Speed of sound with altitude

[Chris71]

Am I right in thinking the sonic velocity is lower at low altitude (for the same temperature, humididty etc) and this is why planes are typically quicker at high altitude?

Coicidentally I ask this because I've just been talking to a chap who's forthcoming car is faster than his day (previous) daily transport, which happened to be a Typhoon. I'm just recapping the physics.

[tank slapper]

The speed of sound is slower at higher altitudes, but the reason aircraft are generally faster at altitude is because the density of air is a lot less than it is at sea level.

[MarkK]

Chris71 said:

Am I right in thinking the sonic velocity is lower at low altitude (for the same temperature, humididty etc) and this is why planes are typically quicker at high altitude?

Coicidentally I ask this because I've just been talking to a chap who's forthcoming car is faster than his day (previous) daily transport, which happened to be a Typhoon. I'm just recapping the physics.

Speed of sound gets lower with increasing altitude. Planes are not neccesarily faster at altitude but their Mach number is higher for a given airspeed as Mach is a ratio of airspeed to speed of sound.

[Chris71]

tank slapper said:

The speed of sound is slower at higher altitudes, but the reason aircraft are generally faster at altitude is because the density of air is a lot less than it is at sea level.

Thanks. I had a notion that the drag characteristics changed at supersonic speeds and hence the ground speed was higher for a given mach number.

It's all a bit complicated - I think I'll leave it at 'lower air is denser' to avoid embarassing myself any further.

[dr_gn]

Chris71 said:

I've just been talking to a chap who's forthcoming car is faster than his day (previous) daily transport, which happened to be a Typhoon.

Irrespective of whether he's driving at sea level or at 65,000ft, that must be one hell of a car...

[tank slapper]

Chris71 said:

Thanks. I had a notion that the drag characteristics changed at supersonic speeds and hence the ground speed was higher for a given mach number.

Supersonic aerodynamics are complicated and beyond my rudimentary knowledge, but the Mach number is just the speed of sound made relative - ie Mach 1.5 is always 1.5 times the speed of sound, regardless of the actual speed of sound. The reason for using it, is that the speed of the aircraft relative to the speed of sound, is much more relevant to how it behaves when approaching the speed of sound than at slower speeds.

[Simpo Two]

Chris71 said:

Coicidentally I ask this because I've just been talking to a chap who's forthcoming car is faster than his day (previous) daily transport, which happened to be a Typhoon. I'm just recapping the physics.

He has a car that's faster than a Eurofighter?

Coincidentally the Thrust SSC team are building a new land speed record car using a Typhoon engine - is that the one?!!

[Sam_68]

dr_gn said:

Chris71 said:

I've just been talking to a chap who's forthcoming car is faster than his day (previous) daily transport, which happened to be a Typhoon.

Irrespective of whether he's driving at sea level or at 65,000ft, that must be one hell of a car...

I suspect that it doesn't corner very well, though.

IIRC the speed of sound drops from about 760mph at sea level to 660mph at 36,000ft.

[Chris71]

Simpo Two said:

Chris71 said:

Coicidentally I ask this because I've just been talking to a chap who's forthcoming car is faster than his day (previous) daily transport, which happened to be a Typhoon. I'm just recapping the physics.

He has a car that's faster than a Eurofighter?

Coincidentally the Thrust SSC team are building a new land speed record car using a Typhoon engine - is that the one?!!

The stats its bringing up are just spectacular - 50,000 radial g at the tip of the wheels, for example; 1,200 kg/metre squared peak aero forces on the bodywork; 10 miles (including acceleration and braking stages) in 100 seconds. Oh, and 130,000bhp. That's even more than TVR claimed for its later creations.

[dr_gn]

Sam_68 said:

dr_gn said:

Chris71 said:

I've just been talking to a chap who's forthcoming car is faster than his day (previous) daily transport, which happened to be a Typhoon.

Irrespective of whether he's driving at sea level or at 65,000ft, that must be one hell of a car...

I suspect that it doesn't corner very well, though.

IIRC the speed of sound drops from about 760mph at sea level to 660mph at 36,000ft.

And it's presumably 0 mph in space.

[Papoo]

The speed of sound varies only with temperature. Therefore, in an isothermal layer, the your M number at a given speed will not change. It is a largely irrelevant measurement in any regard other than to allow aviators to know when they are approaching an aircrafts critical mach number (the speed at which the airflow over some part of the airframe becomes supersonic, or indeed to alert aviators of their relationship with M1 - in the trans-sonic range, aerodynamics can become a little exciting.

[Sam_68]

dr_gn said:

And it's presumably 0 mph in space.

That's why in space, no one can hear you scream

Actually, since space isn't a complete vaccuum, I assume that if you screamed really, really loud, people would hear you... very slowly?

[speedtwelve]

Aeroplanes have a maximum, limiting Indicated Airspeed (IAS) which they can achieve. This is irrespective of altitude. Most Boeing airliners are around 350 kts IIRC. Now 350 kts doesn't sound very fast, and at low-level if the power levers were left all the way forward our Boeing would quite quickly exceed its design max speed, although it would only be doing Mach 0.6 or something. At higher altitudes for a given actual 'speed' (TAS, True Airspeed), IAS decreases and Mach No. increases. So the 350 kts IAS becomes less limiting, as at 37000' the jet may have an IAS of 180 kts, a TAS of 580 kts, and a Mach no. of M0.88. At altitude, lower air density = less drag, more speed and higher Mach No. for a given Indicated Airspeed. Jet engines are also much more efficient at high altitude and airspeed.

A lot of 'supersonic' military aeroplanes struggle to go faster than M1.0 at low-level, especially if they are carrying external stores. The World outright low-level airspeed record was held for years by a Tornado F2 on flight-test out of Warton, which achieved 800kts (920 mph) with no pylons and a very 'clean' airframe.

All the M2.0+ fighter aeroplanes need to be up in the stratosphere to reach those Mach nos. None of them will go remotely as fast at low level. It's reckoned that at the end of their service life, some of the RAF's Phantoms wouldn't go supersonic at low-level such was the state of the airframes after 20+ years of dings and speed tape!

[Le TVR]

Papoo said:

The speed of sound varies only with temperature.

It is proportional to the square root of absolute temperature. Altitude temperature is not a linear function so what happens to 'local mach' is the following:

Troposphere - local mach drops from 340 m/s to 295 m/s at the tropopause.
Tropopause - local mach remains constant at 295 m/s.
Stratosphere - local mach starts to rise again from 295 up to 330 m/s at the stratopause.
Stratopause - local mach remains constant at 330 m/s. It starts to drop again in the Mesosphere.