basic flying wing c.g. calculations?
08-28-2004 | 09:48 AM
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From: harrison, AR
basic flying wing c.g. calculations?
good day!
i am new to r.c and aero dynamics and was wanting to build some simple flat plane flying wings out of fanfold or depron.
is there a basic equation to determine c.g. of a flying wing?i heard that 15%of m.a.c. is a good start but i dont know what that is and if it would be the same for planes of different shapes, sizes, etc.
i built one based on tom hunts elipstik and it flew great on prescribed cg but other models did not fly when i applied the same c.g locations.( did not seem to be any where near 15% mac on the elipstik model...but i dont know how to calculate..)
help stop my confusion... i am running out of foam!!!
my power system is a gws 350 d geared and a 7cell 600 mah pack with a 10x4.7 electric prop.
i am new to r.c and aero dynamics and was wanting to build some simple flat plane flying wings out of fanfold or depron.
is there a basic equation to determine c.g. of a flying wing?i heard that 15%of m.a.c. is a good start but i dont know what that is and if it would be the same for planes of different shapes, sizes, etc.
i built one based on tom hunts elipstik and it flew great on prescribed cg but other models did not fly when i applied the same c.g locations.( did not seem to be any where near 15% mac on the elipstik model...but i dont know how to calculate..)
help stop my confusion... i am running out of foam!!!
my power system is a gws 350 d geared and a 7cell 600 mah pack with a 10x4.7 electric prop.
08-28-2004 | 01:18 PM
#2
RE: basic flying wing c.g. calculations?
If no one has said it already...
WELCOME TO RCU! ! !
Flying wings will become very unstable as the balace point moves back to the 25% chord mark. There's no one perfect point but it's generally recognised that between 15 to 20% is a good plank style flying wing balance point. How close to the magic neutral point you get to will depend on your flying abilities and how you set up the controls. But for first flights 15% is a good starting point.
For swept back wing type models this magic % changes and with extreme sweep angles and narrow chords the neutral point is not that far off the 30 to 40% used by conventional models. But in these cases the sweep angle is replacing the fuselage by moving the tips of the wings back to where the stabilizers would be on a conventional layout. A Zagi type flying wing is sort of in the middle but leans more to the flying plank style of wing thanks to it's wide chord and moderate sweep angle. I'd say a 25% balance on THAT style of wing is quite safe.
For your Ellipsoid type layout there is some sweep angle but it's minimal. Treat it like a retangular plank and go with the forward balance at first. Once flying test the stability by using the dive test. Basically you trim for level hands off flight. Then push it into a 40 degree dive and then let the stick go back to neutral. The model should pull the nose back up all by itself. How tight a pullup you have shows how pitch stable the model is. For a 30 inch span model I like to go for a pullout to level from a 40 to 45 degree dive that takes about 50 feet of height. Note this is dive to level, not to nose up. It'll pull through level to a nose up and then stall and dive again but it's the first part of the first pullup that counts. With a 15% setting it'll probably nose up faster than this. So move the balance back a couple of % and retrim for level flight and do the test again. As the balance (CG) moves back and you retrim for less up to level the flight the pullups will get longer and shallower. The elevator will also get more sensitive to pitch commands as well so you may have to reset the amount of elevator throw. Do this through the mix in the transmitter instead of at the control horns. The pitch may be sensitive but you'll still need full throw for rolling.
Good luck.
WELCOME TO RCU! ! !
Flying wings will become very unstable as the balace point moves back to the 25% chord mark. There's no one perfect point but it's generally recognised that between 15 to 20% is a good plank style flying wing balance point. How close to the magic neutral point you get to will depend on your flying abilities and how you set up the controls. But for first flights 15% is a good starting point.
For swept back wing type models this magic % changes and with extreme sweep angles and narrow chords the neutral point is not that far off the 30 to 40% used by conventional models. But in these cases the sweep angle is replacing the fuselage by moving the tips of the wings back to where the stabilizers would be on a conventional layout. A Zagi type flying wing is sort of in the middle but leans more to the flying plank style of wing thanks to it's wide chord and moderate sweep angle. I'd say a 25% balance on THAT style of wing is quite safe.
For your Ellipsoid type layout there is some sweep angle but it's minimal. Treat it like a retangular plank and go with the forward balance at first. Once flying test the stability by using the dive test. Basically you trim for level hands off flight. Then push it into a 40 degree dive and then let the stick go back to neutral. The model should pull the nose back up all by itself. How tight a pullup you have shows how pitch stable the model is. For a 30 inch span model I like to go for a pullout to level from a 40 to 45 degree dive that takes about 50 feet of height. Note this is dive to level, not to nose up. It'll pull through level to a nose up and then stall and dive again but it's the first part of the first pullup that counts. With a 15% setting it'll probably nose up faster than this. So move the balance back a couple of % and retrim for level flight and do the test again. As the balance (CG) moves back and you retrim for less up to level the flight the pullups will get longer and shallower. The elevator will also get more sensitive to pitch commands as well so you may have to reset the amount of elevator throw. Do this through the mix in the transmitter instead of at the control horns. The pitch may be sensitive but you'll still need full throw for rolling.
Good luck.
