Airfoils
You’ll often see the letters “NACA” followed by a string of four or five digits, in reference to a particular airfoil. What is the meaning of the numerical designators referring to commonly used airfoils?
- Their various digits indicate camber and thickness.
- They are arbitrary numbers assigned in sequence.
- They represent when they were certified; they’re Julian dates.
- It means such a reference is obsolete; NACA became NASA in 1958.
Answer: A wing is designed to produce lift and minimize drag, and its shape is the most important feature in fulfilling this role. Almost any cross sectional shape (meaning the airfoil) will produce lift. Many present-day airfoils are wing cross sections described by the early NACA organization (the National Advisory Committee for Aeronautics, chartered in 1915, operational from 1917-1958, and from which the National Aeronautics and Space Act of 1958 created NASA). When NACA began its investigation of airfoils in the 1920s, they started by examining the most successful airfoils of the time. Very quickly they observed significant commonality among popular airfoils, even though they had at first appeared quite different, and they also realized that simple mathematical equations could represent an airfoil, just by specifying the camber line and thickness distribution. As it was described in NACA’s landmark report “The Characteristics of 78 Related Airfoil Sections from Tests in the Variable Density Wind Tunnel” thickness distribution is a single equation, while the camber is usually two joined quadratic equations. As you probably guessed by now, it’s choice “A”.
The NACA four-digit “foil family” was developed in 1932, and combined three sets of digits: the first digit represents the value of the maximum camber (the amount that the airfoil curves, in percent of the chord width); the second digit is the location of the maximum camber from the leading edge in tenths of the chord width, and the last two digits denote the maximum thickness of the airfoil in percent. They have been tested in many wind tunnels over the years. For example, the NACA 2412 (which you’ll see on most Cessna 172s) has a camber of 2% of chord with maximum camber occurring at 40% of chord from the leading edge, and a thickness/chord ratio of 12%. Most airfoils for light aircraft have 2% to 4% camber, thickness ratio of 12% to 15%, and maximum thickness at around 40% of chord. Symmetrical airfoils have only thickness, and no camber. NACA four-digit symmetrical foils are listed as “NACA 00_ _” (and used on rudders, boat keels, etc). NACA five-digit foils were developed in 1935. The primary difference is the use of a different camber line (having more forward camber). In a five-digit airfoil, 1.5 times the first digit is the design lift coefficient in tenths, the second and third digits are one-half the distance from the leading edge to the location of maximum camber in percent of the chord, and the fourth and fifth digits are the thickness in percent of the chord. Modified NACA four and five-digit series designations include additional numbers (such as after a hyphen), indicating other attributes such as roundedness of the leading edge, or design lift coefficient. Later advances involved further six, seven, and eight-digit designators and involved specifying desired pressure distributions over the airfoil and then mathematically deriving the shape producing it; these airfoils were not generated using some sets of analytical expressions as the earlier series. But speaking of earlier, they still say “NACA”; the name stuck, even though there isn’t any more NACA.
For the Birds
When birds “come in for a landing” what is the major factor influencing their landing direction?
- obstacles
- Birds almost always land in the same direction in which they have been flying, regardless of wind.
- Just like us, it’s wind; birds almost always land into the wind to minimize their forward speed and maximize the braking action afforded when they suddenly spread their wings to catch the wind and stop.
- There are no determining factors.
Answer: C. Only a birdwatcher can prove it: watch some, sometime. There are exceptions though, such as when a large bird like an albatross has to land on a small island in strong crosswinds, in which case it will crab, and then make a “crosswind landing”.
More Stuff For the Birds: No DF Steers For Feathered Friends
Birds, like pilots, don’t always have a tailwind. As a matter of fact, they have the same problems we do. How do they handle crosswinds in flight?
- They continuously crab while en route.
- They drift off course, and make a series of corrections as they go.
- trick question: Birds do neither. Actually, they get blown off course, and then when in sight of their destination, they “home in” on it visually. In cases where greater distances are involved, they use the Earth’s magnetic field to do the same.
- They get lost, then walk the rest of the way.
- We have no idea.
Answer: First, the long-winded (pardon the insufferable pun) version: This is one question which actually doesn’t have a simple answer; the subject still generates controversy and discussion among students of bird migration. Typically, a migrating bird is observed at one place and time, and knowing the wind at its altitude, scientists can determine the bird’s course and ground speed, as well as its heading and air speed. What they usually don’t know is the bird’s destination (lacking flight plans, and all), which leaves them guessing as to whether it is experiencing wind drift from a fixed heading, or whether it is adjusting its heading (and/or its speed) to compensate for the wind (however completely) in order to “fly direct”. With satellite tracking however, it is possible to follow large birds over their entire course of migration. Because many birds exhibit a high degree of fidelity to specific nesting or migratory destinations, once a given individual has been followed for a complete “there and back” cycle and enough data has been accumulated, there will be better answers to the question of where it was trying to go.
The answer is, at least for the present, that birds are sometimes drifted completely by crosswinds, and sometimes correct completely. Larger, faster-flying birds, such as waterfowl or shorebirds, are less subject to drift than slower birds (like small songbirds). In order to assess the amount of drift, most birds need some visible ground reference. Birds flying over large bodies of water appear to experience wind drift. They may use waves (also however influenced by the wind), but this seems to result in only partial correction for the drift. There is evidence that birds may allow themselves to be drifted or to compensate for drift depending upon where they are in their migratory journey. The closer to their destination they are, the more likely they are to correct because precise navigation at that point is obviously important. Earlier in the trip, drift experienced during one flight might be compensated by drift in the opposite direction on another flight, or lighter winds on a subsequent flight might make compensation less costly. This stuff can all be subjected to optimality modeling, and to some degree, the birds do seem to behave according to the predictions of the models.
And the short answer: Choice E. We have no idea (yet).
