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  #11  
Old 04-24-2007, 11:36 PM
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garnt.piper garnt.piper is offline
 
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Smile Power v. Altitude

Convention has it that piston engine power varies with the ratio of seal level air density to density at altitude (commonly designated sigma, the density ratio) raised to the power of 1.2 . For turbines to the power of 1.0, i.e in direct proportion to the density ratio.

Hope this helps reduce reinventing the round thing.
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  #12  
Old 04-25-2007, 03:53 AM
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Kevin Horton Kevin Horton is offline
 
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Quote:
Originally Posted by fodrv7
Kevin,
When we flew out of Hong Kong (Tropics) to Alaska (Arctic) we had no idea about "Cold Temperature Altimetery" until it was bought to our attention by the Canaks.
Subsequently, a section was inserted in the manual for corrections to the Minima under very significant IAS minus temperatures.

Do you want me to dig up the data and send it to you.
Hi Pete - I know all about cold temperature altimetry, so I don't need the data. But thanks anyway. It doesn't have too much relationship to the TAS we would get at a given OAT, unless you know something I don't.
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  #13  
Old 04-25-2007, 04:32 AM
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Kevin Horton Kevin Horton is offline
 
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Quote:
Originally Posted by garnt.piper
Convention has it that piston engine power varies with the ratio of seal level air density to density at altitude (commonly designated sigma, the density ratio) raised to the power of 1.2 .
Unfortunately, this does not produce the same power vs temperature relationship as the Lycoming power charts. If we only change temperature, and leave the pressure the same, the Lycoming power chart says the power changes by the square root of the density - i.e. we use a power of 0.5, not 1.2. I don't claim to understand why this should be so, but I have to start from the assumption that Lycoming should know how the power produced by their engines varies as you change the conditions. Once I get some data to analyze, hopefully we can figure out which is correct - Lycoming, or one of the rules of thumb, or some new rule of thumb.
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  #14  
Old 04-25-2007, 05:27 AM
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Default i doubt it.

Kevin,
I doubt there is anything I know about Performance that you don't know.

What i was thinking about of course related to Altimter error; not TAS.

And of course i meant to write "significant ISA minus temperatures", not "significant IAS minus temperatures".
Pete.
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  #15  
Old 04-25-2007, 07:20 AM
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Kevin, great thread!

I have been testing my RV8 using the 4-legged GPS method presented on the National Test Pilot School website for the last 4 years. I always do the test at the same density altitude. The temperature (and the resulting pressure altitude vs density altitude) absolutely does make a difference. I get about 2 kts faster in the winter than in the summer.

I will look through the data (I have a giant Excel spreadsheet) and try to pick some examples to send you. It is unfortunatly complicated by the numerous mods I have made, trying to make the airplane go faster.

It is also complicated by my living in the mountain west, where smooth air without verticle movement is hard to find.

John
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  #16  
Old 04-25-2007, 08:22 AM
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Default Water injection

As I understand the use of water injection in big supercharged reciprocating engines, it is used as an anti-detonation measure. They ran pretty high MAPs (2 atmospheres) achieved with big super- or turbochargers, so the induction air was pretty hot. This despite inter- or aftercoolers. So injecting water reduced the air charge temperature and prevented detonation.

This effectively increased the power, because without water injection, they would've had to reduce the boost to avoid detonation. Less boost -> less oxygen -> less fuel -> less power.

That does not alter the fact that a cubic metre of moist air has less oxygen than the same volume of dry air.
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  #17  
Old 04-25-2007, 10:08 AM
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Kevin Horton Kevin Horton is offline
 
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Quote:
Originally Posted by Nuisance
I have been testing my RV8 using the 4-legged GPS method presented on the National Test Pilot School website for the last 4 years. I always do the test at the same density altitude. The temperature (and the resulting pressure altitude vs density altitude) absolutely does make a difference. I get about 2 kts faster in the winter than in the summer.
I'd love to see that data John, if we can somehow find points at a similar configuration.

My first cut at trying to estimate the effect of air temperature on power and TAS suggested that the TAS should be a bit higher in warm air and lower pressure altitude (i.e. summer) than it would be in cold air and higher pressure altitude (i.e. winter). This differs from what you report, so I obviously have some more head scratching to do, or maybe I made a calculation error (first one this year, I swear), or Lycoming's temperature correction is out to lunch, or you've remembered it backwards.

My calculations did have estimates for ram air recovery and air box pressure losses. I doubt the effects of any errors here would be enough to turn my predictions completely around, but I'll need to look into this some more. Do you have any idea on how much MP pressure loss there is in your air induction (filter + snorkel), and how much ram pressure recovery you see? For example, how much lower is the MP during the take off roll than it is with the engine stopped on the ramp? If you did a high speed pass down the runway, quite low, how much higher would the MP be than it would be during take-off? And, what would the speed and altitude be during that high speed pass? Do you have a stock air induction system, or have you changed it?
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  #18  
Old 04-25-2007, 05:23 PM
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Kevin Horton Kevin Horton is offline
 
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Out of curiousity today, I went up to our technical library and pulled out the Operator's Manual for piston engines from several different manufacturers to see how they handled temperature and humidity corrections. None of them mentioned a humidity correction at all. Every manufacturer had a different way to present a temperature correction. Lycoming has a correction that is based on the square root of the standard temperature over the test temperature. Continental says to add or subtract 1% power for every 6 deg F difference from standard on most engines. But some of their engines have a 1% correction for every 10 deg F. Rotax has a power correction that is strictly based on density altitude. The big Pratt radials just say to add a certain amount of additional MP for temperature changes (they are supercharged, so you don't have full throttle at low altitude - you set the throttle to get a desired MP).

The laws of physics are the same no matter who built the engine. So it is a bit puzzling to see very different approaches to correcting for non standard temperature. I don't see how all the approaches can be correct. We need some test data to help clarify things.

Over lunch I did a search for old NACA reports on the effect of humidity on piston engine power. I found one from 1933 that seems to shed some light on the issue - The Effect of Humidity on Engine Power at Altitude. I need to study it some more, but my initial impression is that it confirms Alex Peterson's suggestion that the effect on power is completely explained by the displacement of O2 by H2O. I could build a spreadsheet that does this correction, but it probably wouldn't be very useful, as we don't really know what the humidity levels are at altitude, unless we happen to be close to one of the sites where weather balloons are sent up.
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  #19  
Old 04-26-2007, 02:44 PM
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Default What GRT does

I asked GRT what they do. Temp is used. This is the formula to which they referred me.
http://williams.best.vwh.net/avform.htm#Mach
I only offer this to round out the discussion.
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  #20  
Old 04-26-2007, 03:44 PM
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Kevin Horton Kevin Horton is offline
 
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Quote:
Originally Posted by hevansrv7a
I asked GRT what they do. Temp is used. This is the formula to which they referred me.
http://williams.best.vwh.net/avform.htm#Mach
I only offer this to round out the discussion.
hevansrv7a - I'm having difficulty putting your comment in context, so I'm not sure what you are trying to tell us. Which of the formula on that page did GRT refer you to? Which part of the discussion should we look at when we consider that formula?
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