Josh Cohn wrote:Lift is generated by deflecting air downwards (actually perpendicular to the glide path). At higher speeds the deflection is spread out over a longer distance per unit time. This gives the air less opportunity to flow spanwise around the wing, which would generate tip vortices.
You can feel and see this effect if you skim your hand over the surface of a pool slower and faster.
Adrian Thomas wrote:To generate lift, the wing accelerates air downwards, applying (inducing) a downwards velocity (v) to the mass (m) of air it encounters in unit time (t) so that in steady flight mv/t = total lift.
At higher speeds the wing encounters more mass of air. m is larger so v must be smaller if mv/t = total lift.
The kinetic energy lost to the wake is 1/2mv^2. So at higher speeds where m is larger and v is smaller the kinetic energy lost in the wake is lower. Therefore induced drag decreases as speed increases.
Florianopolis wrote:Don't forget your lift vector also has a backwards component, which is more backwards at higher angles of attack.
The3hadow wrote:Florianopolis wrote:Don't forget your lift vector also has a backwards component, which is more backwards at higher angles of attack.
Yes, but then I ask why's that? There's no direct physical explanation why the backwards component (induced drag) is greater just because of a higher AOA. The backwards component is greater because the effective lift is higher when increasing the AOA while keeping the speed constant or because the speed has to be lower when increasing the AOA to keep the effective lift constant. Both lift (pressure difference) and speed (see explanations above) give the direct physical explanation for the resulting backwards component / induced drag - at least for me.
In other words: the lift generated by the wing doesn't tilt rearwards because of the wing tilting relative to the free stream (AOA) but because lift (pressure difference) itself results in induced drag and airspeed has an impact on the induced drag as described in the previous posts.
mxaxai wrote:I'd explain it like this: on an ideal wing, you have high pressure below and low pressure above the wing, creating lift. If you tilt the wing (increase AOA), your pressure difference increases (more lift) but some of the low pressure is now behind the wing, whereas some of the high pressure is in front of the wing. This "pushes" the wing back.
I don't think that this is the reason for increased induced drag at low speeds/high AOA, though.
mxaxai wrote:I don't think that's 100% accurate. That sounds like induced drag is a direct result of the tilt of the wing:
"some of the low pressure is now behind the wing, whereas some of the high pressure is in front of the wing. This "pushes" the wing back."
That would also mean, if you tilt / increase the AOA of a 2D wing, i.e. a wing with infinite aspect ratio, you would also get induced drag but that's not the case.
B737200 wrote:One last thing, if I remember correctly, the vortices do not cause the induced drag/downwash, the downwash causes the induced drag and the downwash also causes the vortex to form. Therefore the vortex is a consequence of the phenomena which causes the induced drag, it isn't the root cause of the induced drag. I mention this because you seem to be basing your thought process on the vortices as a starting point which is in incorrect, assuming that my memory serves me well...
The tip vortices are coupled with the bound vortex when circulation is induced with lift. The effect of this vortex system is to create certain vertical velocity components in the vicinity of the wing. The illustration of these vertical velocities shows that ahead of the wing the bound vortex induces an upwash. Behind the wing, the coupled action of the bound vortex and the tip vortices induces a downwash. With the action of tip and bound vortices coupled, a final vertical velocity (2w) is imparted to the airstream by the wing producing lift. This result is an inevitable consequence of a finite wing producing lift. The wing producing lift applies the equal and opposite force to the airstream and deflects it downward. One of the important factors in this system is that a downward velocity is created at the aerodynamic center (w) which is one half the final downward velocity imparted to the airstream (2w).
The effect of the vertical velocities in the vicinity of the wing is best appreciated when they are added vectorially to the airstream velocity. The remote free stream well ahead of the wing is unaffected and its direction is opposite the flight path of the airplane. Aft of the wing, the vertical velocity (2w) adds to the airstream velocity to produce the downwash angle ϵ (epsilon). At the aerodynamic center of the wing, the vertical velocity (w) adds to the airstream velocity to produce a downward deflection of the airstream one-half that of the downwash angle. In other words, the wing producing lift by the deflection of an airstream incurs a downward slant to the wind in the immediate vicinity of the wing. Hence, the sections of the wing operate in an average relative wind which is inclined downward one-half the final downwash angle. This is one important feature which distinguishes the aerodynamic properties of a wing from the aerodynamic properties of an airfoil section.
Source wrote:Near the wing the bound circulation due to lift leads to an up-wash ahead of the wing and downwash behind the wing similar to the flow produced by a two-dimensional lifting wing of infinite span.
A very important effect is generated by the flow due to the vortex pair that comprises the wake. The semi-infinite sheet of vorticity distributed in the wake produces a downward velocity component in the free-stream ahead of the wing, at the wing and far downstream [...]
The downwash by the wake leads to a reduction in the angle-of-attack of the wing relative to the free stream, reducing the lift. In addition the downwash rotates the oncoming flow vector at the wing leading to a component of drag [...]
The downwash produced by the circulation of the bound vortex of the wing is zero at the wing and therefore does not contribute to the induced drag.
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