Weight of air in?

[Loose rivets]

The things I never knew while operating crew

Board as blazes, I picked up an AA brochure from the seat pocket, and was amused to note that one of the new generation of engines breaths in a ton of air a second - (at 350 mph)

I find it very difficult to believe that this a literal statement.

While we're (hopefully) on the subject, I'm upgrading my home in the US to a four or five ton A/C, does anyone know what this actually means?

Cheers LR

[Genghis the Engineer]

Just a quick back-of-envelope sum. Let's say we are doing 350 mph as you say, and has an intake diameter of 3 metres.

A 3 metre diameter implies a cross-section of about 7m². 350 mph equates to 156 m/s.

So, 7 x 156 = 1095 m^3/s of air.


Standard sea-level air density is 1.225kg/m^3.

So, 1095 x 1.225 = 1342kg.

Which is 1 1/3 metric tonnes per second.


So, by back of envelope guesstimation, a tonne a second seems about right.



With regard to your AC, does it imply the mass of air that it can keep "conditioned"?. 4 tonnes would be 3,265 m^3, or about 35,000 cubic feet. That would equate to an air mass of something around 44ft x 44ft x 18ft - which I'd guess is probably the size of the air mass in a typical modern american house.

G

[Thunderball 2]

...fascinating stuff, but that's at sea level of course. Go to 35,000' and the air density drops to less than 1/100th of the sea level value. So even at 600 mph, according to the back of my beer mat, the mass flow rate only equates to about 15 kgs per second.

This seems incredibly low - how can you impart enough energy to 15 kgs of air to get it to produce thousands of kilos of thrust? (albeit that altitude thrust ratings are obviously massively less than at sea level).

While we're off the subject, how much mass-flow is (was) going through the J58 of an SR71 at 80,000' every second, even if it is bowling along at Mach 3? A few ounces, you'd think.

(Also, if you're burning (say) a kilo per second in the big fan, isn't 1 kilo of fuel to 15 kilos of air a bit rich?!).

[kansasw]

A ton (yankee spelling) of air conditioning capacity is the amount of refrigeration it takes to turn a ton of 32F (yankee measurement) water into a ton of 32F ice in 24 hours, IIRC.

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Go ahead, ask the next question. Or google Latent Heat of Solidification for the answer.

[Genghis the Engineer]

Sorry Thunderball, but at 35,000 ft the air density drops to about 35% of it's sea level value, which would bring my crude back of envelope sum down to about half a tonne per second - still in the same ballpark.

G

[FakePilot]

But at 35,000 feet your TAS would be in the neighborhood of 450-500 mph, thus correcting Khan-san's estimate? I realize the original poster said 350 mph, but perhaps this was groundspeed?

[Loose rivets]

Fascinating stuff...I have obviously become one of those pilots that has forgotten more than most people know - ( see amusing link in Rumors and news. how many hours..... -) ..still, I think the brochure mentioned 15,000 feet, and the speed was in MPH for the sensibilities of the average punter. So, a nice compromise.

As for A/C, well having single handedly torn the replacement unit out from a house that was being demolished, (dodging bricks and a big pointy thing that was smashing through the walls,) there was little technical data available, except that it was a certain ratio bigger that the gasping unit in my Texas home. A recent 95F and 95% rh prompted me to get on with it.

Having not been able to find the answer, of course as soon as I posted, I found the tons as an equivalent of BTH...so, if I can find my old Aristo, I'll equate the figure to a cube of cold water.

Thanks all. LR

[Genghis the Engineer]

TAS factors inversely with the square root of relative air density. So, if we're at the altitude giving 35% (or 0.35) the inverse square root of that is about 1.7.

So, back to my quesstimate of half a tonne per second, the correction Fake Pilot quite correctly makes increases this to roughly 3/4 of a tonne per second. (And 350 mph becomes just udner 600 mph TAS).

Add in the suction from the are outside the direct tube that my crude estimate used - and you're going to be back around the tonne.

G

[Thunderball 2]

Genghis,

"Sorry Thunderball, but at 35,000 ft the air density drops to about 35% of it's sea level value, which would bring my crude back of envelope sum down to about half a tonne per second - still in the same ballpark".

You're right - must have been a long day yesterday. I read metres as feet;

Height Temperature Pressure Density
(m) (C) (hPa) (kg/m3)
0000 15.0 1013 1.2
1000 8.5 900 1.1
2000 2.0 800 1.0
3000 -4.5 700 0.91
4000 -11.0 620 0.82
5000 -17.5 540 0.74
6000 -24.0 470 0.66
7000 -30.5 410 0.59
8000 -37.0 360 0.53
9000 -43.5 310 0.47
10000 -50.0 260 0.41
11000 -56.5 230 0.36
12000 -56.5 190 0.31
13000 -56.5 170 0.27
14000 -56.5 140 0.23
15000 -56.5 120 0.19
16000 -56.5 100 0.17
17000 -56.5 90 0.14
18000 -56.5 75 0.12
19000 -56.5 65 0.10
20000 -56.5 55 0.088
21000 -55.5 47 0.075
22000 -54.5 40 0.064
23000 -53.5 34 0.054
24000 -52.5 29 0.046
25000 -51.5 25 0.039
26000 -50.5 22 0.034
27000 -49.5 18 0.029
28000 -48.5 16 0.025
29000 -47.5 14 0.021
30000 -46.5 12 0.018
31000 -45.5 10 0.015
32000 -44.5 8.7 0.013
33000 -41.7 7.5 0.011
34000 -38.9 6.5 0.0096
35000 -36.1 5.6 0.0082

...but all credit to the designers of the SR71's J58.

