Accurately calculating mach from a Raven Altimeter

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drew
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Accurately calculating mach from a Raven Altimeter

Postby drew » Fri May 25, 2012 3:44 pm

Hi All,

Just curious, does anyone have a recommended way to accurately calculate the relevant Mach number from the reported velocity a Raven altimeter produces? I've been using the following page today:

http://www.luizmonteiro.com/Altimetry.aspx

I've used the last of the Mach number calculator and entered my relevant temperature and velocity parameters.

Any comments?
Andrew Hamilton
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Re: Accurately calculating mach from a Raven Altimeter

Postby PK » Fri May 25, 2012 3:50 pm

Not sure how anything on that page is relevant? The pressure changes with altitude are basically linear for the first 15000'. The transonic compressibility compensation calcs are for pitot air speed sensors yes?
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Re: Accurately calculating mach from a Raven Altimeter

Postby drew » Fri May 25, 2012 4:14 pm

Yea, I'm not going to argue with you PK. Given what you've said and the fact that most if not all of our motors have finished their burn before 15000' I'm assuming I should just use tables? I was also under the impression that temperature did have an impact on Mach numbers but to be honest I'd willing to go off your advice.
Andrew Hamilton
AMRS 28 L3
AMRS Records Committee Chairman
Max Alt AGL - 23,908ft - K300 - Balls 22
Max V - 2,488 ft/s, ~Mach 2.2 - M2250 - THUNDA 2015

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Re: Accurately calculating mach from a Raven Altimeter

Postby PK » Fri May 25, 2012 8:02 pm

So, back to basics: What data do you have, and what information are you trying to get from it?
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drew
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Re: Accurately calculating mach from a Raven Altimeter

Postby drew » Mon May 28, 2012 3:41 pm

Sorry for the late response, spent the entire weekend trying to finish my staging project.

Basically I want to definitively calculate a Mach number from a Raven's output. For instance, my flight at Williams last year (Firestorm on a K550) produced the following data.

Parameter Min Max
Average PreLaunch Altitude (ft) 1042.00
Temperature (F) 71.25 71.73
Velocity (Accel-Ft/Sec) 0 1151
[Altitude (Accel-Ft)] 0 11110
[Altitude (Baro-Ft-AGL)] -4 10267
[Altitude (Baro-Ft-ASL)] 1038 11309
[Velocity (Accel-Ft/Sec)] 0 1152
[Velocity (Accel-MPH)] 0 785

I guess my main problem is figuring out the associated altitude when compared with my maximum velocity figures and knowing which equation to apply. I hope that makes sense.
Andrew Hamilton
AMRS 28 L3
AMRS Records Committee Chairman
Max Alt AGL - 23,908ft - K300 - Balls 22
Max V - 2,488 ft/s, ~Mach 2.2 - M2250 - THUNDA 2015

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Re: Accurately calculating mach from a Raven Altimeter

Postby PK » Tue May 29, 2012 7:29 pm

drew wrote:I guess my main problem is figuring out the associated altitude when compared with my maximum velocity figures and knowing which equation to apply. I hope that makes sense.


For this you need to go to the charts. Look at the altitude at Vmax and either calculate the density and hence local speed of sound, or just look it up.

Worth mentioning that we are assuming that there is some value in expressing velocity as a multiple of the speed of sound at the altitude at which the velocity was achieved. This would just be a convention and, unless you were using your data for a transonic aerodynamic analysis of the airframe, no more valid that comparing it to the speed of sound/my new bike/the crap the comes out of Lotterings mouth at ground level...

One can make the argument that the ground level reference is more useful in the normal context of the use of this information. ie "Your rocket sucks! Mine did Mach 3.2 and you only managed 2.9".

When using local mach numbers this statement could be incorrect if, for example, Mach 2.8 was achieved at 20,000' with a 40,000' feet apogee whilst the other rocket hit 3.2 at 2,000' before going just as high....
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Re: Accurately calculating mach from a Raven Altimeter

Postby OverTheTop » Wed May 30, 2012 7:40 am

I'll throw another fly in the ointment here. The temperature logger will have a thermal time constant so measurements will be slightly behind the actual values. :( Given it is a high-speed bird this effect will be even more pronounced.

To get around this problem look at the temperature at that height on the way down. Should be closer to the real value since the descent rate is much slower than ascent.
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Re: Accurately calculating mach from a Raven Altimeter

Postby strud » Mon Jun 04, 2012 8:55 am

If you run a pitot tube &associated pressure sensor and have another way of determining velocity, I believe you will then be able to calculate density and then Mach number.

Does the raven have provision for another analog input ?

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