Supersonic, Zoom Climbs - Possible?
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Right. Looking at the Mach meter, 1.4 seems to be around 800 knots at 30,000 feet. In any case, this table suggests about a 100 knots difference in the speed of sound between altitudes:
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Thank you Cik! That finally makes some sense, will try it.
EDIT: Yes, indeed. The HUD mach number is in the range of 1.88-2, while the airspeed indicator dial and HUD airspeed scale read 700-800 knots. Now, (just for curiosity sake), I am wondering how to set the airspeed scale to true airspeed.
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edit:
sorry, guess you hadn’t read my last post. no hostility here ~
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Must have missed whatever you were planning as a response, but a big thank you for pointing out the use of CAS. Think that has been misleading me a long while now.
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NP bro
i’m only paying it forward. someone else wrote a 3 paragraph essay on how it worked when i was confused. for reference the switch is on the right side of the cockpit, where your knee would be. 3 position switch, defaulted to up (cas) middle position (TAS: true air speed) and bottom position (GN SPD: ground speed) as you climb, you’ll notice your CAS decreases sharply, because of the much thinner air. your plane will handle like a whale, even at mach 1.2 because there’s so little air to manipulate. you’ll be going much faster though, and your engine will be sipping something like 4000~ fuel throughput to maintain speed. if you are going to travel a long distance, it’s almost always worth it to make the climb to angels 30.
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Many, many thanks! After flipping the appropriate toggle button on the HUD controls, true airspeed now reads 1200 knots, corresponding to Mach 2 in level flight, which the HUD also displays. Wow, to think something so simple was so confusing, this has been plaguing me for months!
Now to see if some of the climb/altitude behavior makes more sense with a usable airspeed indicator.
Thanks again, pushing back up to 1100 knots at 40,000 feet again, and very happy!
EDIT: Yes, indeed altitude behavior is much more comprehensible, with much more intuitive speed/altitude relationships. Managed to retain 1000 knots at 60,000 ft without embarking on a zoom climb, and the plane definitely launches upward better from its 50,000 ft service ceiling.
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In a nutshell, it’s very difficult to tell how “fast” you are really flying. In relation to what? The ground? If you flew in the direction that the earth is spinning, and flew as fast as it’s spinning, would you be moving at all? These are all considerations that have to be understood before you can contemplate “speed”. CAS is important to understand how your aircraft will perform in a specific altitude regime but it won’t tell you how “fast” you are actually going. It’s important to understand these concepts before you can break the “sound barrier” multiple times over
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I’ve gotten the BLK52 up to M1.95 or maybe M2.00 in level flight. Check your HFFM/HAF performance charts; the line goes across the 2.0 threshold under narrow conditions.
The maximum ceiling of temporary excursions above sustainable altitude is an interesting one. Energy charts express both height and speed in the same units. Specific power “Ps” is expressed in feet per second climb. So 100 fps Ps represents the ability to raise 1 kg of airframe 1m higher against gravity 9.8 m/s² in a certain time (higher Ps, lower time). It takes a certain power (energy per time) to increase the potential energy of the aircraft by moving it farther from the Earth’s center. Ps 100ft/s also represents a power which is able to accelerate the aircraft.
The minimum time to climb profiles are made under the assumption that if you maximize Ps, you maximize energy entered into the system. It doesn’t care if you are using this power to go faster or higher. The idea is that Ps curves have “negative curvature” and you’ll reach a maximum value at some speed. If you pitch to maintain that speed you will go higher to avoid going faster and leaving the max Ps curve peak. It’s assumed that adding speed and adding height are adding the same substance and that you are free to convert freely with 100% efficiency between the two with no delay. Of course in reality it isn’t. Mach 10 at ground level doesn’t translate with 100% efficiency to convert all of that speed to altitude instantly.
Energy charts can be “bumpy” like humps on a camel. It is possible to exist in a steady state at do different height/Mach points on the graph just fine but for it to be impossible to exist on a point directly between those two zones. The transonic region has a higher drag than the sub or supersonic region usually. In some cases you have to take an interesting path to go “under the dip” in the graph. This is were diving to get into the faster regime comes from. Usually this “follow the valley around the mountain” path in the F-16 is near M0.9.
Back to the original question maximum altitude depends on two things:
1. Total energy
2. Converting as much speed energy to altitude energy as possibleBeing at maximum sustained altitude is a tall launching point for a zoom but your zoom potential here is zero. It is the sum of the altitude at each point on the sustainable chart plus the zoom potential that we wish to maximize. Then there is the zoom technique to consider. Too harsh and drag saps the energy so it won’t become altitude. Too gentle and gravity*time saps the energy. The ideal transient zoom profile maximizes the ratio of climb rate to energy loss. This is a difficult problem for my brain.
The FLCS is not a significant hindrance to pushing the envelope except perhaps at extreme AOA. These energy regimes generally are smooth and require minimal flight surface movement. Indirectly the FLCS is a benefit because the airplane can be designed without traditional draggy stability concepts.
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Many, many thanks! After flipping the appropriate toggle button on the HUD controls, true airspeed now reads 1200 knots, corresponding to Mach 2 in level flight, which the HUD also displays. Wow, to think something so simple was so confusing, this has been plaguing me for months!
Now to see if some of the climb/altitude behavior makes more sense with a usable airspeed indicator.
Thanks again, pushing back up to 1100 knots at 40,000 feet again, and very happy!
you are welcome. keep in mind though that CAS is a very useful mode, because it provides a metric on how your airplane will handle independent of altitude. if you are in a fight and ask yourself “can i turn into this MIG-29 and get an advantage?” CAS will tell you how your aircraft will behave. corner airspeed will be at 400~ kn regardless of altitude. it just confuses people because it’s the default mode.
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I’ve gotten the BLK52 up to M1.95 or maybe M2.00 in level flight. Check your HFFM/HAF performance charts; the line goes across the 2.0 threshold under narrow conditions.
Yes, indeed, I am pushing similar performance now.
maximum ceiling of temporary excursions above sustainable altitude is an interesting one…
Being at maximum sustained altitude is a tall launching point for a zoom but your zoom potential here is zero. It is the sum of the altitude at each point on the sustainable chart plus the zoom potential that we wish to maximize. Then there is the zoom technique to consider. Too harsh and drag saps the energy so it won’t become altitude. Too gentle and gravity*time saps the energy. The ideal transient zoom profile maximizes the ratio of climb rate to energy loss. This is a difficult problem for my brain.
Exactly, and true airspeed seems to be a much better indicator as to how much energy is available.
The FLCS is not a significant hindrance to pushing the envelope except perhaps at extreme AOA. These energy regimes generally are smooth and require minimal flight surface movement. Indirectly the FLCS is a benefit because the airplane can be designed without traditional draggy stability concepts.
Thank you very much for confirming this.
EDIT: Yes, Cik, CAS is definitely useful for aerobatic/combat situations. Personally, I just feel like starting with zoom climbs and similar maneuvers to get some idea as to the energy/altitude tradeoffs involved with a (hopefully) more accurate flight sim than what I am used to.
EDIT2: Definitely, TAS is proving much easier to understand going into a zoom climb. Pushing 70,000 feet easily now, without needing to depart from controlled flight.
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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.