Throttle curve - bottom point
last updated 1-1-2008

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A common question is why set the bottom throttle curve point to 15% in certain radios. Lets take a look at one of the current popular radios, the Spectrum DX7. This radio has a function called 'Throttle Recovery' that is used to kill the engine. For this function to work the throttle trim (which is digital) is held down until the on-screen trim reaches the bottom. At this point it only takes one click up to move the trim position back to the idle position. So this means that with the throttle stick down and trim all the way down the carburetor should be fully closed, aka 0% open. Here is where the problem comes in. If you set the throttle curve to 0,40,50,60,100 then look at the IN/OUT values shown on the radio you will see that with the throttle stick up the IN value which represents the throttle stick position is IN=100 and OUT=100. OUT is the amount going to the servo. So IN=100 OUT=100 seems right... and it is, however if you move the stick down you will get IN=0 OUT=-15. This means with the stick and trim down the output to the servo is -15 and stick up is 100. That is a total of 115.

This just does not match up logically (or mechanically as you will see later). 100% of stick travel should be 100% of servo output... not 115. Percentages should be 0 to 100, 115 does not fit in this range and as you will see it is not a technically correct setup. Can you get by with setting the servo up with a bottom throttle curve of 0%, sure, it does not mean the helicopter will not fly. It does mean that the throttle curve values you will need will not match what is typical. In other words while other people will set the middle throttle curve point to 50% for a typical hover, you will have to set yours around 60%. You will also lose precision... but it will work.

So what is normal? Normal would be when the throttle curve of the radio matches the carburetor of the engine. I want 0% of the throttle curve to represent the carburetor is open 0% (aka closed), 50% in the throttle curve would mean the carburetor to be open 50%, and 100% throttle curve point would mean the carburetor is fully open (aka 100%).

To do this simply put the bottom throttle curve at 15% before doing the servo setup. By setting the bottom throttle curve point to 15% this will produce IN/OUT values of both 0%. Now you will have IN/OUT values that both range from 0% to 100%. You will also notice on the throttle curve graph that you have a straight line now from the bottom left corner to the next point. With the throttle curve set at zero on the bottom point you will have a flat line.