John
Here is a short version of the high school rules:
- Monoplanes only.
- 50 cm wingspan limit.
- 7 cm chord limit.
- 30 cm max stabilizer span.
- 4.5 cm max stabilizer chord.
- Prop diameter max of 18.5 cm.
- Mass, less rubber, no less than 7 grams
- Mass of rubber motor no more than 2 grams.
My design (LittleSquare) was worked out in Bernie Hunt's design spreadsheet. The plan is in the files section of this group. It uses the maximum dimensions listed above. The CG is about 1/4" behind the wing with motor attached. The calculated static stability margin is 72%. Based on very limited flying experience so far it is quite stable. More experience will hopefully be gained this weekend.
Bill Gowen
Decatur, GA USA
----- Original Message -----
From: John Barker
To: Indoor_Construction_at_yahoogroups.com
Sent: Tuesday, November 14, 2006 3:17 PM
Subject: RE: [Indoor_Construction] cg/stab size
Neil
What follows is a stab in the dark as far as solving your stability problem
on the SO model because you have not provided much detail and when I tried
to find the SO rules on the Internet I was greeted with the amazing
statement that they were not allowed to be published!.
The dominant factors in longitudinal stability are the Tail Volume (the
moment arm multiplied by the tailplane area) and of course the CG position
as a percent of the wing chord. You say that you already have the maximum
permitted tailplane area and I guess there may be a limit on fuselage length
that prevents an increase in moment arm. It would seem to me therefore that
your first choice to improve stability would have been to move the CG
forward. At a wild guess I would say from its present position (which you
describe as behind the trailing edge) to about 90% of the wing chord.
However you say that you have reduced the tailplane area, which will reduce
the Tail Volume and make the stability even worse, unless you do something
else as well. That something else must be an increase in moment arm or a
forward movement of the CG. If the fuselage cannot be lengthened then I
suggest that you move the CG forward to around 70% of the wing chord.
More information might lead to better advice!
I have just noticed the 'stab in the dark' in the first line. It was
unintentional but I am now quite proud of it and I must concede that the
English English version of 'a tailplane in the dark' does not have the same
ring.
John Barker - England
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Received on Tue Nov 14 2006 - 13:49:00 CET
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