I am building a twin f-4 with two bvm 91 engines, the problem is if i use the in flight mixture control i will need 9 channels and i only have an 8 channel transmitter, in the past planes i have had i had i9n flight control but never really used it when in the air, only on the ground before takeoff, whats everyones opinion on this, would it be ok to go without the in flight mixture control and just adjust them on the ground. thanks

I would not fly a DF without inflight needles on both motors. It is rarely used in flight, but it very well could save you from hurting a motor. As a last resort, I might consider putting both inflights on one channel. You could richen both motors if one went lean.

i also thought about using both needles on one cahnnel, what does everyone else think, i will probably run the motors a tad rich anyways with the twin setup, i just am not sure what to do, i dont really have money for a 9 or 10 channel radio right now.

I allways use the in-flight mix. I set it up for full power on takeoff and once I get up and going I richen it up a bit, and then half way into the flight I richen it some more. As you burn fuel the head pressure goes down, so the mixture starts to lean in, so you have to compensate.

I would put them on the same channel, set the needle on the ground and then use in-flight capability to richen them up and keep them that way through the flight.

Steven

I never used in flight mixture, if you have tanks around the carb and less than 1 PSI tank pressure they will work fine. You never have problems with the parts you leave off.
Darryl

what do you mean tanks around the carb and 1 psi tank pressure, not sure what you mean. thanks

You have two tanks, one on each side of the power plant. The tank starts
just behind the fan mounts and centered "up and down" on the carb. Next you have 3/4 to 1 PSI pressure to the tanks when full throttle.
Darryl

I am afraid that what you are describing is an old fallacy. By changing the position of the tank you can control the initial head pressure, but as the fuel burns off the head pressure will still change the same amount. The fuel tanks are about 5" tall, so from the beginning of the flight to the end the pressure will change by about 5 inches of water (convert to alcohol). That will cause the engine to start to run leaner as the head pressure decreases.

Steven

Center the tank and have a 2.5 inch change, seems kinda weird to want 5 inches.
Darryl

Center the tanks and the head pressure is +2.5 inches at the start and -2.5 at the end, Total change = 5.0 inches. No matter where you put the tank the total change will always be the same. Centering the tanks does make life easier in other ways, but it does not cure the leaning out during flight.

Doing the math - 1 inch of alcohol creates 0.02926 psi, so with a 5" tall tank the pressure at the bottom of a full tank is 0.1463 psi. If you have 1 psi pressurization from the pipe that means that the fuel delivery pressure to the carb is going to reduce by about 15% from the start to the end of a run. That will create a noticeable change in the mixture.

Steven

"You canna change the laws of physics, Captain!"-
Mr. Scott to Cap'n Kirk

Your right but being 2.5 inches rich to 2.5 inches lean was ok for me.
On an second item when you adjust the high speed needle do at max RPM not widely open throttle. Then watch the oil trail and it will remain fairly constance. Not all installation will turn max RPM at rest.
Darryl

GUYS,,,PLEASE;
just don't make it too complicated ,I had in fligth mix, and never use !!( just set the needle of each motor at the time ,and (ONLY) with when fuel tanks half full ,so "no more richen mixture at half fligth ) and just please don't forget to pull the glow plug element out ,just a tad ,and re -center ,then ,,,,,,,,,if you can iddle for 15 seconds and go full throttle you are set ,and just don't try to get the last 300rpms, out of your motors ,just remember when flying keep your trim (high) and on the middle for landing), in my years of DF ,I never had a "flame out ???
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