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  #61  
Old 03-15-2006, 09:54 AM
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RV - GREAT stuff - i sorta figured they had it in - on the internet at lots of places they use that to correct the PR
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  #62  
Old 03-15-2006, 02:25 PM
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Quote:
Originally Posted by rv6ejguy
This is fairly close (24.8) to my standard figure of hp divided by 8.07 will equal mass flow in lbs./min.
I think i calculated 21.something - calcs are at home
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  #63  
Old 03-15-2006, 04:20 PM
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Is that actual, or corrected for the turbo maps?

I calculate:

2600 rpm => 24.5 lb/min corrected air flow
2800 rpm => 26.4 lb/min corrected air flow

PR = 3.11
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  #64  
Old 03-15-2006, 10:43 PM
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just some thoughts in spreadsheet form - looking to duplicate 85 degree SL performance at 25000 feet
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  #65  
Old 03-16-2006, 06:50 AM
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My calculated barometric pressure at 25000 feet is different. I get 5.45 psi.

I also thought you were pressurizing to 31 inHg, not 29.92, this would be 15.2 psi, not 14.7.

I also assume 1 psi drop accross the intercooler and .5 in Hg (0.24 psi) vacuum drawn at the compressor inlet.

Pressure ratio = (15.2 + 1.0)/(5.45 - 0.24) = 3.11
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  #66  
Old 03-16-2006, 10:51 AM
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before i went to 31.3 MAP i just wanted to look at 29.92, the next adjustment to 31.5 shouldn't be too bad.

my standard baro at 25000 feet is 52% of SL - smells right? I'm takin it slow to reduce errors and to make sure my thick headed skull understands it
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  #67  
Old 03-25-2006, 03:06 PM
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Ok, mike was right on the baro - i had it wrong - fixed it.

question i now have PR of 2.13 to achieve performance at 25000 feet at -30 with an outlet temp (including a temp rise of 167 degrees F to a turbo outlet temp of 138 degrees) what is the turbo outlet pressure?

spread sheet attached with column needing formula - i'm a dam accountant, these formulas are driving me nuts.

the good thing - 2.13 pressure ratio - easily attainable
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  #68  
Old 03-25-2006, 04:48 PM
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ignore it - i was mixing k and f
AHHHHHHHHHHHHHHHHH
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  #69  
Old 03-25-2006, 05:01 PM
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OK, i think unmixed
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  #70  
Old 03-26-2006, 11:28 AM
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OK, i know i am boring - stupid calculations when other have done and have offered to do them, but it is just my way - gotta learn and understand it. have no choice in the matter - that is just the way my brain is wired.

As many here know turbo maps, those funny lookin charts that tell you how a turbo ought to perform, are all done at about sea level and i THINK 85 degrees F.

I have been skirting the issue of actually delving into the formulas for years now, gathering understanding of general principles and operation until the last few weeks. BIG problem for me is physics, did not take them in school so the most rudimentary principles are a stretch for me. I beeeeeen stretchin.

My main thought in this process is that as you go up the air gets thinner and colder. because it gets colder it gets denser. simple enough, but not when you are untrained and not when the places on the internet that calc this stuff do not seem to properly take it into consideration.

That was my goal.

The next item was to choose a level of performance to shoot for at 25000 feet. the basic calcs for pressure ratio indicated a 3 to 1 PR. looking at turbos - this is beyond the surge line - over-spinning the turbo and destructing it. so i picked a target. my engine is rated at 31.5 MAP (manifold air pressure) at no particular temperature.

As we all know the engine performs better at lower temp - more ox = more HP. I chose 31.5 at 85 F. I set my goal low figuring that to attain that level of power at 25000 at -30 F was a conservative goal.

I calculated the weight of a cubic foot of air being used at 2800 rpm at those conditions. 22.36

That was my goal after much work i found that the weight of a cubic foot per minute of air at 25000 feet was 10.48, a ratio of 2.13 to 1. THAT WAS a ratio i could live with - turbos love that ratio.

Now i new i could take the air presented to the turbo and compress it 2.13 times and have the air the engine needed, but what was my temp after compression and what was my post turbo pressure.

well i calculated a temp rise using the CO2 model that is so popular and divided it by .75 for the efficiency of the turbo and then I new that (thank you so much rv6guy) the amount of heat would be more so i added a fudge factor and increased the heat by 20% and i came up with a heat rise of 301.71 F and a post turbo temp of 272. HOT

Now what was the pressure mmmmm 42.26 is what i calculate. hot and allot of pressure. now come the inter-coolers, mmmmm the charts say 75% efficient - sure I'll get that - yeah right - so i used 50% and with that i get a MAP of 33.55 and a MAT (manifold air temp) of 121 F

