Machalot Posted March 26, 2021 Share Posted March 26, 2021 (edited) Let's back this up with some actual numbers. I started with this chart: threw in some basic flight mechanics and a dash of Prandlt-Glauert, and turned it into this spreadsheet: (UPDATED) UPDATE: Found an error in the Mach number transformations. Updated the figure and text with a corrected spreadsheet. Note the highlighted cell (I22). There is more than 3 times as much drag at A-End at Mach 0.758 than at A-Start at Mach 0.344. This is why acceleration tapers off even though thrust is increasing. I would be happy to provide the spreadsheet if you want to check my work or play around with it. Edited March 28, 2021 by Machalot "Subsonic is below Mach 1, supersonic is up to Mach 5. Above Mach 5 is hypersonic. And reentry from space, well, that's like Mach a lot." Link to comment Share on other sites More sharing options...
SpaceMonkey037 Posted March 26, 2021 Share Posted March 26, 2021 3 hours ago, Machalot said: Let's back this up with some actual numbers. I started with this chart: threw in some basic flight mechanics and a dash of Prandlt-Glauert, and turned it into this spreadsheet: Note the highlighted cell (I22). The drag increase factor from A-Start at Mach 0.344 to A-End at Mach 0.758 is over 7! This is why acceleration tapers off even though thrust is increasing. I would be happy to provide the spreadsheet if you want to check my work or play around with it. wow! You've put way too much effort into this haha. Great work! Link to comment Share on other sites More sharing options...
Machalot Posted March 26, 2021 Share Posted March 26, 2021 (edited) On 3/26/2021 at 3:22 AM, SpaceMonkey037 said: wow! You've put way too much effort into this haha. Great work! Ha, yep, too much time and effort. But I became an engineer because I enjoy this kind of stuff! Just as a point of interest, my very approximate methods in the spreadsheet predict a top speed at MIL of Mach 0.92, but in a test in DCS I was able to reach Mach 0.97. This is probably because the Prandtl-Glauert rule tends to over-predict drag very near Mach 1. I also used the US Standard Atmosphere while DCS uses the International Standard Atmosphere. Not sure whether that makes it predict faster or slower. UPDATE: Corrected an error in the Mach transformation formulas for the lift and drag cofficients, and now the predicted max speed at MIL is Mach 0.96, while DCS gives a max speed of Mach 0.97. Pretty good for a back-of-the-envelope calculation! Edited March 28, 2021 by Machalot "Subsonic is below Mach 1, supersonic is up to Mach 5. Above Mach 5 is hypersonic. And reentry from space, well, that's like Mach a lot." Link to comment Share on other sites More sharing options...
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