As we discovered last month, there’s much to be said about fixed-pitch props for certain installations. Indeed, there’s a whole range of aircraft for which the fixed-pitch prop is the only choice. The engines either will not support an inflight adjustable, variable-pitch prop or the airframes speed range is sufficiently narrow that a constant-speed prop makes little performance sense. And there are aircraft with a broad speed range and high performance that just wouldn’t be efficient with a fixed-pitch prop.
Its those in-between cases that are difficult, and many builders fight the internal battle of low weight and cost against higher performance and greater installed weight. But at some point-generally at the 200-horsepower mark-a constant-speed (or inflight-adjustable-pitch) prop makes more sense, and you just have to swallow hard, find the cash and put up with the weight penalty. The performance will be worth the effort.
Terms and Conditions
When we talk about constant-speed propellers, we really mean that there is a way to adjust the prop blade pitch to maintain a specific engine speed regardless of the aircrafts speed or the engine load (within reason). How this task is accomplished depends on the props individual technology-and your engine, less so than your pocketbook, will be the determining factor.
Some engines have so-called solid crankshafts, which cannot be used with conventional hydraulic props because there’s no way to get the engine oil out to the prop. Or your engine might have a prop-speed reduction unit (PSRU) that does not accommodate a hydraulic prop. For you, there’s still an option of an electrically driven prop.
This type of propeller uses a small electric motor in the hub to rotate the blades in their ferrules, thereby changing blade pitch. Most of the common electric props use slip rings to transfer the motivating electricity from the crankshaft side to the spinning prop. An upside of the electric prop is that you can use it on virtually any engine that meets its power and crank-flange layout. The downside is that electric props are typically quite slow to adjust, and even when fitted with an external constant speed controller, they can have difficulty keeping up with rapid airspeed changes. Many pilots set the prop in cruise in the constant-speed mode and then lock the pitch in place unless the atmosphere or airspeed dictate change. In this sense, an electric prop is definitely not indicated for aerobatic use.
Hydraulic constant-speed props take advantage of engine oil to change blade pitch-see the sidebar Politics of the Governor on Page 40. And in that sense they’re essentially interchangeable. The primary variables you’ll be considering are:
- Specific flange style and bolt size. Smaller engines use the SAE 1 bolt circle. Continentals small four-cylinder engines (not including the IO-240) up to the O-200 use the SAE 1, while Lycomings O-235 could have an SAE 1 or the larger SAE 2 bolt circle. Starting with the O-290, all of the larger Lycomings use the SAE 2 bolt circle. Bushings are pressed into the Lycoming crankshaft flange. For the larger engines, these will be inch, while the smaller engines-all the little Continentals and the Lycoming O-320 and below-all use 7⁄16-inch bolts.
- Number of blades.
- Horsepower capability.
- Airfoil section and overall diameter.
- Blade composition.
Spotlight on the Details
The first step in choosing a prop amounts to a bit of chicken-and-egg question. If you already have the engine, check to be sure it has the correct bolt circle and bushing sizes for its displacement and power-nothing worse than a shiny new prop arriving only to find it wont physically fit. All of the major prop manufacturers are set up for the right combinations, so this isn’t much of an issue unless you have an engine with an uncommon combination of parts.
Horsepower capability is easy to figure, as all of the manufacturers provide the range their props are capable of handling. Don’t forget to figure in any performance enhancements for your engine. If the manufacturer rates a prop for 200 hp, its foolish to try to feed it 210 or 220 hp without checking in first. For some makers, the power limit has some give; for others-and particularly those that have done regimented testing-the upper limit is the true limit.
Already we’ve started to narrow the field, and now its time to consider the number of blades and diameter. (Well get to composition last.) Generally speaking, the more blades you have (for a given diameter), the greater the initial takeoff and climb will be at the expense of cruise performance. Additional blades put a greater volume of air into motion at low vehicle speeds, resulting in generally better static thrust. All that thrust turns into drag at some point, and there’s your compromise.
