KITPLANES Magazine, January 2002

Murphy's Radial Rebel
Bushplane pilots don't have to leave it to Beaver.
By Ken Armstrong

Murphy's new SR3500 boasts a gross weight of 3500 pounds, but with
the M-14P radial, power isn't a problem.

The big radial coughed to life dispatching a swirling plume of smoke that danced through the towering trees on the remote grass strip. The pilot guided the six-place taildragger's tail against the verdant barrier that marked the terminus of the short one-way strip. With nine pistons pumping and temperatures and pressures in the green, the throttle was quickly firewalled, and the high-wing bushplane surged forward in a rush to leave its earthly bonds.

Chief Pilot Robin Dyck points out a doubler that was installed to strengthen
the SR3500 for its increased weight.

    The deep drone of the 98-inch propeller and reesonant exhaust pulsated off the rangy cedars lining the strip, but the cacophony was left behind by the rapid acceleration. No need to lift the tail as two notches of flap coupled with the immense wing flung the big plane skyward in a tail-down attitude in less than 600 feet. The pilot maintained the steep climb as Big Bird accelerated to 80 knots while climbing 1900 fpm.

Sound like a de Havilland Beaver taking off? In fact, this was my first flight in Murphy Aircraft's new SR3500 with its 360-hp Russian M-14P radial providing the exciting performance. It may perform like a DHC-2 Beaver, but this is a new fast-build kit aircraft.

Kit versions of the SR3500 will have a composite cowling with integrated oil cooler.

    This radial-powered aircraft is special in two ways. First, it is an extension of the SR2500 Super Rebel line with a 500-pound increase in operating weight to 3500 pounds. Second, the standard Lycoming O-540 engine of 250-300 hp has been replaced with a round engine and all that entails by way of nostalgia-not to mention significantly enhanced performance.

Big Lungs
    Radial engines have always been popular with the bush pilot set. This particular radial dominates the overseas market so much that there are more M-14P's in service than all of the engines ever produced by Continental and Lycoming combined. Reliability is not an issue. These powerplants are operated in Russia by pilots who often don't know the difference between the words spinner and empennage. (Of course, this may have something to do with the fact that they don't speak English.) And these backyard mechanics typically practice a form of maintenance called minimal, yet the engine still proves dependable with a TBO of 1500 hours.
    Part of this reliability stems from the engine loafing at a compression ratio of 8:1. And low-pressure supercharging provides rated power at 35 inches of manifold pressure, which helps balanced the combustion aarea pressure between power strokes. Low compression leads to lower operating costs because auto fuel is the fuel of choice. Occasional doses of 100 LL avgas provide the lead that ensures the valve train remains well cushioned and lubricated..

The radial has an oil reservoir (right).

    A gearbox or propeller speed reduction unit (PSRU) allows the engine to turn up to 2950 rpm to create lots of power while the propeller turns at a more efficient 1940 rpm. When dyno'd, these engines typically produce 390 hp, so it takes a large-diameter, wide-chord set of blades to absorb the thrust. The 98-inch diameter dictates that the SR3500 will never be a tricycle gear aircraft. I reckon the prop tip clearance is 8-10 inches, so operators of the prototype need to ensure that they don't lift the tail too much for landings.
    The company has logged hundreds of hours with trips to Sun 'n Fun, Oshkosh and Alaska and will incorporate two modifications to the kits. First, it will lengthen the maingear legs to provide an additional 4 inches of height. Second, it will replace the aluminum cowling on the radial with a composite creation that's more attractive and will also enclose the oil coolers that are currently slung underneath.
    A significant note on this engine is the low cost. Murphy Aircraft paid $18,000 U.S. for this new engine, and that's likely half the cost of a new 300-hp Lycoming.

