Mosquito Directional Indicator - Strange Behaviour

[450Devil]

I have searched the forum and been unable to find any mention of this issue.

The issue is that the Directional Indicator moves when RPM is increased and decreased - as if the electrical system is supplying more or less power to the DI depending on engine RPM. This makes it difficult to maintain a flight path when the DI is effected by changes in engine RPM.

Attached are five photos showing the DI

a. Before Startup - Engine Off

b. At Startup

c. At 1000 RPM

d. At 1500 RPM

e. At 900 RPM

This was on the South Atlantic map with the Mosquito parked on the Rio Grande apron.

I tested the Spitfire, P51 and P47 but these aircraft's DI did not move with either increase or decrease in engine RPM.

[RodBorza]

They do change in the P-51 and P-47, if the engine RPM is low enough so the vacuum pumps do not work. Above a certain RPM they work fine.

I tested as you showed and couldn't replicate. Below 1000 rpm it will be different of course, due to the vaccum suction being too low. Above 1000 rpm, it works independent of the rpm.

Tested it on the ground, parking brake set, rpm lever full in, and varying only the engines boost. 

[Art-J]

@450Devil Does it happen on other maps as well?

I tested the Caucasus cold start mission and the gyro behaved like RB explained above.

[Terry Dactil]

Here is a great video that shows just how complicated the DI is.

It shows that there are lots of built in errors due to the physical properties of a gyro that need to be corrected.  One that may be significant is the correction for the latitude error due to the earth's rotation. This is a small weight on the gyro axis that causes a correcting precession. However this depends on the gyro rigidity, which depends on the gyro rpm, which depends on the system suction, and that depends on engine rpm. I doubt ED would code all of this to any precise accuracy, and it is the one area where the DI can be affected by engine  rpm. I suspect that if it is done at all, it is just gyro rpm (rigidity) is proportional to engine rpm, and this is slightly incorrect at low rpm.

I don't see this as much of a problem anyway. We don't fly around wildly varying engine rpm. Also the DI is expected to drift over time, and good airmanship requires periodic checking and resetting against the magnetic compass.

 

[Ramsay]

I'll bump this thread as I came across this DI error while testing.

I seems to me that for hot starts on the ground and in the air, the MagVar correction applied to correct the Gyro DI has the wrong sense i.e. it's being subtracted when it should be added and vice versa.

I see this when spawning in a 1944 Normandy map (MVar = -11.9°W)

The DI should read 110° but instead reads 86° = 98°T - 12° (should be 98 - (-12) = 98 + 12°)

https://forum.dcs.world/topic/318891-compass-question/?do=findComment&comment=5147957

... and on the default 2016 Caucasus map (MVar = +6.6°E)

The DI should read ~90° when spawning in but instead reads 103° = 96°T + 7° (should be 96 - (+7) = 96 - 7° = 89°)

Tested using DCS Open Beta 2.8.2.35759 using Normandy (21st June 1944) and Caucasus (21st June 2016) maps.

Mosquito Spawn DI MagVar and Compass Check, 2016, Caucasus.trk

[J13 Serenity]

Hey
On a Spitfire and Mosquito, apparently, Vacuum gyros only work fine if RPM is above 1200
Also, if, for any reason, you keep your RPM below 1200 for about a minute or so, your Attitude indicator will never correct itself again (From my testing)
If you try to make turns on the ground with RPM below 1200 - the Gyro compass indicator will have ever growing error...

you HAVE to keep your RPM above 1200 (at least on one engine) at all times while on the ground, otherwise no Artificial horizon.

Edited July 26, 2023 by J13 Serenity

[Bozon]

5 hours ago, J13 Serenity said:

Hey
On a Spitfire and Mosquito, apparently, Vacuum gyros only work fine if RPM is above 1200
Also, if, for any reason, you keep your RPM below 1200 for about a minute or so, your Attitude indicator will never correct itself again (From my testing)
If you try to make turns on the ground with RPM below 1200 - the Gyro compass indicator will have ever growing error...

you HAVE to keep your RPM above 1200 (at least on one engine) at all times while on the ground, otherwise no Artificial horizon.

 

Interesting, this could explain a few things, like why sometimes the artificial horizon never correct itself and in other sorties it is OK - sometimes while taxiing I do pull the throttle back to slow down into a turn, and RPM drops to below 1200 during the maneuver. So if the quoted above is correct, the state of my AH during the sortie will depend on how I behaved during taxi. I’ll have to test that.

In any case this seems incorrect. I mean AH not functioning at low RPM and suction is OK, but it should recover just fine after RPM is increased.

[J13 Serenity]

I could not find a good enough answer in the documents I searched for.
May be the Gyro tumbles in such a way as to not being able to recover?
Or may be its just another bug...