This diagram plots the Ma, Altitude and Specific Excess Power (SEP) of a specific military aircraft:
Why does a lower SEP allow for higher altitudes and a larger range of flight speed?
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3 Answers
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Why does a lower SEP allow for more higher altitudes achievable
Power can be represented in term of dimensional analysis by an additional height the aircraft can climb in a given time or by the additional distance it can move in the same given time (power is the ability to convert energy in a given time). Definition of SEP:
Measured in feet per minute or feet per second, it represents rate of energy change—the ability of an airplane to climb or accelerate from a given flight condition.
Available specific excess power is found by dividing the difference between power available and power required by the airplane’s weight.
The red curve represents an excess of power of 150 m/s. It means you have a lot of power available. You can use this power to accelerate (moving horizontally on the graph) or to climb at the same speed (moving vertically), or a combination of the two. In both cases you reach the green curve which is a lesser SEP, meaning you have now less power available to do the same.
At some point you reach the blue curve, the SEP is exhausted, your altitude is the maximum achievable at this speed. You might still have the possibility to use the wing at a better efficiency, which means targeting another speed closer to Mach 0.95.
Best efficiency:
Moving closer to Mach 0.95 you regain the possibility to climb (the blue curve is higher). So the highest achievable altitude is obtained around Mach 1.
Conversely if you want to fly the fastest, you see it can be achieved at the altitude of about 11km.
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$\begingroup$ Alrighty, thank you so much for the elucidation!! $\endgroup$ Commented Jul 15 at 9:47
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2$\begingroup$ It looks like the vertical scale is in meters, so best mach number is at an altitude of about11000 meters. As sound speed decreases with altitude, the best speed may be a little lower. $\endgroup$ Commented Jul 16 at 0:57
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I think you've got this backwards. If you follow the blue curve, that shows the combinations of mach number and altitude where zero excess power is available. At lower altitudes for a given mach number, there is generally more specific excess power available (not always- as you can see in the bulge in the blue curve in the supersonic region).
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$\begingroup$ Oh, yes, you are absolutely correct! So basically at lower altitudes, we can reach up to 150 m/s SEP, whereas at really higher altitudes only to 0 SEP, right? $\endgroup$ Commented Jul 15 at 9:47
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$\begingroup$ @CheesyBeqa Yes. The blue curve is where the plane can only barely remain level, and the other curves the plane can climb or accelerate. $\endgroup$– ChrisCommented Jul 15 at 16:33
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Previous answers are fine but I'm choosing to add some information in the form of an answer rather than a comment. The graph shown is for a specific weight, thrust setting, and load factor (presumably 1). Any one of those change and the curves change. More broadly, you want to search on the term energy maneuverability theory for more information.
