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| General RC Aircraft Discussion Discuss anything RC related |
Topic Review (Newest First)
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| 09-07-2015 09:58 AM | ||
| RCBYAIR |
Re: EDF jets - no wind makes it tough Here you go buddy, http://usatoday30.usatoday.com/weather/wdensity.htm http://www.physlink.com/Education/AskExperts/ae652.cfm |
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| 09-04-2015 10:50 AM | ||
| lhasa |
EDF jets - no wind makes it tough in a Hot & Humid Day Very good insights. Thanks Kevin. Richard C. |
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| 09-03-2015 07:10 PM | ||
| kcaldwel |
Re: EDF jets - no wind makes it tough Quote:
Lets say your airplane flies 100kph. A 1km leg (just for easy math) would take 36 seconds. 2km forward and back would take 72 seconds at a constant 100kph speed. If you have a 50kph headwind, the upwind 1km leg over the ground (50kph ground speed) takes 72 seconds, while the downwind leg (150kph ground speed) is only reduced to 24 seconds. A 2km forward and back circuit would take 96 seconds. If you need to make the upwind and downwind times similar like you may have to do to keep your pattern maneuvers symmetrical relative to the ground, you have to fly much faster into the wind. Or you just spend a lot more time going upwind, and don't make it all back on the downwind leg. Either requires more fuel for the same forward and back distance over the ground. I doubt your pattern maneuvers end up with all the climbing into the wind, and descending with the wind. If you climb going downwind, and descend going upwind, the airplane actually loses more energy than it would in no wind, so that would require more fuel as well. Most likely you about break even on the climbing and descending energy gain/loss with or against the wind. An EDF may well be flown with most of the climbing done into the wind, with diving high speed passes that are more impressive going downwind. That sort of flight pattern would gain energy from the wind gradient. Kevin |
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| 09-03-2015 07:02 PM | ||
| lhasa |
EDF jets - no wind makes it tough IN A HOT & HUMID DAY Thanks Gordon. Enjoy! Richard C. |
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| 09-03-2015 06:52 PM | ||
| Trumpetman |
Re: EDF jets - no wind makes it tough Richard, it would be interesting if you had the chance to fly this weekend with the same battery in calm and cool conditions. Do the same kind of flight and let us know what flight times you get. I'm going to a contest this weekend and I'm hoping for calm and cool conditions as well! Have a great long weekend 
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| 09-03-2015 06:33 PM | ||
| lhasa |
Re: EDF jets - no wind makes it tough Still remembered it was above 30C, feel like 40C with the humidity! Hope everything will cool down. Happy flying this long weekend! Richard C. Toronto, Ontario |
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| 09-03-2015 05:54 PM | ||
| Trumpetman |
Re: EDF jets - no wind makes it tough I'm not going to argue about full scale aerodynamics or dynamic soaring. We are talking about the power needed to fly R/C models. Models do not spend much time climbing or descending through wind gradients (other than dynamic soaring which I submit is an extreme example). I fly IMAC (scale aerobatics). My practice flights consist of the same maneuvers flown with the same plane in the same order and at approximately the same altitudes and speed. On a calm day I can fly the sequence for 14.5 minutes before I need to land. On a windy day I can only fly the sequence for about 12.5 minutes. I carry enough gas (128 ounces) that, on a calm day, I can fly close to three times through the sequence. At one contest a couple of years ago the wind was really howling and I deadsticked on short final after only flying through the sequence the required two times. Sorry, I just don't buy it. My experience tells me that there was something else going on if he noticed a marked reduction of flight time on a calm day. |
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| 09-03-2015 11:17 AM | ||
| kcaldwel |
Re: EDF jets - no wind makes it tough Quote:
If there is a gradient with altitude, it takes less power to climb at the same rate going upwind, or the same power will result in an increased climb rate. The airplane is continuously gaining airspeed as it climbs into the higher wind speed going up wind. It would continuously lose airspeed climbing through the gradient gong down wind. For example, say the wind speed increases 1kph per meter of altitude. If the airplane gains a meter, it picks up 1kph of airspeed basically for free. That extra airspeed can be used to increase the climb rate, or use less power to climb at the same rate. The opposite is true climbing downwind through a gradient. "For a typical modern aircraft V may be about 600 ft/sec during the climb. Thus even a wind gradient as small as 0.01 sec -1 can change the rate of climb by nearly 20 per cent. For aircraft climbing at higher speeds (including supersonic aircraft), the effect of a wind gradient will of course be greater. The larger wind gradients occurring at heights below about 1,000 ft wilI also have a greater effect ; this is considered below." naca.central.cranfield.ac.uk/reports/arc/rm/2953.pdf That is how dynamic soaring works in extreme wind shear cases. Unpowered gliders can go over 500mph extracting energy from the wind shear by climbing in to the gradient, and diving down wind through the gradient. I also found one page of an article on the subject by my old aeronautics prof, Bernard Etkin: http://arc.aiaa.org/doi/abs/10.2514/...urnalCode=jans And the USAF Flight Testing Manula has a section on how to correct for increased climb performance into the wind through a wind gradient: https://books.google.ca/books?id=uQo...adient&f=false Kevin |
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| 09-03-2015 10:43 AM | ||
| Flypaper |
Re: EDF jets - no wind makes it tough Gradient has little to do with it in that respect. On a particular flight, if you were to climb to a certain height and saved, say 100 ma. and you were to turn downwind and climbed to the same height, it would require an extra 100 ma. So at the end of the flight the result would be the same flight time as on a calm day. On a windy day you need more throttle manipulation to handle the turbulence. I have found, on a windy day and landing, at about 4 ft altitude, the wind speed drops drastically and the plane drops to the ground through lack of lift. I learned to touch down a little hotter to compensate for this. Gord. |
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| 09-03-2015 10:20 AM | ||
| kcaldwel |
Re: EDF jets - no wind makes it tough Yes, high temperature and to a lesser extent high humidity, increases the density altitude. This means the airplane will behave as if it is flying at a higher altitude when it is hot and humid. The stall speed will increase, propeller or impeller performance will decrease and internal combustion engines will make less power. Calculating the density altitude should always be done before flight in a full size airplane, so you know how long the take-off and landing might be due to less wing lift and poorer engine and propeller performance, and how your climb rate will be affected. The ancient Cessna 150 I flew for my license wouldn't climb past about 3,000' AGL in Ontario on some of those hot and humid summer days, and the take-off roll with two people and full tanks was very long. https://en.wikipedia.org/wiki/Density_altitude Density altitude calculator here: https://wahiduddin.net/calc/calc_da_rh.htm Kevin |
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