correct as-is Erratic and inaccurate altimeter

[450Devil]

As the Mossies top two roles were high altitude and low altitude day / night penetration of enemy territory, an accurate altimeter was a requirement. The altimeter in this module is neither accurate nor consistent. To test this I suggest you set up a simple mission in the ME with a player Mossie @ 220 knots and ME standard altitude (6562 feet). On spawning in go to Active Pause and outside view and not the altitude. Then go back into the aircraft and adjust the altimeter to the real altitude. Then resume flight and descend the aircraft to 1000 feet on the altimeter. Go back to Active Pause and outside view and note the shown altitude which will be much less than 1000 feet. Not only does an inaccurate altimeter make it difficult to fly at a selected altitude but it also makes it very hard to low level bomb as there is a real danger of the aircraft sustaining blast damage due to the aircraft being lower than the indicated altitude on the altimeter.

[Nealius]

Are you referencing the ctrl+Y info bar altitude? If so, that altitude is not adjusted to baro pressure. When setting my altimeter to mission QNH it is accurate enough for me to traverse the channel at 50ft. 

Edited September 29, 2021 by Nealius

[grafspee]

Yeah, real night time mission during ww2 was done by pausing the game read your alt from out side source and apply it in to your altimeter

[Hiob]

"Schätzeisen"...

[Olddog]

If I remember right,  an altimeter error of + - 75 feet is legal for IFR flight (FAA). I believe VFR is more. 

That is with today's equipment. I don't know what it would have been in the 1940's.

If the props are throwing dirt, tree limbs, grass or water on the windshield, you are low enough

[Nealius]

That's one thing that really needs tuning with the Mossie: the adamantium trees. Mossie pilots talked about coming home with leaves and tree branches stuck in their radiators. Clipping a single leaf in DCS results in instantaneous death. 

[Hiob]

17 minutes ago, Nealius said:

That's one thing that really needs tuning with the Mossie: the adamantium trees. Mossie pilots talked about coming home with leaves and tree branches stuck in their radiators. Clipping a single leaf in DCS results in instantaneous death. 

I think, that's an unrealistic demand. One Tree, one object, one collision model. I think we need to choose between "ghost trees" (even more unrealistic imho) and adamantium trees.

There will always tradeoffs with limited computing power.

[Nealius]

Make the hit box confined to the trunks 80-90% of the way up the tree. problem solved. 

[hazzer]

Hi all, 

 

Just want to add my thoughts on the altimeter being erratic:

The altimeter appears to be the same one as in the spitfire, which I find also sufferers this issue. 

 

The altitude needles jump up and down with pitch movement and very little actual vertical speed. It makes it pretty difficult to maintain as set altitude as any correction you make gets exaggerated by the altimeter dropping 1000ft then correcting to your actual altitude. 

 

Can someone confirm if this is correct or not? None of the other warbirds other than the spitfire have this issue. In the Cessna I fly the altimeter was prone to bouncing up and down 200ft due to an issue with it and had to replace it. It wasn't fit for purpose! 

 

On 9/29/2021 at 9:50 AM, Olddog said:

If I remember right,  an altimeter error of + - 75 feet is legal for IFR flight (FAA). I believe VFR is more. 

That is with today's equipment. I don't know what it would have been in the 1940's.

If the props are throwing dirt, tree limbs, grass or water on the windshield, you are low enough

The altimeters were generally very similar to the ones in aircraft today so probably similar ability. As I have said the biggest issue (for me) is how a slight change in pitch makes the altimeter jump up rapidly before correcting. That just doesn't appear to be accurate. 

[Rolds]

There are two real world aviation issues which are reproduced in the mosquitos altimeter which posters in this thread do not appear to be aware of.

The first: Barometric altimeters are configured with an assumed standard rate of change in pressure corresponds to a certain date of change in altitude, but on any given day this assumption is not accurate. A standard rule of thumb is that a decrease of 1 inch of mercury corresponds to 1,000 feet but on any given day this will not be accurate, so an altimeter is generally only dead on if you are sitting on an airfield of known altitude and have calibrated the altimeter to match. Thus when you calibrate the altimeter to the God's truth altitude shown on the F2 view for example, then change altitude and compare again, you will see a difference has appeared. I can't say if DCS implementation of this phenomenon is 100% accurate, I've never done any in depth testing, but it is generally accurate to the real world and my guess is that it is dead on because this is a well known phenomenon (I learned about it in ground school while doing my private pilot license).

 

Two: barometric pressure altimeters use a moveable diaphragm to compare pressure inside the instrument to the outside pressure measured from the static air port (which is generally on the side of the plane as you don't want ram air pressure affecting the reading). That diaphragm can move around freely as you would expect, and it reacts to inertia just like anything else. Thus when you maneuver the aircraft it can give a false reading as the diaphragm is moving in response to aircraft movement, not change in the difference in pressure between outside atmosphere and that inside the instrument. Again can't say if the implementation is exactly right in DCS, this one depends heavily on the actual instruments used, but this effect does exist in real aircraft. 

[Nealius]

6 hours ago, Rolds said:

an altimeter is generally only dead on if you are sitting on an airfield of known altitude and have calibrated the altimeter to match.

Does this mean it's better practice to set the altimeter to airfield elevation rather than to QNH pressure from the briefing/METAR/ATIS?

Edited October 14, 2021 by Nealius

[Rolds]

That would be to set the altimeter to QFE which I hear is sometimes done but never came up outside the classroom in my limited experience with general aviation.

[Nealius]

I may have phrased my question poorly. 

