Meaning of ISA/Concorde

[rjemery]

I am reading Brian Trubshaw's Concorde: The Inside Story (Sutton Publishing, 2000, 0-7509-2393-8). On page 123, Trubshaw writes: After entry into service the availability of an uprated engine showed that the intake was operating super-critically during cruise at ISA +5° with matched ramp angles of 7½ to 8°. In terms of efficiency, this was not good since the intake was operating at maximum capture.

Could someone kindly explain that passage in layman's terms, especially the meaning of the ISA acronym?

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R. J. Emery

[WhoNeedsRunways]

R :

ISA stands for International Standard Atmosphere. It says that on an ISA standard day, the sea level pressure is 1013.2 Millibars / HectoPascals ( for the new boys ), the sea level temperature is 15degrees Celcius, and the air density is 1225 grams per cubic metre. Also, it assumes a temperature drop of 1.98 degrees Celcius per 1000' up to 36000 feet ( or thereabouts ).

Where it comes into play in aviation is mostly in aircraft performance. If the temperature increases, or the air is less dense, or the pressure decreases, all aircraft and their engines tend to perform worse than ISA, and if the temperature drops, pressure increases and density increases the opposite is true. It can be used by manufacturers to provide charts based on an ISA standard day, then appropriate corrections to the one table is all that is needed, rather than lots of tables for lots of different combinations of temp., pressure and density.

Is that a help ?

As to the Concorde stuff, are you there, WOK ?

[rjemery]

Yes, WNR, that is a big help! Thanks for your input. Incidentally, given your username, are you a helicopter pilot?

And who is WOK, may I ask? Someone highly knowledgeable about the Concorde? If so, I hope he would enlighten me on a number of a/c design and flight questions I have.

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R. J. Emery

[WhoNeedsRunways]

Yep. I fly helicopters for fun, and WOK is a Concorde pilot who must be getting sick of going to Filton to do sims so often . . .

And I'm just waiting for someone to correct any mistakes I've put into my reply to test other people's knowledge . . .

[This message has been edited by WhoNeedsRunways (edited 03 November 2000).]

[WOK]

It's nice to feel wanted!

The operation of the ramps and other systems is well explained in the many books about the SSC so I shall only go into briefest detail here -

The successful design of the ramp control system was the absolute key to Concs success over competing designs.

The ramps' job is to decelerate the airflow so it is subsonic as it reaches the engine. They do this by moving up and down, focussing a succession of shockwaves onto the lower lip of the intake. The passage of the free stream through these shockwaves decelerates it and greatly increases its pressure.

The position of the ramps needs to be adjusted whenever the engine demand changes, the speed changes, the alpha changes or the temp changes. They are controlled normally by dual controllers - the first digital full authority computers ever used in a commercial a/c.

If the shockwaves are focussed aft of the lip the condition is known as 'supercritical'. Efficiency will be reduced in this state (as by a 'subcritical' condition)

I've not read the book but your quote suggests that the ramps did not have enough authority to deal with the upgraded engines at ISA>+5. A problem long since sorted.

Because the engines run, basically, to an RPM in the crz and because, of course, the overriding airframe concerns are mach no. and temperature, the conc is very sensitive to OAT - negative ISA = good, positive ISA bad.

Helpful?

[WhoNeedsRunways]

WOK :

Just trying to earn brownie points with you :)

[WOK]

Maximum brownie points earned by being a 6ft tall, 36-24-36, blonde, female P51 owner set to inherit a brewery.

Got to start somewhere, though. :)