Mach, TAS & Weight
 

Hello,

Looking though the cruise section of my performance manual, I found something a little unusual.

According to this data, while cruising at a constant mach, my TAS increases with weight. Either I'm forgetting something very basic here, or I'm missing something fairly important.

How does weight affect the relationship between TAS and Mach no.?



Edit: For some reason, I can't upload an image. My manual shows that at FL 370 & M.76, TAS is 437kt at 50,000lb and 432kt at 30,000lb

Just tossing this out there but a change of 5 KTAS with a 40% change in weight seems like it might be attributable to uncorrected pitot/static installation error. At 50.000 lbs the AOA would be significantly higher than at 30,000 lbs at the same indicated mach number at a constant altitude and temperature, So maybe any uncorrected installation error could result in a small inaccuracy in IAS and therefore indicated mach number and computed TAS. If the indicated mach number and the computed TAS are both reliant upon the IAS value and the IAS accuracy varies somewhat with AOA, then the different AOA associated with the different weight may account for the difference in computed TAS at various weights. Just a thought...

westhawk

TAS is TAS. No position error, no instrument error, no compressibility error...

But wait, maybe mach number is indicated mach number. That could explain that for a given indicated mach number, true mach number is different, and so is TAS

just a hypothesis. Here in pPrune there are real experts in all this issue of speeds, pitot systems, errors, etc... They will be here soon! Let's wait

At fl 370 the speed of sound equals 573.57 ... So if you round up your Mach number from .755 (Tas 433) to .764 (Tas 438) there is a 5 kts difference but still flying Mach .76 :) ... If you use 573.0 as a reference speedd of sound your Tas will be 432.6 and 437.8 respectively ... Maybe the target during the flight tests was an accurate n1/fuel flow / SR and the Mach number the resulting value of this target :) With a heavy airplane you should(most probably) fly .764 to have a stable airpeed and with a light .755 in order to achieve the best range
(always flying .76).Always talking about a constant Mach number table .. Right ???

Respect. That makes sense.

Why aren't you reading the paper though?