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What is the principle behind flight of airplanes? Need a simple answer for grade 7 student. Thank you

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  • $\begingroup$ Stick your hand out the window of a car and... $\endgroup$ Aug 1 at 15:06
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    $\begingroup$ Physics has principles, but they don't specifically refer to airplanes. There is not one law of physics that is "the principle" that explains airplanes. $\endgroup$
    – user7915
    Aug 1 at 18:47
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    $\begingroup$ Obxkcd: "airfoil". $\endgroup$ Aug 1 at 20:46
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For Seventh Grade?

How about:

Because air consists of tiny little particles called atoms, or molecules, and when an airplane moves through the air, it collides with these particles and they bounce off the airplane and push on it, just like if you hit, or push on something with your hand.

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    $\begingroup$ This is actually quite a good description for how lift works in the supersonic regime. Subsonic, not so much. $\endgroup$ Aug 1 at 21:43
  • $\begingroup$ This is exactly how lift works at any speed. If it's not impact of air molecules bouncing off the surface of the aircraft, then what is the mechanism of force transfer? Lift is a force. Where does the force come from, if not from the impact of the air hitting the aluminum.? $\endgroup$ Aug 2 at 1:48
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    $\begingroup$ @CharlesBretana Of course the air molecules will hit the wing, but they also interact with each other. That interaction is crucial for subsonic airflow around the wing. Neglecting it is an incorrect over-simplification. Therefore, explaining lift on the molecular level is not going to work. $\endgroup$
    – Bianfable
    Aug 2 at 9:08
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    $\begingroup$ This answer is actually quite brilliant, hard to make it any simplier... $\endgroup$
    – Jpe61
    Aug 2 at 12:20
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    $\begingroup$ @CharlesBretana Unfortunately, your description of lift on a molecular level is still incomplete (what about the upper surface of the wing?). Please read this excellent answer by Peter Kämpf to get a better understanding of how lift actually works. $\endgroup$
    – Bianfable
    Aug 3 at 8:20
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Airplanes fly mainly because their wings push air downwards. As the air is being pushed down, an upwards force pushes the wing up.

The downwards movement of the air is achieved by two methods, both of which are used on airplanes:

  1. The leading edge of the wing is higher than the trailing edge, making the wing act as a wedge. The angle between the line from leading edge (front edge of the wing) to the trailing edge (rear edge of the wing) and the airflow coming towards the wing is called the angle of attack.
  2. The wings profile is such, that it forces the upper airflow go faster than the airflow under the wing. When these airflows meet at the trailing edge, the faster airflow coming from the top tilts the airflow down.

A portion of the lift comes from the faster airflow on top of the wing: as the airflow accelerates, the pressure of the air goes down, causing suction on the top of the wing.

To achieve lift, the airplane needs to go forward, so it either needs an engine to push or pull it, or it needs to glide downwards.

To get more lift, airplanes either raise the angle of attack (raising the leading edge even more higher than the trailing edge), or they go faster. To get less lift, the do the opposite. Adjusting lift will, of course, make the plane go up or down.

If the angle of attack gets too high, the air can no longer follow the curve of the wings upper surface, and the wing will lose its lift. This is called a stall. A wing stalls if the aircraft is going too slow, therefore all aircraft have a minimum safe speed.

Obviously, there is also a maximum speed for airplanes, usually limited by the airplanes structure which can no longer stand the stresses the fast moving air causes.

