charlee22009
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Joined: 6/28/2015
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Hi, Question.
Take the Su-27P Flanker (or any of a number of 4th generation fighters from the database)
Band 4 (36000-40000) has a 'cruise' speed of 480 knots. The listed 'cruise' speed of all the other altitudes is the same. 480kts. I understand this to be 'indicated' Airspeed. Indicated here would make the *most* sense... as this is a measure of the approximate 'dynamic pressure' of the oncoming air used to generate lift.
And... as IAS and TAS is about the same at very low altitudes, like near sea level... 480 knots implies that indicated airspeed (for cruise or best range) is around 480 near sea level. Regardless of whether it's 480 'True' or 'Indicated' in the database... that much is clear. (There are a number of other potential factors as well in that number, maximizing 'range'... involving things like engine RPM and resulting efficiency... but let's not go there yet. Perhaps the developers can highlight some more of their methodology?)
For example, at a given 'angle of attack' (say for 'cruise' or max range) a given 'dynamic pressure' (the speed of the oncoming air taking into account thicker or thinner air density...) will result in a given amount of 'lift'. If the airplane is heavier it needs 'more lift.' If it's lighter it needs 'less lift.' (Generally these 'Angles of Attack' are set for the particular airplane... where some angle is best for range, one for endurance. Range 'angles' are typically near 3 degrees for most airfoils.)
Therefore... to simplify things... at a given weight (say near max takeoff weight) the aircraft needs the same amount of 'dynamic pressure' - in other words indicated airspeed, which *shows* dynamic pressure - to provide a certain amount of lift for that weight. Where lift equals weight.
- If 480 knots is approximately the required airspeed, for cruise, just after takeoff (say at low altitude)... then it's approximately the right cruising speed (that *indicated* airspeed displayed inside the cockpit) at *any* altitude. This is because, like I stated earlier, the Angle of Attack for *best cruise* doesn't really change (for a particular airplane of course). It doesn't change with weight or generally change with altitude either (with subsonic flight.)
Now... since air becomes less dense at higher altitudes... the plane has to fly 'faster' through the air to get the same 'air pressure' needed for lift. That airspeed (or *true airspeed*) varies with density.
Doing some math (just an equation)... if the IAS is 480knots near sea level ("cruise")... then it should be *about* 480knots IAS at 36000 ft... which then becomes about 881 knots True Airspeed. Which makes it 'supersonic' just to cruise. This makes me think that that '480' (cruise near sea level) is too high? (It would be interesting to get the developers' methodology on this... as it's possible that higher turbine/engine RPM would result in better efficiency... therefore requiring higher 'speeds' even at low altitude for best range???)
Or take a different, 'lower' example.
Say at 12-24 thousand feet... that 480 knots indicated airspeed... is best for 'cruise'. (Says so in the database) That IAS is actually about 640 knots *True Airspeed.* (If we take the 'standard density' at around 18000 feet... we can easily convert indicated to true.) If the Speed of Sound (in the 'standard' atmosphere) at 18000 ft is about 620 knots... then that 480KIAS becomes->>>640KTAS which becomes->>>Mach1.03.
While this is possible in a fighter, it seems kind of 'fast'? (For a range-maximizing cruise speed? -IF the database numbers were in KIAS to begin with??)
So while this seems 'fast'... maybe it's only because the original '480' knots (at or near sea level) it too fast in and of itsself? I know that number is a deliberate number, but I'm really interested to know where it comes from.
I just have some questions about the designers' methodology in choosing what performance parameters, where, and why... in order to make the simulation work. It doesn't have to be 'perfect' and it's certainly not 'simulating' one or 2 airplane... but ***maybe there are some things which could be added***???
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For example, in how the developers figured the "Fuel Consumption" in the database for aircraft.... While turbojet and turbofan efficiency increases at higher altitudes (giving generally better performance *both* in terms of range and endurance) - it *would be awesome* to have more information in the database.
For example the 'specific range' listed in the database. The "fuel consumption" is basically the 'specific endurance' - that is the amount of gas used per unit of time. This is listed. HOWEVER, If the TAS (which is ALREADY presented) is *then* taken into account, it allows you to find the distance covered while that specific amount of fuel is being burned... This is **specific range**. This would be a phenomenal thing to see in the database. The given amount of fuel used per unit distance (ie KG/NM). This would be really awesome, a huge help to the player/scenario developer, and it would be easy to implement (ESPECIALLY if the database speed are already in KTAS.) I think a spreadsheet could *easily* calculate and present this data - just base on what's already presented.
Anyway, thanks for reading this post and thinking about it. I really like CMANO as it's an excellent simulation and very thought provoking (VERY!) But I also think that feedback from the community is important and I'm interested in how all of these mechanics have come about or have been implemented. It's incredibly interesting.
< Message edited by charlee22009 -- 2/20/2018 9:48:02 PM >
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Questions about 'KTAS' and 'KIAS' numbers in database, ... - 
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