||Pietenpol Sky Scout|
|THE PIETENPOL SKY SCOUT
by Chad C. Wille
(As published in EAA SPORT AVIATION magazine, February 1998)
When Bernard Pietenpol sat down to design the little single seat version of his successful 1929 Aircamper, he was engaging in a bet with himself. His Model A Ford powered two seater had splashed onto the pages of Modern Mechanics magazine with the force that was to see it popularized through the 1930's. But even though the Model A engine was a cheap alternative to aircraft engines it was still expensive in those "Great Depression" years and one did not run right out and buy it from a Ford dealer. Having once calculated the current value of a dollar, compared to what a dollar would buy in 1932, I asked my father if he thought it sounded about right. "Maybe $6 back then would be worth $100 today. But its far easier for anyone to lay hands on a hundred dollar bill today than six dollars back then." And so Mr. Pietenpol was besieged with letters from young men wanting to use not the expensive, new, Model A engine but the old Model T, built in the tens of millions and available in junk yards everywhere. Instead of rejecting the idea out of hand, and knowing that the Aircamper with two seats was just too much airplane for the little T engine, he bet himself he could design an airplane that would fly one person on the 30 hp. available. Using a shortened span of the standard Aircamper wing he went ahead.
The Sky Scout did all that was asked of it on its meager, heavy horsepower. It may have had the climb of a leaking submarine, but otherwise handled well and cruised along at a respectable 55-60 mph, which feels plenty fast when the air rushes over you in an open cockpit. With Pietenpol's own undercambered airfoil it got off the ground quickly and could land in any short space. Bernie had proved to himself, and anyone else who might be interested, that the Model T engine could be made to fly well. But aside from the prototype very few of them were built. The trouble with most conventional single seaters, even today, is that for a very small amount of extra work, one can have two seats. So the popular older brother, the Aircamper, remained one of the mainstays of the lightplane movement of the Depression and the Sky Scout was largely forgotton. Had it not been for the EAA reprints of the early Flying And Glider Manuals few people except those close to Mr. Pietenpol would have thought that he had bothered to design and fly a single seater after the success of the Aircamper.
My own Sky Scout project began with the finding of an engine. A short advertisement hidden in the local paper sent me 50 miles out of town to a farm in the foothills of the Coast Range in western Oregon. An old gentleman had a Ford Model 'C' for sale. Outwardly identical to the Model A car engine, this was a stationary industrial engine that ran a pump for a mining operation. It sat in a corner of his barn now, hay sticking out of the water pump and exhaust ports, faded to a rusty grey color. "I worked for the mine for a while," he said, pointing vaguely up into the mountains. "The motor burned up and I was one of the fellers that took her down to a shop to have her overhauled." He didn't give a clue as to when any of this happened but I got the impression that he had been a much younger man. "I put her back on and hooked her all up and just got it running when the mine shut down. It doesn't have a days running time on her! Well we all went home, but I never got paid my last paycheck, see? So I just waited a few months and went back up there and took that engine home with me cause I knew it was good and it would just sit and rust out there."
The Model C is a heavy engine. It has a fully forged counterbalanced crankshaft, unlike the Model A, and more iron filling in the head chamber to increase the compression. Its weight bare is 240 lbs. After dragging it into my shop I pulled the head off, and the pan, and it looked clean and oiled on the inside. I toyed with building an Aircamper around it, but having an 0-200 powered Aircamper already, took the urgency out of that idea. I was used to hauling 250 lb. passengers with ease in the front cockpit, in fact for over 20 years I rarely flew the Aircamper solo. The idea of struggling aloft behind the Model C seemed charming, but dangerous. And would my poor passenger appreciate what they were getting themselves into? I mounted my engine on a castored pallet and over the years, moving it further out of my way, eventually into another shop, I put it out of my mind.
As the years went by there were other airplanes being built, a couple gyrocopters, a Sopwith World War One fighter, an antique glider restored, and an original design 3 seater 'Cub.' And also welded parts and propellers for Pietenpol customers. The Pietenpol has always been a popular homebuilt, has gone through several surges of interest over the years, and now has two devoted type clubs and newsletters and its own organized fly-in. More and more, roughly starting in the 1980's, people were experimenting again with the Model A engine. More and more the propellers I was building for customers were the original design Model A props and I began to get some good feedback on the Ford engines. Some were flying 100 hrs and more with no trouble of any kind and their owners were satisfied. Some carried passengers, some not, depending on the weight of the ship. (Most men today are not the skinny lads of the hungry Depression years.) But more and more a new generation was proving that the Model A engine was a success. When the day came in the Spring of 1989 that I had run out of airplanes to build or fix, I turned back to the Model C I had tucked away.
I reread the Flying and Glider Manual article on the sky Scout. I had never actually seen a Sky Scout but the pictures looked darn cute. In the article I found Mr. Pietenpol almost apologizing for the Model T engine, and recommending the Model A instead. "The Model A is a wonderful motor for the small plane, and may be used in this ship, and advise its use it you have not a Model T all rebuilt for aircraft use," he had written in the 1933 Manual. I was hooked.
