Trim only sets AoA

[tcasblue]

Actually, I just want to confirm that this is the case. And subsequently, because of the balancing of forces, it also sets the airspeed. Power then determines rate of climb— positive, negative, or level.

Back to basics.

[Intruder]

Technically, pitch trim sets the angle of incidence (angle with reference to the longitudinal axis of the airplane) of the horizontal stabilizer and/or elevator. Then the "balancing of forces", as you describe it, causes the airplane to settle into a particular attitude.

Some fly-by-wire modes can indeed seek a stable angle of attack, and this is valuable for takeoffs and landings of naval fighters on aircraft carriers.

[Vessbot]

Originally Posted by tcasblue

Actually, I just want to confirm that this is the case. And subsequently, because of the balancing of forces, it also sets the airspeed. Power then determines rate of climb— positive, negative, or level.

Back to basics.

In a simplified picture, yes. Trim sets AOA and therefore airspeed. A complicating factor in a more fleshed-out picture is that thrust also effects trim. Under-wing engines have a nose-up thrust couple, in other words increasing thrust is the equivalent to trimming for a higher AOA, a.k.a. a slower airspeed. Opposite for high-mounted engines. GA prop planes generally also have a nose-up thrust couple, from the downwash over the horizontal tail. There are also other more minor contributors to trim, like landing gear. (Drag below the CG, tends to pitch down.)

It is false that trimming sets an attitude as Intruder mentioned above. It sets AOA.

Also, it's true in a simplified sense that thrust sets climb/descend angle. But in reality, let's say you're trimmed and stabilized level at an intermediate thrust setting, and then increase thrust while touching nothing else.

1. The first thing that'll happen is that it'll pitch up or down depending on the direction of the thrust couple, and begin to tend to stabilize around the new trimmed speed (which is slower if you're in a 737 or the like).

2. Over the longer term, as a separate effect, the higher thrust will increase the speed, which will increase the lift, which will deflect the flight path upward, establishing a climb. In other words, half of a phugoid oscillation. This initial climb will probably be steeper than the new steady state (i.e., it overshot), so the rearward component of weight will decrease the speed, which will decrease the lift, which will deflect the flight path downward, completing the first phugoid cycle. This will continue for at least a few cycles of decreasing overshoots, until it stabilizes at the new steady state climb angle (might take as much as a few minutes) commensurate with the new excess thrust.

Obviously this is not a nice way to fly, so in real life the climb is established in conjunction with the practiced use of the right amount of elevator and trim.

The new steady state speed, again, will be that established by the trim change from the thrust couple. If you were in an aerodynamically "perfect" plane with no thrust couple (no downwash effect, jet engine that's not high or low but goes perfectly through the CG) then the trimmed speed wouldn't change and would be exactly what we started the whole exercise with.

[gearlever]

Originally Posted by Vessbot

In a simplified picture, yes. Trim sets AOA and therefore airspeed. A complicating factor in a more fleshed-out picture is that thrust also effects trim. Under-wing engines have a nose-up thrust couple, in other words increasing thrust is the equivalent to trimming for a higher AOA, a.k.a. a slower airspeed. Opposite for high-mounted engines. GA prop planes generally also have a nose-up thrust couple, from the downwash over the horizontal tail. There are also other more minor contributors to trim, like landing gear. (Drag below the CG, tends to pitch down.)

It is false that trimming sets an attitude as Intruder mentioned above. It sets AOA.

Also, it's true in a simplified sense that thrust sets climb/descend angle. But in reality, let's say you're trimmed and stabilized level at an intermediate thrust setting, and then increase thrust while touching nothing else.

1. The first thing that'll happen is that it'll pitch up or down depending on the direction of the thrust couple, and begin to tend to stabilize around the new trimmed speed (which is slower if you're in a 737 or the like).

