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Supersonic Level Flight Envelopes in FSX
This covers the basic physics of supersonic aircraft, why there are so few in fsx with realistic flight envelopes, and how to fly a realistic one to maximize its performance.
I tried to follow the KISS principle, yet still cover everything you need to know to fly high and fast, so the text is quite dense with information.
1. What is a Flight Envelope?
A flight envelope is a description of what an aircraft is capable of reaching in terms of speed, altitude, or load factor (G). There are various ways of describing these 3 variables on 2-D plots. This post will concentrate on mach and altitude.
2. The Basics
Starting from rest on the runway, acceleration is equal to thrust divided by mass. As an aircraft gains airspeed, drag increases and opposes thrust, thus reducing acceleration. Eventually drag will completely counter thrust, and the maximum speed will be reached.
Acceleration is related to the rate that kinetic energy is gained. This energy can also be directed to potential energy, which is altitude. Therefore an aircraft with strong acceleration will have a strong climb rate. This is why rail trains don't do well on hills - they don't accelerate or decelerate well.
Thrust decreases with altitude. The turbojets are used in fighters because they are able to produce good thrust at high altitude and speed, but they still have their limits. Maximum KIAS decreases at high altitude, resulting in less thrust.
Turbojets need air to produce thrust, and generally the lower and faster you fly, the most thrust they produce. However, they can take in too much air and overheat at high KIAS. This limit is somewhere around 800KIAS.
Drag decreases with altitude, which is why aircraft are more fuel efficient at high altitude. Too high, however, and wings struggle to create enough lift at the reduced maximum KIAS. This results in an increased angle of attack, and even MORE induced drag. To fly high, you need to fly very fast to keep KIAS reasonable.
These factors create the basic boundaries of the flight envelope in speed and altitude.
2. Mach Effects
Near mach 1 there is a significant increase in drag. Swept wings and clever designs have reduced this effect and even provide a decrease in drag past the sound barrier, but the transonic region remains draggy in all aircraft.
The transonic region is so draggy that even the SR-71 struggled to break through it, and sometimes performed a diving maneuver to accelerate through it.
At sea level, fighters don't like to fly supersonic. There's high drag, fuel use, and not a significant speed advantage. 800KIAS as about the limit for airframes and engines, which keeps aircraft from breaking completely free of the high drag mach region.
Just before the high drag region, wings provide very good lift. Fighters turn very well at mach 0.7 - 0.8.
Mach 0.9 is a good speed in fighters, from sea level up to near ceiling. It provides peak acceleration, climb rate, and turn efficiency.
Above mach 1.15 or so, drag may decrease, allowing the aircraft to accelerate better, but with limited turn performance. This can only be done at higher altitudes that provide lower KIAS.
3. Air Temperature
Air temperature decreases with altitude up to about 36000ft. It then remains approximately constant up to 60000ft before decreasing further.
This is important because engines produce more thrust at lower air temperatures.
This is why the maximum achievable speed in fighters is usually at 36000ft.
4. Putting it together
The level flight envelopes of most fighters follow this distinctive shape. The graph shows the the boundaries at Military Thrust and Maximum Afterburner.
Some key points to note are:
The aircraft struggles to go supersonic at low altitude.
The dip in the top from mach 1.0-1.2 is caused by the increased mach drag.
Mach drag creates a 'wall' that prevents the aircraft from flying supersonic without significantly more (afterburning) thrust.
Peak acceleration (not shown) is just before that wall, at mach ~0.9 (and at sea level).
Maximum achievable speed is at 36000ft.
The maximum speed at low air temperature is often limited by the design of the airframe or engine.
5. The Do's and Don't's
Do cruise at mach 0.9
Do climb at mach 0.9
Do climb before going supersonic
Don't fly at mach 1.05-1.25
Don't fly beyond design limits ~800kias
6. What's wrong with FSX?
Most supersonic fighters in fsx fly way too fast at low altitude, and way too slow at high altitude. This is due to errors in fsx, resulting in incorrect thrust. Some designers have tried to include mach drag, but not many. Combined with inaccurate induced drag, most fighters in fsx fly more like UFOs than turbojet aircraft.
