Mark:
Thanks. Your explanation makes sense. I will follow your advice.
I had the pleasure of breaking into indoor flying some years ago with some mentoring from Cezar Banks on Wright Stuff (WS) Science Olympiad (SO) airplanes.
The rules for the WS event had no overall length limit for the airplane, but the chord of the wing typically had a relatively small maximum, like 10 cm or in some years, 7 cm. The max wing span under the WS rules was typically 40 cm or 50 cm.
Cezar used a really long tail boom on his SO planes. The overall length of his WS planes, including the motor stick, would be 80 - 100 cm. Cezar positioned the main wing so that the CG was way behind the trailing edge of the wing.
Cezar's long WS airplane would fly very slowly (for a 7.0 gram airplane with relatively high wing loading) but if it hit the ceiling, recovery was poor and you gave up a lot of altitude. It flew nose up and used the stab for lift. The stab of a WS plane was subject to limits that made it relatively small.
So if you used Cezar's design you had to reliably fly no-touch, or move your CG quite a bit forward. In a Category I flying site, Cezar could beat even the best WS flyers by about a minute. He was so generous with his time and expertise, and very humble about his abilities.
The overall max length in the LPP rules is relatively small compared to the max wing span limit. I will try setting the CG on my LPP at 70% aft of the LE and see how it flies. I am nearly finished with my second LPP and I am on-budget to get slightly below the 3.1 gram minimum so I can balast up. I put extra care into wood selection for the motor stick, leading edge spar, trailing edge spar, and tail boom this time around. I used very light balsa for the ribs, stab and rudder. It might have been Cezar that told me that you wouldn't start the Boston Marathon with two bricks in your pockets!
Thanks again Mark. I love having access to experts via this site.
--- In Indoor_Construction_at_yahoogroups.com, "Mark F1diddler" <f1diddler@...> wrote:
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> --- In Indoor_Construction_at_yahoogroups.com, "calgoddard" <calgoddard@> wrote:
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> > Cezar Banks' LPP plan says the CG should be at 70%. How is that calculated? I know he recommending the center of gravity of the airplane (with the rubber motor mounted) should be a certain distance from some reference point that yields the specified percentage. What is the reference point? The wing has a five inch chord and he shows the CG at about three inches behind the leading edge. But that does not yield 70% in terms of a fraction.>>
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> Can't explain the discrepancy but 70% usually means 70% of the chord, going back from leading edge of wing. On LPPs it's convenient to drop a mark on motorstick vertically from CG line. If you can get adequate stability from more than 70%,(such as 75%) give it a try. Let the stopwatch judge.
> MB
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Received on Fri May 27 2011 - 17:58:21 CEST
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