[Ill.u.s.tration: Figure 34.--Modified pistons after endurance run. U.S.

Navy test, 1931. (Smithsonian photo A48325D.)]

Combustion Chamber: In 1931 the contour of the cylinder head was changed slightly. This improved the combustion efficiency to the extent that the stroke of the fuel pumps could be decreased about 15 percent. The specific fuel consumption then decreased about 10 percent. In addition the compression ratio was reduced from 16:1 to 14:1.[20]

These changes were designed to eliminate smoke from the exhaust at cruising speed, and to reduce it at wide-open throttle.

Valves: A two-valve-per-cylinder model was built, but not put into production. It featured more horsepower (300), a higher rate of revolutions per minute (2000), and a better specific fuel consumption (about .35 lb/hp/hr).[21]

Capt. Woolson designed the production model with a single large valve for each cylinder. This was done in order to shorten the development period, for it is easier to design a single valve which serves both the intake and exhaust functions than one valve for each function. Not only are there fewer parts, but more important, there are no heat-dissipating problems. Although the single valve is heated when it releases the exhaust gases, it is immediately cooled by the incoming air of the next cycle. This cooling advantage is not shared by a valve which only pa.s.ses exhaust gases.[22]

Cylinder Head: Ribs were added to increase its rigidity (compare fig. 32 with fig. 33).

Engine Size: A 400-hp model was developed in 1930. It was not put into production.[23]

Comments

Comments of Aeronautical Engineers: These comments appeared in _Aviation_ for February 15, 1930, just a month before the Packard diesel received its approved-type certificate. They were in answer to the question, "What is your opinion of the probable early future of the compression ignition type of engine in aircraft powerplants?" Most of the engineers were enthusiastic about the diesel engine's future in aviation; however, neither George J. Mead nor C. Fayette Taylor shared their colleagues' opinions. Mead's prophesy was accurate except for his discounting the diesel's role in lighter-than-air craft. Taylor was correct in implying that there was a future for the diesel in powering airships.

George J. Mead (vice president and technical director, Pratt & Whitney Aircraft Company):

Compared with the present Otto cycle engine, the Diesel powerplant weight, including fuel for a long-distance flight, would apparently be less. It is doubtful whether there would be any saving if the orthodox engine were operated on a more suitable fuel. Inherently the Diesel engine must stand higher pressures and therefore is heavier per horsepower. A partial solution of this difficulty is the two-cycle operation, which seems almost a requirement if the Diesel cycle is to be considered at all for aircraft. For any normal commercial operation in the United States there seems to be little or no improvement to be had from the Diesel. After all, it is not entirely a question of fuel cost but payloads carried for a given horsepower. It seemed at one time as though the Diesel was particularly desirable for Zeppelin work. Now that blau gas has been introduced, which obviates the need of valving precious lifting gas, the Diesel cycle seems much less interesting for this purpose. There may be a reduction in fire hazard and radio interference with the Diesel cycle, but it is doubtful whether it will be used in view of these considerations alone.

C. Fayette Taylor (professor of aeronautical engineering, Ma.s.sachusetts Inst.i.tute of Technology): "I believe that the compression ignition engine will continue to remain in the experimental stage during the year 1930. I should expect its first really practical installation to be in lighter-than-air craft."

P. B. Taylor (acting chief engineer, Wright Aeronautical Corporation): "I believe the compression ignition engine is probably the type which will eventually supersede the present electric ignition units. This development will come slowly and will not be a solid injection engine."

Henry M. Mullinnix (former chief of powerplant section, Navy Bureau of Aeronautics):

The advantages of compression-ignition, including reduced fire hazard, more efficient cycle, elimination of electrical apparatus and hence of radio interference, elimination of carburetion problems, and other benefits less evident, would seem to outweigh the difficulties encountered in metering and injecting minute quant.i.ties of fuel at the proper instant. Although the Diesel engine suffers upon comparison with the Otto cycle engine in flexibility there seems to be a definite field for employment of Diesels and a gradual extension of their use may be predicted.

John H. Geisse (chief engineer, Comet Engine Corporation): "I am firmly convinced that the Diesel engine in the future will not only maintain the advantages of Diesel engines as they are now known, but will also be lighter in pounds per horsepower than the present Otto engines."

Lt. Cdr. C. G. McCord (U.S. Navy, Naval Aircraft Factory): "The use of compression ignition in due time appears to be a.s.sured; but increase in weights above those of present Otto cycle engines, to insure reliability, must be expected."

L. M. Woolson (aeronautical engineer, Packard Motor Car Company): "There is no question that the compression ignition aircraft engine will in time offer severe compet.i.tion to the gasoline engine. There are, however, many basic problems to be solved for the solution of which there exists no precedent."

