Aircraft pitching forward when gaining speed during takeoff but also forward when stalling?

Question

Aircraft with conventional landing gear are tilted back when at stop/low speeds on the ground, but then when they gain speed they pitch forward (so they're level). Is this because of increased speed and lift from the rear surfaces? I assume it's related to aerodynamics.

However when the aircraft is flying and stalls, doesn't it also pitch forward (nose down)?

This is confusing to me because why would it pitch backward (nose up) due to slow speeds on the ground but pitch forward (nose down) when at slow speeds in the air?

I obviously don't have any piloting experience. The reason I'm asking is I'm making an arcade-ish flying game and I'm using lift and gravity and other forces in the physics engine. I need to figure out why planes are behaving like that so I don't have to hardcode the plane on which way to pitch depending on whether it's on the ground or in the air.

Answer 1

For airplanes with conventional landing gear, their center of gravity is located behind the main gear. This is what allows them to sit on their tail on the ground. You are correct that as they gain speed the air flow over the tail eventually becomes enough to lift the tail and bring the plane into a more level attitude.

Once the airplane is in the air, the wing is what's supporting most of the weight, not the main landing gear. When a wing stalls, the center of pressure (where the remaining lift is being produced) moves backward. This pushes the tail up and the nose down. The stall also changes the air flow over the tail, reducing the downforce there and making it even easier for the tail to move up and the nose to move down. See the answers to this question for more detail on why airplanes tend to pitch down when they stall.

Answer 2

However when the aircraft is flying and stalls, doesn't it also pitch forward?

Yes correct, but for a very different reason.

Without entering in mathematical and aerodynamical details, when an airplane flies the following aerodynamic forces and moments act on it:

• Lift $L$ created by the wing and acting at a distance $X_L$ from the CG;
• Pitching-down moment $M$ created by the wing;
• Lift $T$ created by the tail and acting at a distance $X_T$ from the CG.

Lift $L$ and moment $M$ make the airplane pitch nose-down (just think the CG like if it were an hinge) so that a negative lift $T$ is normally required from the tail in order to give a counteracting (nose-up) pitching moment:

_{An A350-1000 in flight. Source, modified by me}

Now, when the airplane stalls this equilibrium is upset since:

• $L$ decreases;
• $M$ increases (normally quite a lot); and
• $T$ decreases.

The net effect of these modifications is an increase of the total pitching-down moment which is no more counteracted by the tail's pitching-up one.

I need to figure out why planes are behaving like that so I don't have to hardcode the plane on which way to pitch depending on whether it's on the ground or in the air.

I fear you have to differentiate between when the airplane is on ground and in air since the landing gears do change the behaviour quite a bit.

Answer 3

Why would it pitch backward (nose up) due to slow speeds on the ground but pitch forward (nose down) when at slow speeds in the air?

Short answer: Because the main landing gear gets in the way.