As far as paper aeroplanes are concerned in this article, a directionally stable plane means a plane that can keep flying straight in one direction without ‘slipping’ (moving sideways) or unintentionally yawing (turning) to one side as it flies. This was always one of the key goals that I sought to achieve with the design of my paper aeroplanes. Fortunately, there are a number of aerodynamic tricks you can use to improve the directional stability of your planes. Some of these techniques are more complex than others; however, here are some of the simplest:
1. Use the basic calibration technique for countering yawing, outlined here.
This is the most important technique if you’re using a design that has already been tried and tested, like the planes on the main page of my site. However, if you want to experiment a bit more, or are designing your own planes, then you should also consider either:
2. When you fold the wings, make sure the back of the fuselage is higher than the front.
3. Give the plane large fins towards the back of the plane (fins at the front will have the opposite effect).
Of the latter two techniques, I generally recommend making the fuselage higher that the back rather than making large fins, simply because it’s harder to get the fins perfectly straight than it is to get the fuselage perfectly straight. However, both of these techniques can be applied to almost any paper aeroplane without making considerable modifications to the fundamental design.
So why do these things work?
Well let’s look at a side view of a paper aeroplane, in this instance, the classic dart.
In this design, the centre of gravity is right in the middle of the longitudinal axis of the plane, because there is no more paper (i.e. no more mass) at the back of the plane or at the rear, the only difference is that the folds are more concentrated at the front than at the back. This means that the fuselage of the plane is considerably higher at the back than it is at the front. In turn, this means that the section of the plane in front of the centre of gravity has a considerably smaller surface area than the back.
This means when the plane starts to move sideways in the air rather than straight, the back of the plane will encounter more air resistance than the front. The plane will then rotate around the centre of gravity into the direction of the airflow, like a weather vane.