DuPage Free Flyer - Flight Theory

Lift - A Tale of three Foamies

Details: Category: General-Flight Theory General-Flight Theory; Published: 27 January 2021 27 January 2021

How do you make an effective Wing out of a sheet of paper, or a sheet of plastic foam? You can leave it as it is, cut it into a pleasing shape, and put a power source on it big enough to force it through the air at a speed sufficient for lift. Kites have this shape, with wind for power. You can form the sheet around an object so that the sheet takes a permanent curve, called an under camber. The wings on the Wright flyer had this shape, and they certainly preferred it to a flat plate. You might form your sheet into a recognized efficient airfoil, maybe a Clark Y section, as used on the Spirit of St. Louis or a Piper Cub. You might carve or mold a thick sheet into the section, or form a thin sheet around ribs and a spar. Both these methods require more work, and add more weight per unit area, than just cutting out a flat plate.

There is another way to make a wing out of foam sheet, that was discovered by Dick Kline and Floyd Fogelman when they were looking for more efficient, i.e. longer gliding, paper airplanes, now called a Kline-Fogelman airfoil. Folding a paper airplane makes a wing with a step, and Dick and Floyd found that they got better performance with the step in the right place. Since then, when R/C modelers discovered blue insulation foam sheet or foam core poster board made a good combination with clean electric power for building lots of fun planes, their discovery found a home.

http://theparkpilot.org/kline-fogleman-airfoil-design

While adding an extra strip to a flat plate foam wing might be justified as a reinforcement, or possibly a way to increase the critical angle for stalls, others might argue that Kline-Fogelman wings don't live up to expectations. This doesn't deter enthusiasts from building planes that fly with such wings, and doing it with a minimum of fuss.

https://en.wikipedia.org/wiki/Kline%E2%80%93Fogleman_airfoil

My Gym Stick has an under camber airfoil, probably best at low speeds, and a thin K-F strp, probably just a reinforcement. My EZ-Fly has a more prominent K-F strip, and flies just fine. My NutBall has no K-F strip, and might be a more stable flyer if it had one. I surely would have used less glue repairing its wing's leading edge.

What is Lift

Details: Category: General-Flight Theory General-Flight Theory; Published: 09 January 2021 09 January 2021

Airplanes rise into the sky because they transfer the momentum they have from forward motion into an upward component called lift. According to Newton, if the airplane pushes the air, the air pushes back. This works for all shapes of wings, but for some better than others, when the combination of angle of attack and drag are accounted for. There is another effect, discovered by Bernoulli, that aids lift, especially with airfoils of efficient shape. We need to account for the air going over the wing, that is changing speed and direction.

There is a Wikipedia article that goes into this a bit deeper than we might like, but has some interesting flow simulations included. https://en.wikipedia.org/wiki/Lift_(force)
NASA summary pages relating to lift refer to a simulation program, without a lot of explicit references to equations: https://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html
One of them discusses the wrong way to explain the relation of Bernoulli's equation and lift : https://www.grc.nasa.gov/WWW/k-12/airplane/wrong1.html
Another one says that wing shape aids in deflecting the air flow over the upper wing surface downward, thus increasing lift. Flaps use this strategy to increase lift at low speed.
https://www.grc.nasa.gov/WWW/K-12/airplane/shape.html
This NASA summary discusses the relation of lift and a wing's "angle of attack" with respect to the air. https://www.grc.nasa.gov/WWW/K-12/airplane/incline.html
Here is an MIT student's report on the Bernoulli effect as it applies to wings: https://web.mit.edu/2.972/www/reports/airfoil/airfoil.html