HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old May 11, 2012, 03:34 PM
Build something.
Nodd's Avatar
United States, CT, Fairfield
Joined Mar 2012
669 Posts
I Need Help with the CG
Okay I'm trying to figure out where my CG needs to be. I've visited half a dozen online CG calculators, plugged in my numbers & of course they don't seem to agree with each other...

Wing Only CG Calculator Results
Calc A: 1.71" back from LE
Calc B: 1.71" back from LE
Calc C: 1.71: back from LE

Wing & Tail CG Calculator Results
Calc D: 4.48" back from LE
Calc E: 3.22: back from LE

Here's the glider's data if you wanted to give this a try...

Wing Root Chord: 9"
Wing Tip Chord: 4"
Wing Sweep: 0
Wing Half Span: 55"
Tail Root Chord: 7.25"
Tail Tip Chord: 3.75"
Tail Sweep: 8"
Tail Half Span: 12"
Distance between Wing LE & Tail LE: 32.5"
Please enter what ever %MAC or Static Margin you feel would be good.

I'd appreciate help with this as I'd like to avoid having an iffy CG for the maiden flight. Thanks.
Nodd is online now Find More Posts by Nodd
Reply With Quote
Sign up now
to remove ads between posts
Old May 11, 2012, 03:51 PM
Theoretical Modeler..
plane_tech's Avatar
Joined Oct 2007
2,083 Posts
A general rule of thumb that I use is 25%-30% of the wing chord at the root.
plane_tech is online now Find More Posts by plane_tech
Reply With Quote
Old May 11, 2012, 03:51 PM
Registered Idiot
Keenan smith's Avatar
United Kingdom, London
Joined Dec 2010
6,824 Posts
Put at 3.2inches.. From the LE That's by my calcs seems right and by the chuck glider of my venom...
Keenan smith is offline Find More Posts by Keenan smith
Reply With Quote
Old May 11, 2012, 03:56 PM
Balsa builder and balsa basher
RC Trainer plane's Avatar
Joined Apr 2012
683 Posts
I would have thought the CG is about a cm behind the main spar
RC Trainer plane is offline Find More Posts by RC Trainer plane
Reply With Quote
Old May 11, 2012, 04:10 PM
Registered User
boingk's Avatar
NSW, Australia
Joined Feb 2011
2,120 Posts
Quote:
Originally Posted by Nodd View Post
Yes electronics can have all the programmable features in the world but without a programming manual what's the point? I love HobbyKing but sometimes, well umm, yeah. Anyway thanks once again for the advice. I spoke with our LHS RC guru about your 1 amp per servo rule & how you felt 3 amps wasn't going to cut it for a 100" full house e-glider. All I can say is he raised one eyebrow, Spock style & scoffed at the idea. Still I agree its better to air on the side of caution. More amps than I probably need = one less thing to worry about. Thanks again for the help.
My Blue Series actually came with a manual, go figure! Probably because they're a direct clone of Mysterys product.

No worries on the advice, I've been flying for a year and building for most of it - I know experience is usually earnt the hard way!

Anyway, my thoughts on servos/RX circuits is that all it takes is ONE servo out of the six to load up, jam or jitter and then you could push the circuit over the edge. I've seen this happen on the ground during testing due to a stiff pushrod/sleeve pair and its not fun! Overkill on the current allowance and you'll strip a servo long before you blow the BEC up.

As far as the linear/switch debate goes, switchmode BEC is far superior and yeah you'd most likely get away with 3A. Still, a switch-mode 5A is even better and the weight penalty is marginal.

Cheers mate - boingk
boingk is offline Find More Posts by boingk
Reply With Quote
Old May 11, 2012, 05:17 PM
Red Merle ALES
Curtis Suter's Avatar
United States, Mt, Helena
Joined Apr 2002
5,334 Posts
I put your numbers into Sailplane Calc and got 3.68" aft of the root leading edge. This is assuming the model has no trailing edge or leading edge sweep, which I think it does but the numbers weren't given. Sailplane Calc only needs leading edge sweeps and chord changes along the wing and it'll automatically calculate trailing edge sweeps. This would make a big difference. I also didn't know the angle of the V's which of course affects the size of your horizontal stab if you will, so I chose 100 degree angle between the V's.

I've attached the Excel spreadsheet, it's in a .zip file so you'll need to download it then extract it. It works fine with Free Open Office but I used Excel to put it together. The formulas are from Dr. Simons book Model Aircraft Aerodynamics. I've used this spreadsheet for close to 18 years or so and it's proven quite accurate, assuming the proper numbers are input.

If will give you a good balance point to start a safe first test flight. Once you have your trims and flight controls sorted then you can perform the dive test to fine tune your balance point further aft.

Instructions are on the "Articles/Files" page at www.TailwindGliders.com

Curtis

PS Don't use the numbers given till the wing chords/sweeps and V-Tail angles are confirmed!
Curtis Suter is online now Find More Posts by Curtis Suter
RCG Plus Member
Reply With Quote
Old May 11, 2012, 07:13 PM
Build something.
Nodd's Avatar
United States, CT, Fairfield
Joined Mar 2012
669 Posts
Thanks everyone for the CG help. Combined with advice from my mirror build threads it seems most folks are recommending a CG of around 3" back from the LE. That looks right to me & apparently the numbers back it up. I added a little over an ounce of lead to the tail & she's now sitting pretty on my CG stand.

