# How come deflected flaps cause a nose-up pitching moment in high-wing aircrafts and a nose-down pitching moment in low-wing aircraft?

I imagine that the increased downwash due to the new camber of a high-wing aircraft causes a down-force on the horizontal stabilizer which in turn causes a nose-up pitching moment. However, I read that the increased drag also causes a nose-up pitching moment in high-wing aircraft. I don't understand how though. The drag acts on the center of pressure but how does that cause a nose-up pitching moment in a high-wing aircraft. It's assumed that the center of pressure lies aft of the CoG. When the center of pressure moves further aft due to flap deflection, then how can increased drag cause a nose-up pitching moment? The lever arm w.r.t. CoG grows and would rather cause a stronger nose-down pitching moment.

• "How come deflected flaps cause a nose-up pitching moment in high-wing aircrafts" - I remember a pronounced pitch-down change in attitude with full flaps in the C152 and C172, but that was a long, long time ago. Memory may be playing tricks on me. May 22, 2023 at 18:22
• @WayneConrad at least in the C150 I flew, adding flaps created a nose-up pitch moment that had to be counteracted with nose-down elevator. The flaps-down attitude ended up more nose down than with flaps up. May 22, 2023 at 19:08
• @CameronSS That's makes sense, thanks for the explanation. I never flew those planes, but was a passenger, so likely only the end result was really noticeable to me, with the initial pitch-up moment being compensated for by the pilot without me knowing it. But I distinctly recall how much it seemed like I was looking straight down after flaps were applied in those planes. May 22, 2023 at 21:57

The center of gravity is a point in three dimensions, not simply "aft or forward" but also located somewhere "up or down." Since drag is opposite to the aircraft's motion, the relevant lever arm is not how forward or aft the center of gravity is, but how far above or below it is from the center of pressure.

If the center of pressure on the wing does not correspond to the center of gravity, then drag produces a torque around the center of mass, corresponding to a nose-down pitching moment if it is below the center of gravity, and a nose-up pitching moment if it is above.

In a typical high-wing airplane, the wings are above the center of gravity, and so drag on the wings causes a pitch up moment, which is increased when flaps are deployed. The opposite is true for low-wing airplanes.

Lowering flaps should cause a pitch down moment of the wing, but a secondary effect of downwash on the horizontal tailplane can affect the actual pitching motion of the aircraft.

This is markedly noticeable on the high wing Cessna 172 when flaps are deployed, resulting in a pitch up motion.

With low wing aircraft, especially with a T-tail, lowering flaps will not have as great an effect on the tail. In this case, only the pitching moment of the wing is affected, resulting in a pitch down tendency.

Typically with flaps on any kind of wing, they change the camber of the wing when deployed, causing the center of lift (CL) the shift forward on the chord line, resulting in a marked pitch up. This requires the pilot to trim the aircraft nose down in order to compensate for this. Conversely, when flaps are retracted, the center of lift shifts aft on the chord line, requiring nose up elevator trim to compensate. This phenomenon does not change based on the position of the wing in regards to the fuselage, but can be a little different, depending on the type of wing flaps used.

• Shouldn't a backward shift of lift cause a pitchdown moment? May 22, 2023 at 5:29
• Assuming that we're talking about trailing edge flaps, why would deploying them cause the CL/CP to move forward? Since the camber is being increased at the aft of the wing, shouldn't the CP move aft? May 22, 2023 at 8:51