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  #41  
Old 08-25-2017, 06:36 PM
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DanH DanH is offline
 
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Quote:
Originally Posted by Bicyclops View Post
Using online density altitude calculators, it appears that I am seeing something like an inch or two more MAP than the expected absolute pressure.
Remember, the calculated available MP would be equivalent to measuring at a point before the filter, fuel control venturi and butterfly, intake manifold plenum, (if it has one), and the intake tubes, all of which offer some restriction. Since we actually measure at one of intake ports in a cylinder head, there will be a significant difference between what is available and what is seen on the EFIS. There is no good way to guesstimate the total intake tract loss since everybody's system is different, but there are examples; my notes say an RSA-5 fuel control alone is responsible for a pressure loss of about 12.75" H20 at 1560 pph, or 0.94" Hg. An FM-200 is about 4" H2O, or 0.3" hg, same conditions.

So, if you're seeing more on the EFIS than any reasonable calculation, well, it would have to be magic.

Quote:
So, for my flight at 13500', 74degree, 30.07 alt setting , I see a calculated absolute pressure of just over 18" and my MAP gauge was reading 20.5 at wide open throttle.
Yep, that would magical. If I use your inputs with an assumed a 3.5"D intake and IO-360, pressure before the filter would be about 18.1" Hg. Mr. Short's report of 17.4 displayed on the panel is probably pretty accurate.

BTW, 74F at 13,500 feet? Is it really that hot over California?
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Dan Horton
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Barrett IO-390

Last edited by DanH : 09-17-2017 at 06:29 AM.
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  #42  
Old 08-25-2017, 09:02 PM
Bicyclops Bicyclops is offline
 
Join Date: Oct 2012
Location: LA, California
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Well, I can't prove any of it, so far and I don't want to be bragging about inaccurate numbers. I'm just going by what my instruments show. That's why I want to hear other people's numbers.

Yup, it was hot. It was 98 when I took off from Chino and there is almost always an inversion. It was 104 at 3500' where I was trying to get some fuel nozzle data. Bumpy too. It was a relief to climb to the cool cool air of 74 degrees. ;-) It was hot the whole time I was flying off my 40 hours - 2, 3, even 4+ hrs a day. I was also commuting 65 miles one way and had to stop down and annual our 6A about halfway through. Wrung me out pretty good. I was trying really hard to make Osh.

I'll have to measure my inlet. It is bigger than standard - maybe about 4.5" tapering to where I could get a short length of 4" scat onto it. I ran out of room behind the prop and had to make it larger to not constrict at the dome of the filter. I'm running an FM-150.

One factor I just thought of - the inlet is right, and I mean right, behind the prop. The 74RV has a pronounced twist and some extra chord at the root. The folks at Whirlwind told me that they had reports that this prop cooled very well on the ground. That might account for some extra MAP. Didn't Kent Pacer note some of that? Where's my copy of Speed with economy?

Next time I go out to the airport I'll check the MAP against field elevation. If it is off, I think I can put an offset into the EFIS. Gotta put a big window in for the missus this weekend. Sigh.

Ed Holyoke


Quote Yep, that would magical. If I use your inputs with an assumed a 3.5"D intake and IO-360, pressure before the filter would be about 18.1" Hg. Mr. Short's report of 17.4 is probably pretty accurate.

BTW, 74F at 13,500 feet? Is it really that hot over California?[/quote]
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