How to balance your model aircraft properly

Well you have built this all singing, all dancing masterpiece model aircraft from scratch. But now the problem is where to place the centre of gravity? So many aircraft modellers find themselves in bother on first flight because the balance is just not where it should be.

It all hangs in the balance

Aircraft can be balanced anywhere from in front of the wing leading edge to 50% chord or more and still be flown successfully.

Aahh you say ... Not so ! It will never get off the ground, or will be uncontrollable if it does.

Well, have a look at the canard, Centre of Gravity somewhere between the canard and wing leading edge and flies beautifully. At the other extreme you will find the so called 3D types with huge tailplanes that can carry part of the flight load, a bitch to land smoothly but will do all of the 3D manoeuvres you may wish.

A couple of modern jet fighter types are balanced so far to the rear that computers are required to anticipate the next pitch error and input corrective controls before the error becomes apparent.

Centre of Gravity (C of G)

I think it’s about time we exploded some of the myths surrounding suitable placement of Centre of Gravity (C of G) for your beautiful new model.

Centre of Gravity placement is not an exact science. If it were there would not be any aircraft with front and rear seats or rear baggage compartments. The aeroplane will be easy to manage if the balance falls within a certain range. How to determine this range is the purpose of this article.

Assuming that when you design your Model you have given the wing a small positive incidence relative to the tailplane, say about 1 to 2 degrees, you will want to set the C of G so that when the aeroplane flies it will do so with a neutral elevator. The objective here is to ensure that under all flight conditions the tail carries no positive load. During normal flight there will be no noticeable difficulty if the tail is carrying part of the flight loads but when the airspeed drops closer to the stall, say in a landing approach the smaller tailplane can no longer carry its portion of the aircraft weight it will stall and drop sharply, (rearward C of G) thereby quickly increasing the angle of attack of the wing until it stalls ! Ah well, back to the drawing board.

So now we can see why the aircraft needs to be balanced at or forward of the centre of lift of the wing. So what happens if the centre of gravity is placed further forward ? The tailplane will now have a negative load (will try to rise) as the airspeed drops off the tail will rise reducing the angle of attack of the wing and the effect will be that the aircraft speed will increase until the tailplane can again carry the load. It will be self-correcting and therefore stable.

Moving the C of G further forward will not be a problem until the extra Negative lift generated by upward deflection of the elevators is insufficient to control the rise of the tailplane. This really just means that in the slow flight speeds of approach and landing there may be insufficient elevator command to round out (level the aircraft) and it will nose in to the airstrip a bit. This is really how lots of modellers land anyway, (before the aircraft is ready). The answer is to land at a higher airspeed where the elevators can still carry the load. Just before the tailplane stalls.

So how do we locate the Centre of Gravity range so that our beaut new model will fly successfully?

We will look at a typical wing to see the various points of interest in determining C of G. As can be seen from this borrowed illustration, stability is adversely affected by moving the C of G too far to the rear, like balancing on a ball.

The reverse becomes more apparent if too far forward, it becomes more difficult to manoeuvre the aircraft from its stable flight.

With a constant chord wing the best C of G range will be between 25% and 33% chord. If you want your model to be really stable, and not so sensitive in elevator, you should err toward 25%. If the model has a heavy wing loading it may be beneficial to move the centre of gravity even beyond the 25% nearer to 20% for first flight. This is to ensure that it is more difficult to lift the nose too high at low flying speeds, there by inducing the stall.

Tapered and Swept wings

When dealing with tapered wings or swept wings, whether tapered or not we must first determine the location span wise of the mean aerodynamic chord.

As you would expect with a tapered wing, this line will be nearer the wing root as the tapering section becomes smaller nearer the tip and can carry less of the aircrafts weight.

The crossed lines are obtained by extending the tip chord lines each side of the wingtip by a dimension equal to root chord “A”, and “A” and extending the root chord lines by the tip chord “B”, and “B” and drawing the cross from each of these points as shown. Now we can draw the line parallel to the direction of travel through the centre of the cross, where that line intersects the leading edge is the point from which the 25% and 33% figures are obtained for the C of G range.

Now we have a starting point for the all-important first flight and from which to make minor adjustments.

The pilot will do well to remember that the neutral stability point is not set in concrete at 35% chord. So many factors have an influence; wing thickness, wing section, leading edge thickness and laminar flow ratios are just a few. Therefore, particularly for your maiden adventure flight, be conservative and set the balance nearer to the 25% point.

How do I find the location of the centre of gravity accurately you ask? Easy !

1. Take 4 pieces of strong string and hang them from a single point source in the roof of your workshop.
2. On the bottom of two of these strings add a couple of wire hooks located conveniently above the floor so that your model can be hung from them. These can be hooked onto the main undercarriage legs of your inverted aircraft.
3. Now take string number 3 and add a hook in the same way, but tied so that it can be adjusted up or down the string. Hook this to the tailwheel or nose wheel, as the case may be.
4. Next place a spirit level on the underside of the tailplane and adjust the string so that the tailplane is level (horizontal).
5. Now take the fourth string and hang upon it a plumb bob such that it comes to rest just clear of the belly of the model.
6. And there you have it, accurate location of the Centre of Gravity !

It now becomes easy to relocate batteries and other components to make some adjustments. If extra ballast is going to be required placing this temporarily in locations where it can conveniently be fixed will be easy as well.

And while you are on the job, lateral adjustment to the centre of gravity can be accurately done as well. All while the model hangs, makes it so easy to see what influence your ballast is having to the C of G location.

Now to the final positioning of the C of G so that the aircraft will fly as you want. Finding the sweet spot might be the best way to describe what we are after.

Setting the trim

The trim should be adjusted so that at about ½ power the aircraft will fly straight and level, increase to full throttle will result in a climb, while reducing to low throttle will cause the model to reduce altitude.

When you have this trim set, fly the aircraft straight and level at ½ throttle and then introduce a gentle dive of about 30%. When the airspeed has increased, return the stick to neutral and watch what happens. If your balance is about right the aircraft should continue in the same dive, or slightly begin to recover.

If however the dive increases (becomes steeper) upon release of the stick, then the aircraft is tail heavy. If on the other hand the Aircraft climbs sharply upon release of the stick, it will be nose heavy.

The reason for the increase in descent rate in a tail heavy aeroplane is that, when it is set up for level flight at half throttle, an increment of down elevator had to be added to hold the tail up for level flight. But when you induce extra airspeed in the shallow dive, the elevator becomes much more effective after return to “neutral”. So the dive is increased.

Of course the opposite occurs when the aircraft is nose heavy. Now the increasingly effective up elevator induces a zoom.

Tune your aeroplane with these balance factors firmly in mind and your creations will fly as they should. Whatever the cause of any misadventure, it will not be due to a poorly placed centre of gravity.

Go out there and have some fun !