PDA

View Full Version : Magpie Question-How to Set CG?


jatekman
04-03-2007, 06:45 AM
I finished building my 2nd plane, the Magpie, after learning on the Hobbico Sky Fly. How, exactly, do I "set" the Center of Gravity? According to instructions it should be .25" to .50" from the wing spar. So do I mark this location under the wing and hold a finger on each side of the wing and move the Li-Po battery until it balances correctly? (This is how I checked the balance on the Sky Fly, but the battery could only go in one spot on that plane and I simply added a few coins to the tail to balance). I attempted to maiden the Magpie w/ the battery mounted on the nose of the fuse after reading about another fellow wattflyer's success in doing so, and it seemed to be a good location; however at hand-launch take off I sort of froze when I heard the motor cut out for a second upon full throttle application so I cut all power (wasn't smart) and the plane dropped like a rock straight down (which made me think it may be nose heavy?). Luckily it did not damage the motor shaft and after I cleaned it up a bit it and put a new prop on it seems to still be working fine, but now I'm extremely hesitant to try again. I experimented w/ different battery locations and how the plane would balance w/ the battery on the right side of the plane but even if I moved it almost completely to the front side of the fuse it still seemed a little tail heavy. So after much trial and error, the top of the nose fuse still seems to be the best bet for the battery-it balances perfectly there if I am doing it correctly (just holding the plane up by a finger under each wing at the spot explained above). I've also tested the motor and it seems to be fine, so maybe if I hadn't let off power upon the cut-off I thought I heard, it would have been ok, but the plane still seemed more "3D-ish" compared to the ol Sky Fly and not as stable on take off, so I'm a little afraid to try it again in fear of slamming straight down into the ground again and bending the shaft. Is the top of the nose a good spot for the battery in terms of balancing? I also checked the lateral balance and added a coin into a slit in the wing tip per the instrucitions and it seems ok, too.

Thanks,
John

Murocflyer
04-03-2007, 07:14 AM
John,

The correct CG is probably the most important thing you need to be concerned about on a maiden flight. Proper control surface trimming won't do any good if the plane is not able to fly if the CG is off.

Finger tip balancing is OK, but your best bet is a CG Tool. You can either make one (like the picture below) or buy one like this one here (http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXK259&P=7). I found it much easier, faster, and cheaper to make one.

First and foremost, verify the CG mark/location in the instructions.

The easiest way to do CG checks is to mark the wing at the recommended CG location, set the battery where it is "supposed to go" and set the plane gently on your CG tool. Then slowly remove your hand. If it sits level, great! Consider yourself very fortunate. If the nose falls forward, then slide the battery back some. If the tail falls first, then slide the battery forward slightly and re-check. If the battery can't be moved then throw that plane way and buy a new one. Just kidding. You will have to add small amounts of weight to the opposite end of the plane that drops first.

Once you get the hang of it, it'll be easy.

Hope that helps and good luck. Let us know how it goes.

Frank

PS Posts are easier to read if you break them up in smaller paragraphs instead of one big one -at least it is for me anyway.

PPS How 'bout some pics of you Magpie?

Grasshopper
04-03-2007, 07:16 AM
I finished building my 2nd plane, the Magpie, after learning on the Hobbico Sky Fly. How, exactly, do I "set" the Center of Gravity? According to instructions it should be .25" to .50" from the wing spar. So do I mark this location under the wing and hold a finger on each side of the wing and move the Li-Po battery until it balances correctly? (This is how I checked the balance on the Sky Fly, but the battery could only go in one spot on that plane and I simply added a few coins to the tail to balance). I attempted to maiden the Magpie w/ the battery mounted on the nose of the fuse after reading about another fellow wattflyer's success in doing so, and it seemed to be a good location; however at hand-launch take off I sort of froze when I heard the motor cut out for a second upon full throttle application so I cut all power (wasn't smart) and the plane dropped like a rock straight down (which made me think it may be nose heavy?). Luckily it did not damage the motor shaft and after I cleaned it up a bit it and put a new prop on it seems to still be working fine, but now I'm extremely hesitant to try again. I experimented w/ different battery locations and how the plane would balance w/ the battery on the right side of the plane but even if I moved it almost completely to the front side of the fuse it still seemed a little tail heavy. So after much trial and error, the top of the nose fuse still seems to be the best bet for the battery-it balances perfectly there if I am doing it correctly (just holding the plane up by a finger under each wing at the spot explained above). I've also tested the motor and it seems to be fine, so maybe if I hadn't let off power upon the cut-off I thought I heard, it would have been ok, but the plane still seemed more "3D-ish" compared to the ol Sky Fly and not as stable on take off, so I'm a little afraid to try it again in fear of slamming straight down into the ground again and bending the shaft. Is the top of the nose a good spot for the battery in terms of balancing? I also checked the lateral balance and added a coin into a slit in the wing tip per the instrucitions and it seems ok, too.

