Strictly speaking, our training aircraft’s engine doesn’t produce thrust. Our engine produces power.
The intricacies of thrust and power get a little complicated but we need a basic understanding in order to get our head around maximum range airspeed, which relates to thrust, and maximum endurance airspeed, which relates to power. We’ll try to simplify this…
Our training aircraft’s propeller forces air backwards and this creates thrust, which accelerates the aircraft forwards. The engine itself (the part hidden inside the nose) creates power, and this power is used to turn the propeller.
Any power produced can be expressed as:
Power = Force x Velocity
Since the force created by our propeller is thrust, and our velocity is the aircraft’s true airspeed (TAS) we can swap these terms and re-write this equation for our aircraft as:
Power = Thrust x TAS
We’ve also learnt that to fly straight and level, our thrust must be equal to drag. So our propeller must produce at least as much thrust to equal drag (i.e. Thrust = Drag). We can then say that the minimum power we require to fly straight and level is:
Power Required = Drag x True Airspeed
All this boils down to us being able to create a Power Required Graph. We can multiply the drag value at every airspeed in straight and level flight by our true airspeed to create the Power Required graph:
Since our fuel consumption depends on our engine power setting (not thrust!), the airspeed that will give us the minimum fuel consumption is the airspeed that requires minimum power. With minimum fuel consumption, we will be able to fly for the longest possible time – and we now have our maximum endurance airspeed.
This maximum endurance speed is always slightly slower than the VMD speed (minimum drag, maximum range speed) we learnt about in the Drag section of this course. This is because level flight can be maintained at a slightly slower airspeed, and thus a moderately lower power setting.