Center of gravity

[weasel]

this might seem a daft question, why is a low wing aircraft turned upside down to obtain the balance point, & a high wing is kept the right way. can anyone enlighten me, on this subject.. thanks weasel....

[Tim Mackey]

It doesnt HAVE toi be done like this....it just makes it easier to actually balance the thing as the weight of the fus and gubbins is lower down as it were - think pendulum effect

[Terry Whiting]

weaseI, I  think  if you tied balancing  both types of model first upright, then inverted, it would answer your own question.

[001]

If you screw a small cup hook into the wing on the balance point, you can balance the plane laterally, by hanging it from a ceiling or door frame, if your low winger has a canopy this can be quite difficult. (Not recommended for quality scale jobs.)

[Martin Harris - Moderator]

The technical answer is that the Centre of Gravity is a 3 dimensional point somewhere above, below or on the point shown on the plan where all the weight can be said to be acting.  On a high winger it is somewhere below the wing and on a low winger somewhere above it.

 

If you hang an object from any point, a theoretical plumb line would pass through the C of - G.  Do it from any 2 points and the plumb lines will only meet at one point - the C of G.

 

If you attempt to balance an object by supporting it below the C of G, the slightest movement will displace the C of G point above the pivot and gravity will tilt the object more and more as the effective weight moves away from the pivot.  Support it above the C of G and gravity will pull the C of G back into line.

[Former Member]

[This posting has been removed]

[LHR Dave]

if I have balanced my low wing seagull spacewalker the right way up is this wrong?

[Martin Harris - Moderator]

No - but you've done it the difficult way as it would needed to be EXACTLY on the C of G with no air movement and with the model absolutely still - unless your balance pivot was too sticky to move properly, of course...

[Tim Hooper]

What everybody else said!

Mind you, if your low-winger has a little bit of dihedral it's often possible to balance your model by supporting it under the very tips of the wings.  In fact, it's often practical to push map-pins into the tip blocks on the stated CG position, and use these to accurately set your CG.

 

tim

[Terry Whiting]

Well guys being a builder I always make provision for balancing by building in a hardpoint which is capable of screwing in a small eyed hook.

This method reveals ALL, where as  commercial  balancers I have seen only balances  'pitch'

 A perfectly balanced model in all axis,  should eliminates the need of  'trim', if trim is required then it  is usually an aerodymamic problem, due to thrust line or a warp.

 

I have balanced many ARTF models for club members who look a little worried when I say I will have to  make a hole through their wing.

 A  1.5mm hole through the wing near as possible to the recommended  B/P. I try not to drill through a rib.

Next you require 4 things.,

 

(1) a quare of 1.5mm ply which can span two rib bays, with a hole is drilled dead centre,  (2) a piece of pushrod wire about 70mm long, making a loop in one end,

(3) a screw clamp removed from  a 10amp electrical connector   

(4) a screw driver.

 

For a high wing model the necessary hole is made through the wing, the pushrod wire is carfully  pushed through from the top of the wing, and the ply support is placed  on the wire from under the wing, slid on the electrical connector ,and tighten.

Fix the wing to the fuselage., It doesn't matter if the wing is not sitting perfectly due to the ply, a long as it is secure.

Support the model by the looped wire.

 

Now you are ready  to balance 

 

To balace the wing horizontally, I use self adhesive lead strip, the type use for imitation lead light windows  

 

A low wing model is as above, but balanced inverted.  

 

This is my prefered method, taking  time in balancing properly can be very rewarding.

All my models are balanced either way, screw hook or hole method including gliders.

Those of you worried about the screw eyed method don't be, I have tested  a  screw eye in 6mm ply fixed in a vice, and using a spring balance fishing scale, when it read 20lb

I said good enough .

 

TJW