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ae5x
03-21-2006, 11:16 PM
Well I got my 1st plane (Slow Stick) in yesterday's mail and spent a few hours putting it together at work today (great job, eh!). All that's left is to rework the control linkages so that the rudder & elevator are in their neutral positions when the transmitter's sticks are centered - and I have to balance it.

The manual says the CoG should be 3 3/4" from the wing's LE. How do I determine if the CoG is at that point, ie what am I looking for. Do I place each side of the wing on some object that touches the wings only at that particular measured distance from the LE and then check that it stays there w/o sliding or tilting fore or aft? Or do I want a slight upward tilt...? Or is this measurement just a ballpark guesstimate that I'm making too big of a deal of at this point?

Thanks for the help,

Don Sims
03-22-2006, 12:43 AM
You've got the general idea of the plane being balanced at that point. For my planes I mark the underside of the wing on both sides then use two fingers at that point if I'm at the field. At home, I use a Great Planes CG machine.

As I recall on a Slowstick, be sure the leading edge of the wing is in the correct spot also!

panzerd18
03-22-2006, 05:21 AM
On most planes the Center of Gravity is just forward of the Center of Pressure.

flypaper 2
03-22-2006, 09:43 PM
Balance as said above at the 3 3/4 point and the fuse should be level or slightly nose down. Move the battery back and forth or add weight to get it.

TManiaci
03-23-2006, 01:20 AM
How do I determine if the CoG is at that point, ie what am I looking for. Do I place each side of the wing on some object that touches the wings only at that particular measured distance from the LE and then check that it stays there w/o sliding or tilting fore or aft? Or do I want a slight upward tilt...? Or is this measurement just a ballpark guesstimate that I'm making too big of a deal of at this point?

When the plane settles to a level "flying attitude" you are on the CG point. On smaller planes, it's good enought to do this with your finger tips. When you get close, it won't slid on your fingertips anymore. If you are off an 1/8" or so, it's not really that critical. Use the battery location to adjust. In any case, a plane will fly easier if it is slightly nose-heavy. Tail heavy can make a plane pretty difficult to fly, but generally more aerobatic. I usually maiden on or slightly in front of the factory CG point, as they are pretty good about giving a conservative balance point.

adhoc
06-21-2006, 07:54 PM
Okay, as someone who started a couple of months ago, on my third plane, a few thoughts...

Maybe this thread should be made sticky; as I've done some searches, there is no clear thread on how to measure/set CG (this thread is the closest I've found)...
On low wing planes, flip the plane upside-down, and balance the tops of the wings on your fingers (or whatever)... trying to do it from under a low wing is very tricky/touchy...
Re: recommended CG vs a little forward: My Multiplex high wings do in fact fly better a little nose-heavy, but when I maidened my GWS Funny Park (a low wing) that way, it was kinda fast and wanted to dive; when I backed off to recommended CG, it flew fine. So don't second guess the factory recommended CG... like the man said, they are pretty conservative, so use them as a starting point.
One thing I've noticed is that it is somewhat tricky to get both balance points (fingers, etc.) lined up evenly (the same distance from the leading edge of both wings). Seems like it is relatively easy to line up the one closest to your eye, but the one on the other side of the plane is then hard to eyeball. This could be an argument for NOT using your fingers, but balancing on something stationary, so you can walk around to both sides of the plane and make sure both balance points are the same. Any hints on good ways to do this?

Solid Hit
06-21-2006, 08:50 PM
This could be an argument for NOT using your fingers, but balancing on something stationary, so you can walk around to both sides of the plane and make sure both balance points are the same. Any hints on good ways to do this?

I've got a 4x4 (heavy for the balance) with 2 holes drilled for the insertion of two pencils. I use pencils due to the eraser tops. That takes the place of my fingers without the cost of a "machine" to do the same thing.


