# Similiar speed but different mach numbers?!

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Salaam

According to some online sources, the  max speed of Su-47 (Berkut) is about 2400 km/h while its max mach number is 1.6.

Meanwhile Su-27 has almost the same max speed but its max mach number is 2.35.

Could you please describe that how the max speed of two aircrafts could be almost the same while their mach number being so different?

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Greetings SAH - The reason that speeds and Mach numbers are different for different aircraft, is because the Mach number is related to the speed of sound - which can vary according to altitude, air pressure, and temperature.

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The speed of sound is a function of the density of the material it is being transmitted through.

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Which is altitude and temperature. Vmo and Mmo. are both limiting . In practice you climb on a varying airspeed limit and then change to a Mcrit  Mach Number limit when it applies..  Nev

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• 1 month later...

The speed of sound is purely a function of air temperature. So if you are on the ground in Siberia at minus 60 degrees celsius the speed is the same as minus 60 degrees at say 40,000'.

From memory the formula is:

38pointsomething x the square root of the absolute temperature.

Absolute temp being zero at ~minus 273°.

Your Mno is the ratio of your true airspeed/local Mno. At sea level and ISA (+15°C) at 250kts indicated your TAS is 250kts and the Mno is 250/661~0.38M.

As you climb out at 250IAS your TAS will progressively increase (eg+16%@10,000') as the air DENSITY decreases. Assuming a standard atmosphere the local speed of sound will decrease with temperature until it is ~573kts@36,090'-the tropopause where the standard temp is -56.5°C. Typically in the cruise the numbers you will see might be: 36,000'/245KIAS/M0.78/460KTAS

There is a lot more to the story but I won't bore you with that.

Edited by Cyrano
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Sorry but the speed of sound is not purely a function of the temperature.  It is a function of the density!  In a gas temperature is a significant contribution to the density.

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There’s bucket loads of info re this subject on the net but watching a vid of the U2 spy plane recently the cockpit camera showed an IAS 110 kts at 70000’👍

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That's about what I would expect. The U2 is like a large wingspan glider  flying in very thin air  Nev

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If you look at 25:02, you will get a brief glimpse of the PFD. 104KIAS/70,000' and M0.707.

The air is indeed thin!

The best of TOP GEAR.

Edited by Cyrano
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I’ve taken the jet I drive to 49K, thought that was special but 70K is almost out of this world😉

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2 hours ago, Flightrite said:

I’ve taken the jet I drive to 49K, thought that was special but 70K is almost out of this world😉

I've been to FL430 many times and M1.5 on one occasion. We share many diverse experiences, all different but none better than another!

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