Angle of Attack in vertical turbulence

[MrManH]

Hi everyone,

I read that the AoA increases in both a downdraft and updraft. I can visualize that being true in a downdraft. If we were to take an aircraft trimmed for straight and level cruise flight, it would suddenly pitch up and at the same time follow a downward trajectory, the chord line and flight path/relative wind would be moving away from one another, so the AoA would be increased.

However I fail to visualize how the AoA would be increased in an updraft. The aircraft would pitch down while at the same time following an upward trajectory, wouldn't this bring the chord line and flight path/relative wind closer to each other and therefore reduce the AoA?

Thanks!

 

[dmspilot]

The updraft itself increases the AoA. As in, you're flying straight and level and the relative wind is coming from in front if you, now you hit a sudden updraft, you have wind hitting the airplane from below--that's an increase in AoA.

Not sure I buy your downdraft argument. Why does a downdraft cause the airplane to suddenly pitch up?

 

[MrManH]

The tail due to its greater arm (distance from CG) has greater torque than the wings. So if the airplane gets hit by a downdraft, the same down force will have a greater effect on the tail as it does on the wings, this unbalances the airplane pushing the tail down and therefore raising the nose.

 

[dmspilot]

The tail due to its greater arm (distance from CG) has greater torque than the wings.

This is the correct explanation for why changes in airspeed produce nose-up or nose-down pitching moments. A sudden increase in airspeed causes the wing to produce more upward lift, and the horizontal stabilizer to produce more downward force, but the horizontal stabilizer wins due to the greater lever arm.

I am not sure the same is true for a sudden downdraft. However, if I were to assume you to be correct, I guess the answer to your original question is that an updraft increases AoA instantaneously by virtue of the updraft itself, whereas a downdraft does not increase AoA until the aircraft reacts aerodynamically, which is not instantaneous due to inertia.

 

[exncsurfer]

Hi everyone,

I read that the AoA increases in both a downdraft and updraft. I can visualize that being true in a downdraft. If we were to take an aircraft trimmed for straight and level cruise flight, it would suddenly pitch up and at the same time follow a downward trajectory, the chord line and flight path/relative wind would be moving away from one another, so the AoA would be increased.

However I fail to visualize how the AoA would be increased in an updraft. The aircraft would pitch down while at the same time following an upward trajectory, wouldn't this bring the chord line and flight path/relative wind closer to each other and therefore reduce the AoA?

Thanks!

I would think if someone said that, that they meant the updraft increased + AOA, and downdraft increased negative AOA, as in more negative(from above the wing).

 

[MauleSkinner]

However I fail to visualize how the AoA would be increased in an updraft. The aircraft would pitch down while at the same time following an upward trajectory, wouldn't this bring the chord line and flight path/relative wind closer to each other and therefore reduce the AoA?

Thanks!

You're assuming that the pitch "correction" of the aircraft is instantaneous...it isn't. There's still inertia to overcome.

 

[MrManH]

I would think if someone said that, that they meant the updraft increased + AOA, and downdraft increased negative AOA, as in more negative(from above the wing).

Funny you say that because whenever I draw it down to illustrate the chord line going down and the relative wind going up, the two always cross and I'm indeed seeing a negative AoA.

This is the correct explanation for why changes in airspeed produce nose-up or nose-down pitching moments. A sudden increase in airspeed causes the wing to produce more upward lift, and the horizontal stabilizer to produce more downward force, but the horizontal stabilizer wins due to the greater lever arm.

I am not sure the same is true for a sudden downdraft. However, if I were to assume you to be correct, I guess the answer to your original question is that an updraft increases AoA instantaneously by virtue of the updraft itself, whereas a downdraft does not increase AoA until the aircraft reacts aerodynamically, which is not instantaneous due to inertia.

How would the AoA be increased though since the updraft lifts the tail from below which results in the nose dropping down?

You're assuming that the pitch "correction" of the aircraft is instantaneous...it isn't. There's still inertia to overcome.

If inertia is a factor, I'd say it applies more to the direction of flight (forward motion of the airplane) than it does to how the airplane is balanced in flight. I'd see the pitch disturbance as ~ instantaneous. It wouldn't be any different than suddenly pulling back or pushing on the control wheel, the airplane responds immediately as far as I can tell when flying.

 

[MauleSkinner]

If inertia is a factor, I'd say it applies more to the direction of flight (forward motion of the airplane) than it does to how the airplane is balanced in flight. I'd see the pitch disturbance as ~ instantaneous. It wouldn't be any different than suddenly pulling back or pushing on the control wheel, the airplane responds immediately as far as I can tell when flying.

There's also rotational inertia...and the more mass exists at a distance from the CG, the more it takes to get rotation started.

