Does sudden downdraft increase or reduce drag?

Question

Let say wing section in wind tunnel operate at 10° AoA, somehow we force downdraft that reduce wing effective AoA to 6°.

Of course lift sensor will show reduction in lift. Will drag sensor show increase or reduce in drag?

Answer 1

In the steady state a 6 degree AoA Will normally have less drag than 10 degrees unless you have a very unusual section. However, what will happen during the transition depends on the nature of the airflow at that point, which is uncertain.

Answer 2

If the change in AoA is fast enough, then strange behaviours start to appear.

The usual aerodynamic characteristics like $C_l$ or $C_d$ which are seen in any standard plot are obtained modifying the AoA of the airfoil in a quite slow way. For example, for a conventional NACA 0012 they look something like that (plot from this answer):

This way of testing reveals only the so called "static" aerodynamic characteristics of the airfoil. Anyway, if the AoA is changed relatively faster, then those plots change quite drastically. Said $\omega$ the frequency at which the airfoil is made oscillate in the test section, a number called "reduced frequency" can be defined as:

$k=\frac{\omega c}{2V}$

Every time that $k$ is bigger than some 0.05, then also "unsteady" aerodynamic phenomena emerge which cannot be disregarded. Unsteady aerodynamic phenomena with $k$ bigger than 0.05 are normally encountered on rotary wings (helicopter and wind turbine rotors) airplane wings at high speed (aka flutter) and civil structures in strong wing (light poles, bridges, power lines, ...).

The following plot (from this NASA technical note plus a couple of arrows from my side) shows how the lift coefficient changes when the same NACA 0012 of before is made oscillate at $k=0.065$:

Now the following can be seen:

• increasing the AoA, $C_l$ increases but following a line (blue arrow) which is higher than the static line (in black); the dynamic lift coefficient is higher than the static one, the AoA being the same;
• when AoA decreases, lift decrease but following yet another line (red arrow) which lies lower than the static one; the dynamic coefficient is lower than the static one, the AoA being the same;
• the lift coefficient at stall is now more or less 2.5, up from the 1.5 for the static condition; that's a lot more!

This behaviour is termed "dynamic stall". Physically, what happens is very similar to how a delta wing generate a quite high lift at subsonic speeds: a vortex is continuously shed from the leading edge and travels down the upper surface till the trailing edge; this vortex helps the airflow in remaining attached to the surface even beyond (static) stall.

Answer 3

OK, some very interesting answers about transient effects and lift theory, but what happens in the next few seconds?

a downdraft can be considered an "anti-thermal"

A downdraft will cause an aircraft to lose altitude unless power is increased. Here, the effect on fuel consumption is identical to increased drag.

Aerodynamicly, the shift in relative wind may cause a brief reduction in drag and lift. However, the pilot must compensate by increasing AOA (to where it was originally), as well as power, to maintain altitude.

"Sudden downdraft" at its extreme is seen in microbursts, which are very dangerous at low altitudes above ground level.