I’m wondering about the impact of downwash on lift generation. I've read that, because downwash creates lift according to Newton's third law of motion, increases in downwash due to greater AoA (especially when augmented by devices such as slots and slotted flaps, which use various aerodynamic effects to increase the amount of downwash generated) result in increased lift. But I've also read that ground effect increases lift by decreasing downwash, so I'm confused how this works. Can someone clarify the effect of downwash on the amount of lift generated and tell me whether I've got it right or not? Thanks!
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3 Answers
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Downwash is behind the wing and is the result of pressure differences between the top and bottom of a wing. Although the energy conservation bookkeeping says the plane goes up and the air goes down, the main driver of lift is pressure differential. True to Bernoulli, the angled bottom wing slows and increases air pressure beneath the wing. This is "bottom lift" that even simple flat plates have.
Airfoils also deflect air upwards and away from the top of the air foil, creating additional lift above the wing. Downwash is created by the movement of air from higher to lower pressure as the wing goes by, filling the void where the wing was. This constant process is known as circulation.
Ground effect may not be directly related to downwash, but may be indirectly, as the ground and the downwash stream may create enough of a "fence" to slightly raise the pressure under the wing. A Cessna 172 has a wing area of $174 feet^2$.
$174 feet^2 × 144 inches^2/foot^2= 25056 inches^2$
A pressure increase of 0.01 psi from ground effect adds 250 extra pounds of lift!
Ground effect is also said to be caused by the wing tip vortices being disrupted by their proximity to earth.
answered Feb 15, 2020 at 18:21
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$\begingroup$ So the lift gain by downwash is some kind of very small „addition” to overall lift that is mainly generated by pressure differential by Bernoulli? I asked about downwash in ground effect because in atpl books and atpl questions there is mentioned that in ground effect downwash is reduced, so the induced drag and then our lift is less inclined backwards and the overall lift increases. Am I getting this right? $\endgroup$– KonradFeb 15, 2020 at 19:07
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1$\begingroup$ It depends on what theory you wish to believe. As a pilot, ground effect is real. Down wash is after the fact (of lifting the plane). And some say bottom lift is not Bernoulli, but momentum mv of the wing colliding with the air. I think any downwash hitting the ground is even more after the fact, and downwash itself does not create lift, it is the result of it. However, if you look at a plane with an AOA of 12 degrees 20 feet off the ground, one may imagine a compression wave hitting the ground and bouncing back to the wing. This "ground effect" makes sense as much as the other explanations $\endgroup$ Feb 16, 2020 at 0:42
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This is a most excellent question. I've struggled with this for a while. From a Newtonian perspective, there must be momentum transfer from the flow to the wing in order for the wing to gain lift, which necessitates in the generation of downward airflow. Let's call this downward airflow downwash.
However, from the circulation theory of lift, which holds incredibly well for low Mach aircraft, downwash is the source of induced drag (note that this theory does not account for viscous drag or form drag from flow separation). Furthermore, this drag is the result of reduction in lift, due to downwash; increased downwash equals more reduction in lift and more induced drag.
Consider the elliptical lift distribution (which for all intents and purposes, can be considered a typical wing distribution for low Mach lift, without loss of generality), the downwash over and behind the wing is constant across the span of wing, and can be expressed as (Ref: Anderson, Fundamentals of Aerodynamics):
$$(1) : w=\frac{SC_L V_\infty}{\pi b^2}=\frac{C_L V_\infty}{\pi A}$$
where $w$ is the downwash, $C_L$ is the lift coefficient of the wing, $S$ is wing reference area, $b$ is the wing span, $V_\infty$ is the free-stream airspeed, $A$ is the aspect ratio.
Induced drag, for an elliptical distribution, can be expressed as:
$$(2): C_{D_i} = \frac{\pi w^2 A}{V_\infty^2} = \frac{C_L^2}{\pi A}$$
So what do we get from all of this? From equation (2), induced drag is directly proportional to the square of downwash for any aspect ratio. From equation (1), the lift coefficient is proportional to downwash. By increasing aspect ratio, downwash, and therefore induced drag, can be reduced asymptotically to zero, while the wing's lift coefficient stays unchanged.
In other words, while downwash is necessary for lift, it also results in (induced) drag; increasing the aspect ratio makes downwash more efficient at generating lift and reduces (induced) drag.
In ground effect, the exact same thing happens as the reaction from the ground makes downwash more efficient at generating lift. This is manifested, once again, from a reduction in actual downwash for a given lift coefficient.
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$\begingroup$ Work on winglets is pointing towards removal of draggy turbulent areas from the wing. Could it be that increased pressure under the wing is pushing downwash further aft? Would this re-direction be less draggy, a bit more thrusty, or both? $\endgroup$ Feb 17, 2020 at 15:47
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1$\begingroup$ @Robert Not sure what you mean by the first part. A winglet in the Whitcomb sense converts induced drag from the winglet into forward thrust. But winglet is always inferior to an equivalent span extension if there's no restriction on span. $\endgroup$– JZYLFeb 18, 2020 at 6:48
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$\begingroup$ "Removal of draggy turbulent areas" is consistent with laminar flow theory, "clean wings", golf balls, rear mounted props, reverse tapered trailing edges, rounded wing tips, Hoerner wing tips, not mounting engines (or munitions) above wings, mono wings, thin wings, just to mention a few things we all have at our disposal to build a better plane. "Restriction on span": too bad 747 size seagull wings can't be built, or can they? $\endgroup$ Feb 18, 2020 at 10:56
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Strictly, the more wing lift is created the more the downforce on the air. They are equal and opposite forces.
Yes, the increased downforce usually results in greater downwash.
But if the ground is in the way then the downwash gets spread sideways and its net downward component is reduced. This raises the local pressure between wing and ground, increasing lift.
So in normal flight the downwash is a consequence of lift, while in ground effect the added lift is a consequence of downwash.
