Bill Palmer
Well Known Member
RV-8A Builders/Owners/Pilots,
Last Thursday, after I read the May 2006 Sport Aviation article "Reciprocals Rotaries Nose to Nose" by Van's Ken Scott, I e-mailed Van's regarding the apparent poor performance (relatively speaking) of Van's 200hp RV-8A "Tweety." I questioned "Tweety's" inability to approach Van's published performance specifications and, particularly, "Tweety's" inferior performance in a one-on-one fly-off with "Dilbert," a 180hp(+), FADEC-engined RV-8. Today, I was pleased to receive a response from Ken Krueger, Van's Aircraft Engineer, as follows:
"Bill, you are not making correct comparisons between numbers."
"When Van's quotes a number for 75% cruise, it is for operation at wide-open throttle at 2700RPM at 8000 ft Density Altitude. We ran the side-by-side cruise tests at 8000 ft Pressure Altitude but the temperature was warmer than standard. Also, we ran at 2500 RPM as opposed to 2700 RPM as in our 75% cruise scenario. Finally, the side-by-side ground speeds do not account for the effects of wind. The GPS ground speed was strictly for comparing different aircraft flying under the same atmospheric conditions."
"When Van's quotes a number for rate of climb, it is for operation at wide-open throttle at 2700 RPM at Sea Level density altitude. We timed the side-by-side climb test from brake release to 8000 ft. This means that your calculated rate of climb to 8000 ft includes time for take-off and acceleration to best rate of climb speed. Also, your calculated rate of climb is an average climb rate which reflects the fact that climb rate diminishes with density altitude. Finally, we ran the side-by-side climb tests at 2500 RPM as opposed to 2700 RPM as in our "quoted" max rate of climb scenario."
"The engine in "Tweety" has a lot of hours on it and it is definitely not putting-out rated horsepower. The engine in "Dilbert" is FADEC-equipped and, if anything, is putting out more than rated horsepower. Also, the prop on Dilbert is one of Hartzell's "Blended Airfoil" propellers which is very efficient indeed."
"I think that what you are seeing in "Dilbert" is an aircraft that performs better than our published numbers. What you are seeing with "Tweety" is an aircraft that, when new, performed up to our published numbers. The numbers you derive from the side-by-side fly-off are NOT valid for comparison to Van's published numbers for the reasons I have outlined in the previous paragraphs of this response."
"Ken Krueger, Engineer, Van's Aircraft"
My personal "take-aways" from Ken's response is that I'm definitely equipping my 200hp, angle-valve engine with electronic ignition on at least one set of plugs, building as light as I can, and installing a blended-airfoil, constant-speed Hartzell propeller if, or when, the propeller is approved for angle-valve engines. I wish I was already flying so I could challenge "Dilbert" to a duel, but I'm still many months away from first flight. Is there anyone on the list who has a strong, light 200hp RV-8A to challenge "Dilbert" now?!
Best Regards,
Bill
Last Thursday, after I read the May 2006 Sport Aviation article "Reciprocals Rotaries Nose to Nose" by Van's Ken Scott, I e-mailed Van's regarding the apparent poor performance (relatively speaking) of Van's 200hp RV-8A "Tweety." I questioned "Tweety's" inability to approach Van's published performance specifications and, particularly, "Tweety's" inferior performance in a one-on-one fly-off with "Dilbert," a 180hp(+), FADEC-engined RV-8. Today, I was pleased to receive a response from Ken Krueger, Van's Aircraft Engineer, as follows:
"Bill, you are not making correct comparisons between numbers."
"When Van's quotes a number for 75% cruise, it is for operation at wide-open throttle at 2700RPM at 8000 ft Density Altitude. We ran the side-by-side cruise tests at 8000 ft Pressure Altitude but the temperature was warmer than standard. Also, we ran at 2500 RPM as opposed to 2700 RPM as in our 75% cruise scenario. Finally, the side-by-side ground speeds do not account for the effects of wind. The GPS ground speed was strictly for comparing different aircraft flying under the same atmospheric conditions."
"When Van's quotes a number for rate of climb, it is for operation at wide-open throttle at 2700 RPM at Sea Level density altitude. We timed the side-by-side climb test from brake release to 8000 ft. This means that your calculated rate of climb to 8000 ft includes time for take-off and acceleration to best rate of climb speed. Also, your calculated rate of climb is an average climb rate which reflects the fact that climb rate diminishes with density altitude. Finally, we ran the side-by-side climb tests at 2500 RPM as opposed to 2700 RPM as in our "quoted" max rate of climb scenario."
"The engine in "Tweety" has a lot of hours on it and it is definitely not putting-out rated horsepower. The engine in "Dilbert" is FADEC-equipped and, if anything, is putting out more than rated horsepower. Also, the prop on Dilbert is one of Hartzell's "Blended Airfoil" propellers which is very efficient indeed."
"I think that what you are seeing in "Dilbert" is an aircraft that performs better than our published numbers. What you are seeing with "Tweety" is an aircraft that, when new, performed up to our published numbers. The numbers you derive from the side-by-side fly-off are NOT valid for comparison to Van's published numbers for the reasons I have outlined in the previous paragraphs of this response."
"Ken Krueger, Engineer, Van's Aircraft"
My personal "take-aways" from Ken's response is that I'm definitely equipping my 200hp, angle-valve engine with electronic ignition on at least one set of plugs, building as light as I can, and installing a blended-airfoil, constant-speed Hartzell propeller if, or when, the propeller is approved for angle-valve engines. I wish I was already flying so I could challenge "Dilbert" to a duel, but I'm still many months away from first flight. Is there anyone on the list who has a strong, light 200hp RV-8A to challenge "Dilbert" now?!
Best Regards,
Bill