When I meet other paper aeroplane enthusiasts (generally in smoke filled rooms, or dimly lit car parks) a debate I occasionally get into is whether putting wing-tip fins on paper aeroplanes is a good idea.
As can be seen by a quick glance, some of the paper aeroplanes on the main page have wing-tip fins, and some do not. This is because I don’t think there’s any one-size-fits-all answer here: it all depends on the plane and the maker.
Fins can be beneficial because they can be used to change various aerodynamic properties of the paper aeroplane – which can be used to good effect in skilled hands. However, fins can be a bad idea, especially if you don’t know what you are doing; and/or are not very precise with your folds; and/or make them in the wrong place.
Let’s look at a selection of the things fins can do:
1) Move the centre of lift to optimise pitch stability.
Fins can move the centre of lift of a paper aeroplane. By turning a bit of wing into a bit of fin, you largely nullify it as a lift producing surface. If the paper aeroplane has swept or delta wings (which is the case for most paper aeroplanes) then the fins will be towards the back of the plane. And if you create fins at the back of the plane, you also reduce the lift at the back of the plane – whilst the lift at the front of the plane stays the same. The relative position of the lift hence shifts forward.
This could be a good or bad thing depending on the plane. If your paper aeroplane tends to nosedive, then its centre of lift may be behind the centre of gravity along the longitudinal axis of the plane. Shifting the centre of lift to approximately meet the point of the centre of gravity can help it maintain level flight. However, if the centre of lift is already ahead of the centre of gravity – meaning it has a tendency to pitch upwards – then shifting the centre of lift yet forward will only exacerbate the problem.
2) Shift the centre of gravity of the plane up or down the vertical axis.
If you make a paper aeroplane with fins that point downwards, you are shifting mass down the vertical axis of the plane. This is a good thing for enhancing the keel effect and is why, when I do make paper aeroplanes with fins, I generally make them so they point downwards. The exceptions to this (paper aeroplanes such as The Piranha) generally come about because it would be impractical to turn the fins downwards on such planes.
3) Aid directional stability.
If the paper aeroplane has fuselage that tapers towards the back (such as The Piranha), it will tend to lack directional stability unless fins are present. The plane will easily yaw and bank as it flies, potentially humiliating you in front of crowds of onlookers. The reasons for this are explained in an earlier post on directional stability.
The inclusion of large fins that are PARALLEL to the centre line of the plane, will help correct for this. You are creating vertical surface area behind the centre of gravity, and in all likelihood, one with a long moment arm.
4) Increase wing rigidity.
If the wings of your paper aeroplane tend to twist, or flop a about a bit, adding a fin can help increase rigidity. This helps them maintain their shape, producing more predictable flights. This is why I include fins on paper aeroplanes like The Squirrel
So all good you might think? Yes? Yes?
5) Mess up a plane’s directional stability.
Fins can mess with a paper aeroplane’s directional stability for two reasons:
a) They are not made so they are parallel to the centre line of the plane. This is obvious, but often ignored. The simple solution is to make your folds more precisely.
b) They are made ahead of the centre of gravity. This generally only happens in planes with canards, like some variants of paper aeroplanes like ‘The Spyder’. Essentially, by putting fins near the nose of the plane, you’re doing the opposite of what is recommended here.
6) Increase drag.
If a fin is not completely parallel to the centre line, it may increase drag. This itself often happens for two reasons
(i) they are simply made badly
(ii) aeroplasticity -or, to use more simple language: the plane changes shape during flight.
This is especially a problem for paper aeroplanes like the dart. During the launch phase of the flight, aerodynamic forces will tend to push the fuselage together all the way along the plane; however, as the paper aeroplane starts to slow down, the fuselage will come apart in a V shape at the back. This can turn a pair of fins into a pair of airbrakes. This is why fins should usually be avoided in dart-type planes.
7) Act as accidental elevators.
If your paper aeroplane has a fin that is not parallel to the centre line of the plane (either due to imprecise folding or due to the effects of aeroplasticity) AND it is not perfectly vertical (either due to imprecise folding or aeroplasticity) then it can deflect air upwards or downwards. This can have the same effect as tweaking the backs of the wings upwards or downwards… however, the problem is it’s difficult to spot, so you might be creating extra up or down elevator without knowing it.
Ok, so I think I’m spent on the aerodynamic factors to consider with fins, so here are some take-home lessons:
– Fins need to be parallel to the centre line of the plane, so fold them precisely and avoid them in planes like The Dart. Otherwise, you’ll just create drag, cause unwanted yawing moments and mess with your plane’s lift.
– You should generally have your fins pointed down rather than up. This shifts the centre of gravity lower on the vertical axis of the plane, good for enhancing the keel effect and lateral stability.
– Well made fins can aid directional stability on *some* planes (especially those with fuselage that tapers or disappears at the back of the plane), but they will only help if made precisely and towards the back of the wing, not on canards.
– Fins can shift the centre of lift of a plane to aid pitch stability. Increasing the size of the fins at the back shift of the plane shifts the centre of lift forward. Reducing the size of (or eliminating the fins) fins can shift the centre of lift backwards.
– Fins can increase wing rigidity. This is useful if you notice that the wings of your paper aeroplane do not hold their shape during the plane’s flight.