B-17 Electronics
The B-17 is actually a simple 4 channel airplane. I did some transmitter programming and used a 10 channel RX because that is what I had but you could do this with the simplest 4 channel computer radio. You could even use a non-computer radio if you added a rudder servo, but at this point I am not even sure if a rudder is necessary.
The power harness has 6 each XT-60 connectors to accept input from up to 6 batteries in parallel. This is mostly to provide nose weight. Just one 3S, 3.3Ah battery would be adequate for flight. I have a lot of these batteries that I use for many projects so it is just a matter of convenience. The power harness has 4 each XT-60 connector outputs that provide power to the 4 ESCs. The ESCs are from the Power Pack D (Standard Quadcopter), currently $100 in the FT store. This includes 4 ESCs, BL Heli 20A XT-60 and 4 motors Emax 2213-935kv (CW Threaded Shaft) as well as 8 propellers, 10x4.5 CW and CCW 4 each plus some tools. I have measured about 6 pounds of static thrust with this setup so it should provide sedate performance with a 12 pound airplane.
The motors do not come with the standard “X” type motor mounts but you could bolt them directly to the plywood firewalls if you wanted. I used the X type motor mounts so I could more easily remove them for use in other projects.
Note that the battery connection to the ESCs is short, and the wires to the motor are long. It is important that the ESCs not be placed next to the motors with long wires to the battery. This would quickly destroy the ESC’s due to the “water hammer” pounding of pulse currents on their input capacitors.
Also note that all motors, ESCs, etc. are left in the stock configuration. No wires are cut or extended, and no connectors are changed. We will want to use this gear in many future projects.
The power harness also has a JST connector output which feeds a switching BEC (Battery Eliminator Circuit) which is set to 6V output. The higher voltage is intended to help make up for any losses due to the long servo extensions. Any reasonable BEC will do, even a linear BEC. It is probably possible to use the linear BEC inside one of the ESCs. It is probably also possible to just plug all 4 ESCs into the RX so the BECs are all in parallel. In my experience it does no harm to parallel linear BECs of this type, but always read and follow the manufacturer’s recommendations.
The wires from the ESCs to the motors are 16 AWG (American Wire Gage) stranded hookup wire from the hardware store. There is no need to use expensive wire from the hobby store. This sort of wire is flexible enough for the application. The motor wires are 20 AWG and the 16 AWG hookup wire has a lower resistance per foot by about 2.56 times, which helps to make up for the extra length. The motor “extension cables” have a 3mm male bullet connector on one end, and a 3mm female bullet connector on the other, and are just taped along the bottom of the wing.
The 2 aileron servos are physically arranged so they can be driven by a single channel and a Y harness, but as a matter of convenience I have them plugged into 2 separate RX channels that output the exact same signal. The two Elevator servos are are driven by a single RX channel with a Y harness in the tail. The motors on the left side could be connected together with a Y harness, but I used 2 identical RX outputs, the same for the 2 right motors. I have 30% rudder programmed into the motors for differential throttle yaw control. There is no physical rudder or rudder servo as I don’t think it is needed, but one could easily be added.
I am using the main RX and two separate satellite receivers. This is probably overkill but I am concerned about all the metalized (silver) plastic acting as an RF shield. One receiver is on the left side below the wing, one on the right side below the wing, and one on top of the fuselage. Hopefully this gives a clear path to one of the antennas from every angle.
I am using one heavy duty Y harness in the tail to drive the 2 elevator servos. This connects to a long custom servo extension back to the RX. All of the more or less permanent connections between RC connectors are tied off with dental floss. These large aircraft can flex in flight and pull connectors apart, even if the wires are taped down. It is a good idea to leave a tiny bit of slack where they connect to prevent this. There are 2 long custom made servo extensions from the aileron servos to the RX. All of these custom servo extensions were made from 1 foot heavy duty extension cables that were cut in two and spliced with a length of heavy duty 3 wire servo cable. Each individual wire is cut about ½” apart so the solder joints do not overlap, and a single piece of shrink tubing can be used to cover all 3 joints. This technique avoids having to buy a crimping tool for the contacts inside the RC connectors.
The 4 servos are standard Flite Test Tough Tilt Servos – Emax ES3154. They have a lot of mechanical slop in the output shaft but should be adequate for this application. The output spline fits JR style servo arms and I am using very long heavy duty servo arms. This is necessary because the tape hinges on the side opposite of the control horns. These long geometry linkages are a good thing because they reduce slop and stress on the linkage.
When the plane is assembled for flight, 6 bullet connectors and 1 servo connector must be connected for each wing half. These connections are made at the wing root near the leading edge. There is 1 servo connection to be made for each horizontal stabilizer half as well. It doesn’t matter if any of the servo connections are reversed. The motors might run backwards, but it is a simple matter to swap any two wires for any motor to fix them.
I added the Quanum Pocket Vibration Telemetry voltage meter with alarm (869.54Mhz FM).
https://hobbyking.com/en_us/quanum-...try-voltage-meter-with-alarm-869-5mhz-fm.html
It just plugs into the balance lead of one of the batteries. This thing will probably fly for 45 minutes so battery power is not a problem, but I am hoping to pass the transmitter around as part of the fun.