Best L/D and most efficient AOA
Hi everyone,
Actually I am just a single PPL pilot, and I have a theoretical question. I can not find the answer and I am confused a little bit. That's why I am here to ask for help.
These are 4 statements on L/D max and AOA for max L/D:
· L/Dmax AOA produces the minimum total drag
· At L/Dmax AOA, parasite drag and induced drag are equal
· L/Dmax AOA produces the greatest ratio of lift to drag
· L/Dmax AOA is the most efficient angle of attack
These above can be found in so many aviation literature.
In the following I will be speaking about prop common light aircraft.
When you fly straigh and level at Vmd, your L/D is at max, you fly at the most efficient AOA, and this is the speed for best range and minimum drag. Vmd is a quiet low cruise speed.
If you increase your airspeed in level flight to reach a normal cruise speed, the AOA will decrease and the plane leaves the most efficien AOA.
But you can find statements about it wich confuse the hole thing (I mean confuse to me). They say at the most used cruise speed, the plane will fly at the AOA of the best L/D. And the horizontal axes will be paralell to the relative airflow, thanks to the right incidence angle.
Well, now at wich speed will fly the plane at the most efficient AOA?:
- at Vmd or
- at normal cruise
Futhermore I have made a little flight test (in a software), and monitored the specific values of different situations. The aerofoil best L/D is at 3 deg. Here are the results:
Max Cruise
power: full
IAS: 130 kts
AOA: 2 deg.
L/D: 12,5
Normal Cruise
power: 2300 RPM
IAS: 110-120 kts
AOA: 3 deg.
L/D: 14,5
Level flight at Vmd:
power: 1500 RPM
IAS: 85 kts
AOA: 7 or 8 deg.
L/D: 15,5 (max L/D)
Now you can see at level flight at speed Vmd the L/D is max. But the AOA is not the L/Dmax AOA of the aerofoil.
At normal cruise the AOA refers to the most efficient AOA of the aerofoil, but the L/D is not the best.
May I ask you to enlighten me about this contradiction, please. I really need a help here.
Thank you in advance
FA
Hungary