weather and lift

[osprey]

HOT weather vs COLD
dose an aircraft have less lift in HOT weather vs Cold weather
i know it makes a difference on real aircraft but does it have the same effect on our r/c aircraft.
and is it a BIG difference or a small difference ?

i say it does and one other person in our club sys it don't.

[Cory]

Physics is physics. The plane will be flying at a lower density altitude when it's cold. This will make more lift for a given true airspeed....however, though that is still true, you won't find the lift from the strong thermals that happen more in the summer, so it kind of depends on what kind of "lift" you're talking about. It also depend where you live. Someone in a warm climate won't see as much of a seasonal difference as us northerners.

[JetPlaneFlyer]

Yes, lift is proportional to air density. When it's cold air density is higher so lift will be greater. Drag will also increase by the same proportion.

[Jurgen Heilig]

HOT weather vs COLD
dose an aircraft have less lift in HOT weather vs Cold weather
i know it makes a difference on real aircraft but does it have the same effect on our r/c aircraft.
and is it a BIG difference or a small difference ?

i say it does and one other person in our club sys it don't.

It has the same effect on our model aircraft. Whether it makes a big difference or a small difference depends on the temperature difference between what you consider cold-/hot weather:

http://en.wikipedia.org/wiki/Density#Changes_of_density

:) Jürgen

[CloudyIFR]

This is a good question actually, and all the responses are correct. I hope this one is too. ;)

As a ground based pilot we reference ground speed instead of airspeed. The model flies by airspeed and NOT groundspeed. My point is when the density altitude goes up or if the you fly at a higher altitude or hotter weather, that's what causes an increase in density altitude then the model will fly faster across the ground, assuming everything else is equal.

Not sure what the difference is, but I do think you'd notice it when flying at sea level on a cool day versus say 5,000' on a hot day.

Curtis
Montana

[Ralph Walton]

You can get more lift in warm conditions. If you're into slope flying you'll find for a given windspeed, cold air has less tendancy to blow up a slope and would rather blow around the slope if it can. Winter sloping often has alot of "horizontal" wind associated with it.

Warm slopes in summer often get a slope/ thermal phenomonon happening (slermal) that can result in alot of lift for minimal wind.

High humidiy lowers air density too, lowering lift. I used to think it would be the opposite, but water vapour is lighter than air. I guess thats how clouds get up there

[Jurgen Heilig]

You can get more lift in warm conditions. If you're into slope flying you'll find for a given windspeed, cold air has less tendancy to blow up a slope and would rather blow around the slope if it can. Winter sloping often has alot of "horizontal" wind associated with it.
...

I think we should leave slope soaring and thermal soaring out of the equation.

From an F3A contest in France about a week ago:

A day of very mixed fortunes for a lot of pilots. Day started with virtually no wind, baking hot and clear blue skies. Good you think? No the air was very thin and models just seemed to sag in the air.

:) Jürgen

[JaRaMW]

You can get more lift in warm conditions. If you're into slope flying you'll find for a given windspeed, cold air has less tendancy to blow up a slope and would rather blow around the slope if it can. Winter sloping often has alot of "horizontal" wind associated with it.

Warm slopes in summer often get a slope/ thermal phenomonon happening (slermal) that can result in alot of lift for minimal wind.

That would strictly not be a property of hot/cold air alone, but rather of stable/unstable air layering. You'll usually find unstable layering conditions more often during summer since the sun heats up the ground with a lot more power than in winter and this heating of the lower air layers creates an unstable layering, which again is necessary for thermals. The slope then helps to trigger the thermal.

That's for thermal lift. But when it comes to aerodynamic lift, you'll quickly find density variations in the order of 10% and more between a "hot" and "cold" day. This means the same order of lift variation at a given speed => lower stall speed in cold air. Same with propeller thrust, it's just a rotating wing after all. Combustion engines output more power in cold air due to the higher charge density.

[fnev]

Here is a useful link: http://www.denysschen.com/catalogue/density.asp

You loose over 20% air density when flying @ 6000ft with 90º F from sea level at 70º F. The V^2 is going to change in the same proportion to maintain strait and level flight. So, if your stall speed was 20 MPH at sea level it will be 22+ MPH at 6000’ @ 90º F.

[osprey]

i was not thinking about gliders
i was thinking more about takeoffs with a powered plane
as in less left = longer runway needed with less lift HOTTER air temps

[Cory]

Yep, that's correct.

[BMatthews]

Yep, it's basic training in full size craft that you need to reduce the max takeoff gross weight for hotter temperatures.

[vintage1]

The difference between sea level and 5000 ft is so marked, the when I lived in S Africa, different octane petrol (gasoline) was sold on the High Veldt (5000 ft high plateau) than at the coast..you could hear the engine pinking at sea level..but boy, it was noticeably more powerful.

And they never took off the Jumbos from Johannesburg at midday..too marginal.

Also, thick moisture laden air is also denser and more lifty than dry air.

[JetPlaneFlyer]

Also, thick moisture laden air is also denser and more lifty than dry air.
Vintage,
That would appear logical on the face of it, however in reality the reverse is true.. Humid air is less dense than dry air because the molecular mass of water is less than the mass of the air molecules it displaces.

google it...

Steve

[CloudyIFR]

Another way to say it is that airplanes fly in air and a not water. ;)

Curtis

[vintage1]

Vintage,
That would appear logical on the face of it, however in reality the reverse is true.. Humid air is less dense than dry air because the molecular mass of water is less than the mass of the air molecules it displaces.

google it...

Steve

I guess thats why rain falls upwards. I always wondered.. :D

[Cory]

i was not thinking about gliders
i was thinking more about takeoffs with a powered plane
as in less left = longer runway needed with less lift HOTTER air temps

One exception to this is if you are flying an electric plane. An air breathing engine will ingest denser air and make more power, and the prop and wings will make more thrust/lift. However, cold batteries can offset these wing and prop advantages.

The humidity affecting density altitude thing is correct. It gave helecopter pilots just one more enemy to contend with in Vietnam, for example.