Model Glider Weight and Balance
by Gary Binnie
Hi folks, I was asked to write an article on a method of weighing large model gliders after I wrote a short ‘how to’ on RC Groups, so here it is!!
My background is 20 plus years service in the Royal Air Force as an airframe engineer and a lifelong aero modeller, I currently work in Formula 1 aerodynamics. During my military service I took up full-size gliding as a hobby, became an instructor and tug pilot, but perhaps more relevant to this article I applied my aircraft engineering knowledge to maintaining club gliders as a volunteer inspector. Later I took up a full time post as a gliding instructor/tug pilot/engineer for four years (it got me away from the ‘fast jets’ and there are worse jobs!!).
I have a couple of 4 meter span model gliders and recently I refitted one with new radio gear and battery. Realizing that I needed to check the center of gravity (abbreviated to c.g. from now on) I had two choices, lift it on my finger tips in the traditional way or buy one of those neat but relatively expensive balancing rigs.
A third choice came to me, why not try and weigh it like I used to weigh the full-size gliders? It was a wet afternoon so I tried it and it worked!!
The importance of the center of gravity position:
I would think that model pilots and builders reading this will already know about the importance of the c.g. (some people call it the ‘balance point’ and they are technically correct) and may have found out the hard way but others may not so I thought I would add a few words on the importance of the c.g..
All full-size gliders have a range of the c.g. for flight to allow for different weight pilots and the addition of water ballast in the wings and tail fin (vertical stabilizer) where it is used.
Model kit manufacturers usually give a set position or also a range, like the full-size.
Let’s look at the effects in flight of various positions of the c.g., they are the same for both models and full-size (ever seen ‘Flight of the Phoenix?!!).
A c.g. close to the forward limit (or beyond it!) i.e. nose heavy will have the following main effects:
- Ineffective elevator with slow control response in pitch, may not ‘flare’ out after the landing approach unless carrying extra speed!
- Reluctance to stall or spin (you may be looking for this characteristic though).
- Difficult to slow down and trim to a normal gliding speed.
Aft or rearwards c.g.:
A c.g. close to the rear limit (or beyond it!) i.e. tail heavy will have the following main effects:
- Very effective elevator, ‘twitchy’ or uncontrollable in pitch. Worse at higher speeds on aerotow.
- Prone to stalling/spinning.
- Unable to trim for higher speeds.
Of the two possibilities the forward position is safer and is why model sailplane instructions often recommend a forward position for early flights, moving it rearwards in stages later if desired.
Full-size glider weight and balance:
It is obviously important that full-size gliders are weighed accurately to determine the empty c.g. (calculations are done afterward to determine the minimum and maximum cockpit weight) and, not so obviously, to determine the weight of ‘non-lifting’ parts for structural strength/load factor reasons (wing bending loads).
Gliders are generally weighed when they are new and after repairs/repainting. National regulations apply. They are also sometimes weighed with the pilots on competition grids to make sure that they are not overloaded!! Once they are weighed a sheet is completed for the logbook and a placard fitted in the cockpit with the loading limitations.
The process of weighing a full-size glider involves weighing all the components separately (fuselage, wings and tailplane) then rigging it and weighing it as a complete machine.
Before the advent of electronic scales large spring balances were used (and probably still might be at some establishments!), the difference being the accuracy and ease of weighing a glider on top of scales rather than slinging it from above by the hangar roof beams or small cranes. Modern glider factories have load cells set into the floor, even easier.
The glider is placed in the flying attitude using an inclinometer, datum lines are drawn using chalk on the floor and distance measurements are taken (or are sometimes given by the manufacturer). These distances (the ‘moment arms’) are combined with the measured weights to work out the c.g..
For more on the subject of full-size glider weighing here is a link to the British Gliding Association procedure: http://www.gliding.co.uk/bgainfo/technical/ampmanual/4-1.pdf
Model glider weight and balance:
Can we use this method for model gliders? Don’t see why not, in fact it is much easier as we don’t need to worry about cockpit limits or load factors. For very large model sailplanes it may well be the only way as we can’t physically lift them by hand.
Here goes then, there are three steps to the process:
- Weigh the model at two points on the fuselage and do some measuring of the moment arms.
- Do some calculations.
- Add nose or tail weight to correct the c.g. if needed and reweigh to check.
I weighed my elderly Multiplex ASW 22 with two electronic kitchen scales, one under the tail skid and the other under the nose using a balsa former to fit the contour.
Using a modified version of a Microsoft Excel spreadsheet for full-size weighing it produced a c.g. of 109 mm aft of datum, tail heavy as I was looking for 85 mm. You can do the maths manually but using a spreadsheet is easier and should remove any human error. The formula for calculating manually are given in the lower right of this image.
On another tab of the spreadsheet is a moment arm calculator, I entered the actual c.g. and then entered a guessed amount of nose weight until it arrived at 85 mm. 205 grams were needed, I added that, weighed it again and it came up with a c.g. of 83 mm. Close enough for me and 2 mm on the safe side!! Without any maths just keep adding ballast and reweighing.
Just to confirm the maths I lifted this 4 kg glider onto my fingertips and it balances perfectly on the two dots that I placed under the wings.
There are some ‘variables’, the measurements need to be accurate and the glider should be in the flying attitude per the full-size). On the full-size the centers of the weighing points are ‘plumb bobbed’ down to the floor and chalk marked, as is the datum position and a center line. It’s worth also checking the calibration of the scales with a known weight; the full-size scales I used were calibrated regularly.
If you have a wheel you can use that as your front weighing point and use the first formula
I hope that was informative or at least interesting, I believe it is correct to the best of my knowledge. As a disclaimer please use this method at your own risk but I trust it enough to use on my own models and many full-size pilots have flown very safely in gliders that I have weighed, I even flew some of them myself!!