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Amount of elevator trim required for level flight?


Art-J
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Hello there

In a couple of posts scattered all over the threads in this section, there have been doubts voiced about amount of elevator trim required to keep DCS Mossie flying level (mind you, a different topic to the "sensitivity" one). In some youtube videos which show restored KA-114 being flown, we can see a couple of times that trim settings of between 1/3 and a half division nose down were used for cruise (and sometimes for the takeoff as well). Examples can be found here:

and here:

That is very different to "our" Mossie, which on non-FFB joysticks requires about 2.2 divisions of nose down trim to fly level, or even more on FFB sticks as shown in the other thread.

We don't know what loadouts were used in the real flights seen above and what power settings were applied (at least I didn't investigate these vids frame by frame to try to catch a glimpse of tachos and boost gauges). That being said, to cover wide range of comparison possibilities, I performed two test flights in DCS, with two very different loadouts and three chosen power settings. The same weather conditions and the average altitudes were chosen.

1) Case 1 "light" - 25% fuel only, no ammo for guns, three power settings: 7 PSI @ 2650 RPM, 9 PSI @ 2850 RPM, 18 PSI @ 3000 RPM. Trimming for level flight after every power change, Indicated speeds reached with radiators open were about 240, 270 and 320 mph respectively. First screenshot shows pitch trim required.

2) Case 2 "heavy" - 100% fuel plus drop tanks plus bombs plus full ammo, three power settings as previously, triming for level flight as previously, speeds reached were about 230, 250 and 310 mph respectively. Second screenshot shows pitch trim required.

Even though there surely were some minor trim differences between all these phases, not clearly detectable on a rather simple and coarse trim indicator, they were all around 2.2 div. nose down.

@Yo-Yo, could you please investigate the issue, or explain what the reason for this discrepancy compared to KA-114 might be, or post trim curves data the flight model was based on to cut the speculation why we see what we see?

Thanks in advance!

Cheers.

 

light.jpg

heavy.jpg

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25% fuel is spread all over the place and is uneven. 1 outer tank is empty.

No Mosquito flew with this loadout and given its role probably only ever used 100% fuel load.

Outer tanks had to be fully loaded IRL.

The museum aircraft is not representative.

 

One thing you might like to try.

 

Wind the trim fully forward in level flight and see what happens.

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Edited by Holbeach

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I would assume museum aircraft would need excess nose trim on account of them lacking the weight of four guns and all that ammo. 

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Just three figures for authentic WWII plane.

Directed tail adjustment.

image.png
Metal elevator.

 image.png
image.png
Fabric elevator, tail incidence NEUTRAL
image.png

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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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And, please, think about the take-off trim directed n the Pilot's notes - 0.5...1 tick nose-down. Then take a look at the huge landing gear creating much drag and nose-down pitching moment. Then, take a look to the weight and balance chart to be sure that the loaded plane has 32-34% MAC CoG, i.e. almost neutral so, as the plane is airborne at relatively low speed (high CL, see the diagram), and it is necessary to compensate the moment of retracted undercarriage by significant adding of nose-down trim. So, for flight 0.5-1 tick will not be suitable at all.

 

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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34 minutes ago, Yo-Yo said:

And, please, think about the take-off trim directed n the Pilot's notes - 0.5...1 tick nose-down.

This results in a large pitch-up moment for me on takeoff. I need 2.0+ nose down trim for a controlled takeoff where I can accelerate level to 170mph before climbing. 

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But it's not the best trim after you airborne. As the plane is on the ground, wheels reaction force create a lot of nose-up moment.

 

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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I get a good take off @ 20800 lb with 2 nose down, 1 div right rudder and 1 div right aileron, but that is not the pilot notes recommendation.

 

Aircraft can fly trimmed, "hands off", with 1 100 gal drop tank empty and 1 full, down to 160 mph, before aileron trim is required. IRL.

 

Why is it possible to trim forward and overcome the ability of the elevator to maintain level flight. The aircraft will nose dive.

It can't believe this would be possible IRL. What would be the point?

 

..

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Yo-Yo, thanks for reply. Can't see the graphs very well on my phone, so will take a look at them later, but they'll definitely help to understand this airplane's handling.

i7 9700K @ stock speed, single GTX1070, 32 gigs of RAM, TH Warthog, MFG Crosswind, Win10.

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4 hours ago, Yo-Yo said:

But it's not the best trim after you airborne.

2.0 nose down? To fly level with my stick centered I need more than 2.0 nose down trim at nearly all speeds between 180mph and 300mph. 

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You absolutely right, and this 2+ (-3.5 degrees of trim) is exactly what you need to have according the charts.

But, you did not catch the point that 2+ nose down is not the right trim for being airborne WITH GEAR DOWN.

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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4 minutes ago, Yo-Yo said:

You absolutely right, and this 2+ (-3.5 degrees of trim) is exactly what you need to have according the charts.

But, you did not catch the point that 2+ nose down is not the right trim for being airborne WITH GEAR DOWN.

I missed the part where the topic changed to trim required for gear-down flight. The opening post (and I think most of us) have been talking about trim needed for forward gear-up flight. The second video shows neutral trim in situations where we need 2.0+ nose down. 


Edited by Nealius

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This graph shows exactly the trim you need to fly the plane in typical full loaded condition LANDING GEAR UP.

As we have the plane from WWII, it must fly in accordance to the direction documents and tests relevant to this time, right?

 

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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Re trim in general…
 

I seem to constantly add right aileron trim. I get that there are two very powerful engines, both driving props on the same rotation.  This seems very sensitive to revs adjustment, so suspect that it is absolutely correct?

