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Old Dec 19, 2011, 12:45 PM
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Does cold weather affect planes flight?

I was flying my Clouds Fly glider and my Cessna on Monday when it was 55 degrees out and calm. Both planes were all trimmed out and flying great!

On Sunday I went out with both planes again, only this time it was 25 degrees out and calm. Now both planes needed down elevator trim (lowering the nose) to keep the planes flying level again. The glider required ALOT of down elevator and the Cessna not so much....but both needed the down elevator trim.

Is this the result of flying in the colder air?
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Old Dec 19, 2011, 12:50 PM
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More apt to be the fact that your control rods changed length with temperature (some have a high coefficient of expansion) than from any effect of the more dense colder air. Now, if using an ignition motor or glow motor, the higher density of the air has a slight effect on performance as you get more oxygen per liter of air intake into the combustion chamber.
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Old Dec 19, 2011, 02:13 PM
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denser air is more lift and such. I have retrimmed my Jug for more down elevator than usual as well.
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Old Dec 19, 2011, 02:16 PM
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Hey Air Cooled,
I also do notice a difference with flight on cold days as my foamies seem to "tighten up" a bit with the ailerons, elev., & rudder. I found the pushrod slots also to be a bit tighter, although did not affect movement & the batteries definitely had a shorter flight time. There was nothing with the cold that really affected flight characteristics that may jeopardize control movement resulting in a crash. I noted my normally 10-14min. flights reduced to 7-8min.. I simply adjusted my times & kept my spare batteries in my jacket pocket.

Denny
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Old Dec 19, 2011, 02:36 PM
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Originally Posted by Rodney View Post
More apt to be the fact that your control rods changed length with temperature (some have a high coefficient of expansion) than from any effect of the more dense colder air. Now, if using an ignition motor or glow motor, the higher density of the air has a slight effect on performance as you get more oxygen per liter of air intake into the combustion chamber.
I agree. Someone took pics of an Easy Star control surfaces in warm and cold weather. The difference was very noticeable and pronounced
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Old Dec 19, 2011, 02:46 PM
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But if you trim your plane, that shouldn't matter. There is a definite air density effect with a 30-degree shift in temperature. The effect is compounded by the fact that cold days are usually not as humid - further increasing the air density. You might not see any change in the power system, but all your airfoils are more effective - so, the prop, the wings, the tail - it's all increased effectiveness on a cold day. We have pretty radical shifts around here, and I do have to adjust my gyros for the season - more gain in summer. On a hot day, maybe 100 degrees and 70% humidity, the feeling is like the air just isn't there anymore - fast planes feel really slippery and slow stuff feels under-powered. Once you have eliminated for trim issues, I think it really is the density shift that you're noticing.
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Old Dec 19, 2011, 05:05 PM
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I'm with Jasmine on this one. If control rods did change length they would tend to be shorter with colder temperatures. Since most elevator control horns are mounted on the bottom of the elevator that would pull the elevator down and up trim would be needed to compensate, not down trim.

Colder air is more dense so the propeller, flight surfaces and control surfaces are more effective. So for the same power level the airplane would be faster and tend to climb. That alone would require down trim to compensate for the change in temperature.

Larry
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Old Dec 19, 2011, 10:11 PM
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Originally Posted by jasmine2501 View Post
But if you trim your plane, that shouldn't matter. There is a definite air density effect with a 30-degree shift in temperature.........Once you have eliminated for trim issues, I think it really is the density shift that you're noticing.
Yes, I was able to trim it back out, but I was trying to figure out the "why" part
The Clouds Fly glider took ALOT more correction, but I could still trim it out. This plane does have the elevator horn on the bottom surface, so a colder "shrunken" rod would be the opposite of what I'm seeing.
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Old Dec 20, 2011, 12:13 AM
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The Clouds Fly glider took ALOT more correction, but I could still trim it out. This plane does have the elevator horn on the bottom surface, so a colder "shrunken" rod would be the opposite of what I'm seeing
I would be inclined to think that the foam airframe would contract more than the metal pushrod, which would explain this pretty well. I could be wrong.
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Old Dec 20, 2011, 06:50 AM
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Your right TP. Plastic expands and contracts very much with temp change. If the pusrods were made from metal, wood or carbon fibre, there would be a big difference. Had a 200 cc Quadra motor with a plastic timing ring turning on an aluminum collar. Couldn't figure out why the engine wouldn't run right in the colder weather. Turned out the plastic ring shrunk onto the collar and wouldn't change the timing. I turned down the inside of the ring a few thou and everything was cool. I wonder what some of the things are that have to be done to compensate with the high altitude jets.

Gord.
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Old Dec 20, 2011, 11:58 AM
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Originally Posted by Air Cooled View Post
Yes, I was able to trim it back out, but I was trying to figure out the "why" part
The Clouds Fly glider took ALOT more correction, but I could still trim it out. This plane does have the elevator horn on the bottom surface, so a colder "shrunken" rod would be the opposite of what I'm seeing.
It's a glider, so it probably has a flat-bottom and very efficient wing design. A glider which can take advantage of tiny thermals, can definitely take advantage of a change in air density, even a small one. With the wing generating more lift, you could fly slower, or you could put the CG forward a bit - but it's definitely going to change things. It's specifically designed to be able to use small variations in the air, and the density shift from getting 30 degrees colder is not a small variation.
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Old Dec 20, 2011, 12:00 PM
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Originally Posted by Air Cooled View Post
The Clouds Fly glider took ALOT more correction, but I could still trim it out.
Here I disagree with Jasmine. The difference in the amount of trim is most likely due to a larger static margin. In other words, the CG is farther away from, or in front of, the airplane's neutral point. This makes the elevator less sensitive. The less sensitive elevator would require more deflection to effect the same amount of pitch change. Moving the CG back a bit would compensate for that. It would also make the glider react more readily to changes in air currents making it easier to find thermals.

Larry
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Old Dec 20, 2011, 12:12 PM
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Damn... There really is a complex answer to everything, isn't there?
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Old Dec 20, 2011, 12:43 PM
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Originally Posted by TP16 View Post
Damn... There really is a complex answer to everything, isn't there?
Yes, and you would not see this effect with symmetrical wings or profiles if they are properly balanced. If they are not correctly balanced, then the change in the lift would cause a trim problem.
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Old Dec 20, 2011, 01:22 PM
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"Back in the day", temp and air density made a big difference in how many fully loaded marines a CH-46 would be able to lift off and fly with. The temps were 20 degreees lower at 6:00 a.m. than at 10:00 a.m. and the crew chiefs were constantly recalculating so they could take off with as full a load as possible during a big operation. This could mean a two person difference between early morning and mid day. Lift was always higher the the cooler it was.

Regards,

Hankg
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