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I learnt in class today that fast-moving air over the wing of the plane, which causes a difference in pressure on the airfoil due to something called Bernoulli's principle.

If this is so why cant the engine be placed in front of the wings since the exhaust from the engine can help the plane in take off.

I'm only a beginner at fluid dynamics thus the silly question.

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  • $\begingroup$ Most airplanes DO place the engine(s) in front of the wings. E.g. all non-pusher single engine propellor planes, most twins, a lot of multi-engine prop planes... With commercial jets, I suspect the main reasons for placing the engines in pods under the wing for lighter structure and ease of maintenance. $\endgroup$
    – jamesqf
    Nov 9 '20 at 17:04
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They can be, as on the An-72/74, which has the engines mounted above and forward of the wings in part to increase lift, utilizing the Coandă effect.

One downside is that this makes access for maintenance more difficult, which is a large factor in operation costs.

https://en.wikipedia.org/wiki/Antonov_An-72

https://en.wikipedia.org/wiki/Coand%C4%83_effect

An-72

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    $\begingroup$ The civilian version is the AN-74 $\endgroup$
    – GdD
    Nov 9 '20 at 8:09
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In addition to the Antonov 72/74's, Boeing worked on the YC-14 concept as a replacement for the C-130's.

Boeing YC-14

Also, many seaplanes have to place the engines as far up as possible, to keep the engines and propellers out of seaspray. This has the secondary effect of accelerating airflow over the wing and increasing its effectiveness.

Canadair CL-415

The engine wash is also used to increase the effectiveness of control surfaces, an example for this is the A-10. The A-10 blows jetwash between the rudders and above the horizontal stabilizer, augmenting the efficiency of all these surfaces.

A-10 Thunderbolt

The downside is that the surfaces in contact with jetwash need to be designed to handle the soot and extra heat. The airplane also needs to be able to handle the lift imbalance in case of an engine failure.

F-4 Phantom tail Not a great example, but good to illustrate I think.

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Positioning of the engine can severely affect the aircraft's CoP and CoG, having it directly at wing level must move CoP or CoG in abrupt ways.

Also, if an aircraft engine were to be creating fluctuations of exhaust gas, then the lift of the aircraft will also fluctuate.

Throttle of the engine would not only affect how much thrust the aircraft produces, but also how much lift it is using. If you wanted the aircraft to go faster but not gain any altitude, you would now need to pitch the aircraft's nose further down since a high amount of exhaust gas is being blown into the wings that is essentially adding to the aircraft's relative airflow and (if any) headwind. If a lot of lift is being produced, that is beyond the maximum deflection of the elevator, you may have some trouble maintaining altitude. To combat this, you could use flaps and spoilers, but these shouldn't be used at high airpseed in the first place.

The pilot also has to account for more factors. He/she must now also account for the additional lift that the aircraft's exhaust is providing for the aircraft.

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