7. How can it be fixed?
This is tricky, and involves tables 1502-1507 in the .air file. Basically the engine needs to run higher rpm at higher altitude to produce more thrust. Some 160% N1 and 130% N2 may be required at 36000ft. Aircraft can be given very accurate flight envelopes, but those values will be messed up.
Getting the aircraft to turn properly is another story, and equally complicated.
8. How to climb as fast as possible
This is a good exercise to put the above information to the test.
Takeoff and pull up: You want to build energy (kinetic or potential) as quickly as you can. Peak acceleration is at mach 0.9, which is the speed that energy is gained the fastest. You should first accelerate to near that speed. Avoid bleeding off energy in a high-g pull up. Start a smooth pull up before at mach 0.7-0.8 and accelerate to mach 0.9 during the pull.
Climb: Adjust your climb angle to maintain mach 0.9. In a modern fighter, the climb angle may be 45-60 degrees. If you need a heading change, during the pull and climb is a good time to make it.
Level off: between 25000 and 36000ft by rolling inverted. Maximum speed is reached at 36000, but remember the engines produce more thrust at higher KIAS, so slightly denser air may not hurt acceleration through the sound barrier.
Break the mach barrier: Accelerate to mach 1.25 with minimal wing loading (don't turn, try to set 0AoA)
Climb again: to 36000ft for maximum speed, or higher as to not exceed design limits or to save fuel for a longer run
The F-15 Streak Eagle Time to Climb Records followed the ideal path to reach set altitudes in a minimal amount of time. The Streak Eagle could break the sound barrier in a vertical climb, so the ideal flightpath to 30000m involved a large immelmann.
https://www.youtube.com/watch?v=HLka4GoUbLo
https://www.youtube.com/watch?v=S7YAN9--3MA
For further reading, see Design for Air Combat by Ray Whiteford
This covers the basic physics of supersonic aircraft, why there are so few in fsx with realistic flight envelopes, and how to fly a realistic one to maximize its performance.
I tried to follow the KISS principle, yet still cover everything you need to know to fly high and fast, so the text is quite dense with information.
1. What is a Flight Envelope?
A flight envelope is a description of what an aircraft is capable of reaching in terms of speed, altitude, or load factor (G). There are various ways of describing these 3 variables on 2-D plots. This post will concentrate on mach and altitude.
2. The Basics
Starting from rest on the runway, acceleration is equal to thrust divided by mass. As an aircraft gains airspeed, drag increases and opposes thrust, thus reducing acceleration. Eventually drag will completely counter thrust, and the maximum speed will be reached.
Acceleration is related to the rate that kinetic energy is gained. This energy can also be directed to potential energy, which is altitude. Therefore an aircraft with strong acceleration will have a strong climb rate. This is why rail trains don't do well on hills - they don't accelerate or decelerate well.
Thrust decreases with altitude. The turbojets are used in fighters because they are able to produce good thrust at high altitude and speed, but they still have their limits. Maximum KIAS decreases at high altitude, resulting in less thrust.
Turbojets need air to produce thrust, and generally the lower and faster you fly, the most thrust they produce. However, they can take in too much air and overheat at high KIAS. This limit is somewhere around 800KIAS.
Drag decreases with altitude, which is why aircraft are more fuel efficient at high altitude. Too high, however, and wings struggle to create enough lift at the reduced maximum KIAS. This results in an increased angle of attack, and even MORE induced drag. To fly high, you need to fly very fast to keep KIAS reasonable.
These factors create the basic boundaries of the flight envelope in speed and altitude.
2. Mach Effects
Near mach 1 there is a significant increase in drag. Swept wings and clever designs have reduced this effect and even provide a decrease in drag past the sound barrier, but the transonic region remains draggy in all aircraft.
