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Feb 01, 2009, 06:32 PM
Registered User
USA, CA, Riverside
Joined Mar 2006
232 Posts
Quote:
 Originally Posted by Rolland Benson I know this is a wierd request, but I need some help on this one. I have built a forward swept wing and am having trouble calculating where the CG should be. I entered the sweep as a negative number and the calculator did not like that. If anyone has some ideas on this Thanks in advance.
You could calculate the CP graphically. I think that's related to what he's doing here.

Mark the root chord at 25%
Mark the tip chord at 25%
Extend the root chord aft by the length of the tip chord.
Extend the tip chord forward by the length of the root chord.
Draw a line from the extended forward end of the tip chord
to the extended aft end of the root chord line.
Draw a line from the root 25% mark to the tip 25% mark.

The intersection of those two lines is an approximation of the center of pressure. You'll probably want your CG forward of that point.

It's only an approximation. I'd guess you'll still need to experiment a little.
Feb 01, 2009, 08:29 PM
Joined Aug 2006
2,228 Posts
Quote:
 Originally Posted by TjW Draw your planform. Mark the root chord at 25% Mark the tip chord at 25% Extend the root chord aft by the length of the tip chord. Extend the tip chord forward by the length of the root chord. Draw a line from the extended forward end of the tip chord to the extended aft end of the root chord line. Draw a line from the root 25% mark to the tip 25% mark. The intersection of those two lines is an approximation of the center of pressure. You'll probably want your CG forward of that point. It's only an approximation. I'd guess you'll still need to experiment a little.
Most of that is right but the spanwise line is from 50% at the root to 50% at the tip. The point where the diagonal constructions meet is called the centroid. The chord that passes through the centroid is commonly called the Mean Aerodynamic Chord. It's just a reference point. Calculating the actual aerodynamic center is very difficult

Some new guys have asked about the labels on the input and output boxes so here's a labeled drawing to shorten the learning curve:

# Images

Last edited by nmasters; Aug 22, 2011 at 05:12 PM.
 Feb 08, 2009, 12:38 AM Registered User Shanghai Joined Dec 2008 5 Posts It's very useful thank U~
 Feb 24, 2009, 08:42 AM the answer 42 is Switzerland, AG, Lenzburg Joined Jan 2006 1,911 Posts Hey nice project, I looking forward for new adds :P, would be nice to have the possibilito of includding some extra panels for bit more complicated wings , and somehow in my case the drawing did not change eventhough I put "0" for sweep :P keep on going EZ
 Feb 25, 2009, 05:25 PM Registered User Laguna Beach, CA Joined Dec 2008 184 Posts Does adding vertical stabilizers that go behind the wing change the correct cg?
 Feb 26, 2009, 07:27 AM Registered User east coast aussie Joined Feb 2009 3 Posts i just built a 48" pluto ( foam slope soarer )and with the c/g in plan was 162.5 mm from nose flying at this c/g was extremely difficult because of the elevators touchiness so added weight slowly it became better to fly but could not pull loops so went back to the plans c/g. your calculator brings it to 11.05 cm not 16.25 as in plan any one familiar with these flying wings some input would be appreciated thanks pluto specs in (cm)(wingspan 120.15)(root cord 29) (tip crd 18)(sweep 110)
 Feb 26, 2009, 07:37 AM Registered User east coast aussie Joined Feb 2009 3 Posts correction on last post (sweep 11)
 Feb 26, 2009, 09:16 AM the answer 42 is Switzerland, AG, Lenzburg Joined Jan 2006 1,911 Posts Hello My CG calculator also gives some 11.05 cm at 25% that is a good starting point for a flying wing. I have couple of questions is the Pluto similar to the Bluto?? does it have a fuse?? With planks It is always good to start wiht the GoG forward and slowly move it back till you get you desire flight condition. Because the forward CoG can be compensated with up trim that is reduced everytime you move the Cog Back. on the other hand for a plank you need to have good servos, that come back to center and do not have to much loose conections. the throws shold be rather small from 5 to 9 mm up and down and the mix usally reduces the elevator funtion of the elevons to 15% to 20%. I uploaded a CoG Calculator to other thread, it had worked for me fro many years. http://www.rcgroups.com/forums/showthread.php?t=1003030 EZ
 Feb 26, 2009, 11:33 PM Registered User east coast aussie Joined Feb 2009 3 Posts yes the pluto looks like a dead copy of the bluto and no fuse does have good servos installed but yes i beleive the plans c/g is way nose light but as said after wieghting the nose to flyable couldnt loop will have to persist i guess
 Mar 04, 2009, 05:08 PM Registered User Joined Jul 2007 9 Posts Cg Hello. Do you consider aileron when you introduce root / tip chord? Regards, Mariano
 Mar 04, 2009, 05:14 PM zoom zoom USA, NJ, Mt Laurel Joined Oct 2007 3,691 Posts i include elevons at tip but not at root to take into account the motor cutout
 Mar 09, 2009, 12:45 PM zoom zoom USA, NJ, Mt Laurel Joined Oct 2007 3,691 Posts Z80, would it have been easier to build this program using VB? I was looking through the HTML and this must have taken you forever to code... Good job man!
 Mar 13, 2009, 06:17 AM Registered User New Zealand, Waikato, Cambridge Joined Mar 2009 95 Posts Nurflugel (Ranis) program I'm new to this forum but I've been chatting with Peter Wick on another forum here in New Zealand and he is going through a tutorial to use the Ranis Nurflugel program for selecting the COG for a flying wing among other things, can follow the link here if you are interested http://rcsoaringcommunities.co.nz/viewtopic.php
 Mar 29, 2009, 08:18 AM Proud to eat Kraut ;-) Germany Joined Dec 2003 5,438 Posts Hi! What about an new feature for this useful program, concerning the following problem: Many wing designs have no straight trailing edge, but rudders which are taped behind, thus leading to a gap in the middle. An additional option to define rudder lenght in wingspan direction, and rudder lenght at both ends in flight direction would solve this problem. Cheers, Julez
 Apr 11, 2009, 11:04 AM Dirt Torpedo Chicago Area Joined May 2008 42 Posts Nice tool, Z80. I'll be using it a lot, making various wings. And thanks for keeping all the code in one HTML file instead of using external JS. Easy for anyone to use it offline by saving the web page. en kunstværk...du folkekær ;-)