War Thunder

Accuracy of the Spit FM – with some data to back it up

warthunder 4 - Accuracy of the Spit FM - with some data to back it up
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T.L.D.R -The issue isn't the Spit. The adverse yaw in the spit model seems to be quite accurate given actual accounts. The BF109 directional stability seems to be far from correct though – it's way too forgiving.

As a followup of a post made a few days ago about the Spit's FM and it's directional stability – I thought I'd dig through some of the data to see how close the model of the Spit actually is.

For those not familiar – directional stability is yaw stability – aka how prone the aircraft is to oscillate about its yaw axis.

I – like most people who play this game – don't have any official flight training. As such – we tend to either not know about or ignore some of the more subtle mechanics – such as coordinated turns. Uncoordinated turns generally cause the yaw oscillations that are very noticeable in the Spitfire FM (Especially the mk I and mk II ). However – the frustrating part about this isn't that the Spit tends to yaw with hard aileron input but that its Axis contemporary doesn't! .

From a report about the Spitfire's performance – yaw due to aileron deflection was characterized as follows

In aileron rolls made at 110 percent of the minimum speed with full aileron deflection and with the rudder fixed, about 18° side-slip was developed. The requirement of reference l, which states that less than 20 (percent) side-slip shall be developed in this maneuver, was therefore met. Source

Mid you – this is with the ailerons slammed. Furthermore the Spit was specifically designed to perform coordinated turns at moderate to high speeds without ANY yaw input from the pilot.

The Spitfire is designed to perform coordinated turns at moderate-to-high airspeeds without the need for rudder displacement. The Pilot's Manual advises pilots not to place their feet on the rudder pedals at all at moderate-to-high airspeeds, or when flying with reference to instruments only (blind or instrument flying). Source – This is from a forum post. I didn't go through the trouble of looking for the actual Spit flight manual

Now compare this to the bf109. I have looked pretty hard to try to find a measure of sideslip due to aileron deflection but haven't been able to find any. The closest I can find is this excerpt from Rob Erdos flying a restored 109E where he makes a comment about the pilot needing rudder input to keep the plane's turns coordinated. (Maybe someone knows where I can find actual data on the adverse yaw behavior of the 109.)

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Power was set at 1.15 ATA (atmospheres of manifold pressure) at the recommended climb speed of 250 km/hr. Propeller speed was sensitive to airspeed changes, so a slight pitch reduction was required to stabilize at 2300 RPM. The Daimler engine sounded much smoother in flight. My initial impressions of the aeroplane were mixed. The field of view was poor, necessitating continuous clearing turns in the climb. The greenhouse canopy structure seemed to be slightly obtrusive no matter where I looked. Control response in the climb was satisfyingly light and crisp, with good harmony between pitch and roll control forces. Directional stability was clearly inadequate. Every roll input required conscious pedal coordination.

It seems – at least to me – that the issue here isn't the Spit model. I have been flying the Spits in sim pretty exclusively the past 2 weeks. The control sensitivity is apparent. I find myself barely using more than 20% of the elevator when flying. It takes getting used to but it's actually made other aircraft feel like bricks in the air in comparison. With smooth rolls on turn entry and exit, as was outlined above, there is almost no considerable yaw. I know there's a video in the previous thread comparing the turn exit performances of the two – it's hard to draw any conclusion because we can't see the control input from the pilot. Even so – the yaw out of the turn is damped in less than 2 oscillations and never looks like it exceeds 20%.

As such – it seems to me that the issue with the Spit FM isn't necessarily the averse yaw. Maybe there are other issues but I haven't looked at those in detail but feel free to comment on them below.

Clearly the 109 model needs work though. It's far too forgiving in yaw. At least assuming the above account of it's yaw stability is accurate.

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Side note – for those wanting to fly the Spit – being as sensitive as it is – I don't think you can fly the Spit without 100% control sensitivity. The aircraft responds very quickly to control input and any time delay (which is what sensitivity does for those that don't know) makes it almost impossible to fly.

Source: Original link


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