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Old 2nd Nov 2018, 22:13
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Join Date: Sep 2016
Location: USA
Posts: 451
Originally Posted by ManaAdaSystem View Post
I though I had this annoying system nailed, but when I read what the FCOM says, I’m just getting confused. How can a system trim to return the aircraft to the trimmed speed by trimming opposite the speed change?
That's self-explanatory, anything that changes is returned to the original state by making a change opposite to the first change (i.e., stable behavior). If you added a change in the same direction as the first change, the state will only move further away from the original (unstable behavior).

I think I understand where you're coming from, and if I'm right, it's due to a confusion between two different things that are meant by "speed stability." They're superficially similar (it's what, um, keeps the speed stable, right? duh!) but actually very different. And the distinction is a little tricky and more than a little subtle, but nonetheless important; and if you're having a hard time applying what I pointed out as self-explanatory to speed and trim, it's probably because you're thinking based on the first definition, while the system was designed (and explained in the FCOM) based on the second.

1. The "speed stability" in some pilot training materials:

This assumes that you're controlling for a constant altitude, be it with autopilot altitude hold, or manual elevator control. Whatever is controlling the elevator, and, in turn, the AOA (be it your arm or the autopilot) is free to do whatever it takes with the AOA to maintain that level flight path. Think of it as altitude-fixed, AOA free. So if a gust speeds you up a little bit, the extra drag will then slow you back down to the original speed. If a gust (or momentary dip in thrust, or anything) slows you down, the reduction in drag will speed you back up to the original speed. This works on the front side of the thrust curve. If you're slow enough already to be on the backside, however, then a momentary slowdown will see more drag, which will slow you down even more, which will cause more drag, etc. in a runaway cycle. So this type of speed stability only exists above Vmd. It's unstable when you're slower than that.

2. The "speed stability" in aeronautical engineering:

For this definition you have to completely forget about holding altitude. The airplane is free to climb/descent at the whims of the other factors at play. However, without elevator changes, AOA is maintained. So, opposite the fist definition, think AOA-fixed, altitude free. (Also for the moment, set aside the effects of thrust-pitch couple like the 737 is famous for, or propwash like in most bugsmashers.) This AOA-fixed behavior is the natural behavior of normal planes. After any disturbance (assuming the elevator and/or trim returns to the original position) the AOA will also return to its original position. It may be after a period of alternating overshoots (phugoid oscillation), but eventually it will.

Let's say you're cruising level in the middle of the speed envelope, and you reduce some power (permanently). The power reduction will cause a speed reduction, which will cause a lift reduction, which will cause the beginning of a descent, which will cause a component of weight to align forward parallel with thrust, which will cause the plane to speed up, and where it finishes speeding up will be its original speed.. (If it finished at any other speed, that speed difference would cause a lift other than the weight, and the plane would go through more repetitions of this same cycle until lift=weight. Since weight is fixed, lift is fixed, and the only 2 remaining variables of the lift equation are speed and Cl, and Cl is a stand-in for AOA. So the speed will settle at that speed which is corresponds only to the particular AOA you're flying at.) This is why there's a fundamental relationship between AOA and speed in vertically unaccelerated flight, and why AOA stability implies speed stability under the engineer's definition. To summarize, we pulled the power, but the speed remained the same (albeit under a descent, but we don't care about that). Vice versa, if we increase power the airplane will begin a climb, at the same speed.

This speed stability (definition 2), by the way, is required to exist throughout the entire certified envelope all the way down to stall speed. Remember, so far no electronic or mechanical trickery is involved. This is the natural, aerodynamic behavior of every stable plane from a hand-thrown model to the 747. To change the speed that is held constant we must change the AOA, which is done either by moving (and holding in a new position) the elevator or moving the stabilizer, i.e., actuating the trim. And in some engineering contexts, they don't care whether you end up having to hold a force with your arm or not. (This can be kind of confusing to us, who are used to thinking of trim as the-thing-that-removes-the-arm-force.) It's all just considered changing the trim point, or trim speed, based on the new position of elevator or stab. Now, this new speed will be maintained (again, maybe climbing or descending now, but that's irrelevant).

If I increase the speed this means the STS will trim aft. At the same time increased speed will increase the lift. Aft trim will bring the aircraft into a more out of trim state.
It appears that your scenario is based on the premise of the first definition. You're increasing speed while maintaining altitude, (intentionally) so the trim required is forward. (Faster speed = forward elevator and/or forward trim) But the STS, sensing that you're faster than original, trims aft in an attempt to slow you down to the original speed. This causes the plane to tend to climb which annoys you. (Of course, we're used to the premise of maintaining altitude) But this is where the engineering definition kicks in and you have to realize that the airplane doesn't care about the climbing tendency. It only wants to return to the original speed, which, being slower than the new speed, mandates a climb. This is the behavior of a normal airplane, only a little stronger with the augmented speed stability of STS. Understanding that (non-Airbus) airplanes really behave in accordance with the engineering (#2) definition, will hopefully let you come to terms with the "opposite" behavior.

Last edited by Vessbot; 5th Nov 2018 at 19:53.
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