Why does an aircraft pitch up when the speed increases?
What you ask is: If a model glider is thrown at a higher speed than its trim speed, why does it pitch up?

Short answer: Because the rear horizontal surface produces less lift per area than the forward surface. When flying at a speed different from its trim speed, the combined center of lift of all surfaces is shifted such that it creates a pitching moment around the center of gravity. This pitching moment causes the flight path change.

Note that I try to explain things in terms that work equally well for canards or even flying wings. That may sound odd in places, but needs only one explanation for all cases. For unswept flying wings read forward wing = forward part of the airfoil and vice versa.

Long answer: Let's assume an airplane that is rigged for flight at an angle of attack of 9°. For simplicity, let's assume that both airfoils are symmetric and both surfaces have the same lift curve slope, for simplicity 0.1 per degree. The local incidence is 0° on the forward and -5° on the rear wing. Neglecting downwash effects, this results in a lift coefficient on the forward wing of 0.9 and on the rear wing of 0.4. The zero-lift angle of attack is +1° when the forward wing lift coefficient is at 0.1 and the rear wing lift coefficient is at -0.4.

Let's further assume that the tail surface has 25% of the area of the forward surface. Lift on the forward wing is 90% of weight and on the rear wing it is 10% of weight. The center of lift, therefore, sits at 10% of the line connecting both neutral points and that is where the center of gravity is as well for trimmed flight. Like this:



Now the model is thrown at twice its trimmed speed. Let's assume that the angle of attack is also 9°, but that does not matter much. Lift on the forward surface is now 360% and on the rear surface 40% of weight. Again both forces combine for a common center of lift at 10%, so no pitching moment is created. But lift far exceeds weight, so the aircraft immediately climbs. Climb without pitching motion means that the angle of attack decreases immediately. Therefore, the aircraft will slightly accelerate upwards and settle at a new, lower angle of attack where the combined lift is equal to weight. But how is lift now distributed?

To have lift decrease to a quarter needs an angle of attack change to 25% of its initial value, relative to the zero-lift angle of attack. For this, the new angle of attack on both surfaces has to decrease by 6°. The local incidences result in an angle of attack of 3° on the forward and -2° on the rear wing and lift coefficients of 0.3 and -0.2, respectively. Again neglecting downwash effects, the new lift contributions are 120% on the forward wing and -20% on the rear wing. Like this:



Now the center of lift is 20% of the distance between the two wing's neutral points ahead of the forward wing's neutral point and 30% ahead of the center of gravity. That causes a strong nose-up pitching moment which will let the aircraft pitch up. Together with the initial flight path change of 6° for lift correction this will make the airplane climb until its flight speed drops below the trimmed condition and the condition reverses. Since pitch damping is high, very few cycles are needed to arrive at the trimmed condition but at a height over the launch point which corresponds to the extra energy provided by the high launch speed.

This can be run with different numbers and will work regardless of lift or downforce on the rear surface, provided that lift per area is lower there than on the forward wing.