# Retrotechtacular: Forces Acting On An Airfoil

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.

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.

Airfoil 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.

# Weather station graphs history on webpage

Kudos go out to [Jose] for his work getting so many different components to talk to each other in this Arduino weather station that using a Raspberry Pi to display the data online.

The components shown above make up the sensor package. There’s an Arduino with a custom shield that interfaces the barometric pressure sensor, real-time clock chip, a digital temperature sensor, and a humidity sensor. On top of that shield is an XBee shield that lets this push data back to the base station. [Jose] also rolled in an LCD character display and a few buttons so that the user may view weather data without heading to the web.

A Raspberry Pi board makes up the other half of the XBee pair. It harvests the incoming data from the radio module using a USB to Serial converter cable. You can see the data log on the webpage linked above. Just choose the “LIVE” menu option and click on “Daily” to get a better overview of humidity and pressure changes.

# Force carbonating root beer with dry ice

[Paul] is sick and tired of his homemade root beer being flat. He analyzed the problem with his carbonation techniques and ended up with a method of force carbonating beverages using dry ice.

He starts of by discussing the various methods that are used to carbonate beverages. There’s the old yeast and sugar trick that takes place inside of a sealed bottle. But this takes time, and if you don’t calculate the mixture correctly you could have over or under carbonated bottles (or exploding bottles in the case of glass beer bottling). [Paul] himself has tried the dry ice in a cooler full of root beer method. The problem is that the cooler isn’t pressurized so the carbonation level is very low. You need to have cold temperatures, high pressure, and the presence of carbon dioxide all at the same time in order to achieve high levels of carbonation.

His solution is to use a 60 PSI safety valve. He drilled a hole in a plastic bottle cap to receive the valve. He then drops a few chunks of dry ice in and seals it up. The valve will automatically release the gas as the pressure builds past the 60 PSI mark. What he ends up with is a highly carbonated beverage in a matter of minutes.

If you don’t mind spending some cash you can use an adjustable pressure regulator. This way you can carbonate just about anything.

[Thanks Steven]

# Sensor array tries to outdo the other guys

The team over at the Louisville Hackerspace LVL1 is not going to be outdone when it comes to collecting environmental data. They put together this Frankenstein of sensor boards that lets you collect a heap of data showing what is going on around it.

At the center-left a small Arduino clone is responsible for collecting the data. Data storage is not talked about on their write-up, but if that’s an ATmega328 chip you should be able to work out an easy way to store data on the 1k of internal EEPROM. If that’s not enough, there is an I2C bus included on the board making it easy to add a compatible EEPROM.

The sensor on the bottom left should look familiar. It’s a DHT11 temperature and humidity sensor we’ve seen popping up in projects lately. But wait, there’s also a TMP102 temperature sensor; but that’s not the end of it. A BMP085 pressure sensor also includes a third temperature sensing option. Want to see when the lights go on in the room? There’s a CdS sensor and a TSL230R Lux sensor for that. An op-amp circuit can measure the sound level in the room via one of the Arduino’s ADC pins. And finally, an RTC board is used for time stamping the data.

Obviously this is overkill, and we’re sure it’s meant as a test platform for various sensors. All of them have been mounted on the protoboard and wired up using the point-to-point soldering method.

# Logging bubble frequency and pressure in your fermenter

In an attempt to add technology to his brewing process [hpux735] build a sensor rig that monitors bubbles and pressure during fermentation. What does this have to do with brewing great beer? We’re not sure and neither is [hpux735], but he’s got some preliminary readings to spark your imagination.

The bubble sensor itself was inspired by a SparkFun Tutorial where fermenting wine was monitored with a data logger. It uses an optical gate to detect the passage of air. But the goal here was to combine bubble frequency with internal pressure measurements to calculate how much CO2 is being vented. Perhaps it would be possible to get an idea of how close the batch is to completion based on those calculations. A hole was drilled into the fermenter side of an airlock to take these pressure readings.

This actually works quite well during secondary fermentation when the bubble frequency is quite slow. The hardware is able to discern a pressure difference before and after a bubble has passed the lock. Unfortunately the system breaks down during the vigorous bubbling that takes place soon after pitching yeast. See a few bubble-counting clips in the video after the break.

# Improving a cheap espresso machine

For those of us that would like a good cup of coffee but don’t want to put up with the ‘burnt butt’ taste of Starbucks and don’t have a decent coffee shop nearby, we’ve had very few options. Most of us have been made to suffer with an el-cheapo espresso machine. [Joe] sent in a great build that improves these el-cheapo models and brings them up to the quality we would expect from their more expensive brethren.

For the best pull from an espresso machine, the great [Alton Brown] says 200° F water must be forced through the grind at around 10 PSI atm. [Joe]’s espresso machine can’t build up pressure because the heating element is only active when the lever is in the ‘brew’ or ‘froth’ position. To build up pressure in the water reservoir, [Joe] simply added a pressure gauge to the frothing attachment. When the gauge reads the necessary 10 atmospheres, just move the lever over to the ‘brew’ position and enjoy a nice cup of espresso.

[Joe] has already tested the pressure relief valve of his espresso machine. With the gauge in the way, [Joe] can’t make use of his frother, but a secondary valve could easily remedy that. [Joe] hasn’t published his espresso hack anywhere, but he did email us some pics of his build. We’ve embedded them as a slideshow after the break. Check out the pressure gauge on the frothing attachment and the pressure relief valve below.

# Pressure mapping sensor mat

[imsolidstate] built his own pressure sensitive mat. It utilizes two discs of copper clad board with a piece of foam in between for each of 64 sensors. As the foam gets compressed, the capacitance between the two pieces of copper changes, a measurement that is fairly easy to make with an analog to digital converter. The mat is being used to measure how well a horse saddle fits the animal. Data is read in through a serial port and then mapped using Excel. This prototype proves that the concept works but [imsolidstate] mentions that there’s room to improve the sensitivity and that there could be more noise filtering incorporated into the design.