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Secrets to making paper airplanes fly well

Secrets to making paper airplanes fly well

The "secrets" to making paper airplanes fly well are largely the same adjustments which make hand launched gliders fly well. Most people have the unfortunate idea that a good paper airplane needs no adjustments after the basic folds are finished. All real airplanes have trim tabs to make small adjustments to the plane, and all paper airplanes need small adjustments to fly their best. There are a few basic adjustments and principles which will transform the paper airplane novice into a paper airplane expert. The following flying tips are generally covered in my books, but I go into a little more detail here.

1 Dihedral
One of the most common paper airplane mistakes is to leave the wings folded down at an angle. That is called "anhedral", and it reduces the lateral stability of your paper airplane. What you want is called "dihedral" which is when the wing tips are the highest part of the wing. The resulting lateral stability will help keep your paper airplane flying straight, or perhaps in a gradual turn. With lateral instability your paper airplane will either roll over on its back and crash, or enter into an ever tightening spiral which becomes a spiraling dive. Just remember - keep your wing tips up.

Technically dihedral provides a stabilizing rolling moment due to sideslip. For example if the plane yaws to the left (positive sideslip), the right wing has a slightly increased angle of attack (AOA) because of the dihedral, while left wing's AOA is decreased (this is most easily imagined if you think about 90 degrees of sideslip). The resulting rolling moment is to the left, which is stabilizing. During a level turn, the yaw rate combined with the stabilizing yawing moment due to yaw rate results in a little bit of sideslip, positive for right turns, negative for left. That small amount of sideslip together with a stabilizing rolling moment due to sideslip (dihedral effect) results in the plane wanting to roll out of the turn. With anhedral, the plane wants to roll into the turn, resulting in a "graveyard spiral". The tendency to roll into or out of a turn is called the spiral mode, which is controlled mainly using dihedral. Most real airplanes have to limit the amount of dihedral they use to keep the Dutch roll mode, a rapid left and right oscillation, under control. While dihedral makes the spiral mode more stable, it reduces the damping of the Dutch roll. I have rarely witnessed any Dutch roll problems with paper airplanes, likely due to increased yaw rate and roll rate damping associated with low airspeeds. As a result all paper airplanes should be flown with plenty of dihedral.

2 Weight Forward is Good
The paper airplane balances is called the Center of Gravity (CG), and there is a specific CG position known as the Neutral Point which provides neutral pitch stability. If the airplane has a CG ahead of this point, the plane is stable, if its behind this point its unstable. Naturally all airplanes without computer assisted flight controls need a CG ahead of their neutral point. For rectangular wings the neutral point is ¼ of the distance from the nose to the tail. For delta wings (such as the common dart paper airplane) the neutral point is ½ of the distance from the nose to the tail.

Stability means the plane, if disturbed, will return to its original state. For pitch stability it means the plane will seek a single airspeed. A plane which is unstable in pitch will either pitch up into a stall, or nose dive, but won't settle out anywhere in between. A stable airplane will tend to oscillate up and down a few times, but converge on a steady flight speed. Many typical paper airplane designs are stable, but just barely. As a plane becomes more and more stable, it wants to fly faster and faster. To counter this tendency, up elevator must be used to produce a good trim airspeed. This is why many of the classic paper airplane designs are nearly neutrally stable. Few people realize good pitch stability requires a heavy nose and some up elevator. The classic designs rely on the small inherent "up elevator" effect (positive zero lift pitching moment) resulting from the swept wing, and possibly the airfoil shape. Thus many classic paper airplanes can be flown with no elevator adjustment. Sometimes they fly well, many times they don't, and they always have poor stability.

I like to add a tiny amount of up elevator to the classic pointed nose paper airplanes, to make sure they don't dive. If I have the time and materials, I like to add a few layers of tape or a paper clip to the nose of the plane to improve its stability. Most "square" paper airplanes have plenty of weight in the nose, and require some up elevator to fly well. Actually the amount of up elevator needed on a paper airplane is a good indicator of its pitch stability. Build a paper airplane (any kind) and place a paper clip on the nose. Make a few flights to determine the best amount of up elevator needed. Now move the paper clip back an inch or two, and repeat. The amount of up elevator needed is reduced, and the plane becomes more sensitive to elevator adjustments. When the paper clip has been moved back to a point where you are using nearly no elevator deflection, and you can't get the plane to fly well, you have the CG at the neutral point (try to balance the plane on a finger, the point where it balances is the neutral point).

3 What about the airfoil shape?
Most people who are reading this know that airplane wings are "Cambered" which means they have generally a curved shape, with the top of the airfoil rounded and the bottom fairly flat. As explained in section 3.0, paper airplane wings must be thin to work well. In addition, they need very little camber, and generally any curvature is limited to the front portion of the wing. I have had people ask me why I don't advocate cambered airfoils for paper airplanes in my books. Since most paper airplanes are flying wings, only small amounts of camber are practical, as large amounts of camber create nose down pitching moments which need tails to balance. Generally I do use a little curvature at the leading edge of the wing. I have noticed that paper airplane performance is not noticeably degraded with flat, uncambered airfoils. The reason for this is likely due to low Reynolds numbers. Remember that a large portion of the boundary layer across the front of the wing is laminar flow, but for high lift we need a turbulent boundary layer. The use of a flat uncambered wing produces a large pressure gradient at the leading edge, which likely aids the transition to a turbulent boundary layer, which could likely be the reason for little camber in insect wings. Also, swept wings with uncambered leading edges promote vortex flow just behind the leading edge on the upper surface. Although lift coefficients at these Reynolds numbers aren't large enough to promote a large amount of vortex lift(vortex lift increases exponentially with lift coefficient), any vortex flow likely helps the transition to a turbulent boundary layer.

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