# Trivia Testers : Stalling and Falling

Stalling and Falling

An airplane can be made to stall at any speed. Okay then, at what airspeed will any airplane stall, if the stick (or yoke) is briskly pushed forward so that it enters a rapid descent, and the airplane is “unloaded” to exactly zero gees?

1. By definition, it will immediately stall, regardless of the airspeed (even if it were zero).
2. At zero gees, stall speed is the square root of the cosine of the angle of incidence, multiplied by the quotient of present airspeed to the square of Vs. (Generally, since this works out to just a very few knots, the wings usually won’t stall.)
3. At zero gees, an airplane cannot stall at any speed below Mach 1.
4. Mach, schmach…This is a trick question: The airplane won’t stall, period. It can’t, because at zero gees, the wing doesn’t need to produce any lift!

Answer: D. Don’t you go telling me that I got you with this one!

Slip Sliding Away…
If the tire pressure on your airplane is 25 psi, at what speed could hydroplaning occur?

1. 45 knots
2. 50 knots
3. 54 knots
4. 63 knots

Answer: A. Just like with a car, as you may have heard (but filed away in a dusty cabinet of your memory, hopefully ready to recall when needed) the speed at which hydroplaning can occur is, as weird as it sounds, nine times the square root of the tire inflation pressure. The square root of 25 is five, and five times nine equals 45.

There are several reasons that it’s a good idea to keep this particular trivia item in mind. First of all, tire pressures can vary greatly between different airplanes that you may fly, and even with the same airplane, it can still vary. For a Cessna 172 for example, if you walk out and the tires look soft and squishy–that’s a technical term–with a bulge on the bottom, and you suspect (hah!) that they’re underinflated, you just might have only 25 psi in there, or even less. On the other hand, if you walk out and the tires look rock hard, round all around, and not all of the tread is in contact with the pavement, it might be as high as 50 psi. Of course you’re supposed to keep the tire pressures at the level that the manufacturer recommends, because even pressures that are only 10 psi low would allow your airplane to hydroplane at a lower speed–which you may need, one rainy day.

Several things besides the tires can also influence hydroplaning. Tire treads having grooves that allow water to escape from being trapped between the runway and the rubber would obviously increase the allowable depth of water before hydroplaning could occur. (Depending on tread design, such a critical water depth can range from one to four-tenths of an inch.) The more worn the tire, obviously the more likely it will hydroplane. For you heavy iron drivers out there (all both of you), undercarriage assembly also affects hydroplaning characteristics. With tandem “bogies” there is a kind of clearing action from the front wheels, which reduces the depth of water encountered by the rear ones, therefore delaying hydroplaning and increasing the given depth of water before you and your passengers start taking water skiing lessons. Runway surface is obviously also just as important. Surfaces that have a coarse matrix (or better yet, grooves) will take longer to build up to limiting depths, especially in strong winds. With little or no wind, most runways have an adequate transverse slope for good drainage during all but the heaviest of rain. However, drainage can still be compromised with a crosswind above 10 knots, because a crosswind that is blowing up that slope could allow water to build up higher on that side. This would lead to hydroplaning, or (if your airplane was big enough to straddle the hump) asymmetric braking. Very heavy precipitation rarely lasts long, and waiting a few minutes is usually enough for water to drain off sufficiently to get rid of really critical water depths.

Early Birds
The first wartime aerial photo reconnaissance was used

1. During the War of 1812
2. During the Civil War
3. During WWI
4. During WWII

Answer: C. Most of the recognition and progress in aerial photo reconnaissance generally shows up in the history books beginning with World War One. Although some clever innovator probably made the connection, and it is likely that photography probably took to the air fairly soon after it was invented in the mid-nineteenth century, no vertical or oblique aerial photography captured by balloons from the Civil War period have been found. During the Civil War, there certainly was a US Army Balloon Corps, but most activity (if not all) was limited to verbal descriptions and sketches. Despite its advantage to the Union Army, the unit was deactivated in 1863. (They had a not-surprising tendency to draw enemy fire.) Credit for the first aerial photograph goes to French author and artist Felix Tournachon, who went by the popular name of Nadar. He captured the first aerial photo from a balloon tethered over the Bievre Valley, in 1858. (The oldest aerial photograph in existence is a view of Boston by James Wallace Black, in 1860.) So as far as actual evidence goes, the first verifiable instances of photo reconnaissance work were during World War I. (Early cameras used glass negatives, where the photographer had to change plates between exposures.) By the way, choice A is out; Louis Daguerre didn’t even begin creating images until the 1830s.