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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?

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3 Answers 3

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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.

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  • $\begingroup$ I'm not sure what you're referring to. If AoA is reduced until lift is zero then there should be no tip vortices and so no induced drag, only parasitic. $\endgroup$
    – Frog
    May 7, 2023 at 22:49
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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):

 Aerodynamic characteristics for NACA 0012

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 $k=0.065$:

 Dynamic stall for NACA 0012

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.

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  • $\begingroup$ "Physically, what happens is very similar to how a delta wing generate a quite high lift" - no, not at all. What really happens is that the boundary layer at the rear of the airfoil comes from flow at low AoA while the pressure at the forward part comes from the suddenly high AoA. Separation is delayed and stall shifted upwards. The opposite happens on the way down: Now the thick boundary layer of the high AoA flow reduces lift at low AoA. Rinse and repeat. $\endgroup$ May 7, 2023 at 14:16
  • $\begingroup$ @PeterKämpf: I agree that the phenomenon is more complicated than wrhat I have written, but the main source of the peak in the plot is the vortex shed at the leading edge. Otherwise Leishman is wrong and you should correct him 😉 $\endgroup$
    – sophit
    May 7, 2023 at 14:58
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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.

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  • $\begingroup$ Additionally, sudden downdraft may cause all kinds of turbulence before the new relative wind is established. I would not bet on much in the way of drag reduction. $\endgroup$ May 7, 2023 at 16:57
  • $\begingroup$ @ Robert DiGiovanni agreed - the OP is asking about a downdraft specifically rather than a change in the angle of the wing itself. depending on the cause, the airflow may be relatively smooth or could be highly turbulent. $\endgroup$
    – Frog
    May 7, 2023 at 22:53
  • $\begingroup$ @Frog yea, the whole moving with the airmass thing works against the plane pretty fast. Essentially, one must climb to hold altitude after recovering AoA. Sophits graphs are interesting as to what happens exiting the downdraft. $\endgroup$ May 7, 2023 at 23:45

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