Retrotechtacular: Forces Acting On An Airfoil

floating film title We’ve probably all experimented with a very clear demonstration of the basic principles of lift: if you’re riding in a car and you put your flattened hand out the window at different angles, your hand will rise and fall like an airplane’s wing, or airfoil. This week’s Retrotechtacular explains exactly how flight is possible through the principles of lift and drag. It’s an Army training documentary from 1941 titled “Aerodynamics: Forces Acting on an Air Foil“.

What is an airfoil? Contextually speaking, it’s the shape of an airplane’s wing. In the face of pressure differences acting upon their surfaces, airfoils produce a useful aerodynamic reaction, such as the lift that makes flight possible. As the film explains, the ideas of lift and drag are measured against the yardstick of relative wind. The force of this wind on the airfoil changes according to the acute angle formed between the airfoil and the direction of the air flow acting upon it. As you may already know, lift is measured at right angles to the relative wind, and drag occurs parallel to it. Lift is opposed by the weight of the foil, and drag by tension.

wind tunnel testing

Airfoils come in several types of thicknesses and curvatures, and the film shows how a chord is derived from each shape. These chords are used to measure and describe the angle of attack in relation to the relative wind.

The forces that act upon an airfoil are measured in wind tunnels which provide straight and predictable airflow. A model airplane is supported by wires that lead to scales. These scales measure drag as well as front and rear lift.

In experimenting with angles of attack, lift and drag increase toward what is known as the stalling angle. After this point, lift decreases abruptly, and drag takes over. Lift and drag are proportional to the area of the wing, the relative wind velocity squared, and the air density. When a plane is in the air, drag is a retarding force that equals the thrust of the craft, or the propelling force.

monometer tubesAirfoil models are also unit tested in wind tunnels. They are built with small tubes running along many points of the foil that sit just under the surface. The tubes leave the model at a single point and are connected to a bank of manometer tubes. These tubes compare the pressures acting on the airfoil model to the reference point of atmospheric pressure. The different liquid levels in the manometer tubes give clear proof of the pressure values along the airfoil. These levels are photographed and mapped to a pressure curve. Now, a diagram can be made to show the positive and negative pressures relative to the angle of attack.

In closing, we are shown the effects of a dive on lift as an aircraft approaches and reaches terminal velocity, and that lift is attained again by pulling slowly out of the dive. Remember that the next time you fly your hand-plane out the window.

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Building a Quadcopter with a CNC Mill and a 3D Printer

Quadcopter

Quadcopters are a ton of fun to play with, and even more fun to build. [Vegard] wrote in to tell us about his amazing custom DIY quadcopter frame that uses a commercial flight control system.

Building a quadcopter is the perfect project to embark upon if you want to test out your new CNC mill and 3D printer. The mechanical systems are fairly simple, yet result in something unbelievably rewarding. With a total build time of 30 hours (including Sketchup modeling), the project is very manageable for weekend hackers. [Vegard's] post includes his build log as well as some hard learned lessons. There are also tons of pictures of the build. Be sure to read to read the end of the post, [Vegard] discusses why to “never trust a quadcopter” and other very useful information. See it in action after the break.

While the project was a great success, it sadly only had about 25 hours of flight-time before a fatal bird-strike resulted in quite a bit of damage. Have any of your quadcopters had a tragic run-in with another flying object? Let us know in the comments.

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Japanese Micro Planes

Some very well engineered micro planes(translated) have been buzzing around the net. The goal here is ultra light weight. These suped-up paper planes have a remarkable target weight of around 10 grams (translated). The lighter the micro plane is the slower and more maneuverable it will be leading to some pretty interesting and scary applications. For controls it looks like many of the planes are using infrared receivers/transmitters (much like you would find in a TV remote hint hint). Getting the lightest plane possible has forced the designers to come up with some pretty ingenious tricks. For example, instead of using packaged servos they use a coil of wire wrapped around a rare earth magnet to control the flaps. You can see these home made “servos” in action after the break.

Some have taken a more classic approach and used rubber band power instead of a li-po/motor combo.

[via Make]

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Four generations of motion simulators

We like a good flight simulator but often find the available control schemes lacking. [Roland] not only builds his own controls, but creates full cockpits that add physical motion to the mix. He completed his third generation cockpit last year.  It’s pictured above as well as in video after the break. That design uses a belt system to move the tricked out cockpit.

Now he’s started work on prototypes for generation IV. This time he’s using three Sarrus linkages to replace the belt system.  We saw these linkages yesterday in an extruder prototype and if they can handle the load they should work well for this application. Video of the prototype is embedded after the break but be warned, the lewd thrusting motions are not for the faint-of-heart. [Read more...]

Single-wing flight based on maple seed aerodynamics

one-winged-flight

The Samara Micro-Air-Vehicle is a product of over three years of work at the University of Maryland’s Aerospace Engineering Autonomous Vehicle Laboratory. The Samara is an applicant in the DARPA nano air vehicle program. Unlike the ornithopter we saw in July, this vehicle uses only one wing for flight. A small propeller on a rod mounted perpendicular to the wing provides rotation. The pitch of the wing is changed to climb, descend, or hover.

You can see a video of the flight tests after the break. The sound the Samara makes reminds us of classic alien invasion movies and the use of Verdi’s Requiem for the background music during flight tests (2:43) seems quite fitting. At about 5:45 there is some on board video footage that is just a blur of the room spinning by. This would be much more useful if a few frames per second were snapped at exactly the same point in the vehicles rotation.

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Autonomous helicopter learns autorotation


Stanford’s autonomous helicopter group has made some impressive advancements in the field of autonomous helicopter control, including inverted hovering and performing aerobatic stunts. The group uses reinforcement learning to teach its control system various maneuvers and has been very successful in doing so. One of their latest achievements was teaching the bot the emergency landing technique autorotation. Autorotation is used when a helicopter’s engine fails or is disengaged and works by changing the collective pitch to use the airflow from descent to rotate the blades. The group has more flight demonstrations on their YouTube channel.

[via BotJunkie]

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