The Magneto March: Left, Left, Left-Right-Left….Left, Left, Left-Right-Left…
Perhaps you’ve noticed that key-operated magneto switches are configured to read, from left to right: “OFF-RIGHT-LEFT-BOTH-START” (or something like that). Why is “left” on the right, and “right” on the left? Why isn’t it “OFF-LEFT-RIGHT-“, etc?
- It’s not even an issue; not all are that way. It actually depends on who made the switch; many indeed are also labeled as “OFF-LEFT-RIGHT-BOTH-START”
- Because that’s the way it was always done; i.e., it was purely arbitrary.
- Because each relative position (not its letter label) probably reflects which respective p-lead is grounded.
- The magnetos are intentionally wired as they are viewed from the front of the airplane, and not from inside the cockpit.
Answer: A magneto is both a low voltage alternating current generator and a high voltage coil and distributor, providing for an aircraft what the battery/alternator, high voltage coil, and distributor do, on a car. Magnetos act as a generator and create their own low or primary voltage, then act as a spark coil and transform the low voltage into high voltage. The high voltage spark gets routed to the spark plugs at about the right time to ignite the fuel-air mixture in each cylinder. So magnetos convert rotational energy into high-voltage pulses that are used to fire your aircraft’s spark plugs, without the need for external power from a battery or the electrical system. Because they work even when there’s a total electrical failure, they are a preferred spark source, for obvious reasons. The “left” position is positioned on the right and the “right” position is labeled on the left because this is one way to indicate which of the respective P-leads is grounded (more on that in a minute). What’s a P-lead? The P-lead is a thick shielded wire (about 16-gauge) running from the cockpit ignition switch to the ungrounded end of each magneto coil’s primary winding. (The “P” stands for “primary” by the way.) It’s sole purpose, in a way, is to actually “break” something. Well, it allows the ignition switch to disable a magneto (or both of ’em) by grounding the “hot” side of the primary coil in the rotor of the magneto. (The rotor is like a cam operated spark generator. A spark is generated because of a sudden–and intentional–collapse of a magnetic field in one coil and the induced and stepped up voltage–the spark–in another coil.) When the P-lead is grounded through the ignition switch, these so-called breaker points are unable to interrupt the current flow in the primary coil, making the mag incapable of generating a spark. Maybe it helps to think of those dangly strips you see on some automotive vehicles, which allow stray static electricity “sparks” to be grounded, and get harmlessly dissipated, down to the ground. By the way, I may be covering old material here, but for those new to all this stuff, aircraft engines have two spark plugs in each cylinder and two magnetos. One plug in each cylinder is fired by one magneto. The other mag fires the other set of plugs. The benefits of this “dual ignition” are that if one system fails the engine will still operate, and also that there is some improved combustion of the mixture (with two plugs firing in each cylinder), which increases power output and performance. Okay then… (I must warn anyone who is dyslexic: you’re gonna have a real tough time on this one.)
Taking as an example the Bendix 10-357200 series ignition switches, if the ignition key is in the “R” position, the left lead is grounded (so that it can’t spark), and the right mag is un-grounded, so that it can. (The common way that folks refer to this “enabled” state is that we say the mag is “hot”.) When the ignition switch is in the “L” position, the right lead (mag) is grounded and the left mag is the one that’s un-grounded, or “hot”. (To help you along a bit here, just think of a kid who has misbehaved, and who has then been grounded by his parents: he can’t leave the house…or circulate…or make any more sparks for awhile…does that help?) So thank goodness for small favors; the letter on the ignition switch at least represents which mag you’re “on”. Why they reversed their position in terms of “handedness” would make an interesting study in terms of the lineage of functionality, but my guess is that it was their way of indicating that you’re grounding the mag other than the one you selected. So…to reiterate: when you rotate the ignition switch to that left position (which happens to be labeled “R”), you’re grounding or shutting off the left mag; when you check the mag that is labeled “L” (which, as I have already beaten to death, is on the right), you’re grounding the “right” (not wrong!) magneto. And, finally, when the switch is on BOTH, you are un-grounding both mags. So…the position label on the ignition switch is for the mag that’s activated, while the positions themselves are for the ones that aren’t. So…the answer’s C…for “Clear as mud”…right? (Notice by the way that I used the word ” probably”…you may also come across different ignition harnesses, or other wiring diagrams like this one, which in this case seems to show that whichever mag you select means that’s the one you’re grounding out! The bottom line is, as you always hear, don’t move a prop unless you’re well positioned to get outta the way in a hurry, ’cause the mags could be “hot”.)
Little Bighorn, A Century Later
For what clever but ill-fated design was General George Armstrong Custer’s great grand-nephew best known?
- a reciprocating aircraft engine that ran on methane, and whose emissions consisted of only carbon dioxide and water
- for founding the only public use airport located within the borders of both a National battlefield monument and an active Indian reservation, (in Hardin, Montana), but mostly for crashing his airplane during a landing attempt there exactly one hundred years to the day after the historic battle of Little Bighorn on June 25, 1976. (Actually, the battle took place during both June 25 and 26, 1876, but you can appreciate the irony).
