twgin Posted October 12, 2017 Posted October 12, 2017 I have read most of the material on Level Bombing but still have some questions... I'm flying the Ju-88 mostly but questions would apply to any of the bigger bombers... I have read some (older) comments that the "units" setting in the game could affect things such as differences in what the cockpit altimeter reads vs. what the reference bombsight altimeter reads, i.e. preferences are set to Imperial, but flying a German bomber that uses Metric, discrepancies between the cockpit altimeter and the reference altimeter in the bombsight view happen. As of now (October 2017), it seems the best setting available in preferences is "airplane dependent", and any old bugs due to this issue are now dead ? In campaign missions information given includes elevation of departure airfield, elevation of target, and local atmospheric pressure in millimeters of mercury (mmHg). Elevations given are understood to mean MSL (height above Mean Sea Level). For setting Altitude in the bombsight, the particular altitude we are concerned with is ALWAYS "above ground level" or AGL in real life parlance. In other words, how many meters or feet will the bomb fall after leaving the plane until it hits the ground. It seems to me it would be clearer to not call this an "altitude" but rather refer to it as something like "bomb drop". Is this a true statement ? Two ways of setting the cockpit altimeter: 1) At nearest airfield - when selected (and sitting on the ground at departure airfield) will basically set cockpit altimeter to "zero" regardless of actual atmospheric pressure, cockpit altimeter then reads AGL above the departure airfield for balance of mission. Value for bomb drop can be calculated using the difference between the departure field elevation and the target elevation. For instance, assume departure airfield elevation is 100 meters, target elevation is 200 meters, planned altitude for bomb run is 3000 meters. If before taking off you select "nearest airfield" for the cockpit altimeter, eventually level off at an accurate 3000 meters as indicated on the cockpit altimeter, bomb drop would be 2900 meters as the target elevation is 100 meters higher than departure field elevation. This could be set in the bombsight at any time and the reference altimeter in the bombsight view can basically be ignored. 2) Standard atmosphere - I have seen comments that say using this will set the Kollsman window of the cockpit altimeter to 760 mmHg (the atmospheric pressure on a so-called standard day, equivalent to 29.92 inHg in the US, or 1013 millibars in Europe) regardless of whatever local pressure is specified in the mission. If this is in fact true it is indeed a spectacularly useless setting; I can't see any way it gives any useful information. It would make more sense if, when using this setting, the cockpit altimeter is adjusted to the local pressure specified in the mission. In this case the cockpit altimeter would reliably read MSL altitude and bomb drop is easily calculated with target elevation. Real life pilots set their altimeters in this way every time they get in an aircraft, and after doing so the cockpit altimeter will indicate field elevation (in the example above, after setting the cockpit altimeter to local pressure, the cockpit altimeter would indicate departure field elevation of 100 meters). For a bombing instance, if target elevation given as 200 meters, after setting local pressure before takeoff, bomb drop would be whatever the cockpit altimeter reads minus 200 meters. Again, if the aircraft is accurately leveled off at 3000 meters as indicated by the cockpit altimeter, bomb drop is 2800 meters, which can be set in the bombsight at any time, and still not using/ignoring the reference altimeter in the bombsight view. So I guess the questions would be: Anything written here wrong ? What is set on the altimeter when using the Standard Atmosphere option (the Kollsman window is visible but tiny in game, perhaps I can check this...) ?
Inkophile Posted October 12, 2017 Posted October 12, 2017 (edited) The bomb sight altimeter (which is the only one you should use. As far as I am aware there are no more issues with using that regardless of units, but someone please correct me if I'm wrong) is not affected by which reference altitude you use (sea level or ground level), and will always show AGL altitude to the aim point from your dialed in flight altitude. Basically we have an intelligent bomb sight, which I personally find boring, but since the in-game map has exactly zero altitude information that's the only way it can be. Can only be remedied if we get much more accurate and informative maps. Edited October 12, 2017 by Inkompetent
1CGS LukeFF Posted October 12, 2017 1CGS Posted October 12, 2017 The bomb sight altimeter (which is the only one you should use. As far as I am aware there are no more issues with using that regardless of units, but someone please correct me if I'm wrong) is not affected by which reference altitude you use (sea level or ground level), and will always show AGL altitude to the aim point from your dialed in flight altitude. Basically we have an intelligent bomb sight, which I personally find boring, but since the in-game map has exactly zero altitude information that's the only way it can be. Can only be remedied if we get much more accurate and informative maps. The bombsight altimeter shows altitude above sea level, and target altitude is given in the mission briefing in the current campaign mode.
twgin Posted October 12, 2017 Author Posted October 12, 2017 This is kind of why I asked the question... Inkompetent says that (what I call) the reference altimeter in the bombsight (in the bombsight view, the altimeter on the left) reads what I called "bomb drop" directly, regardless of any other setting used. This says whatever value shows here should just be dialed in over on the right side of the bombsight. But what does "dialed in flight altitude" mean ? If an accurate description, this really is kind of boring... LukeFF on the other hand, says that this reference altimeter (in the bombsight view, the altimeter on the left) always shows MSL altitude (regardless of other settings ?) which must be modified by the target elevation to get a "bomb drop" number to dial in over on the right side of the bombsight. Which, if either, correctly describes the behavior of IL2 ? How to find out ? a little aside, airfields, targets, i.e. things on the ground, do not have altitudes, but rather elevations... a pretty small point but using more precise language might clear up confusion on these matters...
