When descending , an aircraft never goes in free fall, even if negative angle of attack is large. Does that mean the net downward force is not gravity, but less than gravity, due to certain lift acting at all negative angle of attacks?

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    This question is unintelligible. A large negative angle-of-attack would create a downward loop. That is very possible. If the wing is at the zero-lift angle-of-attack the aircraft would go into freefall. That is also very possible. Yet the language of the question suggests otherwise. No on should be answering this question because it is posed it a manner implies things are true that are actually not true. – quiet flyer Oct 23 at 3:03
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    I really don't understand why so many people are answering this question, and why their answers are getting upvotes. I though that the way this site works is that a question that contains contradictory or false statements-- statements not congruent with reality-- should be edited, not answered. – quiet flyer Oct 23 at 3:45
  • Re "an aircraft never goes in free fall", that's not strictly true. See e.g. the "Vomit Comet" used for astronaut training: en.wikipedia.org/wiki/Reduced-gravity_aircraft Can be done in other planes, too - even sailplanes. – jamesqf Oct 23 at 4:16
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    @quietflyer, the question is self-contradictory, because its author is confused about the proper definition of the terms. It should be answerable by clarifying what they actually mean (we would need some feedback from them though regarding whether the existing answers clarified anything and what remains to be unclear). – Jan Hudec Oct 23 at 5:09
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    @Sachin Are you aware of the difference between pitch angle and angle of attack? I based my answer on the suspicion that you do not, but from that comment I still can't tell. Negative lift would be where you push so hard on the yoke that your head would hit the ceiling if you hadn't buckled up; is that what you're asking about? – Sanchises Oct 23 at 16:07

You seem to be confusing angle of attack and pitch angle. The pitch refers the orientation of the aircraft: where is the nose pointing? The angle of attack is the angle of the wings relative to the incoming air stream. Both can take on any value independent of each other. For example, one could imagine an aircraft falling straight down, but wings level. The pitch angle is then zero, but the angle of attack is 90 degrees!

Similarly, the pitch angle can be negative for a continuous steady state descent (and often is for a steep descent). However, you correctly identify that an aircraft on approach generally does not go into a freefall. The lift acting on the airplane is still (approximately) the same. A positive angle of attack is thus still required. As a result, the aircraft will be descending even steeper than where the nose is pointing, resulting in a positive angle of attack (the angle between the pitch vector and the even steeper velocity vector).

The Newton's second law of motion tells us that acceleration of object equals sum of forces acting on it divided by its mass. In a steady descent, an aircraft is not accelerating, it is just flying with a constant velocity pointing obliquely downward. Therefore the sum of forces acting on it must be zero.

Now there are four main forces acting on an airplane:

(Image from How It Flies, chapter 4 Lift, Thrust, Weight, and Drag)

While drag is slanted slightly upward in descent, and thrust might be depending on the resulting pitch attitude, in normal descent the only force with large upward component is still lift, and therefore it must still be almost equal to weight to get the forces in balance.

The lift is only decreased when initiating descent, so the plane needs to accelerate downward in order to change direction. However usually it only decreases by maybe 10–20%. If it decreased more, you'd surely notice: if it decreases to zero, you'll feel weightless (e.g. in the Vomit Comet) and if it went negative, you'd raise from your seat as the aircraft would be accelerating downward faster than free fall, but you only have gravity pulling you down, so you'd lag behind it.

Besides the above linked, you may also want to look at the chapter 2 Angle of Attack Awareness and Angle of Attack Management of the abovementioned How It Flies book, for detailed treatment of the relationship between Angle of Attack and pitch, and any other section too; it does quite a good job explaining the physics related to flying.

  • I see different ways the lift is illustrated. Some say the lift is "the force that directly opposes the weight of an airplane", meaning the lift is opposite to gravity, i.e. perpendicular to the horizon. Some illustrate lift as perpendicular to the aircraft's chord line, so not necessarily perpendicular to the horizon (when the aircraft is pitched at an angle). In the diagram you show above, the lift is perpendicular to the wind direction. So confusing. Is there a universal definition, or people use the word "lift" loosely in different ways? I wish I know how to attach pictures here. – gadfly Oct 22 at 21:27
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    @gadfly, the universal definition is as in this picture: lift is the component perpendicular to relative wind, drag is the component along relative wind. Most images show level flight, so the distinction is not clear in them. – Jan Hudec Oct 22 at 21:31
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    @gadfly, also the force generated by airfoil due to circulation is perpendicular to it, but that includes the induced drag and gets decomposed to lift, perpendicular to the relative wind, and the induced drag, parallel to the relative wind. – Jan Hudec Oct 22 at 21:35
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    @gadfly, the lift induced drag is induced by lift in the sense that any way of producing lift over finite span also produces this drag. It is not a component of lift, but the term “induced” would not be used for describing that anyway. – Jan Hudec Oct 23 at 4:58
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    @gadfly, relative wind is indeed the wind that you feel coming at you due to your motion, but that means you have to be moving relative to the air mass. If natural wind blows in the same direction as the aircraft's travel, the aircraft will maintain its speed relative to the air mass, because it needs that to generate lift, and it's speed relative to the ground will be sum of the aircraft airspeed and the wind speed. – Jan Hudec Oct 24 at 5:07

The short answer is Yes. In a "normal" descent, (i.e. wings level positive G) there will still be a positive AoA. All that needs to be done to initiate a descent from a state of equilibrium is to reduce a little power and adjust pitch nose down enough to maintain airspeed. This will initiate a downward acceleration, but once the rate of descent has steadied out the wings will still be producing lift equal to the weight of the aircraft that they are opposing.

