machmeter

[flipside]

Can someone please help I can not find much info on how it works. Firstly on an aircraft without adc that has barber pole on asi is there simply an imput from altitude ie no linked temp input and is temp used on machmeter that gets imputs from adc.
any help appreciated thanks

[john_tullamarine]

Suggest you track down a text on instruments for the details of the innards ..... there is no need for a temperature input as Mach can be expressed in terms of altitude and CAS only. The equation comes down to something along the lines of

M = (5((qc/ps+1)^0.286-1))^0.5

(presuming that I have counted my brackets and put them in the correct spots).

Just think of the machmeter as a fancy ASI ....

[AIRBUS ERROR]

flipside & JT,

Temperature has a HUGE effect on Mach number... for instance at FL360 the temp is -57 Celcius and M1 is 290m/s... at ground level and ISA conditions M1 would be 340m/s... in other words on a very cold day at ground level at the North Pole, M1 would be @ 290m/s. For confirmation I strongly suggest the latest edition of A.C Kermode's 'The Mechanics of Flight.' A machmeter is NOT a 'fancy ASI', no matter how tempting it is to think like that. Imagine what would happen if you disregarded temperature in a dive at M0.9, and hit a serious temperature inversion? Indicated Mach might not rise, but true Mach would, possibly causing a shock stall. Bottom line, temperature is included in Mach number calculations.

AE

Edited for spelling lapses of author ;)

[john_tullamarine]

Airbus Error,

Respectfully suggest that you revisit Kermode ... or, better still, any of the standard undergrad basic aerodynamics course texts ... Kermode is a bit simplistic for other than a generalist overview approach to life .... indeed your examples, for instance, must unsettle your position ... the mach number quoted is the same for both temperatures (M=1) .. although the OAT is drastically different ..... this would seem to indicate that mach is not terribly interested in the OAT.

Speed of sound, however, certainly depends on temperature -

a/a0 = root(T/T0) with temperatures in degrees absolute ...

Mach, however, is not interested in temperature ....

Very common misunderstanding ...

Mach compares static and dynamic pressures. The equations are certainly different ... but the ASI and the machmeter are, in fact, doing similar sorts of things ...

[Tinstaafl]

Temperature is irrelevent to how a Machmeter determines Mach No.

In the maths of it, Temp. ends up being cancelled across both sides of the equation during the process of deriving the formula on which a Mach meter is based.

I *used* to be know how to derive it. Not any more... <sigh>

[flipside]

Thanks for your help guys, all is appreciated

[AIRBUS ERROR]

Guys,

Apologies for the misunderstanding, JT you're quite correct, I was looking at actual velocity rather then Machmeter reading. However, consider what the speed of sound would be at absolute zero temp... look it up and you'll find some interesting answers.

Regards,
AE

[oxford blue]

The CAS/Mach No relationship is independent of temperature. However, the TAS/Mach No relationship depends ONLY on temperature. This confuses people.

Most pilots are only interested in the TAS/Mach No relationship. On most aircraft you use your CAS to determine TAS up to a certain altitude, then use the Machmeter beyond that altitude. So, in general, you're not terribly interested in the relationship between CAS and Mach No, because you don't need to use it. Again, on most aircraft which operate above or near the tropopause (ie, most airline medium to big jets), you climb using an initial CAS until you hit a certain Mach No, then continue the climb by holding the Mach No (I know that lots of you will write in with examples of aircraft which are the exception, but, nevertheless, it's a pretty general rule).

Because of the temperature range you actually practically encounter in realistic atmospheres, Mach 1 is nearly always between about 550 and 650 knots TAS. At around 30 - 35000 feet, it's nearly always very close to about 600 knots.

This means that, provided the head/tailwind component is fairly small in comparison to your TAS, the Machmeter becomes a mile-a-minute meter. Suppose you're at M 0.8 and you've got a head/tailwind component of 20 knots or so. You can disregard that for all practical purposes compared with your TAS of about 480 knots. 480 knots is 8 miles a minute. Mach 0.7 is 420 knots. That's 7 miles a minute. Mach 0.6 is 360 knots. That's 6 miles a minute - and so on.

So, if you're doing M0.7 and the DME shows 77 nm to the next reporting point, provided the wind's reasonably light, you've got 11 minutes to run - just like that!

[flipside]

OB
I like the rule of thumb, got any more gems