Best L/D and most efficient AOA 1

[gundernak]

Dear Sirs,

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. If this thread does not fits here, than I apologize and kick me off.

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 enlight me, please. I really need a help here.

Thank you in advance

FA
Hungary

 

[HerkyJim]

Here is a link to the best combination of theory and practice that I have ever seen

http://www.av8n.com/how/#contents

Maybe this will help..,

 

[ccheric]

I hope I can provide a couple headers for you to start with:

1. Most efficient AOA for the airfoil is mostly like not the same as the same the as the most efficient AOA for the airplane. You have the fuselage, tail surfaces and their trims.

2. There are 3 cruise modes: the first one you have is for "max speed", second is for "max range", third is for "max endurance" I believe (might has switched, but you can easily find out). Now, if you go into the flight mechanics a little bit, you can derive the conditions required for "max range" and "max endurance", you will notice they are different.

If you start from these, you will get to the solutions real quick. Have fun!

** As I recall, max L/D is the same as the farthest glide without power (--> sailplanes/ gliders); add power, you get max range of the airplane.