DCS Spitfire : trim gauge correct readings ?

[Topsy]

Does the elevator trim gauge on the DCS Spitfire show the correct settings ?

Firstly I should tell you that I am not a real world pilot and I only have basic knowledge about aerodynamics. My aim here is not necessarily to gripe about bugs, but more to understand if my interpretations are right and if not why.

An apparent inconsistency occurs on take-off, where the AM Pilot Notes, suggest to trim 1 notch nose down at normal loads - and this works fine in DCS. What is strange, is that when raising the undercarriage one would expect that without the friction of the wheels on the runway and the drag on the landing gear, that the aircraft would naturally tend to pitch up. However in DCS it is necessary to quickly add nose up trim to get the aircraft to fly level - this is counter-intuitive.

When lowering the gear and flaps for landing in DCS it is necessary to add more nose up trim - this seems perfectly logical.

When flying at a cruise setting of 6 boost and 2650 rpm at around 260 mph the DCS Spitfire flies level with the elevator horn balance slightly raised above the tailplane horizontal centerline. This seems to be correct as when looking at photos of real Spitfires in level flight their elevators are often trimmed slightly down with the horn balance slightly raised. However in the DCS cockpit we see about 3/4 notch of NOSE UP trim on the gauge. This again seems counter intuitive as with an elevator trailing edge trimmed slightly downwards one would imagine that the aircraft is being trimmed to raise the tailplanes, ie it is being trimmed nose down.

So how does one explain that the aircraft is physically trimmed slightly nose down while the gauge indicates slightly nose up ?.

At greater trim settings the gauge correctly translates what is effectively happening to the aircraft a-o-a, so why does it seem to show an inverse reading at cruise settings ?

Does this mean that the Spitfire mainplane profile induces variations in pitch at different speeds ?

For information, my non scientific home made table of trim settings at various flight phases & speeds with relevant DCS AoA.

Flight Phase	Mph	Kn	AoA DCS	Cockpit Gauge	Alt Ft
Aircraft on ground	0	0	11,8°	na	na
Cruise level flight	265	228	- 0,2°	up 0,8	3,000
Climb 2K/min	160	143	+ 2,0°	up 0,9	10,000
Gear Down level flight	160	145	+ 2,6°	up 1,2	2,000
Gear & flaps down level flight	140	114	-0,7	up 2,6	2,000

 

To conclude, I guess that DCS more or less correctly models the elevator trim action, as when winding the trim up or down it does have the correct effect on aircraft pitch and the visuals of the tabs tend to match with the real aircraft maximum angles of 20° up and 7° down. Is the issue more to do with the in-flight centering of the cockpit gauge ? Or have I missed something ?

Does the seemingly bizarre effect in DCS exist in the real aircraft ?

[Art-J]

First of all, could you please post in non-formatted text? For those of us who use dark/night forum theme, your post displays black text over almost-black background (see attached image), which makes reading somewhat difficult I'd say . Unless one drags the mouse over to select the text, which is not convenient solution either

Now, back on topic - the reason for behaviour you described is explained on page 63 of DCS Spit manual. The gauge, as simulated in DCS (and Il-2GB for that matter), is calibrated in a way that neutral / zero trim is marked by pointer aiming two notches UP from level. Thus, for example, the aforementioned 3/4 above level actually means 1 1/4 nose-heavy (which corresponds to elevator horns being up). Sounds counter-intuitive and there are guys on the forum having serious doubts about whether it is correct implementation, but - it is what it is, and since the Spit module is a few-years old, it's not going to change. 

Mind you, external 3D model shows tab being flush with the elevator when pointer IS level, but remember the visual model animations in DCS are NOT tied to flight model and cannot be used for physics evaluation.

Cheers!

P.S. - watching videos of restored Spits, for example on excellent Dave Hadfield's channel, is inconclusive - sometimes pilots cruise with pointer a bit up, sometimes with 1 notch down for high speed aerobatics - all restored warbirds are rigged a bit different it seems, so unless we've got access to some old factory drawings, we won't know how the gauges used to be set up in the 1940s.

Edited January 18, 2023 by Art-J

[Topsy]

Hey Art-J, thanks for your response.

Sorry about the black on black, was unaware of the pb as I tend to prepare texts elsewhere then copy/paste.

Good you pointed me back to the manual which I hadn't re-read recently. The fact that the plane flies 'neutral' (in DCS) with 2 notches of Nose Up is almost what I get at certain speeds. Pitch trim in my setup may be altered by the fact that I use a replica control column with asymmetric fore & aft throw in an effort to replicate the movements of the real thing. This means that my rest position could be slightly forward of true center (ie giving nose down input in the sim) - I am not sure how well the Windows calibration takes asymmetric centering into account.

The DCS Spit manual p109 states that the gauge indicates the position of the trim tab. However in the image shown this seems wrong as with the trim tab up, the elevator goes down and the aircraft pitches nose down. Unfortunately I don't have a copy of drawing 30034 SHT 9. Nearest i've got of the Instrument panel is 30034 SHT 10 which effectively refers to drawings 30034/9 or 30034/42 for this gauge.

However I take your point about this being a sim, it not being necessarily perfectly true to life and that the module is now getting old. DCS do announce from time to time updates on WWII modules. On the Spitfire there are tons of threads about the startup procedure being different to the AM Pilots Notes, plus other issues. I have posted about the DCS control column cinematic which incorrectly rotates fore & aft around the elevator control rod attachment. Even if Nineline responded to this, it doesn't seem to bother people much as we rarely look down into the well in front of the seat - but it does effect the relatively feeble amount of column fore & aft visual throw in the sim. These "errors" frustrate Spitfire lovers and we are envious of the time and effort that DCS spends correcting seemingly minor details on other perhaps more modern aircraft.

Regarding modern warbirds, I guess they fly at meetings with minimal amounts of fuel, they hopefully don't carry war mission loads, so they are surely trimmed differently to wartime aircraft.

Anyway thanks again for your time and interest.