Trivia Testers : Breaking the Sound Barrier

Breaking the sound barrier in your 172?
How fast would you need to go in order for at least part of your Skyhawk to exceed Mach One?

  1. That’s easy: 662 knots, at sea level, on a standard day.
  2. about ninety five knots past Vne. (You’d never get even that far, however.)
  3. There are actually parts of almost any light piston airplane that are already exceeding the speed of sound in level cruise flight.
  4. a bit over 400 knots

When It’s Okay To Bend the Rules
When may a pilot descend below the glide slope on a visual (or even an instrument approach) and land on the displaced threshold?

  1. anytime normal bracketing maneuvers are performed for the purpose of remaining on the glide slope
  2. any time the approach is flown in daylight visual conditions
  3. only in an emergency
  4. only at certain airports at which the normal regulations have been waived by the FAA Air Traffic Division in order to accommodate aircraft requiring a greater runway length, and only under certain conditions (namely, those in choice B)

Aerial scythes
Airplane propellers generate most of the noise that is the source of complaints (at least those against piston and turboprop airplanes). The noise becomes much worse as propellers reach transonic speeds. If airplane wings can spoof critical Mach numbers by means of their sweep-back, why aren’t the propellers on small airplanes also swept back?

  1. Hydrodynamics isn’t intuitive, and neither is the behavior of fluid motion. The fact is that sweep-back totally loses its effectiveness on smaller scales, where airfoil chords are measured in inches, rather than in feet.
  2. Although it is true that small airplane propellers could benefit to some extent from even a small degree of “back sweep”, generally most low speed airplanes wouldn’t benefit significantly from it.
  3. Most people couldn’t afford it. Scimitar-shaped propellers are actually common among racing planes, but at a cost of about $20,000, they are beyond the reach of most.
  4. They don’t need to be. Most people don’t realize it, but the very property of increasing tangential velocities at greater and greater distances from the center of rotation effectively gives propellers a forward sweep, which actually accomplishes the same thing.

The Answers…

Breaking the sound barrier in your 172?
Answer: You may have already guessed which part was the most likely one to go supersonic: your propeller. Taking a C172 of average vintage, say a 1981 172P, it might have a 75-inch prop and a maximum allowed engine speed of 2700 rpm. Translated into a tip velocity at the start of a short field takeoff (say while the brakes are held and it isn’t yet moving), that would be about 884 feet per second, or around 523 knots. (That’s already fast enough to make quite a racket!) To calculate the forward speed needed for the resultant velocity vector to equal Mach 1 involves simply involves a bit of trigonometry. Assuming a standard temperature of 15 degrees Centigrade, at which the speed of sound is about 662 knots, the necessary forward speed works out to about 683 feet per second…or about 405 knots (choice D). Since the speed of sound in air is dependent almost entirely on temperature, and because it goes down about one and one-seventh knot for each Centigrade degree decrease in temperature…I guess you’d want to pick a cold day. Taking the silliness level down a peg though (or maybe that should be up a peg), in reality the air accelerated over the propeller airfoil, because it travels at a speed considerably greater than its speed of flight, would reach supersonic speeds well before it did. The so-called critical Mach number for an airfoil is often much less than one, such as 0.7. (In the case of this propeller, that would mean you would create sonic booms on the run-up pad, because its tip speed is already close to four-fifths of the speed of sound.)

When It’s Okay To Bend the Rules
Answer. First of all, CFR Title 14, Part 91.129(e)(2) says that for large or turbine-powered airplanes approaching to land on a runway served by an instrument landing system, they shall fly at an altitude at or above the glide slope between the outer marker or point of glide slope interception and the middle marker. For the rest of us, 91.129(e)(3) says that an airplane approaching to land on a runway served by a visual approach slope indicator shall maintain an altitude at or above the glide slope until a lower altitude is necessary for a safe landing. At the end of sub-paragraphs (e)(2) and (e)(3) it says basically what’s in choice A. But that won’t hold the lawyers back, if you prang something “behind the lines”. And the conditions in choice B are nowhere near sufficient to justify such a thing. (I don’t know about you, but every flight instructor that I’ve ever flown with has pretty much branded into my forehead the fact that one is not supposed to land on a displaced runway threshold.) Choice C is of course always a thumbs-up. (Got fire? Well heck, then you can land on the closest taxiway, if you think it will help!) If you picked it, give yourself half a point for basic common sense. But you don’t get the whole goody bag, because the most correct response would be if you had choices C and D. Yes, D. First, a displaced threshold is similar to a runway, but is designed as a safety net for pilots to use in their calculations for the amount of room needed for a takeoff or landing. It is generally available for takeoffs in either direction, or for landings from the opposite direction. It is not designed for use specifically for a landing, but rather provides extra “rolling” room. So what gives? Well, there are actually airports at which these rules have been waived. Where might you see this? The northeast U.S. Airport/Facility Directory has a Special Notice page for Boston-Logan International Airport’s runway 4R stating that, during daylight hours and under VFR meteorological conditions, the full 10,005 feet of the runway (and not just the measly 8,840 feet past the landing threshold) is available to even large and turbine-powered aircraft. (It does also caution operators to be vigilant for vessels traversing the Boston Inner harbor Channel, so watch out!)

Aerial scythes
Answer: There are a few advantages. Although swept-back rotating airfoils experience span-wise stresses on the blades, due to centripetal force not being parallel to their length, propellers with some type of sweep-back have been researched and designed. They’re shaped a bit like scimitars. On the more pronounced versions, there are usually eight or more blades, and they have actually been used on some high performance turboprops. (Some piston propellers have a slight sweep-back, but they’re not all that eye-catching.) They are expensive to design and manufacture, and swept-back airfoils generate less lift and thrust than straight ones. However, they do generate less noise and drag, to a small extent. The correct answer is B.