Building an electric pre-rotator for your whirlybird is a very simple process – once you know what to watch out for. I installed a Honda motorcycle starter on my Eich JE2 which I got out of a salvage yard for thirty dollars.
The first thing that I didn’t realize, however, was that there are two different types of starter motors, those that turn clockwise and those that turn counter-clockwise. I had previously gotten a Bendix to drive my pre-rotator ring gear from Dick Wunderlich.
When I got my starter motor I realized that it turned the wrong way. A phone call to a couple of gyro friends who had already installed one of these jewels for a pre-rotator unveiled that they had each gotten a motor that turned a different direction.
When I quizzed the first friend about these Honda motors turning the wrong way he said, “Yeah, I know. You’ve got to get a reversing Bendix from Wunderlich.” The second guy I talked to said, “Mine turns the right direction.”
Another trip to the junk yard disclosed that there are two different ones, and even though they are DC motors, they can’t be reversed by changing the wire polarity. My motor is a 700 watt unit and it requires the reversing Bendix.
Most of the Honda starter motors that I could find were 600 watt units, and they turn the right direction for a direct-drive Bendix. The best solenoid to get for it is the one off the motorcycle. It’s a lot smaller than a Ford Bendix that some guys use on them.
I went to several electrical shops to buy some 4 gauge wire to hook it up, but couldn’t find what I wanted (fine strand wire). I finally found it in a welding shop for about 60 cents per foot. The square red button on my dash is a pre-rotator button.
It is wired through the ignition switch so that no one can spin the blades up until the switch is on and the engine is running. Pretty snazzy looking dash, eh? I finally found a good digital tachometer at Pep Boys.
It’s a reverse LCD which is backlit at night with a red light when I turn on my navigation lights. It shows up well in bright sunlight (unlike the LED tach that I tried), and is super accurate at a cost of about $42.00.
The other gauges are: Hobbs meter, amp gauge, oil pressure, water temp, altimeter, airspeed indicator, and compass.
To hook the starter motor to the Bendix I used a 12 mm socket which fits over the shaft of the starter motor with a little tap of a hammer. A hole was then drilled through the socket and shaft (do this with the socket on the shaft – one operation) and a 3/16 inch bolt was put through it.
I cut a piece of 3 / 8 inch square aluminum which is about 1 inch long and used it to connect the end of the socket with the bottom of the Bendix. The Bendix that I received didn’t have the square hole in the drive shaft quite in the center of the shaft which I didn’t notice until I had cut about 1/2 inch off the end to shorten it.
As a result it shakes a little, so check yours before you do any cutting on it. For a grounding strap I bought a foot of large shielded coax cable from a surplus house. The shielding was copper braid, so after stripping it off it made a very nice grounding strap for about 25 cents.
Finally, I installed my bicycle computer for a rotor tachometer. This is a Cat-Eye Cyclometer with a cadence counter. I’ve had some trouble getting it to start reading, but when it works it seems to be very accurate.
My present rotor blades start flying at about 315 RPM. The rotor tachometer upgrades every 4 seconds which is fast enough to check the flying speed of your blades, but not fast enough to check small variations under varying load conditions.
Building An Electric Pre-Rotator: The end result?
I’m very dissatisfied with the electric pre-rotator. I’ve had other guys tell me theirs spins the rotor blades at about 75 RPM and some guys tome me about 150 RPM. Unfortunately, I have not talked to anyone that had a rotor tachometer installed so they are only guessing.
My rotors only turn at about 56 RPM with my bird at a standstill. If it was rolling forward the movement of air through the blades would help spin them up. And it takes about a full minute to a minute and a half to get them up to that speed.
That takes a severe toll on my battery which is a 12 amp/hour motorcycle battery backed up by a 35 amp alternator. After running my pre-rotator it takes about 20 minutes of running the engine to get the battery charged back up again.
The amp needle is always all the way to the right when I’m pushing the pre-rotator button. All-in-all, I’m disappointed with mine, but my friends seem to be pleased with theirs. I will be putting my chain saw engine on my ship for a pre-rotalor this summer.
EDITORS NOTE: It is possible to spin 23 ft. metal blades up to 100-150 RPM with an electric pre-rotator, but not with a 700 watt starter motor. (Lon’s rotor diameter is 28 feet). You also need a hefty battery to handle the large current drain.
Mr. Lawrence,
What controller is used to power the starter motor?
Thank you.
Hi Gordon, if you read through the article you will see the comments made about what motor is used…
“Another trip to the junk yard disclosed that there are two different ones, and even though they are DC motors, they can’t be reversed by changing the wire polarity. My motor is a 700 watt unit and it requires the reversing Bendix.”
“Most of the Honda starter motors that I could find were 600 watt units, and they turn the right direction for a direct-drive Bendix. The best solenoid to get for it is the one off the motorcycle. It’s a lot smaller than a Ford Bendix that some guys use on them.”
This is an older article and there are many good options available now – it’s an individuals choice as to what suits them and their craft as to which motor is used.
Kind regards Adam
Want to add a electric prerotor to my Benson. Can some one give me a recommendation on a component list ? Starter, solenoid, switch, drive wheel, etc. Thank You.