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Old 05-15-2019, 07:55 AM   #121
amazingme
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Originally Posted by Curly View Post
That's correct in first test they pulled out of the dive with the trim system. In the second test they did not use the trim system and pulled out of the dive with the stick alone. The purpose of the tests conducted on Feburaruy 15 to March 12 1943, is explicitly stated in the notes.

"This trials started with idle dives to estimate an horizontal stabilizer trim setting that makes it possible to recover a dive with the stick. This setting was +1 degree 15 minutes and was blocked in the direction nose heavy by a stop to assure the same settings at every flight." "... After this the plane was pulled out of the dive just using the stick." As indicated by the chart on the right with the trim setting of +1 degree 15 minutes.

The reason why they wanted to know the trim position was because they considered pulling out of a dive with the trim alone unreliable.

"pulling out with the horizontal stabilizer trim is a potential danger (high g acceleration increase in the pull-out) so dive recover should be achieved without changing the position of the horizontal stabilizer."

"If you trim the elevator just .5 degrees more nose heavy the force in the direction push is less but it is not possible to recover just by using the stick, it is necessary to use the horizontal stabilizer trim. At high speed the horizontal stabilizer trim is heavy and just jerkily moveable." The reason these test were conducted was to "investigate insufficient elevator controls at high mach numbers". They were trying to make the craft safer by making it possible to recover from a dive with the stick alone.

They also ran into problems with this trim setting when testing powered on dives. The pilot could not maintain the necessary amount of push force needed to keep the aircraft in a dive with the power on.


“In a full throttle dive despite previous trim to +1.7° just 30° angle and IAS Va = 650 km/h at 6 km altitude was reached because the elevator trim was frozen, and the stick force was too high to push it farther forward”

"unlike idle dives it was not possible to hold the angle of dives at full throttle with the same trim setting, because the upward torque of the engine. Trim tabs were set to nose heavy to reach a similar force like in idle dives." Thus, a powered on dive with the trim set to +1 degree 15 seconds is only recoverable with the stick, if the trim tabs are set to a nose heavy position.

So why use the trim tabs to adjust the stick forces on the elevator during the powered on dive tests? It's the easiest way to change to hinge moment coefficient. So that desired elevator trim position can be achieved with same amount of force. As the hinge moment depends linearly on the tail angle of attack, the elevator deflection and the deflection of the trim tab angle.

Also, the notes indicate that there is a stopper on the trim system. So maybe it was quicker to just adjust the tabs then to adjust the stopper. Also we know from the report that the trim gauge "has an insufficient resolution" to make the small adjustments needed here.

Again, the purpose of these tests was to determine a trim position that allowed the pilot to recover from a high speed dive using only the stick. Since the differences between a successful and unsuccessful result is less than .5 degrees of trim; Adjusting the trim tabs seems like the easiest and most reliable to be able to achieve the desired test results.

The trim tabs are needed in the dive because nose down trim is limited by the stopper. During the dive elevator wants to return to the return hinge moment to zero, as the controls are reversible. The stick is pushing back against the pilot during the dive. Due to the high speed and increasing mach effects, the pilot cannot exert enough force on the stick to maintain a powered on dive at desired angle, <60, with the trim tabs set to neutral and the elevator trim limited to +1, 15'.

By decreasing the hinge moment via the trim tabs it is possible to deflect the elevator sufficiently to make the craft maintain the dive and be recoverable with the stick alone. As the pilot was incapable of pushing or keeping the elevator at the correct postion to maintain a dive angle greater than 60 degrees.

"During first flights the position of the stop unit was at +1°45'. The elevator forces at this stabilizer position were not sufficient to reach a dive angle greater than 60° at 100% power. Therefore, the surface area of the static trim tab was doubled."



Changing the trim tab position changes the hinge moment, thus also changing the derivative dhm/disma.

The hinge moment of the elevator is Che = Che0 + dChe/d Alpha with respect to Alpha ht + d CHe / d sigma with respect to sigma e + dChe / dSigma Trim tabs with respect to Sigma of the trim tabs.
So by altering the trim tabs trailing edge down you change the hinge moment.
q.e.d. Bravo!!
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Old 05-15-2019, 10:02 AM   #122
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Originally Posted by Yo-Yo View Post
Because trim tab and stab do the same thing regarding the hinge moment. Because the derivative dmh/dsigma (hinge moment vs elevator deflection) remains the same regardless of what way you shifted zero moment and you have to apply the same force to get the same g at the same speed.
No they dont, in school academics you are free to use it as approximation for small angles. In the real world you get kicked in the ass.

