5=94x5=94 is a bit on the big side, it would take over 500hp = to turn it at 2000rpm.

I=92ve just made up a 54=94long by 3.6=94 = 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. =A0A typical prop is. . .what?. . .3"x= 8" at the hub, for a total cross-section of 18sq.". =A0A 5"x= 5" beam would give you 25sq.", and the typical prop is quickly na= rrowed down to a very flat section. =A0And your not worried about weight, s= o you could use something strong but heavy, like oak or hickory.

But, you could also use an aluminum bar. =A0McMaster-Carr has a 5"x5&q= uot;x6' 6061 aluminum bar for $700 (http://www.mcmaster.com/#aluminum/= =3D933sgh). =A0A 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. =A0Package (crate?) it correctly, have it validated against a stan= dard dyno, and it could be rented out to other builders.

A composite bar could be built in a few days. =A0Again, 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", ra= ther than going batsh!7 crazy, just to save materials. =A0Either way, it wo= uld 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??? =A0If 191 HP = can do that I will be surprised, but would not want to be close enough to w= itness it! =A0[:<O

=A0
Bill B

=A0
------------------------------------------------------------------------
*From:* Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] *On Behalf= Of *Andrew Martin
*Sent:* Saturday, September 25, 2010 11:56 AM
*To:* Rotary motors in aircraft
*Subject:* [FlyRotary] simple dyno

=A0
Whilst researching propellers, I came across this gem of info from www.jcpropellerdesi= gn.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 prop= eller with a square cross section of 1/15th of the length of the bar, you c= an measure the rpm and calculate the HP available.

Power in HP =3D 0.0532 (diameter/450)5 (RPM/1000)3

=A0
As I need a load on the engine when I run it I=92m going to give this a go,= 1350 x 90mm at 7000 rpm will give me 191hp, that=92ll do if I can get it.<= br> --
Regards

Andrew Martin
Renesis RD1c

West Australia

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Regards

A= ndrew Martin
Martin Ag
275 Newmarracarra Rd
Moonyoonooka WA 6532
0427477144
08 99241145
andrew@martinag.com.au