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RPM @ 75% power is dependent on prop pitch. It has a minimum in the range you are guessing, could be higher, can't be much lower.

 

Yeah, I get that. I was kind of assuming a "Roger's optimization method" of 5600-5700rpm WOT at cruise altitude, which for me is 3000-4000msl. For me that means 75% is 5200rpm at 3-4k, so I'm guessing at 5000msl the same throttle setting would be 5000-5100rpm.

 

But when they quote 75% power at altitude, are they talking total power (meaning you'd be at WOT at high altitude), or available power (meaning you'd run at the same throttle setting you'd be at in a lower altitude, but just running lower rpm)? I realize in your case CT it's all WOT to get decent performance, I'm just wondering what Rotax means by 75%.

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But when they quote 75% power at altitude, are they talking total power (meaning you'd be at WOT at high altitude), or available power (meaning you'd run at the same throttle setting you'd be at in a lower altitude, but just running lower rpm)?

 

First, a quick diversion since we have pilots and wannabe pilots of all levels here. This will be academic for most here.

 

In a normally aspirated (not turbo/supercharged) engine, when the piston moves down, something has to cause the cylinder to fill with the fuel/air mixture, and that something is atmospheric pressure. That starts out at around 30" of mercury at sea level, but then drops about 1" per thousand feet. So, at 10,000', let's say, there's only about 20" of pressure to fill the cylinder, resulting in much less "stuff" going in and much less power coming out.

 

Second, it's a shame that many manufacturers have moved away from graphs and started showing performance data in tables.

 

A picture is worth 1,000 words (click to enlarge):

 

Cruise%20Performance%20Chart%20(True%20Airspeed),%20Langley%20Flying%20School.gif

 

The numbers will vary from plane to plane, but the general shapes of the curves and altitude/power ratio should remain pretty constant.

 

Want full power and top speed? In this Cherokee there's one and only one place to do this - sea level, for about 138 mph. Climb even one foot above sea level and speed starts immediately tapering off.

 

Or let's say you've decided on 75% power. That can give you a speed of about 122 mph at sea level, but about 132 mph at about 7,600'. That would be the most efficient place to cruise, all thing being equal, since the fuel burn should be about the same but the speed much higher. Note that above 7,600', the plane cannot maintain 75% power, even at full throttle.

 

One more example - you want to conserve fuel as much as possible, and decide 55% power with its reduced fuel consumption is where you want to be for maximum range and efficiency.

 

Quiz: at 55% power, what's the Cherokee's maximum speed and at what altitude would you find it?

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First, a quick diversion since we have pilots and wannabe pilots of all levels here. This will be academic for most here.

 

In a normally aspirated (not turbo/supercharged) engine, when the piston moves down, something has to cause the cylinder to fill with the fuel/air mixture, and that something is atmospheric pressure. That starts out at around 30" of mercury at sea level, but then drops about 1" per thousand feet. So, at 10,000', let's say, there's only about 20" of pressure to fill the cylinder, resulting in much less "stuff" going in and much less power coming out.

 

Second, it's a shame that many manufacturers have moved away from graphs and started showing performance data in tables.

 

A picture is worth 1,000 words (click to enlarge):

 

Cruise%20Performance%20Chart%20(True%20Airspeed),%20Langley%20Flying%20School.gif

 

 

My take-away from that chart is that the Cherokee wheel pants kind of suck. ;)

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Quiz: at 55% power, what's the Cherokee's maximum speed and at what altitude would you find it?

 

According to the chart it's WOT at 12,000 feet and 118mph...HOWEVER, you would burn so much fuel climbing up that high, I'm not sure in the end that is the correct answer for anything but the longest flight legs.

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Indubitably.

 

But haven't most CT owners found similar pathetic gains from wheel pants?

 

That seems to be the case. The Sonex wheel pants in conjunction with gear leg fairings are good for 10-15mph, and I know some of the LoPresti stuff have good numbers, so they CAN help if properly designed. But those cases seem to be the exception rather than the rule.

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http://mauldinaviation.com/Documents_files/LS_Proformance_Supplement.pdf

 

Flight Design document, and it can't be correct. 1st I thought it was a translation issue, maybe they were labeling MCP (max continuous power (94hp)) but using percentage of available but the hp column clearly shows more power than is available.

 

Something's not right.

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Most graphs are not actually in flight and flown numbers. They are hypothetical on paper and provides that everything is at max and perfect conditions. That isn't real life. The Rotax engine looses 3% per 1000' above sea level and proper temp. Even if you could actually get a real 100 hp, which you can't especially with a ground adjustable prop and local atmospheric conditions, then you will loose 36 hp at 12K'. That will put you down to 64 HP. It would be nice if all these graphs posted how they got their figures and what they are based on, but they are not actual flight test.

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Most graphs are not actually in flight and flown numbers. They are hypothetical on paper and provides that everything is at max and perfect conditions. That isn't real life. The Rotax engine looses 3% per 1000' above sea level and proper temp. Even if you could actually get a real 100 hp, which you can't especially with a ground adjustable prop and local atmospheric conditions, then you will loose 36 hp at 12K'. That will put you down to 64 HP. It would be nice if all these graphs posted how they got their figures and what they are based on, but they are not actual flight test.

 

You are right in general but this graph overstates power across the board, these numbers are just wrong. You cannot realize 88% at 8,000', fill in the blanks their underlying formula have an error.

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