Return to Answer

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 posses any well defined 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).

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).

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

The answer is actually quite simple: airplanes do not pitch nose-down in stall... they basically always pitch nose-down (at least airplanes of conventional design and without an automatic stability system). That's why normally the horizontal stabiliser has to generate a downward lift in order to create a counteracting (nose-up) moment.

We are all well aware of lift and drag coefficients. For a classical airfoil like the NACA 2412 (used for example for the wing of the Cessna 172), $C_l$ (underlined in blue) and $C_d$ (in red) have the following trends:

Now, 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, that is, it is normally nose-down. And that's why, again, the horizontal stabiliser has to normally produce a downward lift i.e. a nose-up moment.

But what happen at stall? Just before of it (let's say from 8 to 16°) the pitching moment goes a bit up (toward zero). This seems to be a good news but it's not: incrementing AoA makes the wing pitch-down a bit less; less pitch-down means that the wing tends to go nose-up therefore incrementing its AoA. So in that region just before the stall, an increase in AoA generates an ever increasing AoA: a vicious circle has just started pushing the wing nearer and nearer to the stall region!

As soon as the stall region is reached (at some 16°), the wing's pitching moment becomes suddenly more and more negative, almost five time more negative than just before the stall! This drop in $C_m$ sums up to the sudden drop in $C_l$ and the rise in $C_d$. What does all that imply at stall?

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; anyway the fact that the airplane is falling down will make it gain speed again, counteracting the drag increase;
3. as just seen, the nose-down tendency increases making the nose of the airplane going down.

What about the horizontal stabiliser? It is (should be) designed to stall well after the wing so it is still producing lift but:

1. because the aircraft is falling down, the local AoA seen by the horizontal stabiliser increases; and the wing is producing also less lift i.e. less downwash: this reduced downwash also increases the local AoA on the horizontal stabiliser;
2. anyway because of the nose-down movement (i.e. tail-up movement), the local AoA decreases.

So the horizontal stabiliser will increase or decrease the nose-down tendency according to which of these two effects is predominant. I don't know which one is going to win but for sure by then the pilot will have already properly set the horizontal stabiliser to counteract the stall.

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 posses any well defined 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).

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

The answer is actually quite simple: airplanes do not pitch nose-down in stall... they basically always pitch nose-down (at least airplanes of conventional design and without an automatic stability system). That's why normally the horizontal stabiliser has to generate a downward lift in order to create a counteracting (nose-up) moment.

We are all well aware of lift and drag coefficients. For a classical airfoil like the NACA 2412 (used for example for the wing of the Cessna 172), $C_l$ (underlined in blue) and $C_d$ (in red) have the following trends:

Now, 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, that is, it is normally nose-down. And that's why, again, the horizontal stabiliser has to normally produce a downward lift i.e. a nose-up moment.

But what happen at stall? Just before of it (let's say from 8 to 16°) the pitching moment goes a bit up (toward zero). This seems to be a good news but it's not: incrementing AoA makes the wing pitch-down a bit less; less pitch-down means that the wing tends to go nose-up therefore incrementing its AoA. So in that region just before the stall, an increase in AoA generates an ever increasing AoA: a vicious circle has just started pushing the wing nearer and nearer to the stall region!

As soon as the stall region is reached (at some 16°), the wing's pitching moment becomes suddenly more and more negative, almost five time more negative than just before the stall! This drop in $C_m$ sums up to the sudden drop in $C_l$ and the rise in $C_d$. What does all that imply at stall?

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; anyway the fact that the airplane is falling down will make it gain speed again, counteracting the drag increase;
3. as just seen, the nose-down tendency increases making the nose of the airplane going down.

What about the horizontal stabiliser? It is (should be) designed to stall well after the wing so it is still producing lift but:

1. because the aircraft is falling down, the local AoA seen by the horizontal stabiliser increases;
2. anyway because the nose-down movement (i.e. tail-up movement), the local AoA decreases.

So the horizontal stabiliser will increase or decrease the nose-down tendency according to which of these two effects is predominant. I don't know which one is going to win but I really hope that by then the pilot will have already properly set the horizontal stabiliser to counteract the stall.

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

The answer is actually quite simple: airplanes do not pitch nose-down in stall... they basically always pitch nose-down (at least airplanes of conventional design and without an automatic stability system). That's why normally the horizontal stabiliser has to generate a downward lift in order to create a counteracting (nose-up) moment.

We are all well aware of lift and drag coefficients. For a classical airfoil like the NACA 2412 (used for example for the wing of the Cessna 172), $C_l$ (underlined in blue) and $C_d$ (in red) have the following trends:

Now, 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, that is, it is normally nose-down. And that's why, again, the horizontal stabiliser has to normally produce a downward lift i.e. a nose-up moment.

But what happen at stall? Just before of it (let's say from 8 to 16°) the pitching moment goes a bit up (toward zero). This seems to be a good news but it's not: incrementing AoA makes the wing pitch-down a bit less; less pitch-down means that the wing tends to go nose-up therefore incrementing its AoA. So in that region just before the stall, an increase in AoA generates an ever increasing AoA: a vicious circle has just started pushing the wing nearer and nearer to the stall region!

As soon as the stall region is reached (at some 16°), the wing's pitching moment becomes suddenly more and more negative, almost five time more negative than just before the stall! This drop in $C_m$ sums up to the sudden drop in $C_l$ and the rise in $C_d$. What does all that imply at stall?

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; anyway the fact that the airplane is falling down will make it gain speed again, counteracting the drag increase;
3. as just seen, the nose-down tendency increases making the nose of the airplane going down.

What about the horizontal stabiliser? It is (should be) designed to stall well after the wing so it is still producing lift but:

1. because the aircraft is falling down, the local AoA seen by the horizontal stabiliser increases; and the wing is producing also less lift i.e. less downwash: this reduced downwash also increases the local AoA on the horizontal stabiliser;
2. anyway because of the nose-down movement (i.e. tail-up movement), the local AoA decreases.

So the horizontal stabiliser will increase or decrease the nose-down tendency according to which of these two effects is predominant. I don't know which one is going to win but for sure by then the pilot will have already properly set the horizontal stabiliser to counteract the stall.