Let's say you get an ATIS pressure of 29.87 at an airfield with elevation of 250ft MSL. If you set the altimeter to 29.87, there could be errors like what you mentioned where the altimeter wouldn't read 250ft at 29.87. So in that scenario, is it better practice to set the altimeter to 250ft instead of setting it to 29.87?

Edited October 14, 2021 by Nealius

[Frederf]

RAF WWII were rather keen on QFE for landing operations. I wouldn't be surprised if historical practice was QFE for that plane by that nation at that time.

Every altimeter setting guarantees that the altimeter is accurate at best to one and only one elevation. For QFE that's the airport surface (reads zero). For QNH that's the airport surface* (reads actual). For QNE it's nowhere in particular (pressure altitude). For QFF that's sea level (reads zero). For everywhere except the calibrated elevation the altimeter will be wrong. Sounds crazy but altimeters are wrong for every altitude except one.

*Every QNH is tied to someplace. There is no "the QNH" but "the QNH for this place." Every elevation has a different QNH even if the atmosphere is uniform. An airport at 7000' elevation will usually have a different QNH than an airport at 2,000' elevation. The "QNH" in the DCS briefing is actually QFF which is equivalent to QNH for sea level. You will set the DCS "QNH" value at a high altitude airport and it will be wrong because it's not a QNH for that location.

The amount of error is proportional to the elevation distance from the calibration point and the degree of which the real atmosphere is different than the assumption built into the instrument. The usual relationship modifier is temperature. If it's colder than standard and the calibration point is below you then you're lower than the altimeter indicates. In the traffic pattern the error might be 10' but at high altitude it can be 1,000s of feet. The closer temperature (and other factors) are to the standard atmosphere the wider range of elevations around the calibration point will be good enough.

For bombing purposes the crew have three options:

1. Do the calculation to find a new altimeter setting which reads accurate at the intended bombing altitude.
2. Do the calculation to find the displayed instrument value which will coincide with the intended bombing altitude.
3. Accept the accuracy loss due to altimetery error.

Examples:

1. Desiring to bomb from 15,000' it might be necessary to set 801mm pressure even if QNH is 755mm and QFE is 732mm such that the altimeter reads 15,000' when at 15,000'.
2. Desiring to bomb from 15,000' it might be necessary to fly at an altimeter reading of 16,170' under QNH or QFE or QNE such that one is actually at 15,000'.

In real life I believe correct procedure is to set the reported altimeter setting even if it doesn't match field elevation. The idea is that if everyone is on the same setting then at least relative deconliction is assured. But I've never seen it be off much. What's legally required might depend on the regulations and what kind of flying you're doing. If they aren't close then usually that instrument is to be considered in need of maintenance. Obviously setting known elevation is good enough practically and you shouldn't lose any sleep doing that unless you're an actual commercial or military pilot.

As for extra sensitive altimeter, that's usually an issue with vertical speed instruments, not altimeters. And the issue is older VSIs would lag causing confusion for the pilot. Newer designs are more immediately responsive which helps with fine adjustments. Altimeters tend to lag (showing the past) for example this is a common correction when dive bombing since the instrument will read a higher altitude than you actually are (it hasn't caught up yet).

I've never heard of a jumpy altimeter which exaggerates actual changes. That's weird. Are you sure it's not just altimeter lag and you're chasing it half out of phase?

[Nealius]

44 minutes ago, Frederf said:

The "QNH" in the DCS briefing is actually QFF which is equivalent to QNH for sea level. You will set the DCS "QNH" value at a high altitude airport and it will be wrong because it's not a QNH for that location.

That used to drive me insane on NTTR at Nellis. Now I finally know what was causing it. 

 

45 minutes ago, Frederf said:

I've never heard of a jumpy altimeter which exaggerates actual changes. That's weird. Are you sure it's not just altimeter lag and you're chasing it half out of phase?

It's definitely something specific with the Mossie and Spitfire, which isn't suprising that they both would have the issue given they use the same instrument. In the P-47, P-51, Bf109, and Fw190 the altimeter needles are steady, and lag behind the needle on the VVI. In the Spit and Mossie this is reversed: The VVI lags behind the altimeter. The slightest pitch up or down and the altimeter needle will lurch up or down in large increments. Which sounds kind of like the inertial movements Rolds mentioned, but then why does it only affect the Spitfire and Mossie, and not any of the other warbirds (or even modern jets)? 

[Yo-Yo]

Yes, Spitfire altimeter does have this "weird" behavior, but it's a feature we had the opportunity to implement for this plane. For some planes we have full data that allows to recreate separate part of instrumental error from static and dynamic vents. So, as you change AoA the systems creates errors, you see at altimeter.
Mosquito at EA  uses, I thnk,  Spitfire's system that is not correct.
For the planes we have no exact info for it's better not to make this effect than to create something fantasic.

[Nealius]

17 minutes ago, Yo-Yo said:

So, as you change AoA the systems creates errors, you see at altimeter.

This must explain why I occasionally see the altimeter jump while parked on the ground in missions with wind and turbulence. I see similar behavior with the AoA instruments on various modern modules. 

[Holbeach]

The altimeter and VSI are virtually the same, except that the VSI has got a hole in it.

The altimeter will detect via the static and retain change in pressure in its bellows making it stable.

The VSI will detact via the static and release that pressure through a hole in its bellows.

This will make it liable to hunt and make it twitchy.

The fact that the Mosquito is rather sensitive in pitch, tends to exagerate the effect, but the altimeter seems to be the same as the VSI, rather too twitchy in our Mossie .

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