crhammond
Posts: 392
Joined: 8/31/2005
From: EdmontonAB, CANADA
Status: offline
|
I found this on-line ... if you want to calculate it. There are a number of on-line calculators that can gett he CG of each wing and then you can use the method mentioned here to figure out the CG for the top-wing having more area. Of course someone has flown this plane and had to figure this out so it would be good to know their experience!
(From Don Stackhouse)
We have a section on our website:
www.djaerotech.com
called "Ask Joe and Don" that deals with all sorts of questions, including this one. Just go to the AJ&D page and type "C/G of a biplane" or something like that into the search engine, and it will come back with a list of links to which of the over 300 articles refers to that phrase. You might want to browse a bit while you're there, you might find other useful articles on other subjects as well.
C/G involves a number of factors, including the tail moment arm and the tail area. The starting point is the Mean Aerodynamic Chord ("MAC"  and Aerodynamic Center ("AC", for an individual flying surface it's located on the MAC of that surface, typically at about 25% back from the leading edge) of the combined wings. From there you find the AC of the whole aircraft (including the tail). You may or may not want to include some correction factors for the effects of aspect ratio and airfoils on the diferent flying surfaces, although in my experience these effects tend to cancel each other out for most models.
So how do you find the AC of a pair of wings? First find the AC's of the individual surfaces, then use a weighted average method to find the AC of the combination.
For example, lets assume we have a biplane (a "sesquiplane" in this case, where the upper wing has twice the area of the lower) with an upper wing of 200 sq.in., and a lower wing of 100 sq. inches (total are of the two is therefore 300 sq.in.).
On a side view, draw a line from the AC of the top wing to the AC of the bottom wing. The AC of the combination will lie on that line, closest to the larger of the two surfaces, at a distance proportional to the ratio of their areas. In this case it will lie 1/3 of the way from the top wing to the bottom wing (that's 100 divided by (100 + 200)).
Once you have the combined AC of all 300 square inches of wing together, you can find the AC of the combination of that plus the tail using the same method. This AC for the whole aircraft (assuming the effects of aspect ratios of the different surfaces, plus the effects of their different airfoils, can be ignored) is going to be the approximate location of the "Neutral Point" ("NP" , the C/G location at which the plane's stability is neutral (i.e. it goes where you point it, with no tendency to come back to the original attitude after it's been disturbed).
If you put the C/G ahead of the neutral point, you will have positive stability. The amount that the C/G is ahead of the NP is called "static margin" and is a measure of how much static stability you have. It's typically expressed as a percent of the MAC. If you calculate the static margin in %MAC of an airplane which has an amount of static stability you like, and use the same static margin on your new model, the static stability of the new model should be similar, assuming the two models are similar enough in general type and arrangement that other complicating details don't get in the way.
_____________________________
Wings Level.
|
Printable Version
All Forums >> RC Airplanes >> Giant Scale Aircraft - 3D & Aerobatic >> RE: Bigairplanes.com 41% Pitts 12 
RE: Bigairplanes.com 41% Pitts 12 - 
