Mach / Thrust Question
 

Could anyone point me to a reference/explanation for the following statement:

At a fixed pressure altitude, a set amount of thrust will produce a specific Mach number (i.e. the properties of Mach account for any variance in temperature).

Further to this, I'd appreciate any pointers as to the variables that keep this from being the case with a propeller aircraft (horsepower vs thrust, prop efficency variable with speed, etc).

Appreciate any thoughts that you can provide (or more importantly, a good reference...)

I can't point you towards a good reference but the sentence means that the speed of sound varies with air temperature. I think the sentence is incorrect and actually meant to say "...properties of Mach accounted for..."

Speed of sound in air is calculated as
c = (331.5 + 0.6 T) m/s
Where T is the air temperature in degrees Celsius.

See Old Smokey's post in a previous thread on temperature/Mach relationships (and the rest of the thread, too).

As he states, temperature doesn't affect Mach or EAS. Since Mach and EAS are basically all you need to calculate drag, it follows that a fixed thrust will give a fixed drag, and since the drag-to-(Mach&EAS) relationship is unique (for a given Mach-to-EAS relationship, which is satisfied by the fixed altitude constraint), it follows you'll get a single value of Mach/EAS at a given altitude.

Thank you for the point...I was searching the wrong forum. That was exactly what I was looking for...

Oh well...it was that kind of day anyway.

Shiny switches forward, rubber side down.

Cheers,

t355

There is one other variable that must be fixed for the above Mach/Thrust relationship to be true - all up weight. For a given thrust, as AUW increases, stabilised Mach number will reduce.