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Jr Falcon CG question

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Jr Falcon CG question

Old 12-27-2007, 12:20 PM
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cortacesped
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Default Jr Falcon CG question

Long time ago I started the construction of a Jr. Falcon based on the plans provided by G. Rock / D. Fritzke (thanks). Now I have it almost ready to flight but I have a problem, when I put all the components in place to check the CG it results to be behind the wing spar. Just to level I added some lead weights to the nose until it become balanced with 100 g (3.5 oz. apx) extra weight in the nose.
Do I have to keep that weight in the nose or according to what I read in this thread: http://www.rcuniverse.com/forum/m_34..._1/key_/tm.htm The airfoiled stabilizer corrects the problem in flight ?
Thanks for your attention.

Regards

Claudio
Old 12-27-2007, 12:59 PM
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g-rock
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Default RE: Jr Falcon CG question

Claudio,
Contest free-flight models with lifting stabs have their CG at almost the trailing edge of the wing. Carl shows the CG to be 2-5/8" behind the leading edge. In other words, I kept faithful to his original drawing.
Gene
Old 12-27-2007, 01:20 PM
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rainedave
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Default RE: Jr Falcon CG question

Mine is also balanced back pretty far (because of the airfoiled stab, that Gene pointed out). I think it's at about 45% or so. This also helps level out your rudder turns. If you balance it at 25-33% it will pitch down when you apply rudder. Mine doesn't require very much elevator pressure at all to make nice banked, level turns.

David
Old 12-27-2007, 03:01 PM
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cortacesped
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Default RE: Jr Falcon CG question

Thanks for the comments. It´s a relief to hear that. I was worried, because after putting so much effort in keeping light, it was such a pity to ballast the plane with that useless weight. Regards.

Claudio
Old 12-27-2007, 09:21 PM
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Default RE: Jr Falcon CG question

Heavy elevator input implies nose heavy airplane. The JR. Falcon will balance right about on the rear of the wing spar. If you use 28%-35% of the wing chord as a balance range you will be close. Just remember you also have to have the wing and horizontal stab aligned correctly. Pitching when applying the rudder is an indication of nose heavy also in that when you drop a wing you effectively loose lift and thus the nose being heavy will drop.
Old 12-28-2007, 05:50 AM
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Default RE: Jr Falcon CG question

Thought this might help answer some questions about Carl's suggested CG Location.
Gene
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Old 12-28-2007, 06:18 AM
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Default RE: Jr Falcon CG question


ORIGINAL: skylane42

Heavy elevator input implies nose heavy airplane. The JR. Falcon will balance right about on the rear of the wing spar. If you use 28%-35% of the wing chord as a balance range you will be close. Just remember you also have to have the wing and horizontal stab aligned correctly. Pitching when applying the rudder is an indication of nose heavy also in that when you drop a wing you effectively loose lift and thus the nose being heavy will drop.
The later Falcon's Mk111 and pattern ships of the 70s? had a swept back fin (vert stab) and rudder to help the nose drop situation. I understand that modern gear has the facility to dial in some up when using the rudder or even aileron for a turn.



old git - - - - aka John L.
Old 12-28-2007, 06:21 AM
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old git
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Default RE: Jr Falcon CG question


ORIGINAL: skylane42

Heavy elevator input implies nose heavy airplane. The JR. Falcon will balance right about on the rear of the wing spar. If you use 28%-35% of the wing chord as a balance range you will be close. Just remember you also have to have the wing and horizontal stab aligned correctly. Pitching when applying the rudder is an indication of nose heavy also in that when you drop a wing you effectively loose lift and thus the nose being heavy will drop.
The later Falcon's, Mk111s and pattern ships of the 70s? had a swept back fin (vert stab) and rudder to help the nose drop situation. The angled rudder gave some effect of lifting the nose. I understand that modern gear has the facility to dial in some up when using the rudder or even aileron for a turn.



old git - - - - aka John L.
Old 12-28-2007, 06:26 AM
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Default RE: Jr Falcon CG question

I measured g-rock's image and it puts the CG at 38.9% back, which I'm sure is on the forward side of the "safe zone." So, 40-45% back is about right for a slightly more lively and nimble ship.

BTW, I always thought Carl Goldberg designed the Falcons; apparently not:

http://www.rcuniverse.com/forum/m_6664729/tm.htm

David
Old 12-29-2007, 10:52 AM
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Yuu
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Default RE: Jr Falcon CG question

OOOPS !! Wait, guys... this started as a "Jr. Falcon" thread... and data has been injected for the Much Larger 'Falcon 56'...... be careful which data you pick up.....
Old 12-29-2007, 01:25 PM
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Default RE: Jr Falcon CG question

Yuu,
The Falcon 56 and the Junior Falcon appear to use the same airfoil and that's why I mentioned it. From the Internet address given, you can get the coordinates for this airfoil.
Gene
Old 12-31-2007, 09:48 PM
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tweedy
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Default RE: Jr Falcon CG question

I have learned the hard way that trying to fudge on the aft side of a designers recommended CG is a recipe for disaster, it is always safest to start with the most forward cg and gradually work your way back a little at a time. Competition free flight models can get away with an aft cg because they have a non-symmetrical lifting stab that is very large in comparison to the wing and provide a portion of the overall lift. It might be noted that a lifting stab on a full size airplane will prevent it from being given an airworthiness certificate because it has been shown to make a dive un recoverable.

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