Hi mav many ways of doing this. I fly my falcon with all functions including an extendable nose leg (for take off) and smoke, with just a DX7.
I had run out of channels, so couldnt fit an electric pump and link it to the U/C up selection, as motor would run dry when smoke fuel gone. I went for a pheumatic sysyem. I use an extra U/C air tank and a plastic RADWELD bottle, which holds i think 300mls. I conect a servo and tiny BVM air switch, via Y lead, to the U/C chanel. When the gear is down, servo is well away from the air switch. When gear selected up, the servo stops, just short from operating the air switch. I now flick the Rudder Mix switch on the transmitter, which I slaved to the U/C channel. My mix, gives virtually the same throws on the rudder, but the U/C channel, moves just a touch more and operates the air valve. Obviously the Air switch can be replaced with a microswitch for electric operation. I have used this method with mechanical U/C valves and the excellent jetronics electroinc valves, with great effect.
There is not much info around on smoke, but I posted on here my setup and findings a long time ago, you could do a search. Also on youtube, (falcon120 (I am tim777jet)) you will see my comparison between MT smoke fluid and diesel and also my ground test.There was some talk about any flame getting past the turbine, during accel, igniting the smoke fluid, but have never seen this on my home made turbine, or any one elses. MT fluid is slightly blacker than diesel, so does show up a bit easier than diesl on a whitish sky. Against blue sky, diesel is fine. I just use diesel. Have tried, food colouring dyes and cooking oil and other concotions to no avail. If you want coloured smoke, it can only be done by spraying dye, into the evaporating diesel you are spraying into the jet efflux. It can not be added into the mix, so I have read. Apparently green is the best colour to have, you have to use textile dyes and the mess is terible, once stained you cant get it off. I have not experimented any further with colour, but if you get some results, please post them up.
ref the spray pipe, i have tried spraying the fuel across the jet efflux at 90 degrees, along with it and back up into the tail cone. it seems 90 degs across is best for me. I do not get any residue on the tail, fins booms or engine.
I use I think, a 3 or 4 mm piece of brass tube crushed down at the end @ 0.020thou. I have heard that the Xaco model knife blade is the correct size to squeeze the tube down to.
When I apply the smoke fluid, my 100 psi air, flows into the RADSEAL bottle and pushes the fluid out the brass tube. The RADSEAL bottle fits nicely just in front of the main tank just fwd of my main tank. I fly from a rough grass , very short patch and have no noticeable difference in take off run with the fluid on board. My smoke lasts about 30 secs and tails off as the pressure drops, but good fun, especially when the sky is blue, Hope that helps, cheers tim