RCU Review: Sig Four Star - 40 E-Powered

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    Contributed by: Fly RC Magazine | Published: September 2004 | Views: 34376 | email icon Email this Article | PDFpdf icon

    This review appears courtesy of Fly RC Magazine.

    Article by: Steven Horney
    Photos by: Walter Sidas

    Convert the SIG 4 Star 40 to E-Power
    Simple guidelines from a pro

    Electric power has really taken the market by storm over the past few years, but most of the attention has been going to park flyers and indoor planes. As it turns out, electric power works just as well for bigger planes, including the very popular .40 size planes flown at club fields. This article will give you basic guidelines for making a successful electric conversion, and show you a good example system as well. I’ll be converting my formerly glow-powered Sig 4 Star 40, a popular sport plane that makes a great electric plane, but the tips presented here will apply to many other planes of similar size as well.



    Plane: Sig 4 Star 40
    Manufacturer: SIG

    Wingspan: 59.75 in.
    Wing area: 604 sq. in.
    Length: 47 in.
    Flying Weight: 6 lb., 2 oz (98 oz) with 16 cells
    Wing loading: 23.4 oz/sq. ft. (16 cells)

    Motor: AXI 4120/18 motor
    Controller: Jeti 70-3 Opto
    Cells: 16 Sanyo NiCd RC 2400mAh cells.
    Prop: Graupner CAM 14x9.5 folder

    Top RPM: 7500
    Full Throttle Power: 38 amps, 608 watts; 6.2 W/oz., 99 W/lb.

    Radio: Multiplex Cockpit transmitter from Hitec RCD USA, Hitec 555 receiver, 3 standard size Airtronics servos, 4-cell 500A receiver pack

    (Power system and prop provided by Hobby Lobby International.)

    Why the 4 Star 40 is a Great Conversion Candidate

    Almost any plane can be converted to electric power, but some airplanes are definitely better candidates than others. The biggest key is wing loading —how much weight each square inch of the wing carries. Since the electric power system will weigh more than a comparable glow system (assuming you are using NiCd batteries), it stands to reason that a glow airplane with a relatively light wing loading will make a better electric conversion. A wing loading of less than 23 oz./sq. ft. is desirable (that’s equivalent to one pound or less per 100 square inches—a good rule of thumb to remember).

    After years of experience, my Sig 4 Star 40 is no longer a thing of beauty, but it is a rugged, everyday flyer that performs superbly as an electric conversion. I’ll work on the aesthetics of the motor mount in the future, but for now it showcases another attribute of electric power – no need for fuel proofing!

    Looking at this practically, if you fly with 16 2400mAh NiCd batteries you’re adding approximately 1.5 pounds to the airframe. Suppose your airplane has 600 square inches of wing area and weighs 6 pounds (wing loading of 23 oz/sq. ft.). Adding 1.5 pounds to your airplane will give it a rather heavy wing loading (29 oz./sq. ft.); it will probably have a fairly fast landing speed and may tend to snap if slowed too much. On the other hand, if the glow weight is listed at 5 pounds or less and your plane has 700 square inches of wing area, there’s plenty of potential for an electric conversion (wing loading would increase from 16.4 to 21.4 oz./sq. ft.). When converting to e-power, there will be some additional weight savings along the way (no fuel tank or fuel, fuel proofing is not needed, servos can often be smaller, motor mounts are lighter), but simply adding battery weight to the glow weight is a good means of making a quick estimate of a plane’s suitability for electric power.

    How much power do I need and how do I calculate the power required?

    Power estimation and motor selection are probably the biggest sources of confusion for the electric neophyte, especially with the bewildering ways of describing motors. Part of the problem is that electric motors are generally much more versatile than glow motors; a single brushless motor can potentially be flown in a small aerobat on 7 cells, then pumped up to 24 cells with the right gearing to give serious power to a .60 size airplane. Consequently, it’s hard to recommend a single setup, since a variety of approaches will work well. Rather than go into an exhaustive study of all that’s involved in motor systems, I’ll give you a few rules of thumb and then show you a power system that works well.

    When choosing an electric power system, we generally focus on the batteries first, since the batteries actually provide the power. The motor simply converts battery power to rotary motion. Two rules of thumb have long been used in electric circles. The first was proposed by Keith Shaw, and it’s often referred to as the Watts per Pound rule. Power is measured in watts (multiply voltage times current in amps to get watts). Dividing by the total weight of the plane will yield a W/lb. number. Basic flying generally requires around 50 to 60 W/lb.; good aerobatics require 80-plus W/lb. Going over 100 W/lb. will provide some serious vertical performance.
    For a good estimate, multiply the battery current by the number of cells (1 volt per cell is a good estimate when NiCd cells are under load) to obtain the watts being produced. Of course, you’ll need an Astro Flight Whatt-meter (or a voltmeter and a shunt) to measure
    current. The system I’m recommending draws close to 40 amps at full throttle.

    A simplified rule of thumb was proposed by Matt Orme (formerly of Aveox, now producing and marketing Razor Motors). According to “Orme’s Law,” use 1 cell per 50 square inches of wing area for trainer-like flying; 1 cell per 35 square inches for solid aerobatics. Although simple, this rule works surprisingly well. [Editor's note: these rules of thumb apply primarily to models powered by glow engines in the .20- to .60-size range.]

