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Help needed for CG

Old 01-30-2021, 08:47 PM
  #1  
jb48
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Default Help needed for CG

Hi everyone. I am about to finish a new airplane of my own design. Since it's my first tapered wing, where the leading edge is straight and trailing edge tapers from root to tip. I would appreciate if a fellow builder/flyer could calculate the MAC and the CG at 25% and at 30%.
Span is 60 inch, root chord is 8.75 in., tip chord is 6 in. Sweep from root trailing edge (8.75 chord) to tip trailing edge (6 in. chord) is at 5 degree angle. Thanks a lot.
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Old 01-30-2021, 09:10 PM
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solentlife
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Need more than that ...........

Basically need all dimensions including Wing TE to LE Tailplane ... Tailplane chord and spam etc.

There are many free online and downloadable CoG calculators ...........

Or PM me and I'll send you a few ...
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Old 01-30-2021, 10:25 PM
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quorneng
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jb48
As the taper is not that severe I would simply set the CofG at 25% of the mean chord. The mean chord is 7.475 so 25% of that is 1.87" back from the leading edge and then flight test from there,
As solentlife points out setting a CofG on its own will not guarantee success unless all the other dimension and angles are acceptable. The 'best' CofG position is depends on quite a few things including how you want the plane to fly. ..
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Old 01-31-2021, 12:17 AM
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ron_van_sommeren
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CG calculator
www.ecalc.ch (18languages)
→ Toolbox
→ cgCalc

Markus Müller, the eCalc_toolset developer, drives a Boeing/Airbus to earn his hobby money.

cgCalc has the blessing of Bob Parks. This is what he wrote about cgCalc.
Originally Posted by Bob Parks
Markus, Yes, I am being fussy about this! And I very deliberately picked a very difficult case (canard/tandemwing) to test your calculator! For relatively normal airplanes, you already have what I think is BY FAR the best thing I have seen on the internet. I will certainly refer people to it.
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Old 01-31-2021, 01:07 PM
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jb48
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Thank you all for replying. One thing I cannot understand is that some cg calculator consider only the main wing while some others take every measurements of the whole plane into consideration?
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Old 01-31-2021, 06:28 PM
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solentlife
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Originally Posted by jb48 View Post
Thank you all for replying. One thing I cannot understand is that some cg calculator consider only the main wing while some others take every measurements of the whole plane into consideration?
Let me ask a question in reply ...

What would you believe ? A calculation taking into account tail as well or just wing alone ?

I noticed that years ago when I downloaded / used various online .... so now I basically run through a number of calculators and see what average is. Or I just go for 30% chord on a monoplane ... and a combo of the two wings on a biplane !!
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Old 02-01-2021, 02:20 PM
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jb48
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OK, Solentlife and Ron van Somme, I took maybe one hour to do the complete work for cg calculation. I used the Aircraft Center of Gravity Calculator from rcplanes.online/cg_calc, which seems more user friendly than the one who divides the wing in multiple panels. I used ruler and square to find the precise dimensions and filled the spaces of the program. Quite interesting, especially that the program gave me exact plan form of the aircraft, (once I discovered that for straight leading edge and forward sweep of trailing edge I had to enter negative wing sweep distance).
The calculator gave me a lot of data. For instance, for a static margin of 15% (recommanded for first flight), it suggests 2.14 in from root leading edge. I rounded distance to 2 in. which is the average of Quorneng suggestion (1.87 in) and the calculator 2.14 in.
Thank you all for your suggestions.
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