Concorde/speed of sound
Discussion
Always thought speed of sound was in the 600s (mph), and looking up the figures for Concorde, which could exceed Mach 2, they say 1354 mph, Mach 2.04. So around 677 mph.
And yet, looking up the 'speed of sound', I'm given 761.2 mph, which is considerably more...
Is it they're quoting a different figure for the high altitude to make Concorde seem quicker than it actually was?
Or am I missing something else?
Thanks
And yet, looking up the 'speed of sound', I'm given 761.2 mph, which is considerably more...
Is it they're quoting a different figure for the high altitude to make Concorde seem quicker than it actually was?
Or am I missing something else?
Thanks
robbyd said:
So basically it didn't achieve mach 2 at sea level numbers... where many people might be comparing it with...
Airliners don't tend to do too well at sea level.B1 for example can go low and fast but still its nowhere near mach2
Would be amazing to put one of the ex bucaneer nutters in the concorde and have them push it at 50ft
Do say if this doesn’t make sense, I’ll try to explain it in layman’s terms - but for the OP, we need to know loosely what True Air Speed is first.
Know 2 things to begin;
that calibrated/indicated airspeed (CAS) is the rate at which the aircraft is hitting air molecules (the lower you are, the denser the air, the more molecules you hit, and vv). If the air is really dense, you’ll hit loads of them as you fly through the air at a given speed.
True Airspeed is the speed at which the aircraft travels through a a body of air regardless of air density.
So with that info, you can figure that at a constant CAS, the TAS will be lower at lower altitudes than higher ones.
Mach number is the relationship between True Air Speed (TAS) and the local speed of sound (which is almost entirely affected by temperature). It’s proportional to the square root of the temp. So, at 40,000ft it’s about 100 knots slower (TAS) than on the ground; about 570 vs 660kts.
The important point there is that those speeds are TAS.
On the ground, where TAS roughly equals CAS, then Mach 1 is a huge aerodynamic load on the aircraft (remember CAS is the rate that air molecules hit the aircraft). At 40,000 it may only be 300kts CAS. That’s why loads of jets can fly supersonic at altitude - the airframe only ‘feels’ CAS. So for a M2.0 Concorde, an ‘aerodynamically felt’ speed of M2.0/1300kts low down would tear the wings off it, but it’s actually only 350kts EAS (which is a corrected CAS) up there at 60,000 feet.
The relationship (assuming temp decreases with altitude) is:
For a constant TAS, as altitude increases, CAS decreases and Mach number increases.
Hope this helps/wasn’t too confusing. There’s a few details left out, but I figured it best that way. There’s also a few aerodynamic foibles when aircraft transcend the local speed of sound, but I don’t think that’s relevant to what’s being asked.
Know 2 things to begin;
that calibrated/indicated airspeed (CAS) is the rate at which the aircraft is hitting air molecules (the lower you are, the denser the air, the more molecules you hit, and vv). If the air is really dense, you’ll hit loads of them as you fly through the air at a given speed.
True Airspeed is the speed at which the aircraft travels through a a body of air regardless of air density.
So with that info, you can figure that at a constant CAS, the TAS will be lower at lower altitudes than higher ones.
Mach number is the relationship between True Air Speed (TAS) and the local speed of sound (which is almost entirely affected by temperature). It’s proportional to the square root of the temp. So, at 40,000ft it’s about 100 knots slower (TAS) than on the ground; about 570 vs 660kts.
The important point there is that those speeds are TAS.
On the ground, where TAS roughly equals CAS, then Mach 1 is a huge aerodynamic load on the aircraft (remember CAS is the rate that air molecules hit the aircraft). At 40,000 it may only be 300kts CAS. That’s why loads of jets can fly supersonic at altitude - the airframe only ‘feels’ CAS. So for a M2.0 Concorde, an ‘aerodynamically felt’ speed of M2.0/1300kts low down would tear the wings off it, but it’s actually only 350kts EAS (which is a corrected CAS) up there at 60,000 feet.
The relationship (assuming temp decreases with altitude) is:
For a constant TAS, as altitude increases, CAS decreases and Mach number increases.