A Spruce and Fir fuselage was laid up and completed in 10 days. Within another 3 Weeks all the controls were installed and all the motor mount fittings welded up and bolted on. I was anxious, like most builders, to get the ship done in the least possible time. But then the project stopped. I was called to a flying job up in Alaska, with just enough time to pack my bags and hop an airline north.
The flying season in Southeast Alaska is short, about 7 months, but intense. Flying can start at 6 am and go all day. "Day" can last till 2 am, and the sun never completely sets. The only work I managed on the Scout that summer was to spend a stolen hour here and there to cut some fittings and hinges, a little filing and drilling. That summer job led to another flying for a large West coast helicopter operator, living in a travel trailer and moving every few weeks from Washington, to California, Montana, and Idaho. Within the confines of my trailer there was a great deal that could be done on the Scout and for the next two years I built ribs, a center section for the wing, gas tanks, landing gear and all the tail surfaces. Half of my trailer was a workshop with bench, vice, tools, saws and hardware all carefully placed in a small area. One campground owner in the California Sierras even lent me his welding tanks and wouldn't let me pay or refill the tanks. "They've sat here 10 years, I never use 'em, forget it," he said.
I would often raid the hardware store in some little town looking for the right kind of hinges, screws, glue, copper tubing, and the other thousand parts a homebuilder needs. And here is where the Pietenpol shines. Designed for 1930's farm boys with no money to buy fancy parts and equipment, one can scrounge the necessary stuff just about anywhere. Todays kitplanes may provide bubble wrapped completeness right down to the cotter pins, but that convenience takes away all the fun of searching for parts. The process of building a plane from plans can be more interesting, even more rewarding than the finished flying machine. After all, flying is not all the aviation is about.
My propane stove doubled as a heat source to solder my fuel tank. Fabric covering was done outdoors on a picnic table wherever I happened to be, to the delight of the other campers, who were amused that a person could build an airplane in a travel trailer. (Though they never really believed any of it.) When the time finally came to assemble the wings I realized there was a limit to what could be done on the road and I quit my job to spend a few years finishing the Scout at home.
Work began immediatly on assembling the ribs and spars. I had taken great care on the rest of the airplane to keep weight to a minimum, and special work was done to keep the wings light. This included planing the Fir spars down to 11/16" and saving 8 lbs. The leading edge was heavily routed on the back side accounting for another couple pounds. The false spars for the ailerons, as well as the aileron main spar and reinforcing spars were also treated this way. Rather than a single piece wing, a center section and two panels were built up for ease of transport. The wings, as well as the rest of the airplane were covered in the very light Stits process with new generation fabric and finished with Stits Polyurethane Enamel in colors I thought were in keeping with the era of the airplane.
The mounting of the landing gear to the fuselage turned out to be a headache and the only trouble spot on the airplane. I liked the look of it with its long spring shock strut and thought it would be an improvement over the rather tight bungee cord of the Aircamper gear, But the geometry was worth scratching your head over. Because of the rather extreme curvature of the lower longeron blending into the engine cowling where the front gear leg attaches, the axle would actually move fore and aft (toe in and out) as the gear vees moved up and down! This could be minimized with just the right amount of spring tension in the shock strut, which required building the strut last, or estimating the final weight of the airplane ready to fly with pilot. I estimated, came out a little heavier in the end, added shims to set the wheels so they didn't sag outward and the strut worked soft and troublefree in the end. But it took a very long time to get it right, including making a jig to test the compressed strength of various springs. Ultimatly a die spring of heavy 1/4" coil was required on each strut. In his article Mr. Pietenpol shows a picture of the Scout taxiing, its gear badly splayed out and says mysteriously, "You may also build the type of landing gear that is used on the Aircamper. You will find that this is much easier to make.........."
The plans called out for 26" x 3" wheels and tires. This is a 20" rim and a size not available in the U.S. My father had used a pair of 26 x 3 wheels from a 1926 BSA motorcycle on his Aircamper for 15 years and except for the tires they were in good shape. Tires were finally discovered in England, smooth rib tread, and make a very nice addition to the lines of the airplane. A 20" wheel is noticeably larger than the common 19" often used, and makes the airplane stand up that much taller, making it look larger than it actually is. I never use brakes for steering on an old taildragger, as I've seen it result in too many wrecked airplanes. But the minimal brakes from the motorcycles wheels were used, attached by cable to a single lever in the cockpit that operates both simultaneously. This allows you to bring the ship from walking speed to a full stop on a paved taxiway. There is no need for brakes for a mag check runup, since, with single ignition, there is no need for a runup. If the engine is running, you are ready to go.
As the airframe and wings came together, it was time to dig into the engine. I found the poured babbit bearings in good shape, machined the crankshaft to accept a rubber seal, took up on the shims for the rods and mains and so had a working lower end. I had a machine shop surface the cylinder head and cut the faces for the new valves. Since this engine has a pressure oil gallery it was easy to run the two oil lines to the front main bearing as called out on the plans. The only modification made was to use adjustable valve lifters. (Ford originally set valve gap by grinding the end of the valve.) These proved to be unnecessary as the heavy iron block does not expand and contract like an aluminum air cooled engine, so valve gap remains constant. While getting a magneto drive worked out I decided to use battery ignition just to get the engine running initially. The distributor on the Model C looks like the 'A', but has a centrifugal advance that cuts in at about 900-1000 rpm, which gives a good retarded spark for hand propping. The whole system worked so well that I decided to fly it that way. Instead of a charging system I used 2 motorcycle batteries ($3 each!) and a master switch to select either one. There is plenty of reserve battery power for long flights.
