Last month we told of the Air Hogs Power Hawk, available from Toys-R-Us stores in the USA. We noted at the time that although this RTF model had three propellers, it had only two motors, one on each side of the center line at the trailing edge of the top wing and a third prop, in the nose, which is simply a free-wheeler, able to spin in the wind, but with no power to drive it.
The original unmodified Power Hawk
Although the Power Hawk flies very well with the two motors, I thought it would be more interesting if the nose prop was also powered. I therefore looked for a suitable motor and found one, the 7 mm diameter orange Slicks motor available from Bob Selman Designs. I decided on a direct-drive setup for this motor, rather than a gear drive to the prop, for simplicity. Although I could have used the original Power Hawk prop, I decided to go with one of my favorite props – the U-80 – about the same size and weight. This pushed firmly on the shaft of the motor so that further simplified the arrangement.
In making this addition to the Power Hawk, my goal was to have the added components come out at about the same weight as last month’s Power Hawk, which used a couple of pennies, plus a carbon rod nose boom, for better flying qualities. This additional weight came to just 6 grams.
The motor and prop combination I chose came to 3.3 grams. So I looked for a single cell Lipo battery I could use to drive the motor (I didn’t want to use the original Power Hawk battery and get involved with tricky receiver wiring modification it might involve). I found that the 70 mah Minium Lipo battery that comes with the ParkZone Cessna RTF to be ideal: it weighs just 2.3 grams. This, added to the motor and prop, came out to be 5.6 grams.
U-80 prop, BSD motor, Cessna 70 mah Lipo battery
With about half a gram more that could be added, I thought that another nice touch would be to add the very light main landing gear from the ParkZone Cessna. That landing gear mounted easily, just behind the original Power Hawk battery. All it took was to slit, with a razor knife, the bottom of the fuselage ½” ahead of the Power Hawk receiver unit and to push the landing gear into the slit. I secured the gear with a drop of foam-compatible CA glue at each l.g. leg.
The original Power Hawk, unmodified except for landing gear addition
Underview of model with 3rd motor and battery installed
The next step in this modification was to provide a connection between the motor and the new battery. The battery comes with a neat little connector but I didn’t have its mate, which is the one wired to the Cessna receiver unit and I didn’t want to cut it off. I searched and found them at Dallee Electronics of Leola, Pennsylvania. They sell a pack of five male and female sets that are used for model railroading. You can look them up at www.dallee.com and the pack, which sells for $ 11.95, is called their #957 two pin Ultra Miniature Connector Pack.
The way I first operated the third motor, which is independent of the Power Hawk RC system, is to turn on the Power Hawk transmitter, connect the third motor plug to the Cessna battery (the motor then runs at full power), then switch on the Power Hawk RC system. I use the control stick to hold the two wing mounted motors at medium speed. I then hand launch the Power Hawk and control it as usual, though without the need for full power on the wing motors since the nose motor provides considerable thrust.
The nose motor ran for about 9 minutes before it slowed down noticeably. At about 8 minutes I controlled the Power Hawk so as to descend and land, so that the nose motor battery could be unplugged before its battery ran down too much and harmed itself and became unable to be recharged.
Nine minutes is a lot of flying and landing by that time is reasonable. During the landing phase the P-H motors can be run at low speed, which causes the model to descend but still with turning ability because the P-H motors are still running, which is necessary to provide directional control.
The initial flight of the tri-motor showed a major increase in thrust, for outstanding climb performance. Yet the plane flies well on low power, as it did before, so that it can fly on the nose motor alone, needing power from the other two motors only when turn control is need. So it’s like having a model that flies free flight with the one motor, or RC with all motors running; a fascinating combination.
I then added a timer to the nose motor battery circuit. The purpose of this was to provide an automatic cutoff of the nose motor before the end of the flight, so I wouldn’t have to keep count of the minutes it was running before needing to be shut down. The timer, which is one I got from Nick Leichty of www.microflierradio.com is one that allows the running time to be preset and it also allows the motor rpm to be preset as desired. The timer sells for $20,weighs less than a gram, will handle up to 4.5 amps current draw.
Closeup view of the timer which has rpm and flight time controls
The timer, wired between the motor and its battery, can be set from 0 to 5 minutes flight time.
The flight procedure now is to set the timer for about 5 minutes, with close to maximum rpm. The timer starts when it is plugged into the separate battery for the nose motor, then I switch on the RC system for the receiver and the wing motors. Then, with all three motors running, I hand launch the model. With all motors running the Power Hawk has a brisk climb.
The timer is mounted with double sided tape on the left side of the model nose
View from below, with timer installed
I let it get up to about 20 feet high then cutback the power of the two wing motors just enough to let the model fly level or in a gentle climb, with turn control by the differential thrust of the wing motors as called for by the side to side transmitter stick position. At about 50 feet high I cut the power to the wing motors so that the model flies free flight – with no turn control – on the power of the nose motor alone.
Close up view of the Timer installed on the left side of the model nose
If necessary, I’m able at any time to energize the wing motors to climb higher and/or to turn as desired. This interplay of switching from free flight to RC mode and back again makes for interesting flights with a combination of hands-off free flight and on-off, more or less, radio control. It’s all just for fun, to see what combinations of more or less control can do.
After the nose motor shuts off by the timer, there are still a few minutes of radio control left to play with, without the extra thrust of the third motor. This provides an extra element of interest to the flight and essentially lets the Power Hawk fly as originally intended, under radio control with power on, or gliding free flight without power.
The modified tri-motor Power Hawk, ready for another flight
All in all, making a tri-motor out of the twin-motor was a fun experience, simple to do and adding another dimension to the experience of flying the Power Hawk. It’s a great plane and among the best of the many RTF planes I’ve tried to date.
RC Micro World
November 2007
