I’ve been out of R/C for 20 yrs and am getting back into it. I purchased a GPs PT 40 and all has gone well, but I have one question. On the initial CG set up, which is 4-7/64” aft of the leading edge, they give me a range of ¼” fore and aft. So far so good. I set up fulcrum points on the bench at the wing tips, and the plane will balance anywhere within this range but at different attitudes. There is no mention of leveling or a leveling point, and no mention of a longitudinal C/L on the plans. Shouldn’t balancing be done to a level point determined somewhere longitudinally?

Thanks for any help, explanations, or suggestions.

When suspended at the CG it should appear to flying level. It's best to locate the CG at the fuselage rather than wingtips. I find it amazing that they give the CG location in 64th's It is certainly not that critical.

On my trainer and low-wing trainers I had them balanced to a slight nose down attitude when checking the CG. There's the old saying "A nose heavy plane flies bad; a tail heavy plane flies once."

Having a slight nose-down attitude means if you lose power, the plain will want to gently dip forward. This will allow more ability to keep airspeed and air flowing over the wings, creating lift.

Either way, like they said above it's not THAT critical. Get it as close as you can, and make adjustments based on how the plane flies. The manual is just a safe starting point.

FALSIFY,
THE WAY I HAVE BEEN BAL. ALL OF MY PLANES IS TO PICK THE PLANE UP BY ETHER THE WING TIP'S OR LIFTING IN THE SAME DISTANCE BACK FROM THE LEADING EDGE NEXT TO THE FUSE BUT MAKE SURE THE WING IS BALANCED BEFORE YOU FLY IT MEANING CHECK THE BALANCE OF THE WING WITH THE WING OFF THE FUSE BY HOLDING THE CENTER SECTION OR WING JOINT AND SEE IF IT FALLS OFF TO ONE SIDE IF IT DOES CORRECT IT IVE SEEN PLANES GO DOWN BECAUSE OF THAT,,../ BUT WHAT YOU WANT IS A SLIGHTLY NOSE HEAVY BALANCE WITH THE FUEL TANK (EMPTY) ARE YOU FLYING OFF OF GRASS OR PAVMENT
USE DAVE BROWN LITE WHEELS 3 TO 3.5 IF YOU ARE FLYING OFF GRASS GOOD LUCK AND IF YOU NEED A
INSTRUCTOR LET ME KNOW I'M NOT TO FAR FROM YOU


JIM

Thrust line of the engine - unless you have downthrust built in. It's the datum line of the horizontal axis from the center of the prop/spinner straight back along the longitudinal axis (marked "W.L. 100" on the drawing in the below website) . . . but unless you have it marked on the building plans it's like TedMo said: "appear to flying level". Most models have a pretty flat horizontal stabilizer and unless you have built other than 0º incedence into that it is a good reference line for the horizontal.

http://www.pilotfriend.com/training/...ing/wt_bal.htm

Be sure to balance it laterally, too. That is, suspend it from propshaft and a point along the linear axis at the rudder and make sure one wing half is not heavy (more often than not one side is due to wood density & glue differences).

Thanks for the quick replies guys.

Charlie, the lateral balance has been taken care of, the engine has 4-1/2 deg of down thrust and the horizontal stab and wing both have +1 deg angle of incidence, and the wing is washed out per building jig. I think I will level at the fuselage saddle to wing root junction, using that as the longitudinal datum, minus the angle of incidence. I’m probably nit picking here but I love it.

Six Guns, We could use a little of that Global Warming huh?

Thanks again guys. I’m glad I found this site. Things sure have changed in the last 20 yrs![sm=thumbs_up.gif]

Hi,

Try this device, it takes any guess work out of CoG providing the plans/destructions are accurate.

http://home.mindspring.com/~the-plum...%20Machine.htm

Not sure if the CoG is accurate from the plans or destructions?[]
Try this:

http://www.inlandsloperebels.com/Calc/cgcalc.html

Cheers,

Colin

I use something similar to the Vanessa in the above link and I guess the photos explain it better than words. The two marks on the wing are the suggested range from the instructions. I just eyeball the model from the side to have it very close to parallel to the ground and then from the front/back to level the wings. The pendulum pointer actually points towards the true CG which is somewhere inside the fuselage. If the pointer is off to one side of the centreline then it's indicating a lateral unbalance so all that's needed is to add weights to the lighter wing tip until it's centred and then you know how much to fix permanently. Same with fore and aft CG, just put weights where they'd be permanently fixed until it's in the range you want.

Crash...What am I missing?. I built the device as shown. Wherever I drop the wings of the plane on my "loops"..... the plane stays in one spot. Forward and the plane appears tail heavy...back and the plane appears front heavy. I can see it working well in the "F" configuration, but not the others. Does the loop of string have to be slippery mono or something? thanks Rich

Hi rich,

You twist the dowl, (either forwrd or back), that the loops are suspended from until the model is level. Lock the dowl in place with the thumb screw and assuming it is still level the plumb bob points to the CoG. I have used fine manilla cord that isn't slippery on mine as you want it to stay in place not slide.

Cheers,

Colin

I like this "Venessa" cog method, but I'm a little confused about the one instruction "adjust the model to a level flight attitude". Does that assume one uses a bubble level, like the one that came with my TopFlite CG Machine? Or doesn't it matter? I'll have to try this the next time time I balance a model.

All good suggestions on balancing and CG. But please read my post below on the GP PT-40 trainer - I have some experience with this. Please make sure your plane balances on the wing spar! I found that the plans and instructions are incorrect. Above all get some help at your local club. Have an instructor check out your plane, check the CG and perform the maiden flight.
Good luck. BTW, you will love the plane once it's setup correctly!

http://www.rcuniverse.com/forum/m_58...tm.htm#5823388