Bryan Wood
Well Known Member
Often the performance specs are referred to in these lists for the 7A vs. 9A and there are some quirks that have me puzzled. I'll start with the 7A because as far as I can tell the numbers listed make sense, but the ones provided for the 9a don't add up. My question is why? For these examples I'm using the posted numbers for solo weight and 75% power for both airplanes.
Van's numbers for the 7A are 190mph on 160hp, 198mph on 180hp, and 205mph on 200hp.
After doing some learning about speed increases the math isn't terribly difficult to calculate how much the speeds will increase with additional horsepower on the same airframe. Simply put, the speed increases with the cube root of the power increase. So increasing the horsepower from 160 to 180hp is a 1.125% power increase. (180/160=1.125) The cube root of 1.125 is 1.040 and when this is multiplied by the 190mph of the 160hp RV-7A the result is 197.6, or rounded to 198. This is exactly what Van publishes, and when using the same formulas the numbers match Van's numbers for the 200hp plane also. Try it for yourself, here is a link to a cube root calculator.
http://www.csgnetwork.com/cuberootcubecalc.html
Now if you use this process to check the numbers for the 9a they are not coming up even close to what the factory publishes. If you start with the 135hp and 173mph and then jump the power to 160hp the numbers are significantly slower than the factory numbers. This is puzzling me. The only thing that I can come up with is that the props available for the smaller engines are not up to snuff.
According to the math the 160hp 9A should achieve 183mph at 75%, but the factory claims 187mph. Do any of you speedsters have an explanation for this? The factory plane is faster than 183mph. I know this personally having done my transition training in it. The CAFE Foundation report has it going even faster than what they publish. So why are the numbers skewed?
Best,
Van's numbers for the 7A are 190mph on 160hp, 198mph on 180hp, and 205mph on 200hp.
After doing some learning about speed increases the math isn't terribly difficult to calculate how much the speeds will increase with additional horsepower on the same airframe. Simply put, the speed increases with the cube root of the power increase. So increasing the horsepower from 160 to 180hp is a 1.125% power increase. (180/160=1.125) The cube root of 1.125 is 1.040 and when this is multiplied by the 190mph of the 160hp RV-7A the result is 197.6, or rounded to 198. This is exactly what Van publishes, and when using the same formulas the numbers match Van's numbers for the 200hp plane also. Try it for yourself, here is a link to a cube root calculator.
http://www.csgnetwork.com/cuberootcubecalc.html
Now if you use this process to check the numbers for the 9a they are not coming up even close to what the factory publishes. If you start with the 135hp and 173mph and then jump the power to 160hp the numbers are significantly slower than the factory numbers. This is puzzling me. The only thing that I can come up with is that the props available for the smaller engines are not up to snuff.
According to the math the 160hp 9A should achieve 183mph at 75%, but the factory claims 187mph. Do any of you speedsters have an explanation for this? The factory plane is faster than 183mph. I know this personally having done my transition training in it. The CAFE Foundation report has it going even faster than what they publish. So why are the numbers skewed?
Best,