Reference/C of G - Finding A Safe CG

<body topmargin=0 leftmargin=0 marginheight=0 marginwidth=0> <table cellpadding=0 cellspacing=0 border=0><tr> <td valign="top" rowspan="2" width=156 BGCOLOR="#FFFFFF" TEXT="#080000" bgproperties="fixed" background="00e9cc00egerdp.jpg"> <div> <B> <STYLE>  </STYLE>  </B></div> <bR> <bR> <bR> <bR> <bR> <div> <B> <TABLE BORDER="0" CELLPADDING="2" CELLSPACING="1" WIDTH="97" BORDERCOLORLIGHT="#C0C0C0" BORDERCOLORDARK="#808080" FRAME="BOX" RULES="ALL" HSPACE="0" VSPACE="0" > <tR> <tD VALIGN=TOP HEIGHT= 313 ><NOBR><div> <A HREF="index.htm" TARGET="_top" TITLE="Home Page"><U><B>Home</B></U></A></div> <div> <A HREF="id2.htm" TARGET="_top" TITLE="Archive"><U><B>Archive</B></U></A></div> <div> <A HREF="id5.php" TARGET="_top" TITLE="Contact"><U><B>Contact</B></U></A></div> <div> <A HREF="id6.htm" TARGET="_top" TITLE="Downloads"><U><B>Downloads</B></U></A></div> <div> <A HREF="id7.htm" TARGET="_top" TITLE="Events"><U><B>Events</B></U></A></div> <div> <A HREF="id8.htm" TARGET="_top" TITLE="For Sale"><U><B>For Sale</B></U></A></div> <div> <A HREF="id9.htm" TARGET="_top" TITLE="Galleries"><U><B>Galleries</B></U></A></div> <div> <A HREF="id14.htm" TARGET="_top" TITLE="Games"><U><B>Games</B></U></A></div> <div> <A HREF="id15.htm" TARGET="_top" TITLE="Links"><U><B>Links</B></U></A></div> <div> <A HREF="id16.htm" TARGET="_top" TITLE="Magazine"><U><B>Magazine</B></U></A></div> <div> <A HREF="id17.htm" TARGET="_top" TITLE="Newsletter"><U><B>Newsletter</B></U></A></div> <div> <A HREF="id18.htm" TARGET="_top" TITLE="Reference"><U><B>Reference</B></U></A></div> <div> <A HREF="id88.htm" TARGET="_top" TITLE="Video"><U><B>Video</B></U></A></div> <div> <A HREF="id19.htm" TARGET="_top" TITLE="Weather"><U><B>Weather</B></U></A></div> </NOBR></tD> </tR> </TABLE></B></div> <bR> </td> <td valign="top" colspan=2 height=100 BGCOLOR="#FFFFFF" TEXT="#080000" bgproperties="fixed" background="00f00640egerdp.jpg"> <bR> <bR> <bR> <div> <I><B>Reference</B></I></div> </td> </tr><tr> <td valign="top" height=380 colspan=2 BGCOLOR="#FFFFFF" TEXT="#080000"> <div> <FONT SIZE="2" COLOR="#000000" FACE="Arial" > <STYLE>  </STYLE>  </FONT><A HREF="id65.htm" TARGET="_top" TITLE="Reference/BMFA C Test"><U><B><<</B></U></A><B>   </B><A HREF="id79.htm" TARGET="_top" TITLE="Reference/CCs to Ozs/Pints"><U><B> </B></U></A><A HREF="id79.htm" TARGET="_top" TITLE="Reference/CCs to Ozs/Pints"><U><B>>></B></U></A></div> <div> <TABLE BORDER="0" CELLPADDING="25" CELLSPACING="1" WIDTH="602" BORDERCOLORLIGHT="#C0C0C0" BORDERCOLORDARK="#808080" FRAME="BOX" RULES="ALL" HSPACE="0" VSPACE="0" > <tR> <tD VALIGN=CENTER HEIGHT= 1698 WIDTH="550"><div> <B>Finding a Safe C.G.</B></div> <div> It's all very well designing and building the model of your dreams but if you have no way of working out the C.G. you're going to be taking an awful risk with the "third of chord" method on that all important first flight. It may be fine on the average sport type model that usually has a long tail moment arm and generous tail area, but on a scale model it could quite easily result in disaster. Here's a method for finding a safe Centre of Gravity position for those critical first flights. I first saw this method twenty years ago in an excellent book by Gordon Whitehead called "Radio Control Scale Aircraft". There's a bit of working out to do but nothing serious.  