N. N. Tilley (chief engineer, Kinner Airplane and Motor Corp.):

Considerable development of the compression ignition type of engine for aircraft will be required before it is commonly available. It is believed that the weight per horsepower must be equal to, or less than, that of the present type of engines, in order to interest the public, since rapid take-off, rate of climb, and speed are desired, rather than low fuel consumption or high mileage. Most flights are of few hours duration. It is believed that flights must be of over five or six hours duration in order to show any advantage of Diesel engines (with low fuel consumption) if appreciably heavier than present engines. Also the difference between Otto cycle and Diesel becomes slight as the compression ratios come closer together.

Comments of Flight Crews: The preceding comments were made by engineers thinking primarily of the commercial possibilities of the diesel.

Following are comments by flight crewmembers about the operating characteristics of the Packard diesel. The former were largely optimistic. Most of them were only familiar with the aeronautical diesel as a design project and therefore did not have the practical experience necessary to understand all of its limitations. The latter were pessimistic, as they knew firsthand various shortcomings of the engine which only became apparent when it was operated.

Clarence D. Chamberlin, pioneer pilot:

My only experience with the Packard diesel was in a Lockheed "Vega"

which I owned back about 1932. The Wright J-5 had been replaced with the 225 hp Packard Diesel. My main complaint was the excessive fumes. When I would come home at night my wife would greet me with, "You have been flying that oil burner again." It was so bad that pa.s.sengers' clothing would smell like a smoky oil stove for hours after a flight.

Looking backward, it is my guess that the Diesel would have had only a limited period of acceptance even if all mistakes had been avoided. It is easier and cheaper to get performance with lighter and more powerful engines and longer runways than by refining the airplane. Fuel economy of an engine has ceased to be the deciding factor. Higher utilization of a high speed Jet at least in part offsets the inefficient use of fuel. The only time the Diesel had a chance was from the middle 20's perhaps on thru WW-2 for certain things due to gasoline shortage. To sum it up, the thing that licked them worst was the use of a single valve for inlet and exhaust making it impossible to collect and keep the fumes out of the fuselage.[24]

Ruth Nichols, prominent aviatrix:

I was flying Chamberlin's diesel-powered Lockheed, in which a month before I had made an official alt.i.tude record for both men and women in aircraft powered by an engine of that type. The record, I believe, still holds. It was a rugged, dependable plane whose experimental oil-burning engine nevertheless had a number of bugs.

For one thing, it was constantly blowing out glow-plugs used for warming the fuel mixture, and when that happened long white plumes of smoke would stream out, giving spectators the impression that the ship was on fire. For another, the vibration was so bad that out of 10 standard instruments on the plane, 7 were broken from the jarring before my return. The diesel fuel also produced a strong odor in the c.o.c.kpit, the fumes so permeating my luggage and clothes that my public appearances during the tour always were highly and not very agreeably aromatic. Having a strong stomach, I soon became accustomed to the fumes, but another pilot who ferried the plane between cities for me on one occasion ... was almost overcome. On arrival he said, "I wouldn't fly that oil burner another mile."[25]

[Ill.u.s.tration: Figure 35.--Ford 11-AT-1 Trimotor, 1930, with 3 Packard 225-hp DR-980 diesel engines, right side view of right engine nacelle.

(Smithsonian photo A48311.)]

Richard Totten,[26] airplane mechanic:

The Ford Trimotor was the poorest of the lot. It was inherently noisy and slow, and with the Packards installed it was on the point of being underpowered. It was almost impossible to synchronize the three engines, and the beat was almost unbearable. It was not flown much but it made a fine conversation piece standing on the airport ap.r.o.n....

The Waco taperwing developed the unnerving habit of breaking flying and landing wires from the vibration, and most of the time sat on the hangar floor with its wings drooping like a sick pigeon. In flight the open c.o.c.kpit filled with exhaust smoke and unburned fuel and the pilot would land after an hour's flight looking like an Indianapolis 500 Mile Race driver....

The Stinson "Detroiter," the Bellanca "Pacemaker" and the Buhl-Verville "Airsedan" were the most successful ships and were the most used. The "Airsedan," in which Woolson was killed, was his favorite ship, and the one I believe that was the most flown.

