Curtiss OX-5

By Chad Wille

(As published in the Summer 2002 issue of TO FLY magazine)

If you are captivated by ancient airplanes, you have read of the Curtiss OX-5 engine, seen pictures in history books, or heard a discussion where it was mentioned. For a time, during the explosive growth of private aviation, from 1927 when Lindbergh crossed the Atlantic until the beginning of World War II, the OX-5 was everywhere, as common as Continental and Lycoming engines are today. Every airport had them, every mechanic knew their valve timing and clearances from memory, every pilot learned to fly behind one. It is a testament to the fast pace of the 20th century that a single generation can be completely involved with machinery that their own children will know nothing about and find archaic. That is what happened to the once pervasive OX-5.

When I was growing up, I read of the engine that powered the airplanes at my father’s airport. But I never saw one in an airplane myself. I never heard one run. Occasionally someone would speak of the OX-5, but it was always second or third-hand information, generally the butt of a joke, the story ending in a big laugh. Other cultures tend to revere their history. Americans often prefer to laugh at it. For 33 years as a pilot and mechanic I wondered if those negative stories were true.

Attending an antique airplane fly-in today gives a skewed sense of the appearance of open cockpit biplanes. Most of them have high powered Wright, Lycoming and Continental radials installed. In most cases this is not the way they left the factory. Waco, the biggest and most successful manufacturer of biplanes, produced the Waco 10 for 5 years, from 1927 through 1931, and of the 1500 straightwings produced, fully 1200 of them left the factory with the Curtiss OX-5 engine, 75% of the total production. Clayton Bruckner, president of Waco, was so dedicated to the engine that he headed off government attempts to render it obsolete during 1927 when the Curtiss Jenny was banned from commercial flying. His efforts helped to assure that the OX-5 received a Type Certificate, issued in the Category 2 approval. Aside from economic reasons, (by then the surplus Curtiss was cheap), the engine must have been a good one for the nation’s biggest manufacturer to dedicate the bulk of their new production to it.

Now, seventy-five years later, we are faced with opposing concepts. Either the OX-5 was produced in huge numbers, over twelve thousand engines, and logged millions of hours of flight time ; or..... the Curtiss OX-5 was a pathetic, wheezing, unreliable design that couldn’t possibly have accomplished any of that.

That was the question in my mind concerning the OX-5. I wanted to learn first-hand which side of the story was true. And when the chance came, I bought my airplane sight unseen and 2000 miles across the country.

My Waco 10, also known as the GXE after Waco began 3 letter designations in about 1929, was owned and barnstormed by John Danicourt of Breckinridge, Minnesota from 1929 through the summer of 1941. He logged 1350 hours on the OX-5 engine it left the factory with, and he kept meticulous logbooks of every preflight inspection, every valve adjustment, every changed spark plug. These entries are fascinating to read. In the days before electrical systems and hour meters each flight was entered into the log to keep track of the time run on the ground, the time in the air, and the total time. There are many dates showing four, five, six hours flown. And 30, 50, 60 passengers carried. On the 4th of July, 1935 he flew 110 passengers. Two weeks later on July 21st he logged seven hours and carried 135 passengers. Anyone who has flown rides, 2 people at a time, knows that is a lot of hard flying in one day. Those fading ink entries and hundreds more like them make a strong case for reliability. Twice in 12 years he broke a propeller and noted it in the logbooks. Once he bent tubing in the landing gear truss. But there is not a single entry mentioning a forced landing in all that time.

My Waco was saved in its original condition after WWII by Shelby Hagberg who stored scores of old airplanes on his Iowa farm and even opened a museum for a while. Many of the antiques now in the U.S. once belonged to Hagberg. Restoration was completed in 1976 by John Tiffany, also of Iowa. Since then, the airplane has led the pampered life of an admired antique and has flown another 70 hrs, also without failure. But eventually the airplane changed hands, was disassembled and stored for 9 years until the owner passed away.

The OX5 is a beautiful engine in its original form. Curtiss nickel plated all of the steel as well as the brass and copper parts of the engine. This included all the nuts, bolts and washers and even the outside of the cylinders. This was very advanced for its time, aircraft engines generally being merely painted. The aluminum case was left bare, giving the engine a shiny, silvery color. Nickel plating has a yellower tone than modern chrome plate, but doesn’t last as long and often has oxidized away in the ensuing 85 years. Most OX5 engines on display today suffer from a quick overall coating of silver paint that doesn’t do justice to the original lustrous plating and is not the way the engine was built.