twgin Posted October 12, 2017 Author Posted October 12, 2017 I dialed up a BOM campaign mission... given in the briefing was departure airfield elevation of 151 meters (called an altitude in briefing !) and atmospheric conditions of 764 mmHg. In the cockpit (Ju-88), I chose the "nearest airfield" option and both cockpit altimeters dialed themselves to "0". This is expected behavior. Switching to bombsight view, the reference altimeter read 151 meters (this is correct airfield MSL elevation, as LukeFF said, this altimeter is reading MSL despite the setting on the cockpit altimeters) Back in the cockpit, I then chose the "standard atmosphere" option and the cockpit altimeters dialed themselves to a Kollsman window value of 1013 millibar, resulting in an indicated airfield elevation of only 111 meters (wrong elevation, some 40 meters too low, see below). Switching to bombsight view, the reference altimeter still read 151 meters, or correct airfield elevation ! So: I think LukeFF is correct in that the reference altimeter on the left in the bombsight view always reads actual MSL altitude regardless of cockpit altimeter settings. Bomb drop to be dialed in on the right will always be the altitude indicated on the reference altimeter on the left MINUS the target elevation reported in the briefing. Answered my own question, I did... For those interested, the "standard atmosphere" option for the cockpit altimeter is useless... in this case local pressure as reported in the briefing was 764 mmHg (millimeters of mercury). The cockpit altimeters of the Ju-88 (there are two of them !) use a different unit system: millibars. A local pressure of 764 mmHg should translate into a Kollsman window value of 1019 millibar. In fact, when the "standard atmosphere" option was chosen the Kollsman window adjusted to 1013 millibar (and a wrong indicated elevation of 111 meters). 1013 millibars is equivalent to 760 mmHg is equivalent to 29.92 inHg; all are representations of atmospheric pressure on a "standard" day. It would make more sense for the program to set whatever local pressure is specified in the mission briefing; although this might be problematic and individual missions may not specify a value for this... So: In game I would set the altimeter option to "nearest airfield", the altimeter will read correct AGL altitude above your airfield for the balance of the mission. Use of the "standard atmo" option means that whatever is indicated on your altimeters is off by some unknown amount, unless local atmosphere just happens to be standard. 2
twgin Posted October 12, 2017 Author Posted October 12, 2017 interesting links: https://aviationglossary.com/kollsman-window-altimeter/ https://www.boldmethod.com/blog/lists/2017/05/the-5-types-of-flying-altitudes/
AndyJWest Posted October 12, 2017 Posted October 12, 2017 Yup, you seem to have answered you own question, but just to confirm this, you can air-start a bomber in the QMB by one of the Kuban coastal airfields, with 'normal' difficulty settings: this will start you over the sea. Keep the auto-level engaged, and turn towards land. The bombsight reference altimeter reads at whatever you airstarted at, the cockpit instrument reads a bit less (presumably reading the altitude relative to whatever airfield you are supposed to land at), and the HUD shows the height above ground - which changes rapidly as you cross the mountains. If you set the correct wing speed and relative direction, and then look at what the yellow aiming circle is doing as you cross mountains, it moves backward and forwards depending on your height relative (presumably) to the calculated impact point from where you are at that instant. The correct bombsight setting must then be your altitude above MSL, minus the target's elevation above MSL. Note though that small errors in altitude from high-level drops have relatively little significance as the bombs are going to be descending at a steep angle. being out by a hundred feet or so probably isn't going to matter, given all the other variables.
1PL-Husar-1Esk Posted October 12, 2017 Posted October 12, 2017 If I remember correctly there is key for reset altimeter on ground before take off. If there is any use of it ?
56RAF_Roblex Posted October 12, 2017 Posted October 12, 2017 So to summarise, level bombing in MP will never be accurate because you need to know the target elevation above sea level and that info is not known? I cannot agree that the differences are minimal though. Dropping bombs on an airfield that is 200m above MSL without taking that into account is the same as dropping bombs on a target on the beach which is at the foot of a 200m cliff and doing it from the landward side. All the bombs will impact a long way back from the cliff edge where the target is.
Monostripezebra Posted October 12, 2017 Posted October 12, 2017 So to summarise, level bombing in MP will never be accurate because you need to know the target elevation above sea level and that info is not known? I cannot agree that the differences are minimal though. Dropping bombs on an airfield that is 200m above MSL without taking that into account is the same as dropping bombs on a target on the beach which is at the foot of a 200m cliff and doing it from the landward side. All the bombs will impact a long way back from the cliff edge where the target is. https://www.youtube.com/watch?v=IumhmB6F3U0 If I remember correctly there is key for reset altimeter on ground before take off. If there is any use of it ? TO be correct there is a key that toggles between ISA-value and QFE for the cockpit altimeteres. As pointed out here, the ISA is of limited value, because it is not ment for relating aircraft to ground, but aircraft to aircraft. It does not tell you an accurate height above ground. QFE means it shows your altitude above the measuring point (your airfield... or in BoS any airfield that is toggled), if on the ground it should go to 0. Error is in the elevation figures, but it works fine to use for bombing in MP.. if you add some guesswork and account for increased error differences on winter maps. https://www.youtube.com/watch?v=pThPE4NVCOI
AndyJWest Posted October 12, 2017 Posted October 12, 2017 So to summarise, level bombing in MP will never be accurate because you need to know the target elevation above sea level and that info is not known? I cannot agree that the differences are minimal though. Dropping bombs on an airfield that is 200m above MSL without taking that into account is the same as dropping bombs on a target on the beach which is at the foot of a 200m cliff and doing it from the landward side. All the bombs will impact a long way back from the cliff edge where the target is. What do you consider 'a long way'? a 200 metre error in altitude, with the bomb path 15 degrees from the vertical (about right from the data I have for high-level drops) would place the bomb 53.6 metres off - equating to an error of about 1/5 of a second in timing of the drop, at the aircraft speeds we are dealing with. As for not knowing the target elevation in MP, that is down to the person writing the mission briefing.
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