I leave needle settings alone for months at a time.  I do see a little difference between first and subsequent runs sometime. I wonder if there is some castor film in the NVA which gets washed out during the first run, making that run a hair leaner than subsequent runs.  I think if you have plenty of power, a 100 RPM either way due to weather or whatever is not a problem.

I do use after run oil after each session, I think that is why its hard starting in the cold and takes a few seconds for the oil to burn of before starting. I always wait till the engine is cool to the touch before re- starting.

Is there any rule of thumb for needle setting when flying the same plane several times in a row. My first flight the motor always runs the best and then the second and third flights are not as powerful. Do you lean or richen the needle more with each consecutive flight?Also have been using Sig 10% fuel during the summer should I change to a different nitro percentage when the weather is cold? What is the rule of them for deciding nitro percents? This is on an OS 25 LA with the venturi and back plate modifications made per members of this site.

  Unless the conditions are changing or you have some sort of cooling issue, you shouldn't have to change the needle on consecutive flights. What can happen, however, if you misuse a tach by continually trying to hit a "number", you could have problems. Some airplanes needle consistently on the ground on a second flight, and some don't. Mine typically doesn't. We routinely fly 2 flights back-to-back when practicing, to save time moving airplanes on and off the circle. Mine will run ""normally" on the first flight, but on the second (with no attempt to let it cool, just long enough to fill it up), it will start and run quite a bit faster/leaner. It will stay that way until about the 3rd or 4th lap, as the in-air cooling drives it back to equilibrium. If I was to try taching it to a fixed number on the second flight, I would end up opening the needle to slow it down, then when I took off it would slow down a lot and feel weak. If I need a change on the second flight, I will tweak it open-loop (i.e. just open or close it about what I thought it needed) without paying attention to what it sounds like.

   Of course as the air temperature or density changes over the course of the day, you will likely have to chase the needle up and down to keep it constant. Typically, you will have to close the needle to keep the same speed since it usually warms up over the course of the day. It's not a problem if you are just out there practicing all day, but at a contest, this can be an issue. At this years TT, I was good until the sun came out and the temp went up drastically. I knew to tweak it up, but since I had flown about 20 flights all year, I guessed wrong on how much, didn't do it enough, and ended up with a functional but not very positive speed - and then I not only got off the team, Bill even beat me for #4. Very frustrating but that was an example of not being fully engaged and it costing me - in the *hardest contest in the world*, you can't overcome that.

   As the temperature changes, I change the nitro and pretty much nothing else. I think changing it until you get the right run time is a good start. If you run 6:30 on 10%, and it gets cold, and you start running shorter, lower the nitro until the same volume gets you about the same run time. Some people actually ONLY change the nitro and not the needle as the day goes along. It's easy to mix fractional percentages in the syringe - if you want 12.5%, take 2.5 oz of 15% and 2.5 oz of 10%, shake it up, and put it in there.

    Brett

 Hi.

 Unless there is several months between flying sessions, I doubt that dry oil residue is the reason. The most usual reason is simple: At first flight, the engine compartment and fuel tank is cold and the viscosity of fuel is higher. The next time you fill the tank, it heats up  the fuel a little, thus richer setting.
 For the same reason, if fuel tank is badly insulated, laptimes sometimes get slower towards the end of flight.
 Also, it can take surprisingly long time of running to fully remove all the after-run oil from engine. Before day's first flight,  allways rinse my engine with 10 millilitres of fuel and burp it untill it's dry again. L

  .... Some airplanes needle consistently on the ground on a second flight, and some don't. Mine typically doesn't. We routinely fly 2 flights back-to-back when practicing, to save time moving airplanes on and off the circle. Mine will run ""normally" on the first flight, but on the second (with no attempt to let it cool, just long enough to fill it up), it will start and run quite a bit faster/leaner. It will stay that way until about the 3rd or 4th lap, as the in-air cooling drives it back to equilibrium. If I was to try taching it to a fixed number on the second flight, I would end up opening the needle to slow it down, then when I took off it would slow down a lot and feel weak. If I need a change on the second flight, I will tweak it open-loop (i.e. just open or close it about what I thought it needed) without paying attention to what it sounds like. ..

   

I wish I had been thinking of this a month ago.....

I usually dont mess with my needle too much. I fly a little quick in the cool air in the morning and if the temp goes way up I may have to go in on the needle a .25 turn or less. I run 10% nitro 99% of the time and I use a tach and my ear to set the engine. I have a target Number on the tach but if it sounds good I just fly.

Derek

"and then I not only got off the team, Bill even beat me for #4"

Even Bill beat you eh?

That did not come out the way I had intended! 

You know those stock PAs work pretty good...

    Yeah, that's why I gave it to you. Nothin' but the best for you, little buddy!

     Brett