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I understand that trimming an aircraft at a particular airspeed e.g. 100kts will keep the aircraft flying at that airspeed (100kts) even if you were to e.g. Lower power. This is apparently accomplished by the pitching down of the trimmed aircraft.

But I am not too sure if I understand the rationale behind the pitching down. I hope to seek advice on whether what I understand is correct:

When you elevator trim to fly unaccelerated straight and level flight (all forces balanced) at e.g. 100kts, you are relieving the pressure needed to keep your aircraft in that angle of attack that produces enough lift to counter the weight, at that particular airspeed (because any increase or decrease in airspeed also will affect lift, so speed will affect the amount of lift that needs to be created by the AOA to counter the weight so as to stay level). So if your speed starts to drop, the amount of lift that your AOA must make = increased. But since you have trimmed elevator to that position means the AOA is fixed. So what happens is that the aircraft will pitch down (trade height which is potential energy for kinetic energy so your speed increase), then your speed will be back to what it used to be and your that trimmed AOA will continue to be able to supply that exact amount of lift, which together with the lift at that same airspeed is just right to counter the weight. So the aircraft will be back to level flight again.

Since power is lowered, I would expect the aircraft to continue to lose speed and the whole cycle of pitching down then back to straight and level will go on and on.

Does this make any sense?

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You are very perceptive to note that for a given angle-of-attack, a return to the exact airspeed that allowed level (horizontal) flight would indeed imply a return to level (horizontal) flight. But that's not what we expect to happen when we reduce the power. Hopefully this answer will help you to understand why not.

When you elevator trim to fly unaccelerated straight and level flight (all forces balanced) at e.g. 100kts, you are relieving the pressure needed to keep your aircraft in that angle of attack that produces enough lift to counter the weight, at that particular airspeed

Exactly right.

(coz any increase or decrease in airspeed also will affect lift, so speed will affect the amount of lift that needs to be created by the AOA to counter the weight so as to stay level)

Technically this is not quite right. The amount of lift that is required for the flight path to stay level rather than bending up or down is the same at any speed, but the AoA needed to create this lift varies with airspeed. Maybe that's what you actually meant to say, or something close to it.

So if your speed starts to drop, the amount of lift that your AOA must make = increased.

It may be that the word you are looking for is not "lift", but "lift coefficient". Increasing the AoA increases the lift coefficient. Lift is proportional to lift coefficient times airspeed squared. To maintain horizontal flight as we slow down, we need the same amount of lift, which means we need a higher lift coefficient, which means we need a higher AoA.

Or maybe you simply meant to say something like "if we hold angle-of-attack constant and decrease airspeed, then lift will be decreased, which means lift will have to somehow be increased again to bring things back into balance." This is exactly true, regardless of whether that "somehow" involves an increase in angle-of-attack and lift coefficient, or a return to a higher airspeed at the same angle-of-attack and lift coefficient that we had before.

But since you have trimmed elevator to that position means the AOA is fixed.

Yes, at least to a first approximation. We'll assume that is exactly true for the purpose of this answer.

So if your speed starts to drop

The speed is dropping because we've reduced power and thrust, so drag is greater than thrust, temporarily causing the net force acting along the direction of the flight path to be non-zero.

the amount of lift that your AOA must make = increased.

Yes, the drop in speed has caused a loss of lift, and lift must somehow be increased again to bring things back into balance.

In the meantime, lift is temporarily less than weight. There is now a net force acting perpendicular to the flight path (i.e. downward), so the flight path bends (curves) downward.

Since the angle-of-attack stays constant, as the flight path bends downward, the aircraft must pitch down.

As the flight path bends downward, gravity (the weight vector) gains a component that is partly acting parallel to the flight path in the forward direction, and this is what drives the increase in airspeed till it is approximately back at the initial value.

