Changing props for summer weather

[that rat]

I managed to brave the 95 degree and 40% humidity weather to go flying today and it seemed like my plane wasnt climbing as good as it was before. This could all just be my imagination. But anyway I got thinking warmer more humid air is less dense. So that means less thrust and posibly less current draw mabye? Does that mean i should/could run a bigger prop on hot humid days?

Or I guess more generally how does air density effect flying? Does it all just cancel out?

[Sparky Paul]

Less dense air should let the prop turn faster.... electric motor..
Less dense will affect the plane's aerodynamics.. it will need to fly faster.

[Gary Warner]

...Does that mean i should/could run a bigger prop on hot humid days?....

On most of my planes, I'm usualy prop'ed for max current. I resist the desire to increase the prop on hot days because the heat levels on my motor/battery setups may go too high. I just put up with the 10 or 15% reduction in performance.

If your setup leaves room for more heat in the power system then increaseing the prop would be fine, up to the heat limits.

Gary
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[vintage1]

It is a fact that hot air supports aircraft less well. Its worthwhile checking to see whether a bit more pitch will help the model along.

Examine flight times in and out of hot high places - like Mexico City and Johannesburg - to see when the big birds choose to take off fully laden. Its seldom midday ;)

[Airboatflyingshp]

If there are any Aussie bush pilots/desert flyer's out there or ground crew they will tell you that some very famous aircraft have had to be uprated quite dramatically to cope with the hot air flying conditions lift and power can become very marginal.......a bit like flying at high altitudes but with the added problem of heat build up.

You will need to create more thrust and lift all things been equal whilst maximising cooling of the motor controller and cells if your set up is marginal or your wing loading high it will be more noticeable. There is a link to high altitude flying advice http://www.stenulson.net/rcflight/floatfly.htm
Ive borrowed from a thread I set up in Waterplanes. you could always ask them for their ideas.

[Langenase]

Hi
Here we fly F5J class (National Rules) electric gliders (7cell 2/3SC size, usually 7'WS, free motor, ESC, wing load, area, etc...) at altitudes between 3' ASL and 2700' ASL and indeed we change our propellers according to temperature/humidity/height above sea level. For us is essential to maintain climb performance/Amps drawn ratio constant, because of needed minimum number of flights (6 flights, max. 45sec motor each) without battery change...

[Gary Warner]

Yesterday my Cherokee was flown at 90 deg F an 90% RH. This thread was brought up at the field as the Cherokee was a bit 'off' in performance. This plane is running at max temp (16 pounds, Astro 40, 24 cells, large GB and 18x10 prop) on normal days. It even has a muffin fan on the back of the motor that does it's best to cool the motor off.

When the guys at the field put their hands on the motor after a flight, no one was recomending a bigger prop. Again, IF your setup is already at its limits, proping up for hot, humid days might overheat the power system. Now, just flying at higher altitudes in cooler, dryer air is another thing and proping up might work.

I guess what I'm trying to make clear is the difference in proping up for high heat/RH versis just flying at higher altutudes.

Gary
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[Langenase]

Gary

I don't know the altitude ASL of your flying site, but from data of your National Weather Service and estimating 500 feet altitude, 90ºF, 30.4 inches Hg and 90% RH your relative density of air is 91.5%.....so you theoretically should need to prop-up almost 1/2" in pitch. If your motor/ESC/batt temperature is near the limits, that´s another matter. In relatively big electric airplanes (2-3 Kg, 400-600W) we usually employ one or two computer fans and carefully placed NACA type cooling ducts. It works! Our summer temperatures easily reach 105ºF 80% RH :)