2. Over the longer term, as a separate effect, the higher thrust will increase the speed, which will increase the lift, which will deflect the flight path upward, establishing a climb. In other words, half of a phugoid oscillation. This initial climb will probably be steeper than the new steady state (i.e., it overshot), so the rearward component of weight will decrease the speed, which will decrease the lift, which will deflect the flight path downward, completing the first phugoid cycle. This will continue for at least a few cycles of decreasing overshoots, until it stabilizes at the new steady state climb angle commensurate with the new excess thrust.

Obviously this is not a nice way to fly, so in real life the climb is established in conjunction with the practiced use of the right amount of elevator and trim.

The new steady state speed, again, will be that established by the trim change from the thrust couple. If you were in an aerodynamically "perfect" plane with no thrust couple (no downwash effect, jet engine that's not high or low but goes perfectly through the CG) then the trimmed speed wouldn't change and would be exactly what we started the whole exercise with.

All true, thx Vessbot.

Very sad most pilots will never ever experience just an airspeed rise due to more thrust WITHOUT pitching up, like B727.

[OK4Wire]

Very sad most pilots will never ever experience just an airspeed rise due to more thrust WITHOUT pitching up, like B727.[/QUOTE]
[QUOTE]

I thought you were going to say "like the Harrier"!

[Vessbot]

Originally Posted by gearlever

All true, thx Vessbot.

Very sad most pilots will never ever experience just an airspeed rise due to more thrust WITHOUT pitching up, like B727.

I'd rather fly a pitch-up airplane than a pitch-down airplane (my current one being the latter). There's an elegance in the climb/descent change being commanded, matching the pitching caused by the throttles.

[Derfred]

Originally Posted by tcasblue

Actually, I just want to confirm that this is the case. And subsequently, because of the balancing of forces, it also sets the airspeed. Power then determines rate of climb— positive, negative, or level.

Back to basics.

I would argue that trim should only be used for one thing: to achieve neutral elevator force.

Elevator to set desired AoA, thrust to compensate for the new AoA, then trim out the elevator force.

[Vessbot]

The meaning of "trim" has branched out (as meanings tend to do) to mean not just the mechanism (tab, etc.) that neutralizes the elevator force, but also simply the final elevator+stab position (regardless of what combination of forces put them there) and what effect does that have on the airplane. (Like used in the Stability and Control section of Aerodynamics for Naval Aviators, p. 264)

The effect of elevator deflection on pitching
moments is illustrated by the first graph of
figure 4.13. If the elevators of the airplane are
fixed at zero deflection, the resulting line of
CM versus CL for 0° depicts the static stability
and trim lift coefficient. If the elevators are
fixed at a deflection of 10° up, the airplane
static stability is unchanged but the trim lift
coefficient is increased. A change in elevator
or stabilizer position does not alter the tail
contribution to stability but the change in
pitching moment will alter the lift coefficient
at which equilibrium will occur. As the ele-
vator is fixed in various positions, equilibrium
(or trim) will occur at various lift coefficients
and the trim CL can be correlated with elevator
deflection as in the second graph of figure 4.13.

But even in the strict sense of the way you use the word, the question of the thread is: once the elevator is in a certain place and your hand is off the stick (aka trimmed), what will the airplane do after disturbances? I.e., what parameter will it tend to maintain? And the answer is it will maintain AOA and therefore airspeed.

[BluSdUp]

Vessbot

I hear what You say about the elegance of the pitch power copeling , on the 737-800 I found it rather excessive and ridicules. Then it becomes second nature.
I do worry about the transition for the P3 Orion crew to the P8 Poseidon.
I have seen some nice low passes filmed from flightdeck inspecting trawlers and the likes at 100 feet with steep turns from the P3.
Dont know about the Electra , but the 738 takes no prisoners with rapid thrust reduction without massive pull and trim.
I am sure someone will mention this to them,,,,,

Regards
Cpt B

[gums]

Salute!

Some good points, Vess.
I should hasten to add that your final statement is only accurate for non-FBW aircraft. I also heartily endorse your first statement:

The meaning of "trim" has branched out (as meanings tend to do)

This is especially true for any plane that has an elevator on the horizontal stab, whether fixed or trimmable (as many cargo designs exhibit).