    Setting up the 4 Star 40

    Let me show you how I applied the above rules of thumb to arrive at my current electric conversion setup for the Sig 4 Star 40. Originally a glow airplane powered by an OS .46, the only changes I made to my 4 Star 40 for electric power were removal of the fuel tank and engine mount and the addition of the batteries and electric power components.

    The Sig 4 Star 40 has a listed glow weight of 4.75 pounds and a wingloading of 18.1 oz./sq. ft. Note that the 4 Star 40’s listed weight gives a 1.25-pound margin before hitting the magic 1 lb./100 sq. in. wing loading rule.
    A plywood plate (with triangular reinforcement) glued to the front of the cut-down “ears” on the 4 Star 40 serves as a mount for the AXI motor. Thin washers keep the motor off the plate enough to prevent rubbing of the shaft retaining clip.
    This is a very powerful motor that needs to be mounted securely. The folding prop helps prevent damage to the motor shaft in the event of a serious crash.

    I’ve found that 16 cells provide a good balance between weight and power in a plane the size of the 4 Star 40. It’s also relatively easy to mount two 8-cell packs in the fuselage. As you can see in the specifications, my 16-cell setup brings the airplane weight to around 98 ounces, or just over 6 pounds. Going with the 1 lb. per/100 sq. inch rule, you can see that the 4 Star 40, with 604 square inches, is right on the money. This particular plane will fly well even with higher wing loadings, but it’s a very sweet flyer at this weight. As for power, assuming I’ve propped for 40 amps, that leaves me with around 640 watts and 107 W/lb., a very good number for powerful aerobatics. Using Orme’s Law, I should be running 17 cells for great aerobatics, so I’m very close to that rule of thumb as well. The end result? A well-mannered airplane capable of great aerobatics!

    Estimating Flight Time

    Great aerobatic performance no longer equates to short flight times as it once did. The use of Sanyo 2400mAh cells yields around 8 minutes of good aerobatic performance, with reasonable throttle control, in my 4 Star 40. Few glow flyers keep their planes up more than 10 minutes (as shown by several informal measurements), so practically speaking my 4 Star 40 offers glow performance and flight times without the mess and hassle of running a glow engine. Flight time can be estimated by multiplying the amp hours of the battery (a 2400mAh battery is 2.4 amp–hours) by 60 minutes to get amp-minutes, then dividing by the average current draw. In this case, the 2400’s yield 144 amp-minutes. If the average current draw is 18 amps, you’ll get 8 minutes of flight.

    The high torque design of the relatively light AXI 4120/18 motor allows it to put out a tremendous amount of thrust.

    I’ve powered my 4 Star 40 with several electric power systems over the past few years, all of which have worked very well. My current configuration is powered by an AXI 4120/18 motor and Jeti Advance 70-3 Opto controller carried by Hobby Lobby. Jeti controllers are simple to use and very reliable, and the new Advance series goes a step further with programmable timing, lower rpm “idle,” and improved efficiency.

    A 3/16-inch balsa doubler glued against the top of the fuselage interior serves as a base for the batteries. Two 8-cell 2400mAh battery packs are attached to each other with Velcro, then attached to the base plate with Velcro. A Velcro strap ensures everything stays in place. The speed control wires run through the firewall to the motor, while the controller itself is allowed to “float” in the battery compartment. Anderson Power Pole connectors ensure good power flow between batteries and controller, plus they allow the two battery packs to be joined in series to make up a 16 cell pack.

    AXI motors have a huge amount of torque for their weight due to their unique design. Instead of the typical brushless motor design which places the magnets on the rotor and spins them inside the windings on the housing, the AXI’s have an “outrunner” design—the magnets are mounted to the housing (on the inside wall), spinning around the windings which are fixed on the inside. The shaft runs through the middle of the windings and attaches to the rotating housing at the back plate. These motors are designed to spin very large props without a gearbox. AXI motors must be mounted at the front of the motor (generally behind a firewall or a plate), since the entire outside housing rotates. Most new brushless motors, such as the AXI I’m using in the 4 Star 40, are sensorless designs. Reversing these motors is easy; just switch any two of the three leads and the motor is reversed. Motor timing and commutation are all performed in the controller.

    Before we get into the details, let's look at the good news in terms of flight performance. In the electric conversion that I will describe, an AXI 4120/18 brushless motor spins a Graupner CAM 14x9.5 prop at 7500 rpm and 38 amps on 16 cells, providing a huge amount of thrust. This system provides about 4 minutes of full throttle power, but you can fly up to 8 minutes with a mix of good aerobatics and cruising flight. Takeoff is almost immediate, and the power available will allow the 4 Star 40 to fly knife-edge, extended verticals, and even perform torque rolls. It performs better than it did with the glow engine in it!

    Electric power isn’t for everyone or every plane, but I think most pilots who opt to power their planes with electrons will find the experience very satisfying. Current technology places electric performance nearly on par with glow performance, without the disadvantages of noise, fuel, starting hassles, etc. What could be better than glow performance and electric convenience in a .40 sized airplane?

    This article appears courtesy of Fly RC.

    Fly RC Magazine is published by Maplegate Media.

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    Fly RC magazine.

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    Hobby Lobby International, Inc.,
    (615) 373-1444

    Astro Flight Inc.,
    (310) 821-6242

    Hitec RCD USA, Inc.,
    (858) 748-6948

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