Why does an aircraft pitch up when the speed increases?

Question

Can anyone please explain why aircraft pitches up when the speed increased? (please consider an aircraft, Wing AC, CG and Tail AC lies on a line.)

The ideal explanation I'm looking for should have something to do with the static margin.

Here I am talking about a powered or unpowered aircraft, trimmed for steady flight, reacting to an EXTERNAL velocity perturbation which increases the airspeed. (Like a sudden, substantial, sustained increase or decrease in the speed of the wind, e.g. due to wind shear.)

I have seen that when faced by this kind of a perturbation, the aircraft tries to slow down/keep the airspeed unchanged by increasing the pitch angle. My question is how it happens

Peter Kampf wrote:

However, I can imagine what might have happened to let you observe a pitch-up. This needs several conditions: A propeller-diven airplane with the propeller in the front A sufficiently large static margin so the empennage produces a down-force. Speeding up by opening the throttle.

Let me clarify these are not necessarily the cases for what I'm asking, 1. If we throw a glider hard enough, which is trimmed for a certain airspeed it will pitch up and climb. 2. At least for now I think it doesn't matter whether the tail produces down-force or upward force. Say the velocity is increased by factor of 2 and the forces on both wing and tail increased by a factor of 4. Equilibrium is still maintained as moment around CG has not changed by this. (and this is what stands against my observation)

Mike Sowsun wrote:

The horizontal stabilizer always provides a downward force to balance the forces of lift and weight with the center of gravity. This also provides stability because if the aircraft pitches down and starts to speed up, the increased airflow over the tail will result in more downward force and cause the nose to rise and the aircraft to slow.

I think there is no need for the tail to make downward lift always. The tail can be uplifting as well. Anyway, if we agree that to be the case for now, when the aircraft gains airspeed, the flow speed over the wings increases as well as tails. Isn't it? What I can't understand is, what makes the aircraft pitch up when both the wing force and tail force increased by the same factor, due to increase of airspeed.

Any further input is much appreciated.

Answer 1

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.

Answer 2

The horizontal stabilizer always provides a downward force to balance the forces of lift and weight with the centre of gravity. This also provides stability because if the aircraft pitches down and starts to speed up, the increased airflow over the tail will result in more downward force and cause the nose to rise and the aircraft to slow.

As the aircraft further slows the decreased airflow over the tail will cause the nose to drop and the airspeed to increase again.

This pattern will then continue in a "Phugoid motion"

Answer 3

The aircraft will stay at the trimmed angle of attack with an increase in airspeed but will accelerate upward due to the increase in lift,which is dependent on airspeed (there is no pitch-up, which is a misnomer for this case- pitch-up being defined as an increase in AoA). As the craft rises up, it continues in a constant AoA curve until the longitudinal speed decays to zero, at which point it falls down of course- causing an abrupt change in the wing angle of attack which results in a stall. As the craft tries to regain the trim AoA due to its inherent stability, its motion becomes a series of climb, stall, and descent oscillations, due to attempts to damp out AoA overshoots in the process. Note that with sufficient initial launch speed it will continue in its constant AoA curve in a loop which will continue on the reverse side until its trim airspeed is once again regained- without any series of climbs and stalls-since the AoA was maintained in the first place.

The explanation here is for a free-flight model with preset control surfaces, no power or constant- thrust power.

Answer 4

I think I've got the answer. The key is, When the speed increased, it appears to the aircraft that the angle of attack has decreased.

Once the angle of attack decreased, the rest works exactly as normal longitudinal stability case. http://adg.stanford.edu/aa241/stability/staticstability.html

Answer 5

• You ask "What I can't understand is, what makes the aircraft pitch up when both the wing force and tail force increased by the same factor, due to increase of airspeed."

• It is critical to understand that an imbalance in pitch moment-arm between the wing and the tail is only required to cause a CHANGE in the rate of pitch rotation, not to cause a pitch rotation. In general, you aren't going to be able to account for pitch rotation by looking for an imbalance in pitch moment-arm between the wing and the tail. Any explanation that involves an imbalance in pitch moment-arm between the wing and the tail, is getting down into the nitty-gritty details about the cause of a CHANGE in the pitch rotation rate, and how this CHANGE in pitch rotation rate then restores the balance in pitch moment-arm between the wing and tail.

• Taking a larger view, it may be sufficient to simply understand that to a first approximation, an aircraft tends to trim to a constant angle-of-attack, because any deviation from the trim angle-of-attack tends to set up an imbalance in pitch moment-arm between the wing and the tail, which creates a pitch torque that changes the pitch rotation rate, which leads to a change in angle-of-attack. (Later we'll see why this statement is only an approximation-- why an aircraft deviates somewhat from the trim angle-of-attack during a pitch "phugoid" oscillation.)

• What you are asking about is called "speed stability".

• Many attempted explanations of this phenomenon suffer from the following flaw-- they suggest that if we add weight at the CG of a glider, this will cause the glider to trim not only to a higher airspeed, but also to a different angle-of-attack. This is not accurate.

