Subject: centrifugal force calc
From: Paul
Date: 3/15/2001, 7:24 PM

Paul wrote:


John Lynch wrote:


Hi Paul,
I was just reviewing your recent e-mail to Steve regarding propeller centrifugal force calcs.  We have been doing this ourselves and have come to considerably different numbers than you have.  Our reference for this work is Airplane Propeller Principles by Wilbur C. Nelson from 1943.  In his version of the blade centrifugal force calculation, he calculates "delta C.F." which is the blade centrifugal load per inch of blade length.  These blade loadings are then plotted against radius, and the area under this curve is the centrifugal force.  We used ANVIL 1000MD's area analysis to plot and calculate the area under the curves.  Our analysis (matches his) and results in a total centrifugal force of about 14,000 lbs for a 7 pound composite blade, and a force of just over 40,000 lbs for a 14 pound aluminum blade (both 34 inches long at 3000 rpm).  I'm attaching an excel spreadsheet showing the calculation.  One thing that tripped us up was the use of mass of these segments rathe!

r than

weight. The volume x density of each 1" segment was then divided by 386 inches/sec^2 (same as 32.16 ft/sec^2 but converted to inches).  You might notice also that the total centrifugal force is actually maximum at the blade root and minimum at the tip (as each segment must "hang on" to the ones outboard of it).  Your table shows this going the other way.


Regards,
John




Thanks John. Looks like I got it all wrong. Will take another look at
my program source code when I get a chance. The prop shank we are building is
rated for a 25,000 pound load at 14,000 psi stress level so it looks like
we are still OK.

Paul Lamar


Vance can please you look at this for me and tell me where I went wrong?

Paul Lamar
 
The AirCraft Rotary Engine NewsLetter.              Powered by Linux.

http://home.earthlink.net/~rotaryeng/            http://www.linux.org

blade-cent-force.bas
'calculates the centrufugal force on the blades
'as function of the radius.

OPEN "centforc.dat" FOR APPEND AS #1
CLS
dia = 70                     'prop dia in inches

weight = 6                   'blade weight in pounds
deltaweight = weight / (dia / 2)      'weight per inch of blade
RPM = 3000

PRINT #1,
PRINT #1, DATE$,
PRINT #1, TIME$
PRINT #1,

PRINT #1, "Total weight of blade "; weight
PRINT #1, "Constant cord weight of blade per inch "; deltaweight
PRINT #1, "Diameter of prop"; dia
PRINT #1, "            RPM"; RPM
PRINT #1,

PRINT #1, "radius in      MPH       force          totalforce"
    PRINT "radius in      MPH       force          totalforce"

FOR radius = 1 TO 35

PRINT #1, radius,
PRINT radius,

mass = deltaweight / 32                            'pounds  per inch

vel = ((((radius * 2) * 3.1416) / 12) * RPM) / 60   'in feet per second
PRINT USING "####"; vel * (60 / 88);
PRINT #1, USING "####"; vel * (60 / 88);

force = mass * (vel) ^ 2 / radius

PRINT USING "##########"; force;
PRINT #1, USING "#########"; force;
totalforce = totalforce + force

PRINT USING "##########"; totalforce
PRINT #1, USING "##########"; totalforce
NEXT
CLOSE
SYSTEM

SHANK.gif