Thursday, November 30, 2006

AV - Vernon Payne

Most of you have never heard of Vernon Payne. He’s best known as the designer of a spiffy little biplane called the Knight Twister. Rag & tube fuselage, solid spruce spars, plywood skins on cantilevered wings. Slicker than snot on a door knob. First flew about 1929 and continues doing so today and very well, too. Ask him nice, Vern would build you a set of wings. Fuselage, too.

For a short while when he was moving out of his place north of Escondido I worked for Vernon. Not what you’d call a real job even though the sweat was real enough. Mostly helping him prepare for the move, which involved finishing a set a wings he was working on, prepping a fuselage and other minor chores.

Skinning the leading edge, Vern soaked each piece of plywood, fitted it to the wing and let it dry in place. The old barn we worked in was good for that sort of thing, hot and dry and no distractions other than Taddy, the shop cat who liked to watch.

After the ply had dried to the required shape Vern would check the fit with a bit of chalk, make any adjustments. The fitted panels of the beautiful double-tapered wings were glued all at once using Weldwood ‘Plastic Resin’ and nailing strips. Lots of nailing strips, most of which were also pre-molded to match their particular rib, both of us working away with our tiny tack hammers, taking our time but not wasting any, doing it right so we wouldn’t have to do it over.

And we’d talk. Vern had boots that was older than me but I wasn’t no kid; I knew who Gilmore was and the joke about aviation-grade horsemeat, which meant Vern could talk without having to explain every other word. I suspect he needed the companionship as much as the help, working all alone out there in that old barn north of Escondido, having to finish those wings and move a life-time of stuff because the Yuppies were kicking him out.

I usually have a sketch book with me, a life-long habit. Vern didn’t like it when he saw me sketching the engine arrangement of his latest project, a two-place VW powered thing he called the Dolphin. I gave him the book, told him to tear out anything he didn’t want me to have. He flipped through it and saw Taddy curled up atop the blueprint cabinet and a chiaroscuro study created by a slice of sunlight falling across a steel tube fuselage. And of Vern too, bent over the wing, pensive look on his face as he examines a tip rib smaller than a pocket ruler. He gave me back the sketch book, didn’t tear anything out.

We got to talking about landing gear, which kind was best; that sort of thing. It was an interesting subject to me because I’d just made a composite gear leg out of wood, fiberglas and good intentions that busted all to hell when I did a drop-test. Vernon told me how the CAA used to have a standard formula for acceptable landing gear strength and a drop-test calculation based on the gross weight, a Jesus factor and the stalling speed of that particular aircraft. Then he said, ‘I just used the worse-case, which was fifty inches.’ Later, I made a note of that on the corner of a sketch of a landing gear. - - Worse case = 50" - - I don’t know exactly when that was. Back in the eighties.

About three years ago I wanted to compare the performance of different types of landing gear for a particular design and made up a drop-tester that allowed precise control of the height and the angle at which the wheel contacted the ground. The gear leg bolted to a plate on the end of a long arm that was raised by a little derrick using a screw-thread winch off a boat trailer. I used a glider hook as the release mechanism and welded a tray to the top of the arm so I could stack on plates of lead and scrap iron for the weight. It’s quite an affair. Really shook the ground when it hit. And busted everything I tried. (Some of you may have seen pictures of the knee-action gear leg I posted over on the Fly5k mailing list. There were a number of others. All initially failed the drop-test.)

I soon became quite the expert at drop-testing. As I gained experience I began to understand how the load gets distributed through the structure and saw ways to make the gear legs stronger without adding too much weight. But always some. It was pretty obvious that most landing gear used on modern airplanes couldn’t even come close to that ancient CAA requirement. By the time I got something that was strong enough to withstand the worse-case fifty-inch drop, it was huge. And heavy. More suitable for the NYP with a full load of fuel than a single-place do-it-yourself puddle jumper with a converted VW on the nose. Fiberglas offered some advantages, as did oleo-pneumatic systems but their cost and complexity put them beyond the means of a first-time builder on a tight budget. It was all rather discouraging.

A couple of days ago I was digging through a file looking for... I can’t remember what... when I came across a blurry copy of a 1930's article by Raoul J. Hoffmann, the aeronautical engineer who crunched the numbers for Matty Laird. Hoffman was one of my dad’s heros and it was a good choice. The article was titled ‘Landing Gear Shock Stresses’ and included the usual boilerplate formulae, a couple of graphs and a few column-inches of text. I’d seen it before, hadn’t looked at it too closely since I’d already decided to emulate Vernon Payne’s method of using the worse-case drop height. But as I scanned the article something jumped out at me.

“...drop from a height in inches equal to .38 times the calculated stalling speed in miles per hour... (but) ...not over 15 inches for conventional airplanes.”

FIFTEEN inches. Not fifty.

I sat down and ran the numbers for my puddle-jumper. Maximum applied load not to exceed 5.5 times the gross weight (I’ve been using 6.0). But the drop test need not exceed fifteen inches. And I'd been using fifty! No wonder my gear legs came out like something off the Dreadnaught.

- - - - - - - - - - - - - - - -

Taddy came to live with us after Vern and his wife moved into a place that didn’t allow pets. They would drop by now and then to say hello, more to Taddy than us but they were nice visits. Vern passed away a few years ago but will never be forgotten. His little bipe is a rare combination of art and science, as was Vernon Payne himself.

