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Is reducing throttle/power already enough? Or do I have to adjust the pitch as well? If so, I suppose pitching up as well? If pitching up is needed as well, is it because as speed decrease, there is less aerodynamic force acting on the tail and so the tail down force is less which pitch the nose down? Or is it because as speed decrease, lift decrease, so pitch up to create more lift?

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To maintain altitude, lift must be equal to weight. The first question is, does slowing down reduce the amount of lift we have?

We can look at the lift formula for the answer:

$$\mbox{Lift} = C_l \times 0.5 \times \rho \times V^2 \times S$$

Airspeed is a part of this formula, represented by V. Therefore we can say that if we slow down, we will have a reduction in lift and therefore descend, if we don't change anything else.

So what can we change to maintain altitude? Working from the end of the formula: S can be increased with flap in some aircraft, but this is inefficient and cumbersome. Then, ρ is air density which is outside of our control. That leaves Cl, which is essentially a function of camber and angle of attack.

Again, camber could be increased with flap, but the best way to increase Cl is to increase our angle of attack. We can increase this via a pitch adjustment. So there we have our answer - if we want to maintain altitude while slowing down, we must either increase flap or, the preferred option, increase our angle of attack via an increased pitch attitude.

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Consider the four forces acting on an aircraft in equilibrium:

enter image description here

Drag must exceed thrust in order to decelerate. Presuming that you don't have speed brakes to increase drag, the only way to slow down while keeping lift and weight equal is to reduce thrust.

As the aircraft slows, the wing will produce less lift, requiring an increase in AOA to maintain lift equal to weight. This will further increase induced drag, necessitating a power correction once the target airspeed is reached.

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  • $\begingroup$ Re "necessitating a power addition once the target airspeed is reached." -- not if initial and final speeds are on the "front side of the power curve". $\endgroup$ Aug 29, 2023 at 20:03
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    $\begingroup$ @quietflyer, I changed to "power correction". But... unless you know the exact power setting for your new target airspeed you will likely need to add more power than what was used to decel. That's what I meant, not that the power would be higher than the first, faster speed. $\endgroup$ Aug 29, 2023 at 20:06
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If you reduce forward thrust then you implicitly reduce the lift generated by the wing in its current configuration. So to maintain altitude you must pitch up or otherwise change the configuration of the wing - which in itself will change the attitude.

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Is reducing throttle/power already enough? Or do I have to adjust the pitch as well?

It depends on the aircraft. It also depends on whether you are flying on the "front side" or the "back side" of the power curve.

If you are flying on the back side of the power curve, you are going to need to increase power to maintain altitude after you slow to your desired airspeed.

As for pitch inputs--

Most aircraft, at least to a first approximation, have reasonably strong speed stability, meaning that they tend to maintain the trimmed airspeed, even as the engine power output is changed. This is a secondary result of their tendency to maintain the trimmed angle-of-attack. So normally, if we are trimmed for some given airspeed, we expect to have to exert some forward pressure on the stick or yoke to maintain a faster airspeed, and we expect to have to exert some aft pressure on the stick or yoke to maintain a slower airspeed-- especially if power is held constant.

But a closer look will reveal that many aircraft have some tendency to move toward a higher angle-of-attack (and a lower airspeed) as power is increased, and a lower angle-of-attack (and a higher airspeed) as power is decreased. Most high-winged aircraft with tractor (front-mounted) engines fall into this category.

In such case, if you operating deep on the "back side" of the power curve, if you start out trimmed for level flight, it's conceivable that you might have to apply some forward pressure on the stick or yoke as you increase power after slowing down, to maintain your new target airspeed.

Some other aircraft--especially some with engines mounted on a pylon above the fuselage-- have some tendency to move toward a lower angle-of-attack (and a higher airspeed) as power is increased, and a higher angle-of-attack (and a lower airspeed) as power is decreased.

In such case, if you are operating on the "front side" of the power curve, if you start out trimmed for level flight, you'll likely have to apply some forward pressure on the stick or yoke in order to maintain your new target airspeed, after you decrease power to slow down.

But if you are just asking about the aircraft's pitch attitude-- slower flight is almost always associated with an increase in angle-of-attack.1 If the flight path remains horizontal, the aircraft's pitch attitude will always be shifted in the nose-up direction.

Footnotes:

  1. Why "almost"? Consider this "outlier" case-- "hovering" flight. Google "model airplane 3-D hover" if you've never seen something shaped like a "conventional" airplane, with no rotor blades or vectored thrust capability, hang on the prop in a stationary zero-airspeed hover. Here the pitch attitude is 90 degrees, but the angle-of-attack of the wing in relation to the total airflow (including the propwash) is zero degrees (or if the airfoil is non-symmetrical, whatever slight negative angle-of-attack is associated with zero lift.) Nearly the same applies to flight at extremely low airspeeds, well below stall speed, where the plane is "creeping" slowly along in an extreme nose-high pitch attitude. It appears to be the case that no full-sized aircraft lacking vectored thrust, rotors, etc can be flown in this manner at present.
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  • $\begingroup$ "If you are flying on the back side of the power curve, you are going to need to increase power to maintain altitude as you slow down." I'd suggesting rewording slightly to avoid reading like you need to add power to slow down. You must first decrease power to initiate the slow down, then add more power to maintain airspeed given the higher AOA needed to maintain altitude. $\endgroup$ Aug 29, 2023 at 20:16
  • $\begingroup$ @MichaelHall -- thanks, done $\endgroup$ Aug 29, 2023 at 20:24
  • $\begingroup$ Let me know what you think of my edit. (again, the 4 forces... power counters drag, not weight) $\endgroup$ Aug 29, 2023 at 20:31
  • $\begingroup$ @MichaelHall -- I decided to roll it back. The edited statement is not necessarily incorrect, but I find it more natural to think of using pitch inputs to control airspeed and power changes to control vertical speed. Especially on the "back side" of the power curve. $\endgroup$ Aug 30, 2023 at 10:53
  • $\begingroup$ Ok, I agree when flying an approach, but all trimmed up during cruise do you really adjust throttle to hold altitude? How much time do you really spend on the backside? $\endgroup$ Aug 30, 2023 at 14:02
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is reducing power enough, or do I have to adjust the pitch as well?

The pilot is generally ready to do both at the same time, but it helps to know pitch (angle of attack) and power requirements beforehand.

Dig into your Pilots Operating Handbook and find V min power. This is the airspeed your plane uses the least amount of throttle in level flight (for a given flaps setting).

Any airspeed faster than this will require more power.

If your V min power for flaps zero (clean) is 60 knots, and airspeed is being reduced from 100 knots to 80 knots, you will simultaneously reduce power and pitch up to hold altitude, then re-trim for the new airspeed.

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  • $\begingroup$ This is actually similar to what is done at the end of the downwind leg for recreational aircraft, where airspeed is reduced from pattern speed to approach speed. $\endgroup$ Aug 31, 2023 at 23:52
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Pitch for speed. Throttle for rate of climb.

So, first you adjust your pitch trim to achieve the desired speed. Then observe the rate of climb indicator. If positive (most likely), then decrease throttle to reduce to zero.

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  • $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Aviation Meta, or in Aviation Chat. Comments continuing discussion may be removed. $\endgroup$
    – Ralph J
    Aug 30, 2023 at 4:42
  • $\begingroup$ This is correct if one is flying on the backside of the power curve. Here it is safer to pitch up first to desired airspeed. Chopping throttle first in slow flight and pitching up can easily lead to a stall. $\endgroup$ Sep 1, 2023 at 14:56

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