Where's the Beef?
    Murphy has accomplished a miracle with the SR3500's 500-pound increase over the SR2500 Super Rebel. The empty weight of the SR3500 was increased only 2% to prepare it structurally for more powerful engines, higher speeds and, of course, the 17% payload increase. That's a bargain.
    Surprisingly, there is little apparent difference between the two series even though most of the changes are to the outside of the structure where they should be obvious. Knowledgeable Super Rebel builders might notice the doublers added to the wing and horizontal stabilizer roots. A much beefier set of rudder and elevator horns have been incorporated, and the undercarriage has been strengthened. Structural modifications were also made to handle the greater thrust from bigger engines and the higher flight speeds.

This heavy hauler still offers spry performance.

    The additional 65 pounds of modifications were so successful, the SR3500 is approximately 35% stronger than the SR2500. Specifically, the structure was statically loaded to 8400 pounds on each wing (%.3 G) and the torsional load tests were equivalent to a 220-mph dive with a 4-G pull-up with full aileron deflection. Few kit manufacturers conduct this form of stress testing on their airframes.
    The great news for SR2500 builders is that the beef-up kit is available for $3000 U.S.. These mods allow a gross-eight increase to 3500 pounds and use of engines up to 400 hp. Because the modifications are essentially external, there is little difficulty installing them, according to the company. This is the sort of modification that greatly enhances an aircraft's function and resale value.

Why Buy?
    It's difficult to load a big plane such as the SR3500 with passengers on short notice, so our flight was accomplished at 900 pounds under gross. Company chief pilot Robin Dyck has logged more than 150 hours in the plane at full gross weight (and higher for testing) and advises there is little change in climb or cruise performance with a full load. Knowing Dyck as long as I have leaves me with no doubt as to the veracity of his observations.
    With every airframe/engine combination,, there are tradeoffs. The radial is 80-100 pounds heavier than a Lycoming and burns slightly more fuel, but it provides a lot more thrust for takeoff and climb. For owners who want to maximize useful load, the selection of the lighter Lycoming O-540 allows a greater payload--at the cost of runway and cruise performance.
    Radials require some special attention before start-up as oil tends to pool in the heads of the lowest cylinders. Fluids are not compressible, so attempting a start without draining this oil could blow a jug or two. A unique bleed system allows the pilot to turn over the propeller blades while pumping any accumulated oil out the exhaust. (This is why operators of aircraft such as Beavers carry a can to catch the few ounces pumped overboard. Think of it as part of the nostalgia.)
    Another unusual aspect of the pre-start check is to ensure the air reservoir is up to approximately 800 psi. Air pressure? Yep, this engine is cranked with pressurized air that is supplied by an engine-driven compressor known as an AK-50. Go figure. Part of this system includes a shower-of-sparks ignition for superior starting characteristics. The shower has been used in many aircraft as it overcomes the problem of normal magneto systems that provide a spark that is too advanced and might cause a kickback on start-up or just hard starting. Not so on the M-14P. To crank, one simply presses the start button, and when the engine fires, the mag switch is turned from off to both. The oil that drained into the exhaust is immediately converted to smoke for that reassuring proof of a successful start, although you won't see anything behind the aircraft for a few moments. The magic of the radial on the ground is the reassuring rumble and hint of beaucoup horsepower under the cowling. For the remainder of the flight, I was enchanted by the harmonious hummm.

