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Power drops off w/ higher altitude?


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In any of the FA-18 missions in either Caucuses or Persian Gulf I have a problem while climbing from a carrier launch. I usually go into AB for the launch. Wheels up immediately I leave the deck. Flaps up at about 250k (they may have come up automatically but I move the switch anyway) and out of AB 275-300. While in AB I can climb at 15 degrees and still accelerate. Once out of AB I drop down to 10-12 deg climb & get to maybe 425k. Say I want to get to 30,000ft. Past 10-15,000ft the speed starts dropping off from the 400+ so I reduce climb to about 7deg and can hold 375 for a while. Then I notice I have dropped below 375 so I reduce to 5 deg. Once around 25,000 altitude I have to drop climb to 3 deg or speed keeps dropping. Is this normal behavior? If not what should I check for?

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Assuming your speed is still in CAS, the higher you go, the lower the number becomes (assuming TAS = constant). CAS is basically the speed that shows you how the aircraft performs at the current position.

 

Trying to keep 375 kt CAS at 30'000ft will be hard, as this is almost M1.0.

 

If you want to cruise at 30'000ft, (check your FPAS page), you should lower your CAS speed. FPAS usually shows a Mach number between 0.6 and 0.8 or something like that, which is around 300kt CAS.

 

So, knowing that the CAS will drop with increasing altititude, you have to life with the fact that it happens and that you have some 300kt or less at 30'000ft without afterburner.


Edited by razo+r
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At altitudes higher than around 20K, is better to measure your speed using the MACH number instead of kts, gives you a more realistic number, I mean, as they told you, you can be 350kts and still going supersonic at high altitudes.

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Totally normal and realistic behavior.  You need to understand what Calibrated Airspeed (CAS) is, and understand the air is thinner as you climb, resulting in reduced CAS as well as reduced engine power.

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Yes air speed is not a great way to measure how fast your jet is moving across the planet, mach or ground speed is much better. Air speed is great for knowing how your aircraft will “handle” for a given maneuver.

 

The air speed you see on your HUD is a better indicator for how much air resistance (or wind) your jet is seeing.

 

To test this, fly at 100ft above the ocean with full afterburner. Record maximum CAS and MACH. Next, climb up to 40,000ft and do the same afterburner test. You should see a much higher MACH at higher altitude with much lower CAS.

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Wow, this is all quite interesting. How does it fit in w/ the often heard, so I assume true, thought that jets get better economy at higher altitudes. I always figured that was because the air is thinner so offers less drag. But if the power is also dropping, is it dropping more slowly than the drag? I will do some research so I can get a better understanding of this. Mach# is right there in the HUD, is ground speed available on the GPS maybe?

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Check the FPAS page on the DDI for optimum altitudes for both range and endurance.

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3 hours ago, CBStu said:

Wow, this is all quite interesting. How does it fit in w/ the often heard, so I assume true, thought that jets get better economy at higher altitudes. I always figured that was because the air is thinner so offers less drag. But if the power is also dropping, is it dropping more slowly than the drag? I will do some research so I can get a better understanding of this. Mach# is right there in the HUD, is ground speed available on the GPS maybe?

At a given true airspeed (assuming roughly similar Mach number), there's about 1/2 as much drag at 22,000 ft and 1/4 as much drag at 40,000 ft, due to proportionally thinner air compared to sea level. (source: https://ntrs.nasa.gov/api/citations/19770009539/downloads/19770009539.pdf)

 

To maintain airspeed, thrust must equal drag, so the thrust required for steady flight is reduced by the same amount as drag.

 

Thrust Specific Fuel Consumption (TSFC) is the rate of fuel flow required to generate each pound or Newton of thrust.  For a sampling of different turbofan engines shown in the plot below, the TFSC  increases a bit between sea level and cruise altitude, but by less than 2x.  

 

So at higher altitude, even with a slightly higher TSFC, there is significantly less drag, therefore less thrust needed, which means a net decrease in fuel consumption.

 

image.png

(from p. 12 of this paper https://www.researchgate.net/publication/332013573_OpenAP_An_open-source_aircraft_performance_model_for_air_transportation_studies_and_simulations)

 

 


Edited by Machalot
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"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."

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