Ignorance is mankind's normal state, alleviated by information and experience. Much of that experience is negative; we learn to do things right by doing it wrong. If fate gives us another shot at it, we do it differently the next time around. Once we learn to do things right we become the local expert, the fellow who can show you how to kill a grizzly with a spear or attach a propellor to a pulley hub. But as Smokey Yunick once said, “Most experts aren’t.” And I’d just proved him right because nothing leads us astray faster than the things we think we know. Vern was a pro; he obviously said ‘fifteen’ and I heard it as fifty.


Photo of the Knight Twister courtesy of Steen Aero

AV - Ignition Timing

With regard to ignition...

Got a match? Gopher, kitchen, safety... any match will do. You’ll need a few of them for what follows.

Strike a match and measure how long it takes for the chemicals to burn off. Just hold it vertically and count-down starting from the pop of ignition until all of the chemicals are gone. You may chant ‘one-potato, two-potato...’ if you wish :-)

Do that several times and you will see that for same amount of chemical, it takes the same amount of time.

Now try to make it burn faster. Or slower. Blowing (gently) on the flame should give it more oxygen whilst holding it in the steam from a kettle should give it less but the odds are neither will effect the burn-time because the chemicals are a balanced mixture of fuel and oxidizer. That’s what’s referred to as a ‘stoichiometric’ mixture.

As with the match, the fuel-air mixture in a gasoline-fueled internal combustion engine does not explode, it merely burns. Or should :-) If it does explode (ie, detonatation) you’ve got a serious problem on your hands.

Although the match experiment isn’t very precise it offers a hint that combustion of a given quantity of mixture not only takes a certain amount of time, that amount of time is virtually fixed for a given quantity of material. If we set aside the temperature of combustion, which I am doing deliberately for the purpose of this explanation, the only way to alter the amount of time it takes to burn a given quantity of fuel is to alter the composition of the mixture. The key point here is that for a given engine and within the parameters already mentioned (ie, temperature and mixture ratio) combustion takes approximately the same amount of time regardless of engine rpm.

Now consider a spark-ignited Otto-cycle engine.

Even with a cylinder of large displacement, when the fuel-air mixture is compressed, combustion takes only a few thousandths of a second - - a brief flash is all you’ll see through the quartz head of a Test Engine. What happens during that brief flash is the heart & soul of understanding internal combustion engines..

During that brief flash all of the fuel combined with all of the oxygen to produce a given quanta of heat, raising the temperature of the residual gases in the combustion chamber, most of which are nitrogen, to several thousand degrees, at least momentarily and nearest the core. But that brief flash of heat also serves to raise the pressure inside the combustion chamber. Which is good. But only if the pressure rises in an orderly fashion - - and only if the peak pressure occurs after the piston has reached the Top Dead Center point of its up & down movement. If peak pressure occurs too early we might as well go home.

A little bit early isn’t too bad. It wastes power but the engine will still run. Here’s why: Each cylinder of an Otto-cycle engine has only one power pulse for every two revolutions of the crankshaft and that pulse lasts for less than ninety degrees of rotation. The energy needed to rotate the engine through the other 630 degrees has to come from other cylinders or some storage mechanism, such as a flywheel. Whatever method is used, it is sized for the slowest speed at which you want the engine to run, meaning there will always be some amount of excess energy at any higher speed. When peak pressure occurs a little bit early some of that stored energy will be used to get the piston past TDC. Under those conditions the engine’s efficiency is low and fuel consumption is high but the thing will still run.

But if the pressure peak occurs too early, there won’t be enough energy in the system to overcome the timing error and the thing will fail to run, often signaling it’s disgust with a back-fire.

By the same token, we don’t want the pressure peak to occur too late. If the pressure does not peak until the piston is already descending - - which it will do even without a power pulse, thanks to the momentum inherent in the Otto-cycle design - - much of the pressure we’ve worked so hard to produce will be dissipated without doing any useful work; the amount of torque available at the output will fall. When peak pressure occurs too late, the engine will still run but not very efficiently in the thermal sense, and its top speed will be limited, since any increase comes at the a further reduction in torque.

Notice here the distinction between initiation of ignition - - when the spark occurs - - and the moment of peak pressure. Although sequentially related these are two separate events, the interval between them determined by a number of factors such as the shape of the combustion chamber, the octane rating of the fuel, the point of ignition and so on. Most confusion associated with engine tuning stems from addressing only ignition timing and ignoring the timing of the resultant pressure curves.

It should be obvious that an efficient engine is more desirable than an inefficient engine. An efficient engine burns less fuel to produce the same power as an inefficient engine. Efficient engines also tend to last longer. What isn’t so obvious, especially with an antique design such as the air cooled Volkswagen, is that a remarkable improvement in thermal efficiency may be achieved by focusing the keenest attention to the myriad details which contribute to its inefficiency, such as the timing of the cam, valves and ignition, proper waste-heat management and so forth.

At the very least this message should have made two things immediately apparent: Ignition must occur at some time prior to the need for peak pressure, and the precise moment of ignition must vary according to the rpm of the engine.

Which is why I don’t use magnetos. Or any other ignition system having a fixed firing point.

Yeah, I know - - it flys jus’ fine. The question you gotta ask yourself is, how much better could it fly - - and how much fuel are you pissing away.