If there is a large negative AoA as the question stated, (assuming wings level) the aircrew would experience negative G forces and the aircraft would accelerate downwards at 32 ft/sec squared, PLUS any additional downwards force produced by the wings as a result of the negative AoA. Suffice it to say that this would be a rather brief transitory state.

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    Re "The wings are still producing lift, just a little less lift than the weight of the aircraft that they are opposing": no, unless the plane is accelerating downwards, the lift of the wings is still equal to the weight of the aircraft. Although maybe a small part of the lift is due to the downward speed, so one might call that part an upward drag force. – Marc van Leeuwen Oct 22 at 19:22
  • Your answer makes sense, because the aircraft reaches a steady state. (not accelerating) However, I would seem to me that that forces cannot be equal. Consider the classic diagram of an aircraft turning at 60deg AOB. Load factor must be increased to 2Gs so that the vertical component equals the weight or the aircraft will descend in the turn. How do you account for this? If it in balanced flight and holding a constant altitude, and then is descending at a constant airspeed, something has changed. What has changed? – Michael Hall Oct 22 at 22:18
  • @MichaelHall What changed is that during some period between the steady state of flying straight-and-level and the steady state of descending at a constant rate, the aircraft briefly produced less lift than its weight, causing it to accelerate downward. Once the desired vertical speed was reached, the lift was returned to equal the weight in order to maintain a constant rate of descent. On the flip side, as you slow the descent, the aircraft will actually be producing more lift than its weight, even though it's still descending, as it would be accelerating upwards. – reirab Oct 22 at 22:45
  • @MichaelHall More to the point, acceleration is directly proportional to net force. Current velocity is irrelevant to that relationship. – reirab Oct 22 at 22:46
  • @reirab, thanks - good explanation. I edited my answer above to reflect this truth. – Michael Hall Oct 22 at 22:47

Unless you’re flying some sort of unlimited aerobatic airplane capable of such rapid pitch changes that you can achieve a negative angle of attack, your angle of attack will always remain positive as the relative wind changes in direction when transitioning from straight and level to a descent. A descending airplane is subject to the same forces and loads as any other airplane is. However in descent, gravity does provide a component of its total which acts in the direction of other thrust forces, propelling the aircraft forward (this is also how a gilder works) as the potential energy of the airplane is siphoned off and converted into work to counteract aerodynamic drag forces.

  • Gravity always acts downwards. How can gravity provide a component? – Sachin Chaudhary Oct 23 at 9:47
  • Orient your frame of reference with the aircraft’s axes and you’ll understand why. – Carlo Felicione Oct 23 at 14:00

During a steady (non-accelerating) descent, the sum of the vertical components of lift and drag equal the weight of the aircraft. The total lift (aerodynamic force perpendicular to the aircraft's path with respect to the air) will be reduced. Thrust and drag may may also be reduced depending on the air speed.

In an aircraft that is descending at a constant rate, not only is there lift acting on it, but that lift is equal in magnitude to the aircraft's weight. If the lift were less than the weight of the aircraft, then the aircraft would be accelerating downwards.

The only times that lift is not equal to weight are the times when the aircraft is accelerating upwards or downwards. That is, when the aircraft changing its rate of climb or descent. Any time when the rate of climb or descent is constant, the lift is equal to the entire weight of the aircraft. As Jan Hudec's answer mentions, this is a necessary consequence of Newton's Second Law.

As others have mentioned, remember that pitch angle and angle-of-attack are two totally different things. Pitch angle (angle of the nose relative to the horizon) can be quite negative while still maintaining a positive angle-of-attack. During a normal descent, angle-of-attack is always positive.

Note: For purposes of this answer, I'm defining 'lift' to be "the sum of all forces acting on the aircraft in the opposite direction of gravity" in order to keep things more simple.

  • Any particular reason for the downvote? – reirab Oct 24 at 14:24

When descending, an aircraft never goes in free fall,

Not strictly true. Freefall just means falling at the same rate you would accelerate under gravity. The passengers on the vomit comet experience free fall even after the aircraft has peaked and has started to descend. Aerobatic maneuvers can often result in zero or even negative G forces.

...even if negative angle of attack is large.

Angle of attack is not actually relevant here. If the wings fell off your aircraft, you would still descend and you still wouldn't experience sustained freefall.

Does that mean the net downward force is not gravity, but less than gravity, due to certain lift acting at all negative angle of attacks?

Yes and no. The upward forces acting on any object descending through the air can include drag from air resistance, lift from aerodynamic effects, and thrust from engines etc. The downward forces obviously include gravity, but could also include thrust from engines and negative lift from aerodynamic surfaces. So while lift (positive or negative) from angle of attack is a factor, it is only one of many.

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