# Technical question on flight model. Alpha-dot derivatives

• In the Aerofly FS wiki, I see that the AeroWing class has an attribute for downwash (it's not well explained, actually).

I assume this controls the amount of downwash an AeroWing receives from itself or from another AeroWing (e.g. the downwash produced by the main wing and acting on the horizontal stabilizer).

My question is this: is the actual downwash of one wing on another calculated with or without lag due to the distance between the AeroWings?

An example to better understand the issue: let's say the aircraft AoA impulsively goes from 2 degree to 4 degrees. Even if the main wing downwash doubles, it will take some finite time before the increased downwash travels downstream and "hits" the horizontal stabilizer.

In the conventional stability-derivative-based flight models, this is accounted for using the so called "alpha-dot" derivatives.

These alpha-dot derivatives affect the short period dynamics of the aircraft, and if not accounted for, produce a virtual aircraft that is more stable than it should be in reality.

Does Aerofly FS accounts for this time lag of downwash? In case it does not, are you planning to add it in future?

• The downwash of one wing on another like the wing's downwash acting on the horizontal stabilizer uses the current value of the downwash at the wing (multiplied by a factor that can be adjusted for the relative position of the stabilizer to the wing), so there is no delay.

However, the computation of the wing's downwash includes a small delay. This is done for numerical stability and because the flow field around a wing doesn't change instantaneously even for sudden changes in angle of attack. The value chosen for the delay is kind of a best compromise, a little too long for the wing acting on itself, a little too short for the stabilizer.

This will be addressed when we move to the next version of the aerodynamic interactions between different parts of the airplane, then also allowing interaction between multiple elements.

• Very nice to know!

Thanks Murmur for your always pertinent questions and IPACS for the great work you're doing with this simulation platform!