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#1
09-01-2007, 06:08
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Speed of Sound
It seems that quite a number of BM's are very knowledgeable about Aircraft. I have one for you.
On a recent trip to Bangkok with EVA the flight time from Heathrow was 9 hours 20 mins. (From stand to stand). The distance is 6000 miles. Allowing for the take off and landing and a bit of circling I reckoned that we were in the air at speed for about 9 hours. An average speed of about 660 MPH. What is the speed of sound at 10,000 metres. At Sea level it is about 769 MPH... at -71C and 10,000 metres.. I always thought it was about 660 MPH... lol. So can a 747 really fly this close to the speed of sound.. (surely not). Were there some other factors which allowed us to travel so quickly, eg tail wind. Just interested Mac Last edited by Mac : 09-01-2007 at 06:20. 
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#2
09-01-2007, 06:22
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i think only PABLO can answer this.
Most of the time a 747 mach speed at cruise level is about 0.80 @ 37.000 ft i believe
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#3
09-01-2007, 06:25
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Altitude in it self means very little, it is the temperature that matters.
At -71°C the speed of shound is approximately 638 MPH, where at 20°C it is 768 MPH.
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#4
09-01-2007, 06:39
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Quote:
Cant recall formulae just at the moment, but it would be higher than 660mph, probably up around 670mph, using the parameters you provided, but there are numerous external factors which can affect this. The speed of sound is very much dependent on the relative air density, and the air density is dependent on the air temperature. One thing which confuses passengers, is on those airmap displays the speed of the aircraft displayed is the ground speed, and not the speed of the aircraft through the air. So dont be confused in thinking that you are travelling at or even close to the sound barrier, it ain't going to happen in a 747. Cruise speed is measured in Mach numbers, with the normal cruise of a 747 in the 0.87 range, with Mach 1.0 being the big bang. Obviously a good tailwind will push you along quite nicely. Luckily no one has to worry about calculating any of this stuff, its all done by computers. Quote:
Technically YES, operationally NO. There was one such incident which comes to mind, a China Airlines 747SP went supersonic over the Pacific when they somehow managed to loose it, luckily they managed to recover.
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#5
09-01-2007, 06:45
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Only a steep dive could push you through mach 1 in a 747 !
but i dont want it to happen on my flight out to l.o.s.! 
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#6
09-01-2007, 06:48
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Stevem, here's the link to that China Airlines incident.
Airliners.net Photos: China Airlines Boeing 747SP-09
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#7
09-01-2007, 07:09
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Quote:
The effect of the wind is massive, at that altitude you can easily get 80 mph winds and more.
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#8
09-01-2007, 07:27
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And just to confuse things a little more ... EVA don't operate with 747s on the London-BKK route, they use 777s. Two engines (but BIG f@&£ers!) and no 'hump'. They park outside my office window every evening at Heathrow.
I find it hard to believe that an EVA flight did London-BKK in 9h20 when it is normally 11h20. They must have taken some serious short-cuts while en-route. It normally departs at about 21.30 (London time) and should arrive in BKK at 15.50 ... are you telling me that it arrived at about 13.50 (BKK time)? FWIW, when I came to LOS in December last I flew with Thai A/W. At Heathrow, the EVA was on the next stand to us. It 'pushed' first, and took-off immediately before us. When we landed (in out Thai B747) at BKK the very next plane to land was the EVA from London. I suspect that the Thai flight was given priority when approaching BKK, but it was still nice to see that we arrived first. Is there anywhere which would document the flight and how long (short?) it took.
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#9
09-01-2007, 07:33
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Quote:
Speed of sound in air and the temperature calculator. No air pressure table density of air calculation acoustic impedance air density - sengpielaudio
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#10
09-01-2007, 07:40
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Selective C&P works wonders. Reread my post, thats basically what I have said.
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#11
09-01-2007, 08:22
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Yeah it's basically the same thing because of the relationship between the different units. It's really poor article IMO, following the same logic you could equally say that temperature has no effect on the speed of sound because really it's determined by the kinetic energy of the air molecules. Whatever way you choose to express what's going on it's all derived from the same fundamentals...
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#12
09-01-2007, 08:23
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Just to add more confusion. I worked on B727s for a while. They are the fastest passenger aircraft, but really fuel hungry. Later Boeings were designed for more fuel efficiency v speed. I think the 727 would cruise at MAch 0.89. This is all from memory, which is getting jaded, so don't use this to solve a betting argument....
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#14
09-01-2007, 16:13
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Faster the better, 12hr flight with no smoking... id be farting out the window and flapping me arms if i thought it'd shave a few mins off
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#15
09-01-2007, 17:45
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Aircraft speeds we have TAS thats true air speed, and then we have GS which is ground speed, only GS will change depending on wind, with a strong tailwind you will get a high ground speed, and the opposite with a head wind.
