It’s a question that I don’t think is asked enough; quite simply put, it’s exactly the same as on any IC engine. The throttle servo linkage, and arm setup in your aircraft will have a massive impact on the behaviour/characteristics of your throttle control on your aircraft. Read on, for a simple way of ensuring maximum control and a linear throw!!!!
As I fly mainly large petrol engines we will deal with these in this example.
When I first started with these large engines, my engine supplier who shall remain nameless, would ship all of their engines (even the competition series) without any throttle attachement arm. And to add insult to injury the OEM Walbro factory “stub” was also “case hardened” which made drilling it so much fun!! So my apprenticeship was born, as you had to make your own arm and solder this on! It was apparent that arm length on the carb and the servo was critical along with the arm geometry. If I was to just slap a push rod on any old way, then the result would be a non linear throw and therefore give a non linear throttle response. Ed note: It’s pretty obvious how I learnt that!
My method on my Desert Aircraft 170 Boxer is as follows:
1. Although this wonderful engine does come with a pre mounted arm I still like to make my own. Prepare a new carb throttle arm extender to 28mm, secure this with one bolt so that you can still adjust its pivot. On the servo end use a 26mm aluminium servo arm.
2. Switch on your radio and connect up the throttle servo and move the throttle stick to half way
3. Put the servo horn on so that it is in the middle of its arc.
4. Open the engine carb up to half way by looking at the butterfly valve, ie in the middle of its arc.
5. Now adjust the carb arm extender so that the arm is parallel to the servo arm.
6. Fix the Carb arm extender with a second fixing point and thread lock.
7. Keeping everything in this position measure up the distance between the arms and make a push rod to this dimension.
8. Connect up the push rod with ball links and thread lock.
When you move the transmitter throttle to idle, the butterfly valve will be almost shut; inversley when you open the TX to full throttle, the butterfly valve will be almost fully open.
Now you will have to increase the ATV on your radio above 100% to get it to go to full and to close completely.
I have never had to program a throttle curve when I have used this method as my throttle is using the maximum amount of steps between closed and open, so I must be doing something right!!!