hevansrv7a
Well Known Member
http://home.cogeco.ca/~n17hh/PrinceEllipticalProp.html
Van's Air Force
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Testing a new prop design - part 1
- Thread starter hevansrv7a
- Start date
hevansrv7a
Well Known Member
hevansrv7a
Well Known Member
This is about horsepower
I did some more testing today (a nearly perfect day for it) while using my Catto 3-blade. I will write that up as an addendum to the first installment. But here's some HP information which is largely independent of the prop.
I was at 10,000' density altitude, 8540 pressure altitude, temp about 51 degF. My power meter said 75% while going 170 KTAS. My engine was at 2690 RPM x 22.6". My atmosphere table says that 22.6" is equivalent to about 7500'. Or said another way, I was getting a net 0.84" of boost (over ambient) from the ram effect of my front-facing snorkel. The density of the air at 10,000' is 73.85% of standard. The pressure at 22.6" is 75.5%. Let's ignore the -10 RPM. That means that my power reading of 75% (best power mixture) is very reasonable. At least so it seems to me.
My fuel flow was 10.5 gph or maybe a little higher (readings are not as stable as I'd like). Superior says my best SFC is .48. Maybe .50 is reasonable. Let's use 6 pounds per gallon (this varies more than I like but you have to use something; CAFE says 6.02). So my 10.5 gph is 63 pounds. If my SFC is .50 then my HP is 126 which is 70% of 180. If it is 0.48 then the HP is 131.25 or 73%. Close enough?
Note this was at 10,000, not 8000'. My readings at DA 8000 were 2720 x 24.3, 81%. 12.45 gph, approx. 56 degF, PA 6680. TAS was about the same. An SFC of .48 gives 86%. SFC of 0.50 gives 83%.
Note that I am ignoring all those altitude-compensated power charts from the manufacturer. Superior admitted to me that theirs is copied from a Lycoming chart and turned 90 degrees, but there are some errors in it. Anyhow, Lycoming doesn't specify how they configure the motor for the test. The use of fuel flow and SFC was suggested by Walter Atkinson of GAMI and I agree with him that it's a more reliable method but still not completely precise. Superior's SFC number is not a constant so we don't really know what the SFC is at a given moment, only a reasonable range of values. At least one of Kevin Horton's spreadsheets which are supposed to nearly match Lycoming data does say 81% at 8000'. Walter and Kevin have been very generous with their time and expertise. I hope I'm not misinterpreting what they told me.
Lonnie Prince has agreed to build one test prop per Jan's design - to start. I checked with a leading aero expert and the formula Jan is using is essentially correct. My other expert does point out that the "devil is in the details" when looking at the corners of the prop and the constants in the formula. I plan to validate the first test prop using aviation scales to measure torque and will report that when I can.
So far, lots of data, some of it highly suggestive, but no rock-solid proof, yet. More to come. As always, I'd welcome and appreciate comments from the guys who actually understand this stuff!
I did some more testing today (a nearly perfect day for it) while using my Catto 3-blade. I will write that up as an addendum to the first installment. But here's some HP information which is largely independent of the prop.
I was at 10,000' density altitude, 8540 pressure altitude, temp about 51 degF. My power meter said 75% while going 170 KTAS. My engine was at 2690 RPM x 22.6". My atmosphere table says that 22.6" is equivalent to about 7500'. Or said another way, I was getting a net 0.84" of boost (over ambient) from the ram effect of my front-facing snorkel. The density of the air at 10,000' is 73.85% of standard. The pressure at 22.6" is 75.5%. Let's ignore the -10 RPM. That means that my power reading of 75% (best power mixture) is very reasonable. At least so it seems to me.
My fuel flow was 10.5 gph or maybe a little higher (readings are not as stable as I'd like). Superior says my best SFC is .48. Maybe .50 is reasonable. Let's use 6 pounds per gallon (this varies more than I like but you have to use something; CAFE says 6.02). So my 10.5 gph is 63 pounds. If my SFC is .50 then my HP is 126 which is 70% of 180. If it is 0.48 then the HP is 131.25 or 73%. Close enough?
Note this was at 10,000, not 8000'. My readings at DA 8000 were 2720 x 24.3, 81%. 12.45 gph, approx. 56 degF, PA 6680. TAS was about the same. An SFC of .48 gives 86%. SFC of 0.50 gives 83%.
Note that I am ignoring all those altitude-compensated power charts from the manufacturer. Superior admitted to me that theirs is copied from a Lycoming chart and turned 90 degrees, but there are some errors in it. Anyhow, Lycoming doesn't specify how they configure the motor for the test. The use of fuel flow and SFC was suggested by Walter Atkinson of GAMI and I agree with him that it's a more reliable method but still not completely precise. Superior's SFC number is not a constant so we don't really know what the SFC is at a given moment, only a reasonable range of values. At least one of Kevin Horton's spreadsheets which are supposed to nearly match Lycoming data does say 81% at 8000'. Walter and Kevin have been very generous with their time and expertise. I hope I'm not misinterpreting what they told me.
Lonnie Prince has agreed to build one test prop per Jan's design - to start. I checked with a leading aero expert and the formula Jan is using is essentially correct. My other expert does point out that the "devil is in the details" when looking at the corners of the prop and the constants in the formula. I plan to validate the first test prop using aviation scales to measure torque and will report that when I can.
So far, lots of data, some of it highly suggestive, but no rock-solid proof, yet. More to come. As always, I'd welcome and appreciate comments from the guys who actually understand this stuff!
rocketbob
Well Known Member
Hi Howard,
I just reread your post and I have a question for you. How are you accounting for critical mach at the tip, if you believe that the diameter should be increased?
In Paul's designs he was careful about staying below critical mach at the tip to avoid the drag penalties that are a result of exceeding mach. If you're getting 2720 rpm and its quiet it sounds like there just needs to be some corrections in the helix angle.
I just reread your post and I have a question for you. How are you accounting for critical mach at the tip, if you believe that the diameter should be increased?
In Paul's designs he was careful about staying below critical mach at the tip to avoid the drag penalties that are a result of exceeding mach. If you're getting 2720 rpm and its quiet it sounds like there just needs to be some corrections in the helix angle.
