Basic Guide To Model Aircraft Design

MODEL AIRCRAFT DESIGN is not as hard as you may initially think; in fact, it is only as hard as you wish to make it. In this question I shall be outlining some of the basic terminology and theories that you should know before starting a design. But before I get stuck in I should mention a good rule of thumb to keep in mind whilst designing…

If it looks right, it'll fly right.

I am going to be looking at the most common aircraft configuration, i.e., a 'tractor' type with the tailplane to the rear of the wing, as seen in the plan view picture, left. Starting with the wing, there is a simple ratio that you'll need to know. The ratio of the wingspan to the wing chord is known as the 'aspect ratio'. This is a very important ratio, since the greater the aspect ratio the more efficient the wing will be (hence gliders have long skinny wings). This is down to tip vortices, basically the greater the aspect ratio the smaller the percentage of wing that is affected by the tip vortices, thus more lift is produced. In general you may find 4:1 to 8:1 maybe typical for a sports model and perhaps 8:1 to 15:1 for a model glider. The drawing shown has an aspect ratio of 4:1, i.e. the span is 4 times that of its chord. The tailplane area is calculated by a simple function of the wing. Tailplane area should be around 15-25% of the wing area depending upon the moment arm length. The longer the moment arm the smaller the tailplane can be. A good starting point for the moment arm would be 1.5 times the wing chord combined with a tailplane at 20% of the wing area.The fin size is again calculated via a percentage of the wing. As a rule it should be about 10-15% of the wing area, but again it does depend on the moment arm length. If in doubt it is always best to start with a fin that is too big and possibly trim it down after if it is oversized. Once again apply the rule of thumb, if it looks right ...! Moving on; the length of the nose is not too important but is usually at least one wing chord long. Generally the nose is made long enough to get the model to balance correctly but remember that too long a nose only adds weight and is more vulnerable to damage when landing.

You will find that many aircraft have dihedral built into their wings. Dihedral is used to stabilise in the roll axis when in flight. This works due to the components of lift acting upon the wings i.e. if a model with dihedral drops a wing, that wing half that dropped will produce more lift in the vertical plane since its component of lift will be acting more vertically when compared to the other wing half. This in effect results in more lift being produced by the wing half that dropped and therefore that wing will rise up again. There are three types you should know about, dihedral, tip dihedral and polyhedral. The most effective of these being polyhedral, however also the hardest to build. Generally polyhedral and tip dihedral is more common on rudder and elevator thermal soarers, whereas dihedral is more common on sport models and gliders that incorporate ailerons. The amount of dihedral to use does depend on the model type; anything from 0-12 degrees total is common, obviously the more you have the more stable the model, but be warned too much can cause problems.

There are four main types of aerofoil used; symmetrical, semi- symmetrical, non-symmetrical and under-cambered. The type you use does depend upon on the model. Symmetrical i.e. the same shape on top and bottom, are used for aerobatic models so that they can fly either way up with very little trim change, though these can be a little awkward to build on a flat board. Non-symmetrical is good to use on a glider or vintage type model where good lift is required. Semi-symmetrical is the middleman and a good compromise between lift and aerobatic precision; these are generally a good choice for a semi- aerobatic sport models. Finally, under-cambered aerofoils, mostly used on early vintage models but fell out of favour due to being a little difficult to build. However more recently with new building materials such as sheet depron foam, under-cambered sections are once again widely used especially in slow flying indoor models.

Once you have decided upon the aerofoil type, you will need to decide what wing incidence or angle of attack to use it at. The angle of attack is a measure of how many degrees by which the leading edge of the aerofoil has been raised. This value can be anywhere from 0 to 6 degrees depending upon model type. The greater the angle of attack the more 'floaty' the model will be. For example, free-flight models and vintage models may well use 3 to 6 degrees, thermal gliders may range from 1 to 3 degrees and an aerobatic model 0 to 0.5 degrees.
Finally, you will need to balance the model. Generally a model should be balanced at 1/3 average wing chord, i.e., 1/3 back from the leading edge. However, be aware that aerofoil choice, wing incidence and wing shape will all affect the balance point for a given model. The best way to test a design and find the balance point is to make a chuck glider of your design from balsa sheet around 10" to 12" span. This will allow you to find the centre of gravity, check the flying surface proportions are correct and see just how the design works in flight.
I hope this may tempt you into making you own design and even if it's only a chuck glider then it's been worthwhile. However, this is only a brief overview to some areas of model design and for the more serious amongst us I would recommend buying a good book. One I would recommend is Model Aircraft Aerodynamics by Martin Simons, published by Argus Books, and I'm sure there are many others out there.

Should you decide to build a design or are in the process, send Reg a picture and I'm sure he would love to use it on modelflight.