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It's the one on your engine; the propeller. A lot of us forget that it's a wing, or better yet, a whole kit of them, usually arranged in pairs. (Yes, there are single-bladed props. A tear-drop shaped counter-weight adorns the other side. But except for low Reynold's number events, such as model airplanes, the hoped-for improvement failed to materialize.)
Your propeller converts torque into thrust. It does this by accelerating a slug of air. Air has mass and while it's pretty thin stuff, if you can accelerate it to a significant velocity, that soft summer breeze can become a tornado or hurricane. Unfortunately, with a tractor-type engine installation a good percentage of your airplane is embedded in that tornado. And along about there you remember that Drag increases as the Square of velocity. That means you and your fuselage, with all its lumps and bumps and intersections is a major problem, especially when trying to generate thrust with a Volkswagen engine.
To stop being part of the problem and start being part of the solution you need to do two things. The first is to find a prop that is the best possible match for your power-plant and airframe. The second is to do whatever you can to produce a wide thrust-slug rather than a narrow one.
Solving the first problem is pretty easy, if you got lottsa money. You just keep buying props and test-flying them until your improvement peaks-out and you start going down hill. That's when you glom onto the Last prop you tested and put it back on the nose of your bird.
But you may not be able to do very much about the second part of the problem. The only way to produce a wider thrust slug is to sling a longer propeller. But if your engine doesn't have enough thrust to spin the longer prop, you're stuck. But hang around; there's a couple of things you can do that can be of real benefit.
When selecting a prop most of us track down someone having the same airframe and power-plant and use whatever prop they happen to be using. Sometimes that even works out. But not always. His airframe could be seriously out of trim. Or it could have up to twice the parasitic drag as yours. And if it's a Volkswagen engine, most of them are victims of the Horsepower Myth, which is okay for dune-buggies but hilariously wrong when bolted to the nose of an airplane. Even so, there's no doubt his prop will fly your plane if it manages to fly his, but unless your engines are a good match there's the possibility you've just bought a pig in a poke. And a properly made propeller doesn't come cheap.
Gathering more information about other guy's props & engines is a step in the right direction but the chances are, they did exactly what you are doing, which means you could all be barking up the wrong tree. Don't believe it? Okay. It's your plane and you are the Mechanic in Charge, not only of your plane but of your life. So good luck in the Contest :-)
One thing you've got going for you is the fact that propellers are relatively easy to build, meaning you can roll your own for a fraction of the cost of buying one. And as soon as you finish the first one, you get started on the second, because for your research to be useful you should only change one thing at a time.
Didja get that? You carve yourself a 58x34, balance it to a gnat's ass and go fly. Your test flights follow a carefully worked out routine in which you quantify the propeller's performance in your particular machine by measuring such things as Take-off Run, Time to Climb to 1000 feet, average speed over a known distance and so-forth. While all that's going on you're spending your evenings carving another prop in which you change only ONE of the propeller's four basic characteristics; that is, diameter, pitch, Blade Area, and Blade Area Distribution. (Other factors are: Weight, Stiffness and Airfoil.) And while you're doing all that you may want to look up the recommended propeller for the Continental A-40. Rated at 37.5hp and having a displacement of only 1834cc (sound familiar?) the A-40 was typically between 60" and 68" with a pitch that varied between 37" and 43".
As to the things you can do to improve your engine's torque, one of them is to simply use a longer connecting rod. Here's why: Each time the piston reaches Top- or Bottom-Dead-Center, it reverses direction. Depending on the ratio of rod-length to crankshaft stroke, the piston may literally stop. The crankshaft is still rotating but for a number of degrees the piston is motionless. The longer the connecting rod, the longer the piston will dwell at TDC.
