One of the hardest habits to overcome when training new pilots is the automatic instinct that throttle equals speed. From our earliest childhood years we are used to the idea in cars, bikes, boats, etc., that the throttle is a pure speed controller. We naturally carry this habit into aircraft and if you are getting too slow on final approach the first habit is simply to open the throttle. With trainers more than any other type of model this can be disastrous due to the strong nose up pitch leading to a hammerhead stall. The reverse of this habit of course is to close the throttle and assume that therefore the plane is travelling slowly when in fact it is still hurtling along. If elevator controls the speed, how come all the diagrams show drag being balanced by engine thrust? Surely more thrust must mean more speed until drag once again balances thrust?

So it looks like throttle is intimately tied up with the speed. Gliders can go slow and gliders can go very fast indeed. How can a glider go fast when it has no throttle? Simple, the pilot leans forward on the elevator. Ah you say, but the glider is losing height and to go faster it loses height faster. Not true actually! It loses air height faster and that is not the same as height above the ground. Imagine our glider in cruise at 40mph, still air, losing height at 200feet per minute. The pilot leans forward on the stick and flies at 80mph losing height at 500fpm. Now he brings it back to the 40mph cruise and enters a thermal. By sheer coincidence this thermal is rising at 200fpm so the glider stays at the same height, stays at 40mph and appears to be experiencing a miracle – with no engine it is flying level and maintaining speed. In truth it is descending through the air and is still losing energy at the rate of 200fpm even though the height above the ground does not change. What has happened is that the thermal has brought in new energy at the rate of 200fpm and is balancing the rate of loss of energy. Now the thermal increases in power to rising at 500fpm. The glider pilot has a choice – continue at 40mph and rise at 300fpm, or lean further forward on the elevator and go all the way up to 80mph and not lose or gain height over the ground yet be losing energy in the air at the rate of 500fpm. Do you see what is happening – the pilot uses the elevator to control the speed, it is the input of new energy from the thermal that controls whether or not he goes up or down at that speed. Different speeds cause the plane to lose energy at different rates, so to maintain level flight the plane must replace that energy at different rates, and that is what the thermal does for a glider. If you want a glider to come down you do not put the nose down. You open the airbrakes, which is the glider equivalent of throttling back. A power plane is simply a glider with a built in and controllable thermal! To control the speed I use the elevator, that alters the rate of energy loss which the engine is there to balance so I must alter the rate at which the engine puts in replacement energy. To go faster I push forward on the stick and to stop the loss of height I add a bit more power. Eventually we reach a speed in level flight at which the throttle is fully open. I can’t go any faster in level flight, but I can go faster, often much faster even though I have no more throttle to open, I just have to accept the loss of height.

So you see, throttle is intimately tied up with speed but is not how we control the speed. The throttle controls whether or not we are climbing, level, or descending at that speed.

Harry