Altitude 0 25000
Adjustment 1 0.389367979
Baro at altitude 29.92 11.65
PSIA at altitude 14.7 5.72
Standard Temp change at altitude F 59 -30
LBS cu FT 0.076505291 0.035935713
Inlet Temp at Kelvin 288.15 238.71
lbs used per minute at above engine spec 22.31 10.48
PR needed for full power 1.00 2.13
Temp Rise F with .75 inter-cooler ineff 0 140
Calculated temp rise F 1 251
temp for IR and turbo conduction 1 302
outlet temp F 60 272
outlet temp at K 289 406
turbo outlet pressure PSI 14.77 20.75
turbo outlet pressure IM 30.08 42.26
inter-cooler efficiency at 50% kelvin drop 0.42 83.65
IM Temp K 288.58 322.35
IM at F 59.77 120.56
IM pressure 30.04 33.55

So this tells me that at 25000 feet to get 85% performance with a MAT temp of 120.56 i need a MAP of 33.55. So a pop off valve set to 31.5 will rob me of power

also i was planning of a post turbo pop off valves of 31.5 and these calcs tell me i will have a post turbo pressure of 42.26 and if i pop it off at 31.5 - i be screwed - no power

I think the calcs are right - i think i converted the k to F to K etc, etc at the right times and used the correct formulas.

Is this the way it will look????? or does it all happen so fast and fluid that it never reaches these calculated pressures? (i know the temps happen.)

My purpose - to choose the correct turbos and pop off valves and to plot actual versus calculated once it is up and running to verify that i chose the correct pop off valve springs and operating parameters at lower levels so i do not self destruct at my silly goal of full power at 25000 feet
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  #71  
Old 03-26-2006, 12:44 PM
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A bit hard for me to follow here with these mix of units. The big thing here is to get the density ratio calculated to plot compressor inlet mass flow at your calculated PR. This is the point to use on the compressor map.

By the way, I'd never use a pop off valve to regulate boost pressure. This will cause an increase in N1 and higher charge temps. The pop off can be a safety device if you wish but use a WG to control boost.

Even 50% intercooler effectiveness is unlikely at 25,000 feet without a lot of inlet area. Trying to pull full takeoff power up here will tax the air and oil cooling as well and is probably not realistic. The turbo can supply the MAP to do it. The engine may not be too happy if it cannot rejectthe heat generated in the thin atmosphere.
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  #72  
Old 03-26-2006, 01:00 PM
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absolutely understand - others have said along with you that as you go higher - the cooling becomes more difficult to attain. Ima tinkin that even though you have the same mass air flow at 200 ias at 25000 that you have at sl ias of 200 that the speed of that mass is 50% higher and that the speed with which the air passes through the inter-cooler and cooling fins on the engine stops it from absorbing the heat. the air simply does not hang around long enough.

The calcs show a 40 degree cushion to the 160 F pre-ignition danger limit, i will plot and chart it carefully to make sure i stay far away from it.

as far as the pop off valve - mmmmm - I'm using waste gates to control boost - you don't like the safety of a pop off valve set at say 35 on the IM, just in case?

the mix of temps - just came from the spread sheets to simplify the formulas. instead of using all K or converting in the values inside the formulas i put a row in to allow simpler formulas. all the K's can be deleted or hidden for ease of review
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  #73  
Old 03-26-2006, 02:26 PM
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Yes, my post said by all means use a pop off as a secondary safety if you wish. Wastegates are very reliable and you always have the throttle to reduce MAP if needed and probably an ECU boost limit fuel cut to protect the engine too. I've never used pop offs myself.

The IAS/ TAS thing and mass flow has been thrashed before. Not sure people have grasped this concept quite yet. A Cozy will not indicate anywhere close to 200 kts. at 25,000. As a really general rule of thumb, you will have somewhere around half the available mass flow for cooling under any given flight condition at 25,000 compared to SL. As the atmospheric pressure is 37% of SL, the temperature probably much lower and the TAS will be quite a bit higher, this is a round, general figure.

So as compressor discharge temp increases with altitude at a given MAP, intercooler effectiveness drops, providing a double whammy to affect intercooler discharge temps. This goes without saying that engine cooling air and oil cooling are also affected. Some turbocharged GA aircraft require that the cowl flaps be open in the climb at high altitudes and even in cruise sometimes if the OAT is well above standard.
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  #74  
Old 03-26-2006, 03:58 PM
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heh heh heh 200 mph and i get ya on the cooling. cooling and not pressure ratio will probably be my limiting factor.

it will be interesting to plot the actual versus the calculated as i go along. hope i will be able to project 15000 after i have 12500 under control and 17500 after i have 15000 under control and so on and so forth.

next i am going to do the intercooler calcs on actual specs. i think i can fit 2 - 11x8 inch intercoolers in two p51 style scoops
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  #75  
Old 06-09-2006, 05:06 PM
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Default New turbo book

The third or new replacement edition of the old Hugh MacInnes "Turbochargers" HP book, re-written/ updated by Mark Warner is now available to those who wish:

http://www.amazon.com/gp/product/155...108648-9016723

Mark and I collaborated on several sections in the book including a section on aircraft turbocharging which features some text and photos of my flying Subaru installation.

I have not seen the final product yet but hope to receive my "hot off the press" copy any day now. Hopefully this will be of some aid to those contemplating turbos.
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