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To create a lift, the airflow speed on top of the wings should be higher than the airflow speed on the bottom of the wings.

But when you keep the engine on the bottom of the wings, wouldn't it hurt the lift as air flows faster on the bottom of the wings?

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To create a lift, the airflow speed on top of the wings should be higher than the airflow speed on the bottom of the wings.

No, that's not true.

In order to create lift, the pressure on top of the wings must be lower than the pressure on the bottom of the wings. The airflow speed doesn't matter.

I'm guessing your thought process is something like this:

  1. To create lift, the pressure on top of the wings must be lower and the pressure on the bottom must be higher.
  2. According to Bernoulli's principle, faster moving air has lower pressure and slower moving air has higher pressure.
  3. Therefore, to create lift, the air speed on top must be faster and the air speed on the bottom must be slower.

However, point 2 here is wrong. Bernoulli's principle does not say that faster moving air has lower pressure. What Bernoulli's principle says is that given two points a and b, if

  • the fluid flow is steady, and
  • the fluid flow is incompressible, and
  • viscosity is negligible, and
  • gravity is negligible, and
  • the points a and b are on the same streamline, and
  • the fluid is moving faster at a than at b,

then the fluid has a lower pressure at a than at b.

Since the air above the wing and the air below the wing are not on the same streamline, Bernoulli's principle doesn't tell you anything about the speed of the air going above and below.

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  • $\begingroup$ In figures posted here, i.imgur.com/01yDUem.png, which one is a better design. In Fig.2, the air blown by the engine is passing through a tunnel. Whereas in Fig. 1, the air blown by the engine is directly touching the bottom of wings as it moves back $\endgroup$ – mks Nov 3 '19 at 21:03
  • $\begingroup$ Well, just because two fluid particles are not on the same streamline doesn't mean Bernoulli can't be applied here. If there's no energy/momentum source (and assume all the other stuff you've mentioned), then the total pressure at far field must be equal for the two particles at two streamlines. If it gains static pressure at some location along the streamline, then its speed must decrease, and vice versa. The proportion of the change must hold for both streamlines. $\endgroup$ – JZYL Nov 4 '19 at 22:44
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You're going by an old outdated lift theory, still taught in a lot of places. The wing induces a very large package of air to move down as it's going along, and this newtonian action/reaction of this package of air being induced to move down is most of "lift". The Bernoulli part is important, because the pressure differential is a factor and it also is part of what encourages the air above the wing to move.

Anyway, most of the air mass that is motivated to move downward is above the wing. You can put stuff under the wing and it has little effect on this, but put stuff above the wing and the whole process of making a large package of air extending half a span above the wing to move down is disrupted.

This is why you see airplanes like Skyraiders completely festooned with junk underneath the wing, but which has little effect on the wing's lifting ability. Put all that stuff above the wing, and it'd never get off the ground. Same deal with engines.

enter image description here

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  • $\begingroup$ What I am trying to say is that the air is blown by the engine is touching the bottom of the wings and moving with high speed. Wouldn't it impact the lift? $\endgroup$ – mks Nov 2 '19 at 2:23
  • $\begingroup$ The propeller wash is adding energy to the entire lift generation process above and below the wing so lift is increased in the area behind the prop. On a turboprop with 12 or 14 foot propellers, there is a huge effect and power changes have a near instantaneous effect on sink rate. On a single the effect is there but less pronounced. It's one of the reasons power on stall speed is less than power off. $\endgroup$ – John K Nov 2 '19 at 2:42
  • $\begingroup$ Thank you John for your response. Unfortunately, I still did not understand how keeping the engine under the wings helps the lift. Can you post some diagrams describing airflow? $\endgroup$ – mks Nov 3 '19 at 17:59
  • $\begingroup$ My point was that the velocity of the air above and below and the resulting Bernoulli effect isn't how most lift is created. It's the large flow field below and mostly above being redirected that creates the lifting force. Having a jet engine under the wing doesn't help or hurt with lift significantly. Putting the engine under is a better location because putting it, or any other body, on top is very disruptive to the upper flow field. $\endgroup$ – John K Nov 3 '19 at 18:10
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    $\begingroup$ You want a short tail pipe. Early jet airliners put engines buried in the wings with a long tail pipe. There are thrust efficiency losses with long tail pipes and structural difficulties with designing spars around the engines' bulk. Boeing put the engines outside the wing because it's more efficient and moved them forward on pylons because they act as mass balances for the wing. Exhaust farther forward or farther aft doesn't make much difference. You are assuming the jet blast under the wing must create strong suction that inverts the lift force, but that's not how the lift is being made. $\endgroup$ – John K Nov 3 '19 at 21:46
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To create a lift, the airflow speed on top of the wings should be higher than the airflow speed on the bottom of the wings.

That is correct. Lift is the result of a pressure difference between upper and lower side, and pressure is proportional to the inverse of speed squared if no energy is added.

But when you keep the engine on the bottom of the wings, wouldn't it hurt the lift as air flows faster on the bottom of the wings?

Note the point about adding energy in the paragraph above. As @JanHudec correctly points out, speed in the engine exhaust stream does not indicate suction. The pressure in the exhaust stream is still higher than on the upper side of the wing and lift is not diminished.

But friction drag is proportional to speed, so the higher speed on the upper side will cause more friction drag from the engine mount and nacelle. Placing the engine on the lower side puts it into a comparatively slow airstream. Also, the engine intake will slow down the air ahead of it (ram effect) and since the air that flows around the intake ends up on the lower side, this blocking effect of the engine intake causes less de- and acceleration of air, lowering losses. Adding the greatly increased volume stream of the hot exhaust gasses also keeps pressure up since all the volume behind the nacelle can be filled with exhaust gas. Adding the engine, therefore, slightly increases lift.

But the most important reason is maintenance and accessibility. The low-hanging engine is easy to reach and to inspect. That is a major reason why the "classic" layout that started with the Me-262 and was carried over to the passenger jets is still preferred. There have been designs with their engines mounted above the wing (for noise abatement), but that never caught on.

The next reason is cabin noise. By shielding the loud exhaust stream from the cabin by placing the wing between both, passenger comfort is significantly improved. This was especially important for the early jets with their high exhaust speeds.

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No, because the air is still turning the same way. It might even help a little bit, depending on the wing shape.

  1. To create lift you need lower pressure above the wing than below, but the difference in speed is the effect, not cause of this. The cause is that the air would like to continue moving straight due to inertia, and the pressure decreases above until it can pull the air down along the surface, while it increases below until it can push the air out of its way.

  2. The air behind the engine does not have lower pressure. Remember that the Bernoulli's equation is just formulation of conservation of energy for fluids. So it applies when the flow is accelerated without adding energy. Then the pressure decreases to compensate. But adding energy is exactly what the engine does. So in this case the increased kinetic energy of the flow is added by the engine, and the pressure does not decrease (well, it might, depending on the nozzle design and operating conditions, but the goal is that it does not, because that way the engine is most efficient).

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