Why do airplanes usually pitch nose-down in a stall?

Question

Why do airplanes usually pitch nose-down in a fully-developed stall?

I've seen this seemingly-simple question discussed on other aviation forums, but there doesn't seem to be a single agreed-upon answer, at least that covers all or most cases.

Here is why I find this question confusing: When we say that airplanes are usually "nose-heavy," what we mean is that the center of lift is aft of the center of gravity. So the lift generated by the wings, in addition to counteracting weight, also produces a pitching moment, rotating the aircraft about its lateral axis, nose-down / tail up. This pitching moment is counteracted by the horizontal stabilizer (or canard wings) to maintain a level attitude.

In a stall, the wings produce significantly less lift. So my intuition would be that the nose-down moment would also be significantly reduced. And indeed, if the wings stalled before the tailplane, I would expect the downforce produced by the tailplane to be dominant, and the nose to pitch even further up. This is obviously contrary to experience.

Here are some of the answers I've seen proposed elsewhere on the internet:

• "The CG is heavy so it falls down."

This doesn't seem like a very good answer. It's not just that the airplane is falling, it's that it's rotating, and all rotation happens about the CG. So the portion of the airplane that is pitching downward is the portion that is forward of the CG, and the portion that is pitching upward is the portion that is aft of the CG. What causes this pitching moment, as opposed to the airplane falling in its previous nose-up stalled attitude?

• "The plane would fall in its previous attitude if it were in a vacuum, but because the engine is heavy, it has more inertia and is less affected by air resistance, so it falls faster."

Maybe? I don't know enough to know whether or not this is right.

• "A stalled wing still produces some lift, and as it stalls, the center of lift moves aft. The greater arm results in a greater forward-pitching moment. So it is the 'nose-heaviness' being exaggerated in the stall."

This also seems plausible, but I don't know. Since stalls and stall recovery are so fundamental to basic flight instruction, I would expect this question to have a straight-forward answer, but maybe it just doesn't? Or maybe there's something I'm missing. Curious to hear people's thoughts.

Thanks!

Answer 1

You are quite close to the answer

And indeed, if the wings stalled before the tailplane, I would expect the downforce produced by the tailplane to be dominant, and the nose to pitch even further up. This is obviously contrary to experience.

Your assumption is that the tail keeps producing downforce. However, if the main wing stalls, the relative airflow changes. The plane goes down, so the relative airflow suddenly comes from below instead of straight ahead. The angle of attack on the tailplane increases from slightly negative (downforce) to positive. This obviously rotates the plane nose down.

Answer 2

A properly designed aircraft must pitch down in a stall in order to lower the angle of attack of the wing, re-establishing the lift produced by the upper wing.

When an aircraft stalls, the bottom of the wing still produces some lift by simple action/reaction. The center of pressure moves back towards the center of the wing. The top of the wing is kaput because flow has separated. So this mechanism and the loss of downwash on the tailplane drops the nose.

Huge amounts of forward CG are not required for this to work.

However, rotation of the nose to reduce wing AoA is a race against the aircraft's increasing vertical drop velocity, which changes the wings relative wind to favor higher AoA.

With a tiny paper airplane, rotation easily wins because the surface area to mass ratio is very high. With a huge swept wing airliner, there is more of a tendency for the relative wind (from the drop) to change faster than the plane can rotate.

thus the legend of forward CG is born

However, the combination of rearward shift of Cp on the wing combined with reduced downforce from the tail causes the nose to drop.

relaxing elevator back pressure helps the tailplane do its job

Now we come to the dangers of abusing the rearward limit of CG.

if both the wing and rear tail plane stall, you may be in big trouble

Setting the rear tailplane "decalage" to a lower AoA than the wing allows the main wing to stall first. However, if CG is too far back, the tailplane must produce upforce.

This creates the very real possibility that a sufficient change in relative wind can cause both wing and tailplane to stall.

if both airfoils stall, the center of vertical drag is all that is left to rotate the nose down, which is why "old time" aircraft have large, low aspect delta shaped horizontal stabilizers.

Stall behavior and recovery technique varies with aircraft, but abuse of CG limits is universally dangerous.

Answer 3

It's by design so that the aircraft can recover from a stall. So that it is possible to recover from a stall, the CG of the aircraft is placed ahead of the center of lift. Suppose the CG was too far aft, the plane could fall flat (flat spin?), or be unrecoverable after a stall. There are aircraft with poor stall tendencies. A stall is a failure condition, but you do not want it to be unrecoverable.

Answer 4

The wing of a GA airplane is designed so that the root of the wing (i.e. the part of the wing close to the fuselage) stalls before the wing's tips. These has mainly two positive effects:

1. when the root stalls, its turbulent wake impinges on the tailplane shaking it and warning the pilot of the incipient stall;
2. the outer part of the wing keeps on lifting and the ailerons keep on working.