My study of the engine started immediately after trucking the airplane home and finding that only 2 cylinders had anything resembling compression. The removal of all the cylinders, pistons and connecting rods was an exciting experience, for if you love old engines, you love digging into the heart of them and learning the interesting quirks of the design. The OX was designed at the very beginning of aviation, 1910. It evolved through several improvements resulting in more power. By 1918 the 90 horsepower, dash 5 version was installed in the Curtiss JN4D built under government contract, so many placed on order that the giant Willys Motor Corporation was called in to augment Curtiss’ own production.

The lower end of the engine, the crankshaft, rods, aluminum crankcase, timing gears and with an exception, the camshaft, all follow the sturdy construction techniques one would find in a modern aircraft engine. Even the sizes of all the parts seem typical with anything built today. Internal combustion engines have been refined in 100 years but the basic parts haven’t changed much. The layout, a 90 degree water cooled V-8, was common long after Curtiss pioneered it and stayed popular in cars for decades after. Water cooling for early aircraft engines was a good choice since airspeeds were slow while power output was high. The radiator kept temperatures cool and uniform within a variety of cowlings.

But there are oddities about the Curtiss design and they start with the cylinder. I will go out on a limb to say that it is the only cylinder ever built, for airplane or automobile, that was symmetrical. That is, it could be rotated 180 degrees and the valves, intake and exhaust ports, the cylinder hold down studs and the valve gear could all still be mounted and used without any changes. A single outlet on the monel water jacket is the only way to tell the proper orientation. And that outlet could easily be unsoldered and moved if necessary. Whether or not there was a reason for this symmetry is anyone’s guess. But it resulted in an interesting problem. The valves were now arranged in a perfectly straight line with the lifters. There was only room for one rocker arm to operate one valve. How then to operate the second one? The clever mechanics of the day found a way, almost unique in engine design. They would use a normal pushrod to open the exhaust valve, and have it run inside a tube connected to a yoke that would pull the intake valve open. That’s right, pull the valve down. It was a ‘Rube Goldberg’ solution, that any lesser designer would have ignored while it was still on paper. But it worked. The intake valve lifter and camshaft design for this system is unique and the parts must be studied carefully to understand its workings. The benefit was it gave the OX-5 overhead valves, which results in better breathing, combustion and power output. The disadvantage was that the long exhaust rocker arm pushed sideways on the valve, causing guide wear. Before long an aftermarket roller rocker arm was available from the Leslie C. Miller Company, Los Angeles. When installed, the engine became "Millerized." In his advertising Miller claimed 3500 hours flying behind the OX-5. His system also stopped intake valve spring fatigue and provided "one shot" lubrication of the bushings. But the roller rocker arm was the big advantage, one that most aircraft engines still do not have today.

There are other delightful peculiarities. Instead of a dipstick, there is a float connected to a pointer on the side of the crankcase. A single glance and you know how much oil you have. Three and a half to four gallons is recommended. To add oil you can choose either of two filler necks, which have screened covers and double as crankcase breathers. One is mounted at the front of the crankcase, the other at the back. Remember, this engine powered the famous Curtiss pusher and when mounted facing aft, the filler near the prop was handiest.

And there is the lubrication of the valves. The Curtiss manual states, "The valves should be lubricated before every important flight." I will leave the reader to interpret which flights today are important and which are not. But in an era of pilot training (1910-1918) where some student hops would only be a few feet off the ground and the length of a long field, this comment is probably not as naive as it sounds. On the Waco it requires a ladder, the removal of the top cowling and four kinds of lubricants. Grease for the rocker arm bushings, engine oil for the springs and pull down straps, Marvel Mystery oil for the valve guides, and spray lubricant for the Miller rollers. If you are quick, it takes 20 minutes. I do this before every flight, important or not, and the manual says it is good for "several hours." With almost 5 hours fuel on board, the oiling then becomes the limiting factor in range.

