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Old Jun 12, 2003, 03:01 AM
John Houvener
John Houvener
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[EFLT] watts needed

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I see where inquirey was made and question raised as to how many watts needed for certain size airplane, and if any charts are available. I think any chart to cover all case's would be five feet long, however, if a modeler is really serious about sizeing the correct motor and prop for the airplane he has in mind there are some real handy formulas available--contact Ken Myers who is president of the EFO and also the NL editor of the Ampeer newsletter which you can get via subscription or down load by your kornputer. Contact Ken at If you go this route get yourself a calculator that will give you sq. rt., and cube rt. and also will have the X2 button on it. Poke X2 twice for 4th power. The casio model fx-300W is available at a K-mart store at a reasonable price. Ask Ken for the formulas from the Mar. 2001 issue.

I will give you a quick example of an airplane to size up from one of his formula examples--670 low-wing, mid-wing or high wing higher performance type.
Desired target wing loading--cube rt. of 670x2.5=21.88 oz/sq. ft.
" " weight=670/144x21.88=101.78 oz or 6.36 lb.
Output power required=55(watts lb.) x 6.36=350 watts .This is watts at the prop which is the true power required. Your Astro watt meter which you connect between the battery and ESC will give you watts in. I divide the output watts needed by the eff% of the motor to give me the watts in needed. I use .85% for a brushless motor, .75% for a good cobalt brushed and .65%(hopefully) for a brushed can ferrite motor. I will use a Astro cobalt in above example, therfore 350 watts / .75=466.66 watts in needed. now as to the prop needed to do this.
Prop Dia.-(sqrt.((101.78x1.15)/Pi))x2=12.2 rounded to 12
Required RPM=1/3(means cube rt.) (350/((12/12)x2-x2x(9/12)x1.31))=7.089KRPM or 7000 therfore find a motor that will turn a 12x9 prop 7000 plus RPM. This is minimum for higher performance airplane, I probaly would look for about 550 watts in, to allow for various makes of props.

His formula does go on to show you what power system wt., radio system and completed airframe wt.(30.53 oz.) should be. You can use different motor charts, E-calc or Moto-calc to check out motors to use. Using the above example, we find that 466.66 watts in / 6.36 lbs.=73.37 watts per lb. in. Also use the (-(( signs exactley as shown. / means divide, x means multiply, poking X2-X2 will give you 4th power.

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