So get those calculators out and get started. First, here's the formula for quick reference. </div> <DIV ALIGN="LEFT"></DIV> <div> <IMG SRC="36ba0200.png" border=0 width="416" height="32" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <div> All measurements to be taken in INCHES. (Relief all round!) For tailless aircraft and delta's the C.G. is simply the Average Chord divided by 6. The first thing to find is the Average Chord of your wing. For a constant chord wing it's just the chord. But for a tapered or swept wing it's a bit more difficult. They say a picture is worth a thousand words so take a look at this: </div> <DIV ALIGN="LEFT"></DIV> <div> <IMG SRC="20a37e30.jpg" border=0 width="311" height="227" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <div> Simple enough. For biplanes, triplanes etc the Average Chord is measured from the Leading Edge of the leading wing to the Trailing Edge of the trailing wing. View the two wings in planform as one, and if there's taper or sweep apply the method above. Another pic' I think! </div> <DIV ALIGN="LEFT"></DIV> <div> <IMG SRC="36d0bce0.jpg" border=0 width="267" height="206" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <div> The red line denotes how we must view the two wings as one  for the purposes of the Average Chord measurement. For the Wing Area part of the calculation you must add together the combined areas of all wings. For eliptical wings divide the Wing Area by the Wing Span to give the Average Chord. Next is Wing Area and Tail Area. Simply multiply the Wing Span by the Average Chord. Next is the Tail Moment Arm. To get this figure measure the distance between a quarter of the Average Chord of the wing to a quarter of the Average Chord of the tail. Work out the average chord for your tailplane in exactly the same way as the wing. More pix!</div> <DIV ALIGN="LEFT"></DIV> <div> <IMG SRC="20e74eb0.jpg" border=0 width="372" height="235" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <div> Now we have all the relevant information it's time to apply the formula. I'll use make believe values that apply to that lovely little aircraft above.</div> <div> Span = 60 inches.</div> <div> Average Chord = 10 inches.</div> <div> Wing Area = 600 sq inches.</div> <div> Tail Area = 108 sq inches.</div> <div> Tail Moment Arm = 25 inches.</div> <DIV ALIGN="LEFT"></DIV> <div> ................................</div> <div> First - Average Chord divided by 6:  10 divided by 6 = 1.66</div> <div> Second - Tail Area multiplied by 3:   108 multiplied by 3 = 324</div> <div> Third - 3 x Tail Area multiplied by the Tail Moment Arm:  324 multiplied by 25 = 8100</div> <div> Fourth - 8 x Wing Area:  600 multiplied by 8 = 4800</div> <div> Fifth - 8100 divided by 4800 = 1.68</div> <div> Sixth - 1.66 + 1.68 = 3.34. <FONT SIZE="3" COLOR="#006600" FACE="Arial" > </FONT></div> <DIV ALIGN="LEFT"></DIV> <div> <IMG SRC="36fab1f0.png" border=0 width="427" height="31" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <div> So the Centre of Gravity for our model is 3.34 inches from the leading edge when measured along the average chord line. You will notice, of course, that this position is almost exactly one third of the average chord!! Ho hum...</div> <div> Why not download the C.G. Calculator program on the <FONT SIZE="2" COLOR="#0000CC" FACE="Arial" ><A HREF="id6.htm" TARGET="_top" TITLE="Downloads"><U>Downloads</U></A> </FONT>page? <I>G :-)</I></div> <div> FF 11.03.01.</div> </tD> </tR> </TABLE></div> <div> <A HREF="id65.htm" TARGET="_top" TITLE="Reference/BMFA C Test"><U><B><<</B></U></A><A HREF="id65.htm" TARGET="_top" TITLE="Reference/BMFA C Test"><U><B> </B></U></A><B>  </B><A HREF="id79.htm" TARGET="_top" TITLE="Reference/CCs to Ozs/Pints"><U><B> </B></U></A><A HREF="id79.htm" TARGET="_top" TITLE="Reference/CCs to Ozs/Pints"><U><B>>></B></U></A></div> <bR> </td> </tr></table></body>