I usually fly with zero percent nitro at sea level. What would be a good nitro percentage for flying on 400 meter altitude to achieve the same power?

I usually fly with zero percent nitro at sea level. What would be a good nitro percentage for flying on 400 meter altitude to achieve the same power?

   "400 meters" is about 1300 feet, I would suggest 10% to start. If it runs short on the same volume of fuel, go down to 5%. The one hedge I will have is that using any nitro at all will greatly improve the power so you may have a wildly disproportionate effect of even tiny amounts. If you baseline 10%, changes above and below that are generally pretty linear.

    You should be able to tell immediately if you get it right, because you will not need to move the needle very much to get the same RPM and the same "setting" (i.e. it will be running at the right speed at the right point relative to the 4-2 break point). You *should not* have to change the head shims, venturi, etc ,since all you are trying to do is replicate the same combustion conditions. Obviously, if it starts detonating or acting "fierce" then you will have to add a head shim, but no more than .005. But I have run plenty of FAI-fuel engines on 10%+ with no real issues and stunt engines are generally very conservative to begin with.

    If you need some intermediate value between 0 and 10, you can mix it in the syringe. Say, if you are want to run a little less than 10, pull 1 ounce of FAI fuel into the syringe, then pull the rest from the 10% can. Since you really don't know what you need, it doesn't really matter if you know the exact percentage. People have in fact done this *on every flight*, where then more-or-less never change the needle and just adjust it for temperature/pressure/altitude by changing the mix based on experience, as an alternative to needle adjustment. I haven't tried to do that, I always reserve the possibility of fine needle adjustments, but over a very narrow range.

    For reference, this is what I use for various conditions:

Sea level, 50ish degrees  10% with a pitch reduction (3.6 VICE 3.75)
Sea level, 70-110 degrees, dry, 10%, 3.75"  (baseline)
980 feet (Muncie) , 70-100 degrees, humid, 15%, 3.75
2200 feet (Tucson), 70-80 degrees, dry, 20%, 4.1

   You might need different corrections but the general trends should be clear. 1300 feet is definitely enough to warrant a change.

     Brett

   

Thank you for the fast replies.
The contest was yesterday in Wegnez, Belgium. Altitiude 300-400 m. Participation from Belgium, France and Germany.

My engine is a ST51 with some modifications. Prop is 2 blade Brian Eather 13-5, not undercambered.

It is not easy to acquire Nitromethane separately in Belgium. So I usually purchase a premixed fuel with 25% nitro. Not clear whether this figure is based on volume or weigth. Density of nitro is 1.14 g/cm2. So not too much of a problem. I adjust the oil content of this fuel to 22%, resulting in a nitro percentage of about 22%.

Yesterday at the contest site, there was only room for 1 testflight , So I made a mix of 1/3 22% fuel and 2/3 FAI fuel. This results in about 7% nitro overall. Seemed in the ballparc, based on your replies. Thanks again.

I have the habit of adjusting the needle after the flight (in stead of before) . So this testflight was leaner than I was used to. 4.9s laptimes. So the effect of the nitro in combination with reduced airpressure (990 mm hg) was noticable. Flight time was excessive as well. So I set the needle 3 clicks richer for the competition flight with the same mixture and same fuel load. No more time for additional experimenting. Only 1 F2B circle, so no room for training.

Plenty of power, decent laptimes (5.2s) and engine shut off 8 laps after the clover leaf. Well within the 7 minutes. Perfect.

Second flight went well as well. Even better what scoring is concerned.

For the 3th flight I made a stupid mistake. I want the pressure in my fueltank constant on atmospheric pressure. Probably this also attributed to the leaner setting of the test flight. To achieve this, I avoid ram air effect on the uniflow inlet. Therefore I use a small sealing plug on this uniflow inlet with small holes drilled in the side, so propwash is not creating pressure build up inside the tank. For the 3th flight I forgot to re-install the plug after fueling. The result was a much richer flight than I expected. Too slow for a competitive pattern and the engine quit 1 lap after the clover leaf...

I recently measured the ram air effect and it is about 1 inch of water pressure...

Puzzling that you'd adjust the NV after a flight, but I'm guessing that's because you can't afford to warm the engine up before setting the NV after starting for the official flight.

   I frequently set it right after a flight, depending on how fast or slow it might have felt. In-flight performance is far better indicator than measuring with a tach or something. So, tweak it faster or slower right after the flight. I also tweak it a priori based on whether the temperature went up or down before the flight. I only check it by ear and tach, for the most part.

    Bear in mind that my entire adjustment range on my various standard fuels and locations is less than 1/4 turn since about 2007 (since I have been running the same engine and needle since then) aside from fuel tests, including various location from sea level to 2200 feet, and temps from upper 40's to about 110. I know about what kind of RPM to expect with different props, from about 9700 to 10300 rpm at launch. At the NATs this year, I maybe moved it a total of about 30 degrees from fast to slow over the week.

     Brett