# Power changes and trim forces. related to position of Center of Mass (CoG), or Center of Pressure?

Theoretically (and Actually) do power increases cause a pitching tendency (and a need to adjust pitch trim), because the thrust line is not aligned with the Center of pressure, or, as I understand it, because the thrust line is not aligned with the center of mass (CG)?

I was recently told that the Cessna 182 is particularly sensitive to abrupt power changes in the landing phase (at slow airspeed) because it is a high wing aircraft, and the thrust line is below the Center of Pressure. i.e., Increases in Power generate a pitch up moment, requiring down trim to compensate, and reductions in power generate a pitch down moment, requiring nose up trim to compensate.

This strikes me as wrong, I believe it the alignment of the thrust vector with the center of mass of the aircraft, (the Center of Gravity (CG) as it is called), that determines the moment generated by thrust vector.

• I believe the propeller on the 182 affects the elevator more. As you pull back power, the propeller slows the airflow hitting the tail. The effect of the tail is greatly reduced and the nose will drop. Sep 20, 2020 at 22:46
• @Wbeard52, yes this makes sense as well. This effect could also be part of the cause of the pitch moment. Sep 20, 2020 at 23:24

Your intuition is correct. Each of the three principle axes of the aircraft originate at the center of gravity. Therefore, all rotation in either pitch, roll, or yaw occur around the CG.

The center of pressure is a place along the chord where the lifting force acts on the wing. It is not a pivot point like the CG is. In fact, an increase or decrease of lift will actually produce a pitching moment around the CG if they are not co-located along the longitudinal axis. This contributes to the relative stability (or instability) of an aircraft.

Well I have an issue with this definition as I've seen this happen in low wing and high wing airplanes. I don't know if the effect is more pronounced in a Cessna than a Piper as I've never personally flown a Cessna but I know it happens on a Piper.

I've heard a different story about it. When you trim your aircraft you set a combination of power and pitch and that gives you a particular airspeed that allow for level flight.

When you abruptly increase power your velocity can not increase as fast due to mass inertia and newton's second law. On the other hand pitch is quite free to change and it will adapt to the new power setting in order for the aircraft to stay at "equilibrium" regarding the power, pitch, speed relation.

We know that power is used to compensate drag and drag is a function of speed and AOA. Therefore taking out all other parameters we can get the following relation. $$Power = AOA * speed^2$$ With a quick increase of power and the speed remaining constant you need a quick increase of your AOA that will put the nose up and therefore increase pitch.

Who "told" you this? When in doubt, start with gliding. Does the high wing have any "abrupt pitch tendencies". This Cessna is about as rock solid as you can get as a safe design aerodynamicly.

Using the CG as the center of aerodynamic torque, in stable flight, all pitching moments = 0. Looking at the front of the plane, at any given speed, the position of the center of drag in relation to the CG will contribute to trim. This means that less tail down force is required to "zero" any pitching force from the wing center of upward lift being behind the CG.

So we can isolate the issue to when power is applied and conclude the prop blast (or lack of it) will affect pitch by increasing airflow over the elevator. But downthrust angle is designed to compensate of this effect.

So if a 182 is experiencing "abrupt pitch changes" one may wish to have the mechanic check the engine mounts. Another possible cause could be the CG is too far forward, which should be checked preflight.

Actual flying experience in the 182 at altitude will be a great way to sort out any alledged vices. It has slightly more power than a 172 (160/180 hp vs 230 hp)

• I apologize if I am mis-readng you, but if you are denying that there is a pitching moment from power changes, well, that is not even in issue. I have 300 hours in the 182, and I noticed this immediately on the first 182 sortie I flew after transitioning from the 172. On final to land, pulling power causes an immediate pitch down (before airspeed decreases), requiring nose up trim to compensate, and increasing power causes a pitch up that requires nose down trim. I do think prop wash over the elevator may be the explanation for this, but why is it not as bad in the 172? Sep 21, 2020 at 13:03
• You may have misread my post, specifically my use of the word "abrupt". I did not say there were "abrupt" pitch tendencies, I said there were pitching moments when making "Abrupt" Power changes. This is a common effect. In the F-4 Phantom, for example, the engines were canted downwards at the back specifically to ensure that the thrust line passed close to the CG (vertically) to avoid this, because it is a significant issue. Sep 21, 2020 at 13:09
• @Charles Bretana it pitches down when power is taken out for the exact same reason it pitches up when power is added: (more powerful prop wash effects). Granted, with more power and speed, you move everything to the middle (right on up to the X-15). Downthrust is the cure for high wings (look at a Catalina). It's important to know the relative impacts of design (and to have a good mechanic). Sep 21, 2020 at 15:46
• @Charles Bretana the pitch down when power out may be in there as a safety feature because the 182 is heavier, but, as you have noted, it means that full power on pitch up tendency has to be carefully watched. Even in a 172, a little "push" on the yoke helps to hold Vy (and certainly a lot more going into cruise). That is from the high wing (relative to CG). Sep 21, 2020 at 15:56
• In cruise, depending on the aircraft the elevator is actually creating negative lift to have equilibrium. But when you increase power you still have a nose up attitude change. Following your theory we should have the opposite happening Sep 21, 2020 at 20:35