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When the pitch of a plane increases (facing upwards), it has a positive angle of attack, and when it decreases to face downwards, it has a negative angle of attack.

Increasing the angle of attack would increase the lift coefficient as a result and vice versa. If all other factors remain the same, then the force of lift would increase as a result.

What I don't understand is that when the pitch is increased, wouldn't the direction of the lift be facing backward, which would push the plane backward?

Lift Free Body Diagram

When the pitch is decreased, the relative direction of lift would face forward. Won't this not only propel the plane forward but also increase the force of lift for the plane?

For planes with a very low thrust-to-weight ratio, won't this effect be magnified? Is there some component of force I forgot to consider?

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  • $\begingroup$ What I don't understand is that when the pitch is increased, wouldn't the direction of the lift be facing backward, which would push the plane backward?” Yeah, your hunch is correct, we just call this effect “induced drag” instead of backwards lift. $\endgroup$ Aug 26, 2023 at 13:34
  • $\begingroup$ @MichaelHall -- So induced drag is zero if AoA is negative? (For a cambered airfoil, it is possible to sustain upright flight at slight negative AoA.) $\endgroup$ Aug 26, 2023 at 13:42
  • $\begingroup$ @quietflyer, no. If the wing is producing lift it is producing induced drag. We both know that, so not sure of your point? $\endgroup$ Aug 26, 2023 at 13:45
  • $\begingroup$ @planes: please wait at least 24 hours before choosing an answer in order to give all the people around the world the possibility to come up with an answer 🖖 $\endgroup$
    – sophit
    Aug 26, 2023 at 19:40
  • $\begingroup$ BTW, the square/diamond shapes don’t help your question. Why not draw actual airfoil shapes? $\endgroup$ Aug 27, 2023 at 17:58

4 Answers 4

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When the pitch of a plane increases (facing upwards), it has a positive angle of attack, and when it decreases to face downwards, it has a negative angle of attack.

Well, sort of. Strictly speaking, that sentence is only completely true if the flight path is exactly horizontal, and the wing incidence is zero. But a negative-angle-of-attack generally isn't compatible with sustained horizontal flight. Read on for more.

You do understand that pitch attitude and angle-of-attack can be two very different things, right? Pitch attitude is the angle between the longitudinal axis of the aircraft and the horizon, and angle-of-attack is the angle between the wing chord line-- or in some uses, the longitudinal axis of the aircraft-- and the direction of the flight path. Even if wing "incidence"-- the angle between the wing chord line and the longitudinal axis of the aircraft-- is zero, angle-of-attack will often be very different from pitch attitude. If you are under the impression that any time we see an aircraft in a nose-down pitch attitude, the wing must be at a negative angle-of-attack, then you need to do some re-thinking. Again read on for more.

Your sentence would be better changed to something along the lines of-- "If the direction of the flight path stays constant, then an increase in pitch attitude corresponds to an increase in angle-of-attack, and a decrease in pitch attitude corresponds to a decrease in angle-of-attack."

What I don't understand is that when the pitch is increased, wouldn't the direction of the lift be facing backward, which would push the plane backward?

You need to understand that the Lift and Drag vectors are defined orthogonal (perpendicular) to, and parallel to, the direction of the flight path, respectively.

Not the wing surface (or "chord line").

The flight path is also the direction of the (free-stream) airflow, which we call the "relative wind" -- this is the wind or airflow that the airplane "feels" in flight, regardless of the direction that the external, meteorological wind is blowing-- and that's why the vectors are defined that way.

Take a look at this related answer: Can we show through simple geometry rather than formulae or graphs that the best glide ratio occurs at the maximum ratio of Lift to Drag?. Note that the Lift vector is not tilted backwards because the aircraft is drawn nose-high, rather it is tilted forwards because the flight path is descending. If the flight path were horizontal and the wings were not banked, and the wings were at a positive-lift angle-of-attack, the Lift vector would point straight up, regardless of the pitch attitude of the aircraft.

Of course, the angle-of-attack can be decreased to the point where Lift is zero, or negative. In the latter case, if the aircraft is initially upright and the flight path is initially horizontal, the Lift vector will initially point straight down.1 Obviously, that is not compatible with continued horizontal flight-- the flight path will immediately begin to curve (bend) earthwards (downwards), and the orientation (pitch attitude) of the aircraft will immediately start changing as well!