Evaluation
    Our evaluation flight was from the seldom seen Butler/Howroyd Field between the city of Victoria, British Columbia, and its international airport. This one-way, 1700-foot tree- and berm-lined strip is slowly being devoured by gravel-hungry machines and has become a short field that chases away many a pilot. For the Super Rebel, it's a large airport.
    After a short warm-up and engine checks, pressure on the differential brakes lines us up for the northerly departure with the south end trees almost touching the trailing edge of the elevator.
    Due to the diminutive length of the strip, I apply power quickly and am pleasantly surprised that a small amount of rudder easily holds the torque and slipstream effects in check. This isn't due to low power. Acceleration is right-now rapid, and by holding the tail-down attitude, the SR3500 simply lifts off the strip at approximately 50 mph. Our camera man is stationed 600 feet down the runway, and we climb steeply as we pass him.
    Power is set for a cruise climb at 2400 rpm (1580 rpm propeller) as we quickly claw our way to altitude for higher air work. The first clearing turn is delightful with lighter than anticipated aileron pressures. Dyck advises that the aileron horns have been widened, and because they're ahead of the hinge line they aerodynamically reduce the amount of control force necessary to create a given turn rate. I note that rapid aileron input requires a touch of rudder to overcome adverse yaw, just as the Beaver aircraft do.
    The noise level at cruise is reasonable, and this is with the prototype's thin Lexan windshield (production kits will have thicker material with bulging acrylic windows as an option). The cockpit is super size, allowing large adults to sit in comfort. Visibility is good in most directions. The slightly higher cowling with the round engine was no problem for either Dyck or me, and we are somewhat shorter than average pilots.
    We level off at 2000 feet and find our indicated speed rises quickly to 130 knots with a fuel flow of 17.9 gph. At altitude, the true airspeed should have no trouble reaching 135-140 knots at this power setting. The supercharged engine is capable of providing higher cruise speeds right up to flight levels that require oxygen. It is quite possible that the composite cowling intended for production kits will enhance oil cooling and cruising speeds, as it will be aerodynamically cleaner.

Kit versions of the SR3500 will have a composite cowling with integrated oil cooler.

    How slow can you go? Pulling power back at altitude results in rapid speed decay due to the increased drag of the huge propeller. This is handy for approaches, too. Our first stall investigates flapless flight at idle power. Finally, as the needle nudges 40 knots, the nose nods downward ever so gently. Forty knots would be an excellent stall speed with full flaps extended and a modicum of power--never mind no flaps.
    When we set up for a dirty stall with approach power selected, the airspeed indicator registers near 0 before the nose drops approximately 15-20 degrees straight ahead. A number of kit airplanes are capable of carrying large loads, but I can't remember one that also offers such forgiving slow-speed handling.
    With these absurdly low stalling speeds, it seems ludicrous to set up on final approach at 55 knots for landing. However, it gives us lots of safety margin for gusty winds and, in this case, a 5-knot crosswind. We fling all the flaps and set up for a steep approach to simulate obstacles; the SR3500 flies the approach as if it were on rails--too easy. Another takeoff and landing confirm that the aircraft is easy to fly and has virtually no tendency to swing, even in the light crosswind. These steep approaches allow us to clear the berm and after touching down we can easily stop in 600-800 feet on the grass with light braking.

The radial contributes to the airplane's nostalgic feel.

Comparing Values
If you need an aircraft with the capacity to carry 1500-pound loads into short, rough fields or perhaps into remote lakes, the SR3500 is a contender. Moreover, this Super Rebel is much cheaper than factory-built Beavers and Cessna 206 six-place aircraft. (Canadian builders are currently allowed to carry no more than four people in an amateur-built aircraft. However, sympathetic legislators may increase this to six seats.)
A number of Alaskan pilots have likened the SR3500 to a deHavilland Beaver. However, the Super Rebel is easier to fly and maintain, and it's certainly cheaper to operate. Hard to say why anyone would want to pay more for the factory aircraft unless they have no time to build. Also, putting a Beaver or Cessna 206 on certified floats can cost an arm and a leg and may furrow your brow. The SR3500 can don various forms of "rubber footwear" for a fraction of the cost--and the kit comes standard with floatplane fittings.

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For More Information, contact Murphy Aircraft at Unit 1, 8155 Aitken Road, Chilliwack, B.C. Canada V2R 4H5; call 604/792-5855; e-mail mursales@murphy.com, or visit the web at www.murphyair.com.

For engine information, contact Geosoco Industries, Inc., 629 Airport Rd., Swanton, VT 05488; call 802/868-5633; fax 802/868-4465.