balancing a bipe CG
 

i previously posted this in a another thread and never got an answer about it. i'm building an SMP right now (Smith mini plane), the CG indicator is about a 1/4 inch behind the leading edge of the lower wing. being a bipe, when i put it on the CG machine, all the weight is above the scale, making it very difficult to set the CG. with regards to setting CG on other planes, low wings go on the scale upside down, and high wings right-side up. if i draw a vertical line up to the upper wing from the lower on the CG mark, is that going to affect the CG of the plane or is it still going to be correct?

RE: balancing a bipe CG
 

If the tail is kept horizontal, the recommended CG will be somewhere along that vertical line, and between both wings.

RE: balancing a bipe CG
 

When I established the static CoG (center of gravity) of my SIGHog Bipe I used a harness that looped over the prop hub and thru the top hinge of the rudder / fin. The harness is of stainless safety wire so it doesn't pull through the eye bolt in the ceiling. I used a incidence meter to ensure that the flat surface was at 0 degrees (horizontal) - the only way that Iknow of to ensure that the plane is at 0 degrees - my eyeballs lie to me constantly. http://www.rcuniverse.com/forum/js/f...ssed_smile.gif I dropped a plumb bob from the eye bolt and suspended it about 1/16" above the wing. A Sharpie was used to mark the actual CoG for fore-n-aft and laterally. Weight was added to the high wing at the servo bay until the lateral CoG was at the center of the wing. Weight was added at the firewall or the rear of the horizontal stab until the fore-n-aft CoG was closer to SIG's foward specification.

So much for static CoG. Now it's up to dynamic CoG by flying the plane, observing the behaviour and moving weight.

General ROT(rule of thumb) - to move the CoG 1/8" forward (nose heavier)add 1 oz. of weight at the firewall. To move the CoG 1/8" backward (tail heavier) add 1/4 oz. of weight at the rear of the fuse. Ikeep a bunch of lead with ouble sided tape in my field box along with paper towels, Simple Green cleaner and alcohol to clean up the tail after flights. The same can be done at the firewall. If you have a tank hatch - all the better for hiding some weight just behind the firewall.

pic 1 - Bipe hung out to dry - takes 15 minutes for it to settle down (don't turn on the heat or air conditioning!)
pic 2 - static CoG marked along with SIGfront and back recommendations
pic 3 - 1/4"added to the tail moves the static CoG back about 1/8"

Ihope this helpshttp://www.rcuniverse.com/forum/js/f.../msn/75_75.gif

RE: balancing a bipe CG
 

Yes, if the horizontal stabilizer is level when you draw that vertical line and impose the recommended CG onto the top wing, you will be ok. If the plane is sitting on its gear when you draw that line, the CG will end up being forward of where you want it.

SeamusG, I like your explanation. You are basically describing a Venessa rig which has been discussed and plans shown on RCU several times. It works regardless of the nay sayers. And it doesn't care how many wings you have on your airplane nor does it care whether the airplane is upright or inverted. In this case, since the plans from Sig indicate the CG on the bottom wing, he can hang it inverted and work from the bottom wing if he wishes or he could impose that point on the top wing and work with the airplane upright. His choice and it won't make a bit of difference to the finished product.

RE: balancing a bipe CG
 

Since I did my CoG some time ago (and memory DOESN'T server me :D) my version of the Vanessa rig uses the prop hub and the end rudder hinge (top or bottom). It may be that my Hog Bipe would not hang inverted so I (may have) had to do it upright. I really like the use of a soft sling as described in the Vanessa thread! Now that I'm confident with the results - might be time to "upgrade" my version to a "softer" version.

Maiden tomorrow for the Hog Bipe with its Saito 100. Yum, yum ...

RE: balancing a bipe CG
 

Saito 100 in a Hog Bipe? I love it. I have a Saito 100 in a Seagull Ultimate that hasn't flown yet. We fly on Wednesdays and Sundays, so hopefully this week will see it in the air. The Seagull Ultimate is quite a bit larger than the Hog Bipe and I'm kind of wondering if it's enough. It should be perfect in your Hog. What size prop are you gonna start with?

RE: balancing a bipe CG
 

Initially a 14x7/13 Zinger. I used a 13x6/10 successfully with a Saito 91. But I see an APC on the near horizon.

RE: balancing a bipe CG
 

I just put a YS FZ 110 on my 4Star60. Geezzzzz! Guessing that it would do pretty well on the Seagull Ultimate.

RE: balancing a bipe CG
 

<span style="font-family: Comic Sans MS">that's great.sounds aboutright, but for some reason the two wing thing sent my mind up river. thank you for answering me.</span>