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    $\begingroup$ "When these airflows meet at the trailing edge" -- This sound suspiciously like the "equal transit time" theory, which is incorrect (and imo takes the student further from understanding the idea). On reading that point a second time, I don't think it's what you meant, but you may want to clarify. $\endgroup$
    – yshavit
    Aug 2 at 0:07
  • $\begingroup$ A humble suggestion: When explaining to a 7th grader, it helps to use simpler terms like "front of the wing" instead of "leading edge" etc. If I was in 7th grade, I'd also want to know why the air at the top moves faster: physics.stackexchange.com/questions/13030/… $\endgroup$
    – Nav
    Aug 2 at 0:18
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    $\begingroup$ There are no two methods, just one. The leading edge being higher than the trailing edge is essential, but the air flowing over the wing being faster is just effect of the pressure decreasing as the air is made to follow the downward sloping surface. It's not a separate contribution, it's the mechanism how the force is transferred to the wing. $\endgroup$
    – Jan Hudec
    Aug 2 at 4:08
  • $\begingroup$ Yshavit & Jan I'm trying to keep this as simple as possible, while not drifting too far from the truth. By no means do I subscribe to the equal transit theory, but referring that would have complicated the answer. I think the wing profile is very essential, thats open for dicussion but maybe not here. Nav I thought terms leading edge and trailing edge were self explanatory, but I think your suggestion is better than my original guess. Edited. $\endgroup$
    – Jpe61
    Aug 2 at 12:16
  • $\begingroup$ Hmmmm... It would be nice to know why my answer was wrong. I mean I really don't need the extra two points the downvote takes, but really, what is so wrong here? $\endgroup$
    – Jpe61
    Aug 2 at 12:25
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As Jpe61 wrote, the important thing is that the air gets pushed downwards. Contrary to what's often said, it doesn't really have anything to do with the Bernoulli effect.

Flow on the underside of the wing is quite easy to understand: air collides with the surface, which it can't penetrate, so it is forced to change its path from moving horizontally, to moving diagonally down along the wing.

Wing underside air flow

At the point where the path bends, a downwards force needs to act on the air molecules. By Newton's third law, the opposite force must act on the wing, and that's what pushes the plane up.

Easy, right?

...Well, that's actually not sufficient. Consider a wing shape like the following, where you also have the downwards path-deflection, but still don't get any net lift:

A wing example that wouldn't generate lift

You can see there's no lift from the fact that the air flow after the wing is again horizontal, i.e. no vertical momentum has been imparted on the air.

Note in particular that two commonly mentioned factors are not sufficient to generate lift: 1. front edge of the wing higher than aft edge 2. upper surface longer cord than lower surface.

So the crucial thing is that you manage to keep the air streaming diagonally downwards after passing the wing. That's where the importance of the upper surface comes in: it must form a smooth enough curve so the air will follow it, streaming tangentially. In this case, the streams above- and below the wing will both be moving downwards at the trailing edge, so they can just merge again and continue diagonally.

wing in proper operation

But why would the air even bother curving down above the wing? After all, there's nothing in the way there that would prevent it from moving on in a straight line.

The reason is ambient air pressure. If the molecules did move in a straight line, there would be a whole region of vacuum above the wing. In the boundary between this vacuum and the above air stream, there would be a huge density- and pressure gradient, and that would push air molecules down into the vacuum region. This is a very violent process; it happens when space capsules re-enter the atmosphere at hypersonic speeds, but not in airplane wings.

Instead, with a proper wing profile at appropriate speed and AOA, you get the smooth flow over the wing, which involves no vacuum. It does however involve a lowered pressure right over the curved part of the wing. It is the pressure gradient between this low-pressure region and the higher-pressure ambient air above that maintains the curved flow. And the low-pressure region adds lift too, in addition to the excess-pressure lift from the lower surface.

It also causes a suction effect: higher-pressure air from in front of the wing is accelerated as it gets sucked into the low-pressure region. So the air flow is faster there – Bernoulli's effect. This is, however, not really relevant to the principle how lift works.

At too high AOA, or too low airspeed, or insufficient wing camber, something different happens though: the flow stalls, and instead of moving along the upper surface you get uncontrolled turbulence. This creates strong pressure fluctuations, which also disrupt the flow from under the wing behind the trailing edge. Ultimately the whole flow after the wing is then turbulent and without the downwards momentum, that's why a stalled wing produces no lift or drastically reduced lift.

stalled wing

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    $\begingroup$ +1 This is the only sensible answer around that simplifies the more advanced explanation, but does not diverge from it. $\endgroup$
    – Jan Hudec
    Aug 2 at 5:17
  • $\begingroup$ I would edit the second picture a bit: the airflow over the wing is faster than below the wing, so the arrowhead should be further out the back. As for the second pic, dunno, maybe the same... Allthough this might step in the region of "too much". $\endgroup$
    – Jpe61
    Aug 5 at 14:05
  • $\begingroup$ @Jpe61 the arrowhead position has nothing to do with how fast the airflow is. $\endgroup$ Aug 6 at 14:37
  • $\begingroup$ Oh but of course it doesn't, but it could. $\endgroup$
    – Jpe61
    Aug 6 at 15:24
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In short, airplanes fly because the shape of their wings and the attitude they move through the air at generate lift.

