# Why do airfoil's trailing edges point downward?

The trailing edge of and airfoil is mostly pointing downward. Is there any reason for that? Down wash increases drag and down wash is due to the direction of air pointing down. If the trailing edge is made horizontal (or maybe pointing upward), they can produce less down wash and Kutta Condition can still be satisfied.

What are the specific reasons for this shape?

• Downwash is the result of lift (or vice versa, depending on how you look at it). – DeltaLima Mar 27 '15 at 13:53
• There is no lift without downwash, so it's usually considered to be pretty important. This gets back to that whole Newton's Third Law thing. A perfectly horizontal wing (at 0 AoA) creates no lift and a wing that deflects air upwards would create negative lift. – reirab Mar 27 '15 at 14:31
• @reirab Just to clarify, a perfectly horizontal symmetrical airfoil generates no lift. There are airfoils designed to lift at 0 and even negative angles of attack. – StallSpin Mar 27 '15 at 17:10
• @StallSpin Yes, correct. By "perfectly horizontal wing," I meant that the wing is not deflecting the air stream upward or downward (i.e. it's a symmetrical wing at 0 AoA.) – reirab Mar 27 '15 at 17:46

You are right, the trailing edge does not need to point down. Take symmetric airfoils - here the trailing edge runs parallel to the airfoil chord. Or take reflex airfoils (like the HQ 34 of the SB-13 tailless glider): Here the trailing edge is indeed pointing upwards, and still this aircraft flies.

But to create lift with as little drag as possible, it helps that the trailing edge points slightly down. Why? Because then it will be least in the way of the desired local flow. Lift is created by accelerating air downwards. The wing deflects the air flowing over it, and the trailing edge should reflect this deflection angle.

But - as always - too much is not good: The Eppler 417 airfoil of the SB-7 glider was an extreme layout with too much rear camber. It is said that pilots could still wipe off raindrops near the trailing edge after landing when they had crossed a shower an hour earlier. This means that airflow separated well ahead of the trailing edge on the upper surface, and the raindrops would not be blown away in the separated flow. Flow separation increases drag, and this effect shows that the shape of the Eppler 417 had too much downward camber at the trailing edge.

If the plane is large and heavy, its airfoil's rear camber can be higher - an extreme case is shown below. This is an early supercritical airfoil designed by McDonnell-Douglas, and the highly cambered rear part allows it to integrate very effective Fowler flaps. Flaps help because they allow to change the direction in which the trailing edge is pointing: Low lift coefficients require no flap deflection, or even negative flap angles in gliders, and the higher the lift requirement grows, the more the flaps will be extended, pointing more and more downwards.

The same is true for control surfaces: Depending on the desired lift change, their trailing edge will point up- or downwards. See below for an example where I plotted the pressure distribution for three flap deflections in one graph. Upper and lower surface pressure are shown by color-coded lines, and lower lines belong to the lower surface. Dashed lines show the inviscid pressure, and solid lines the pressure distribution with friction effects added. The wider two lines of the same color are apart, the more lift is created. Note the contour plot below, which follows the color scheme of the pressure plots.

If there are slots to re-energize the flow, extreme trailing edge angles are possible and help to create lots of lift at low speed, which helps airliners to get into small airfields. See below the triple-slotted flaps of the Boeing 727, which was designed for regional traffic from and to small airports.

• As usual Peter. I find there's almost nothing left to add after one of your answers :) +1 – slebetman Mar 30 '15 at 9:02
• If lift is produced by accelerating air downward, then higher the downwash higher the lift. Lift induced vortex also produce more downwash and hence should provide more lift, right? But, most of the books say that lift induced vortex reduces the effective AOA as downwash is more and hence, lift reduced. How is eff AOA is reduced if there is a downwash? – Selva Apr 9 '15 at 15:04
• @Selva: The downwash goes together with an upwash in front of the wing. The suction field above the wing accelerates and bends the airflow - look here for a longer explanation. Maybe this would be a good question of its own. – Peter Kämpf Apr 9 '15 at 15:15
• I will make it as a new question, as it will help others – Selva Apr 9 '15 at 15:24
• @yo': Do you see the bumper on the rear fuselage? That will make grand contact first, protecting the flaps. – Peter Kämpf Mar 30 at 13:43

This question discusses different ways to explain why wings generate lift. The Newton explanation (wings go up by pushing the wind down) can explain why a wing trailing edge points down. This will increase the angle at which the air is directed downwards, and thus increases lift.

• I read that ground effect is due to decrease in downwash angle. If as you said, downwash increases lift, then ground effect should decrease lift. But that is not the case. Correct me if i am wrong. – Selva Mar 27 '15 at 14:03
• The ground effect combined with Newton's approach of explaining lift is covered here aviation.stackexchange.com/a/11396/4197 – ROIMaison Mar 27 '15 at 14:08
• So, if I understand correctly, ground effect reduces induced drag by reducing the downwash angle. But at the same time, lift should also reduce, but it is compensated by the cushioning effect (high pressure build up between lower surface and ground) Am I right? Please correct me if I am wrong – Selva Mar 27 '15 at 14:22
• You can see that the resulting force ($F_{res}$) is under an angle (≈ the downwash angle, $\alpha_{eff}$). The vertical component of this resuling force is lift $(L=F_{res} \sin{\alpha_{eff}})$, the horizontal component is the induced drag $(D=F_{res} \cos{\alpha_{eff}})$. The ground effect reduces the downwash angle (therefore putting the resulting force more upright). This increases lift, and decreases drag. – ROIMaison Mar 27 '15 at 14:54
• @Selva: You are exactly right. – Peter Kämpf Mar 28 '15 at 13:26

Down wash increases drag and down wash is due to the direction of air pointing down. If the trailing edge is made horizontal (or maybe pointing upward), they can produce less down wash and Kutta Condition can still be satisfied.

There's a very important reason why we don't want to try to decrease the amount of downwash the wings generate: the amount of downwash is equal to the amount of lift!

An airplane in straight and level flight must produce an amount of downwash equal to its own weight. If it produces less downwash than that, it will fall. There's no way around it.