Why do planes fly at a high angle of attack when flying slow?

I've noticed that when a plane does slow flight, the pilot increases the angle of attack (AOA). What is the purpose of this?

– mins
Sep 1 at 16:54
• For a complete picture: Why does an aircraft pitch up when the speed increases?
– ymb1
Sep 1 at 17:45
• @mins I'm pretty sure that's a military fighter aircraft, not any sort of airliner. Sep 3 at 7:45
• @mins That too, but look at the proportions of the aircraft. The cockpit, wings, and tail are all far too large for an airliner, proportionally speaking. Sep 3 at 17:06
• @nick012000 and everyone... that's an F/A-18 Hornet. A fighter aircraft with an extreme flight envelope. This maneuver is likely part of an airshow or something similar. You can tell it's a Hornet and not the Super Hornet by the shape of the air intake, being straight up/down from this perspective, Super Hornet's intake would appear slanted forward 45* from the picture's perspective. So it's likely a F/A-18C Hornet. Sep 3 at 18:24

Lift is (among other things) a function of the airspeed and the angle of attack of the wing. Hence, if you reduce your speed, you have to compensate the associated lift loss by increasing the angle of attack...

• Is it just the AOA that is important or is the upward angle of thrust vector important too? In other words, would it matter if the engines where still pointing horizontally (and still behind the center of mass somehow)? Sep 2 at 17:59
• @johnDanger The component of the engine thrust which is pointed upward does contribute but this should be considered trying to make the best of a bad situation The wing is much more efficient and any thrust you divert from the horizontal to the vertical removes thrust that could otherwise be used to propel the much more efficient wing. You are supposed to "fly with on the wing". Ideally you have the engines continue to point horizontally while the wings increase in AOA but there are great mechanical challenges with a hinged wing or engine compared to just tilting the whole aircraft. Sep 2 at 18:57
• The vertical component of the thrust is important, and adds to lift... For angles of attack beyond the critical stall value, it may be higher than wing lift... Sep 2 at 19:00
• @DKNguyen But, for a small angle, the gain of vertical thrust is first-order while the loss of horizontal thrust is second-order (much smaller), so it seems it is always worth tilting the engine somewhat even though the wing is much more efficient. Sep 2 at 21:44
• @nick012000 The open question is what is the Lift/Drag ratio for any given aircraft. If you know that number, you can do a lot to figure out whether it is the AOA or the vectoring of the thrust that gives the most effect. It's really something you can't figure out in general. You have to figure it out for the model of the aircraft. I'd point out that non-military aircraft can fly slow too. And they all rely on high angles of attack to do it. Sep 3 at 14:04

In order to maintain level flight, an airplane must generate lift equivalent to its weight. The lift generated by a wing diminishes rapidly as the airspeed decreases. To compensate, the angle of attack must be increased to maintain the necessary lift. This can be done until the the critical angle of attack is reached, at which point things get exciting.

• Where "exciting" = "stall" --> "fall from sky". Sep 3 at 16:13

It's a great step that you know about the acronym AOA while still not understanding well how it works! That's nice; but anyway, try to exercise the answers here using your hand as a wing through the wind when someone (ELSE, lol) drives for you. You'll notice that in order to keep you hand flying at slower speeds, you'll have to increase the angle. This is the AOA that you mentioned.

For an aircraft it is the same.

If your mind is more mathematical, imagine flight as X+1=3; if you remove X from the equation, you must add more value anywhere so the wrong sentence 1=3 becomes the right sentence 3=3.

Remember that a higher AOA brings more drag, which in turn will require some more thrust to avoid the speed to decrease. Speed decreasing with the same AOA means that altitude is decreasing too.

And this bring us to another nice curiosity: airplanes can fly with TWO different speeds using the SAME amount of thrust. One, lower, speed is the one where the thrust matches the total aircraft drag when the bigger player is the angled shape of the aircraft facing the wind at a higher AOA. (It is, by the way, a flight that is very unstable; any input adding AOA will take a toll on speed and make you lose height; any lost height will give you back some better angle shape and increase your speed.) And the second, higher speed, is when the same thrust now matches the aircraft flying at a much better angle with a smaller 'shape against the wind', but the major player of the drag is now from the wings (lift-induced) and overall pressure against the flying body fighting a faster wind force.

Complicated, but always fascinating.

• (Thank you the corrections, admin! Essential for a non-natural english speaker and great opportunity to learn more.) Sep 2 at 12:36

As explained in various posts on StackExchange, at the end of the day the one thing that matters for lift is that air gets persuaded to move downwards (say, with velocity $$v_\text{down}$$). The downwards-acceleration of an air pocket of mass $$m$$ as it passes the wing corresponds to a momentum change of $$m\cdot v_\text{down}$$, and the same and opposite momentum is imparted on the aircraft, lifting it up.

If you decrease the airspeed, this is affected in two ways: the downwards component will scale down as well, and less air passes the wing per time unit. Both can be counteracted by increasing the AoA:

• The steeper angle means the air is sent on a steeper down-trajectory, i.e. with a relatively higher downwards component.
• The higher frontal area causes more air to be deflected downwards.

This assumes that the air keeps streaming along the wing smoothly. If you let the speed get too low and/or increase the AoA too much, the flow will stall, which decreases lift again while drastically increasing drag. Most planes can generally not cope with this well, but fighters like the F-18 can simply power through it. Note that the high AoA also makes the engine point downwards, so you get some lift even without relying on the wings at all.

The lift a wing generates is directly proportional to

• the density of the air
• the square of the airspeed
• the Angle of Attack (AOA) (more or less linearly) (until it stalls)

In non-accelerated flight, where the lift is roughly equal to the weight of the plane, the product V2AOA will be a constant, or AOA will be proportional to 1/V2.

As the plane flies slower and slower, the AOA has to increase to maintain the lift. Keep below the AOA for stall, result happiness. Increase the AOA beyond the stall, you have a bad day, if you don't have the height to recover.

In order to maintain level flight, the aircraft must push air downwards to create a force that supports the aircraft in the air. Lets consider a period of one second of time:

• An aircraft going fast slices through a large amount of air, and only needs to push that air down a bit.

• An aircraft going slow slices through a small amount of air, and needs to push this air down a lot.

Considering the wings as a wedge, the change in the amount the air needs to be pushed down can be achieved by varying the steepness of the wedge, this steepness is essentially the angle of attack.

This is a crude simplification, but gets you going in the right direction.

• The four forces of flight are lift, thrust, drag, and weight. I don’t see pushing the air down on that list. Sep 2 at 19:00
• Smartypants 🤣 Explain lift then. Sep 2 at 20:02
• Oh, wait, you don't need to. Leftaroundabout already did that in his answer. Lift comes form, wait for it... air being forced down @JScarry Sep 2 at 20:04