Kitplanes Feature

® There’s a perfect prop for every plane. BY LEROY COOK 26 KITPLANES January 2019 www.kitplanes.com & www.facebook.com/kitplanes Propellers, like wing spars, are among the few parts on an airplane that abso- lutely, positively, must not ever break. Under continual stress while absorbing engine power and converting it into thrust, the lowly propeller has to keep performing its function, year in and year out, with scant attention paid to it. Catastrophic propeller failure imme- diately produces severe vibration as the engine continues to run with an out-of- Sensenich ground-adjustable prop on an RV-4. balance prop, and the resulting stresses can lead to failure of the engine mount. one revolution of the propeller under from the hub. At the blade’s tip, the speed If the engine departs the airframe, the perfect conditions, assuming that it’s of the airfoil’s movement through the air airplane becomes uncontrollable due turning in a medium devoid of slippage is vastly different than it is near the root, to the shift in CG. And yet, most of the or resistance. Think of a screw boring thus the tip requires a minimal amount of time we just give the prop a light caress itself into soft wood. Usually expressed pitch and thickness as the effective speed during our preflight inspection and say in inches, a propeller’s pitch is com- nears supersonic flow. Tip speeds in excess “Yep, it’s still there.” monly quoted in conjunction with the of Mach .75 result in loss of efficiency. A Propellers deserve a little more diameter, as with a 72/56 prop being propeller with too much angle of attack, respect. The design and creation of these one with 72 inches of disc diameter and or pitch, retards rotation and places an exquisitely shaped rotating airfoils is as 56 inches of perfect forward movement inordinate load on the engine. One with much akin to art as to science. The effi- in one revolution. For aircraft certifi- too little pitch, on the other hand, allows ciency of some propellers can reach into cation, propellers of a certain size and the engine to overspeed. the 90% range, as they screw their way pitch are part of the approved equip- Changing airspeed away from a into thin air while pushing or pulling ment, and a minimum-permissible designed peak performance target the entire aircraft. Nothing could be static (full-power runup) rpm may results in less efficiency. At low speed, simpler—or more complex. The sim- be specified as part of the limitations. engine rpm may be limited by the large plest of all aircraft propeller types is, Static rpm proves that the engine and bite of a high propeller pitch, as in “high of course, the humble fixed-pitch two- propeller combination is capable of angle of attack,” while at a high diving blade airscrew, found on basic airplanes producing the thrust suitable for flight. speed, the rpm may rise to beyond red- optimized for low-cost operation and Proper choice of propeller pitch is criti- line as the engine becomes unloaded. uncomplicated piloting. cal to achieve maximum efficiency. There Owners of airplanes with fixed-pitch are actually several pitches and blade propellers sometimes exchange props Perfect Pitch thicknesses existing in the twisted airfoil to maximize one or the other edge of The term “propeller pitch” refers to along the propeller’s length. The adver- the performance envelope, referring to the amount of forward motion that tised pitch is usually measured at a mid- a “climb prop” as one with a low pitch could theoretically be achieved in point blade station about 75% outward angle allowing extra rpm for takeoff, or a “cruise prop” with a higher pitch to optimize speed rather than climb. The Best of Both Obviously, it would be desirable to be able to change a propeller’s pitch, giving the best of both worlds without having to swap between two or more propellers. The first efforts to do this were ground- adjustable arrangements, with the blades held in a hub that permitted their shank to be rotated for pitch change. Such propellers are still available today. In-flight pitch changing came next, using various methods of “shifting gears” from climb to cruise. The Kop- WhirlWind composite propeller on a trigear RV. pers Aeromatic propeller, used on light Photos: LeRoy Cook, Paul Dye, and courtesy of the manufacturers KITPLANES January 2019 27 aircraft of the 1940s, balanced centrifu- Propeller Corporation in the 1940s, it gal force against airspeed to adjust pitch was reportedly the most efficient pro- for best performance without cockpit peller design. But its weird look, with control. Aeromatic props may be return- a counterbalancing weighted stub ing to at least the experimental market; on one side of the hub, kept it from the rights to the design are now owned becoming popular. by Tarver Propellers