As the airspeed increases, the flight path stops bending down. Actually in real life we are likely to see the airspeed slightly "overshoot" and then decrease again. As the airspeed "overshoots" it provides the excess lift needed to curve the flight path up toward something closer to horizontal flight-- but not exactly horizontal. Since you've reduced power, you'll end up in a descending glide.

then your speed will be back to what it used to be and your that trimmed AOA will continue to be able to supply that exact amount of lift

Since you've reduced power, you are now in a descending glide. To a first approximation, your statement above is true. It is a good approximation for shallow to moderate glide angles. But if you want to know the full truth, it is that the lift vector is slightly smaller in a glide than in level flight, which means the airspeed is slightly lower in a glide than in level flight at the same angle-of-attack. For more, see the vector diagrams in this related answer. But you really don't need to understand this to answer your question. The most important thing to understand is that being in a glide rather than in level flight is the only way we can accommodate the fact that drag is now greater than thrust. Again see the vector diagrams in the linked answer-- just mentally substitute "drag minus thrust" for "drag" in the vector diagrams and you'll get the idea.

which tgt with the lift at that same airspeed is just right to counter the weight. So the aircraft will be back to level flight again.

No, it will be in a stabilized glide. The weight will be balanced by the vertical component of lift, plus the vertical component of (drag minus thrust). See the vector diagrams in this related answer for more; just mentally substitute "drag minus thrust" for "drag".

Since power is lowered, I would expect the aircraft to continue to lose speed and the whole cycle of pitching down then back to straight and level will go on and on.

You are starting to touch on the question of the pitch "phugoid" oscillation but that really has nothing to do with the core of your question. The most important thing to understand is that reducing the power while holding angle-of-attack constant-- or while holding airspeed constant -- technically both cannot happen at once but don't worry too much about that-- will allow the airplane to end up in a stabilized glide, with the weight supported by the vertical component of lift plus the vertical component of (drag minus thrust).

A couple of more notes in closing--

First, this business about the aircraft tending to trim to a slightly lower airspeed in a glide than in level flight, if angle-of-attack stays constant -- you'll rarely be able to detect this effect in actual practice. For example, in an airplane with a propeller on the nose, if you change the power setting without touching the yoke or the elevator trim control, the change in the propwash over the tail will often cause some amount of change in angle-of-attack, which will drive a change in airspeed. In jets, offset thrust lines can cause a similar effect. These sorts of effects are likely to completely dwarf the change in airspeed caused by the fact that for the same angle-of-attack, the lift vector, and therefore the airspeed, must be very slightly smaller/lower in the glide than in level flight. Note that in the vector diagrams in the answer linked above, the lift vector is really not very much smaller than the weight vector as long as the glide angle is not too steep. And since lift is proportional to airspeed squared, only a very small reduction in airspeed would be needed to cause that amount of reduction in the lift vector while holding angle-of-attack constant. Still, when the (drag minus thrust) vector is not zero, it must be accommodated in the vector triangle as shown in the linked answer, which means that lift must in fact be a little less than weight.

And second, if you make your power reduction very gradually, the aircraft will stay close to a steady-state condition. You'll smoothly transition to the final steady-state glide and final airspeed (which for all the reasons noted above, may or may not be a little different from the airspeed you had at the higher power setting). On the other hand, if you abruptly make a large power reduction, you'll see the airspeed drop significantly, and then the nose will pitch down quite far, and then the airspeed will "overshoot" well above the final steady-state value, and then the nose will rise above the final attitude for the glide, and then the airspeed will "undershoot" the final value. This is the pitch "phugoid" in action, and you might see several cycles before everything settles down into the steady glide.

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    $\begingroup$ Your last paragraph makes a very good point. In a 172 the reduction in trim speed from going to cruise power to full power can be several kts in my experience depending on the CG location. When adding large amount of power I got in the habit of applying a little tweak of trim to compensate right away. I used to play around in a CRJ Flight Training Device (no montion/visuals fixed simulator, but otherwise the same as the Level D) and observe trim changes with thrust. With the tail engines, adding thrust added a few kt to trim speed, if you had the patience to wait for the phugoid to finish. $\endgroup$ – John K Apr 17 '20 at 17:34
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    $\begingroup$ (Thnx, now refers to second-to-last paragraph as one more has been added.) $\endgroup$ – quiet flyer Apr 17 '20 at 18:13
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This makes sense and is approximately what happens if you reduce thrust during trimmed straight and level flight. The aircraft will go though a series of damped oscillations, called 'phugoid' until it finds its new equilibrium state. The new state will be the original airspeed, but now in a constant descent.

