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Old 05-14-2019, 08:34 AM   #111
Yo-Yo
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Quote:
Originally Posted by jcomm View Post
Question for Yo-Yo...

- Why would then, the German engineers, have chosen to install those trim tabs in the K-4, actually a pair of trim tabs easily seen ?

- Why would the reports mentioned by Curly refer the reasons for it's use ?

DCS is trying to simulate as close as possible the 109 K-4. You find that it doesn't make sense to have that solution, and it is counter-intuitive, so the decision is to simply truncate it by creating a fixed ( at zero deflection ) elevator trim tabs, because you think ( ? ) that's how the ww2 engineers, ground crew and pilots should have chosen to use it ?

I would accept a justification based on complexity of implementing this feature now, that the module is final, due to the fact that you are 1 ( one ) and there is a LOT of work to do because we all want more ww2 modules, and heck, as much as I am critical here, I also am here because DCS is second to none in details, interaction with modeled systems, precision - it's an Air Combat Study Sim, it's THE AIr Combat Study Sim, and pretty much the only one we have at that level and you should be "blamed" for that :-)

I just don't think that questioning the evident is of any use here in this thread. Both rel4y and Curly have done a REMARKABLE job in providing well founded justifications for the importance of modeling those elevator trim tabs.
The modelling must not be counter of fundamental laws. Any exceptions, that sometimes are possible to seem agains the laws must have concrete-hard documental proofs.
Sometimes, some speculations based on unproven facts or "I think it must be so" approach are deeply wrong (remember, please, X-wind propwash effect on the wing with flaps ).

So, 109G various trim curves from Soviet and German sources are in good accordance and give opportunity to obtain right data for various parameters critical to longitudinal behaviour and now it's the best way to have 109 as close to the real as we can.

The only point I would agree is the 30% increasing of gear ratio, giving 30% decreasing of required forces, but, as I mentioned above, the values of force limits in the simulation are set with a good volutarily approach, so 30% more... 30 less - nothing would be closer to real life taking in account that pilots are very different.

And, by the way, if we increase these limits the percentage of broken wings will be significantly higher.
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Old 05-14-2019, 08:37 AM   #112
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Quote:
Originally Posted by Yo-Yo View Post
Ok, but it is general information, but we are interesting in 109 IN DIVE and in (as far as I can see) full power trim in level flight.
So, your point, probably is that keeping 1.25 degrees to prevent a plane form unrecovered dive (the report gives us the direct proof of this reason) the trim tab magically put elevator down at lower speed, lower altitude and would never affect high speed/Mach diving... Hate doing it but I have to disappoint you - it's impossible, because the same report stated that trim tabs were used to make high speed power-on diving possible AT HIGH-SPEED/HIGH Mach. And, by the way, it was not mentioned if the trim wheel was used to recover after bending the trim tabs to nose-down.. I think, using the simple logic - yes.

So, my question is still actual.

And by the way, have you seen mentioned CRUISE seting as +1 degree?
Yo-Yo please.. lets stop this nonsense. You asked me to name an engineering difference in effect between trim tab and stab. I gave you several. The question was are stab and tab effects the same, because that is what was said earlier in this thread. NO, they are not. Do you disagree?

Then I gave two other pressing reasons why we need trim tabs on the 109. Did I not adress your question here? Do you honestly not see a reason to add trim tabs? My point is not what you put in my mouth but this! The second one is already addressed.

Quote:
Originally Posted by rel4y View Post
First and most bluntly, because its historical. Thats simply how the 109 was constructed and used operationally. In the late models they were doubled in size because they were important features of longitudinal stability. Could they have redesigned the elevator mechanism to be more reasonable? Surely, but changing the production lines most likely wasnt feasable, so they improved on what they could work with. I mean, what are we simulating here, our own personal opinion how the 109 should have been designed & operated, or the real deal?

Second, ...

Third, the maximum trim range clearly increases and it would be possible to trim the aircraft at combat cruise. In german manuals combat cruise (in contested airspace) was considered to be maximum permissible continous rating (1,35 ata for K-4) and not maximum fuel efficiency. In air combat patrols you would want to go as fast as your motor possibly allows, since at any moment a P-51 or Yak could jump you. And I am sure pilots would not have liked their aircraft to pitch up constantly because the lazy mechanic has not set the trim tabs to anything but neutral.
Quote:
And, by the way, it was not mentioned if the trim wheel was used to recover after bending the trim tabs to nose-down.. I think, using the simple logic - yes.
Actually no, it was constant throughout the dive. There is even a graph about it and the passage in the text is three lines long. I would not mind translating the text for you if nobody has done yet. Nobody disputed that they set the stab different from 0° btw, just that it wasn't changed and elevator reversal doesn't occur at stab 1,15°. I can quote myself like six times stating exactly that. Btw they changed the grease in the stab mechanism.



