Balancing a bipe
01-23-2006, 01:13 AM
#1
Senior Member
Thread Starter
My Feedback: (4)
Join Date: Oct 2002
Location: The Dalles,
OR
Posts: 273
Likes: 0
Received 0 Likes
on
0 Posts
Balancing a bipe
How do you find the CG on a bipe? do you turn the plane over and balance on the top wing or what? The wing stagger doesn't seem to allow for balancing right side up. Any help or suggestions would be appreciated. BTW, the plane is a Sig Hog Bipe. TIA, Tom
01-23-2006, 08:41 AM
#2
Senior Member
RE: Balancing a bipe
I've got an Ultimate. Both wings are swept and they are staggered. The manual gives the suggested CG as a location on the top wing. I measured that out on the top wing and marked it with a permanent marker. (the little dot is virtually invisible) I then took a long straightedge and projected that location to the wingtips and put dots on each. One finger under each wingtip dot works great.
Now, how do you find where a CG should be if you don't have a suggested one? There is a great little application available on the internet that works for monoplanes. It also works biplanes, although it doesn't give you the instructions for inputting their slightly different measurements. http://www.geistware.com/rcmodeling/cg_super_calc.htm How to use it for a biplane?
You measure all the things it says to measure but with a slight difference. For the wing LE measurements, just measure back to whichever wing is farther forward, and for the wing TE measurements, measure from whichever TE is farther back. You're basically looking at the planform and viewing the two wings as one. That's the quick and dirty way and it's not necessarily perfectly accurate, but works fairly well, considering what the CG actually is, a starting point.
The slightly better way to calculate a biplane's CG would be to do all the measurements as if the top wing was the only wing on the airplane. Plug those measurements into the calculator and write down it's output. Now do all the measurements as if the bottom wing was the only wing on the airplane. Get those results and average the CG locations. You really only need to do this double work if there is stagger. But it works well.
Whichever way you do the work, you wind up with a location that's best projected onto the top wing. High wing airplanes and "high wing" biplanes are most easily (and accurately) checked for balance with the top wing. Low wing airplanes are the ones that are most accurately and easily balanced after turning them upside down. Truth is, you can balance for CG either upside down or rightside up, because however you hold them, they'll only balance with the CG "straight down". However, if they're teetering and barely stable, it's almost worthless to try and guess where that CG is.
Now, how do you find where a CG should be if you don't have a suggested one? There is a great little application available on the internet that works for monoplanes. It also works biplanes, although it doesn't give you the instructions for inputting their slightly different measurements. http://www.geistware.com/rcmodeling/cg_super_calc.htm How to use it for a biplane?
You measure all the things it says to measure but with a slight difference. For the wing LE measurements, just measure back to whichever wing is farther forward, and for the wing TE measurements, measure from whichever TE is farther back. You're basically looking at the planform and viewing the two wings as one. That's the quick and dirty way and it's not necessarily perfectly accurate, but works fairly well, considering what the CG actually is, a starting point.
The slightly better way to calculate a biplane's CG would be to do all the measurements as if the top wing was the only wing on the airplane. Plug those measurements into the calculator and write down it's output. Now do all the measurements as if the bottom wing was the only wing on the airplane. Get those results and average the CG locations. You really only need to do this double work if there is stagger. But it works well.
Whichever way you do the work, you wind up with a location that's best projected onto the top wing. High wing airplanes and "high wing" biplanes are most easily (and accurately) checked for balance with the top wing. Low wing airplanes are the ones that are most accurately and easily balanced after turning them upside down. Truth is, you can balance for CG either upside down or rightside up, because however you hold them, they'll only balance with the CG "straight down". However, if they're teetering and barely stable, it's almost worthless to try and guess where that CG is.
01-23-2006, 10:46 PM
#7
Senior Member
Join Date: Oct 2002
Location: Philadelphia,
PA
Posts: 2,065
Likes: 0
Received 0 Likes
on
0 Posts
01-23-2006, 10:53 PM
#8
My Feedback: (1)
RE: Balancing a bipe
Balance is usually somewhere about 1/3 of the "mean aerodynamic chord". measure from a point along the centerline starting at the leading edge of the forward most wing to the trailing edge of the rearward most wing. Take a third of this measurement back from the leading edge of the forward wing. It's not precise, but will get you in the ballpark. Make it a little nose-heavy from this point, then shift as needed for the flight characteristic you want (twitchy or solid)
01-24-2006, 12:20 AM
#9
Senior Member
Thread Starter
My Feedback: (4)
Join Date: Oct 2002
Location: The Dalles,
OR
Posts: 273
Likes: 0
Received 0 Likes
on
0 Posts
RE: Balancing a bipe
Dsegal. Thanks for the info. Certainly made the sling method clear. Think I'll try that only using pullys to run the string through to make it a little more sensitive. Using the fingertips as a double check sound good also. Thanks a bunch, Tom
01-24-2006, 09:13 AM
#10
Senior Member
Join Date: Oct 2002
Location: Philadelphia,
PA
Posts: 2,065
Likes: 0
Received 0 Likes
on
0 Posts
RE: Balancing a bipe
> Think I'll try that only using pullys to run the string through to make it a little more sensitive. <
I don't think that is a good idea. You need enough friction at the top of the sling so that the model is not sliding all over the place. You will have enough trouble levelling the model and damping the yaw oscillations without adding pitchup or down allowed by a pulley. Note that you are not looking for shifting of the strings around the support point. What happens is that the entire hanging assembly moves laterally to match the center of gravity and the strings give you all the freedom you need. And to get an accurate reading the model must be level.
At my age the brain is slow. I just remembered that lately I have not run the strings through the eyehook. but instead have used two separate slings- one to the nose end and one to the tail end. I then have to fiddle with the position of the slings on the fuselage to get it levelled. Perhaps some wiser head can comment on the wisdom of this or make a better suggestion.
I don't think that is a good idea. You need enough friction at the top of the sling so that the model is not sliding all over the place. You will have enough trouble levelling the model and damping the yaw oscillations without adding pitchup or down allowed by a pulley. Note that you are not looking for shifting of the strings around the support point. What happens is that the entire hanging assembly moves laterally to match the center of gravity and the strings give you all the freedom you need. And to get an accurate reading the model must be level.
At my age the brain is slow. I just remembered that lately I have not run the strings through the eyehook. but instead have used two separate slings- one to the nose end and one to the tail end. I then have to fiddle with the position of the slings on the fuselage to get it levelled. Perhaps some wiser head can comment on the wisdom of this or make a better suggestion.
