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  #16  
Old 01-03-2005, 02:14 PM
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Default Mass flow

We need to work in the same units which are inches of mercury absolute for ease of calculation. SL pressure is 29.92 inches. 11.12 inches at 25,000 feet. 11.12 over 29.92= .372 the density of SL so .372 of the SL mass flow at the same TAS at 25,000 feet.

Higher velocity means higher dynamic pressure at the face of the heat exchanger. Assuming good exits, this means higher Delta P across the HE and thus higher mass flow through the HE.

If you have a problem at 5,000 feet with high temps in the climb, you will have a really big problem under the same conditions at 15,000.
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  #17  
Old 01-03-2005, 02:19 PM
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Quote:
Originally Posted by rv6ejguy
If you have a problem at 5,000 feet with high temps in the climb, you will have a really big problem under the same conditions at 15,000.
If you keep the IAS constant, should not it be the same problem at both altitudes?
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  #18  
Old 01-03-2005, 02:23 PM
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Quote:
Originally Posted by rv6ejguy
IAS is the difference between dynamic and static pressure. Mass flow is not the same. TAS is roughly 180 at 120 IAS at 25,000 feet but air density is only about 11 inches Ab. Mass flow is therefore about 55% that of sea level excluding temperature. Your climb rate at 120 IAS at 25,000 would be very low.
But isn't AIR FLOW the same. IAS is related to the number of air molecules being pushed against a diaphram relative to ambient and MAF is the same.
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  #19  
Old 01-03-2005, 03:44 PM
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Default mass flow

Nope. If you work out the numbers you will still have a lot less mass flow at altitude even if you maintain the same IAS. Avoiding the math and just thinking logically, an Atmo (non turbo) engine loses power in direct proportion to pressure assuming the same temperature because hp is directly related to mass flow. At 25,000 feet a 200hp SL engine would have only 74.7hp. Of course doing 180 true vs. 120 true offsets the density loss by a factor of 1.5 but the density difference is 2.67 times less at FL250.

Dynamic pressure varies as the square of the velocity. Mass flow varies proportionally with velocity.

It would be impractical to be climbing at 120 IAS at 25,000 as you might see 2-300 fpm because the nose would be pretty low to maintain this speed even with a turbo engine.
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  #20  
Old 01-03-2005, 04:38 PM
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I'm not arguing the praticality or feasibility of climbing at 120 kts IAS at 25,000 ft. In fact, the engine or what it is doing does not matter.

1 atmosphere = 29.92126 inches Hg = 14.69595 PSI = 101.325 kpa.

Pressure is mass over an area. Isn't that what the airspeed indicator really show?

If I stick a MAF sensor outside the plane (in unrestricted air like the pitot tube), won't the flow as indicated by the MAF sensor be proportional to the IAS always? At 120 kts IAS, I'm going to have twice the flow as at 60 kts IAS.
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  #21  
Old 01-03-2005, 05:28 PM
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Default Mass flow

At a given altitude, mass flow is proportional to velocity so yes, if you go from 60 to 120 at the same altitude, mass flow is double. The ASI reads the difference between dynamic and static pressure. Stactic decreses with both speed and altitude.

I worked out an example at 120mph and SL with a 1 square foot area duct. It would pass 13.46 lbs./sec. At 120IAS/ 180TAS and 25,000 feet, it would pass 7.52 lbs./sec.
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  #22  
Old 01-03-2005, 07:38 PM
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I can understand how it would take less air mass at a higher altitude to get a diaphram instrument to read the same airspeed... the resistance on the back of the diaphram reduces as you go up because of the lower "static" air density. To what extent an airspeed indicator uses a diaphram verses spring tension... I dunno. Spring tension obviously would not be affected by reduced atmospheric pressure, or gravity for that matter.

But the MAF sensor is supposed to measure air flow (by mass, not weight) and mass does not change at altitude or in outter space for that matter. If the MAF sensor is somehow affected at increasing altitude, that would be different. But even automotive sensors are designed for 15000 ft so I don't think they are.

Please continue...
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  #23  
Old 01-03-2005, 08:48 PM
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Air hits harder at higher speeds (velocity squared), but the force that air hits at is irrelevant. The only thing that matters is the number of molecules in contact, and the temperature difference of those molecules.