Thanks,
John


Hi John,

It sounds like you have the right idea. You can use your fingers or a couple of pencils with the erasers pointing up. It might have "dropped like a rock" because it stalled for lack of airspeed instead of the CG being wrong. I have a Magpie AP and it is balanced at .25 behind the spar. You can either move the battery up to the front of the nose as you mentioned or you would have to move it back and add weight to the nose to get it balanced. If you do put the battery on top of the nose, I would make sure it is strapped on good so it can't slide off to one side or shift forward or back.

Once you have the battery strapped in place, check the CG again to make sure it still balances. I put a pencil mark on the fuselage to line up the front of the battery so I get it in the same place very time.

You also need to find out why the motor cut out at launch. It shouldn't cut off at any point (unless you hit the LVC). Check all your connections and make sure you're not over amping the motor or ESC too.

AEAJR
04-03-2007, 12:18 PM
I purchased the CG machine. I will use finger tips to test but I do my REAL balacing on the CG machine.

Solid Hit
04-03-2007, 03:31 PM
I purchased the CG machine. I will use finger tips to test but I do my REAL balancing on the CG machine.
I agree. I had been putting off buying one opting for the finger tip or pencil method instead but after seeing how much more precise the CG machine can be I'm finally going to buy one.

It needn't be a "must have" like a watt meter is but something to pick up along the way.

BTW, all the above advise is excellent.

Murocflyer
04-03-2007, 10:20 PM
No need to buy one unless you really want to. I'm sure you have all that you need right in your garage.

Frank

smokejohnson
04-04-2007, 12:08 AM
FWIW I had a scratch built foamie that I knew I had the set up right on but did not fly right. I did the glide test in some tall grass and ended up moving the COG almost an inch. Flew great after that. I never did figure why my COG was different from the plans, didn't really care.;) I will use the glide test for all my planes from now on out until I am a more experienced builder/pilot.

Murocflyer
04-04-2007, 06:03 AM
That's true Smoke. I have found that the CG mark can be a little off from what is called out in the plans. Best bet is to get or make a CG tool, make sure it balances at the recommended mark, do a search on the forum(s), look for any CG issues posted by other guys, and if not, fly her first like that. You can make small adjustments from there to your liking.

For example, I have a Gambler AG (DLG) which I set up to balance at the recommended CG. Flew it and it flew OK. Moved the CG mark back about .375" and it flew much better.

You have to adjust to your tastes. Another good example is the CG mark on the SS. The SS is a good plane to experiment with how the CG affects an airplane's performance.

Frank

jatekman
04-04-2007, 06:45 AM
Thanks for all of the replies-the information you guys provide is greatly appreciated!

Grasshopper, you are right, I need to do a much more thorough pre-flight check next time and also test the motor/ESC like you said...do I need a watt meter to test to see if I am over amping anything? I assumed everything would be ok because the brushless set-up I am using was recommended by Mountain Models.

I think excitement took over when I saw the perfect weather day and I bolted out the door thinking only of flying. It all happened way too fast! Using 20/20 hindsight, I should've maintained full throttle even after hearing the "skip" in the motor instead of immediately shutting down, maybe that way I could've at least had a more gentle return to the ground, but I didn't want to ruin any of the electronics...live and learn I guess.

Thanks again to everyone for providing helpful info for uninformed newbies,

John

Murocflyer
04-04-2007, 07:07 AM
John,

A watt meter (http://stevensaero.com/shop/product.php?productid=16705&cat=0&page=1)is a very useful tool. It allows to you to mix and match motor, battery, prop, ESCs without the risk of burning something up. You don't have to have it if you stick to recommended setups, but how many folks do for long? Pretty soon you will want to be experimenting with different combos, and then you will need one for sure.

Frank

AEAJR
04-04-2007, 11:03 AM
=

Grasshopper, you are right, I need to do a much more thorough pre-flight check next time and also test the motor/ESC like you said...do I need a watt meter to test to see if I am over amping anything? I assumed everything would be ok because the brushless set-up I am using was recommended by Mountain Models.
==

Thanks again to everyone for providing helpful info for uninformed newbies,

John

If you are using a set up recommended by MM you can be confident it is properly balanced. But I would encourage you to get a wattmeter when you can afford one. Very very valuable tool.

Murocflyer
04-08-2007, 09:35 AM
John,

Any updates? How did it go?

Frank