indoor plane flown in gusts and winds... it's still going to be tossed and blown about.
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Old Jan 22, 2013, 07:45 PM
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I don't agree that it's the higher wing loading necessarily. I think it's more to do with speed. And since planes with higher wing loading have to go faster, there's an "apparent" correlation. A plane with a wide speed envelope has an advantage in the wind. Wing loading on a Formosa is fairly low (8-10 oz./sq. ft.), but it's my go-to plane on windy days.

I think the OP is onto something when he talks about the plane's shape. The Formosa is a particularly sleek airplane, a classic pattern airplan. I think that has a lot to do with its wind handling.

I have plenty of other planes with lower wing loading than the Formosa, but they're all draggy -- none of these have a top speed near what the Formosa can do. So they're not as good in wind.
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Old Jan 22, 2013, 08:03 PM
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Originally Posted by rafe_b View Post
I don't agree that it's the higher wing loading necessarily. I think it's more to do with speed.
Higher wingloadings require more speed to remain in flight. Weight has very much to do with it.

mw
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Old Jan 22, 2013, 08:09 PM
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Originally Posted by rafe_b View Post
I don't agree that it's the higher wing loading necessarily. I think it's more to do with speed. And since planes with higher wing loading have to go faster, there's an "apparent" correlation. A plane with a wide speed envelope has an advantage in the wind. Wing loading on a Formosa is fairly low (8-10 oz./sq. ft.), but it's my go-to plane on windy days.

I think the OP is onto something when he talks about the plane's shape. The Formosa is a particularly sleek airplane, a classic pattern airplan. I think that has a lot to do with its wind handling.

I have plenty of other planes with lower wing loading than the Formosa, but they're all draggy -- none of these have a top speed near what the Formosa can do. So they're not as good in wind.
Think of it this way... throw a clay brick in a 40 mph wind and then throw a styrofoam brick and see which one is affected by wind.
Speed is NOT the determining factor.
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Old Jan 22, 2013, 08:17 PM
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Slopers add weight to fly (faster) in higher winds. It takes wing loading to penetrate a nice stiff wind.
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Old Jan 22, 2013, 08:17 PM
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Livermore, CA
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I live in a windy area, 12-14 normal with gusts to 25 on some days.
After building over 100 planes to fly in the wind, I have found that because the wind here is gusty and will sometimes sheer quite a bit, that;

Symmetrical airfoils are best, and undercambered are the worst.
Swept wings add a dihederal effect, but dihederal and polyhederal wings allow the wind to get under them and tip them.
Of course, extra power is always a friend.
Tractor(puller) prop is more stable than a pusher.
Larger control surfaces deflected less than smaller ones deflected more, disturb the air less.
Then the less profile the fuselage has the better. That's why Flying Wings do well.
Mid or high wings are usually always more stable.
And EPP foam helps with the unexpected gusts and sheer.

Putting all this together I built a 23" EPP tractor for windy days.
Flying weight it about 3ozs, Yes, just 3ozs!
It handles 15mph winds real well and doesn't get blown away, except after its landed.
It could handle more wind, but the 10g outrunner pulls it to about 15-16mph and then it needs more power.
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Old Jan 22, 2013, 09:41 PM
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Originally Posted by DustBen View Post
Think of it this way... throw a clay brick in a 40 mph wind and then throw a styrofoam brick and see which one is affected by wind.
Speed is NOT the determining factor.
But a brick is a draggy shape, and neither of these objects has thrust to produce airspeed.

I say, speed is the determining factor. As airspeed increases, the relative effect of wind decreases. We know that there are very light planes that do very well in wind. But they need to be sleek. A sleek plane can have very low wing loading yet still go fast.

I used to hear this all the time from the old men at the club (the first club I was part of...) You need heavy planes to fly in wind, they'd say. I'm not buying it. In 15+ mph wind, I'd rather fly my 'Mosa than one of their club trainers, at twice the wing loading.
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Old Jan 22, 2013, 10:47 PM
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i think it is both speed and weight . i have a big super sky surfer and i have a diamond 2500. sky surfer is bigger and heavier but the diamond has both weight and speed and fly's way better in wind. heck maybe its the wing design not sure .
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Old Jan 23, 2013, 12:43 AM
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Aberdeen
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+1 on the first post. if your goal is that the plane should not get tossed around by turbulence then wing loading is the far and away most important issue.