The planes with only moving stabs and no "elevators" still result in a stable pitch/ AoA by moving that stab. But I don't know of any commercial airliners with such a configuration. And the planes with the all-moving stab seem to be fighters or unique, single purpose planes.

As far as your last statement,

But even in the strict sense of the way you use the word, the question of the thread is: once the elevator is in a certain place and your hand is off the stick (aka trimmed), what will the airplane do after disturbances? I.e., what parameter will it tend to maintain? And the answer is it will maintain AOA and therefore airspeed.

Yep! And just remember that the particular FBW laws for your plane move the control surfaces as required for the commanded gee or AoA or even speed without regard for AoA.
I haven't flown an FBW type with an elevator at the rear end of a stab. So I do not know how the control laws for those decide whether to change the incidence of the stab or crank in some elevator deflection, or both.

Nevertheless, a good discussion. And last I flew, "trimming" was intended to relieve control pressure/movement in order to achieve an attitude or AoA/speed/gee.

Gum sends...

[DaveReidUK]

Originally Posted by gums

The planes with only moving stabs and no "elevators" still result in a stable pitch/ AoA by moving that stab. But I don't know of any commercial airliners with such a configuration. And the planes with the all-moving stab seem to be fighters or unique, single purpose planes.

The TriStar had a stabilator, with control column movement directly altering the stab incidence. It had elevators too, but they weren't directly controllable, being simply geared to the stab.

[Vessbot]

Hi Gums! Yes absolutely, everything I said goes out the window for FBW airplanes with C* control law (classic Airbus, F-16) which maintain a blend of attitude rate and G.

But it still stands, in a modified way, for C*U planes (777, 787, A220). It seeks to maintain a speed (like a non-FBW plane) but even better: eliminates the effect of thrust-pitch couple so the trimmed speed doesn’t change, damps oscillations, and maybe other clean-ups I’m not aware of. So while not acting to maintain AOA, it still ends up doing it as a byproduct.

Now I’ve never been trained in a FBW plane of either type and this is just from doing a lot of reading, so I hope I’m not talking out of my APU exhaust on the topic. But I would absolutely love the chance to fly a C*U plane and experience this in person.

Originally Posted by BluSdUp

I hear what You say about the elegance of the pitch power copeling , on the 737-800 I found it rather excessive and ridicules. Then it becomes second nature.

I do worry about the transition for the P3 Orion crew to the P8 Poseidon.

I have seen some nice low passes filmed from flightdeck inspecting trawlers and the likes at 100 feet with steep turns from the P3.

Dont know about the Electra , but the 738 takes no prisoners with rapid thrust reduction without massive pull and trim.

I am sure someone will mention this to them,,,,,

Regards

Cpt B

You remind me of a point, where in all the discussions of the MCAS crashes you see all these reflexive quips about how they should have pulled the thrust levers back to slow down to unload the stab, as the obvious move that both crews missed, leaving the commenter bewildered. But it’s only “obvious” on the naive premise that thrust controls speed and ignores what we’ve discussed here, that trim determines speed and thrust determines climb/descent angle... as well as its own effect on trim.

So this ubiquitous solution to the problem of being severely mistrimmed nose-down and excessive on speed while too low on height, is an action that...

1, immediately pitches down due to the thrust pitch couple
2, trims to a higher speed due to the same
3, establishes a descent due to reduction of excess thrust

... which seems more than a little misguided. I brought this up in one of the MAX threads in Rumours & News and got accused of being a nonpilot impostor.

Now I don’t know if this consideration went through their minds and they dismissed it as an option upon reasoning, or (I think, the more likelier) that they were brainlocked. But if I was in their scenario I wouldn’t pull back the thrust either.

I acknowledge that some time between 1 and 2 above there would be a speed reduction, but only temporary and, by my intuition, small, before reversing. I’ve never flown any Boeing, but you being a 737 pilot and conversant in this topic, I’m curious for your take on it.
​​​​​​​

[gums]

Salute!

