Climb Speed and Aerodynamics Question

[Switch625]

I’ve been looking at the charts in the A1-F18AC-NFM-200 for some realistic climb profiles. It looks like climbs are performed at a constant KCAS airspeed, then at some altitude transition to constant Mach Number.

 

Ex: Fig 11-23 – Climb Speed Schedule: with a drag index of 75, climb at 425 KCAS until 25,000ft, then climb at Mach .83

 

From a quick Google search, i found that civilian aircraft do this to avoid exceeding the Mach limit of the airframe (Mmo).

 

https://aviation.stackexchange.com/questions/35408/what-is-the-reason-for-changing-the-speed-reference-ias-or-mach-number-with-al

 

But surely a supersonic fighter jet wont be in danger of reaching an airframe Mach limit!?

 

Whats going on from an aerodynamics point of view, that makes the switch form constant calibrated airspeed, to constant mach number ideal?

 

 

Thanks!

[Ramsay]

Whats going on from an aerodynamics point of view, that makes the switch form constant calibrated airspeed, to constant mach number ideal?

AFAIK, it's not, it's an easy rule of thumb.

 

FPAS 'Climb' prompts a optimised climb speed at Mil Thrust that varies with weight, altitude, OAT, etc. during the climb, as altitude increases it converges to a mach number - the one found in the 'rule of thumb' climb tables.

 

In DCS I use 360 KIAS/M0.8 as the DCS Hornet struggles to fly the FPAS climb profile.

Edited March 2, 2019 by Ramsay
Remove speculation on DCS Hornet's drag.

[Flamin_Squirrel]

But surely a supersonic fighter jet wont be in danger of reaching an airframe Mach limit!?

 

Absolutely, but flying supersonic requires a lot of power. Far more efficient to cruise at subsonic speeds. The FPAS page gives you optimum altitude and mach numbers to fly at.

[Below Average]

If you used 350 KTS and intercept .82 nm you will be ok.