jason_peters Posted October 12, 2019 Share Posted October 12, 2019 How come I can get the Mach number in the HUD showing around 1.9 but the speed on the left of the hud is showing 600 (ish) the 2 seem to be disconnected assume I'm being stupid in some way :) --------- System: i7-8700k @ 4.9GHz; Nvidia RTX 2080ti; 32GB (2 x 16) DDR4 @ 3333Mz Ram; ASUS ROG Strix z370-E; SSD Drive; Oculus Rift-S; Thrustmaster HOTAS Warthog Link to comment Share on other sites More sharing options...
billeinstein Posted October 12, 2019 Share Posted October 12, 2019 (edited) It depends on your altitude and temperature. And check your TAS please. Edited October 12, 2019 by billeinstein [sIGPIC][/sIGPIC] Link to comment Share on other sites More sharing options...
razo+r Posted October 12, 2019 Share Posted October 12, 2019 Mach depends on altitude (and temperature). M1.0 at sea is like 661 knots. M1.0 at 40K feet is like 573 knots. https://en.wikipedia.org/wiki/Mach_number Then, also take a look at this: https://en.wikipedia.org/wiki/Indicated_airspeed Link to comment Share on other sites More sharing options...
Ramsay Posted October 12, 2019 Share Posted October 12, 2019 How come I can get the Mach number in the HUD showing around 1.9 but the speed on the left of the hud is showing 600 (ish) the 2 seem to be disconnected assume I'm being stupid in some way :) The speed on the left is IAS (Indicated Air Speed) not True Air Speed. In your example the forces on the aircraft are equivalent to 600 knots at SL, even though the aircraft is likely at higher altitude and moving much faster though thin air. This chart shows ~600 CAS = Mach 1.9 @ 42,000 ft = ~1095 KTAS i9 9900K @4.9GHz, 64GB DDR4, RTX4070 12GB, 1+2TB NVMe, 6+4TB HD, 4+1TB SSD, Winwing Orion 2 F-15EX Throttle + F-16EX Stick, TPR Pedals, TIR5, Win 10 Pro x64, 1920X1080 Link to comment Share on other sites More sharing options...
Bouli306 Posted October 12, 2019 Share Posted October 12, 2019 Mach depends on altitude (and temperature). M1.0 at sea is like 661 knots. M1.0 at 40K feet is like 573 knots. https://en.wikipedia.org/wiki/Mach_number Then, also take a look at this: https://en.wikipedia.org/wiki/Indicated_airspeed You are talking TAS not CAS as indicated on the airspeed dial and normally the HUD (indicated with a C next to the speed tape). In calibrated airspeed M1.0 at 40000ft is around 300kts. Link to comment Share on other sites More sharing options...
jason_peters Posted October 13, 2019 Author Share Posted October 13, 2019 thanks all the chart in post 4 is very helpful to bring all the comments together never knew there was such a difference between Mach and CAS --------- System: i7-8700k @ 4.9GHz; Nvidia RTX 2080ti; 32GB (2 x 16) DDR4 @ 3333Mz Ram; ASUS ROG Strix z370-E; SSD Drive; Oculus Rift-S; Thrustmaster HOTAS Warthog Link to comment Share on other sites More sharing options...
Ala13_ManOWar Posted October 14, 2019 Share Posted October 14, 2019 Mach depends on altitude (and temperature).According to theory, in fact Mach number only depends on temperature. Obviously altitude changes means temperature changes, but altitude per se does nothing. S! "I went into the British Army believing that if you want peace you must prepare for war. I believe now that if you prepare for war, you get war." -- Major-General Frederick B. Maurice Link to comment Share on other sites More sharing options...
Ala13_ManOWar Posted October 14, 2019 Share Posted October 14, 2019 How come I can get the Mach number in the HUD showing around 1.9 but the speed on the left of the hud is showing 600 (ish) the 2 seem to be disconnected assume I'm being stupid in some way :)What aircraft and conditions? Sometimes probes get stuck and readings are false. Turn on pitot heat, for instance. S! "I went into the British Army believing that if you want peace you must prepare for war. I believe now that if you prepare for war, you get war." -- Major-General Frederick B. Maurice Link to comment Share on other sites More sharing options...
probad Posted October 15, 2019 Share Posted October 15, 2019 (edited) the higher up you go the more sparse the air molecules are the faster you have to go (true airspeed) to collect the same number of air molecules per second (indicated airspeed) as down lower now even though you're catching fewer air molecules, because your airplane is impacting them so fast the air molecule still can only get out of your way so quickly (mach) so 200kt up high has your aircraft behaving differently than 200kt down low the interaction between ias and mach meets at the phenomenon known as the coffin corner where the quantity of air molecules are so few that your wings can't keep you afloat, yet the few that you encounter are impacting too fast for your aircraft to deal with, too. this is a major concern for "slow flying" sailplane-like designs like the u-2 near the top of their service ceiling so for matters of divining your aircraft's aerodynamic capabilities at a given altitude, ias and mach are both relevant, they just describe two different factors that your airfoil cares about -- the quantity of air, and the quality of the air. Edited October 15, 2019 by probad Link to comment Share on other sites More sharing options...
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