@TCASBLUE , sorry we all got away from the basic trim question. But a lotta things have changed since those days of yore when I and many others here flew simple planes with simple control surfaces connected to simple cockpit control doofers - sticks, wheels, levers, yokes, etc.

Your basic answer is YES !!! Pitch trim results in an AoA, and that determines the induced drag component of the equation. And thousands of nasal radiators flew millions of landings aboard moving runways using power to control rate of descent, unlike most Air Force folks that "pointed" and used throttle to control speed. In some planes as I flew in mid seventies like the A-7, the thing was very stable with AoA, and tho I landed on concrete it was easy to trim for a speed/AoA, and use throttle to nail the descent rate on approach. The Viper control law with gear down was heavily biased for AoA, but still had the gee command base law. Nevertheless, I could use the trim for AoA and power for descent rate.

In all fairness for the non-FBW folks, and remember that all FBW planes have variations of the control laws. The Viper pitch trim was purely gee unless gear was down, and even then it was gee cmd with heavy AoA and pitch rate bias. The 'bus pitch laws have more stuff cranked into it, but the folks I have talked with say that moving from the Viper to the 'bus was real easy.

Great thread, folks.

Gums ends...

[Uplinker]

Very sad most pilots will never ever experience just an airspeed rise due to more thrust WITHOUT pitching up, like B727.

I never liked having to re-trim every time speed is changed. Obviously it is simple physics, according to the velocity squared term in the lift equation, but I like the way Airbus FBW automatically removes the power/pitch couple when flying manually, and also that no pitch-up input is required from the pilot side-stick when banking to turn. Bliss !

[gums]

Salute!
@ Uplinker!!!!

You must be a spoiled child of the magenta line generation.

BTW, there's no true "manual" in the 'bus, and closest to the old planes is when the sucker is in "direct" law, and you have volts versus cables commanding the elevators and ailerons and rudder and .....

Gums pokes fun.....

[Uplinker]

Hi Gums, nice to speak with you.

Fun taken in good spirit, (but for the record I have flown the Shorts 360, the BAe146, B737-300 and Airbus FBW, amongst others. Airbus has a green line.............. )

I much prefer the Airbus FBW. I see it as a bit like the difference between a manual car transmission, (stick shift) and an automatic. I have driven both, but much prefer the modern electronically controlled autos in today's traffic, since it makes the driving experience much easier and less fatiguing.

I agree there is no true manual in the FBW: The difference can be thought of as: the autopilot is always engaged, and what you actually switch in and out is where the A/P gets its guidance from; the FMGS or your side-stick

.

[Vessbot]

My favorite description is that the pilot is only a voting member of the party.

[BluSdUp]

Vessbot

I am a bit hesitant to make a quick summary of the 737-800 speed stability.
But why not.
Firstly , it is not a very well balanced aircraft and not particularly stable. I have been told the -700 and the 300 to 600 as well is much better.
The -800 has a long fuselage that gives the elevator and the stabilizer a long arm.
Then power changes give rapid pitch changes that You have never ever seen, simply because no other aircraft has been certified with such large power/pitch couple.
The main problem is that anything but a small power change will cause it to overshoot its in-trim speed ( Or AOA if you prefer). And the natural stability is slow to stabilize if uncorrected.

The problem with the second Max accident is that he HAD to reduce power eventually, as the aircraft will go well past say M.95 in level flight at 10 000 feet, remember: 70 ton aircraft and 52 000lbs trust.
Anyway
The closest thing I have ever been to something similar is manual reversion in the sim, ie, no hydraulic for flight-control ,cables only and manual electric stab trim.
And there, it is all about small power input , get in trim and stay there. Descend and climb gently with power change, configure early etc.

With regards to the potential recovery of the latest Max accident one way would be to trim, reduce a bit of pwr, speedbrake. Get acceleration under control, dont sink, reduce power to get speed down towards 250. Then Flap 1 and speed 230, plus minus 20, get in trim, stow speedbrake re-trim.
With Flaps 1 you have a band from ca 190 to 250kts to play with and an elevator that can better overpower the stabilizer. As in above ca 270 to 280 kts : no chance ( with full nose down stab)

230kts is my go to speed in the 737-800 in any and all cases initially with control problems, THEN adjust or as per QRH.

