Written by Dave Lockhart Sports & Aerobatics Column As seen in the Spring 2017 issue of Park Pilot.
>> Aerobatics pilots should never underestimate the value of a propeller. Even the best motors, batteries, ESCs, and airplanes will fall short of their potential if a good propeller is not a part of the equation. Amps, volts, watts, and rpm can provide insight to the health of a power system, but ultimately, if the propeller does not efficiently generate thrust or is not well matched to the power system, airframe, and flying style, performance will be lacking. In 1989, the world of model aircraft propellers was forever changed when Landing Products (apcprop.com) introduced its line of APC propellers. Seemingly overnight, APC became dominant in the RC Pattern (Aerobatics) and Pylon Racing markets. APC Propellers were designed with cutting-edge software/simulation and precisely manufactured from injected fiberglass-reinforced nylon. The days of choosing between a durable plastic/nylon propeller and a high-performance wooden propeller were gone. The APC propeller line expanded and rapidly became dominant in the sport market. Today, APC makes approximately 450 propellers in sizes ranging from 4 to 27 inches in diameter for a variety of airplanes and multirotors. Among its latest offerings are a couple of propellers in the 3-D category: the 8 x 4.1SF and the 9 x 3.7SF propellers. These slow-fly propellers are aimed at the popular 4- to 10-ounce 3-D foamie market—airplanes that are lightweight and agile enough to comfortably fly indoors, but still large enough with sufficient power to fly outdoors in calm to light wind conditions. Although APC has long offered 8 x 3.8SF and 9 x 3.8SF propellers, those were developed to perform well for a broad range of small airplanes. They were heavy and optimized for lower rpm than most current-day 3-D foamie setups. The go-to propeller for 3-D foamies in recent years has been the GWS (gwsprops.com) 8 x 4.3, and the many clone copies that have been produced from it. Although it’s a good-performing propeller, the 8 x 4.3 is notorious for inconsistent balance and is only marginally strong enough for some of the higher power setups. GWS also offers an 8 x 4DD (direct-drive) propeller, which is stronger and intended for higher rpm setups, but it is also heavier and yields shorter flight times on most setups. Larger foamies commonly use the GWS 9 x 4.7; the added diameter produces more thrust to overcome the increased drag of the larger airframe. Several pictures are included that show the differences between older APC SF propellers, newer APC SF 3-D propellers, GWS SF propellers, and GWS DD propellers. Most notably, new APC SF 3-D propellers have the narrowest overall chord (blade width, leading edge to trailing edge) and the thinnest hub. A substantial amount of chord width is retained near the hub to increase torsional and spanwise rigidity.
A comparison of the GWS SF and DD propellers. Top to bottom: 9 x 4.7SF, 9 x 5DD, 8 x 4.3SF, and 8 x 4DD.
Older APC SF propellers were designed to use hub bushings to fit a range of motors, while the APC SF 3-D hubs are molded with a 5mm hole to fit the most popular 3-D foamie motors. The thin hub also makes it easier to secure the propeller with O-ring-style propeller savers. The following are specific weights of the referenced 8-inch and 9-inch propellers: • GWS, 8 x 4.3: 3.98 grams • APC, 8 x 4.1SF 3-D: 4.75 grams • GWS, 8 x 4DD: 5.30 grams • APC, 8 x 3.8SF: 6.92 grams • GWS, 9 x 4.7: 5.07 grams • APC, 9 x 3.7SF 3-D: 5.50 grams • GWS, 9 x 5DD: 7.02 grams • APC, 9 x 3.8SF: 9.17 grams Weight is not the only factor to consider when deciding which propeller to use. With a handheld optical tachometer and an inline wattmeter, I did a couple of quick runups to check the relative static load of each propeller. I used my favorite foamie, an Arrow V.5 from Donatas Design (pauzuolis-rc.com), for the tests. The power system is a NeuMotors (neumotors.com) Neutrino 1215/6Y (1,675 Kv weighing 21 grams) with a Castle Creations (castlecreations.com) Phoenix 10 ESC, and a Thunder Power (thunderpowerrc.com) 2S 325 mAh 70C Pro Force LiPo battery. This data set is consistent with testing that I have done. The APC 8 x 3.8SF and GWS 8 x 4.3 propellers produce similar numbers, which is not surprising given the comparable planforms (overall shape) and airfoil shapes. Historical flight testing also shows that the propellers provide similar performance for comparable flight durations. The edge ultimately goes to the GWS because of its lighter weight. Notable from the data set is the GWS 8 x 4DD, which achieved the highest rpm and the lowest amp draw. Although high rpm is thought to indicate high performance, in this case the high rpm, combined with the low amp draw, indicates that the propeller is slipping or lacking bite in comparison with the other propellers tested. Historical flight testing also shows that the 8 x 4DD does not perform as well in the air as the GWS 8 x 4.3 or the APC 8 x 3.8. When comparing the new APC 8 x 4.1SF 3-D propeller with the older APC 8 x 3.8SF, the new one has significantly less chord at midspan and at the tips. With less propeller area, I expected a reduced load (even with the minor pitch increase from 3.8 to 4.1), but the new propeller actually drew more current. With regard to the 9-inch propellers, I know from previous experience that they are generally not a good match for the Arrow V.5; however, with the relatively narrow chord, I thought the APC 9 x 3.7SF 3-D propeller might be suitable.
Popular slow-flyer APC propellers (top and next to bottom) are compared with the newest APC SF propellers for 3-D and aerobatic flying. Top to bottom: 9 x 3.8SF, 9 x 3.7SF 3-D, 8 x 3.8SF, and 8 x 4.1SF 3-D.
Static testing showed that it produced the highest amp draw and lowest rpm of all of the propellers tested, which is not entirely unexpected because of the larger diameter. It is worth noting that 11.5 amps exceeds the continuous amp rating (10 amps) of the Castle Creations Phoenix 10 ESC. The nature of 3-D foamies, however, is that full throttle is rarely used—and only briefly—and the Phoenix 10 is more than capable of short bursts of higher current. The Neutrino motor is capable of 80 watts, and the loaded voltage of the battery is acceptable, staying higher than 3.4 volts per cell (my personal rule of thumb). It should also be noted that with 3-D setups (except on rare exceptions), amps will drop substantially in the air in comparison with static values. Onto the flight testing of the new APC propellers! I flew each propeller on the Arrow V.5 through my old ETOC (Electric Tournament of Champions) aeromusical routine, starting with the GWS 8 x 4.3 as a baseline. Switching to the APC 8 x 4.1SF 3-D propeller, the airplane accelerated noticeably faster and had a little more top-end speed. The flight time was the same or slightly increased. The APC 9 x 3.7SF 3-D propeller was up next. It hovered with noticeably less throttle stick and accelerated from low speed the fastest. Both of these were expected with a larger-diameter and flatter-pitch propeller. Top-end speed was surprisingly fast and very close to the 8 x 4.1. The flight time was the same or maybe slightly decreased.
| GWS, 8 x 4.3: | 9.0 amps | 8,800 rpm | 7.30 volts | 65.7 watts |
| APC, 8 x 4.1SF 3-D: | 10.0 amps | 7,600 rpm | 7.10 volts | 71.0 watts |
| GWS, 8 x 4DD: | 7.8 amps | 9,800 rpm | 7.48 volts | 58.3 watts |
| APC, 8 x 3.8SF: | 8.9 amps | 9,000 rpm | 7.35 volts | 65.4 watts |
| APC, 9 x 3.7SF 3-D: | 11.5 amps | 7,000 rpm | 6.85 volts | 78.8 watts |