I cited the german report which speaks of Kraftumkehr im Höhenruder, that translates to elevator reversal.



Thanks Curly. I have honestly lost patience by now.

Quote:
Originally Posted by jcomm View Post
P.S.: But probably the developers of this module did have access to the report of the first test above, regarding the idle dives, where the risks of setting the trim tabs are described,
but not to the 2nd part where powered / full throttle dives are referred, and the reverse applies, making the setting of the trim tabs into nose-heavy positions understandable / advantageous.
It is the same test and in the abstract both cases are summarized.
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Old 05-15-2019, 10:22 AM   #123
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Originally Posted by Curly View Post

Changing the trim tab position changes the hinge moment, thus also changing the derivative dhm/disma.
This is a wrong statement because of two reasons: 1) trim tab affected small part of elevator span, and, even more basic 2) considering the trim tab as a control surface for an elevator we must remember that control surface deflection does not affect the slope of the lift vs angle but only shift it upward or downward.
Quote:

The hinge moment of the elevator is Che = Che0 + dChe/d Alpha with respect to Alpha ht + d CHe / d sigma with respect to sigma e + dChe / dSigma Trim tabs with respect to Sigma of the trim tabs.
So by altering the trim tabs trailing edge down you change the hinge moment.
Absolutely right! It is a LINEAR equation. But as you can see there is no difference what way you achieved zero hinge moment - using trim tab , stab or both. Finally, you have to apply the same moment to deflect the elevator to same angle from the zero point.

To see how it works regarding the trimmed CL we must add a couple of extra derivatives of CL, but it will not change anything - in the linear are any trim action regardless of tabs or stab will lead only to shift of force dependance vs elevator deflection. See the graph from the report: the force curve is the same but shifted parallel itself.
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Old 05-15-2019, 04:34 PM   #124
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Ok Yo-Yo,

I believe I'll try a different, much more practical / pragmatic approach!

Implement the adjustable elevator trim tabs, visit Portugal and I'll pay you a dinner with some excellent Portuguese wine !!!

What about that ? Can you refute it ?
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Old 05-15-2019, 08:34 PM   #125
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Hey Yo-Yo,

There is so much smoke screen up I dont even know what we are disagreeing on anymore. So from human to human, would you please give us (your customers, critics and fans) an honest answer? Where exactly do you disagree with us?

We agree that the K gear ratio has changed, correct?

Can we also agree that trim tabs and stab trim in real life are accompanied by slightly different effects, but the approximation you are using is perfectly valid for small deflections?

The trim tabs were doubled in size on the K-4, correct?

For doubling the trim tabs the high speed trial report gives insufficient dive angle at 100% power as a reason, agreed?

After adding double trim tabs to get rid of elevator force reversal, the stab was limited to +1,15°. In that case the dive could be recovered with stab at constant +1,15° with elevator only, correct?

In this test the engineers set the trim tabs nose heavy and the stab to +1,15° to gain nose down authority, yes? (Not sure if this was posted yet, but here is the section of the test report stating that.)

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Old 05-15-2019, 11:54 PM   #126
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Well this doesn’t bode well for the p47 does it
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Old 05-16-2019, 04:12 AM   #127
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Quote:
Originally Posted by rel4y View Post
Hey Yo-Yo,

There is so much smoke screen up I dont even know what we are disagreeing on anymore. So from human to human, would you please give us (your customers, critics and fans) an honest answer? Where exactly do you disagree with us?

We agree that the K gear ratio has changed, correct?

Can we also agree that trim tabs and stab trim in real life are accompanied by slightly different effects, but the approximation you are using is perfectly valid for small deflections?

The trim tabs were doubled in size on the K-4, correct?

For doubling the trim tabs the high speed trial report gives insufficient dive angle at 100% power as a reason, agreed?

After adding double trim tabs to get rid of elevator force reversal, the stab was limited to +1,15°. In that case the dive could be recovered with stab at constant +1,15° with elevator only, correct?

In this test the engineers set the trim tabs nose heavy and the stab to +1,15° to gain nose down authority, yes? (Not sure if this was posted yet, but here is the section of the test report stating that.)

The only thing that is really for sure is 30% difference in the gear ratio.

No, IN THIS CASE regarding hinge moment - definetely no, at least for the deflection you use to maneuver the plane within limits. (By the way, what is finally your opinion about Case A g-load for Bf-109?).