But trust me...if you fly an airplane under a line of CB's at a little under rough air speed, you'll eventually get an updraft that will beep the stall warning.

 

[dtuuri]

...in a downdraft. ...the chord line and flight path/relative wind would be moving away from one another, so the AoA would be increased.

I see the AoA decreasing at the same time as the negative AoA on the stabilizer is increasing. The chord line is controlled by the latter, so it is inclined more upward. The relative wind has moved from ahead to coming from above, so the two are not moving away from each other.

...in an updraft. The aircraft would pitch down...wouldn't this bring the chord line and flight path/relative wind closer to each other and therefore reduce the AoA?

I see the AoA increasing at the same time as the negative AoA on the stabilizer is decreasing. The chord line is controlled by the latter, so it is inclined more downward. The relative wind has moved from ahead to coming from below, so the two are not moving away from each other. So, yes.

dtuuri

 

[MrManH]

I see the AoA decreasing at the same time as the negative AoA on the stabilizer is increasing. The chord line is controlled by the latter, so it is inclined more upward. The relative wind has moved from ahead to coming from above, so the two are not moving away from each other.


I see the AoA increasing at the same time as the negative AoA on the stabilizer is decreasing. The chord line is controlled by the latter, so it is inclined more downward. The relative wind has moved from ahead to coming from below, so the two are not moving away from each other. So, yes.

dtuuri

I think this highlights a fundamental flaw in the way I look at relative wind. Relative wind to me has always been parallel and opposite to the direction of flight, no exceptions; in other words they are the same thing just going opposite ways. What I infer from your logic is that when the wind/air mass suddenly changes direction, it's not before things settle that the direction of flight and relative wind become parallel again?

And to clarify, are you agreeing that in an updraft, the AoA is reduced?

 

[dmspilot]

How would the AoA be increased though since the updraft lifts the tail from below which results in the nose dropping down?

I explained that in Post 2. The tail altering the pitch of the airplane happens later.

I think this highlights a fundamental flaw in the way I look at relative wind. Relative wind to me has always been parallel and opposite to the direction of flight

The direction of flight is with respect to the airmass. An airplane flying straight and level in an updraft has a direction of flight downward through the air, even though it is not descending.

 

[dtuuri]

I think this highlights a fundamental flaw in the way I look at relative wind. Relative wind to me has always been parallel and opposite to the direction of flight, no exceptions;

Right, no exceptions after the plane stabilizes from an upsetting force like a gust of wind relative to the original flight path.

in other words they [ed: direction of flight vs. relative wind] are the same thing just going opposite ways. What I infer from your logic is that when the wind/air mass suddenly changes direction, it's not before things settle that the direction of flight and relative wind become parallel again?

Agreed.

And to clarify, are you agreeing that in an updraft, the AoA is reduced?

The immediate disturbance causes an imbalance, ie, the horizontal tail was keeping the wing at a trimmed AoA then the upward gust both increased the AoA of the wing and reduced the negative AoA of the tail causing the plane to try to reestablish the trimmed condition by pitching down. Did the wing's AoA reduce? From the original, no. From the effect of the gust, yes. Did it take more than an instant? Probably not if the design is proper.

dtuuri

 

[coloradobluesky]

When the speed of air is accelerating that makes a difference from steady state flight. Once stabilized, the up or downdraft doesn't matter in the planes frame of reference to it. Its the same as a crosswind on a a long cross country. The plane doesn't "see" any crosswind, just changes in wind. It is only relative to the ground that a plane has a crosswind. Now if a gust comes along, that changes things. The plane reacts to a gust because it is accelerating or deccelerating wind.

Its a frame of reference problem.

 

[MrManH]

Right, no exceptions after the plane stabilizes from an upsetting force like a gust of wind relative to the original flight path.


Agreed.


The immediate disturbance causes an imbalance, ie, the horizontal tail was keeping the wing at a trimmed AoA then the upward gust both increased the AoA of the wing and reduced the negative AoA of the tail causing the plane to try to reestablish the trimmed condition by pitching down. Did the wing's AoA reduce? From the original, no. From the effect of the gust, yes. Did it take more than an instant? Probably not if the design is proper.

dtuuri

I'm having a hard time understanding this. I just drew something to illustrate how I visualize it, do you agree? In both cases I see the AoA increased.

My drawing illustrates the AoA before the airplane has time to react where the wind suddenly meets the wing at a different angle. I would suppose that very quickly the pitch of the airplane would change due to the imbalance on the horizontal stabilizer, the airplane would pitch into the relative wind and the AoA would be reduced. However there is that split second where the AoA will have been increased momentarily. It also seems that in terms of load factor in this example of straight and level cruise flight, there would be more to fear from a sudden updraft than downdraft.