The total flying weight came out to 54 oz. That gives me a wing loading somewhere in the middle for a thermal duration ship. I'm quite happy with that, was afraid I'd built a flying brick but I guess not. Here's the numbers...

Wing Loading
11.05 oz/sq.ft
4.96 oz/cubic.ft

I want to give out a HUGE THANKS to everyone who's contributed to this build. Couldn't have done it without you. Maiden flight this weekend!
Nodd is online now Find More Posts by Nodd
Reply With Quote
Old May 11, 2012, 07:20 PM
Registered Idiot
Keenan smith's Avatar
United Kingdom, London
Joined Dec 2010
6,824 Posts
When do you predict your maiden to be?
BTW what do ya think of my venom So Far...
Keenan smith is offline Find More Posts by Keenan smith
Reply With Quote
Old May 11, 2012, 07:43 PM
Build something.
Nodd's Avatar
United States, CT, Fairfield
Joined Mar 2012
669 Posts
Quote:
Originally Posted by k.smith459 View Post
When do you predict your maiden to be?
BTW what do ya think of my venom So Far...
Like I said, maiden this weekend.

I'd be happy to discuss your build on your build's thread, not here.
Nodd is online now Find More Posts by Nodd
Reply With Quote
Old May 11, 2012, 07:45 PM
Registered Idiot
Keenan smith's Avatar
United Kingdom, London
Joined Dec 2010
6,824 Posts
Ok ... ill be Waiting to See the maiden..


Sorry Me being Random...
Keenan
Keenan smith is offline Find More Posts by Keenan smith
Reply With Quote
Old May 12, 2012, 01:50 AM
Balsa builder and balsa basher
RC Trainer plane's Avatar
Joined Apr 2012
683 Posts
Waiting for the maiden
RC Trainer plane is offline Find More Posts by RC Trainer plane
Reply With Quote
Old May 12, 2012, 09:46 AM
Joined Nov 2011
89 Posts
Quote:
Nodd Thanks everyone for the CG help. Combined with advice from my mirror build threads it seems most folks are recommending a CG of around 3" back from the LE. That looks right to me & apparently the numbers back it up. I added a little over an ounce of lead to the tail & she's now sitting pretty on my CG stand.

The total flying weight came out to 54 oz. That gives me a wing loading somewhere in the middle for a thermal duration ship. I'm quite happy with that, was afraid I'd built a flying brick but I guess not. Here's the numbers...

Wing Loading
11.05 oz/sq.ft
4.96 oz/cubic.ft
Nodd I have been watching your build. Very good Sir. Thank you for the all up weight, I have been wanting to run the numbers. I was a little worried that your construction was on the beefy side.
I ran you numbers and have calculated that, yours and mine are very close with one addition. You should have a stall speed of +/- 19mph. A nice slow floater.
I would say that you should have the CG spot on. In my experience CG ends up being between 3" and 3" 1/2 right handy of the main spar. Nose heavy = front of spar and tail heavy = backside of the spar.
Before the maiden I like to test the CG. If your an athlete try running and lobbing the plane in the air catching it to see which way it wants to pitch or you can give a toss from knee height.
I hope you take video of the maiden. Maiden videos are always the favorite out of any other flight video except for well, um, maybe, nah I'm not going to be the one to say it. Best of luck.
scallopking is offline Find More Posts by scallopking
Reply With Quote
Old May 12, 2012, 10:14 AM
Registered User
Portugal, Faro District, Almancil
Joined Feb 2008
306 Posts
Absolutely brilliant design there . PLEASE PLEASE PLEASE can you send me some kind of plan for this stunning plane? Been searching for a gull winged boom and pod vtail for ages and its impossible to find one. Already got all my materials just need a design lol. Keep up the good work
MAT1978 is offline Find More Posts by MAT1978
Reply With Quote
Old May 12, 2012, 10:42 PM
Build something.
Nodd's Avatar
United States, CT, Fairfield
Joined Mar 2012
669 Posts
Maiden Flight Video
Well you've all been requesting this so here it is. Enjoy...

Nodd RC - 009 - Sailplane Maiden Flight (6 min 55 sec)
Nodd is online now Find More Posts by Nodd
Last edited by Nodd; Nov 11, 2013 at 11:04 PM.
Reply With Quote
Old May 12, 2012, 11:18 PM
Theoretical Modeler..
plane_tech's Avatar
Joined Oct 2007
2,083 Posts
Nicely done!
plane_tech is online now Find More Posts by plane_tech
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Scratch-built 28" FFF Tiger Moth biplane (first scratch build) mclarkson Foamies (Scratchbuilt) 20 Apr 04, 2012 02:39 PM
Build Log Designing for scratch-built FPV wing... PiperCub49 Flying Wings 15 Mar 09, 2012 09:15 PM
Build Log Scratch built own design foamie... "Whiplash" glydr Australia 30 Jul 09, 2006 05:28 AM
Question Scratch built design? Perseus Large Scale Cars 2 Dec 11, 2005 05:02 PM
How do you design your scratch built jets? Florin777 Electric Ducted Fan Jet Talk 17 Mar 18, 2003 05:34 PM