DICLAIMER
Not blaming the accident crew.

Regards
Cpt B

[Vessbot]

BluSdUp, thanks for engaging despite the reluctance. Your characterization of the severity of the thrust pitch couple matches what else I’ve read about it. However, I still see a disconnect between that, and your suggestion of the consequences of pulling the thrust back in the MAX crashes.

I don’t doubt at all the severity of their high speed problem. But here again what you posit as the solution (“HAD to reduce power”) seems to ignore what you said about the couple, and to be based in the everyday mentality of flying constant altitudes, where thrust is related to speed and the automatic trimming that’s done by the autopilot (or, less often, the manual trimming done by the pilot) falls to the background to the point of being ignored and even forgotten about.

But this trimming, along with the fundamental AOA-speed relationship that it feeds into, is not a side effect that can be neglected but rather is central to the plane’s flight. In order to slow down (level or otherwise), the AOA MUST be increased, which necessitates back elevator and/or back trim. Since they were out of both, this was not possible. To the extent that it was possible (like them tapping into a last-ditch previously unknown reserve of strength) they could have made that same elevator change, meaning the same AOA change, meaning the same speed reduction; with or without a thrust reduction. BUT if they did it without the thrust reduction, it would result in a climb. So they would get the same speed reduction, with the benefit of gaining altitude in this low altitude control emergency, AND WITH LESS RESISTANCE to their aft pull, due to the lack of pitch down couple.

So, to summarize with some example numbers:

4 deg. AOA = 400 knots
5 deg. AOA = 350 knots

And this is true regardless of thrust, and whether they’re in a climb, descent, or level.

Losing that 50 knots would require that same 1 deg. AOA increase, which must be commanded by the elevator regardless of vertical path. Losing that speed by climbing rather than by thrust reduction is A. more possible (lack of pitch down couple) and B. more beneficial (gaining altitude).

---

I guess the best way to simulate this in one's head (or, simulator) is to start stabilized level, at high speed, and in trim. This represents the X pounds of forward stick force due to the runaway-then-cut-out stab, and the -X pounds of aft stick force from your hands. Since X-X is zero, we can represent this by being in trim and you're not allowed to touch the yoke or trim control (since they were already pulling as hard as they can). Now we're in the semi-stabilized condition of the first crash before the final plunge.

So, then, what happens when thrust is reduced?

Now let's modify this by representing that extra superhuman oomph that might have saved them, by pulling on the yoke with the force of one pinky finger hooked around it. Would you be better off simultaneously reducing thrust in an attempt to maintain level flight (and having the thrust pitch couple now counteracting your pinky), or rather leaving the thrust alone and pulling up into a climb, thus buying yourself altitude, time, and options?

---

To put all this in another way: Since pulling into a climb reduces speed just as easy as (actually, easier than) reducing thrust, what would be the purpose of reducing thrust in the MAX crash scenario?

[Stuka Child]

Originally Posted by Vessbot

To put all this in another way: Since pulling into a climb reduces speed just as easy as (actually, easier than) reducing thrust, what would be the purpose of reducing thrust in the MAX crash scenario?

To give your elevators a chance against that massive stabilizer. If you keep the thrust in there, you will continue to speed up and at some point the thrust pitch couple is going to become irrelevant as the aircraft starts going downhill. At high speed going towards the ground with full nose down trim, you might as well not have elevators. Only thing you can do is trim for your life. And depending on how close the ground is, results not guaranteed.

Much better to reduce thrust ahead of time. It's going to want to go downhill, but you should have enough elevator authority to keep it from doing so despite being mad out of trim. At the same time you use electric trim to lighten the pull and if you reach the conclusion that something is wrong with the automatic trimming (which conclusion you probably SHOULD have reached by now), then you cut it off with the switches. You are now in a more or less stabilized state.