FlyingMonkey Posted September 12, 2013 Report Share Posted September 12, 2013 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%. Link to comment Share on other sites More sharing options...
Ed Cesnalis Posted September 12, 2013 Report Share Posted September 12, 2013 75% means 75% of total not available. it is a quantity of power not a throttle setting. Manifold pressure and RPM are the variables, with fixed pitch props you can think RPM at WOT and prop pitch. Link to comment Share on other sites More sharing options...
FlyingMonkey Posted September 12, 2013 Report Share Posted September 12, 2013 That's what I figured, thanks CT. You probably don't even get 75% power at WOT for your normal cruise altitude, do you? Link to comment Share on other sites More sharing options...
Ed Cesnalis Posted September 12, 2013 Report Share Posted September 12, 2013 That's what I figured, thanks CT. You probably don't even get 75% power at WOT for your normal cruise altitude, do you? 75% is available till I'm 400' above the runway then its downhill from there. I have seen close to 130kts at ~ 65hp, where the air is kinda thin. Link to comment Share on other sites More sharing options...
FlyingMonkey Posted September 12, 2013 Report Share Posted September 12, 2013 75% is available till I'm 400' above the runway then its downhill from there. I have seen close to 130kts at ~ 65hp, where the air is kinda thin. I guess that is one bright point, less dense air = less drag = less hp to get the same speed. Link to comment Share on other sites More sharing options...
Ed Cesnalis Posted September 12, 2013 Report Share Posted September 12, 2013 I guess that is one bright point, less dense air = less drag = less hp to get the same speed. it's a bright spot that diminishes with altitude, best efficiency is at 7,500' it goes down from there. Link to comment Share on other sites More sharing options...
FastEddieB Posted September 12, 2013 Report Share Posted September 12, 2013 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): 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? Link to comment Share on other sites More sharing options...
FlyingMonkey Posted September 12, 2013 Report Share Posted September 12, 2013 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): My take-away from that chart is that the Cherokee wheel pants kind of suck. Link to comment Share on other sites More sharing options...
FastEddieB Posted September 12, 2013 Report Share Posted September 12, 2013 My take-away from that chart is that the Cherokee wheel pants kind of suck. Indubitably. But haven't most CT owners found similar pathetic gains from wheel pants? Link to comment Share on other sites More sharing options...
FlyingMonkey Posted September 12, 2013 Report Share Posted September 12, 2013 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. Link to comment Share on other sites More sharing options...
Ed Cesnalis Posted September 12, 2013 Report Share Posted September 12, 2013 A southerly departure from Mammoth requires more than 13,000' Link to comment Share on other sites More sharing options...
Karl Posted September 12, 2013 Report Share Posted September 12, 2013 Found this sometime ago on CTLS . Could not download just took a screen picture. Link to comment Share on other sites More sharing options...
Ed Cesnalis Posted September 12, 2013 Report Share Posted September 12, 2013 That chart uses percentage of max continuous power, which it charts at 94hp. Rotax graphs use max power I believe. Link to comment Share on other sites More sharing options...
FlyingMonkey Posted September 12, 2013 Report Share Posted September 12, 2013 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. Link to comment Share on other sites More sharing options...
FlyingMonkey Posted September 12, 2013 Report Share Posted September 12, 2013 Found this sometime ago on CTLS . Could not download just took a screen picture. Anybody using that chart to get max range will make Roger cry as they cruise along for 885nm at 4350rpm... Link to comment Share on other sites More sharing options...
Ed Cesnalis Posted September 12, 2013 Report Share Posted September 12, 2013 Karl, Who made that chart? 77hp at 12,000' ? how can that be? Link to comment Share on other sites More sharing options...
Karl Posted September 12, 2013 Report Share Posted September 12, 2013 Mauldin Aviation CTLS Specs www.mauldinaviation.com/Specs.html N505MA Flight Design CTLS specifications. ... CTLS Specifications · Manuals / Documents · Calendar of Events ... CTLS Specifications CTLS Performance ... Link to comment Share on other sites More sharing options...
Ed Cesnalis Posted September 12, 2013 Report Share Posted September 12, 2013 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. Link to comment Share on other sites More sharing options...
Dan Kent Posted September 12, 2013 Report Share Posted September 12, 2013 FWIW, It's on the FD USA website also. I remember seeing this when I was researching planes, and was able to locate it again. http://flightdesignusa.com/flight-training/student-pilots/ Link to comment Share on other sites More sharing options...
Roger Lee Posted September 12, 2013 Author Report Share Posted September 12, 2013 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. Link to comment Share on other sites More sharing options...
Ed Cesnalis Posted September 12, 2013 Report Share Posted September 12, 2013 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. Link to comment Share on other sites More sharing options...
Roger Lee Posted September 12, 2013 Author Report Share Posted September 12, 2013 I agree ED. You can learn this in Rotax school too. Rotax will tell you the truth if ask. We talked a little about this last week. 912is verses the 912ULS. Link to comment Share on other sites More sharing options...
johngpilot Posted September 19, 2013 Report Share Posted September 19, 2013 Here is section 5.1.6 of the CTLSi flight manual and the Influence of Altitude on Engine Performance: Link to comment Share on other sites More sharing options...
Ed Cesnalis Posted September 19, 2013 Report Share Posted September 19, 2013 John, That link doesn't work Link to comment Share on other sites More sharing options...
johngpilot Posted September 19, 2013 Report Share Posted September 19, 2013 i think it is fixed now... Link to comment Share on other sites More sharing options...
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