.15 size shrike w/OS32 Prop?
08-12-2002, 11:53 AM
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.15 size shrike w/OS32 Prop?
I have a .15 size Shrike with an OS 32 on it. I am currently running a 9x6 prop with 15% heli fuel. What would be a good prop for the 32? Would an 8x8 be too small? It's very fast with the 9x6, but I want more.
Thanks,
Craig
Thanks,
Craig
09-04-2002, 04:48 PM
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.15 size shrike w/OS32 Prop?
I will try a APC 8x8. I have another one (Shrike) with a .25FP on it. Mine with the .32 is faster at half throttle then the one with the .25FP. The .25 is boring now. I was thinking of throwing the .25fx on it w/pipe. I had to do a lot of mods to the one with the .32. I also have a Whiplash sitting ready to paint. Hopefully it's faster then my Shrike. I like the looks of the Shrikes better though. They look more like a real plane instead of a flying wing.
Later,
Craig
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Later,
Craig
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09-05-2002, 08:01 PM
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.15 size shrike w/OS32 Prop?
jlong,
What do those prop load numbers mean? Does a higher or lower number mean more speed? Does prop efficiency figure into that some how? Inquiring minds want to know.
What do those prop load numbers mean? Does a higher or lower number mean more speed? Does prop efficiency figure into that some how? Inquiring minds want to know.
09-05-2002, 08:38 PM
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prop load
There are many people with alot more knowledge than me to answer this. My simple interpretation may not be entirely correct and i don't proclaim to be an expert by any means.
The load on the engine is determined by the diameter squared multiplied by the pitch. This number is a ballpark way to compare what changing props will do to your rpm's of a given engine. To get more speed you want more pitch, to get more thrust you want more diameter. A larger load will decrease your rpms, a smaller load will increase your rpms. Obviously there are many more variables but this is just a quick way to see where you need to go with the prop dimension to get the desired result.
The original question was about going from a 9x6 to an 8x8. The poster wanted to know if the 8x8 was going to be too small. The load of the 9x6=486, the 8x8=512. The smaller diameter prop is actually a bigger load on the engine and will slow the rpms down. So did he want the rpms up to get the engine in the power band? or did he want more speed? or did he want more thrust? We can say for sure the thrust will be less and the rpms will be less but the speed we cant say because of other variables.
The load on the engine is determined by the diameter squared multiplied by the pitch. This number is a ballpark way to compare what changing props will do to your rpm's of a given engine. To get more speed you want more pitch, to get more thrust you want more diameter. A larger load will decrease your rpms, a smaller load will increase your rpms. Obviously there are many more variables but this is just a quick way to see where you need to go with the prop dimension to get the desired result.
The original question was about going from a 9x6 to an 8x8. The poster wanted to know if the 8x8 was going to be too small. The load of the 9x6=486, the 8x8=512. The smaller diameter prop is actually a bigger load on the engine and will slow the rpms down. So did he want the rpms up to get the engine in the power band? or did he want more speed? or did he want more thrust? We can say for sure the thrust will be less and the rpms will be less but the speed we cant say because of other variables.
09-05-2002, 08:46 PM
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.15 size shrike w/OS32 Prop?
I did a search on "prop load " and found this: (apparently there was a chart that went with it but I didn't find it)
"Engine Load = Propeller Diameter Cubed x Pitch 1. Find your prop diameter in the first column and follow across to the pitch of your prop to find the relative loading factor. The larger the number the more load the prop will place on your engine. i.e. An 11/3 prop with a load factor of 3993 will load the engine the exact same amount as a 10/4 with a load factor 4000. 2. If you want more R.P.M. go to a prop with a lower load factor. 3. If you want more speed go to a prop with a higher pitch. If you want more acceleration/vertical speed go to a prop with lower pitch. 4. If you need more thrust you can also go to a larger diameter prop. "
Based on this and your last post, it makes sense to me now. Its really just a way to compare props to each other. There was an article in one of the model mags recently about this.
"Engine Load = Propeller Diameter Cubed x Pitch 1. Find your prop diameter in the first column and follow across to the pitch of your prop to find the relative loading factor. The larger the number the more load the prop will place on your engine. i.e. An 11/3 prop with a load factor of 3993 will load the engine the exact same amount as a 10/4 with a load factor 4000. 2. If you want more R.P.M. go to a prop with a lower load factor. 3. If you want more speed go to a prop with a higher pitch. If you want more acceleration/vertical speed go to a prop with lower pitch. 4. If you need more thrust you can also go to a larger diameter prop. "
Based on this and your last post, it makes sense to me now. Its really just a way to compare props to each other. There was an article in one of the model mags recently about this.
09-05-2002, 09:02 PM
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.15 size shrike w/OS32 Prop?
Actually if you cube the diameter instead of squaring it the numbers come out different. 9x6 = 4374 and 8x8 = 4096, so the 8x8 would load the engine less than the 9x6, which should mean a gain in RPM. At this point I don't know which is correct, squaring or cubing.
09-05-2002, 09:10 PM
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prop load
It will be different for different engines. Some sites say they use 5th power. The square rule has been proposed for engines of our size. Only way to tell is to test it. I have two jetts, one nelson, four ys, and two os engines. I have just started with these speed planes late this summer. I have alot of props to try out. Plan on collecting data and plotting. Just curious ckoerner, did you tach these props?
09-05-2002, 09:16 PM
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.15 size shrike w/OS32 Prop?
Agreed, testing would be the definitive solution. Although, the artical I found did specifically mention model size props and engines, I didn't paste the whole thing.
