A BlackMac from New England Propeller Division
cuts maintenance expense while it improves performance.

Article from Dec 91 Flying Magazine Written BY J. MAC MCCLELLAN

Most PILOTS KNOW THE ADVANTAGES OF a three or four-blade prop over a two- blader. The extra blades add static thrust, which improves takeoff acceleration and climb. A three-bladed prop lessens-or at least greatly changes the vibration and noise levels in the airplane in a way most people find more comfortable. And a three-blade prop looks better on the ramp. Any airplane looks more powerful, fast and capable with an extra blade. But these are not the reasons U.S. Propeller Service began offering three-blade prop conversions. The real reason is maintainability. The other three-blade prop benefits are a bonus. U.S. Propeller's primary business is prop overhaul at its shops in Connecticut, North Carolina and Florida. Prop overhaul is a business best suited for technical zealots. Props-even on light airplanes are so highly stressed that any corrosion, nick or slight imperfection can lead to failure of the blades or hub. And a prop failure is usually fatal because the severe imbalance of the engine turning with only part of a prop jerks the engine out of its mounts. Once the engine departs, an airplane is usually beyond the bounds of its flyable CG range and cannot continue with a controlled glide. As the general aviation fleet ages. the technicians at U.S. Propeller are finding an increasing number of unairworthy props. Even the smallest amount of corrosion in critical areas of the prop hub or blades demands immediate component retirement. In other words, the prop you thought was working fine when it went in for overhaul may actually be dangerous unairworthy junk. Many of the unrepairable props are of older design dating back 30 or more years. An airplane owner could buy a newly manufactured prop of the old design to replace his unairworthy prop, or buy enough new parts to make the old prop airworthy, but either way he'd still end up with an old prop design that was not as efficient or long-lived as more recently designed props. U.S. Propeller's solution is to obtain an STC for a modern prop and offer it at the same price as a newly manufactured prop of old design. Most of the prop conversions are to three-blade because U.S. Propeller can offer the increased performance and comfort for no extra cost. From the naturally aspirated Piper Arrow that was U.S. Propeller's first three-blade conversion, the airplanes that have been certified since make an ever-growing list.

All conversions use McCauley's Black-Mac series propellers. To obtain a prop conversion STC, U.S. Propeller purchases an analysis of the engine-airframe vibration characteristics from McCauley. This analysis determines that no harmful vibrations or resonant frequencies will result from the new three- blade prop. The test-flying requirements for the prop conversion focus on engine cooling and balked-landing climb performance~ The FAA asked for much more testing on the first few STCs, including V-dive tests VD speed is at least 10 percent faster than redline. But as both U.S. Propeller and the FAA gained experience, the required tests were somewhat simplified. It is conceivable that a prop change can alter cooling flow into the cowling, so the FAA requires a full-power climb test under 100 day conditions. Because 100 days are mercifully rare in Connecticut, the test can be flown under available ambient conditions and a correction factor is applied to simulate century-mark air temperature. During the climb test, no engine temperature limit may be exceeded. This is the same cooling test required for new aircraft certification. The balked-landing climb-rate requirement stipulates that the airplane must climb at a minimum rate in the landing configuration. The required rate is a ratio of the aircraft's stalling speed. Because a three- blade prop provides more static thrust than a two-blader, this test is never a problem for the Black-Mac. I flew in a Mooney 201 with a Black-Mac three- blade conversion and was extremely impressed by the takeoff acceleration and initial climb improvement. And the noise and vibration levels seemed much, much better to me after 2,000 hours of flying in Mooneys with two-blade props. An unexpected benefit was the added drag of the extra blade when trying to slow the Mooney down. Steeper approaches and quicker deceleration to landing- gear extension speed are very noticeable with the three-blader.

The tough, rigid airframe of the Mooney has always transmitted engine vibration into the cabin, but the Black-Mac will change the way you feel about the airplane. If you love two-blade Mooneys, you'll go nuts with a three-blader installed. If you thought the sound level and vibration too high, take another flight with an extra blade installed. It may change your mind. U.S. Propeller people are pleased with the results of all conversions so far, but think the performance change is most dramatic with the Cessna 182s. Climb rates increase by several hundred feet per minute and cabin vibration is dramatically reduced, they say. The one negative of three-blade prop versus two is a slight loss of cruise thrust with the extra blade. This phenomenon increases with altitude, so the pilot of a turbo-charged airplane may notice the loss of several knots of cruise speed while flying in the high teens or low 20s, the pilot of a naturally aspirated airplane won't be able to measure more than a knot or two loss of cruise speed. The more advanced design of the Black-Mac props can sometimes even overcome this inherent cruise efficiency advantage of a two blader. U.S. Propeller pilots found that the new three-blader was actually a couple of knots faster at maximum-power level flight than the two-blade on the 260 Comanche. And takeoff distance, climb, sound level and vibration were also improved as you would expect.

Installation of the Black-Mac conversion involves nothing more than having your mechanic bolt on the new prop and spinner, and, of course, add the required paperwork to the aircraft records. The three-blader will probably he slightly heavier than your old one, but more recently designed props typically weigh less overall, helping to cancel the weight penalty of the extra blade. Bonanza owners will actually welcome the added weight on the nose to help their airplane's chronic AFF-CG problem. The Black-Mac conversions from U.S Propeller range in price from #1.995 fro the two blade fixed pitch prop on a Piper Cherokee 180 and Arch to $5,995 on a Bonanza. The company allows a $300 trade-in on the Cherokee prop and a $500 to 1,000.00 trade on the Bonanza. Other High performance singles have similar prices. The Cherokee Black-Mac Conversion is especially valuable because up to two inches can be trimmed from the new prop to repair damage, while the original had to be junked if the blade tips were even slightly dinged. The company also sells Black-Mac conversions for most light turboprops, offering improved performance, lower maintenance and a quieter ride. The turboprops are converted from three to four blades and prices are understandably higher than for piston powered airplanes, but U.S. Propeller believes the new props can pay for themselves in fuel and prop maintenance savings. The typical pilot opts for the Black-Mac conversion after finding out that his present prop has serious problems. But many others-particularly Bonanza and Mooney owners-simply want the improved performance and comfort. U.S. Propeller will be happy to take your current prop in on trade and may even offer some credit for the airworthy parts of a prop that is otherwise uneconomical to repair.