The British ASI

[bolox]

one thing that has caught my attention since release is the needle behaviour of the British ASI's:- there is a marked drop in indicated speed as G is pulled, something that definitely doesn't appear with German instruments.
I can think of no reason for a difference in the behaviour, both being similar systems:confused:

While we're talking asi's- what is going on with the hurri and blen asi?
i've never seen a British asi(of the period) with dual needles, all having a single needle as depicted in the spit

spit/huuri asi
http://i240.photobucket.com/albums/ff27/bolox00/brit%20gauges/asi/400MPH2.jpg

320 mph asi blen (and many others)
http://i240.photobucket.com/albums/ff27/bolox00/brit%20gauges/asi/340-MPH1-large.jpg

[IvanK]

This bug of the UK ASI needles responding instantly to backstick has been brought up directly with the devs.

[Ataros]

The devs response at il2bugtracker.com
Working as intended, historically accurate. (updated by Ilya "Luthier" Shevchenko and Roman "Mad Dragon" Deniskin) http://www.il2bugtracker.com/issues/2

Interesting details in modelling.

[fireship4]

Finally a bug tracker! It has been needed and easy to get (correct me if I'm wrong) since the start.

[IvanK]

The devs response at il2bugtracker.com
http://www.il2bugtracker.com/issues/2

Interesting details in modelling.

"Working as intended, historically accurate. (updated by Ilya "Luthier" Shevchenko and Roman "Mad Dragon" Deniskin) "

I cant believe that as accurate. I am yet to see any ASI behave in this manner including on some relatively ancient types. Given we are talking about basic Pitot static devices in both RAF and Luftwaffe types why should the only the RAF ASI exhibit this behaviour ?

This DVD

http://ecx.images-amazon.com/images/I/51Z9232BT9L._SL75_AA30_.jpg

http://www.amazon.co.uk/Supermarine-Spitfire-DVD/dp/B000AOX79M

Contains some exceptional in cockpit over the shoulder imagery of a MKI being test flown. This includes a a full low level aerobatic routine in which the ASI can be clearly seen.
There are NO wild reductions in IAS as G is applied. You do see normal IAS reduction as the speed varies throughout the manoeuvres but you don't see ASI needle reductions/fluctuations responding to G/backstick increases.

Here is a screen grab to illustrate the view presented in the DVD test flight the ASI is clearly visible throughout:

http://i40.photobucket.com/albums/e215/zulu64/SpitDVD2.jpg

And another DVD with Cockpit Cam showing a display routine in a Hurricane I
http://ecx.images-amazon.com/images/I/41YnZb5b2HL._SL500_AA300_.jpg
http://www.amazon.co.uk/ITVV-Hurricane-R4118-DVD/dp/B0036U2VGW
Its cockpit viewpoint shown in this screen grab:
http://i40.photobucket.com/albums/e215/zulu64/Hurriasi.jpg
Again NO large ASI swings under G/Backstick. Some minor oscillations in the order of +-5MPH max are evident.

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In the case of large Yaw inputs (particularly at low IAS) then some small oscillation could be expected since the Pitot head on the RAF types is located on the outboard section of the wing. (Port wing in the Spits case) therfore the pitot head would experience local airspeed changes with yawing. e.g. If the YAW input results in the pitot head "advancing" into the airstream (On the Spit with the Pitot head on the outer port wing yawing RIGHT) then you could expect a momentary minor increase in IAS. Once the YAW stabilises the ASI would be constant. The opposite being the case if the yawing moment results in the pitot head retreating in the airstream (in the case of the Spit Yawing LEFT). This is one of the reasons why with aircraft with Pitot heads on the outboard wings that you see differing stabilsed IAS values in a spins to Left and Right.

There is no rational reason imo why this should occur in the pitch axis.

[Ataros]

S!
I am not an expert but maybe a pitot tube has different design or located differently compared to axis ones? What do experts say on WWII historical forums? We have some here in Russia but I am not a member.

Are you sure the videos show authentic instruments?

If yes, could you post the evidence into bugtracker http://www.il2bugtracker.com/issues/2
and ask mods to change the issue status to "new" or add a new status like "updated". We can discuss it here http://www.il2bugtracker.com/boards/1/topics/15?r=17

[IvanK]

Posted on Bug tracker forum asking for the bug to be reactivated,.... and with the devs directly again !

Shouldn't be closed/Rejected subject on Bug tracker imo it should still be open.

Yes these Aircraft are faithful restorations using original components.

