Airspeed question

[EightsOnPylons]

Hello,

Could someone give me an answer on this please.

Question:
If one aircraft is flying M 0.82 at FL 410 and another aircraft is flying M 0.82 at FL 350, which one has the greater TAS (no wind)?

[Johnny Tightlips]

In theory and according to ISA the aircraft at FL 410 would have a TAS of 470 kts and the aircraft at FL 350 would have a TAS of 472 kts. TAS changes by about 1 knot for every 1 degree above or below ISA, your faster if it's warmer and slower if it's colder.

Hope this helps.

[keith williams]

If you wish to avoid looking up tables or carrying out long calculations you could try the following approach.

One method of calculating the TAS at any given mach number is

TAS at any given Mach number (in knots) = Mach number x 38.94 x the square root of the absolute temperature.


If was assume ISA conditions we have

The Tropopause is at approximately 36 000 feet.

Below the Tropopause the temperature decreases at a rate of approximately 2 degrees per 1000 feet altitude increase.

Above the Tropopause the temperature is constant.


FL350 = 35000 feet, which is 1000 feet below the tropopause.
FL410 is above the tropopause.


From the above we should be able to see that the temperature at FL350 is approximately 2 degrees higher than the temperature at FL410.

So the TAS at any given mach number will be greater at FL350 than at FL410.

[EightsOnPylons]

Thanks for replies.

So I have some followup questions:
1. How come the concorde flew around 60 000 ft in that case?
2. In other words it is wrong that 'airspeed increases with altitude'..?

Thanks

[FLEXPWR]

Airspeed does not increase with altitude. Rather the air density decreasing, you will have to increase speed to obtain the same lift (at a given AOA). Therefore your indicated stall speed increases.

If you could put your eyes on a CTM graph (simple graph from ATP theory) referred to as Chicken Tikka Masala, shows how speed and Mach change with altitude compared to one another.

Rest of the question, not familiar with Concorde, but above the tropopause, the temperature moves very little, so no change in the MN for a given TAS, but increasing the altitude would still benefit for fuel saving.

[Dariuszw]

So according to you I should fly next to ground where temperature is highest and TAS "in theory" should also increase.....:ok:
I shell remember that next time I fly my little sexy sport car:E

Ok, lets be serious here.

As altitude increase pressure drops and TAS increases. This is because there is less air to put up resistance against the aircraft moving forward so the aircraft moves faster through the air. Concord could take adventage of this phenomenan all the way to 60.000ft because of special design allowing it to fly above Mach speeds. Most jets fly below Mach speed so they can not continue to fly higher. To check how altitude affect TAS here is a link to E6B calculator where you can input different altitudes and see difference in TAS: E6B Emulator (http://www.csgnetwork.com/e6bcalc.html) Its the last function on that page.

Of course temp also plays a role in this equation but its secondery at best and applies more to Mach definition then to inrease in TAS with altitude.
Any increase in temp or alt will increase TAS but TAS mainly increases due to pressure.

[Johnny Tightlips]

So according to you I should fly next to ground where temperature is highest and TAS "in theory" should also increase.....http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/thumbs.gif
I shell remember that next time I fly my little sexy sport carhttp://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/evil.gif

Ok, lets be serious here.


I was being serious. TAS is higher near the ground for a given Mach number, but of course there is no point flying that low as you are likely to pull your wings off before you would reach any worth while Mach number. If you look at any of the Boeing manuals it says that TAS changes by approximately 1 knot for every 1 degree above or below ISA.

I'm sure your citation manual has this as well......

[Bellerophon]

EightsOnPylons

...How come the concorde flew around 60 000 ft in that case?...

Concorde, when climbing, flew at her (highly variable) VMO, and when I say that, I mean it quite literally, to the knot. She maintained VMO in the climb until she reached her MMO of M2.0 at around FL500.

If, during her supersonic climb, Concorde had levelled-off, at any altitude, she would have been unable to accelerate further as she would already have been at her VMO, for that altitude. So, flying level, at the maximum IAS permitted for that altitude, there were only three ways for Concorde to increase her Mach number.


Firstly, when below the tropopause, climb at a constant IAS and as the OAT fell her Mach number would increase. Most subsonic aircraft do this anyway, climbing at a constant IAS (though not generally VMO!) until reaching their desired Mach number, when they transition to a constant Mach number for the rest of the climb.

Secondly, at or above the tropopause, the OAT would (in theory) remain constant at -56.5°C and would not fall further as she climbed, so this first technique would no longer work. Now, in order to increase her Mach number, Concorde had to climb and utilise the fact that her VMO increased steadily, by 10kts per 1,000 ft above the tropopause, and so by climbing higher she could now gradually increase her IAS, and thus her Mach number.