 

[dtuuri]

I'm having a hard time understanding this. I just drew something to illustrate how I visualize it, do you agree? In both cases I see the AoA increased.
...
It also seems that in terms of load factor in this example of straight and level cruise flight, there would be more to fear from a sudden updraft than downdraft.

See attached. The resultant airflow between the forward motion and gust increases AoA in an updraft and decreases it in a downdraft.

​

Airplanes only have roughly one-half the strength in negative load. Fear a strong downdraft more, I'd think.

dtuuri

 

[MrManH]

Ah yes I see what I missed, the forward motion of the airplane due to inertia changes the relative wind even if the gust came down at exactly a 90 degree angle, correct?

Aside from altitude concerns though, what is there to fear from a downdraft if it doesn't increase the AoA and therefore doesn't increase the load factor?

 

[dmspilot]

Aside from altitude concerns though, what is there to fear from a downdraft if it doesn't increase the AoA and therefore doesn't increase the load factor?

It can increase the load factor in the negative direction, i.e. put negative G-load on the aircraft.

14 CFR 23.333 is kind of interesting in this regard:

§23.333 Flight envelope.

(a) General. Compliance with the strength requirements of this subpart must be shown at any combination of airspeed and load factor on and within the boundaries of a flight envelope (similar to the one in paragraph (d) of this section) that represents the envelope of the flight loading conditions specified by the maneuvering and gust criteria of paragraphs (b) and (c) of this section respectively.
(b) Maneuvering envelope. Except where limited by maximum (static) lift coefficients, the airplane is assumed to be subjected to symmetrical maneuvers resulting in the following limit load factors:
(1) The positive maneuvering load factor specified in §23.337 at speeds up to VD;
(2) The negative maneuvering load factor specified in §23.337 at VC; and
(3) Factors varying linearly with speed from the specified value at VC to 0.0 at VD for the normal and commuter category, and −1.0 at VD for the acrobatic and utility categories.
(c) Gust envelope. (1) The airplane is assumed to be subjected to symmetrical vertical gusts in level flight. The resulting limit load factors must correspond to the conditions determined as follows:
(i) Positive (up) and negative (down) gusts of 50 f.p.s. at VC must be considered at altitudes between sea level and 20,000 feet. The gust velocity may be reduced linearly from 50 f.p.s. at 20,000 feet to 25 f.p.s. at 50,000 feet.
(ii) Positive and negative gusts of 25 f.p.s. at VD must be considered at altitudes between sea level and 20,000 feet. The gust velocity may be reduced linearly from 25 f.p.s. at 20,000 feet to 12.5 f.p.s. at 50,000 feet.
(iii) In addition, for commuter category airplanes, positive (up) and negative (down) rough air gusts of 66 f.p.s. at VΒ must be considered at altitudes between sea level and 20,000 feet. The gust velocity may be reduced linearly from 66 f.p.s. at 20,000 feet to 38 f.p.s. at 50,000 feet.
(2) The following assumptions must be made:
(i) The shape of the gust is—

View or download PDF
Where—
s = Distance penetrated into gust (ft.);
C = Mean geometric chord of wing (ft.); and
Ude = Derived gust velocity referred to in subparagraph (1) of this section.
(ii) Gust load factors vary linearly with speed between VC and VD .
(d) Flight envelope.

 

[MrManH]

Thanks, so the moment the relative wind comes from above the chord line, the AoA is said to be negative?

 

[MAKG1]

Aside from altitude concerns though, what is there to fear from a downdraft if it doesn't increase the AoA and therefore doesn't increase the load factor?

Well, one obvious thing to fear is that the ground is always down.

 

[MrManH]

Well, one obvious thing to fear is that the ground is always down.

Yeah that's one of the things I meant by altitude concerns.

 

[Cooter]

Thanks, so the moment the relative wind comes from above the chord line, the AoA is said to be negative?

Yes. The angle between the chord line and the relative wind is the Angle of Attack. But keep in mind, the AOA doesn't need to be negative to allow a downward force to be imparted to the aircraft. Anytime the relative wind changes changes lift will change as well. So, a downdraft may only decrease AOA from 15deg to 5 deg, but that will result in less lift from the wing and a downward force, i.e. gravity. (assuming straight and level flight).

I didn't read through all of the thread but what seemed to be one of your initial misunderstandings was the effect of a downdraft. The force is applied as a resultant vector, which his diagram showed. It looks like you got that, if I read it right. So, essentially you are only changing AOA by a few degrees in most circumstances. I don't buy into the whole notion of a vertical force being applied to both surfaces. If I understand it correctly, the pitch changes will be a result of changes in lift, and the distance between center of lift and center of gravity.