With new rings, a valve grind and a recharged magneto, my engine starts easily on the first pull and runs smoothly at all speeds. Compression with the 5" piston stroke is impressive and covers a large arc due to the huge wooden prop. You begin the pull with your hands all the way above your head and end with them below your knees. There is a quiet "chuff," like a sneeze and she is ticking away at idle. Once you are used to hand propping the OX, small Continental engines seem like model airplane 2-strokes. Idle is set to only 450 rpm, and with the inertia of the long prop and 8 firing impulses it has no tendency to die. Even at this slow idle, brakes are required to keep 2000 lbs. of airplane and passengers from rolling on pavement. The 102" propeller provides enormous thrust. After starting, a cockpit handle controls the four radiator shutters for faster warm-up. The water is warmed to 120 degrees before increasing the throttle, and indicates 140 to 160 degrees in flight, depending on the outside air temperature.

The rated rpm is 1400 and I cruise the engine at a quiet 1350. It was called 90 horsepower in 1918 but bears no relationship to 90 horsepower today. The equation for horsepower is a multiple of rpm. The higher the rpm number, the higher the horsepower rating. But propellers are more efficient the slower they turn. So for a big, high drag airframe a long prop turning slowly is perfect. In the case of the OX-5, the horsepower rating is meaningless except to compare it to other slow speed engines of its time. It has 500 cubic inches, almost the same as a modern six cylinder Lycoming 0-540 or Continental 0-470 of 240 hp. The OX-5 is a powerful engine.

The takeoff and climb in the Waco is strong and steady and the angle is steep enough that if the engine failed on takeoff over the far end of the runway, you would have to lose some altitude before you turned back or you might overshoot. It takes a lot of modern horsepower to match that kind of performance. Airborne, it fairly purrs along, transmitting that undefinable feeling of reliability that an experienced pilot recognizes separates a good engine from a questionable one.

So why all the horror stories of failures and letdowns? That story might well be entwined with the history of the Great Depression, a time when no money was available to perform the simple maintenance we now consider mandatory ; when most people bought used oil because they couldn’t afford new, and changed it rarely. Alcohol, which soon boiled away, replaced real antifreeze, and few private owners had money to buy spare parts. The 64 water connections occasionally required new hoses and clamps but who had the money to buy them? The crudest, cheapest car gas was poured into fuel tanks from old rusty cans, and the devil-may-care attitude of the "Roaring Twenties" was still fresh in the minds of pilots. Even the kind of basic preflight procedures that we consider common today ; draining the gascolator, checking the air filter, performing a carburetor heat check, were unheard of when the OX-5 was common. None of the major manufacturers provided a gascolator to catch water or dirt in the fuel. There were no quick drains on fuel sumps. There were no air filters for carburetors . And there was no carburetor heat, due to poor understanding of carburetor icing. Wouldn’t any engine subjected to the neglect of that era have failed? Still, the OX-5 soldiered on until WWII, 30 years after it was designed, not because it was a poor engine, but because it was a good one.

After WWII many old biplanes were bought up by crop dusters who quickly cut the old engines off and installed cheap military surplus radials. (Ironically the same reason the OX-5 was used in the first place) These airplanes lasted long enough to be around when the antique restoration movement was started by Robert Taylor and the Antique Airplane Association in the early 1950's. The restorations tended to keep their much newer surplus radials, on the basis of field approvals given by an understanding CAA and later FAA. Those inspectors grew up with the airplanes, knew them intimately and were comfortable with the modifications of changing engines. This system relegated most OX-5's to static museum displays. As the "old timers" in the FAA retired and the specter of liability loomed in the 1970's these approvals came less frequently and now are almost non-existent.

Sadly there are only a handful of original OX-5 powered airplanes left flying today. The engine that trained thousands of American and British pilots in WW1 and powered commercial aviation in the 1920's has been largely abandoned in the modern quest for more performance and speed. But there are a few die-hards who have found the OX-5 to be a satisfying engine, who find it reliable when overhauled and inspected regularly. A few still make cross country flights of hundreds of miles. In this current age where every old biplane has another familiar radial engine, I am thankful to be flying the fascinating and authentic Curtiss OX-5.

Chad Wille
1139 Hiway 148
Corning, IA. 50841
(641) 322-4041
Have questions about your engine? Contact Chad Wille @ St. Croix  (641) 322-4041