Effects of Altitude, Weight & Loadouts on E-M Diagram

[DaleHawerchuk]

I've started a deep dive on fighters maneuverability. I found a brilliant data study by Contact Light . While its summaries are very good, there is still a few things I am not sure of. Since I'm flying the F/A-18C, I will use this example.

Referencing the F/A-18C E-M diagram at page 9, I am not sure of the effects of various factors on the curve & performance indicators. I'm not necessarily looking for precise numbers, but rather general performance trends that could be applied in dogfight situations. I've put my assumptions down here, are these correct?

1. Altitude: since KIAS & Mach already include the effects of air thinning at altitude, my guess is that the speeds themselves (for max/min turn rates and radii) would not vary a lot, but the performance indicators would, effectively "compressing" the whole enveloppe down. 
   e.g. max. inst. turn rate would stay at 310 KIAS, but rate would drop from 25 deg/s. 
   So: Altitude+++, Speeds =, Turn Rates ---, Turn Radii +++, Max climb +++
2. Weight/load (drag): it definitely affects specific power Ps, so I guess that would bring all the Ps curves down within the same envelope? That would have a slight effects on the speeds themselves, but more so on the indicators, maybe not as much as altitude?
   So: Weight/drag+++, Speeds -, Turn Rates -, Turn Radii +, Max Climb ---

I may be completely off-track here, if I am please let me know.

[Smyth]

To learn more about this topic, one good place to look is USAF or USN flight manuals for older aircraft (F-5, F-4, etc). They usually include IRL performance charts at different altitude and/or load. The performance overall may be lower than newer jets like F/A-18 and F-16, but the principals are the same and the details are easier to find for obvious reasons.

Also I'll go ahead and try to summarize, because I've never written this down in one place before and it's interesting to try.

First we have to separate sustained/instant turn, Mach/IAS, and Weight/Drag because they do not always move together. Then note this is specific to a subsonic dogfight in a supersonic jet like the F/A-18:

Sustained turn (and acceleration/climb):

1. Altitude++ -> Turn rate-- , Climb rate--, Best Mach==, Best IAS--
2. Weight++ -> Turn rate--, Climb rate--, Best Mach==, Best IAS==
3. Drag++ -> practically no impact (Negligible for light A-A missile loads; for heavy A-G loads the impact from drag might be measurable, but irrelevant compared to the huge impact of weight).

Instantaneous turn:

1. Altitude++ -> Turn rate--, Best Mach++, Best IAS==
2. Weight++ -> Turn rate--, Best Mach==, Best IAS== (best speed depends on assumptions about airframe strength)
3. Drag++ -> absolutely no impact (not even on the amount of speed lost, because induced drag at ~30 degrees AoA is easily an order of magnitude larger than parasitic drag).

Please note once again, some of this is specific to supersonic jets at subsonic speeds. Subsonic jets like an A-4 or F-86 will also lose performance with altitude and weight, but details like best speed move differently, and drag becomes important. At supersonic speeds, some performance measurements may go up with altitude, and drag is extremely important.

 

[DaleHawerchuk]

Thank you so much for your answer. I have been doing testing on my own and I seem to reach the same conclusions as you.

[Beldin]

On 10/26/2023 at 12:39 AM, Smyth said:

one good place to look is USAF or USN flight manuals for older aircraft (F-5, F-4, etc).

For a bit of a laugh be sure to check out the ejection procedures when looking at the older flight manuals. While not really a "funny" topic the bluntness of the language in the F-14 manual made me laugh.