Quote:
Originally Posted by Yo-Yo View Post
The modelling must not be counter of fundamental laws. Any exceptions, that sometimes are possible to seem agains the laws must have concrete-hard documental proofs.
Sometimes, some speculations based on unproven facts or "I think it must be so" approach are deeply wrong (remember, please, X-wind propwash effect on the wing with flaps ).
Can you tell me how trimming the stab at lets say + 1° and setting the tabs to deflect the elevator ~3° downwards is violating that? Because that would set cruise trim to around combat cruise and is actually what was done in real life. The only thing deeply wrong is your argumentation here.

Quote:
Originally Posted by Yo-Yo View Post
And, by the way, if we increase these limits the percentage of broken wings will be significantly higher.
109G wings were designed to withstand 10G + margin. I can show you a report where 10.5 Gs were reached. If the pilots violates that, its his own demise/death. I also remember earlier in this thread you said that it would have pretty much no effect at all. Why the sudden change of mind?

I am really sorry how the tone of discussion changed, because Id like to get things done in a civilized matter. But I also wasnt the one to call others stupid in the first place. Now it is just a pi**ing match. Yo-Yo, Id like to ask you again very politely to think about what we are saying and I would be happy for a week straight if youd give us elevator trim tabs. I think this will be my last post in this topic.

Btw, I saved this thread on the wayback machine, just in case somebody chooses to delete it for unknown reasons.
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Last edited by rel4y; 05-14-2019 at 10:33 AM.
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Old 05-14-2019, 04:37 PM   #113
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Originally Posted by rel4y View Post
Yo-Yo please.. lets stop this nonsense. You asked me to name an engineering difference in effect between trim tab and stab. I gave you several. The question was are stab and tab effects the same, because that is what was said earlier in this thread. NO, they are not. Do you disagree?
No, the bundle of facts you quoted is not relevant to this case and I explained, why.

Quote:
Then I gave two other pressing reasons why we need trim tabs on the 109. Did I not adress your question here? Do you honestly not see a reason to add trim tabs? My point is not what you put in my mouth but this! The second one is already addressed.
Did you see the clear explanation WHY it was done? I do not.

Quote:

Actually no, it was constant throughout the dive. There is even a graph about it and the passage in the text is three lines long. I would not mind translating the text for you if nobody has done yet. Nobody disputed that they set the stab different from 0° btw, just that it wasn't changed and elevator reversal doesn't occur at stab 1,15°. I can quote myself like six times stating exactly that. Btw they changed the grease in the stab mechanism.

It is not a phenomena named 'elevator reversal". It is "nose tucking" that means that the plane requires nose-up elevator displacement to keep certain AoA as Mach number is growing. And it's the saem you can see in DCS.
Quote:


Can you tell me how trimming the stab at lets say + 1° and setting the tabs to deflect the elevator ~3° downwards is violating that? Because that would set cruise trim to around combat cruise and is actually what was done in real life. The only thing deeply wrong is your argumentation here.
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.
Quote:



109G wings were designed to withstand 10G + margin. I can show you a report where 10.5 Gs were reached. If the pilots violates that, its his own demise/death. I also remember earlier in this thread you said that it would have pretty much no effect at all. Why the sudden change of mind?
Do you know exactly the case A g-load and for what GW it was set?
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Old 05-14-2019, 05:38 PM   #114
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so did the german pilots flying this airframe in WW2 have these elevator trim tabs? If so i would think to be historically accurate they should be on the airframe in this sim? If not then...not. Kind of simple.?
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Old 05-14-2019, 06:35 PM   #115
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The real question is did the pilots tweak them to their own desires, or were they set by the factory/mechanics to be in a specific criteria.

Pilots messing around with things like that on the ground without the ability to change it once in the air could have disastrous consequences.
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Old 05-14-2019, 08:02 PM   #116
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Originally Posted by Shahdoh View Post
The real question is did the pilots tweak them to their own desires, or were they set by the factory/mechanics to be in a specific criteria.

