 # Lift curve slope of the wing

• How is the lift curve slope of the 3D wing calculated? In other words, which of the following 2 cases is actually used by the flight model?

Case 1) the lift curves of the AirfoilRoot and AirfoilTip are directly used to compute the lift of the entire wing, with interpolation;

or

Case 2) the actual wing aspect ratio (possibly corrected with the AspectRatioMultiplier) is used to compute a reduced lift curve slope (or, equivalently, a downwash angle);

In other words, does the actual wing aspect ratio (+ AspectRatioMultiplier) only affects the induced drag, or does it also affects the lift curve slope of the wing?

• The AspectRatioMultiplier is used to halfen the induced drag greated by a pair of wings, where only one end of the wing actually allows the air to flow around it. So the main wings are a pair of wings, they both get AspectRatioMultiplier 2.0 and the horizontal stabilizer has the same logic..

I don't know how exactly all that is implemented, thats at least what I was told when I asked.
I have no clue where that parameter is also used.
From my experience I would say case 1) is true but I was also told that ClAlpha should stay at 2 pi... So there might be some effect of the actual geometry of the wing...

Cheers,
Jan

Regards,

Jan

• From my experience I would say case 1) is true but I was also told that ClAlpha should stay at 2 pi... So there might be some effect of the actual geometry of the wing...

Cheers,
Jan

Ok, I see in the wiki that 6.28 is the suggested value for airfoils Cl_alpha, and I also see there are some values related to the downwash for the aerowing class, so I assume it is calculated depending on the wing aspect ratio.

In any case, should you get further details from developers, it would be nice to know them. • The lift slope of the 3D wing is computed dynamically at simtime, and it's done more like case 2).

Each time the aerodynamic forces acting on a wing are computed, Aerofly FS 2 roughly does the following:

• for each section, the 2D lift, drag and moment curves are interpolated from tip and root values. These curves are modified according to flap and aileron deflections.
• the relative velocity is computed for each section, taking into account the wing's movement, wind, bending and the self-induced downwash (see below) computed the time step before
• relative velocity and the curves are used to compute the lift, drag and moment for each segment
• the downwash is computed for the next iteration. This is where the aspect ratio and the 3D wing as defined by the wing's sections comes in. For a low aspect ratio wing, we have a higher downwash and effectively a reduced lift slope for the 3D wing. The AspectRatioMultiplier is mainly used to tell the left wing that it is just one half of the total wing and vice versa.

So, the key message is: the lift, drag and moment curves are defined for 2D airfoils, and the sim takes care of the lift slope reduction for finite wings automatically.

• the downwash is computed for the next iteration. This is where the aspect ratio and the 3D wing as defined by the wing's sections comes in. For a low aspect ratio wing, we have a higher downwash and effectively a reduced lift slope for the 3D wing. The AspectRatioMultiplier is mainly used to tell the left wing that it is just one half of the total wing and vice versa.

So, the key message is: the lift, drag and moment curves are defined for 2D airfoils, and the sim takes care of the lift slope reduction for finite wings automatically.

Thank you for the detailed explanation IPACS Developer Just what I was looking for. I like how you designed the flight model.

So, I assume the AspectRatioMultiplier is also used when calculating the downwash angle, otherwise only wings designed as a single Aerowing would have a correct lift slope. Thank you again for the explanation.

• Yes, both the AspectRatioMultiplier and the geometric definition by the wing's stations are used to compute the downwash.