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#16
10-01-2007, 00:01
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I thought concorde was the fastest .
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#17
10-01-2007, 00:18
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Speed of Sound
The fastest passenger aircraft ever built was the slightly faster Tupolev Tu-144, a Russian aircraft very similar to the Concorde. However, the Tu-144 was larger and fitted with more powerful engines giving it a maximum speed of about Mach 2.35
Tu-144 suffered many technical and safety problems and was removed from passenger travel after only a few months. It has since been used as a high-speed research aircraft by both Russia and the United States. the seconf fastest passenger aircraft is the Concorde. Its maximum speed is quoted as Mach 2.23, but in actual use it is limited to slightly over Mach 2. Now that both of those planes have been removed from service, there is no obvious record holder for fastest plane in commercial service. All current passenger aircraft cruise between Mach 0.8 and 0.85 with a maximum speed typically between Mach 0.85 and Mach 0.9. Nevertheless, I that the current record holder, even if by the slimmest of margins, is the Boeing 747. We have previously discussed an aerodynamic principle called the area rule that describes how to minimize drag on an aircraft flying above Mach 0.8. In simple terms, the area rule states that the shape of the fuselage should be changed from the traditional tubular shape to more of an hourglass shape when the aircraft cruises in the vicinity of Mach 1. When both the fuselage and wing are added together, this layout creates a smooth distribution of the aircraft's cross-sectional area from the nose to the tail. This smooth distribution of area minimizes the likelihood of forming shock waves over the surface of the vehicle, which minimizes drag. Now take a look at the layout of the 747 and note the large bulbous region located atop the fuselage forward of the wing. This bulge contains the cockpit and upper passenger deck. This distinctive "hump" has the effect of giving the 747 fuselage a shape closer to the hourglass contour described earlier. As a result, the 747 experiences lower drag than a comparable airliner that lacks the bulged fuselage. The aircraft can therefore travel slightly faster than its competitors for the same amount of fuel. If we compare the maximum speeds of airliners, we find that the 747 is quoted as being capable of Mach 0.885 whereas most other airliners can go no faster than Mach 0.87. While the advantage is a small one, it does appear to give the Boeing 747 the distinction of being the fastest commercial airliner in service today. Ref:Aerospaceweb.org | Ask Us - Fastest Airliner and Area Rule
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Quote:
Last edited by Petter : 10-01-2007 at 00:24.
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#18
10-01-2007, 12:18
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do you think this is a factor?
i live in Vancouver BC, which is basically above california now it only takes about 4hrs40min from nyc but takes 6 hrs to LAX from nyc someone told me because vancouver is higher up opn the hemisphere, hence, in reality, a shorter distance seemed to make sense
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#19
10-01-2007, 12:46
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The two distances are about the same and there may be additional factors involved beside the once I would factor. (BTW I have a lot of experience in sound and acoustics, however, my knowledge in this department is only because I fly a lot and gain information as I go). The jet stream which travels mostly in an eastern direction allows for shorter flights in that direction. The jet stream is a lot stronger in Canada and the northern planes of the US, than further south. I would think that is no 1 reason. Next, I assume that the travel times you are giving are gate to gate. The airlines factor in different ground times for different airports, based on typical delays. If it on average takes 15 minutes longer in LAX from gate to wheels up than YVR then that help will account for a longer travel time. In air traffic may be a factor also, but not sure. lastly are the travel time based on the same aircraft type?
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#20
10-01-2007, 12:49
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Simple distances surely..
Whenever you look at a map, to smooth the globe onto a 2d piece of paper the northern and southern lattitudes get compressed.. Creates very different distances to what you would expect.. The alcan highway is 1000's of miles.. Yet when you look on a map you think, hey no sweat.. I drove that puppy for days and days !! Also bear in mind when flying you always use the 'great circle' routes due to earths curves also..
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#21
10-01-2007, 12:53
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LAX - JFK: Nautical Miles = 2145; Statute Miles = 2437 Nearly the same. But you are right a 2D map is hard to judge.
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Last edited by MrDK : 11-01-2007 at 01:09. Reason: Incorrect airport code
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#22
10-01-2007, 20:57
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You can check the stand to stand times for Eva Air on their website
If we know the day and time he travelled, it will be easy enough to check G
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#23
11-01-2007, 01:04
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An example of the wind factor, I always fly Thai Copenhagen Bangkok Copenhagen, the outbound sector to Bangkok last 10 hours and the return flight 11 1/2 hours, and that's due to the prevaling winds at the levels they
normally fly between 30 000 and 40 000 feet. They can't fly at lower levels due to the higher fuel consumption in the more dense air.
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#24
11-01-2007, 11:06
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