22
$\begingroup$

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!

$\endgroup$
12
  • 5
    $\begingroup$ I always figured it was because the stall reduces lift, so the aircraft begins to fall, so the angle of attack increases, causing the tail lift to become less negative / more positive. Not that I know what I’m talking about; just another folk explanation to add to your list, I guess. $\endgroup$
    – zmccord
    Commented Nov 27, 2022 at 4:43
  • 13
    $\begingroup$ They pitch down because that's what they're designed to do. $\endgroup$ Commented Nov 27, 2022 at 17:17
  • 5
    $\begingroup$ @MikeY Yes, of course they're designed like that on purpose. My question isn't "What purpose does it serve to pitch nose-down during a stall?" but rather "What aerodynamic principles cause the nose to pitch down?" $\endgroup$
    – Ethan B
    Commented Nov 27, 2022 at 22:33
  • 2
    $\begingroup$ It's actually interesting to ask the opposite question, "How would I design a reasonably flyable plane so the nose wouldn't pitch down upon wing stall." $\endgroup$
    – MikeY
    Commented Nov 27, 2022 at 22:41
  • 2
    $\begingroup$ Note that the center of pressure is also aft of the center of gravity. If you imagine a dart or arrow with a heavy point and some kind of fletching or vanes at the back, it will always want to travel through the air point first. Similar principle to a weather vane. Another way to think about it is that lift is not the only aerodynamic force on a plane. When there is no lift, the other forces remain. $\endgroup$ Commented Nov 28, 2022 at 2:28

8 Answers 8

31
$\begingroup$

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.

$\endgroup$
8
  • 1
    $\begingroup$ I'm marking this as the answer because it's concise and makes a ton of sense. Honorable mention to @Robert-DiGiovanni, whose answer I believe corroborates this one and adds additional context and info. Thanks! $\endgroup$
    – Ethan B
    Commented Nov 27, 2022 at 20:08
  • 1
    $\begingroup$ @EthanB and Sanchises This is not the whole story: yes, the airplane goes down i.e. the horizontal stabiliser sees a greater AoA but! the aircraft is also pitching nose-down i.e. tail-up and that actually decreases the AoA of the horizontal stabiliser. So there are actually two opposing effects in action. $\endgroup$
    – sophit
    Commented Nov 28, 2022 at 7:37
  • $\begingroup$ @sophit Exactly, once you are properly nose down the tail moment is reduced and the rotation is arrested. $\endgroup$
    – Sanchises
    Commented Nov 28, 2022 at 8:25
  • $\begingroup$ @EthanB and Sanchises: yes correct, I just wanted to highlight that the horizontal stabiliser is not the cause of the pitching down but actually the cure $\endgroup$
    – sophit
    Commented Nov 28, 2022 at 8:30
  • $\begingroup$ @sophit Hm I find that hard to believe but I don't have the numbers on hand either. You'd think a deep stall would be impossible if the main wing pitching moment would be enough to put the nose down, though. $\endgroup$
    – Sanchises
    Commented Nov 28, 2022 at 11:56
14
$\begingroup$

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.

$\endgroup$
2
  • $\begingroup$ This answer addresses the cause-and-effect mixup that seems to have prompted the question. There is nothing about stalling that inherently causes a nose-down moment; the fact is that pushing the nose down is the fastest and most reliable way to recover from a stall, so designers make sure that their aircraft will naturally do that as a result of stall, so that their aircraft help the pilot to recover rather than fighting. It's possible to design an aircraft that doesn't pitch on stall, or pitches up, but it would be very dangerous, so designers (usually) just don't. $\endgroup$ Commented Nov 29, 2022 at 15:37
  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Ralph J
    Commented Dec 3, 2022 at 6:56
9
$\begingroup$

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.

$\endgroup$
3
  • 2
    $\begingroup$ Possibly worth adding a link to en.wikipedia.org/wiki/Stall_(fluid_dynamics)#Deep_stall $\endgroup$ Commented Nov 27, 2022 at 8:04
  • 2
    $\begingroup$ You don't have to design aircraft like this. You can design aircraft to do nearly anything in a stall. It's just that any option other than pitching down is incredibly dangerous (and you will not get a flight cert/flight clearance). $\endgroup$
    – fectin
    Commented Nov 27, 2022 at 22:10
  • 1
    $\begingroup$ Why is CG in front of lift? "To provide the necessary balance between longitudinal stability and elevator control, the CG is usually located slightly forward of the center of lift. This loading condition causes a nose-down tendency in flight, which is desirable during flight at a high AOA and slow speeds." - it's an artifact of the design that also keeps the plane from flipping over in the first place. "Putting [the CoL] further behind increases stability. The closer to the CoG it is, the more maneuverable the craft becomes." $\endgroup$
    – Mazura
    Commented Nov 28, 2022 at 2:50
6
$\begingroup$

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:

  1. 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;
  2. 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):  pitching moment coefficient NACA 2412 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).

 lift coefficient NACA 2412  drag coefficient NACA 2412

$\endgroup$
1
  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Ralph J
    Commented Dec 3, 2022 at 6:58
6
$\begingroup$

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.

$\endgroup$
1
  • 4
    $\begingroup$ I think this is the most appropriate answer. $\endgroup$
    – copper.hat
    Commented Nov 29, 2022 at 9:14
5
$\begingroup$

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.

$\endgroup$
1
  • 2
    $\begingroup$ This is the only correct answer in the whole list. Stalls do not cause the nose to drop. They cause a rise in nose position, (relative to aircraft flight path vector.). It only seems like there is a connection because most of the time we experience stalls from level flight, and the much larger drop in nose attitude due to the downward shift in the aircraft flight path as it drops is much more apparent. $\endgroup$ Commented Aug 20, 2023 at 16:50
5
$\begingroup$

"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.

$\endgroup$
4
  • 1
    $\begingroup$ Well, among the plethora of answers (mine included) this is actually the more logical: they do not full stall in the first place :) $\endgroup$
    – sophit
    Commented Nov 30, 2022 at 13:56
  • $\begingroup$ I finish my comment: they do not deep stall in the first place if they are correctly designed and unless the pilot do so on purpose. $\endgroup$
    – sophit
    Commented Nov 30, 2022 at 15:05
  • $\begingroup$ To clarify my original question slightly, I didn't mean to suggest that the nose necessarily pitched down below the horizon, just that there was typically a pitching moment in the nose-down direction. So "dropping the nose to level" would still be an example of what I'm asking about, if the plane had previously been in a 25 degree nose up attitude to bring about the stall. $\endgroup$
    – Ethan B
    Commented Nov 30, 2022 at 15:32
  • $\begingroup$ @EthanB - It is not something you should rely on. Some aircraft would be just as happy to slip or spin in that situation rather then gently point their nose at or below the horizon. I should have emphasized "usually" instead of "fully-developed". $\endgroup$
    – Paul Smith
    Commented Dec 1, 2022 at 11:15
-4
$\begingroup$

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.

$\endgroup$
2
  • $\begingroup$ "It is normal to nose dive" does not answer why the dive happens, which is the question. $\endgroup$ Commented Mar 7 at 17:48
  • $\begingroup$ Because the Cg is forward on the aircraft , it is stated in my answer. $\endgroup$
    – George Geo
    Commented Mar 13 at 9:36

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .