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How Is The Sound Barrier Measured?  
User currently offlineN659AA From United States of America, joined Aug 2000, 130 posts, RR: 0
Posted (11 years 2 months 15 hours ago) and read 1341 times:

When an aircraft breaks the sound barrier, how is that determined?

True Airspeed, Equivalent Airspeed or Groundspeed? If it's none of the following, could someone tell me the correct barometer? Thanks.




7 replies: All unread, jump to last
 
User currently offlineBuff From Australia, joined Mar 2007, 0 posts, RR: 2
Reply 1, posted (11 years 2 months 14 hours ago) and read 1299 times:

The pilot determines the aircraft's relation to the speed of sound using the Mach Meter, a version of an airspeed indicator.

There are mathematical ways to determine what the speed of sound is for any given air temperature (air temperature is the principle variable in the speed of sound in the atmosphere) in miles per hour or other constant, but the pilot does not need to know that.

Hope that answers your question,

Best Regards,

Buff

User currently offlineN659AA From United States of America, joined Aug 2000, 130 posts, RR: 0
Reply 2, posted (11 years 2 months 7 hours ago) and read 1275 times:

Let me ask the question in a different way.

Assumer temp and pressure are constant as well as your power setting.

You are traveling Mach .99 (Indicated) in no-wind conditions. Now you add 50 knots of tailwind.

Question #1

Does your indicated airspeed change?

Question #2

If the answer to question #1 is no, then the aircraft hasn't broken the sound barrier on an IAS basis. What about True Airspeed? Haven't you broken the sound barrier on a TAS basis?



User currently offlineBuff From Australia, joined Mar 2007, 0 posts, RR: 2
Reply 3, posted (11 years 2 months 3 hours ago) and read 1268 times:

#1: No
#2: No

Neither true airspeed nor indicated airspeed have anything to do with wind.

True/indicated/calibrated/equivalent/Mach AIRspeeds have everything to do with aircraft performance.

Additionally, atmospheric pressure has no bearing on the local speed of sound in the atmosphere.

Best Regards,

Buff

User currently offlineXFSUgimpLB41X From United States of America, joined Aug 2000, 3695 posts, RR: 36
Reply 4, posted (11 years 1 month 4 weeks 1 day 22 hours ago) and read 1257 times:

Pilots isolate a singing sound wavelength compression traveling near them in the air.... give it the finger, and see how fast it can go when they race.


Chicks dig winglets.
User currently offlineSailorOrion From Germany, joined Feb 2001, 2058 posts, RR: 7
Reply 5, posted (11 years 1 month 4 weeks 1 day 19 hours ago) and read 1250 times:

The speed of sound can exactly be determined by that formula:
c=sqrt(kappa*R*T/M)

where sqrt means sqaure root
kappa is an thermodynamic property (adiabatic exponent?) of any matter (it's quite exactly 1.4 for air)

R is the universal gas constant 8,314 [J/(mol*K)]

T is the temperature (in K of course)

M is the molar mass (?) of the gas (about 29 g/mol for air).

If you reach this speed (TAS, not IAS or CAS) you break through the sound barrier

User currently offlineBuff From Australia, joined Mar 2007, 0 posts, RR: 2
Reply 6, posted (11 years 1 month 4 weeks 1 day 14 hours ago) and read 1241 times:

IAS is just a visual representation of TAS. It varies with altitude and angle of attack. As SailorOrion posts, TAS is in fact what matters; IAS/MachMeter indication is how one know's one has reached that speed, short of having a TAS display.

Additionally, subsonic aircraft are limited to a critical mach limiting speed (The B757-200 M0.86 or 0.84 depending on certification). Above this limiting speed, airflow over some part of the airframe may exceed M1.0 due to curvatures built into the airframe.

Best Regards,

Buff

User currently offlineN659AA From United States of America, joined Aug 2000, 130 posts, RR: 0
Reply 7, posted (11 years 1 month 4 weeks 1 day 9 hours ago) and read 1226 times:

My thanks to SailorOrion and Buff for your insight. TAS is the answer I was looking for.

N659AA
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