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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 turning into headwind.)

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.

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    $\begingroup$ I am not familiar with the term 'static margin', but a partial answer for the nose pitch up when airspeed increases is due to the increase in the downward or 'negative' lift over the horizontal stabilizer/elevator due to the higher airspeed. This causes a pivoting action around the center of pressure on the wing thus increasing the angle of attack and moving the CP forward, often, away from the CG. I know it's not the full answer but hopefully this will help. $\endgroup$ – Dawn Breaker Jul 2 '16 at 7:54
  • $\begingroup$ You are wrong. The tail must ALWAYS provide a down force in a conventional aircraft. That is why increased airspeed causes a pitch up. $\endgroup$ – Mike Sowsun Jul 2 '16 at 22:33
  • $\begingroup$ "this is what stands against my observation," what about the stabilizer's moment arm around the CoL? $\endgroup$ – ymb1 Jul 3 '16 at 14:45
  • $\begingroup$ @MikeSowsun: May I respectfully disagree? All what is needed for stability is less lift per area on the tail than on the wing. $\endgroup$ – Peter Kämpf Jul 3 '16 at 16:11
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    $\begingroup$ This question needs to be edited to remove the idea that turning into a headwind will tend to make the airspeed increase. $\endgroup$ – quiet flyer May 29 at 4:10
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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"

enter image description here

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  • $\begingroup$ The horizontal stabilizer produces less lift than the wing but not necessarily negative lift. And your answer does not take into account canard configuration (that works in a similar way) $\endgroup$ – Manu H Nov 7 '16 at 5:37
  • $\begingroup$ The horizontal stabilizer need not always produce a downforce. Many free-flight model airplanes are set up with lifting horizontal stabilizers. $\endgroup$ – quiet flyer May 29 at 3:36
  • $\begingroup$ "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" -- the problem with this line of argument is that it tends to suggest that if you add weight to the CG of a glider, so that it tends to fly faster, it will have some tendency to trim to a higher angle-of-attack, which isn't true. $\endgroup$ – quiet flyer Jun 29 at 12:35
  • $\begingroup$ The real mechanism behind speed stability is more complicated and includes the fact that if the flight path tends to curve upward for any reason, gravity has now gained a component that acts parallel to the drag vector, so the aircraft will tend to slow. $\endgroup$ – quiet flyer Jun 29 at 12:36
  • $\begingroup$ (Yes I see the thread is some years old.) $\endgroup$ – quiet flyer Jun 29 at 12:36
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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.

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    $\begingroup$ Why is a pitch-up "defined" as an increase in angle-of-attack? An increase in thrust will tend to cause an aircraft to pitch up and climb. Yes, the short-term dynamics involved in making this happen do involve a temporary increase in lift due to a temporary increase in airspeed, which may be almost imperceptible to the pilot. $\endgroup$ – quiet flyer May 29 at 3:42
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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

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    $\begingroup$ please consider stop making new accounts, if you register you will be able to edit you own posts. and also do not post comments as answers $\endgroup$ – Federico Jul 3 '16 at 6:16
  • $\begingroup$ I'd suggest that an increase in airspeed looks more like an increase in angle-of-attack, not a decrease in angle-of-attack, to the airplane. Both an increase in airspeed and an increase in angle-of-attack cause the lift vector to increase which causes the flight path to curve upward into a climb. Ultimately of course the aircraft will stabilize in a steady climb with SMALLER lift vector than it had in level flight. $\endgroup$ – quiet flyer May 29 at 3:46
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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.

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

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

  • Ultimately, the plane will 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?

  • Aircraft tends to maintain its trimmed angle-of-attack.

  • Increased speed causes an increase in total lift and the flight path starts to curve (accelerate) upward.

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

  • As flight path curves upward, gravity gains a component acting parallel to the drag vector and against the thrust vector, and the airspeed starts to decrease.

  • The complete process of returning to equilibrium may involve several slowly-decreasing cycles of a pitch "phugoid" oscillation, but this may be more detail than you are asking for.

  • A complete explanation would recognize that angle-of-attack does not stay absolutely constant throughout the pitch "phugoid", both due to rotational inertia in the pitch axis, and due to 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.

  • You may be hoping for more detail on exactly why the airplane tends to maintain its trimmed angle-of-attack. This is not a simple topic. A good explanation of this 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.

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