• Another common flaw in explanations of this phenomenon is an implicit assumption that the flight path will initially remain horizontal after a sudden increase in airspeed (which implies that that the angle-of-attack must be well below the trim angle-of-attack), until the aircraft starts to pitch up.

• Let's see if we can offer an explanation of "speed stability" that does not suffer from either of these flaws.

• Imagine the aircraft is flying into the wind, and the wind suddenly increases by 20 mph.

• Ultimately, the plane may return to equilibrium at its original airspeed and a lower groundspeed.

• However, what happens in the short term due to the instantaneous 20 mph increase in airspeed?

• To a first approximation, the aircraft tends to maintain its trimmed angle-of-attack. Drag is greater than thrust, so airspeed is decreasing, but is still higher than the trimmed airspeed.

• The excess airspeed is creating excess lift, so the flight path starts to curve upward. The excess lift is acting as a "centripetal force" and forcing the flight path to curve.

• As the aircraft tends to maintain its trimmed angle-of-attack and the flight path starts to curve upward, the nose must rise.

• This could be the end of the answer right here. But in case you want to know what happens next...

• As flight path curves upward, gravity gains a component acting parallel to the drag vector and against the thrust vector, further contributing to the rate of decrease in airspeed.

• At some point in the climb, as the airspeed continues to decrease, causing the lift vector to continue to decrease, the force acting "upward" relative to the flight path (i.e. the lift vector) becomes less than the force acting "downward" relative to the flight path (i.e. a component of the weight vector). At this moment the flight path stops curving upward and starts curving downward. The nose starts to fall back toward the horizon, and then falls below the horizon, even as the aircraft is still approximately maintaining its trimmed angle-of-attack.

• When the flight path curves below horizontal, gravity gains a component acting parallel to the thrust vector and against the drag vector. Shortly before this point is reached, the balance of fore-and-aft forces in the aircraft's reference frame must be such that the airspeed will start increasing again.

• Eventually the rising airspeed will increase the lift vector to the point where the flight path stops curving downward and starts curving upward again, as the cycle continues. The whole cycle is known as a pitch "phugoid" oscillation.

• The complete process of returning to equilibrium may involve several, or many, slowly-decreasing cycles of the pitch "phugoid" oscillation.

• A complete explanation must recognize that in actual practice the angle-of-attack does not stay absolutely constant throughout the pitch "phugoid", both due to rotational inertia in the pitch axis, and due to aerodynamic effects caused by the curving flight path and the resulting curvature in the undisturbed relative wind, or to put it another way, aerodynamic damping in the pitch axis. The angle-of-attack tends to be highest near each peak in altitude, and tends to be lowest near each low point in altitude, due to the direction that the flight path is curving at each of these points. (In some aircraft this can be demonstrated by setting up the initial conditions so that the stall horn sounds near each of the altitude peaks, even as the pilot keeps his hands off the controls.) This is not the fundamental driver of the pitch phugoid; in many cases we'd still see a rather similar pitch phugoid even if we manipulated the controls to keep the angle-of-attack exactly constant. However in extreme cases, we can get a full-blown stall break near each of the altitude peaks, in which case the oscillation is unlikely to damp out.

• And here's an even more extreme case to consider: if we trim an aircraft for level flight at 50 knots, and then we dive to 100 knots and then quickly pull out into level flight and release controls to allow the aircraft to return to the trim angle-of-attack, the situation may be so extreme that the flight path may curve well past vertical into semi-inverted flight, or may even describe a complete loop, or the aircraft may run out of airspeed going nearly straight up and violently "whip stall". So there's a limit to the situations that we can expect will lead to a gentle, tame, pitch "phugoid" that smoothly damps out.

• You may be hoping for more detail on exactly why the airplane tends to maintain its trimmed angle-of-attack as the airspeed varies, and also why in actual practice we do see a some deviation from this principle as described above. These are not a simple topics. A good explanation of the basic tendency to maintain a trimmed angle-of-attack may be found in this section of John S. Denker's excellent "See How It Flies" website-- "6 Angle of Attack Stability, Trim, and Spiral Dives" -- https://www.av8n.com/how/htm/aoastab.html . Decalage is key, but the horizontal stabilizer need not actually create a downforce.

• Propwash is another factor that complicates (interferes with) an aircraft's tendency to trim to the same angle-of-attack regardless of airspeed; nonetheless most single-engined powered planes with tractor propellers, jets, gliders, and aircraft of other configurations all behave in GENERALLY the same way, and examples of the dynamics described in this answer can be observed in all of them.

Answer 6

I think it has a very simple logic As static margin is the distance between the center of gravity and the neutral point of the aircraft.Whenever thrust is increased the wings of aircraft provide an upward thrust due to the design and pressure difference.This thrust is then accompanied by the static margin affect keeping an unbalancing force on the rear portion resulting into Pitch moment.