Larry,
If you have to ask, you aren't!
Ok that is the problem. Let me show you what one of my flights looks like. You are looking at Watts (orange) and Pack Volts (grey). The rpm is set to run at a fixed 8100 rpm. You can see that the volts drop through the flight. If you aren't in governor mode, as the volts drop, so does your throttle, and airpeed. Some Timers can compensate by raising the throttle setting during the flight (usually linearly), but I don't think yours can.
Governor mode is much better anyway. When setup this way, the ESC interprets the timer throttle setting as an rpm. There is a little trial and error to get the flight speed you want, but once you have it, it is pretty solid. The rpm will be less than max power, so that when the load goes up in a climb, the ESC can goose the throttle to keep the rpm from dropping. Also when the plane is coming on the downward slope, the throttle is dropped, also keeping the rpm constant.
To set governor mode, you need to program your ESC. I think you can do it with either a RC setup, or with the CC "Castle Link" cable/board and software. It runs only on a PC (unfortunately for me since my machine is a Mac, but I do have access to a PC.) I am guessing that you will need to buy the Castle Link (it comes with everything you need).