# When approaching stall, why does the center of pressure move back?

In an answer to a previous question, Peter Kämpf stated:

Before the lift coefficient of a stalling airplane peaks, the flow over part of the wing will start to separate. Ideally it will do so near the trailing edge of the wing root, and the separation will slowly progress forward and outward as angle of attack increases. This separation will shift the local center of pressure back, such that the aircraft will experience an increasing nose-down moment as it approaches stall.

If separation starts at the back and progresses forward it seems to me that center of pressure ought to move forward. Why does it move back?

• @mins so this applies only to swept wings? – TomMcW May 10 '16 at 22:19
• See page 138, then 156 and following. Actually just before the stall there is a CP move forwards, then a rapid move aft due to the separation at the leading edge (didn't know that) but the latter only on thin wings. – mins May 10 '16 at 23:59
• @mins increase tip AoA (twist) or add fences, so that the tip stalls last and the CP moves forwards Do you have this backwards? – TomMcW May 11 '16 at 1:47
• In an answer to a previous question, Peter Kämpf stated: -- I wonder how many of our questions start out with that statement... I know I've asked more than one of those. – FreeMan May 11 '16 at 12:13
• @FreeMan I was going to say "The great Peter Kämpf." Although it looks like we may have another budding PK in spacegirl1923. Did you read her profile? – TomMcW May 11 '16 at 13:29

The center of pressure is generally not the best way to characterize the aerodynamic forces on an aircraft since it varies with AoA. That is why the neutral point or aerodynamic center is used instead.

Having said that the center of pressure is the location of average pressure force acting on the wing or aircraft. Mathematically it is found by taking the sum of integral of the magnitudes of pressure and shear stress acting on the surface and multiplying it by a distance (for an airfoil from the LE) on the upper and lower surfaces resulting in a moment at the LE, or reference point. This is then divided by the sum of the integrals of the pressure and shear stresses acting on the upper and lower surfaces. My explanation does not do it justice see http://web.mit.edu/16.unified/www/FALL/fluids/Lectures/f03.pdf

It can be somewhat complicated to calculate for an entire aircraft but in general for an airfoil it does move forward up until the point of stall or critical point after which it then moves aft. See https://www.reddit.com/r/aviation/comments/2kibfo/why_does_the_centre_of_pressure_move_rearward/ for a good simple explanation of this.

The movement of center of pressure is highly dependent on the configuration of the aircraft (airfoils are much simpler) and unless you are certain of the specifics of the configuration generalizing the center of pressure movement is inaccurate...

• The movement of overall centre of pressure is dependent on configuration, but as long as you assume the horizontal stabilizer does not stall yet, the general tendency can be derived. – Jan Hudec May 11 '16 at 5:29
• General tendencies for a traditional configurations yes, but once you start adding in wing sweep, twist, canards, and less traditional configurations, generalizations about movement of center of pressure are not a good idea. – spacegirl1923 May 11 '16 at 17:21
• The conditions for down pitch moment when approaching stall are closely aligned to conditions for stability. So while it is certainly possible to come up with design that will behave differently, stable designs will generally pitch down when approaching to stall. – Jan Hudec May 11 '16 at 21:37

It is the overall centre of pressure that includes the stabilizer that moves back.

The reason is that as the flow starts to separate, the lift it generates no longer increases linearly with angle of attack. However, as long as it is properly designed, the elevator is not stalling yet and its lift does still increase linearly with AoA, leading to higher increase of lift on the aft surface and overall down pitch moment.

The centre of pressure of the wing itself should indeed first move forward a little as the trailing edge loses lift. Not too much, because the centre of pressure on the upside is always quite far forward to begin with. And then past $C_{L_{max}}$ it moves aft since the centre of pressure on the underside is about midchord.

• the elevator is not stalling yet and its lift does still increase linearly with AoA, leading to higher increase of lift on the aft surface and overall down pitch moment. But the HS is producing a downforce, especially at a high AoA. Would a higher negative lift not shift overall CP forward causing a pitch up? – TomMcW May 11 '16 at 13:46
• @TomMcW, lift is an upforce. If it is actually producing a downforce (usually is), increasing lift means it now produces less downforce, not more. – Jan Hudec May 11 '16 at 21:21