Pilots messing around with things like that on the ground without the ability to change it once in the air could have disastrous consequences.
That is the real question, as well as, of they used them, how did they set them...
There is a set of FLIR controls in the F-14B, yet it does not have a FLIR, as it was replaced by the TCS. The difference is, we have a good documentation for this change. For the BF.109 K4 there seems not much documentation on late war trim tab use to be around, so we can guess how they "must have been" used, if at all.
I could see some irony in putting the trim tabs in and maybe let people do what they want and complain about the issues deriving from tweaking the trim tabs next?
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Old 05-14-2019, 09:35 PM   #117
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For a given fuel / amno load reference, say 60% fuel in the DCS 109 settings, ISA atmospheric conditions, and a given altitude, what will be the lowest power setting above which full nose heavy stabilator trim will not be sufficient to overcome the pitching up moment ?

Is it within continuous and combat power settings ? Is it emmergency ?

Whatever it is, the pilot will then have to exert a force, constant, pushing on the stick to maintain altitude.

If the trim tabs are then used / set on ground to a given deflection angle, this force will probably be eliminated for those same flight conditions, so, just as rudder and aileron trim tabs are adjusted to save the pilot from the need for constant rudder and / or aileron deflection at cruise why wouldn't it make sense to give him the very same chance regarding the use of down elevator ?
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Old 05-14-2019, 10:07 PM   #118
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Originally Posted by jcomm View Post
(...)Whatever it is, the pilot will then have to exert a force, constant, pushing on the stick to maintain altitude.

If the trim tabs are then used / set on ground to a given deflection angle, this force will probably be eliminated for those same flight conditions, (...)
...and all other flight conditions, as well, as you cannot climb out the cockpit and "reset" the trim tabs. If the alteration to the nose down trim now puts you in a position where you can accidentally get into a dive with no chance to recover, when you "forget" to adjust for your little trim tweak, or worst case can't trim for a safe landing anymore, you die. In real life most pilots I heard from died only once, and that encouraged most of them to be very careful to adjust, tweak and modify an airplane, unless not checked and double checked by an engineer who knows what he is doing...
So sure you can adjust the elevator with the trim tabs, maybe they did... once. May be they considered to limit the trim before or after they tested the trim tabs, so many "if", "could", "maybe", but no "Pilot XY usually adjusted his elevator trim tabs to XYZ, regularly to ease cruise flights above 1.3 AtA" or "the trim tabs were tested with desastrous results when adjusted in the field, so never used, after initial adjustment for neutral trim" or any other statement, other than their existance. Personally, I could live with the addition of the trim tabs and see what happens if people mess with them... just saying.
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Old 05-15-2019, 12:31 AM   #119
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Originally Posted by Yo-Yo View Post
I do not see any logic here - because trim tabs and stab does the same job regarding stick forces. And this report shows that there is no magic separation of effects. From the science point of view, withinlinear area of incidence angles (few degrees, that is sufficient to get necessary G-load) hinge moments due to stab incidence or elevator deflection, stab CL changes caused by stab incidence or elevator deflection - they all presented in LINEAR partial derivatives (depending on Mach for sure).

Anyway, starting and maintaining dive (where this magic tab helps to nose-down elevator deflection) and recovering (where, as somebody wants, the trim tab must do NOTHING to prevent nose-down shift) are both performed at relatively high Mach region. I mean that nothing can be dramatically changed within this region (significant changes start from 0.85 M for typical stab with elevator).
Additionally somebody wants that this magic tab works again at low M for nose-down help... It's too much, isn't it?

And, I think , if Germans have found something magic they would emphasize this effect in the report.

By the way - they DID use stab trim to recover dive (see the left diagram).
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."

Quote:
Originally Posted by Yo-Yo View Post
No, the bundle of facts you quoted is not relevant to this case and I explained, why.
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.
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.

Last edited by Curly; 05-15-2019 at 01:07 AM. Reason: formatting, typos
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Old 05-15-2019, 06:11 AM   #120
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Curly,

I really can't think of anything clearer than your explanation above!

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.

In the end I honestly believe that all of the arguments presented so far are clearly in favor of an update to the current module, even if with low priority given that other modules should
by now be in development and probably taking a lot of time already from the dev team ( ? )
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