- a unique STOL airplane wing design
- for designing and flying the first rotary-wing aircraft whose main rotor (having four blades) could be spun down in flight and function as a pair of forward and rearward-swept wings
Answer: C (for Custer) The Custer Channelwing (“CCW”) was an STOL design wherein each of two engines was placed above and within a half-cylinder shaped airfoil, each of which behaved like the lower half of a venturi. Because the speed of the air drawn through these channels was much greater than the velocity of the airplane through the air, a vastly increased lift was possible at a very low airspeed. (After all, Bernoulli’s Law states that the dynamic pressure of a moving fluid is affected proportional to the square of its velocity!) Although the CCW still needed some small distance (under 200 feet, with no wind) in which to become airborne, only because some speed was needed for its control surfaces to become effective, it could literally lift off while taxiing. The last and most ambitious version, the CCW-5, had an empty weight of 3200 pounds, an approximate cruise speed of about 180 mph, and even though it was the biggest and heaviest of the CCW designs, it had a landing speed of a little over 20 mph. It was even reportedly able to hover in an 11 mph wind. The Custer Channelwing number one, or CCW-1, first flew on November 12, 1942. The only possible technical difficulties were, theoretically, some potential for introduction of asymmetrical loads on the propellers (which was not proven), as they left and entered the lower channeled half of the wing, and an increase in drag from interactions between the channel and propeller which would have limited its top speed to somewhere around 300 knots (350 knots, with minor modifications). Still, it was quite an idea…
Willard Custer was one of the early pioneers in STOL aircraft. He devoted almost 40 years to his design, as well as the company he founded, to make it a reality. Unfortunately, political naïveté, his unwillingness to suffer fools gladly, and his famous lack of diplomacy, which rivaled that of his famous ancestor, caused alienation of potential backers and government contacts. During one particularly frustrating meeting for example, he stood up, pointed a finger at a general, and shouted, “General, you’re just a damned liar!” He was offered many millions for his patents, but wouldn’t let anyone work with it but himself. Only the CCW-5 at the Mid Atlantic Air Museum in Reading, PA, and CCW-1, until recently located at the Air & Space Museum’s Garber Restoration Facility (soon moving to Dulles Airport in Virginia, from Suitland MD) still exist.
Willard Custer was not a pilot, and choices B and D were both fictional. There may however have been some association with methane (choice A) and what Mr. Custer thought of the value and contents of his detractors.
Magic In Numbers
A pilot can use the face of the directional gyro to determine the proper succession of perpendicular ground tracks (or if there is no wind, headings) in order to fly rectangular patterns. If the pilot is familiar with the area, local landmarks often provide assistance. So does judging consistent distances and angles from the runway. Though it has been somewhat over-publicised, there’s also supposedly another trick. What is it?
- Always use the exact same crab angle for each leg of the pattern.
- The distance from the runway to the downwind leg should always be one half mile for each 60 knots of airspeed.
- Turn base at a 45 degree angle for zero wind, and 10 degrees sooner for each part of a headwind that equals one-fifth of your airspeed on final.
- Adding the digits of each no-wind heading (whether it is the upwind, crosswind, downwind, base, or final) will always add up to the same number.
Answer: D. Choice A would make no sense at all (unless the wind direction was exactly 45 degrees from the runway). Choice B? I made it up (as if you couldn’t guess). Choice C looks like it might make sense, actually, but I made that one up, too. On the other hand, choice D, now that one, I didn’t make up. Actually, this so-called “amazing discovery” was supposedly first made (maybe “realized” is a better word)…by a twelve year-old boy! The story goes that while riding in a J-3 Cub flown by his father in New Mexico, Justin Winfield noticed that the digits for the headings flown on each leg had a curious relationship: when added together, he noticed that they added up to the same number! He parlayed this observation into winning third place at his school’s science fair, and then won an FAA award at the state level, as well as an Air Force Gold Medal Award. As an example, let’s say you take off from runway 13, and in a normal pattern, turn a left crosswind. Your no-wind heading is 040 degrees. Turning a left downwind, it becomes 310. Your left base would be 220, and final of course, 130. In each case, the digits add up to four. That so-called amazing fact might’ve astounded the teachers in New Mexico, but it’s not exactly an undiscovered property of a base ten number system wherein adding nine to any two-digit number decreases the ones digit by one and increases the tens digit by the same amount (up to 36, after which, at least for runways, it recycles). And in a similar way, subtracting nine from any two-digit number increases the ones digit and then decreases the tens digit, again both by one.
Of course, it’s easy for right-handed patterns where the “90” gets added to your heading (or multiples of nine get added to the runway number), but wouldn’t you know it, most of our patterns go the other way. The same principle does work in reverse of course, but it’s just a bit trickier.
There’s only one small problem with this brilliant rule. It doesn’t always work. Try it with a runway 01, 02, 10, 11, 19, 20, 28, or 29, and things get a little confusing (if that’s all you’re paying attention to, that is). Of course maybe he knew that and there’s a “modulo nine” clause in Winfield’s Rule. Here is a table of two sets of headings for upwind, crosswind, downwind, base, and final legs, with both right and left-handed patterns for runways in a clockwise sequence starting with 36 and back to 36 again. Notice what happens to this, using certain runways. I think I’ll stick to landmarks and pilotage to keep my own path over the ground on the straight and narrow…