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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Ignored disabled man builds his own damn elevator

diy_wheelchair_lift

There’s an old saying that goes something like, “When the going gets tough, the tough builds their own 5-story wheelchair lift.”

Actually we’re pretty sure that’s not even close to how the saying goes, but when his local council turned their backs on [Dmitry Bibikow’s] request for wheelchair access to his apartment, that’s exactly what he did.

[Dmitry], an avid mountaineer, was injured in a climbing accident that left him without the use of his legs. Unfortunately for him, he and his family reside on the 5th floor of an apartment building that was not handicap accessible. Rather than move out, he asked the local council to install an elevator, which they agreed to.

Time passed, and as the project sank deeper and deeper into a mire of bureaucracy, [Dmitry] began to lose hope of ever seeing an elevator installed. After six years of relying on friends to help him get in and out of his apartment, he took matters into his own hands and installed a chair lift just off the side of his balcony.

According to [Dmitry] it works great, and he can get from the front door to his apartment well before his more able neighbors make it up the stairs. So far, the city council has not said anything about the lift, and he hopes it stays that way.

Robotic helicopter that can grasp a payload

Like the Grand Theft Auto RC missions come to life, this helicopter can grasp objects for transport. They don’t have to be a special size or shape, and it can lift them even if they are not centered. This is thanks to a load-balancing hand (originally developed as a prosthesis) that relies on flexible joints and a tendon-like closing mechanism. As you can see in the video, the light-weight chopper has an on-board camera so that the operator can see what is being picked up. This little guy has no problem lifting objects that are over one kilogram while remaining stable in the air.

[Thanks Paul]

Update: Acrobatic quadcopters team up

We usually envision small wheeled robots when we thing about swarm robotics but these cooperative quadcopters make us think again. This is an extension of the same project that produced those impressive aerial acrobatics. It may not be as flashy, but watching groups of the four-rotored flyers grab onto and lift loads is quite impressive. There is also a shot of one dropping a 2×4 and immediately compensating for the loss of weight. We’re not certain, but it looks like team lifting doesn’t require the 20 high-speed camera rig that the acrobatics did. We’ve embedded the demonstration video after the break.

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