You seem to like physics so this can be proved easily enough with Newtons laws of motion. What we are trying to minimise is unwanted accelerations due to wind catching the plane.

Newton's second law of motion: Acceleration = force /mass

So to minimise acceleration we must increase mass and/or reduce force.

What is the 'force' caused by?.. The force is caused by the wind gusts hitting the plane, the wings have most area so are most effected. We cant reduce the strength of the wind or change it's direction so the only option is to reduce surface area.

Reducing wing area and/or increasing mass serve to increase wing loading.

Unfortunately increasing wing loading has a negative effect on just about every other aspect of flight. not only will it's stall speed be high but the same low acceleration that we strived for means the plane will have poor manoeuvrability.

The other desirable thing is low stability, though this may be counter intuitive. Stability by definition makes the plane try to fly on a constant heading to it's relative wind and at a constant speed. When the heading and airspeed keeps changing due to gusts a stable plane will react by constantly 'hunting' for it's trimmed condition. It will try to 'turn into' any side gust and will climb and dive when any gust changes it's flying speed. A plane with low stability will react much less. So you want low dihedral and you don't want to be excessively nose heavy.



Power and speed in themselves wont stop the plane getting bounced about but you do need good speed for when it comes to flying into wind and not getting blown away backward!
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Old Jan 23, 2013, 07:35 AM
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This sort of question crops up regular, with varying view points.

My view point ?, how often do you see a bird carrying a house brick so that it can fly on a windy day ?.

You need enough power to overcome a head wind.

Plenty of people say they fly their micro models and lightweights on windy days.

Now severe turbulence/gusting is a bit of a different game. I would say then a sleek streamlined model would fly better, than a draggy biplane.
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Old Jan 23, 2013, 07:52 AM
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My view point ?, how often do you see a bird carrying a house brick so that it can fly on a windy day ?
Or an African swallow with a coconut?

I'm sure some saw this coming.
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Old Jan 23, 2013, 08:14 AM
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United States, FL, DeLand
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Why argue about which single factor is important? It's the relationship between the forces that determines the final result. So we have:

Turbulence (not wind, which does not affect a flying plane at all). Here there are two factors involved. Turbulence, which can be expressed as plus or minus the prevailing wind, has an amplitude of highest minus the lowest wind. The ratio between that variability amplitude and the prevailing wind speed means everything. Suppose you have a 10 mph wind with 5 mph variability. That means the wind speed is going up or down 50% in speed! Now take a 60 mph wind with 5 mph variability. That's a an 8.3% variability.

Mass of the plane. This is important because mass increases the plane's inertia, which acts as a force to maintain the direction and speed of the plane. Take a 3 oz plane and a 3 lb plane and hit them both with a gust from the side. The 3 oz plane will respond instantly and the 3 lb plane will respond more slowly. The roll moment will be the same with both planes, other things being equal, and the heavier plane will roll more slowly, giving the pilot more time to compensate.

Frontal area of the plane. This is frontal with respect to the airflow, which may not be on the nose for the instant of a gust.This is where the plane with dihedral or polyhedral takes a hit because that gust from the side changes the apparent wind to the airframe. The wind gets under the dihedral or polyhedral, which has now become frontal area and stuff happens.

Wing loading: This is related to speed. A higher wing loaded plane flies faster than a lower wing loading plane, in general. There are exceptions.

Flight Speed: The higher percentage the wind variability in mph is to the speed of the plane, the more affected the plane is by it. We can illustrate using wind instead of variability. A plane flying 10 mph in a 10 mph crosswind must crab at a 45% angle to maintain course over ground. However a plane flying at 50 mph only needs a crab angle of 11.3 to maintain that same course. Wind gusts would affect the two planes to the same ratio.

The trim of your plane! We don't even mention this one, but it's vitally important. The more forward your center of gravity is ahead of the aerodynamic center of your plane, the more directionally stable it is. Sailplane pilots are always trying to increase their skill and move the CG back. Why? A sailplane with CG forward is very stable. When encountering a thermal, the plane will just fly straight through without lifting a wing, pitching up or down. Lacking instruments, the RC sailplane pilot is depending on the plane to tell him where the thermals are. So they move the CG back to make the plane more responsive (that means less stable too). Now when it's flying, there is a thermal to the right side, the right wing tips up and the plane turns immediately away from the rising air. When the plane flies straight into a thermal the plane pitches up dramatically because it is more responsive (less stable) than with CG forward. This works exactly the same way for powered planes. You may want to have a range of CG positions useful for different conditions.

