This is a spinoff from Walt's "Plett Orbit 25-H Clarification" thread.
The issue of temp and air density for OUR purposes is easily understood and calculated. If you want to dig deeper it gets somewhat complex with some really cool formulas and math. We are lucky to have some genius engineers on this forum (Paul Walker, Brett Buck, our moderator Dean, Igor, and others) they can give a better answer to the complex math than I can. But for the basics the comments below may help. I'm sure others will have comments too.
Dennis asked Paul "is there an area where this issue (temp., & air density affects) is flat"? (Dennis, I am assuming that you meant flat as in not much affect?)
The answer is "yes" and "no".
YES there is a "flat" area: Basically if you live below 1,200" Alt., fly in temps between 45 and 85 degrees, AND fly E power, the affects of temp. and air density is not a big deal. Your plane will feel a little more crisp in the winter and a little softer in the summer, but very little difference. I think that the above conditions cover most CL pilots in the USA. On a 85 degree day you will still be at around 93% of the air density at sea level on a standard 59 degree day. You will not notice much difference in power or flying (unless you are at the very top of the CLPA world, they can tell the difference in performance when a drop of fly poop lands on the elevator during a flight!
)
As Paul mentioned, his E powered planes lap times remained the same on cold and hot days, and that this is due to the decrease in prop efficiency (thin air, fewer air molecules per sq. in.) was offset by the decrease in drag (thin air) on the aircraft. ..... IMHO: If this minor difference bothers you and you want the plane to fly the same in winter and summer you would need to decrease your lap times by around .1 sec in the summer. (this is a WAG, but I think it is pretty close, but only for the narrow alt. and temp conditions above)
NO there is not a flat area outside the above conditions. If you fly out west things change a lot from seasons if you live at high altitudes, or in very cold areas and fly below 40 degrees, then they do have an effect on our planes. If you fly at Denver (5,280') on a 90 degree day, your plane (and your body) thinks it is at 8,483' (that is the Density Altitude for that day) this is only 77% of the air available at sea level on a standard 59 degree day. The affect of this is that our props are about 23% less efficient, our wings and moving surfaces lose almost this much lift and force. Even we duffers can tell the difference now. Because we don't have throttles to help us (yet ;-) we may want to increase our flight speed over the ground (decrease lap times) to compensate. This is the only way we can get back to having enough air molecules pass over our wings per second to get our lift and control forces to be like they are at the lower density altitudes. Plane design and adjustments can help if you live high in the summer, but if your there for a contest only, then more speed is the answer. ..... (BTW: your body also suffers this decrease in efficiency if you are visiting for a contest)
Below about 3,000' density altitude there is not much change, above this alt. then our atmosphere thins out much faster as we go higher. As we can see, by a density alt. of 8,483' at Denver in the summer we are down to 77% of the air at MSL. Our E motor keeps putting out for us but the wet guys have both the prop AND the engine decreased by 23%, they have to experiment to find the right nitro, NV, etc. at a high/hot contest. In fact, because their wet power engine is affected too, they need to make adjustments for even the small density altitude changes at lower alt. and temp changes. But, to each their own. ;-)
If you want to find out the WX (temp, dew point, pressure) you need to calculate the density altitude for your area go here and type in your city:
http://weather.aol.com/forecast/todays/us/ca/hermosa-beach/id/90254If you want to calculate the "density altitude" for your area this is an easy calculator to use:
http://wahiduddin.net/calc/calc_da.htmSorry this was so "wordy", I was too lazy to shorten it.
.... To quote Mark Twain: "I wanted to write a short letter, but I did not have the time so I wrote a long one instead"
Regards,
Topic: Density Altitude and Temp Effects on E CLPA (Read 540 times)
