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5”x5” is a bit on the big side, it would take over 500hp to turn it at 2000rpm. I’ve just made up a 54”long by 3.6” square laminated beam, cost all of $30, calculation shows it should absorb 190hp at 2450 prop rpm, only going to be used for a short while on initial startup so no need to go overboard.
Andrew On Fri, Oct 1, 2010 at 10:10 PM, Ernest Christley <echristley@att.net> wrote:
Why would it need to be wood?
Not that wood would be lacking. A typical prop is. . .what?. . .3"x8" at the hub, for a total cross-section of 18sq.". A 5"x5" beam would give you 25sq.", and the typical prop is quickly narrowed down to a very flat section. And your not worried about weight, so you could use something strong but heavy, like oak or hickory.
But, you could also use an aluminum bar. McMaster-Carr has a 5"x5"x6' 6061 aluminum bar for $700 (http://www.mcmaster.com/#aluminum/=933sgh). A 4"x4" of 6061 is $450, and a 4"x4" of 2024 is $1060. Kind of expensive, but $700 won't buy much dyno time either. Package (crate?) it correctly, have it validated against a standard dyno, and it could be rented out to other builders.
A composite bar could be built in a few days. Again, since weight is not a concern, you could make it a solid layup, but you'd probably want to limit the wall thickness to something simply ridiculous like 1/2", rather than going batsh!7 crazy, just to save materials. Either way, it would probably hold the weight of a dump truck without blinking.
Bill Bradburry wrote:
Over my head!
A 75 inch log that is 5 inches square swinging at 7000 rpm??? If 191 HP can do that I will be surprised, but would not want to be close enough to witness it! [:<O
Bill B
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*From:* Rotary motors in aircraft [mailto: flyrotary@lancaironline.net] *On Behalf Of *Andrew Martin
*To:* Rotary motors in aircraft
*Subject:* [FlyRotary] simple dyno
Whilst researching propellers, I came across this gem of info from www.jcpropellerdesign.com <http://www.jcpropellerdesign.com>
Gustave Eiffel (1832-1923) came up with a simple solution for measuring HP without complex apparatus, By using a /balanced /wood bar instead of a propeller with a square cross section of 1/15th of the length of the bar, you can measure the rpm and calculate the HP available.
Power in HP = 0.0532 (diameter/450)5 (RPM/1000)3
As I need a load on the engine when I run it I’m going to give this a go, 1350 x 90mm at 7000 rpm will give me 191hp, that’ll do if I can get it.
--
Regards
Andrew Martin
Renesis RD1c
West Australia
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Homepage: http://www.flyrotary.com/
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-- Regards Andrew Martin Martin Ag 275 Newmarracarra Rd Moonyoonooka WA 6532 0427477144 08 99241145 andrew@martinag.com.au
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