“I still find it hard to believe that flying works” confessed one of our readers. “Nobody has been able to explain to me yet, in words I understand, where the force comes from to keep planes in the air” he protested. There’s a challenge.
Surprisingly, it turns out he’s far from alone. A full and precise explanation of aerodynamic lift is still a subject of debate in the scientific community. It also appears that some popular explanations are in fact false. So here, we introduce the two main factors involved in staying safely up in the air and, through these, explore some of the main concepts underlying forces and motion more generally.
Force and momentum
The most straightforward part of the explanation holds true when the wing (or any part of the plane) is held at an angle to the oncoming air. When the plane is stationary, there is obviously no effect, but, as it gathers pace along the runway, air streams increasingly rapidly across it. And, as we know, air is not nothing! It has mass (or weight as it’s colloquially known).
Air molecules hitting the angled wing resemble water from the jet of a hose playing on an oblique surface, like the underside of an awning. As you can imagine this tends to force it away but also upwards.
If imagination is not enough to convince you, try running a plate or other flat surface at an angle through bath water or a pond. Of course you’ll feel a resistance against the direction of travel, but there’s also a tendency to rise up. That’s the uplift that occurs whenever a surface moves through a fluid (air or water, for example) at an angle to the motion.
As you can imagine, the steeper the angle the wing makes with the flowing air the greater this uplift is, up to a point. But, at the same time, the greater is the resistance or drag that the air exerts on the wing.
The air rushes along, hitting the underside of the wing and getting deflected back off the wing, pushing it as it does so. The direction of this force on the wing is at right angles to the wing surface (see ‘resultant force’ in yellow). As you can see from the diagram, this force on the wing (yellow) can be thought of as partly acting upwards (purple) – called lift – and partly acting against the forward motion – called drag.
So there we have it, at least part of the uplift is due to air rushing past underneath the angled wing. It’s rather like the force that lifts a water-skier out of the water and up onto the surface initially. See Read More section below for more on Isaac Newton’s insight into forces and motion.
Pressure difference
But there’s also another factor helping lift the wing of an aircraft. It’s due to an effect discovered in the 18th century by the physicist Daniel Bernoulli. When a fluid, like air or water, flows along, it’s speed may be made to vary. This can happen when an obstacle is put in its way – an aircraft wing for example. Bernoulli found that the pressure of the fluid drops if it moves faster and increases if it goes more slowly. This strange and unintuitive effect is in fact a simple consequence of the logic of flow (see explanation in the Read More section below).
Aerofoils
It turns out that both of these forces act on an aeroplane wing – the momentum effect and the pressure effect. With the wing angled to the oncoming air, as in the diagram, you can see it will experience some uplift just due to the force of the air hitting it obliquely.
But, as this diagram reveals, pressure difference also plays a part. This difference in pressure is a consequence of the way the leading edge of the wing parts the oncoming airstream narrowing the layers of air above the wing and widening them below it.
Above the wing, where the layers are narrower, the air flows faster, so the pressure is lower there. Conversely under the wing. This difference in pressure means there is a net upward push on the wing.
If the speed of the air over the wing is sufficiently high, this upward push will be sufficiently large to balance the downward weight of the aircraft. This is how a cruising aircraft maintains a steady level – no net force raising it up or weighing it down.
To get airborne in the first place the plane also requires enough upward force to balance its weight. However it’s travelling more slowly which means the lift force is weaker. To increase the lift at slow speed, the size and curvature of the wing are increased as the first image above shows (as does a glance out of the window as you take off).
Conclusion
Of course there’s more to aircraft flight than the wings. There is the terrific resistance of the air as the plane pushes forward, something you get a tiny sense of when you try putting up an umbrella on a windy day. Imagine the strength of that for an aircraft travelling at 400 mph. It’s the job of the engines – jets or propellers – to provide sufficient force to overcome this.
Image courtesy of NASA
The intriguing point about forces that Isaac Newton latched onto, in contradiction to the received wisdom of the ancients, is that to stay still or to keep moving at a steady speed without turning, no net force is needed. Taking the case of an aeroplane travelling at steady speed at a steady height, you can see how the forces on it simply balance out to zero. Newton’s insight contradicted the ideas handed down from Aristotle and transmitted from generation to generation until the Renaissance period. No force, no source of energy is needed to keep things going – that’s how the stars, planets and moons maintain their celestial orbits. It’s only where your trying to push through air or over a rough surface, as we do all the time on Earth, that you need energy to provide a force to overcome resistance. Newton and his predecessors transformed our understanding, opening the way to modern science and technology.
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Read More
Newton’s insight into forces and motion
Isaac Newton worked out how to understand the size and direction of forces. He realised that the air (or anything else) hitting a surface has to be stopped and then turned around or deflected by the surface. The air (or anything else) has a certain momentum as it hits the surface. Momentum is a blend of mass and speed – think cricket roller at slow speed or bullet at high speed. Afterwards it has momentum in a different direction as it moves way. The surface must have exerted a force on the air to bring about this change in its momentum. Now, we know that a force pushing on something causes a reaction back on the pushing thing. So the passing air kicks back on the surface of the wing. These two insights were captured by Newton in his famous “laws of motion”: a change of momentum produces a force and one body pushing on another causes an equal and opposite reaction back.
Pressure difference across an aerofoil
Imagine a fluid flowing through a wide tube which then narrows. The same amount of fluid must flow out every second as flows in; but to achieve this steady flow the fluid must travel faster through the narrow part so that the same amount passes through in one second as in the wider part.
It’s noticeable in rivers and stream when you see water rushing through a narrow section and meandering through a wider one.
This means the fluid has to speed up as it moves from the wider to the narrower section. To speed anything up – from a ball to a bicycle – there has to be a push to do the accelerating. So the pressure of the fluid must be higher in the wide section than the narrow one to provide the push that speeds up the fluid.
So, as Bernoulli discovered: high pressure is associated with slow speed and low pressure with faster speeds. You can’t have one without the other. It’s not that one causes the other, just that they are two aspects of the same phenomenon.
The diagram reveals just how pressure difference plays a part in lifting a wing upwards. Layers of air are equally spaced before the wing reaches them. But as the wing cuts into them become narrowed just above the wing and widened below it.
This means the air must flow more quickly above the wing to get through these narrow straits. So the pressure must be lower there. The pressure is correspondingly raised under the wing where the layers of streaming air widen out. The result is an upward force on the wing due to the difference in pressure across it.
How an aircraft climbs and descends
Lift force is also exploited to enable an aeroplane to climb upwards or descend. For this purpose small wing flaps at the back of the aircraft, called elevators, tip the plane so that its nose rises or falls. By lowering these flaps the aerofoil effect increases the uplift at the back of plane. This causes the back to rise and the front to dip down, just as pushing down on one end of a see-saw causes the other end to dip down. Place the cursor on the image and press CTRL + click to see this effect in an animation.
© Andrew Morris 2nd December 2019
Getting to Grips with Science 