On Impulse

March 30th I received a text message from Brother Barbeau: ".9 in the Hatz today.  It was a good day."  I was jealous.  When I read Gary's message I had already logged 5.8 hours in the MD-90, and still had (as it turned out) another eight tenths to go---Atlanta, Georgia to Greer, South Carolina.  Don't get me wrong, I love my job; but .9 in the Hatz trumps 5.8 in the airliner almost every time---biplane crosswind landing cold-sweats included!  The Hatz wasn't the only open cockpit biplane cruising around that Saturday afternoon.  Jellystone Air Park neighbor, Mark Accomazzo, was out flying his Great Lakes too. Gary said that when Mark saw the Hatz he asked:  "Is that Joe or Bob?"  I'll save the identity issue for another story.  The big issue here; it looks like spring has finally cracked the edge of an unusually irritating winter. Biplane season is officially open at Jellystone Air Park!

Speaking of spring. . .

Out in Port Townsend, Washington, Summer Martell owns a 1931 Student Prince biplane.  She chronicles her adventures in the Prince, and other airplanes, in her Summersky Blog.  In a recent post Summer recounts her first flight of spring in the Student Prince.  She describes part of the starting procedure for the Kinner radial engine as, "magneto switch to the "L" position."  Other than thinking, I guess that's how you start a Kinner, it never occurred to me to wonder why?  I probably knew the reason back when I took the written, but to be honest, only 1,100 of my 17,000 flight hours---a milestone reached on my last rotation---are in piston engine airplanes, and most of that was before the jet age.  Whatever magneto skills I once had expired years ago!  The important part of the story from my perspective:  Like me, Summer is thankful that spring has arrived.  I sent the link to the Brethren.  Brother Price was the first to respond.  "Why crank with just the left mag?"

It didn't take Brother Baker long to answer. . .

Eddie, on older engines (ours included, sometimes) one mag, usually the left one, is an impulse mag and the other a standard mag. I have Slicks, and they are both impulse mags.

If you know what an impulse mag is, then skip to the end. . . :-)

Back when hand-propping was standard, someone figured out a way to get a hotter spark on the relatively slow engine speed that hand-propping provides. Inside the mag, there is a magical device called an impulse coupling. It has a centrifugal lever/spring action that cocks itself and releases, spinning the mag spark generator faster when the engine is turning slowly, like at hand-propping speeds. It gives you a hotter spark right when you need it--engine start. When the engine speeds up, the centrifugal counterweights inside spin the levers to the outside and the impulse coupling is taken out of the system, letting the regular spark generation system take over. Since mags generate hotter sparks when they are running faster, this is a way of making these engines much easier to start.

As to why only the left? Impulse mags will delay the spark just a touch, backing off the timing when the engine is spun at really low rpm. You don't want the right mag firing at the standard time when you are trying to start an engine on impulse mags--two spark plugs arguing over their timing is not a good thing! This also makes it easier to start the engine when hand propping.
 

So, hence her left mag start. Dad remembers hand propping those older low-compression radials. With our engines, they have a higher compression, and you need a snap to get it through one, and maybe two compression strokes.  One guy--the airport manager in Winslow, Arizona, was so good at this, he could get the engine through four--FOUR--compression strokes when he hand-propped the Luscombe engine. I was impressed! Dad says he simply grabbed the prop--carefully--and walked by the front of the engine and the thing fired!

By the later forties and early fifties, starters had become common, and the cost of the impulse mag had come down to a point where both sides were impulse. No more left-only engine starts.

. . . and that is why Brother Baker is my choice for the next director of the AOPA, the EAA, the FAA, and the Aviation Historical Society of America.