B-25 Mitchell
1/48 Scale
8 Channels
P-51 Mustang
1/48 Scale
6 Channels
2.65", 0.495 gram
Martin's Midge
3.2", 0.98 gram
4", 1.3 gram

February 4, 2011

This is my 8 channel 1/48 scale Mitchell B-25 Bomber, built for the KIEF 2010 indoor electric meeting, where it won the award for "Best Micro Aircraft". It has an 8 channel radio controlling rudder, elevator, aileron, flaps, left throttle, right throttle, retracts and bomb drop. Auxilliary functions are steerable nosewheel linked to the rudder, brakes on main landing gear linked to down elevator, a flashing nav light and red and green wing-tip lights.

The flaps and retracts move slowly over a 2-3 second period, controlled by the transmitter. The retracts lock in both the raised and lowered positions.

This plane was motivated by my development of the Hip-Hop-2 system that supports 8 channels. I felt I needed to show an example of what could be done with 8 channels in a micro plane.

Apart from the fact that it uses 8 channels, for me the most interesting, and challenging, part of this plane is the retracts. The nosewheel retract mechanism is based on a mechanism published by Carl Martin. It uses a pin that slides along a fixed W shaped slot and in a slot attached to the wheel strut. This provides locking in the up and down positions, and allows for a certain amount of variability in the end position of the servo while guaranteeing the wheel will cover the full range regardless. Locking up and down means that there is no back force on the servo in either the up or down position. This much is not new. What is novel is the way in which the nosewheel steering works in conjunction with the retract. Some larger commercially-available steerable nosewheel retracts use a slider mechanism so that the wheel is progressively less affected by rudder movements until the rudder control has no affect on the nosewheel when it is fully retracted. This mechanism can bind if the nosewheel is retracted while the rudder is deflected. On this plane the nosewheel is steered through a lever and two universal joints, so there is no slider to bind, nor any other mechanism to get stuck. There is virtually no slop or backlash. The mechanism can be seen in one of the videos below.

The mainwheel retracts were similarly challenging, this time not because of steering but because of the brakes. The brakes are small sandpaper pads on arms positioned behind the wheels. They operate by rubbing on the rubber tires. When applied they become self-locking as the wheel attempts to turn. This makes for rather abrupt braking, but it is very effective and can hold the plane with both motors at full throttle. The problem with the retracts is that on the B-25 the main gear retracts backwards, so the brakes have to get out of the way. This was achieved by allowing the brakes to move along with the wheels, and providing a slot that lines up with the brake lever when the wheels are full down. The only catch with this is that it will bind if you try to activate the retracts while the brakes are on, but why would anyone do that? The retracts themselves use a different mechanism from the nosewheel. On the main gear the activating pin that runs in a slot in the wheel strut is pivoted about a fixed point rather than running in a fixed slot. This still locks up and down, but it is more dependent on servo endpoint positions.

The other interesting thing about the retracts is the servo. Commercially-available servos small enough for this plane don't have enough throw or force, partly because they have to be fast. Retracts work better with a slow servo, but it has to be quite powerful. So I developed my own linear servo with a 1" throw and twice the gear ratio of conventional servos. This gives about 6 times the force yet weighs only 2 grams. It is rather unusual for a linear servo in that it uses magnetic feedback via a Hall sensor. The magnet and Hall sensor are not mounted in the servo, but are on a torque rod that activates the main gear wheel retracts. This is a technique shown to me by Dave Burley. It provides accurate positional feedback with zero wear, unlike the potentiometers used in most other servos.

The receiver for this plane is an 8 channel Rabbit receiver. It uses a slightly larger package style for the microprocessor to make soldering all those wires easier, so it is about 50% longer than a conventional Rabbit, but the same thickness. It provides both PWM and ESC outputs for the two throttles so it can use either brushed or brushless motors. The plane originally flew with direct drive 7mm brushed motors before the engine nacelles were completed, but it was under-powered. When I added the nacelles it wouldn't even take off. So I switched to brushless motors and now it has sufficient power.

Again this year I didn't quite finish the plane in time for KIEF, so when I arrived the radio wasn't even installed. A very late night in the hotel fixed that and it maidened the next day at KIEF. First attempts were promising but I couldn't get it to climb. I switched the motors for some slightly hotter ones, which helped a little. But then Matt Keennon told me to keep the nose down immediately after take off and fly it out. When I did this it made a beautiful slow scale-like climb out and flew well. At the Sunday noon-time show I was able to show the whole sequence: brakes on, left motor start, right motor start, flaps half down, brakes off, takeoff run, lift off, wheels up, flaps up, fly around, drop bomb, flaps full down, wheels down, touch down, taxi, brakes on, right motor powered down, left motor powered down, just like a full sized plane. Unfortunately not all of this was captured on video. Matt Keennon did a great job making the video below considering he was using my humble iPhone. You can't see any detail but it proves that it flew. The best video of it is on the SKS coverage of the whole meeting, but I don't have permission to reproduce that here.

B-25 Pictures

Taxi out

Takeoff run


Banked turn

Complete turn

Level out


Next to Tx

Rabbit 8 Rx

Servo plug block

Engine nacelle

With P-51

B-25 Videos

A tour of all the functions

A look at the internals

Animation of the retracting nosewheel mechanism

Here's one of it flying at KIEF 2010, video by Matt Keennon

B-25 Specifications
Wingspan 17" (43 cm) 1/48 Scale
Chord 2.25" (5.7 cm)
Wing Area 38 sq.in (245 sq.cm)
Weight 1.50 oz (43 grams)
Wing Loading 5.7 oz/sq.ft (17 g/sq.dm)
Motors 2 * Hexatronix 2 gram brushless outrunners with Turnigy TGY DP 3A 1S speed controllers
Gears None
Propellors GWS 2.5" x 1.0"
Battery 160mAH single cell LiPoly
Radio Custom 8 Channel Jack Rabbit Hip-Hop FHSS, 0.25"x0.4"x0.25", 0.25g (1/100 oz)
Antenna Dipole
Actuators/Servos 4 x HobbyKing linear servos, 1 x Spektrum linear servo, 1 x Leichty servo on ailerons, 1 custom servo on retracts
Construction Durabatics, Depron, paper
Control Rudder, Elevator, Ailerons, Left Throttle, Right Throttle, Flaps, Retracts, Bomb Drop
Auxiliary Functions Steerable nose wheel, brakes on main gear, flashing nav light, wing-tip lights

B-25 Plans
Plans in PDF format
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