Rather then the 4x4, a 2x2 with legs will do the trick. A friend of mine uses his Workmate and clamps the pencils in that. Just be sure to have the pencils at the same height above the frame you use.

adhoc
06-21-2006, 09:15 PM
Okay, here's a more fundamental issue. Seems to me that you can take one of two approaches to balancing CG...

(1) Try to balance the plane at the desired CG point, see which way it tips; adjust weight to correct; repeat...

(2) Find the actual CG by moving where the plane touches the balance points (fingers, etc.) until the plane balances; see how far that is from the desired CG point; adjust weight to correct; repeat...

I've done it both ways. Method (1) takes the philosophy of "I don't care where the actual CG is; I only care which way it is off...", and is simpler. Method (2) is a little more work, but gives you more information about how far off you are (which gives you hints about how to best adjust it)... Thoughts?

redgiki
06-21-2006, 09:34 PM
Here's how Dub Jett measures CG. It works really well for both low and high wings, and you can avoid purchasing a CG machine. In my case, since I like some bigger swept-wing aircraft, the CG machine doesn't work so well anyway.

1. Measure and mark where your CG should be according to plans. Use something temporary.
2. Get two chairs. Set them about body-width apart. If your model is smaller than this, you probably don't need to do it this way.
3. Balance the plane with the fuselage in between the two chairs.
4. Grab two large, flat-blade screwdrivers.
5. Slide your body underneath the plane (hanging between the chairs).
6. It helps to have a helper with this step, in case the plane starts to slide. Gently raise the plane up using the screwdriver tips at the factory CG indicators. If it tips to the nose, scooch the screwdrivers forward and try again. If it tips to the tail, scooch them backwards.
7. Mark where it actually balances, vs. the factory CG.
8. If you're within the range given by the manufacturer, you're done. If you're not, add nose or tail weight to compensate (or adjust battery position).

cbatters
10-10-2006, 01:41 PM
Wrestling with the relationship between proper COG and trim of plane.

Several posts seem to suggest that COG is related to speed (sink rate) of plane but this can be trimmed out with the elevator, the angle of the stabilizer / V tail attachment or by adjusting angle of attack of the wing. I don't believe COG by itself controls glide characteristics.

In theory, if COG is properly set, I believe the horizontal stabilizer would have zero pressure top / bottom.

I can understand that the COG would have a significant effect when the plane approaches stall speed but am having trouble understanding how small differences in COG affects normal flight.

Any insight / links would be appreciated.

(Would also like to know where COG is supposed to be on Challenger and Firebird II planes if anyone knows.)



Clint

jdetray
10-11-2006, 12:50 AM
If the manufacturer suggests a range for the CG point, it is often helpful for beginners to set up the plane to balance at the forward end of the range. In general, a plane that is a little nose heavy is more forgiving than one that is tail heavy.

This advice is sometimes reduced to: "A nose heavy plane flies poorly. A tail heavy plane flies once."

As your flying skills improve, you can move the CG point farther back in the recommended range to make the plane more responsive.

- Jeff

flypaper 2
10-11-2006, 02:11 AM
On a conventional plane the stab is lifting downwards. If you were to cut the stab off you would find the plane flip over on its nose. The reason being the centre of lift is behind the centre of gravity.

cbatters
10-11-2006, 01:56 PM
On a conventional plane the stab is lifting downwards. If you were to cut the stab off you would find the plane flip over on its nose. The reason being the centre of lift is behind the centre of gravity.

This is beginning to make sense. The closer the center of gravity is to the center of lift, the less force the stabilizer has to excert to change the ange of incidence of the wing. I believe will make the plane more responsive to control input on the elevator.

Any input on what affect CG has on the stall characteristics of the plane?



Clint

TManiaci
10-12-2006, 06:52 PM
Any input on what affect CG has on the stall characteristics of the plane?

Clint

I don't believe the CG changes the stall speed or event significantly (in reasonable ranges), but influences how the plane behaves when it stalls... ie. falls off to one side, noses over, drops level, or drops tail. I do know that running significant rearward CG behind the optimum will sometimes aggrivate/enhance a stall with hard pitch (elevator).