CG and control throws- first scratch built - RC Groups
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Jan 07, 2005, 10:39 AM
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CG and control throws- first scratch built

I'm nearing completion of my first scratch built model, the first one I've designed myself. I'm wondering about where I should have the center of gravity placed. Most straight wing airplanes seem to have a balance point maybe 1/3 of the chord distance back from the leading edge. But, tapered wing aircraft seem to vary somewhat. Also, I'm wondering about how I should set up the control throws. Any help's appreciated.

span: 41"
length: 34"
Enya 11 CX engine
All balsa and plywood construction, low wing, semi-symmetrical airfoil
2-3* dihedral
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Jan 07, 2005, 11:28 AM
Think Thermals!!!!
SoCalGliderFlyr's Avatar
Lots of elevator. Very slim ailerons. What type of flying? Sport pattern or 3D?

For ballance about 30% at the root. Might have to move the engine back.

For elevator movement about 3/8" both up and down till you fly it a few times. Might want to put it on a dual rate.

Ailerons abut 3/16" up and down. Usually have these twice as much up travel as down. Couple the rudder to the ailerons via the transmitter.

Ailerons could be half again thier cord length if not twice. Thin ailerons have a bad tendency to stall at high deflections. Think unwanted snap roll.
Jan 07, 2005, 03:57 PM
Visitor from Reality
Lots of 'ifs' here, but try this:
Assuming you have a decent % of stab area to wing - 20 - 25% is real good for all sorts of flying - and your wing section is nothing too radical.

Shoot for a CG at 30% of mean average chord. Draw it up like this on a plan view of the full wing, including the part covered by the fuselage.

Calculate the 30% points of the root and tip chords.
Connect those points with a straight line.
Find the mid-point of that line.
Draw a line from that point to the root chord, at right angles to the root chordline.

That's your 30% MAC point for the wing.

That's also a real eyeball number. Go forwards to 25% for test flights, unless you've done real hard sums to equate your tail area to wing area. Control throws as above, but don't go real crazy on the elevator - that can get you into trouble if you have too much.

Still, exciting test flying is one of the reasons we design our own

Jan 07, 2005, 04:22 PM
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DougC's Avatar
Jan 07, 2005, 08:06 PM
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Thanks for the responses. DougC, the link really helps! I will agree that the ailerons are narrow as shown on the drawing. I actually enlarged them by 1/4 inch while building. This is just going to be a sport, hopefully fun to fly airplane. I will have dual rates. Preliminary balancing, covered plane with servos, engine, and linkages, without fuel, wheels, and without reciever and battery puts the CG right around 25% back from the leading edge at the root. So, it looks like there needs to be a little more weight behind the CG. First flight's going to be very nervous, but thanks for the help!
Jan 09, 2005, 08:15 AM
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ExtremeSkyes's Avatar
Hi Starfire...

don't have any tips here...but I've been playing around with building my own and would be interested to see how you go first flight. I'm into building my third, but my first two are still on the test bench...working out what I'm doing wrong with them. Learning heaps doing it though.

Jan 10, 2005, 09:19 AM
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The first flight might be within a few weeks- depends a lot on the weather here in Ohio. I've got radio equipment partially installed. I still have to attach the landing gear, mount and break in the engine, and make some decisions about balancing. As soon as it flies (if it does) I'll post here.
Good luck with your projects.
Jan 13, 2005, 08:55 PM
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feihu's Avatar

I made some quick calculations from your small sketch which I assume is a scale sketch. Your wing area, horizontal tail area, and moments are fine. Aileron area appears to be less than 10% of wing area but with your 1/4" increase may work out OK. For starters on your first flight, I would suggest about 20 deg deflection for the ailerons and perhaps 15 degrees on the elevators (due to your larger elevator areas).
The 30% CG appears to be at 2 3/4 inches back fron the root leading edge; however I suggest you use the attached graphical solution to accurately find your 30% mac CG location.
One final comment, you might consider making the tailwheel length a little longer so the tail sets a little higher. This will improve the ground handling and reduce the P factor somewhat.
Your design looks good and there's no reason for it to be anything but a good flyer.
Good luck..
Last edited by feihu; Jan 15, 2005 at 05:41 PM. Reason: attachment size
Jan 15, 2005, 07:21 PM
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Aio_1's Avatar
Originally Posted by Dereck
........Calculate the 30% points of the root and tip chords.
Connect those points with a straight line.
Find the mid-point of that line.
Draw a line from that point to the root chord, at right angles to the root chordline.

That's your 30% MAC point for the wing.......
Just a technicality!
That's actually the mean chord rather than the mean aerodynamic chord.
The mean chord doesn't take taper into account so you'll end up with a CG position half way between the tip and root positions for your chosen %chord. For a wing with no taper this is accurate and for small taper it's usually close enough. However the inboard part of the wing has far more area for a highly tapered wing and will have more influence on the CG position. The MAC takes this into account.

Mean Chord = (Cr+Ct)/2 or Area/Span
Mean Aerodynamic Chord = 2*Cr/3*(1+T+T^2)/(1+T)

Cr is root chord
Ct is tip chord
T is taper ratio (Ct/Cr)

I think feihu's graphical solution will give the same result for MAC


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