John... first thing I can see is that I want to say PSI declines at a rate of 0.5 PSI/1000 ft ballpark, so ambient pressure at 10k ft should be 9.7 (14.7-(10*0.5)) on a standard day. Using that ROUGH estimate, 5 PSI ambient is more like 20k feet. (and probably higher than that, given the error in most "rules of thumb".. The lapse rate for Inches Mercury is 1 per 1000 ft, which is what they drilled into our heads in flight training.
 
The second one I am NOT sure of at all, but looking at your spreadsheet and formula's... the sea level data seems workable, but the conversion factor you are using  in box B20 appears to be the conversion factor for standard condition air (59*F and 29.92" / 14.7 psi)... Digging out some books and webpages, it appears that standard conditions at 10K feet include a temp of 23*F/-5*C and an estimated pressure of  9.7 psi/19.7" for a density value of 0.0543 lb/CF
 
I used the calculator at
 http://www.stealth316.com/2-air-fuel-flow.htm for these density calcs, and I couldnt remember the lapse rate for temps, so I dug the standard temp value out of a performance chart in a POH.
 
I hope this helps
Dave Staten

John Slade wrote:

Let's see if I understand this.

To help me read the map, I made a tiny spreadsheet (attached) to show lbs/min at various rpm

This tells me I have 23lb/min at 6000. So...

At SL and MAP 50 I have a PR of (30+20)/30 = 50/30 = 1.66

So on the map I read up from 23 till I hit 1.66 on the y axis and see that I'm on the bottom of the island.

 

Now go to 10000ft and set the MAP at, say 40. My PS is now (10+30)/10 = 40/10 = 4.0 which is off the scale. :(

 

Looking at the (compressor) MAP again I see that my max PR at 23lb is about 1.8, so working backwards my max MAP at 6000 ft is 18 if I want to stay on the island.  To get into higher PR's I need more rpm. I'd need 9000+ rpm to reach the top of the island where the available PR is about 2.5.

 

Doesn't sound right. Where did I go wrong?

 

Comments?

 

 

 

 

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