The Towle TA-3 amphibian flew beautifully, but not for long. It never got a chance to do much as it was a victim of the depression. The Towle was powered by 2 Packard diesels on loan from the Packard Motor Car Company. It was built of corrugated aluminum exactly like the Ford Trimotor. As a matter of fact, Towle had been employed by Ford until Ford cancelled airplane building. Towle got his airplane built at the hangar on Grosse Isle in Detroit, and ran out of money during the flight testing program. He now looked for money to continue with and found a backer in the person of one Doctor Adams, a widely advertised "Painless Dentist" of Detroit. Adams wanted a quicker return on his money than the average backer and he insisted that Towle put the airplane in service so it could start earning some money. At this time the amphibian was beginning to become popular for intercity flying, especially around the Great Lakes region as all of the major cities were located on the waterfront. What was more natural than an airline flying pa.s.sengers right into the downtown area of a city? Thompson was doing it between Detroit and Cleveland, Marquette was doing it between Detroit and Milwaukee, so Adams applied for permission to operate an airplane between Detroit and Cleveland and other cities on the lakes. In those days it was necessary to prove an airplane's reliability by flying a certain number of trips over the proposed route with a simulated payload. This payload was supposed to consist of sand bags, but usually consisted of any mechanic or pilot who happened to be loose at the moment, and who had nerve enough to go along. Mechanics were easier to load and unload than sand bags.

The Towle was in the middle of the qualification flights, and the publicity began to appear about the new airline. Much newsprint was devoted to the fact that the Towle was powered by the new Packard diesel engine, and this, of course, made it the only safe airline since all its compet.i.tors were using the old-fashioned dangerous gasoline. On the last payload trip of the Towle the pilot asked me if I wanted to go along, and of course I was delighted. I neglected to mention that I had been hired by the Adams airline as a mechanic because of my experience in repairing the corrugated skin of the Ford Trimotor owned by my employer, the Knowles Flying Service. The mere fact that I did many repairs to the airframe did not preclude me from getting my share of the engine work too, and since I was already familiar with the Packard diesel, I was quickly hired by Dr. Adams.

The last flight was indeed the last flight. We took off from the Detroit City Airport and when we crossed the Detroit river the pilot decided to land at the Solvay Coal Company docks and fuel up for the opening of the airline the next day. The Solvay Coal Company was the only place in Detroit where diesel fuel was obtainable at the time and all of the diesel powered yachts got fuel there. The pilot was not too experienced in the operation of amphibians, and he put the wheels down as we approached the river. When we hit the water the airplane went over on its back and sunk to the bottom. It came up to the surface again, and we all climbed out onto the keel, and waited for rescue. A police boat came over and took us to the dock. The police sent us to the hospital and then went back and towed the airplane over to the shipyard next door to Solvay. While we were at the hospital, the crane man hooked onto the Towle and lifted it out of the water and gently set it down on the dock.

He was only trying to help, but he inadvertently set it down on its back instead of its wheels. That was the end of the Adams airline. The Packard Company took back their engines. I helped remove them the next day. We dismantled the airplane and trucked it back to the airport where it sat in a state of neglect for some time. The pilot was fired, I lost my job, and Towle lost his airplane.

a.n.a.lysis

Advantages

A Packard diesel advertis.e.m.e.nt which appeared in _Aero Digest_ for June 1930 stated that this engine had three major advantages over its gasoline rivals: Greater reliability because of extreme simplicity of design; greater economy because of lower fuel cost plus lower fuel consumption, permitting greater payloads with longer range of flight; and greater safety because of removal of the fire hazard through the use of fire-safe fuel and absence of electrical ignition equipment.

These were the engine's princ.i.p.al advantages. Others are a.n.a.lyzed here by the author in order of their importance. At low alt.i.tudes the diesel uses an excess of air to eliminate a smoking exhaust; consequently at high alt.i.tudes, where the air is less dense, the diesel is still able to maintain much of its power. In contrast, the carburetored gasoline engine is sensitive to the fuel-air ratio and thus has no surplus air available at higher alt.i.tudes. A malfunctioning carburetor could cause a gasoline engine to cease operating, but an inoperative fuel injector would cause the Packard diesel to lose one ninth of its power, since each cylinder had its own independently operating injector. In practice, however, because of the excessive vibration, the engine was generally shut off immediately after a cylinder cut out.[27] Shielding was unnecessary because the diesel had no electrical ignition system.

Carburetor icing was an impossibility because there was no carburetor.

Any excess lubricating oil in a diesel engine's cylinder is consumed cleanly to produce power. By contrast, such oil in a gasoline engine's cylinder is only partly burned. As a result carbon deposits form that eventually cause malfunctioning of the spark plugs, valves, and combustion chambers. This advantage accrued to the diesel because it utilized an excess of air, and in addition its cylinder walls were hotter. The engine was very clean-running from the standpoint of oil leakage. This was a safety factor since it eliminated the possibility of a fire starting on the outside surfaces of the engine, and in addition it saved the time and money that was normally spent cleaning engines.[28] Since the diesel utilized its heat of combustion more efficiently than the gasoline engine, its cooling fin area could be reduced by 35 percent. This permitted better streamlining. Having less cooling fin area, it warmed up more rapidly than a gasoline engine.

[Ill.u.s.tration: The PACKARD-DIESEL AIRCRAFT ENGINE