Hope this helps/wasn’t too confusing. There’s a few details left out, but I figured it best that way. There’s also a few aerodynamic foibles when aircraft transcend the local speed of sound, but I don’t think that’s relevant to what’s being asked.
robbyd said:
Thsnks - very interesting and I follow it, though a couple more reads would help..
Makes me think about an sr-71 then, doing mach3, but on the edge of space... even slower with regard to sea-level numbers...
Why isn't TAS used as a standard?
Thanks
Fair question! Makes me think about an sr-71 then, doing mach3, but on the edge of space... even slower with regard to sea-level numbers...
Why isn't TAS used as a standard?
Thanks
Firstly - because it’s a direct, easy measurement. The pitot probes measure the pressure at which you’re crashing through the air. TAS requires calculating..
Aerodynamically, IAS/CAS is more relevant (until you get to the upper reaches of the flight envelope); it’s the dynamic pressure (speed) that’ll eventually snap your wings off, or damage control surfaces, allow the flaps or gear to be deployed, stall the wing if you slow too much etc. Your various safety/datum speeds are IAS. (V1/VS/Vmca etc).
TAS is more relevant when it comes to navigation (your ground speed is TAS corrected for wind), and flight higher up. As you may know, if your IAS gets too low, your angle of attack increases and eventually you’ll stall the wing. You’re very interested in low speed margins when you’re low down or in a light aircraft.
High up, you’re very interested in TAS/mach# because you’ll stall by going too fast, too!
Going back to what I said above, if your TAS remains constant, as you climb, your CAS decreases and your Mach# increases.
So, eventually, you’ll get to an altitude where at your TAS, the CAS has decreased all the way down to stall speed, and the Mach# has increased to the limit (which is loosely when the accelerated air over any part of the wing hits M1.0 and high-drag transonic issues occur).
That’s known as ‘coffin corner’, gleefully! You slow down, you stall, you speed up, you stall.
Obviously the useable flight envelope has margins to limit how close you get to that point As an airline pilot, generally higher is better (more efficient) but you’re always monitoring your margins with a thought to turbulence (which can nudge your speed up and down) and how much thrust you have to drag you out of a low-speed occurrence.
Sorry for using IAS/CAS at random. In essence, they’re the same thing. CAS is just IAS corrected for a couple of minor instrument errors.
Edited by Royal Jelly on Friday 10th September 22:28
robbyd said:
Thsnks - very interesting and I follow it, though a couple more reads would help..
Makes me think about an sr-71 then, doing mach3, but on the edge of space... even slower with regard to sea-level numbers...
Why isn't TAS used as a standard?
Thanks
Once you get going proper fast (M2.5+) then the limiting factor becomes aerothermal heating! Basically the air molecules are smashing into the airframe and the resulting stagnation pressure pushes the temperature up. The SR-71 used Titanium to build it's airframe, and used it's own fuel to cool critical parts, and ran with very high system temperatures (esp. hydraulics) which all made it an extremely expensive plane to design, build and maintain.Makes me think about an sr-71 then, doing mach3, but on the edge of space... even slower with regard to sea-level numbers...
Why isn't TAS used as a standard?
Thanks
sorry, temps are in silly units, but those are still some big numbers!
Limiting speed was actually set by the ambient air temperature and by the limiting pre-compressor temperature for the engine compressors, so on "colder days" at altitude, the plane could go faster. Crazily and very counter-intuitively, thanks to the inlet air bypasses turning the engines into scram-jets (where the inlet air is compressed simply by the act of decellerating it, rather than by the engines axial compressor), the engine got more efficient and made more thrust the faster the plane went, this meant that pilots would carefully and continually have to adjust the throttle setting (or the autothrottle setpoint) to avoid the plane accelerating and over temping the engines!
Proper, proper bit of mad engineering!
Royal Jelly said:
monthou said:
Nice pic.
So sound travels at 450mph when air density = zero?
Who knew?
No, but temperature is, by a huge margin, the biggest influence it. Pressure and density are a side-show. So sound travels at 450mph when air density = zero?
Who knew?
But then again, you’re clearly just trying to backpedal from your assertion that the reason for the difference between the speed of sound at high altitude vs at sea level was air density.
It wasn't me.
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