Originally Posted by
gundernak
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
Hello "gundernak",
I am an aerodynamicist and I can confirm that a wing with an aspect ratio of around 10 which uses lightly cambered airfoils (around 2..3% camber ratio) along it's span will find the highest L/D ratio (or glide ratio respectively) around a relatively low angle of attack somewhere near 3. As the relative camber of an airfoil increase and the further towards the trailing edge the camber goes, the lower the AoA for maximum L/D. The lower the camber of the airfoil and the closer the camber's position to the leading edge, the higher the AoA for max. L/D.
As your software tells that the AoA for max. L/D is around 3 degrees, this suggests that you were testing a wing of a given aspect ratio (usually higher than 6) with a cambered airfoil (such as NACA 2412 or 2410), which has a solid confirmation from real life tests, so the "normal cruise" resulted max. L/D AoA is correct.
As airspeed increases and Reynolds number increase, the maximum L/D AoA doesn't vary much and usually varies towards decreasing rather than increasing. As far as I know, at higher speeds the drag becomes more and more AoA sensitive due to compressibility effects, so you can't have lower drag for the same lift (for example) at higher AoA at higher speeds.
If you'd look at most of the airliners as example, the wing's incidence (angle between the wing's root chord and the fuselage's X-axis) is generally found at a value which helps the wing's MAC (mean aerodynamic chord) AoA be exactly the one for maximum L/D during cruise, while maintaining the fuselage's AoA as close to zero as possible (where the fuselage produces minimum drag). The philosophy is simple: make the plane ride only on the surfaces that produce the highest L/D at it's corresponding angle and keep all other surfaces that have bad/low L/D at an AoA where they produce the lowest drag, even if their lifting contribution is null. This is why an airliner's fuselage looks perfectly horizontal (because it actually flies near 0 AoA) during cruise, while the wing is the only thing producing the needed lift at an AoA corresponding to maximum L/D. The maximum L/D is the main ingredient which helps all airliners gain maximum flight range or lowest fuel consumption and also the most economic or highest climb rate. You always want to fly near or at maximum L/D AoA, which depending on weight will correspond to an airspeed and altitude.
It may seem surprising parallel to your test, but an AoA of around 3 is generally found to be that for maxumum L/D during cruise for many airliners.
The fact that in the "Level flight at Vmd" test the software somehow sees the max. L/D AoA to increase so dramatically suggests that there might be some mathematical model limitations within equations used by the software or some sort of "trouble" in it. As far as I know, increasing the maximum L/D AoA can only be possible by varying the wing's aspect ratio (such as a variable wing geometry aircraft as the F-14) and/or it's airfoils shape or maybe by controlling the BL (boundary layer) in a manner towards slightly disrupting the airflow up to an AoA slightly lower than the target AoA for which you'd like the maximum L/D to occur.
The AoA is the major factor (usually the only factor) which affects the L/D ratio, so, there's definitely something strange happening in the calculations for the "Level flight at Vmd".
Regards,
Mav.