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1 hour ago, Yo-Yo said:

shows exactly the trim you need to fly the plane in typical full loaded condition LANDING GEAR UP.

…then why were we talking about gear down flight? I’m thoroughly confused. 

Re: the charts. Where is the speed notated on the charts? I only see the trim angles and what appears to be lift coefficient? How do we translate that to x° trim at y speed? Or at z altitude, as it’s my understanding that a trim setting is only valid for one speed/altitude combination. 


Edited by Nealius

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1 hour ago, Nealius said:

…then why were we talking about gear down flight? I’m thoroughly confused. 

Re: the charts. Where is the speed notated on the charts? I only see the trim angles and what appears to be lift coefficient? How do we translate that to x° trim at y speed? Or at z altitude, as it’s my understanding that a trim setting is only valid for one speed/altitude combination. 

 

Please read carefully my previous post, where was something about landing gear pitching moment. 

Your understanding of trim is not completely right, because at any altitude, gross weight and speed CL can be calculated for level flight. And in opposite,  knowing just CL you can obtain trim for any actual combination of flight parameters including weight and g-load <>1. So, CL relative chart is a proper (and common) way to have generalized trim characteristics.

But sometimes you even have not to calculate something: for example, the curve that is close to horisontal throughout the wide range of CL tells you that this trimmer angle is the same in the wide range of IAS.

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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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11 hours ago, Yo-Yo said:

the curve that is close to horisontal throughout the wide range of CL tells you that this trimmer angle is the same in the wide range of IAS.

Could you point out the horizontal line? For our trim settings of around 2° nose-down, I see it intersecting at CL 0.75 and it's not a horizontal line. I would like to  calculate a speed range for CL0.6 to 0.8 so I can get a better understanding of the chart, but we don't have a lift force (L) to plug into the formula.

image.png.95a50a048bfa4f2763c4b554e11f4a37.png

 

6 hours ago, Olddog said:

I setup a test mission with those numbers and the trim settings were darn close.

 

Can you elaborate how you calculated the numbers to match with the CL in the charts? For the math I have wing surface area, speed, and air density, but I do not have lift force to calculate a CL from my speed. Or vice-versa, a speed from a CL in the charts.


Edited by Nealius

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Can you see the difference between 24.5 and 32.5% MAC?  Why do you mark 24.5 which you never have at normal flight?

Sorry, mistyped curve description: must be  STABLE, slightly stable
image.png

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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22 minutes ago, Yo-Yo said:

Can you see the difference between 24.5 and 32.5% MAC?  Why do you mark 24.5 which you never have at normal flight?

I marked 24.5 because it's the only one that intersects with 2.0 degrees nose-down trim. 32.5 indicates 4.5 degrees nose down trim. Am I misreading the tab angle by equating the tab angle to the trim indicator value in the cockpit, i.e. -2.0 degrees in the chart equates to 2 divisions nose down on the trim indicator in the cockpit? Or does -4.5 degrees tab angle = 2.0 divisions nose down trim?


Edited by Nealius

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Very close.
If the trim angle is +-7.5 degrees (as I mentioned above) and the whole scale is 4 ticks, how many degrees  2 ticks are?

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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18 minutes ago, Yo-Yo said:

If the trim angle is +-7.5 degrees (as I mentioned above)

That wasn't mentioned in this thread, according to a Ctrl+F search. so that's what I was missing. Perhaps a different thread on trim; there's too many to keep track of 😆 

If 7.5 degrees from neutral to 4 ticks, 1 tick = 1.875 degrees. So 2 ticks nose down is -3.75....which doesn't match the chart so I must be missing something somewhere. The chart's -4.5 degrees would be 2.4 ticks, which seems to match my experience in DCS.

Now I wish I could test it to experience the chart first hand, but I can't calculate IAS from Cl without Lift Force. 


Edited by Nealius

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Why 

7 minutes ago, Nealius said:

That wasn't mentioned in this thread, according to a Ctrl+F search. so that's what I was missing. Perhaps a different thread on trim; there's too many to keep track of 😆 

If 7.5 degrees from neutral to 4 ticks, 1 tick = 1.875 degrees. So 2 ticks nose down is -3.75....which doesn't match the chart so I must be missing something somewhere. The chart's -4.5 degrees would be 2.4 ticks, which seems to match my experience in DCS.

Now I wish I could test it to experience the chart first hand, but I can't calculate IAS from Cl without Lift Force. 

 

Why can't you calculate? You have all you need.

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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7 minutes ago, Yo-Yo said:

Why can't you calculate? You have all you need.

CL = 2 * F / (A * ρ * V2)

I need F to solve CL if I have an IAS. I don't see a value for F in this thread. Perhaps I'm using the wrong formula?


Edited by Nealius

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1. 7.5 deg is 4.5 ticks.

2. 4 ib boost @ 2600 rpm is 190 mph IAS. Much slower than normal use and rather pointless.

3. 4 ticks nose down, will overcome the elevator power and send it into an uncontrollable dive?

 

.. 

 

I7 2600K @ 3.8, CoolerMaster 212X, EVGA GTX 1070 8gb. RAM 16gb Corsair, 1kw PSU. 2 x WD SSD. 1 x Samsung M2 NVMe. 3 x HDD. Saitek X-52. Saitek Pro Flight pedals. CH Flight Sim yoke. TrackIR 5. Win 10 Pro. IIyama 1080p. MSAA x 2, SSAA x 1.5. Settings High. Harrier/Spitfire/Beaufighter/The Channel, fanboy..





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