ORIGINAL: daggets
i can understand your analysis....
anyway, as much as many modellers, i don t know how to reprofile a low needle mixture on a carb.... for me, it is a high precision part it is better to don t modify.... especially if you do it ''blind'', with no measuring equipment...
and again.... why do the engine manufacturer did not cure that? it is easyer for them than for us...
helicopter s engine, wich are the same as ours, in most of case, uses 30 per cent nitro... why? to cool the engine?
i ve also noticed that helicopter engine are more reliable than the same on an aircraft..... have i dreamed?
competitors flying f3a classic pattern were using a lot of nitro, same today with 4 strokes..... why?
i just want to understand, i feel like i ve used engine for 30 years, and still not able to tune them....without the recommended mix...
Here is a portion of an engine manual that describes how to re-profile the low speed needle. This works for all Bodemann (Webra-TN carb) style designed carbs
http://www.rcuniverse.com/forum/fb.asp?m=10230872
As for blind if you use your engine you can determine if the corrections work or not. It is no different than adjusting the fuel mixture as you currently do it now. You don't (well I don't) measure the cross section of the needle that is in the fuel metering jet and subtract it from the jets cross section. Nor or do I use a gas analyzer to measure mixture strength. Just make the adjustment and see how the engine responds.
No, I don't think it really is easier for the engine OEMs to do this than us. Like I said there is really no way for the OEMs to know how we are going to use the engine. To make it worse there are no standards by which the fuel are mixed. So again the OEMs have no idea as to how the engine will be used. The high nitro in you heli is not contributing to the reliability of the engine. A lot of that is traced to the cooler operation of the engine because there is more oil flowing as nitro has less power per unit volume. So to get the same power you need more fuel flow. More fuel flow means more oil flow. Also most helis are only at full power for a very short period of time. Most air plane engines are at or near their full power setting for most of the time. When I flew FAI F3A I used low or no nitro in my ships because of the cost factor of nitro. To fly pattern I had to practice a LOT! So the cost of fuel added up FAST! The only 4 cycle engine I recall using (or needing) moderate nitro (20% to 30%) were the YS. She did this because the YS has a rather low compression ratio (CR) head and even less of an effective CR at partial throttle. The four cycle engine was at a disadvantage as for as power and weight going against the 10cc full wave piped engine so the use of nitro was an attempt (band aid) to boost the power levels. Note the rules committee had to give the four cycle a 2:1 displacement advantage even after they tried the logical 1.5:1 to begin with!
I hope this helps with the whys and hows to adjust the mid range by direct intervention with the fuel flow.
This thread might also help.
http://www.rcuniverse.com/forum/m_10..._1/key_/tm.htm
I've been doing this (toy engine stuff) since 1969, and compeditivly (well some might disagree[
] ) since 1980
All the best,
Konrad