|May 11, 2012, 04:34 PM|
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.
|May 11, 2012, 05:10 PM|
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
|May 11, 2012, 06:17 PM|
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
PS Don't use the numbers given till the wing chords/sweeps and V-Tail angles are confirmed!
|May 11, 2012, 08:13 PM|
Thanks everyone for the CG help. Combined with advice from my mirror build threads it seems most folks are recommending a CG of around 3¼" 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...
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!
|May 11, 2012, 08:43 PM|
|May 12, 2012, 10:46 AM|
Joined Nov 2011
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.
|May 12, 2012, 11:14 AM|
Portugal, Faro District, Almancil
Joined Feb 2008
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
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