"Run Time" / Altitude Change.... any Rules of Thumb?

[Chris Fretz]

Getting back to the original question of Run time /Altitude.....Yesterday at Hazelton Pa. with temps in the 90's and 1700' of altitude I ran 15% nitro in my HP40 powered Nobler I had great success. With gusting winds I ran the engine a bit leaner that I normally do. On 4 oz of fuel my run times were 6:30

I did win Intermediate PA but by the skin of my teeth. Chris Fretz( "Liner") flying Intermediate for the first time pushed me all day. We tied for high score on the 2nd round,but I had a better 1st round and got 1st. Chris is a fine flyer, a tough competitor,and a very nice guy. Congrats to Chris,....and to Bernie and Alan for putting on a great meet!

For my first time in intermediate, Skip, I had to show you it was SHOWTIME lol but seriously I had a great time!  For the record I wasn't looking at my watch I was looking at the pattern I wrote on my hand. I only had 5 days to learn it and find a airplane that would run long enough to get through the pattern and not cut out in the clover. I didn't think i was going to make it, I flew the nobler on Saturday and things were working out so I packed up and left. I used Bretts suggestions on filing up the tank and sucking out a certain amount. That worked good for me Thanks Brett.

It was fun keeping you on your feet Skip

Chris

[Skip Chernoff]

I had some 20% with me,but after a morning test flight (which I didn't think I'd have the opportunity to do) I found that the 15% worked and my run time allowed me some extra laps in between stunts. (to try to remember what the next stunt is.....lol)

I did notice that I might have been better off with a 10.5 x4.5 APC rather than the 11x5 Xoar I was running. She seemed to labor a bit while overhead. Could have been a bit to lean.Anyway, I learned a great deal thru this discussion and put it to the test with a practical application. Thanks all for jumping in.

[Skip Chernoff]

Chris when at Brodak's a few weeks ago I watched a few of your "Beginner" flights and thought, this kid's a good flyer. I knew that you would do well as a newbie to Intermediate. A few of the Philly Fliers and myself watched you fly yesterday and we all agreed that you flew well enough to win.....and you will win. Intermediate is a fun class.When I first got back into flying C/L  two seasons ago one of the judges said to me..."You'll never have as much fun flying as you do in Intermediate".

I have found that in Intermediate if your stunts are "recognizable"...(laughing) AND you complete the pattern and get those "pattern points"....you'll be in the hunt every time.

.....and another thing my "Showtime" antics are all in good fun...I even have the judges saying to me me now....Yes Skip,we know it's Showtime!............

[Tim Wescott]

I did notice that I might have been better off with a 10.5 x4.5 APC rather than the 11x5 Xoar I was running. She seemed to labor a bit while overhead.

Laboring while overhead is a really good way to put it -- it means that the thing is too lean.  So if you're going to adjust the engine run by nitro percentage, you should have had more nitro in there.  You might be able to compensate with a different prop, or venturi, or whatever -- but the wisdom that's conventional per a lot of Really Good Pilots is that you settle on a prop/motor/venturi combination, and then you adjust for altitude with nitro.

(I don't know enough to really know -- I never get outside of my little area, and haven't had to worry about it.  I suspect that I'll be in for a rude awakening if I ever make it to the Nats).

[Skip Chernoff]

Which brings us to another interesting question.....what effect does altitude have on prop selection? I'm thinking that "maybe" my mixture wasn't too lean,and for that altitude, temperature,and air density I may have needed a little less pitch....hmmmmm . I really love this stuff...keeps you on your toes!

[Brett Buck]

Which brings us to another interesting question.....what effect does altitude have on prop selection? I'm thinking that "maybe" my mixture wasn't too lean,and for that altitude, temperature,and air density I may have needed a little less pitch....hmmmmm . I really love this stuff...keeps you on your toes!

      That has not been my experience. Very generally, you want more pitch as the air gets thinner, rather than less. The reason is due to several factors - the loss of horsepower from the thinner air can sort of be compensated for by increasing the prop efficiency, and higher pitch props are generally more efficient*. Also, and related, the amount the model moves forward for one revolution of the prop is less in thin air, so if you want to maintain the same RPM (since that's what the rest of the system is tuned for), you have to use more pitch.

   In this case, however, you are changing a lot more than the pitch of the prop - you are also changing the prop type from wood to an APC, which is drastically more efficient in general. So the pitch might not be the key thing you are changing, and it may well have worked better that way.

    This sort of thing is why stunt keeps people going for year after year, there's a million things like this you should probably know about, but no one ever knows them all. Even the true masters still have to hypothesize or guess, then experiment to see what really happens.

     Brett

p.s. "efficient" in the engineering sense of the word, not the vernacular. Efficiency is defined as the ratio of the HP applied to the airplane VS the shaft HP, for our purposes. It's a matter of some debate what a typical level-flight efficiency might be - most of us guess somewhere between 40% and 60%, but it is not the same in all cases.

 Most people equate "efficient" with "better" but that's not generally true for stunt props, in fact, the less efficient the prop is in level flight, the more the efficiency increases in the maneuvers, meaning if you have enough ponies to get the desired lap time, the less efficient it is, the "better" it is.