Hi All,
I have been told that we can achieve best range when flying at best lift to drag ratio speed which is the best glide speed as per Bob Tait CPL aerodynamics book page 5.11
I fly C172 where the glide speed is 68kts, the best L/D ratio speed. now if we fly straight and level at 68kts can we achieve maximum range? because we are still maintaining the best L/D ratio speed, but when we apply power we also need to change the AoA right? so now are we actually achieving best L/D ratio?
Please see the attached range profile from C172 POH, it say at 45% power setting you achieve the best range at 90kts, I am so confused now, in this case how are we achieving best L/D ratio??

Thank you

There is no doubt that the best lift/drag ratio occurs at a particular angle of attack. As long as you fly at that angle of attack you will achieve the best lift drag ratio.

However the speed required to fly straight and level at that angle of attack will depend on the aircraft's weight. Remember also that we are talking here about pure theory and have made some assumptions that may muddy the water in the real world. For example, if we consider only the airframe, the best lift/drag ratio would be very close to the best gliding speed. We can prove geometrically that the best lift/drag ratio is also the best distance/height ratio in no wind.

However, in that argument, we have ignored the requirements of the engine-propeller combination. The engine/propeller combination must also be at it's best efficiency if you are considering level flight. In the end, it becomes a comprise between the airframe's efficiency and the engine/propeller efficiency.

You can be sure that, questions in the CASA aerodynamics exam are considering only the airframe requirement.