Supersonic, Zoom Climbs - Possible?
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Exactly, and true airspeed seems to be a much better indicator as to how much energy is available.
Absolutely not, CAS is there to remind you of how your aircraft will perform AT CURRENT ALTITUDE! Never forget that, as corner remains relatively the same with CAS! Go by TAS and you will die in a dogfight
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Also when gear is out you will automatically read CAS speed on HUD no matter what configuration you’ve set up because of its importance regarding flight behavior.
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Right, right. CAS is useful. TAS is useful too though, as an indicator of what the plane is actually doing. Case in point, just pushed the Mig-25 flight model to Mach 3.05 (1798 knots) @ 55,770 ft. Meanwhile, CAS read out just 1801 knots.
Oh, and pulled off an 85,000 ft zoom climb from there. No doubt even greater altitude is feasible.
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Right, right. CAS is useful. TAS is useful too though, as an indicator of what the plane is actually doing. Case in point, just pushed the Mig-25 flight model to Mach 3.05 (1798 knots) @ 55,770 ft. Meanwhile, CAS read out just 1801 knots. ……
Regardless of what your ‘displayed’ speed is set to (TAS, CAS, etc.), doesn’t your mach readout tell you the correct mach number for whatever altitude you’re at? If so, what do you need TAS for?
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True, I can rely on the Mach number, but the greater precision TAS number does indicate gains and losses more precisely. Most important thing is just realizing that CAS is different from TAS.
Suspect that CAS becomes less relevant at supersonic speeds, assuming most airframes reduce flow to subsonic by the time it reaches the sensor.
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Altitude and TAS (or G/S, jury is out) determine energy state. The math is very straightforward to the point you can compare two airplanes at different speeds and altitudes to see who “wins” the energy game.
Example problem:
Airplane A is 400 KTAS, 20,000’
Airplane B is 500 KTAS
What altitude must Airplane B be at to have equal specific energy as airplane A?Solution:
Convert values for easy calculation
A: 205.8 m/s, 6,096 m
B: 257.2 m/s, ?,??? mEquations for Total, Kinematic, and Position Energies
Es = Ts + Us
Ts = T/m = 0.5*(v^2)
Us = U/m = 9.8*hEs[A] is 80,918 J/kg or expressed as a height 8257 m. For Es **to be equal the sum of its speed energy (33,075 J/kg) and its height energy. By subtraction the height energy must be 47,842 J/kg which is a height of 4,882m.
A: 205.8 m/s, 6,096 m
B: 257.2 m/s, 4,882 mConvert back to knots and feet.
A: 400 KTAS, 20,000’
B: 500 KTAS, 16,017’** -
Right, so I take it TAS is the number to watch closely while pulling zoom climbs and other speed/energy tradeoffs.
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Right, so I take it TAS is the number to watch closely while pulling zoom climbs and other speed/energy tradeoffs.
again, CAS is critical to understanding how your aircraft will perform AT CURRENT ALTITUDE! So take it like this, TAS is NOT the number to watch closely! Example: Watching TAS will not tell you when the aircraft will stall, whether zoom climb or at corner speed. MACH is your friend
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Right, I get that. CAS determines when the aircraft will stall, and in turn, the maximum angle of attack. Nonetheless, exact TAS (as opposed to the slightly more approximate Mach number) is helpful too.
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Right, so I take it TAS is the number to watch closely while pulling zoom climbs and other speed/energy tradeoffs.
All I’m saying the jet’s velocity defines the kinematic component of its total energy. CAS is a poor approximation of true velocity, TAS and G/S are much better. The lesson was that it’s possible to have different kinematic (speed) and potential (height) components and those two components are directly comparable. You can quantitatively compare any speed/height combination to any other speed/height combination.
If you had to watch any speed it’d probably be Mach since the H-M diagrams are in Mach but honestly it doesn’t matter what you reference. What you really want to do is be at “Ps max” which is the point on the H-M diagram where you are feeding the most energy per second into the airplane. If you somehow know that you’re at Ps Max based on CAS then that’s fine. All the performance charts I’ve seen use Mach so that’s what I would use practically.
Break open the HFFM Manual, page 120 through 129. These are Turn-Mach energy diagrams but we can use them for climbing, just only look at the turn rate = 0°/sec at the bottom. At SL look at the tiny “shark fin” Ps contour at the bottom of the graph. Anywhere in that region from 0.88-0.90M you are getting at least 800 fps Ps. We can’t be more exact than that because the Ps contours are only in 200 unit increments. A good estimate looking at all the Ps contours on that graph is that Max is at 0.90M and drops off sharply increasing to 0.905M and beyond. Probably best to stay at 0.89M to avoid that sudden drop.
Next slide, 5,000’. The 800 fps Ps contour is not present. Again we guess that Ps peak is in the 700-750 ft/sec range with a peak near or slightly below 0.90M.
Next slide, 10,000’. Same but less Ps, probably around 650 ft/sec.
15,000’. You get a tiny little 600 ft/sec contour centered ever so slightly slower than 0.90M.
20,000’. Same mach, less Ps.
25,000’. Boooooring!
30,000’. What’s this? The 200 ft/sec Ps contour has two humps! The 0.9M hump looks bigger but the 1.3M hump isn’t far behind.
35,000’. The subsonic and supersonic contours are looking very close indeed.
40,000’. Supersonic Ps contour dominates. 200 ft/sec+ only possible between 1.44 and 1.60M.
45,000’. All Ps is less than 200 ft/sec. Which regime has more power is unclear.Ignoring the possibility of small gains in the supersonic regime, the Ps Max speeds are in the 0.82 to 0.91M range for high and low drag aircraft respectively. You could do the same exercise with the MIL charts and find the fastest profile using only MIL engine power.
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Some people still do not know the CAS-TAS-Mach number relation…