Airfoil: see frontal area

When you're flying you can't just toss out all other considerations other than the one you're interested in. The whole plane goes up in the air, not just its weight or wing loading or airspeed. Each impacts how that plane responds to its environment and their interaction is where things get muddy and interesting.

There are light planes that are great in wind. There are heavy planes that are not so good. All you have to do is, by the design of the plane, make one factor that contributes to ability to handle wind more important than another. Suddenly weight or another characteristic is less important and frontal area, for instance, is more important because of the design of the plane.

Lots of 3-channel sailplanes with polyhedral are good in wind. My Square Soar (ugliness could be a factor there) was great in wind, with an airfoil that penetrated well and a big rudder to keep control of those crosswind gusts that get under polyhedral. It was also ugly and the wind avoided it.
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Old Jan 23, 2013, 08:45 AM
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I vote for wing loading and something not mentioned so far (at least when I was originally typing this post)....airframe cleanness, Speed is less of a factor, And I agree that excess positive stability is not always a good thing for wind handling.

The fact that it is not speed alone is easily proven by flying a lightly loaded model in turbulent wind at the same speed as a model with more wing loading. Done it many times.

When full scale aircraft are designed for low level attack, the best ones invariably have high wing loadings. At 450 knots, something like the Jaguar or F-111 has a MUCH smoother ride in low level wind turbulence than an aircraft with less wing loading like a B-52 or Canberra pressed into the same job at similar speeds.

Airframe cleanliness comes into play when the model is flying in gusty wind conditions. A clean model like a Speed 400 pylon racer or a slope racer or a warbird fighter with retracts that has less junk hanging off of it is less affected by a transient wind gust than a model like a J-3 or a Stearman biplane with wing struts, landing gear fairings, wheels, large windshield fairings and such that catch the transient wind gusts, from any angle.
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Old Jan 23, 2013, 03:48 PM
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Moab, Utah, USA
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Originally Posted by Rockin Robbins View Post
Turbulence (not wind, which does not affect a flying plane at all). Here there are two factors involved. Turbulence, which can be expressed as plus or minus the prevailing wind, has an amplitude of highest minus the lowest wind. The ratio between that variability amplitude and the prevailing wind speed means everything. Suppose you have a 10 mph wind with 5 mph variability. That means the wind speed is going up or down 50% in speed! Now take a 60 mph wind with 5 mph variability. That's a an 8.3% variability.
Have you read what you wrote? If, as you said, wind does not affect a flying plane at all, then the fact that the plane is flying in a 10 mph wind or a 60 mph wind is irrelevant. The only relevant factor is that there is a 5 mph variation. As such, that 5 mph variation will exert the same force on the airplane, and the airplane will react in the same manner and to the same degree to that gust when flying in a 60 mph wind as it would when flying in a 10 mph wind. The ratio between the variability amplitude and the prevailing wind speed means nothing because, as you said, the prevailing wind speed means nothing.

Quote:
Originally Posted by Rockin Robbins View Post
Wing loading: This is related to speed. A higher wing loaded plane flies faster than a lower wing loading plane, in general. There are exceptions.
Wing loading is a factor, if not the main factor, but it has nothing to do with speed. It is all about surface area of the wing, which determines the amount of force a gust of a given velocity has on the wing, verses mass (A=F/M). A higher wing loaded plane does not fly faster than a lower wing loaded plane, it simply has a higher stall speed.

Quote:
Originally Posted by Rockin Robbins View Post
Flight Speed: The higher percentage the wind variability in mph is to the speed of the plane, the more affected the plane is by it. We can illustrate using wind instead of variability. A plane flying 10 mph in a 10 mph crosswind must crab at a 45% angle to maintain course over ground. However a plane flying at 50 mph only needs a crab angle of 11.3 to maintain that same course. Wind gusts would affect the two planes to the same ratio.
Are you really serious? An airplane has no idea what the wind is (as you have already noted above) and it has no idea what its ground track is. So how can it react differently to gusts of equal amount based upon wind or ground track?

Larry
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