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Correct C of G


Ian Lennox
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After completing my first balsa kit (Rans S9 Chaos - Oct 2020 rcm&e) I was disappointed that the plane is extremely tail heavy. I now understand that the C of G should be considered throughout the build process; so may question is:

How do you do this? 

I don't want to wait until the final stages of the next build to check the c of g. I think I'm going to build it again from scratch. 

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Experience Ian, you can judge by the length of the nose and where the given C.G point is, in the case of something like a Hurricane you know you have issues from the off, so it's keep weight down as much as possible rearward of the c.g and plan for the servos/battery to be well forward, sometimes though it will still require lead up front.

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If it's any consolation Ian my first successful r/c model was a Junior 60 built from a Flair kit and powered by an Irvine 20 two-stroke. I had inadvertantly made the tailplane out of the hardest wood in the box so it required one and a half pounds of lead under the engine to make it balance. It still flew well.

 

I made a lighter tailplane later on and was able to remove all of the lead.

 

Junior 60 in Flight.jpg

Edited by David Davis
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Hope I am not overdoing it

But an uncovered, nearly finished fus is well able to show its close balance point without the wing in place.

If it is critical

I build and assemble the main servo tray, as is usual, commonly 3 servos.

I have a rail tacked either side of the inside of the fuz; able to slide the tray.

For I/C, I put the motor and battery in their position, and slide the servo tray too and fro, with the fuz suspended by a rod through the C of G.

Control rods are adjusted accordingly

On many builds, a wing tube is in this position, and you just push a supporting rod through.

A ball park, very close C of G can be determined before final assembly, gluing and covering.I

Final C of G is usually a tiny adjustment after the main components are now fixed.

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Hi Ian, This is not unusual, indeed, its a very common problem, especially if you have a model with a short nose moment (the distance from the front of the wing to the very front of the model)

So what to do? Its pretty well impossible to check during the build, but sometimes there is a note about the problem on drawings, instructions etc.

Build the tail from light materials. Sometimes kits dont select the best wood for each component, go easy on the glue (cyano is good), and dont overdo tailwheels, connections etc. Remember that a small amount of weight at the tail can need a lot at the nose to counterbalance it.

At the pointy bit, move the heaviest bits, batteries etc, as far forward as possible, and if you do need to add some church roof to make things balance correctly, make sure that its as far forward 

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Ian,

 

Might be worth considering the choice of model you select to build? Warbirds and old ic designs tend to have a lot of weight up front and a short nose where more recent designs aka Peter Millers Ohmen was designed for electric. Achieving the C of G in the design for electric plus adding the access hatch was all factored in, but if you go for an older/ic design then its worth considering a few minor changes like longer nose or battery placement as far forward as possible. Its not normally an issue as the lead just replaces what would have been engine etc.

 

The problem arises when you build heavy (in the tail or just everywhere!) and end up with a porker that a lot heavier than it should be as it will need to fly at a higher speed + landings are faster than a lighter version. Some designs are far more tolerant of weight gain and others like my Focke Stick are not, but I am not worried about that as its intended to just go fast!

image.png.13290ba7fcfa72cc43e7aa07e0d091eb.png

 

Experience (not that I have much of that) will tell you how you can build light and remind you at the balancing stage that you haven't. Personally I build the wings first, then the fuselage and then hang bits to see where the C of G looks like it will end up. As mentioned above having things like servos and RX/flight batteries available that can be moved about + keeping options open all help.

 

I did not build the Vulcan, but when it came to finishing ended up with two 4S5000 in the canopy (that's a fair old bit of weight!) to achieve C of G which makes it have a very high wing loading. The result is that it flies very scale + does not mind quite windy conditions, down side is it has to be managed well and glides like a manhole cover (but so what). Every time time it files its reminds most of the club members what can be achieved by Brian (who for most of his life lived with one arm and one leg) RIP.

image.thumb.png.4e52cb31483fadceb8b2d2af462f3c5e.png 

  

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How 'extremely' tail heavy is it? I mean have you actually worked out how much weight is needed to balance it out? 

 

If so, and its 10% or less than the final flying weight of the model and the final weight is not excessive for the model type just forget it. Add the weight and enjoy the model. I did this with my recently tested Hurricane. 2lbs lead in the cowling above the engine and a flying weight of 23lbs. Under 10% lead, fly's very well. 

 

By all means do what you can to minimise weight added at the front end, but the thing has to balance so dont loose sleep over adding some. 

 

Assuming your model is the one i think it is they quote 1.7kg ish as the flying weight so if you have to add 170g of lead and are still hovering around 1.7kg when done, just dont worry. 

Edited by Jon - Laser Engines
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The weight of engine is crucial, and I like to work out roughly what this should be from the get-go, using simple static balance calculations. 


For a scale model, it is usually possible to obtain the manufacturer’s data of the weight of the full-size plane, the weight of its engine and the position of its CG. From these one can calculate, using moments, where the CG of its airframe is relative to the CG. If one then makes the assumption that the CG of the total plane and the CG of the airframe should be in the same scaled positions in the model as in the full-size, one can then make further moment calculations to estimate the required weight of engine to balance the model exactly. One also has to make an assumption about the target weight of the complete model on the basis of its size, and for this I use a desired cubic wing loading. I also include in the moment calculations any components in the model that are not in the same corresponding positions as in the full-size, such as fuel tanks and batteries.


Then I search around for a model engine of the suitable weight, bearing in mind that its scaled CG is usually not in the same place as that of the full-size engine. Also, one has to be careful to include all the clobber on the engine, such as exhaust and ignition systems, propellor and spinner. Manufacturers of model engines often to not include these “extra” items in their quoted weights. If one has the physical engine to hand it is best to weigh it with all the clobber and estimate the position of its CG at the same time. There is a tendency to want to use a model engine that is lighter than the desirable weight, so that its clunky shape can be more easily accommodated in the fuselage, but this will then require extra ballast. This approach can only be taken so far, because the lighter the engine, the more underpowered the model and the more ballast it will need - which is actually the last thing an underpowered model needs! 


As others have said, the position of the CG can be checked as the building proceeds. Bear in mind, though, that sheeting a model airframe will tend to move the CG backwards a little (because the sheeting is roughly distributed around the airframe CG and not the total CG). The same is true of finishing (glassing and painting). 
 

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On a sports model I leave the nose longer than plan and balance the model roughly before fixing the front bulkhead position, rubber bands hold the motor/bulkhead and battery in position and a 'guessed' bit of covering material laid onto the tail to compensate for covering. This is useful when converting a design from IC to electric and worked well for my Ballerina and Tiger Cub, both needing no added lead.

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10 hours ago, Ian Lennox said:

After completing my first balsa kit (Rans S9 Chaos - Oct 2020 rcm&e) I was disappointed that the plane is extremely tail heavy... I think I'm going to build it again from scratch. 

 

How tail-heavy?  How many mm back from the CG indicated on the plan?

 

Do you really need to be so radical as to contemplate a complete rebuild from scratch?!

 

Can you not re-position servos etc further forward, make new lighter tail parts, and add a wee bit of lead to the nose?

 

What does it look like at present - any photos?

 

 

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