Weight and Balance the old fashioned way

Sep 08, 2011 4 Comments by

Setting up a plane for your first flight is a critical step. Most of us fiddle with stuffing all the electronics into the fuselage and then placing it on the center of gravity (CG) stand.  If we are wrong, we move some things around and try it again hopefully getting it right through trial and error.  We can be proactive in our setups though and perhaps with skill rather than luck nail our CG and eliminate some of the frustrations getting plane to balance create.

Modelers are typically provided the weight or All Up Weight (AUW) of their planes by the manufacturer and the CG point.  This, however, leaves many variances available to the flight demons that crash your planes. As an example, the difference in the weight of your servos, electronic gear, motor, and if it is an electric, the battery, can create a large difference in the AUW as prescribed by the manufacturer.  This difference makes getting the CG difficult.  Recently, I was experimenting with electric motors for a 3D plane.  The two choices ranged in weight from 155 grams to 118 grams.  Given that motors have the greatest amount of leverage on the center of gravity this difference in weight is huge.  So the fact is you have a range of weights and an issue of where to put that weight to correctly maintain the CG.

Full scale aircraft are typically designed with a weight and balance chart.  Using a datum which is a fixed point in or ahead of the plane, measurements are given for each item in the plane and their associated weights.  For example, let’s assume we define a datum as the point where the nose cone of the plane touches a fixed point, say the wall.  The engine then should be very close to this point and maybe weigh as I mentioned before 155 grams and is two inches from the datum (wall).  The distance is referred to as the arm.  My next item is the ESC which weighs say 30 grams and is located 5 inches from the datum.  The battery, perhaps the heaviest item we have to install may weigh 80 grams and is located 13 inches behind the datum.  The sum of the weight for each item times the arm equals the moment.  Here is how an aircraft would determine the center of gravity all things being equal.  I have divided the moment total by the weight.

ItemWeightArmMomentCG
Engine155g2310 
ESC305150 
Battery80131040 
 265 15005.66 inches

We now have a CG, but you can see I am going to have problems because 5.6 inches is very close to my firewall and it is likely I can not get this 3D plane to balance with the battery in the present location.  I have to move it forward. There are a few assumptions here including the aileron servos have equal arms so they cancel each other out, the wings and tail are disregarded because they are equalized by symmetry and there are at least two more servos to go in tail.  So let’s add those to the chart to see if we are closer.

ItemWeightArmMomentCG
Engine155g2310 
ESC305150 
Battery80131040 
Elevator servo934306 
Rudder servo934306 
 283 21127.46 inches

Now you can see we are getting closer to leaving the battery at the present location.  There are a few of things you must consider.  First, does the CG as you determined using this method fall within the prescribed CG by the manufacturer?  Second, are the items on the fuselage centerline; thus reducing any arms that don’t cancel each other out laterally but still needing to be considered (tail wheel and nose gear)? And you assume the fuselage is straight and acting within the physical laws of a lever.  Basically, if a fulcrum is placed under the fuselage (the CG point) will it balance?

Try this method.  Weigh everything and measure where you think those items will go.  It is possible you can avoid situations where you can not get a battery far enough forward on some planes or the need to remove the fuel tank to get the receiver battery farther forward under the fuel tank.  A well balanced plane ensures a quality flying experience.

 

David Vaught, Featured News

About the author

A frequent contributor to Model Airplane News and Electric Flight, Dr. Dave is a true RC enthusiast with over 40 years of flying experiences as well as a private pilot license. He flies and writes about everything from ornithopters to giant-scale aircraft, building and flying an average of 20 planes a year.

4 Responses to “Weight and Balance the old fashioned way”

  1. Leonard L Roderick says:

    Very interesing article David. Thank you. I am fairly new to the actual ‘flying’ part of RC. I knew that CG was important, but when I started asking questions, no one could elaborate or give me any new info regarding CG. I even heard ‘just ballpark it’. Your article has given me the info that I have been needing.
    Thanks Again.
    Leonard L. Roderick
    Aurora, Colorado

  2. russell bachman says:

    david—i have a question– if you build your own planes which i am sure you have, what is the best way to figure out where the cg is if you dont hav e someone else to rely on? i have heard 1/3 of the way back from the leadfing edge of you wing is a good place to start, is there a better way. and this dont hold true to jet type airplanes with sweptback wings does it? thanks russell

  3. grimmy says:

    why not do it the easy way works every time attach a string to the tail other end of string to prop nut location lift plane up determine flying angle then use plum bob from finger tips down to plane this gives true CG then when u try it you may have to move items forward or backward to acheive your true CG

  4. David says:

    It would seem likely that CG and a plane in level flight are one in the same. Years ago I built a sling that wrapped two cords over a dowel and then the loops slipped over the wings. The device then has a pendulum that hangs down from the center of the hanger and with that you hang the plane level and mark the CG reference point as the plane would be in level flight. Knowing where this point is allows you to add components.

    To confirm this we need the Mean Aerodynamic Chord (MAC). To get this we take the wing and extend a parallel line on each end of the wing root chord equal to the tip chord and extend a parallel ling on the tip equal to the wing root chord. Then we draw a line between the points that intersects one the wing. This makes a point at which we can measure the CG. This creates the 50% MAC or a stating point for CG.

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