[SR71]

While we're off the subject, how much mass-flow is (was) going through the J58 of an SR71 at 80,000' every second, even if it is bowling along at Mach 3? A few ounces, you'd think.

What I find amazing is that, depending on which source you read, the amount of thrust being generated by the inlet at M=3.2 was between 54-80% of the total.

Similarly at this speed, the length of the engine had grown 6 inches and its diameter by 2.5.

During a ground power run at max AB, the exhaust sections reached 3200F and a football pitch length behind the a/c, the exhaust velocities were still 311F and the air was moving at 150kts.

:ok:

[Thunderball 2]

"...at this speed, the length of the engine had grown 6 inches and its diameter by 2.5."

Who needs Viagra? Just go hypersonic!


"What I find amazing is that, depending on which source you read, the amount of thrust being generated by the inlet at M=3.2 was between 54-80% of the total".

Shades of the "do winglets develop thrust" debate? But I guess what we're saying here is that a massive proportion of the compression actually took place ahead of the face of the first compressor, within the external shock system around the shock cone.

[Bizarrely, J58 was a remarkably quiet intake a low speeds, I seem to remember - so much so that NASA spent money trying to understand why some years ago].

[SR71]

Shades of the "do winglets develop thrust" debate? But I guess what we're saying here is that a massive proportion of the compression actually took place ahead of the face of the first compressor, within the external shock system around the shock cone.

I guess its the three dimensional equivalent of the area velocity relation in quasi one dimensional flow:

dA/A=(M^2-1)*(du/u)

Integrate the pressure over the geometry of a convergent divergent intake and you should obtain the result.

Aerodynamics....full of surprises!

:ok:

[Specnut727]

Engine Air Flow

Another way to approach this may be to go back to the engine manufacturers' specifications. I don't have anything at my fingertips, but I recall seeing mass flow listed in engine specs.

Refrigeration

Going back to basics,

I Ton of Refrigeration = 12,000 Btu/hr (is 'British Thermal Unit' a yankee measurement ?)

1 Btu is the heat required to raise the temperature of 1 pound of water by 1 degree fahrenheit.

[Thunderball 2]

If you go back 20 years or so to the days when the "Flight" directories were fairly serious works of reference, you'll find exactly what you describe. The mass flow rates at maximum sea level thrust range from about 100 kgs per second for the Spey up to 744 kgs/second for the JT9D-7Q. But in the cruise the -7Q, which generates 53,000 lbs on take-off, drops to 8,500 lbs of thrust, and the mass flow rate drops to 285 kgs/sec - very much in line with the calculation by Genghis.

[Is there still a market for a magazine today like "The Aeroplane" was 40 years ago? A subject for another thread on another day, or maybe no thread at all, but the volume and weight of really serious data and analysis carried week after week was formidable. I bought a dozen copies from the mid fifties a few weeks ago - the adverts were more informative of aviation principles than a year's copies of "Flight" (area rule, semi-active radar homing and the principle of the free turboshaft all being explained in the ads of one issue alone)].

[Thunderball 2]

SR71
------------------------------------------------------------------------------------
"I guess its the three dimensional equivalent of the area velocity relation in quasi one dimensional flow:

dA/A=(M^2-1)*(du/u)

Integrate the pressure over the geometry of a convergent divergent intake and you should obtain the result".
------------------------------------------------------------------------------------

I must confess that I haven't got a clue what you're talking about, but that's my problem, not yours.

However, with the greatest respect, I don't think there's anything very straightforward about SR71 performance in relation to the behaviour of the high supersonic shock systems. My (limited) understanding is that the actual flight test programme was a huge surprise in certain areas, and that some aspects of this are still classified today... But I fully accept you may know more.

[SR71]

Thunderball,

I often haven't got a clue what I'm talking about.
:)

A PhD in aerodynamics taught me exactly as you suggest - there is nothing straightforward about aerodynamics and that I had more un-answered questions when I finished than I had when I'd started! They appear to be multiplying all the time.

The relationship I refer to merely illustrates that there is a fundamental difference between subsonic and supersonic flow behaviour in a converging diverging duct....which is essentially what an intake or nozzle is.

Depending on whether M^2 is <1 or >1, if you then look at the relationship between da/A and du/U, you will see something interesting.

In the case of supersonic flow, in order to decelerate the flow to a subsonic flow condition, you need to first, converge the nozzle, then diverge it.

Which is what a supersonic intake does.

If you now draw a quasi one dimensional duct of the former nature, and superimpose the pressure loads on it (they act normal to the surface correct?), then resolve them in the horizontal direction, you will see that its possible for an intake to generate a thrust.

I'm a great SR-71 enthusiast.

You know the flight manual is online?

http://www.sr-71.org/blackbird/manual/

:ok:

[Thunderball 2]

Thanks, SR71, that is a very good explanation - it must be if I understand it!

And fascinating to look through the flight manual - a great link.:ok: