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Speed of Sound

The speed of sound varies with air temperature and is not constant, with colder air resulting in a slower speed. At different altitudes, the speed of sound decreases, making it easier for jets to reach Mach 1 at higher altitudes. Pilots must be aware of Mach numbers as they indicate the aircraft's speed relative to the speed of sound, which affects performance and flow characteristics.

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12 views3 pages

Speed of Sound

The speed of sound varies with air temperature and is not constant, with colder air resulting in a slower speed. At different altitudes, the speed of sound decreases, making it easier for jets to reach Mach 1 at higher altitudes. Pilots must be aware of Mach numbers as they indicate the aircraft's speed relative to the speed of sound, which affects performance and flow characteristics.

Uploaded by

matthewfretwell13
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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What is the Speed of Sound?

●​ Speed of sound = how fast pressure disturbances (sound waves) move through air
●​ Denoted as aa or sometimes cc
●​ It’s not constant — depends on air temperature, not pressure or density!

a=γRTa=γRT​

Where:

●​ aa = speed of sound (m/s)


●​ γγ = ratio of specific heats for air (~1.4)
●​ RR = specific gas constant (287 J/kg·K)
●​ TT = absolute temp (K)

🧠 Key Idea: Colder air = slower speed of sound

🌡️ Speed of Sound at Different Altitudes


Altitude Temp (ISA) Speed of Sound

Sea level 15°C ~340 m/s

10,000 ft ~-5°C ~325 m/s

36,000 ft (FL360) -56.5°C ~295 m/s

✈️ That's why jets hit Mach 1 easier at high altitudes — the speed of sound is lower!

⚠️ Mach Number
Mach=VaMach=aV​

Where:

●​ VV = True Airspeed
●​ aa = speed of sound at current conditions

✈️ Common speed ranges:


Flight Regime Mach Range

Subsonic M < 0.8

Transonic 0.8 < M < 1.2

Supersonic 1.2 < M < 5

Hypersonic M>5

🚧 Compressibility Effects
●​ At high subsonic speeds (M > ~0.3), air no longer behaves like an incompressible fluid
●​ Shock waves start forming around M ~0.8–1.2
●​ Leads to:
○​ Drag divergence (sudden spike in drag)
○​ Flow separation
○​ Control surface effectiveness changes

✈️ Critical Mach Number (McritMcrit​)


●​ First point where airflow over any part of the aircraft reaches Mach 1
●​ Typically M = 0.72–0.78 for standard airfoils
●​ Past this: local shock forms → compressibility problems begin

✏️ Designers delay McritMcrit​with swept wings, thinner airfoils, etc.

🧮 Drag Divergence Mach Number (DDMN)


●​ Mach number where drag rise becomes rapid due to shockwave formation
●​ Marked on drag curve as sharp upturn
●​ Aircraft must be designed to fly below or efficiently through this point

🧠 Why Pilots Care About Mach


●​ Pilots use Mach indicators at high altitudes
●​ TAS ↑ with altitude → but speed of sound ↓
●​ So an aircraft can enter compressible regimes even if IAS is relatively low!

✈️ Example:
●​ At FL350, M 0.82 ≈ ~470 KTAS but might only be ~270 KIAS

✍ Summary Points
✅ Speed of sound depends on temperature, not pressure​
✅ Mach = aircraft speed relative to sound​
✅ Compressibility begins affecting flow ~Mach 0.3

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