But its not quite that simple. As you add blades, you can reduce the diameter of the prop, potentially reducing noise and drag, and increasing ground clearance-important for nosewheel utility aircraft with a lot of power-but you do so at greater expense and with a weight penalty. Often, when a prop manufacturer has a blade that can handle X hp, accommodating larger and more powerful engines requires that same blade used more often. For example, the recommended Hartzell composite prop for the 200-hp four-cylinder Lycoming is a two-blade version with the 7605 paddle; that same blade, times three, is suited for the 260-hp IO-540. There is no cut-and-dried answer here. Look at what others are doing with your airplane type and try to mine data from their stated performance.
Airfoil selection is, of course, proprietary to each manufacturer, but its worth noting that its more difficult to create a thin airfoil from composite than from metal, and while there are several airfoil choices on the market, most manufacturers tailor props for specific airframes mainly through diameter.
Not Made of Cheese
For years, the de facto standard constant-speed propeller for homebuilts was a metal certified prop from a series-built aircraft. Prop manufacturers now produce special versions for homebuilts as Experimental-class props-and they’re doing it in new forms, including high-tech composites. (Remember that to make use of the 25-hour Phase I flight-test period, both the engine and prop must be certified as a pair, and in unmodified form; otherwise, you’re in for the 40-hour test period.) While Hartzell has embraced the homebuilt market with both arms, it appears McCauley couldn’t care less-when we asked for information pertaining to Experimental use of its props, McCauley representatives were dismissive to the point of being rude.
Hartzell, on the other hand, actively develops models and variations for the homebuilt segment. Its recent launch of the ASC-II composite prop for Experimentals (for more, see Around the Patch: Plastic, fantastic, Page 2) follows years of development of blended-airfoil metal props for our market.
Its that decision-metal or composite-that forms the next step. Modern aluminum props are a well-known quantity and surprisingly inexpensive. Hartzells Mike Disbrow says, We have the economies of scale with the metal props, and our [CNC-based] manufacturing allows for reduced cost. They have two shortcomings: They’re heavier than many composite props, and they have a natural frequency that can be excited by engine vibration. For that reason, Hartzell spends a lot of time doing vibration surveys on certain engine/prop combinations, and verifies that during the normal range of operations there are no harmful harmonics. Bear in mind that most of the testing is done on stock or near-stock engines, and variations-electronic ignition, higher compression, different internal balance factors-can change the tendency for, uh, harmonic convergence.
The lure of composite props is the absence of vibration concerns. Even though various composite props are made quite differently, they all share a comparative indifference to engine harmonics. Hartzells ASC-II prop is a carbon-fiber/Kevlar combination on a foam core (the structure is entirely in the composite). AeroComposites also uses a foam core with carbon structural plies in the blade. Popular German manufacturer, MT Propellers, creates composite props from compressed thin layered laminated beech wood in the root and spruce elsewhere, all wrapped fiberglass, carbon-fiber and/or Kevlar (depending upon application), making the MT a bit like a wood prop covered in glass. Whirl Winds process uses carbon fiber placed into a female mold with no core at all; the mold sets the blade shape, and air injected during the high-temperature cure maintains the internal shape.
You knew there was a catch, right? Composite props promise to be smoother, more immune to engine harmonics and lighter than metal props, but the more tangible offset is cash. A Hartzell two-blade metal prop costs about $6600 new (budget also for spinner, prop governor and hoses, if necessary) while the company’s own ASC-II prop starts at $11,000 for the same application. MT is slightly less, and AeroComposites slightly more, but clustered within a fairly narrow range. Whirl Wind appears to be the bargain here, with a two-blade, RV-specific prop at $8665 with spinner and backplate.
The happy ending is that we as builders have an unprecedented choice in constant-speed props; they embody some very high technology and promise real performance. For builders of high-performance aircraft, constant-speed props are about more than bragging rights.