Can anyone direct me to an accessible (as in understandable by dimwits) explanation of the relationships between the above and air density.

I was at a competition recently where I was rather pleased with some of my flights (the ones in cool air) and rather depressed by the rest of my flights (in the surprisingly hot afternoon air). What was obvious was that when it was hot my motor was short of grunt, compared to when it was cool. And worse, this was all in the same location, on the same day, over a period of 6 or 7 hours.

As humidity, temperature and pressure are all reasonably easy to measure these days I'd like the upshot of whatever I can read on the above to result in a cheat sheet that I can use at my next competition.

As I didn't have any way of increasing the nitro content I just had to live with drop in performance, but next time out I'd like to be able to work around that particular problem.

I did notice that all the good guys had their fuel in wine coolers though, which I'll definitely be sorting out for myself next.

Chris

Can anyone direct me to an accessible (as in understandable by dimwits) explanation of the relationships between the above and air density.

I was at a competition recently where I was rather pleased with some of my flights (the ones in cool air) and rather depressed by the rest of my flights (in the surprisingly hot afternoon air). What was obvious was that when it was hot my motor was short of grunt, compared to when it was cool. And worse, this was all in the same location, on the same day, over a period of 6 or 7 hours.

As humidity, temperature and pressure are all reasonably easy to measure these days I'd like the upshot of whatever I can read on the above to result in a cheat sheet that I can use at my next competition.

As I didn't have any way of increasing the nitro content I just had to live with drop in performance, but next time out I'd like to be able to work around that particular problem.

I did notice that all the good guys had their fuel in wine coolers though, which I'll definitely be sorting out for myself next.

  Well, I think the wine part will certainly help.

    I don't do anything special to fuel, it either sits in the car or sits on out in the sun.

      More temperature, more humidity, more altitude = thinner air => add nitro and/or turn needle in, more revs needed in-flight
      less temperature, less humidity, less altitude = thicker air => less nitro and/or move needle out, less revs needed in-flight

        The effective altitude goes up about 1000 feet for a 15 degree increase.

      The rule of thumb is to add nitro until the run time is the same. As the day gets hotter, the engine (left alone) will richen itself up, and you turn the needle in to compensate. If you don't it will fly deeper in a 4-stroke and slower. When you turn the needle in, it uses less fuel. If you add nitro, you will need to open the needle back up, and the fuel consumption will go back up. 5% change should cover the range of conditions. 2.5% is the smallest change I bother with. I carry 10 and 15%, use either one, or mix the two (in the syringe, suck 2.5 ounces from the 10% can, and 2.5 oz from the 15% can, repeat until tank is full).  I wouldn't hesitate to use more if it seemed like it needed it - stunt engines are so far off the "edge" as far as  compression goes that they can usually use all the nitro you can get with no ill effects. Ted ran YS20/20 in his RO-Jett 61 at the 2004 W/C, worked fine. I have run as much as 35% nitro, with startling power, in a 45. And it ran out of fuel before the square 8!

On my airplane, with a PA61, it used ~7.5 oz of 10% for a pattern at sea level and normal temperatures. In Muncie (~1000 feet), on a hot day, it only used about 6 oz and had to run in a dead 2-stroke for the same performance. Go to 15% nitro, and the fuel consumption went back to 7.3-7.4 ounces, needle went right back where it had been, and it ran the same.

    To get a given speed, you need more inflight RPM in hot air - because the prop slips more. But depending on your engine, it might need to be set slower on the ground to get the same or higher RPM. That's because the hotter it gets, the weaker the engine runs and the more it will unload when you launch it.   *Generally* you need to set it faster on the ground, because the prop slip overcomes the weakness of the engine. And if you fix the engine weakness with nitro, you solve the unload issue.

     To start with, if it ran nice in the morning, and you do a test flight and find it weak in the afternoon, particularly if you have to turn the needle in a lot, just go up 5% and set it to the normal RPM (our sound, if you are doing it by ear). If the needle is back where it was in the morning, you are probably good to go and you will get about the same run time. Of course test it with a practice flight if you can, to avoid getting caught out on the run time. And as a long-term goal, pump up the setup so that it might be a little jumpy in the morning, and when it softens up later in the day, you won't have to change anything. You should be able to cover the changes over a day without mssing with the fuel. Around here it's always pretty dry, and sea level, but it goes from maybe 60 in the morning to 100+ at the hot part of the day. That's about the equivalent of a 3000 foot altitude change, but I really don't have to change anything much aside from the launch revs. Morning it might want to be 9600 and in the afternoon it wants to be 10,100, but no other changes are needed. That's about as bad as it will ever get anywhere as far as changes go.

     Brett