Electric Hammer (oz10647)

About this Plan

Electric Hammer. Radio control sport aerobatic model, for electric power with brushed cobalt CP-05 motor and 2:1 reduction gearbox.

Quote: "There's a quiet new member in the Hammer family. With a tweak here, a stretch there, and a motor battery pack thrown in, the author gives us a good looking sport design. Electric Hammer, by Bob Peru.

I was talking to Dick Sarpolus some time ago about extending the Hammer (oz10680) family (which he designed) to include an Electric Hammer. Dick had a look of disbelief like it couldn't be done. That was all the motivation I needed to start the project. The Electric Hammer is a completely new design using different aspect ratios than its predecessors. When the prototype was finished it came out in between the Hammer 20 and Hammer 40 in size. It has a 56 inch span and the ready-to-fly weight is 38 ounces using seven 900 mA Ni-Cd flight cells.

A little theory: the speeds at which electric models fly put them at the lower end of the parasitic drag curve. Induced drag is a function of lift and greatest at high angles of attack. Once the plane accelerates into level flight the induced drag curve levels off to a minimum factor. The real problem or challenge is to conquer the power-to-weight ratio. Very simply, the lighter you build an electric model the better it will fly. In the case of 05 electric motors the all-up weight should not exceed three pounds. Ideally 2-1/4 to 2-1/2 pounds with 400 to 500 square inches of wing area generally will perform well. Select 4 to 6 pound contest balsa for construction, use built-up structure, plenty of lightening holes, light-weight mylar covering, 800 or 900 mA flightpacks, 250 mA receiver pack, light weight 'glider' type equipment, etc.

Onward to the Electric Hammer. Initial bench checks were made with an 8-cell pack, an 05 Mabuchi plain bearing direct drive mo-tor with a 9-4 propeller. The throttle was pushed up and the thrust was good. Hey, this was great! Then suddenly, after 20 seconds or so of running, smoke erupted from the Electric Hammer. Two 800 mA, 4-cell packs were hooked up in series to give 9.6 volts. The 4-cell packs were pre-wired with 26 gauge wire. Needless to say the 26 gauge wire burnt up like a fuse with the 30 amp draw.

Here's where we got help from Larry Sribnick of SR Batteries and Ted Davey of Davey Systems Corp. LS sent a super 900 mA flight pack and 14 gauge super flex hook-up wire. DSC sent an amp meter vital for testing current draw and also supplied the gear box, Part #224A. The gearbox is manufactured by Al-Tec Products and is a new concept in electric motor reduction gears. It utilizes a ring and pinion gear resulting in a more com-pact and efficient gear box. The gear box design is smaller and more compact than any other unit is. Another feature is that the thrust line is only a 1/4 inch above the motor output shaft allowing easier cowling and pleasant fuselage lines.

The gear box was easy to assemble. First, the brass pinion gear is pressed onto the motor shaft in a vice. Next, a nylon mounting plate is screwed in place on the motor end plate. The gear housing and ring gear are installed onto the mounting plate.

It is important to run in both the electric motor and the gear box to seat the motor brushes and adjust the ring and pinion gear clearance. The motor ran cooler after break-in. Also final adjustments of the gear box decreased the battery drain about 4 amps.

The Air Supply Cobalt CP-05 normally handles 7.2 to 10 volts. At 7.2 volts the no-load speed is 30,000 RPM at 2.4 amps. It really winds up. That's why I selected the CP-05 motor and the reduction gearing concept. If you can not locate an Air Supply CP-05, an Astro Cobalt 05 can be substituted. For super performance a Cobalt 15 motor with 10 to 12 cells could be used. The only drawback is the initial cost and the additional battery weight.

Match all the parts you see in this picture and you're on your way to being the pilot for an Electric Hammer. As with all electric airplanes, light weight is mandatory so choose the wood carefully, using only wood stock heavy enough to do the job. The best performance is achieved with just some careful building considerations.

The next few shots show the building sequence for the simple, light-weight wing. First (above) lay down the bottom 1/4 inch square spar and the bottom rear 1/16 inch sheeting on the plans and align and glue on the ribs. Then add (below) the top spar..."

Direct submission to Outerzone.

Update 30/11/2018: Added article, thanks to RFJ.

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