Tech Topic -- Give Me a Brake!  (Rev. A, 9/27/07)

My 1/3 Scale Ercoupe has introduced me to several technologies I have not used at all, or at least not for a long time. The 32 lb. Ercoupe, on its big 5-1/2" air wheels, rolls very freely over the ground. It has a tendency to keep rolling, right off the end of the runway, especially after one of my too-fast, too-long approaches. Also, I could not do a full-stop on the runway before beginning the takeoff roll, which lost me realism points at the recent Scale Qualifier meet at Hemet.

Back in the 1960’s, we used to put a brake on the nose wheel of the .45 - .60 size pattern aircraft of that time. My 10 lb. Taurus-Zeus fiberglass/foam ship, with an Enya 60, had a simple brake someone manufactured, which had an expanding Formica brake shoe in a stationary brake body fastened to the nose gear strut. A metal brake drum rotated with the wheel. A slot in the shoe contained a cam which was actuated by a lever arm, to which we ran a cable from the elevator servo. Full down elevator would pull the cable taut, rotating the cam, which expanded the shoe against the drum, thus applying the brake.

Other schemes used a simple pivoting music wire drag arm, which would contact the nose wheel tire during full down elevator.  Kavan makes a mechanical brake that works similarly.  It requires a double-strut nose gear, as shown below, and is adjustable over a range of strut and wheel sizes.

There was also an electromagnetic brake, made by Aeropiccola in Italy, which used an electromagnet in the stationary brake to drag on a steel disc attached to the rotating wheel. I had an unused pair of these brakes in my parts cabinet. This brake could operate off the receiver battery pack.  Aeropiccola was in business until a short time ago, selling military and ship model kits.  They have now gone bankrupt.

Current braking technology, from Robart, Bob Violett and Glennis, utilizes the compressed air already present on many scale models for retracting the landing gear. The air is piped to a stationary cylindrical brake which has an annular slot in which an o-ring resides. The air pressure expands the o-ring against the inside of a rotating brake drum, inside the wheel hub, to provide braking drag. These brakes, with matching wheels, are quite expensive, and add the complication of the expensive compressed air system if your plane does not already have retracts.

I decided to install the Aeropiccola electromagnetic brakes that I already owned in the Coupe. I hooked up a 5-cell NiMH 1000 mAH pack through a micro-switch that is actuated by the elevator servo on full down command. A standard manual "Super Switch", with built-in charging jack, completed the installation. See the sketch for the wiring diagram. The two brakes draw 600 mA each at 6 volts. These brakes could just barely hold the Ercoupe at engine idle.

Hobby-Lobby offers a Kavan electromagnetic brake that is very similar to the Aeropiccola, but is much larger in diameter. It has an operating voltage range of 4.8 to 9 volts, compared to 3 to 6 volts for the older Italian units. Using the battery pack, switch and wiring already installed in the airplane for the brakes, I adapted the Kavan brake to the Ercoupe landing gear. The larger Kavan brakes also draw 600 mA each at 6 volts, but I had the option of going to 9 volts if the 6 volt system’s brake drag was not enough.  It was not enough, being only marginally better than the Aeropiccola brake.

A 1650 mAH Cermark 8-cell NiMH transmitter battery pack dropped to well below 9 volts under load with both brakes energized, and it works quite well.  The brakes draw over one amp each, but do not perceptively heat up after short periods of engagement.  The transmitter pack weighs about 4 oz. more than the original brake battery pack, but the coupe flies better as the nose weight is increased.  With this battery, the Kavan brakes will hold the Ercoupe still against the thrust of the G62 running at about 1/4 to 1/3 throttle.

As you can see in the picture, the bent up lugs in the rotating steel disc of the Kavan brake are quite small, and they would not engage the spokes in the Ercoupe's 5-1/2" Du-Bro air wheels positively enough for reliable braking.  I soldered short pieces of brass tubing over the lugs, perpendicular to the disc, to effectively extend the lugs to about 1/2" long.  I then drilled the wheel hubs to accept the extended lugs, resulting in positive, non-slip braking.

   (Rev. A) start.        After flying the Coupe in the Scale Masters Qualifier, I wanted to increase the stopping power of the Kavan Electric Brakes.  They will hold the plane against only ¼ to 1/3 throttle, not a very convincing “magneto check”.  The 8 cell transmitter style battery, powering both brakes together, drops to about 8.4 volts with the 2 Amp current drain.

            I consulted Alec at Cermark about this, and he said that these AA size cells were not meant for such high current.  He recommended using NiMH sub-C cells, 3800 mAH capacity, in an 8 cell “hump pack” which would fit in the same space above the engine mount box where the transmitter-sized pack is now located.  This custom made pack weighs about 20 oz., but it may replace the present pack and the propeller flywheel, which together weigh about 16 oz.  Cermark glues the cells together, welds the connections, and then wraps the pack with shrink tubing.

            Alec also recommended using heavier gage leads and better Deans Ultra connectors in the brake circuit, in place of the servo extensions now used. He furnished these, with the new pack, all at a reasonable price, and delivered one day after my order!  Wow! 

            I am also experimenting with an ESC to give proportional control of the brakes.   The incredibly tiny Pixie 7P programmable ESC works, but I am not sure of its reliability.  The ESC plugs into an unused channel on the receiver, and it controls the flow of current to the brakes from the separate brake battery, thus taking the place of the servo-operated micro-switch.  I plan to slave the brake channel to the elevator stick, with full down yielding full braking, proportionally reducing to zero braking at neutral elevator.  Thus, the brakes will work as before, except a proportional braking will be available.

(Rev.  A.) end

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