So you can see that in these kinds of discussions, in any specific case we have to understand whether we're taking it as a given that the flight path is continuing on with no change in direction or speed, or not. If so, all the force vectors must sum to zero (meaning that the force vectors can be arranged into a closed polygon); if not, that constraint is removed.

Here's a more complicated answer that shows the same principal: Does lift equal weight in a climb?. Note that in none of the vector diagrams is the pitch attitude of the aircraft specified. And neither is the angle-of-attack of the wing. But the direction of the flight path (i.e. the climb angle) is. That's all the information we need to draw the Lift and Drag vectors in their correct orientation.2

Footnotes:

  1. Compare and contrast to the figure labelled "Negative AoA" in the original question. Generally speaking, this figure has two errors-- one, the direction of the Lift vector is shown as perpendicular to the wing chord line, not the airflow, and two, the Lift vector is pointing generally upward, not generally downward. To speak more precisely, for a cambered (non-symmetrical) airfoil it is possible for the angle-of-attack to be very slightly negative (just a few degrees) and for the Lift vector still to be in the "positive" direction. But not at the large negative angle shown in the figure--assuming that the figure is intended to represent horizontal flight, and thus a horizontal airflow. If the figure is not intended to make any representations about the direction of flight/airflow at all, then we have deeper problems-- this would suggest that the OP is imagining that "Angle of Attack" is synonymous with "pitch attitude".

  2. There was one simplification made in that answer-- the Thrust vector was assumed to be parallel to the Drag vector, and therefore parallel to the flight path. In reality, the Thrust vector is not defined to be parallel to the flight path, so for any given direction of the flight path, the pitch attitude of the aircraft does affect the direction of the Thrust vector. But not the direction of the Lift and Drag vectors.

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  • $\begingroup$ Both in this question and the previous, the OP seems to be focused on the idea that the Lift vector may somehow propel the aircraft forwards, or slow the plane down. The short answer is "it doesn't, because the Lift vector contains no component parallel to the flight path". At least as viewed from the airmass reference frame. For a bit of a mind-teaser from a different reference frame, see aviation.stackexchange.com/a/75480/34686 $\endgroup$ Aug 26, 2023 at 12:39
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when the pitch is increased, wouldn't the direction of the lift be facing backward, which would push the plane backward?

No.

By definition lift is the part of the aerodynamic force perpendicular to the airflow, not to the body.

The following picture (source) is more correct:

enter image description here

Lift ($C_l$) is always perpendicular to the "Flow direction" and drag ($C_d$) is always parallel. The angle of attack $\alpha$ makes the intensity of lift and drag change but not their direction.

In general a higher (positive) AoA increases both lift and drag, lift in a more or less linear way (in blue in the following plot), drag (in red) in a quadratic way:

enter image description here

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Lift is defined to be perpendicular to velocity. Drag is always parallel to velocity.

If the aircraft is moving 'horizontal', then lift always points 'up' and drag always points 'back'.

We call the Lift / Drag axis system 'Wind Axes' because they are aligned with the freestream velocity (the wind).

We also often use 'Body Axes' because they are attached to the aircraft's frame of reference (the body). If you take a paintbrush and paint an axis system on the aircraft, those could be body axes. We usually call these Axial (from nose to tail) and Normal (out the top of the aircraft).

The angle of attack is (by definition) the angle between the Body axis in the axial direction and the Velocity. I.e. the angle of attack provides the transformation from Body to Wind axes.

When angle of attack is zero, they are the same. So, 'Axial' and 'Drag' are aligned and 'Lift' and 'Normal' are aligned.

Your intuition on lift/drag probably starts here -- but is more properly attacked to Normal/Axial force components.

When angle of attack increases, the Normal force increases -- and while it still mostly points in the Lift direction, it rotates back and now also points in the drag direction.

However, Lift is always perpendicular to velocity. We define it that way.

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From a practical standpoint, yes. The total aerodynamic force created by the wing is directed more aft at a high angle of attack than at a low angle of attack.

Somewhat pedantically, "lift" is defined to be perpendicular to relative wind. The part of the aerodynamic force that is increased in the aft direction is called "induced drag." This is just a matter of terminology- you are exactly correct in your intuition from a physical standpoint. This is why an airplane will slow down if pitched up without adding power- the increased drag does indeed push the plane backwards and cause its airspeed to decrease until a new equilibrium is reached at a slower airspeed.

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