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I know that the absolute maximum velocity of an aircraft is based on the thrust available. However the aircraft is usually limited beforehand by other speed limits such as dive speed and Mach Never Exceed velocity.

So when researching the maximum velocity of an aircraft, is the max speed listed the absolute max speed or the dive speed as that would make the most sense since its the structural limit. The example I am specifically thinking of is the C-130J Hercules which according to this brochure (linked below) from Lockheed, is around 365 kts.

https://www.lockheedmartin.com/content/dam/lockheed-martin/aero/documents/C-130J/C-130Brochure_NewPurchase_May2020_Web.pdf

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  • $\begingroup$ Is that maximum dive speed with the wings still attached or without? I'd think the latter would be somewhat higher... $\endgroup$ Mar 28 at 16:14
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    $\begingroup$ "is the max speed listed..." listed where? In the listing for Vne? Or in the promotional brochures regarding the performance of the aircraft, which would presumably be in level flight? $\endgroup$ Mar 28 at 19:11
  • $\begingroup$ I should have been more clear, I understand that the maximum speed on the brochure is for steady level flight. I was confused as to which speed limit in level flight however. I think I understand now though, thank you. $\endgroup$
    – Bill Shao
    Mar 29 at 13:21
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The maximum cruise speed from the brochure is a True Airspeed value and is based on the highest TAS achieved with the engines operating at the maximum cruise power rating specified by the engine manufacturer, based on some altitude and load condition that is probably buried somewhere in the fine print (we could probably assume the 365 kt value is at max cruise power, max gross weight minus departure and climb fuel, at service ceiling, in ISA conditions).

Note that limitation speeds like Vmo or Vd are based on indicated/calibrated airspeed (CAS, what the pilot sees as a speed on the instruments) and are a function of the dynamic pressure conditions acting on the airframe, regardless of the actual velocity through the air mass (True Airspeed). They are generally a lower value, as TAS starts to exceed IAS as soon as you go above sea level, the difference getting bigger and bigger the higher you go.

So at the 28000 ft service ceiling, the indicated airspeed will only be about 230 kt when the Herc is cruising at a TAS of 365 kt on a standard day with a -40F temperature outside.

The speed limitations will be a margin above the indicated/calibrated cruise airspeed value; if, say, the Vd was 290kt (I'm just picking a number at random), at 28000 ft in standard conditions, that's a TAS of 450, but that's irrelevant for the purpose of the speed limitation because we're only interested in the IAS value insofar as that's what the pilot sees, and it's what represents the actual dynamic pressure effects of that speed for structural purposes (although where the limit is for flutter considerations, that are a function of TAS, the flutter TAS limit is converted to IAS in the Air Data Computer for the pilot to use).

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  • $\begingroup$ The never exceed speed may be a true airspeed limit since flutter depends on true airspeed, though it will still be given as indicated airspeed limit, tabulated dependent on altitude. $\endgroup$
    – Jan Hudec
    Mar 28 at 14:55
  • $\begingroup$ Modified my post thanks. I was only thinking of structural effects. $\endgroup$
    – John K
    Mar 28 at 15:45
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    $\begingroup$ Yeah I took out No. Thanks $\endgroup$
    – John K
    Mar 28 at 18:15
  • $\begingroup$ Thank you for your response! $\endgroup$
    – Bill Shao
    Mar 29 at 13:22
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The 365 knots listed in that brochure is (as noted in the text) the maximum cruise speed. That is the maximum speed at which the aircraft can be expected to operate under normal circumstances in level flight. It is not related to the Never Exceed speed, which depends on the structural limits of the aircraft (as well as air pressure and atmospheric conditions etc.). The maximum cruise speed is interesting for planning purposes, as it gives you an indication of how far you can expect the aircraft to go in a certain time window. It is not, however, very interesting to pilots, who will operate the aircraft with regards to the actual structural limits.

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  • $\begingroup$ "It is not, however, very interesting to pilots, who will operate the aircraft with regards to the actual structural limits." Not if they want to keep their jobs. I imagine that the airline might get rather annoyed at them burning a bunch of extra fuel to get to their destinations marginally faster! $\endgroup$
    – nick012000
    Mar 29 at 10:48
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    $\begingroup$ @nick012000 I'm not saying pilots will try to fly at Vne, but rather that the factors they use to determine which speed to fly are different from the ones used to calculate the max cruise speed printed in a sales brochure $\endgroup$ Mar 29 at 11:46
  • $\begingroup$ Thank you for your response! $\endgroup$
    – Bill Shao
    Mar 29 at 13:22
  • $\begingroup$ @expeditedescent I'm not saying that they'll fly at Vne either - I'm saying that they'll fly at the most fuel-efficient speed, which is likely the cruise speed listed in the manual. $\endgroup$
    – nick012000
    Mar 29 at 23:34

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