Anyway to answer the question at least two other phenomenon should be taken into account:

3. in normal flight (i.e. not at stall) the airflow seen by the horizontal stabiliser differs from the one seen by the wing due to the downwash of the wing which decreases the AoA of the horizontal stabiliser. That means that the horizontal stabiliser has to be set to a higher AoA than if it were isolated;
4. we are all well aware of lift and drag coefficients (at the end of this answer I posted the plots of $C_l$ and $C_d$ for a classical airfoil like the NACA 2412, used for example for the wing of the Cessna 172). Anyway lift and drag do not tell the whole story: to completely describe the aerodynamic characteristics, a third plot is needed, which is as important as the other two but which is normally ignored: the plot of the pitching moment. For the same NACA 2412 it looks something like that (underlined in green): What does this plot tell us? The pitching moment is normally negative i.e. nose-down. And that's why the horizontal stabiliser has to normally produce a downward lift i.e. a nose-up moment. But what happen at stall? As soon as the stall region is reached (at some 16°), the wing's pitching moment becomes suddenly almost five time more negative than just before the stall. This drop in $C_m$ sums up to a sudden drop in $C_l$ and rise in $C_d$.

Now that we have all the needed ingredients, understanding what happen approaching stall is quite easy. So, the root of the wing stalls and basically two main things happen:

• a) pitching-down moment of the wing's root increases as explained in 4.;
• b) the stalled wake from the wing impinges on the horizontal stabiliser shaking it as in 1. and, above all, this stalled wake doesn't generate downwash anymore as defined in 3.

The consequence of a) is that the stalled wing makes the aircraft pitch down; while the consequence of b) is that the AoA of the horizontal stabiliser increases, which lowers its downward lift, which makes the tail go up i.e. the aircraft pitch down. So, the main result of a stalled (root of the) wing is a pitch nose-down tendency.

If no corrective measures are taken, other effects follow:

1. lift drops... and the airplane as well;
2. drag rises; if the stall was entered because of a too slow velocity, higher drag will slow the airplane even more down.

Point 1. is actually not that bad as it might sound since it has two positive effects:

• the fact that the airplane falls down makes it gain speed again, counteracting the drag increase of 2;
• the horizontal stabiliser is (should be) designed to stall well after the wing so it is still producing lift and since the aircraft is falling down, the local AoA seen by the horizontal stabiliser increases again compensating for the decrease due to the loss of downwash as explained in b).

---

Here under the plot of $C_l$ (underlined in blue) and $C_d$ (in red).

Answer 5

A stall is a condition where the wing (or canard, if one is fitted) exceeds the linear lift curve slope so the lift growth with increasing angle of attack is reduced and eventually reversed.

In a longitudinally stable design the tail operates at a lower lift per area than the wing and sees less angle of attack change than the wing because of the wing's downwash. This means the tail will still be comfortably in the linear range when the wing already leaves it.

Now what happens is very simple: The balance of lift between wing and tail is shifted so the tail will produce relatively more lift (in excessively stable designs less downforce), the center of pressure moves backwards and the unchanged location of the center of gravity will pull the tail up.

This nose-down rotation will produce a decrease of the local angle of attack on the tail, but only when the nose-down movement is underway. This is simple pitch damping and helps to limit the rate of rotation.

Answer 6

Why do airplanes usually pitch nose-down in a fully-developed stall?

Because the flight path curves downward. It's really that simple. There's no inherent incompatibility between the idea that the angle-of-attack of the wing is staying at a very high value (i.e. the wing is stalled), and the idea that the nose is pitching downward.

Any discussion of any "imbalance" between the lift generated by the wing and the tail, is really only addressing pitch torque, which pertains to the rotational acceleration in the pitch axis, i.e. the rate of change of pitch rotation. But it's obvious that a non-zero rate of change of pitch rotation can only be sustained rather briefly. Otherwise the pitch rotation rate would quickly "ramp up" to an absurdly high value. And note that any pitch rotation automatically creates a "damping" effect that tends to alter the angle-of-attack of any surface located ahead or behind the CG, and this tends to counteract the pitch rotation-- for example, a nose-down pitch rotation makes the angle-of-attack of the tail surfaces become less positive or more negative.

So, over the long run, it's impossible to sustain an "imbalance" in "lift", or more precisely in the pitching moment generated around the CG, between the wing and the tail. The airplane will always find some angle-of-attack, and some pitch rotation rate, where there is no such imbalance. And so the changes in the direction of the flight path will usually be the primary driver of changes in the aircraft's pitch attitude.

Answer 7

"Why do airplanes usually pitch nose-down in a fully-developed stall?"

They don't.

Many GA aircraft are designed in a way that stretches incipient stalls out way longer or that induce incipient stalls much earlier as both options increase the pilots reaction window. Some GA aircraft are designed to induce pitch down during stalls to makes stall recovery much, much easier, but many will only drop the nose to level rather then actually pitch down. GA aircraft that are not designed to stall gracefully tend not to sell very well as they build a reputation for killing their pilots. Commercial aircraft (usually) have stall detection and warning systems so that the situations that could lead to stalls can be avoided. Actually stalling commercial aircraft outside a sim is not something that is done lightly, and as we have seen, the sim behavior often differed from real-world behavior with tragic consequence. Military and specialist aircraft have no rules.

Answer 8

The CG is in front of the wing (that's why),the CL is about 25 %Mac ( behind the CG )so THIS LOCATION OF THE CG MAKE the nose be pull down like a man tripping on side walk .Doing that the speed increases ,the AoA will decrease and laminar airflow above the top side of the wing is acquired.Pull up and fly again.