Thirdly, at around FL440, Concorde’s VMO stopped increasing and stayed at 530 kts IAS for the rest of the cruise/climb. With no further IAS increase possible, and no OAT decrease likely, the only way now to increase her Mach number was for her to climb at 530 kts IAS and utilise the fact that during a climb at fixed IAS, as the ambient air density decreases, TAS, and hence Mach number, will both rise.

The common thread was that Concorde needed to climb in order to accelerate to M2.00.

[Stuck_in_an_ATR]

@Bellerophon

What was the reason for the Concorde to climb that close to Vmo? Sounds a bit tricky to ride the barber's pole at all times! :)

[Bellerophon]

Stuck_in_an_ATR

...What was the reason for the Concorde to climb that close to Vmo?...

Fuel efficiency.

Very broadly, over much (not all) of Concorde's flight envelope, increasing speed meant decreasing total drag hence better fuel efficiency.

Fuel efficiency in Concorde could be summed up in these words:

"Fly as fast as you can, as quickly as you can, for as long as you can."


...Sounds a bit tricky to ride the barber's pole at all times!...

At first, it was! Then, with practice and a better understanding of the flight envelope, it became a lot easier and not as difficult as it might seem.

I would be the first to admit that some could do it better than others! :O

[Stuck_in_an_ATR]

G'day Bellerophon,

Thanks for the answer. I knew that the Concorde was draggy at lower speeds, but never suspected that the Vmo at lower altitudes would still be at the back side of the drag curve...

BTW, I have found the flight envelope graph you have posted in another thread. Could you explain the reason for those Vmo changes with altitude and weight?

[Bellerophon]

Stuck_in_an_ATR

... Could you explain the reason for those Vmo changes with altitude and weight?...

I’m afraid not, as this is getting in to an area well beyond my own level of technical knowledge!

However, there are one or two learned gentlemen on this forum who were heavily involved in the design of Concorde and who may come along to shed some light on the matter.

[CliveL]

OOOPS!http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/embarass.gif

Posted the response to the wrong thread!!

To save repetition please check the Concorde question thread

Sorry for mix up

[A-3TWENTY]

Bellerophon,

What was a tipical fuel consuption per hour for the Concorde?

[Natstrackalpha]

Well, Toots,

If you are using -56.5 or .6 as a constant and then LSS is based on OAT anyway then the answer is that they (the speeds) are both the same. As Mach = LSS = OAT, ok?

Was that your trick question for the day or did you want the OAT to be with a temp dev (lower)(or higher) thereby giving you lower than -56.6 hmmmm? thus affecting your airspeed. Or was this without ISA, like JSA?

[Bellerophon]

A-3TWENTY

...What was a tipical fuel consuption per hour for the Concorde?...

A total fuel load of 91,000 kg, with a flight time of 3h:20m in the air, was a fairly typical flight plan for Concorde on a LHR-JFK flight.

Reserve fuel (minimum fuel required to be in tanks on landing) was 6,500 kgs, and the (hopefully unused) diversion fuel was usually somewhere around 3,600 kgs, so after an uneventful transatlantic flight Concorde would have expected to land at JFK with somewhere around 10,100 kgs fuel still remaining in tanks.

Allowing for the taxy fuel used whilst on the ground, this means around 80,000 kgs of fuel was burned in the air, giving an average consumption of 24,000 kgs/hr.

However, her fuel consumption could vary a lot, depending on the phase of flight.

Consider the fuel used during her earlier re-heated take-off at LHR:


LHR....ISA....MTOW....V2.... 215 kts....20,700 kg/hr/eng....Total....82,800 kg/hr

Start of Roll to Reheats OFF....83 seconds....Fuel Used: 1,909 Kg / 4,209 lbs / 526 g

You can see why the re-heats were turned off as quickly as possible!

[CPTG747]

if you are stating the IAS are same the AC at FL410 ground speed is greater

[CPTG747]

CONCORDE FLEW AT FL60 BECAUSE air is thinner so less air resistance and it had after burner engines, more efficient at higher. It had to fly at lower levels till burned off fuel, weight, then upwards to FL60. Also long trips as they all were WX is better there, fly over most all WX or easy to go around, but mostly due to EFFICIENTLY, SPEED, AIR TRAFFIC, AND FUEL BURN. I am B-747 Capt A-380 Capt, but much time in F15 and 18s same theory there also. Hope this helps you some. You can also google your questions there.

[M-ONGO]

Are you for real Cptg747? Please stop posting in bold. infact please just stop posting and get back to your sim. Maybe you can play with Darius online...

[Air Soul]

FL60 isn't that high.