View Full Version : C.o.g
03-09-2006, 08:11 AM
I am in the process of assembling a Thunder Tiger E-Hawk that i just bought and have got to the part of setting the centre of gravity. The instructions stress that this is a very important stage and to do it properly. Having not done this before, can someone give me some help as to how to set this up.:confused:
03-09-2006, 12:32 PM
Hello Andrew, Welcome to Wattflyer! The correct center of gravity is absolutely vital to flying a model. First, make small marks on each side of the fuselage (under the wing) where C.G. is shown on the plans. Assemble the plane with all components including battery. Hold plane up in the air and place your thumb and forefinger on the marks, so the plane is suspended by your fingers. You are holding it exactly on the C.G. Does the nose tilt downward? If so, the model is nose-heavy, you will need to slide the battery back and test again. If the tail tilts down, it is tail-heavy: Move the battery forward. When you have established the correct location for the battery to achieve C.G., mark the battery location clearly on the battery tray so you will always have it.
Another method is to drive a pin into the exact center of the wing (spanwise) at the C.G. location. Tie string to the pin and suspend the model. Same nose-down or tail-down method, but this also tells you if one wing is heavier. Lateral and longitudinal balance. For an unbalanced wing, add a small weight to the lighter wingtip until it hangs level.
There is another thread here "Need New Prop For Thunder Tiger E-Hawk 1400" by JakJr. You might want to read that too for other hints. The model flys well enough to be a good aileron trainer and will thermal easily. The stock propeller isn't the best though, the CAM 6" x 3" will perform much better.
03-09-2006, 09:31 PM
Thanks very much for your help Ron
03-20-2006, 01:54 AM
Hold plane up in the air and place your thumb and forefinger on the marks, so the plane is suspended by your fingers. You are holding it exactly on the C.G. Does the nose tilt downward? Ron
I recommend making a balancing stand. Fingers are fine at the flying field when there is nothing else to use, but in the workshop you can balance it better with a balancing stand. Easy to make. Get some 20cm x 10cm 6mm ply, 1 metre of 6mm dowelling and 2 erasers that go on the end of a pencil that are shaped to a point (alternatively you can get the pencils with the rubbers attached and sharpen with a pencil sharpener to a point). Now get the ply and down the length in the middle drill 6 - 8 evenly spaced 6mm holes. Cut your dowelling to 2 10 - 15cm lengths and attach the rubbers to the ends where the wing will be balanced on. Squeeze the dowelling into 2 of the holes in the ply ....... and presto you have an inexpensive balancing stand. BTW the other 4 holes not used are for when you have a larger fuselage to get around to the wing. Mark your balance points and balance away.
Best C of G / MAC website that I use is:
Use it everytime, works everytime ;)
03-24-2006, 05:58 PM
Thanks for the C/G site! Easy to use.
03-25-2006, 10:55 AM
Thanks for the C/G site! Easy to use.
Yeah, it's brilliant isn't it :D . I swear by it now when I am building my own designs. AND it works everytime too. No guesswork :) . Just gotta remember that it's the "MAC distance" that is the C of G at the root chord. I tend to put the center position as 35% (and it works BUT....), what do others put as this figure. I'm interested to know? The program even tells you the Aspect Ratio ...... brilliant huh :) :)
03-25-2006, 04:56 PM
I haven't built a plane using this yet, just went through the calcs; however, will try it after my present project which is electrifying a Guillows P-38 (rubber model). Do you know how it works for a Canard?
03-25-2006, 05:38 PM
Hi Bill, about the C.G, for a canard, this was just posted on the "Foamies" Forum, by carlospq in response to the same question by clipper;
03-26-2006, 05:50 AM
Thanks, Ron! I looked at the site and will be entering some figures based on my own design. from which I made a 1/6th scale model. I had a heck of a time getting the front wing to lift!
04-06-2006, 12:55 AM
Setting the CG, Center of Gravity, is so important that one of the first first RC plane tools I purchased was the Great Planes CG machine:
$22 and I use it constantly, even on planes I have already flown as part of a "routine" tune-up. Worth every penny!
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