The Static Compression Ratio that you built-in to the engine during its assembly, occurs at TDC. Interestingly enough, Ignition has already been initiated -- in fact, the fuel-air charge was ignited quite some time before the piston reached TDC. This reflects the fact that the process of ignition requires a finite amount of time. Not that it's a slow-poke. Even in the largest Big-Bore Stroker combustion only takes a couple of milliseconds. What's interesting is that except for some minor variations having to do with ambient temperature and air density, the process of combustion is virtually independent of the engine's rpm. That means, the faster the engine is running, the earlier we need to light the fire. Even so, in the typical dune-buggy engine the Combustion Space is constantly increasing. Indeed, this is what you want when the engine is bolted to a box full of gears. But this is NOT what you want when the engine is bolted to a fan!
The greater the dwell at TDC, the more time there will be for the process of combustion to occur when the fuel-air charge is at it's maximum compression ratio. With the piston virtually motionless combustion will produce it's maximum temperature, which in turn yields the maximum possible pressure. And it is that pressure which appears in the crankshaft as TORQUE.
To achieve this minor miracle we retain the stock cam shaft and retard it by as much as seven degrees, although -4 is more typical. This causes our maximum torque to appear between 2500 and 2800 rpm, which makes it ideal for slinging a prop. Dune-buggy engines, typically running above 3000 rpm, produce a narrow, high-velocity thrust slug, which works okay in a cleaner airframe. But if you're driving a tumble-weed you're going to need a wide, low-velocity thrust slug. And the only way to get it is to move your engine's torque-band to a lower rpm.
That covers the engine. And should have made it clear that there is an enormous range of differences between supposedly 'identical' engines. Now let's look at the prop itself. I've included a couple of illustrations just to liven things up. The image at the start of this article is of a template used to test the Upper Camber of the prop's airfoil. But don't pay any attention to the dimensions; this template happens to be for a prop only 42 inches in diameter, meant to be used with a 6hp single-cylinder engine.
You don't start 'trying' the templates until you are almost finished fabricating the prop. But such trials are of critical importance to insure each blade of the prop is the closest possible match to the other.
Traditionally airfoil templates were made of brass shim stock, having a thickness of .006" to .010". Nowadays, lo-buck builders use the aluminum from a beer can. If building just a single propeller, you can even get by using paper, such as the cover of a file folder.
Since our prop is going to be operating well below the critical tip-speed of 880 feet per second we can use one of the traditional airfoils such as the Clark-Y, a very good choice for a wooden prop, thanks to its flat bottom and good thickness ratio, which is about 12%. (The Clark-Y, which was used on Lindbergh's 'NYP' and the USA-B, (US Army, version B) which was used on the Piper 'Cub', owe their existence to Col. Virginus E. Clark, one of America's first truly competent aerodynamicists. Colonel Clark (U.S. Army) was a real genius. His research in molded propellers lead to the invention of the 'Duramold' process for producing laminated wooden skins that had significant advantages over riveted metal structures. The Duramold process is what made the HK-1 possible. [HK-1 or 'Hughes-Kaiser #1' more commonly known as the 'Hercules' or 'The Spruce Goose,' even though most of the structure is Duramolded birch rather than spruce.])
The second illustration is of the coordinates for the Clark-Y, which I don't believe is included in Abbott's 'Theory of Wing Sections.'
When I can find the time I will include drawings of the airfoil templates we will need to fabricate propellers for the Chugger. The airfoils will be drawn full-scale (ie, 1:1) allowing them to be used as patterns. Printed by an accurate printer, the patterns will be glued to the metal templates then scored with a scalpel or razor-knife.
Interestingly enough, when working with sheet-metal thinner than about .025" you will find that scoring works better than cutting. The scored metal is simply flexed back & forth a few times, which causes it to fatigue along the scored line and fracture. The tricky bit is in how you make that first all-important score-mark and the secret is, for the thinner the metal, the lighter the amount of pressure. Indeed, for beer-can stock (or .006" brass shim-stock) you don''t us any pressure at all (!) While this would appear to violate common-sense, it turns out that if you use any pressure you will cause the shim-stock to deform long before any scoring takes place, and that the now-deformed shim-stock refuses to fracture along the scored mark.
-R.S.Hoover