Prop Testing for Maximum Speed and Performance
I really enjoy flying RC aircraft of all kinds, but some of my favorites are Formula One pylon racers. I wanted a race plane that I could prop for speed, but I also wanted to be able to try propellers that would produce good torque. I also wanted a plane that would allow me to do some sport aerobatics. One of my favorites is the Kyosho Phantom 70 EP. It is no 3D monster aerobat, but it will do a fair amount of maneuvers, so for my testing this was a perfect aircraft.
Let’s take a closer look at this high-quality Kyosho race aircraft. Both the GP (glow) and EP (electric) versions of this model have high-quality fiberglass parts that include the fuselage, wheel pants and cowl. The EP fuselage is one piece without a cowl but with a large battery access door on the bottom. It has the firewall epoxied to the front of the fuselage and a battery tray glued in right under the hatch opening. You can attach your battery to the plywood plate with Velcro and then bolt this plate to the tray in different locations so you can move the battery to achieve the perfect balance. The wings and stabilizers on both kits are the same using balsa and plywood construction with heat-shrink covering. The hardware package is complete and includes everything you need to finish up your aircraft. The construction manual is type specific and mine included everything on installing all the electric components.
The Axi motor mounts directly to the front of the fuselage, easy and quick. The small air holes on each side of the firewall are all that’s needed to keep the motor cool.
Vent holes on the bottom of the fuselage keep the power system cool.
The build on the plane is very straight forward and construction begins with the lower wing. This is the only wing that has ailerons and the servo space is rather limited, for this I used 2 HS-85 Hitec (hitecrcd.com) servos. Then I hinged the ailerons, then epoxied the four-part gull wing together and attached the aileron servos. Short pushrods make the connection between the servo and the aileron. To complete wing construction, I glued the fiberglass wheel pants and belly panel in place. Even though the gull wing looks rather complicated, the assembly went fast and was rather easy.
After the stabilizer is epoxied in place and the elevators and rudder are hinged, it’s time to install the motor. Before doing that I decided to reinforce the firewall because I will be using a powerful Axi 4130/16 moto rand swapping out props for testing. I stood the fuselage on its nose and used a mixture of BSI 15-minute epoxy and micro balloons to form a bead of epoxy around the firewall. Be careful not to get any this on the firewall face because it will affect the way the motor sits again the wood. I really like BSI 15-minute epoxy because it sets up up faster than the 20-minute stuff but still has plenty of working time. After I bolted the motor to the firewall, I attached the ESC inside the fuselage with Velcro.
Before I epoxied in the stabilizer/rudder servo tray to the inside of the fuselage, I attached the receiver and servos to the plywood tray. The rudder has a pull-pull system for control and I was able to thread in the cables to the servo from the back and attach them to the servo control horn. The tray is glued in using epoxy and micro-balloons. The elevator pushrod has one wire on the servo side with two wires coming out of the back, each one going to an elevator. The easiest way I found to do this is by threading thin wires through the holes in the back of the fuselage and attaching each one to the separate pushrod wire. Then it is just a simple matter of pulling them through to the elevator control horns. All that was left now to finish up the Phantom was to install the top wing, pilot, canopy and decals. I balanced the plane slightly nose-heavy and used the manufacturer’s recommended throws for the control surfaces.
The equipment I used on the ground for testing included the AstroFlight Watt meter and a Cermark One Touch Tachometer.Once the plane got airborne the information was gathered through the Eagle Tree Micro-Power E-Logger (eagletreesystems.com). We clocked the first three passes with a radar gun to get the average speed. Fortunately there was no wind on the test day that would affect the speed. I used a 5000mAh 6s battery pack for each test. I fully charged each battery the day before the test. I flew the plane for 7 minutes and the packs only required 2300 to 3000mAh to recharge them, so I could easily get about 10 minutes of flight time. All of my numbers are taken from the beginning of the flight when the battery has the most power. So let’s see how the testing went.
The ground numbers on this APC (apcprops.com) prop were pretty good with the motor pulling 36.63 amps at 21 volts. This combination was producing about 773 watts at 7,350 rpm. Once in the air the motor was pulling 30 amps at 22.2 volts and producing 730 watts. This prop got the plane into the air rather quickly and, once in the air, the Phantom performed aerobatics very well with a strong up-line before stopping. Our average speed on the three passes with this prop was 73mph.
The Xoar was the only wooden one we tested that is made especially for electrics. This prop is a nice looking blade and should hold its pitch with little or no flexing in the air. The ground numbers were 15.4 amps at 21.9 volts. This combination was producing about 335 watts at 7,650rpm. Once in the air, on average, the motor was pulling 15.3 amps at 21.75volts and producing 335 watts. The plane took up about the same length of the runway to get into the air but once in the air the Phantom performed aerobatics almost the same as the APC prop 15x8E with up-lines almost identical as the other props. This wooden prop seems to pull the plane through maneuvers without slowing down. I was really surprised at how low all of our readings were especially when our average speed on the three passes with this prop was 70mph.
The battery hatch on the bottom of the fuselage is held in place with two screws. As you can see, there’s room for even the largest battery pack!
We tested the rpm of each prop on the ground at the beginning of the flight, after a 10-second motor run.
The ground numbers on this prop were a little higher with the motor pulling 39 amps at 21.9 volts. This combination was producing about 870 watts at 7,230 rpm. Once in the air, on average, the motor was pulling 36 amps at 21.75 volts and producing 808 watts. This prop got the plane into the air quickly (about the same as the 15×8) and, once in the air, the Phantom performed aerobatics really well with a good up-line before stopping. This prop seems to pull the plane through maneuvers without slowing down and maintained good speed around the pylons. Our average speed on the three passes with this prop was 79mph.
MASTER AIRSCREW 15×10
Master Airscrew (masterairscrew.com) has props made especially for electrics; unfortunately, they stop at 13 inches in diameter. This prop is much thicker and heavier than the APC E props, still it produced numbers like these: 42 amps at 21.64 volts. This combination was producing about 935 watts at 7,110 rpm. Once in the air, on average, the motor was pulling 30 amps at 21.12 volts and producing 676 watts. The plane took a little longer to get into the air, but once in the sky, the Phantom performed aerobatics very well with up-lines almost as good as the other props. This thicker prop seems to pull the plane through maneuvers without slowing down and the white tips looked great. Our average speed on the three passes with this prop was 73mph.
The ground numbers on this prop looked good especially with such a large pitch, the motor pulling 36 amps at 22 volts. This combination was producing about 815 watts at 7,380 rpm. Once in the air the motor, on average, was pulling 35 amps at 22 volts and producing 817 watts. This prop got the plane into the air rather quickly and, once in the air, the Phantom performed aerobatics very well with a strong up-line before stopping. Our average speed on the three passes with this prop was 83mph.
I found out that if I wanted some speed out of the Phantom I needed to use the APC 14×12, and this wasn’t unexpected. If I just wanted to do aerobatics and some fun flying with the Phantom then I could use any of the other props with a lower pitch and more blade length for that extra airflow over the surfaces. The surprising thing for me was that if I wanted a longer flight to practice my aerobatics, all I had to do was use the Xoar prop.
Text & Photos by John Reid
Published at Thu, 15 Apr 2021 12:13:19 +0000