The old Ford engine is a bit of a surprise to operate. When pulling the prop through one notices the lack of compression, even on a newly overhauled engine. This is due to the very low compression ratio of the Model A, less than 4 to 1. My engine has a higher compression head, but when the engine is cold, still seems pretty wheezy. After running it feels more like any good small aircraft engine.
It starts very easily and with the carburetor properly adjusted settles right down to a good 500 rpm idle. Since there is no exhaust manifold on the aircraft version, just four short stacks sticking out the side of the block you'd expect it to be loud but it is not. The exhaust has a rather flat sound to it which is not at all irritating, even at full throttle. There is a slightly oily exhaust smell coming back into the cockpit which adds to the thrill of operating this old engine. Of course it does not run very fast, only about 1800 rpm at full throttle, but with plenty of thrust from the 76" long prop. Since the prop speed is low there is very little propeller noise and the whole operation is quite .......sedate. My engine has the counterbalanced crankshaft and is very smooth in flight, much smoother than a Continental A-65. The quiet, smoothness, and massive cast iron weight of the engine makes it seem as if the engine is not being overworked in flight, and adds to a sense of reliability not present in lighter, higher revving aircraft engines. It is easy to see why this engine was a success to Mr. Pietenpol.
The first flights of my ship came in the Fall of 1995. Since I was operating from a 5000 ft. long paved runway I had the luxury of making a great many flights to an altitude of about 75 ft. down the length of the runway, giving myself a month to work out minor problems with the wheels and landing gear and making sure the engine and fuel flow were operating correctly. My engine did have one characteristic that concerned me at first, but eventually I got used to it. At the start of the takeoff run it would missfire as it sped up to full rpms, diminishing until it was running smoothly at liftoff. The Scout is off the ground rather quickly, even when letting speed build to 60 mph, but I always wondered if it would start missing again once I was in the air. It never did. This was probably a characteristic of my engine alone, and I never found a specific cause for it, so after a while just ignored it. Mr. Pietenpol said, "Don't take off with a cold motor, and watch your mixture and your motor temperatures very closely .... Bumpy fields, until you get the hang of the Ford Motor, will make the motor spit on the ground and confuse the pilot."
After six years of part time effort my time came to really fly the little Ford Sky Scout which had been much traveled during her construction. And the flight characteristics of the Scout are just delightful.
The tail comes up quickly with the application of full power and the rudder is extremely sensitive. The response to all the controls is very rapid. Climb is slow but steady. The measured rate of climb is 300 ft/min. (timed by watch) but there is no sense of mushing. The big propeller keeps pulling uphill regardless of wind and sink. Aileron response is quick and the roll rate excellent, could even be considered 'snappy' by any standards. The stall occurs at 40 mph indicated and breaks right wing first, but is easily counteracted with the rudder. The nose must be pointed down and the airspeed let build again during the recovery as it has a very noticeable tendancy for a secondary stall. I have never experienced this on my Aircamper so seems unique to the Scout. The faster roll rate over the Aircamper is the biggest difference between the two airplanes, and is suprising since the Scout has the identical wing chord and airfoil, although the span is 27" shorter. Cruise speed is 65 mph at 1600 rpm, top speed 80 mph.
The glide is not particularly steep and the approach to land is kept at about 55-60 mph since the high drag airframe pays off speed quickly in the flare, touching down at about 40 mph. The Scout, like the Aircamper, handles crosswinds well, since there is no cabin side area to the fuselage and wind seems to blow right across, but one has to be alert on landing and quick on the rudder due to the short fuselage and the tall wire wheels.
Of course, all of that is merely the technical side of flying. One of the reasons we build an antique airplane is to identify with the people and the time from which they came. The joy those builders and flyers had in the "golden age" of homebuilts, experiencing the struggles to get a successful 20 minute flight or a 25 mile cross country, is different than flying a modern airplane for an hour. We have improved airplanes and the science of flight, but have sanitized it at the same time. In duplicating an early airplane and engine we can journey back to the same air rushing through the cockpit and the same bark of the exhaust, the smell of cut grass and barbecues rising to 1000 ft, the particular excitement of flying in that time. We have to solve mechanical problems that are unknown to the modern A&P with parts that come from obscure places rather than a glossy supply catalog. Its very challenging and great fun for those who are interested in it.
Inevitably, the kind of airplane you build attracts the kind of people you might like to talk with and the Scout brings out older gentlemen who had a hand in building or flying an early homebuilt, or knew someone who did "back in the good old days." The stories are often priceless and sadly, will probably be lost when these men pass away.
Mr. Pietenpol's airplanes remain popular not because of their blinding performance but because of their common touch. They are honest and friendly. And with an ever growing following remain a significant part of the homebuilt airplane scene even today.