You can not cool things better by hitting them harder.

Disclamer, I am not an expert and could be wrong.
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  #24  
Old 01-03-2005, 09:32 PM
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Quote:
You can not cool things better by hitting them harder.
You heat them up. Things can get very hot the faster you go. Any deorbiting spacecraft at high speed/thin atmosphere gets very hot.
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  #25  
Old 01-04-2005, 12:44 PM
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Default Mass flow/MAF

There seems to be several things being discussed simultaneously here.

The MAF sensor on the engine measures the mass of air entering the engine. No air temperature sensor is required as in a MAP (pressure) system. There will be less mass going through the sensor as an atmo engine climbs in altitude just as there is less mass available for heat exchangers at altitude. The MAF works just fine at any altitude. It sees a drop in airflow just as it would when you use half throttle at SL. The mass of one molecule of air is the same at 20,000 feet or SL, the problem is that they are much further apart up there so one cubic foot of air has less mass at 20,000 feet as at SL.

The ASI reads the differential pressure essentially between pitot (dynamic) pressure and static pressure. One pressure source is fed to one side of a bellows or diaphram, the other source to the opposite side. Spring pressure is applied to one side as a counteracting force. Dynamic pressure varies as the square of the speed, static pressure on an aircraft is a composite of the ambient atmospheric presssure and the suction effect of airflow across the static port at 90 degrees to the airflow.

Mass flow through a given duct at subsonic velocities is determined by Delta P, in other words the differential between inlet and outlet pressures and the density of air. No Delta P, no airflow. Cut the air density in half and you will get half the mass flow.

Dust is correct in saying that the number (mass) of air molecules passing through a HE determines cooling effect. All the inlet pressure in the world does nothing for you if the air cannot pass through the HE. Exits are just as important as the inlets, a point often missed when it comes to HEs on aircraft.
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  #26  
Old 01-04-2005, 01:26 PM
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Okay... so if DUST is showing 200 IAS at 5,000 ft, and also showing 200 IAS at 25,000 ft (with doggie on oxegen), we know that his true speed at 25,000 is faster (lets throw out headwinds for a moment). The same IAS is due to the fact that fewer air molecules are hitting the IAS sensor at a higher speed than the more molecules that hit the sensor at a lower speed at lower altitude.

So... fewer molecules to transfer heat, higher speeds that increase friction.

But if the MAF readings are the same at low and high altitudes, how can the mass of air flow be different?
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  #27  
Old 01-04-2005, 01:59 PM
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Default Mass flow

Well, if you are indicating 200 in a Cozy at FL250 I would be pretty impressed as that would probably take 500 hp or so! You have it right now I think.

Yes, fewer molecules at altitude means less heat transfer. The friction at speeds below 350 knots can be largely ignored.

No, the MAF reading will be different at diferent altitudes because the mass of a cubic foot of air is less at altitude. The engine is a volumetric device (air pump). If it pulls in 200 CFM of air at full throttle and some rpm at SL or 25,000, this volume remains the same but the mass is only about 40% as much as at SL so the engine power is only about 40% as much.
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  #28  
Old 01-04-2005, 02:49 PM
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Well, I'm hoping a t61 garrett will get me there or close to there
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  #29  
Old 01-04-2005, 04:07 PM
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Default Garrett T61

The T61 would be a very poor match for a 13B at 25,000 feet. You could pull a max of about 28 inches up there before crossing the surge line.
A TO4E with super or standard -40 trim compressor would be good for about 38 inches up there and about 230-250 hp efficiently. Could not throttle back much below 220 hp however as you'd be into surge again. You'll need a good intercooler up here as the compressor discharge temp would be something over 320F even at 0F ambient!

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Well, I'm hoping a t61 garrett will get me there or close to there
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  #30  
Old 01-04-2005, 04:58 PM
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Default 13B turbo dyno curves/ SFCs

Thought you rotary guys might find this interesting. 262hp at 6500rpm 45 inches, SFC .61 lbs./hp/hr.:

http://personal.riverusers.com/~yawpower/sven2.html
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