While there still remains some debate as to exactly how wings do this, science has resolved the reason to a basic principle cause resulting from two different phenomena.

Wings generate lift by altering the momentum of air passing around them. They do this by the curvature, or camber, of their upper and lower surfaces, which accomplish two things. First, airflow which strikes the lower surface of the wing is deflected downward, resulting in a change of momentum and a resultant lift force through the conservation of momentum and Newton’s Third Law of Motion. Think about a child sticking their hand out of a car window with the palm down and fingers pointed toward the front of the car. As the car drives down the road at speed, the child experiences a force on their hand trying to push it upwards and backwards this is a result of air striking the child’s hand on the palm and being deflected downward, just as air striking the bottom of a wing does the same thing.

The second action is that air passing over the top of a wing is accelerated, which causes a drop in pressure over the top surface of the wing due to Bernoulli’s Principle. This low pressure area draws down high pressure air surrounding it altering its momentum and resulting in a lifting force. These two lifting forces constitute the overall force of lift, which counters the force of gravity. As a consequence the airplane can now fly.

The amount of lifting force that we can generate is directly proportional to the square of the airspeed, the density of the air, and the angle of attack, which is the angle between the chord line and the direction of the airflow to the wing.

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  • $\begingroup$ Bernoulli's principle explains the air over the wing the other way around. There is a separate argument why the pressure is decreased (due to inertia and receding surface), and the increase in speed is just due to Bernoulli's principle—which makes it tangential to the lift generation, because you've already lift by the lower pressure and don't need the speed for anything. $\endgroup$
    – Jan Hudec
    Aug 2 at 4:55
  • $\begingroup$ I don’t know if there’s one way or the other. Bernoulli‘s principle is the concept of energy conservation, so in the absence of any other factors aside from velocity and pressure, energy is conserved and will necessitate a change in one if the other is altered. $\endgroup$ Aug 6 at 13:56
  • $\begingroup$ You claim that “air passing over the top of a wing is accelerated, which causes a drop in pressure”, but in fact Bernoulli's principle tells us that the air is accelerated while we already know about the pressure drop! So your claim is not part of the explanation. $\endgroup$
    – Jan Hudec
    Aug 6 at 14:39
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According to Bernoulli's principle: Fluids at higher speeds produce low pressure at that point, which effects the surrounding high pressure to exert force on low pressure area.

The airplane wings are designed so that air speed above the wing is higher than below, causing a pressure difference (low pressure above and high pressure below the wings), Hence pushing the airplane upwards.

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    $\begingroup$ This is a common explanation, but a) the other answers have already mentioned it b) it's highly flawed as such. Bernoulli's principle actually describes that a frictionless laminar flow within a tube will vary its pressure in inverse proportion to how the flow velocity changes. This can also be applied approximately to several quite different situations, but for a wing it's actually more of a side-effect of other phenomena. $\endgroup$ Aug 1 at 21:42
  • $\begingroup$ @leftaroundabout The problem is that people like to have "causes" and "effects" where neither exist. (Personally, I blame Aristotle for creating that philosophical red herring). Newton's laws of motion (correctly) says nothing about whether "forces cause acceleration" or "accelerations cause force." Most of the "disagreements" about how planes fly come down to individuals preferring one of those descriptions to the other. $\endgroup$
    – alephzero
    Aug 1 at 21:47
  • $\begingroup$ @alephzero well, but in this case the causal relation does exist though, and it's the opposite from what's commonly claimed: the curved surface causes a lower pressure, and that then causes the air to accelerate. The latter has no relevance for the principle of lift. If the wing operated in a viscous liquid instead of air, that could cancel the acceleration effect, but you would still get lift. $\endgroup$ Aug 1 at 21:56

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