And, of course, I agree with two last facts that the trim tabs DO help to maintain dive as 1 15' stab alone was not able - there is nothing contradicting with the scientific approach.

But the report tells ONLY ABOUT this fact.

And there is no clear evidences that 1) The dive in the last case was recovered without using trim wheel 2) or the same result could be achieved (or could not be achieved) by carefully setting somewhat between 1 15' and 1 45'.
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Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів
There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.
Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me
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Old 05-16-2019, 04:27 AM   #128
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Originally Posted by rel4y View Post
No they dont, in school academics you are free to use it as approximation for small angles. In the real world you get kicked in the ass.
I see the students's paphos. Please be not so arrogant or you would need to kick the ass of Ostoslavsky and Bushgens... and do not forget that only simplified approximations made F-117 possible.

Anyway, do you have something to disproove linear approach within our case? Real measurements, for example?
I have, by the way. A lot.

(And, please, do not forget to make screenshots of our discussion... )
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Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів
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Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me
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Old 05-16-2019, 05:45 AM   #129
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Originally Posted by Yo-Yo View Post
The only thing that is really for sure is 30% difference in the gear ratio.
there is no clear evidences that 1) The dive in the last case was recovered without using trim wheel 2) or the same result could be achieved (or could not be achieved) by carefully setting somewhat between 1 15' and 1 45'.


For 1 you don't consider, the that report says "This trials started with idle dives to estimate an horizontal stabilizer trim setting that makes it possible to recover a dive with the stick. This setting was +1 degree 15 minutes ."






and that the report notes "the plane was pulled out of the dive with the stick alone."





or that the second chart with the 1 degree 15 minute does not have note stating that the craft was pulled out of the dive with the trim.




To be clear evidence that the craft was pulled out of a dive with a trim setting of 1 degree 15 minutes with the stick alone?


here's the entire page from the report I'm referencing.
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Last edited by Curly; 05-16-2019 at 05:46 AM. Reason: typos
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Old 05-16-2019, 08:05 AM   #130
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THis is the translation, just to be sure we are see all the text and not we want to see:
Quote:
This trials started with idle dives (automatic propeller pitch, n = 1500) to
estimate an horizontal stabilizer trim setting, that makes it possible to
recover a dive with the stick.
This setting was +1°15' and was blocked in
direction nose heavy by a stop unit to assure to have the same settings
at every flight.
Highest speed after peeling off at 10.5 km height above
sea level in a 70-80° dive was IAS Va = 745 km/h at 3.8 km (analysis TAS
Vw = 880 km/h).Following force development at the elevator was noticed:
After peeling off,force into direction “pull” because this trim setting is nose
heavy at cruising speed, when the speed increase “push” to keep the
plane at the angle of dive till TAS Vw = 850 km/h was reached, then the
force was reducing till zero reached. After this the plane was pulled out of
the dive just using the stick.
(Chart) Without peeling off and just pushing
the stick forward it takes to long till the planes is taking speed, and it is not
possible to hold the angle of dive. If you trim the elevator just 0.5° more
nose heavy the force in direction push is less, but it is not possible to
recover just by using the stick, it is necessary to use the horizontal
stabilizer trim (Chart). At high speed the horizontal stabilizer trim is heavy
and just jerkily moveable. Flaring out is soft at the beginning, but it´s
increasing a lot during the course by itself.
Unlike the idle dives, it was not possible to hold the angle of dives at full
throttle with the same trim setting, cause the upward torque of the
engine. Trim tabs were set to nose heavy to reach a similar force
development, like in idle dives.
From this paragraph one can clearly see that 1) the plane can be put in a steep dive at idle with 1_15' and recovered with the stick alone.
2) the plane can not be recovered with a stick alone having stab at 1_45'
3) the plane can not be put in required dive at full throttle with stab at 1_15'
4) not to change the stop at 1_15 they simply added small amount to nose-down trim using trim tabs

If you think that in the last iteration they were successfull having full dive-in capability (idle and full power) and recovered using the stick alone, though they have not summarised the result directly (that is a bit strange for me)- ok, let's suggest it.
They got a fork between 1_15' and 1_45', thus, they fine tuned it with trim tabs, that is quite natural. So, they reduced pulling forces, increased recovering forces but shifted both to the range human can bear.
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Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів
There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.
Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

Last edited by Yo-Yo; 05-16-2019 at 08:40 AM.
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