Trim only sets AoA

[sycamore]

BlSUp ,for those of us who have not flown a 737 ,can you tell us what the main effect would be on popping the speedbrakes at various speeds up tp Vmo/Mmo,and then the combined effect of pulling power back at the same time ...IE can the thrust reduction couple overcome the effect of speedbrakes,as I`m assuming they produce a `nose-up` change,or should do,....??

[Vessbot]

Originally Posted by Stuka Child

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.

​​​​Stuka Child, you address two scenarios: the first one where you do not have enough elevator authority to do anything with, and the second, where you do.

Let me ignore the first for a while, and respond to just the second. To reiterate, you stipulate that “you should have enough elevator authority to keep it [the thrust pitch couple] from doing so [want to go downhill].” To that you say, “much better to reduce thrust ahead of time.” Again I ask, why? I already identified all the advantages of not reducing it. (That you will climb and gain valuable altitude, the value of which hopefully doesn’t need spelling out; and that there will be less resistance to the pitch up than if thrust is reduced. In a scenario where elevator authority is marginal, this is significant.)

What is the advantage of reducing it? You cannot say “to give your elevators a chance,” because that chance is a consequence of airspeed, and airspeed has nothing to do with thrust. You reduce airspeed by increasing the AOA, which establishes a climb if you leave the thrust alone. This airspeed reduction satisfies your “give your elevators a chance” with or without the thrust reduction just the same, so it cannot be the answer.

[BluSdUp]

Speed control on 737-800

Sycamore:
The speedbrake is pretty much pitch neutral, perhaps a slight pitch up tendency (according to the book).
Can be used full up to 320kts then only half until Vmo 340kts as it puts high stress over time on the tail.

Vessbot:
I am still with You on the basic theory, BUT if unable to trim NU for any reason with a out of trim ND stab on the -800 you WILL run out of elevator if power is not reduced to control speed.
So a thrust reduction for a prompt return to say 230kt pluss/ minus , then trim and return to say 70% N1 for level flight or 85 to 90 for a good climb when in better trim and better elevator effect is a must.
The 737-800 would never ever have been certified with the adverse pitch/power couple if certified as a new aircraft Type.
Quite a ride , if You get behind and out of synk with pitch and power. Proven time and again on GA with full N1 and slow trimming. ( the opposite of the above, but still not AOA or speed stable!).


Regards
Cpt B

[Vessbot]

Originally Posted by BluSdUp

Vessbot:
I am still with You on the basic theory, BUT if unable to trim NU for any reason with a out of trim ND stab on the -800 you WILL run out of elevator if power is not reduced to control speed.

But again, if out of trim ND, as you’ve already agreed in another part, reducing the thrust will trim you further ND and exacerbate the problem.

And as a separate effect, the reduction in excess thrust (from zero to a negative value) will command a descent at the trimmed (now faster than before) speed. But, the one possible saving grace for the thrust reduction action, before the descent there will be a temporary speed reduction (that causes the lift reduction that in turn causes the beginning of the descent). But, I emphasize, it’s only temporary and my gut feeling says that it will be relatively small. How small? I don’t know, but I have serious doubts about it getting from 300-350 to the 230 that you posit as the bootstrap point for the rest of the trimming process back to normal.

A very illuminating sim experiment that I absolutely want to see, is to level the plane at 350 knots, trim hands off, cut out the stab, and without touching anything else slam the throttles back to see exactly what happens (magnitude and time) with the airspeed.

[Storm Girl]

Attitude, power, trim.
I like it.

[safetypee]

For powered aircraft the engines are the source of energy.
Wings convert energy to lift.
The elevators distribute the lift.
Trim balances residual force in the elevator control system.

[Vessbot]

Originally Posted by safetypee

For powered aircraft the engines are the source of energy.
Wings convert energy to lift.
The elevators distribute the lift.
Trim balances residual force in the elevator control system.

The way I think of it is, the elevator (AOA) routes the energy toward speed vs. climb angle. (And this is true for powered and glider alike.)

A more complete picture is that you're always getting energy from the engines and losing it to drag, but any leftover amount (excess thrust) goes to the routing control mentioned above. In a glider, that is a negative amount and you still have to choose what to do with it the same way.
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