The transonic region is so draggy that even the SR-71 struggled to break through it, and sometimes performed a diving maneuver to accelerate through it.
At sea level, fighters don't like to fly supersonic. There's high drag, fuel use, and not a significant speed advantage. 800KIAS as about the limit for airframes and engines, which keeps aircraft from breaking completely free of the high drag mach region.
Just before the high drag region, wings provide very good lift. Fighters turn very well at mach 0.7 - 0.8.
Mach 0.9 is a good speed in fighters, from sea level up to near ceiling. It provides peak acceleration, climb rate, and turn efficiency.
Above mach 1.15 or so, drag may decrease, allowing the aircraft to accelerate better, but with limited turn performance. This can only be done at higher altitudes that provide lower KIAS.
3. Air Temperature
Air temperature decreases with altitude up to about 36000ft. It then remains approximately constant up to 60000ft before decreasing further.
This is important because engines produce more thrust at lower air temperatures.
This is why the maximum achievable speed in fighters is usually at 36000ft.
4. Putting it together
The level flight envelopes of most fighters follow this distinctive shape. The graph shows the the boundaries at Military Thrust and Maximum Afterburner.
Some key points to note are:
The aircraft struggles to go supersonic at low altitude.
The dip in the top from mach 1.0-1.2 is caused by the increased mach drag.
Mach drag creates a 'wall' that prevents the aircraft from flying supersonic without significantly more (afterburning) thrust.
Peak acceleration (not shown) is just before that wall, at mach ~0.9 (and at sea level).
Maximum achievable speed is at 36000ft.
The maximum speed at low air temperature is often limited by the design of the airframe or engine.
5. The Do's and Don't's
Do cruise at mach 0.9
Do climb at mach 0.9
Do climb before going supersonic
Don't fly at mach 1.05-1.25
Don't fly beyond design limits ~800kias
6. What's wrong with FSX?
Most supersonic fighters in fsx fly way too fast at low altitude, and way too slow at high altitude. This is due to errors in fsx, resulting in incorrect thrust. Some designers have tried to include mach drag, but not many. Combined with inaccurate induced drag, most fighters in fsx fly more like UFOs than turbojet aircraft.
7. How can it be fixed?
This is tricky, and involves tables 1502-1507 in the .air file. Basically the engine needs to run higher rpm at higher altitude to produce more thrust. Some 160% N1 and 130% N2 may be required at 36000ft. Aircraft can be given very accurate flight envelopes, but those values will be messed up.
Getting the aircraft to turn properly is another story, and equally complicated.
8. How to climb as fast as possible
This is a good exercise to put the above information to the test.
Takeoff and pull up: You want to build energy (kinetic or potential) as quickly as you can. Peak acceleration is at mach 0.9, which is the speed that energy is gained the fastest. You should first accelerate to near that speed. Avoid bleeding off energy in a high-g pull up. Start a smooth pull up before at mach 0.7-0.8 and accelerate to mach 0.9 during the pull.
Climb: Adjust your climb angle to maintain mach 0.9. In a modern fighter, the climb angle may be 45-60 degrees. If you need a heading change, during the pull and climb is a good time to make it.
Level off: between 25000 and 36000ft by rolling inverted. Maximum speed is reached at 36000, but remember the engines produce more thrust at higher KIAS, so slightly denser air may not hurt acceleration through the sound barrier.
Break the mach barrier: Accelerate to mach 1.25 with minimal wing loading (don't turn, try to set 0AoA)
Climb again: to 36000ft for maximum speed, or higher as to not exceed design limits or to save fuel for a longer run
The F-15 Streak Eagle Time to Climb Records followed the ideal path to reach set altitudes in a minimal amount of time. The Streak Eagle could break the sound barrier in a vertical climb, so the ideal flightpath to 30000m involved a large immelmann.
https://www.youtube.com/watch?v=HLka4GoUbLo
https://www.youtube.com/watch?v=S7YAN9--3MA
For further reading, see Design for Air Combat by Ray Whiteford