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  • $\begingroup$ Hello! Thanks for your reply! So the new state will be a constant descent and not a series of straight and level followed by descent then back to straight and level followed by descent and so on? $\endgroup$ – Flightsimrightnow Apr 17 '20 at 10:54
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    $\begingroup$ The end state will be constant descent, but the transition will be a series of pitch/vertical speed oscillations that will dampen out (in stable aircraft). The period of the oscillation will vary between aircraft, generally the bigger the aircraft, the longer the period. The damping factor is low, it can take along time to settle at the end state. $\endgroup$ – DeltaLima Apr 17 '20 at 10:59
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Seems like you have a misconception there: When the vertical component of lift equals weight, this doesn't mean the aircraft will fly level. When the forces balance out, the vertical acceleration will be zero, and thus the vertical velocity will be constant (not necessarily zero).

So after the initial disturbance and the resulting phugoid oscillations (where vertical acceleration oscillates between positive and negative) die down, the aircraft will be in a new equilibrium with zero vertical acceleration and thus a constant rate of descent.

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  • $\begingroup$ Thanks for your reply! I believe in a state of unaccelerated straight and level flight, vertical velocity will also be zero isn't it? $\endgroup$ – Flightsimrightnow Apr 17 '20 at 11:37
  • $\begingroup$ Yes, it will, but only in that case (level flight). $\endgroup$ – TooTea Apr 17 '20 at 11:43
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In Simple Terms ....

Pitch controls speed. Power controls speed and lift. Increasing power for the same angle of attack will produce more speed and hence more lift. Decreasing speed for the same angle of attack will produce less lift.

If you reduce power only and keep pitch the same, then you will lose lift and speed. If you were flying level before, then now you're sinking.

Yes - in reality its way more complicated than this. Its a dynmical balance between all of the controls on your aircraft. If you pull back on the stick then speed gets converted to height until drag washes it all away and you stop climbing. If your center of thrust is not directly in line with the center of drag, then the pitch will change as power changes. (This why 737 Max's crash) Increasing speed increases drag which makes the aircraft less efficient. If you pitch down to maintain speed, then drag goes up and you sink even faster.

Power changes and pitch changes are dynamic forces and most aircraft you will ever fly will stabilize somewhere new. Some take longer than others. Most will tend to wobble about a little till they settle. Don't let go of the stick - look out of the window at the horizon.

If you really want to get a feel for how lift and speed are related, go fly a sailplane for a few hours.

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    $\begingroup$ Welcome to ASE. But, "If you reduce power only and keep pitch the same, then you will lose lift and speed. If you were flying level before, then now you're sinking."-- If you are sinking while holding pitch attitude constant, you've changed angle-of-attack, which the original question suggested should stay constant. The original question was phrased in a way that was based on a constant elevator position (or at least trim setting), not a constant pitch attitude. $\endgroup$ – quiet flyer Apr 18 '20 at 12:15
  • $\begingroup$ Well, to be fair, the question appears to be posted by someone who may not clearly understand the difference between the true angle of attack and the pitch of the aircraft. So yes, the answer is heavily simplified. If you reduce power and don't touch the trim, you'll sink and slow down. You push forward to keep speed up ( or not, you're the pilot). $\endgroup$ – Powerless Pilot Apr 18 '20 at 12:22
  • $\begingroup$ @dalearn-- the guy (or gal) must have chose to put those words in caps for some reason, don't you think? Why take them away? $\endgroup$ – quiet flyer Apr 18 '20 at 12:57

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