in Fallon, Nevada. In experimental aviation, we are per- Simple manual propeller-pitch shift- mitted to build any part of the aircraft, ing, adjusted by pulling or pushing a and that includes carving our own pro- control to shift from a low-pitch take- peller; some dedicated individuals have off/climb setting into a higher-pitched done just that. Unless you are particu- cruise position, was incorporated in larly gifted or inclined toward propeller the Hoffman Dimona motorglider I fabrication, however, it makes more sense Three-blade GT ground-adjustable prop. flew back in the 1980s. One idled the to purchase a ready-made prop from the Dimona’s Limbach engine to reduce dozens of suppliers specializing in them. A buyer’s guide of any type is stress when making the change, and A simple laminated-wood propeller is no bound to have some flaws in it, but when the engine was shut down for longer sufficient for most homebuilts; we’ve attempted to cover the propel- soaring, a third position could move carbon fiber blades, ground-adjustable ler industry as thoroughly as we can. the blades into feather. pitch angles, and even in-flight control- In the accompanying tables, certified Controllable propellers, with vari- lable and constant-speed governing are propellers are listed first, followed by able pitch adjustment using an elec- available to maximize the performance the burgeoning list of non-certified tric motor or oil pressure to position envelope of our homebuilts. propeller manufacturers. The tabula- the blade angle, were developed in the Use of a non-certified propeller, tion includes contact information, 1930s to allow the pilot full control even if the Lycoming or Continental the date of the company’s founding, of optimum engine power, although engine is bone stock, usually requires types of construction, and horsepower rpm still varied with airspeed because an expansion of the Phase I flight test- range available. the propeller was essentially in a fixed- ing period from 25 hours to 40 hours, Some of the larger propeller com- pitch setting once adjusted. This short- just as would a non-certified engine. panies concentrate on type-approved coming was alleviated by coupling the The effect of any pitch adjustments or props for certified engine applications, pitch-change mechanism to a governor other changes needs to be documented while the smaller firms market to the that operated to automatically main- as part of the testing regimen. experimental non-certified user. And tain a constant rpm, the constant- some are doing both, offering a line of speed propeller in use today. Picking a Prop certified props but also more than will- As with choosing a certified Continen- ing to build a custom non-certified pro- How Many Blades? tal, Lycoming, or Rotax engine, there peller for a specific project. The best number of blades to use, fre- are a limited number of suppliers of Established propeller manufacturers quently two blades versus three blades, is certified propellers, perhaps five or six, may sell their products through a distrib- a frequent topic of discussion. With lower which is the reason one finds Hartzell, utor or dealer network, through suppliers horsepower engines, adding an extra McCauley, Sensenich, and MT propel- like Aircraft Spruce, Univair, Wag-Aero blade is largely a matter of sex appeal and lers attached to most of the world’s cer- and SportairUSA, and aircraft kit mak- noise conversion; all things being equal, a tificated airplanes. Hartzell has been in ers may have distributor arrangements to two-blade propeller is more efficient than the propeller business for over 100 years, supply props to their customers. an equivalent three-blade, although the and Sensenich isn’t far behind. Even the three-blade prop will boost thrust at low new kids on the block have a long his- Certified Propeller Companies airspeed during takeoff and climb. How- tory behind them. GT Propellers ever, with increasing horsepower, extra However, the lower-horsepower kit Founded in 1969, GT has produced blades will be needed to absorb the addi- and E/A-B airplanes can be equipped over 30,000 propellers. At last count, tional power. Both the disturbing quality with propellers from a large number of more than 200 propeller variations of the perceived noise and the noise level companies. Some are handcrafted by were available. itself are reduced by adding blades. And, cottage-industry entrepreneurs, each GT fixed-pitch propellers and vari- adding a blade allows the same thrust to with their own following and specialties. able-pitch propeller blades are made be developed with a shorter blade length, Laminated wood predominates as the from a variety of laminated hardwoods reducing tip speed to a quieter level.

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