David,

 

<<<Info copied from Wikipedia>>>

The speed represented by Mach 1 is not a constant; for example, it is dependent on temperature and atmospheric composition. In the stratosphere it remains constant irrespective of altitude even though the air pressure varies with altitude.

 

The Airspeed Indicator (ASI), driven by the Pitot tube, shows what is called Indicated airspeed (IAS). The ASI is calibrated so that IAS corresponds to TAS @ sea level, 15 degrees Celsius and 1013.2 HPa (29.92 InHg) of air pressure - called International Standard Atmosphere or ISA conditions.

When the air around the aircraft differs from said ISA conditions, IAS will no longer correspond to TAS, thus it will no longer reflect aircraft performance. In fact, the ASI will indicate less and less as the air density decreases with altitude and temperature increases.

For this reason, TAS cannot be measured directly. In flight, it can be calculated either by using an E6B flight calculator or its equivalent function on many GPSs. The data required are Outside air temperature (OAT), Pressure altitude and CAS (IAS corrected for installation and instrument errors). Modern aircraft instrumentation use an Air Data Computer to perform this calculation in real time and display the TAS reading directly on the EFIS.

Since temperature variations are of a smaller influence, the ASI error can be roughly estimated as indicating about 2% less than TAS per 1,000ft of altitude above sea level. Thus for a given IAS, the True Airspeed is about 2% higher than IAS per 1,000ft of altitude above sea level. An aircraft flying at 15,000ft with an IAS of 100kt, is actually flying at 130kt TAS, or 130kt through the air.

>>>>>>>

 

Your calculation is not correct since the speed of sound is not a constant.  Use the chart at http://www.tscm.com/mach-as.pdf 

 

Mach is more closely related to TAS than IAS. 

 

I have a Rocky Mountain Instrument's micro-encoder (no longer available) that is also an air-data computer.  Thus, one may display IAS, TAS and Mach because the device measures OAT (corrects for compressibility as TAT) and indicated altitude (after baro correction, also shows Dalt and Palt).  Since modern slick aircraft are all faster than a C172, it would be useful if the designer/manufacturer would describe limits, like Vne, as IAS, TAS and Mach.  After all, modern glass avionics are certainly capable of calculating, displaying and issuing warnings for these values.

 

Remember that one of the wing design parameters is the Reynolds number range (a non-dimensional viscosity measure related to atmospheric composition) in which it is to operate.

 

Altitude, temperature, composition (moisture, etc.) and speed all have an effect on aircraft performance, structural integrity, flutter, yada, yada....

 

Scott Krueger AKA Grayhawk

 

yes,  this whole thing about mach limitting speed is rather new to me also.  i don't really remember being taught about it in initial or recurrent training(though i do subscribe to grayhawk's theory of new info requires old info to leave the brain-lol!).  what i am understanding from this discussion is that vne for lancairs is indicated  airspeed ie 274 kias for a iv.  but the mach speed  for a iv that should not be exceeded due to flutter considerations is 0.57mach ie .57 x 761=433 mph Tas or 382 kTas.  is this correct?  still not quite sure why this mach speed is in tas and not ias.  but if this is true it has definite ramifications for a iv or propet particularly during descent.  despite the few flames, this has been an interesting and practical discussion.

david weinsweig
n750dw propjet
178 hours


>>> bknotts@buckeye-express.com 12/09/2009 8:12 AM >>>
I haven't seen much on Mmo.  In fact, this is the first time I've heard
of a limiting mach number for the IV-P.  My Chilton PFD shows any mach
above .35 at the top of the airspeed tape.  I've seen .44 in cruise at
altitude.  (I giggled like a little girl.)  Now I'm a little concerned
that the "hard" Mmo should be placarded so as not to exceed it in a
cruise power (usual) descent.

Barry Knotts
N4XE, LIV-P, Conti TSIO-550

Frederick Moreno wrote:
>
> 
> When Brent Regan and I were racing his Lancair IV Denver to Oshkosh
> (1996, 97 as I recall), the descent profile was Mach limited initially
> starting at 27,000 feet using a hard limit of Mach 0.58.  This was
> chosen because the factory aircraft was test flown to Mach 0.6 and we
> did not want to enter the unknown.  This Mach number (I had to compute
> it in those early days -- no Mach displays then) was held until the
> IAS built to Vne (274 knots IAS) and this IAS was held to the bottom
> of descent, all occurring at 90-100% power. The  maximum cruise speed
> at 27,000 was about 320 knots TAS (corrected for temperature and
> compressibility effects) and Mach 0.52. As I recall, it only required
> 200-300 feet per minute initial descent at the high power setting to
> drive the Mach number up to 0.58, so the descent started out very
> flat, and the built up as the air got thicker.  I think we hit Vne at
> about 12-14,000 feet and the maximum descent rate near the bottom of
> descent was off scale on the VSI.  It sounded quite different from
> cruise and caused the hair on the back of the neck to stand up a bit
> particularly since much of the descent was in IMC.
>
> 
>
> Key point for you IVP guys: It does not take much nose down high
> altitudes and high power settings to push you up to Mmo, the maximum
> allowable Mach number.
>
> 
>
> Y'all be careful out there.
>
> 
>
> Fred
>
> 
>



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