The 109 Pitot probe is in a similar location to the Spitfire under the outboard portion of the port wing, and yet it does not exhibit the same (bugged imo) behaviour as the RAF types.

[Osprey]

A bug tracker is fantastic as long as there is control - perhaps a few people here who have permission to raise and comment on defects. The last thing we want are biased statpadders to spoil the game in their favour.

[bolox]

IvanK, thank you very much for your input, agrees totally with my limited experience of flying relevant types (Chipmunk and Harvard).

The last thing i wish to do is cause a 'war' over this (or any other issue) and further overload Ilya and the team, however at the very least it is inconsistent with the behaviour of other nations asi's.

anyway for those who want to know a little more how pitot systems work and their errors maybe of interest
http://www.nar-associates.com/technical-flying/airspeed/airspeed_wide_screen.pdf
or for the more technical
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930092348_1993092348.pdf

[Ataros]

Reopened the issue.

[bolox]

some 'evidence'
http://i240.photobucket.com/albums/ff27/bolox00/cod/spitelev.jpg

taken from (fig 8)
http://jsbsim.sourceforge.net/spit_flying.pdf

note it is for a Va but it is a fully instrumented test.
a quick try at replicating these conditions ingame ( SpitIIa 210 IAS trimmed level flight, 2650rpm estimated 2G pull) gives a drop in IAS to ~165mph and ~200 on release. To me that is a huge difference- infact the chart shows a very slight increase in IAS at pull

The graphs of timed 180 turn also fail to show any noticeable corellation between sideslip and IAS, certainly not the ~30mph difference experienced ingame.

Is this sufficient proof to put to Luthier? or are there any errors that anyone can spot that need to be addressed?

[IvanK]

Thanks for reopening the Bug on the tracker site Ataros

Good reference there BOLOX.

[Osprey]

Yes this is a big annoyance and one of the main reasons the 109 can just go vertical to live. Tactically, that aside, can someone please explain how to vote for a bug please? I see lots have done it but I must be dim because I can't work it out. :(

[41Sqn_Banks]

Yes this is a big annoyance and one of the main reasons the 109 can just go vertical to live. Tactically, that aside, can someone please explain how to vote for a bug please? I see lots have done it but I must be dim because I can't work it out. :(

As far as I understand it's only a instrument error and doesn't effect the actual air speed.

[IvanK]

As far as I understand it's only a instrument error and doesn't effect the actual air speed.

Agreed its an instrument display indication bug. Actual IAS is not changing So FM wise it has no impact.

[albx]

Yes this is a big annoyance and one of the main reasons the 109 can just go vertical to live. Tactically, that aside, can someone please explain how to vote for a bug please? I see lots have done it but I must be dim because I can't work it out. :(

go here http://www.il2bugtracker.com/issues/2 if you are logged in then click the green arrow next to Votes... ;)

[Osprey]

Thanks mate, I hadn't registered, it was obvious really..........D'oh!

[bolox]

been doing some more digging on this, mainly trying to be a 'devil's advocate' to punch holes in my theory that the effect as currently modeled is incorrect.

http://naca.central.cranfield.ac.uk/reports/arc/rm/3365.pdf

is the only thing i've found, and while not conclusive it does show some tests between various types, including a German pitot (HNG). This would appear to show small variations between types but not the variations seen ingame, to me they appear to be of almost an order of magnitude out.

anyone an expert on fluid dynamics here care to look these over?

2 other slightly relevant things i've discovered- spitII had a modified pitot fitted- 'single' tube as compared to the earlier 'forked' type (separate pitot and static tubes mounted parallel )
http://spitfiresite.com/2010/04/supermarine-spitfire-variants-the-initial-merlin-powered-line.html/3

also acceptable tolerance for alignment of pitot head in HurriII is +/-2 deg which seems to fit with errors given in first reference, showing less than 1% difference below ~15 deg of off axis alignment and gross errors only occurring above ~20 deg off axis

[ElAurens]

Is it possible that the mechanical parts of the cockpit instrument itself are being affected by G-loading? I know this was a problem with some Smiths tachometers fitted to various racing cars in the 60s. When they came to Daytona to run the 24 hour race the drivers were reporting visual RPM drops when the cars were on the high banking at speed. The G forces were actually pulling the needle down.

Just a thought.

[IvanK]

I don't think so. Firstly its designed to operate in a high G environment. Secondly the needle always reduces even when sitting at the 3 O Clock position and G is applied ... i.e against the force !

There is no Historical record that I know of the current behaviour.