Dead on Arrival:
Porsche PFM

by Alfred Scott

This article appeared in the March 1988 issue of the Falco Builder Letter.

Now that there is a Turbo 260, are you going to offer a PFM Falco option? The speed advantage should be considerable. And, think about the sex appeal of "one of them red eye-talian jobs" with a Porsche engine up front. It should be fairly simple: add a 20 gallon tank in the tail to take care of weight and balance, stretch the cowling by two cylinder widths, modify the center console to eliminate the mixture and prop levers, run a flutter test at 300 knots and we should be home free. Assuming 12 gph and a 250 knot cruise, that would yield a range of about 1250 nm. If you can have the design work done by this summer, I'll wait and build the first one.

Howard Batt
Clearwater, Florida

Where is Don Meredith when you need him? Aw, HOW-ward!

We've all heard the old complaints that our aviation engines lack the sophistication and technology of today's automotive engines. Electronic ignition would be lighter and better than our old designed-for-Case-tractors magnetos. Electronic fuel injection would give us better fuel economy. Better cylinder head design would give us better efficiency and more power. If we could use stock, off-the-shelf, automotive parts, our engines would be cheaper. What if one of the exotic car manufacturers-maybe Porsche!-would build an aircraft engine. Just imagine what kind of engine we would have!

Now we know. And it's a disaster.

Because there is a direct relationship between the number of moving parts and reliability, the first cardinal rule of design is simplicity. Antoine de Saint Exupéry said it so well. "Perfection is finally attained not when there is no longer anything to add, but when there is no longer anything to take away." On this count, the Porsche aircraft engine is a stunning failure. The engine looks like it was designed by NASA. Wherever possible, the elegantly simple design was discarded in favor of adding features and 'engineering solutions' to problems that should have been avoided altogether. It appears that Porsche simply threw engineers at the project the way our government throws money at social ills.

The engine is geared, thus it has a propeller transmission while our Lycoming has none. There is a torsional damper and a cooling fan, while we have none. There are two cam shafts, while we only have one. There are two alternators, and the engine requires two complete electrical systems, including two batteries. There are six cylinders, and everyone knows you can produce a reliable 200 hp aero engine with 4 cylinders. The extra cylinders bring with them a full complement of additional components-valves, guides, pistons, piston pins, spark plugs, crankshaft bearings-and a less rigid crankshaft. The number of moving parts in the engine is difficult to comprehend. Maybe it's because Porsche already had a lot of parts.

The second cardinal rule of aircraft design is light weight. Again, the Porsche is a dismal failure. The engine is very heavy, but the engine weight alone is not the true measure. It's the weight of the entire installation that counts. In the case of the Mooney Porsche, the airplane is about two hundred pounds heavier than the Mooney 201. It would have been more, but Mooney removed the inner wheel well doors and used an exotic $14,000 Hartzell Kevlar prop to keep the weight down. Without such extreme measures, the weight increase would probably have been close to 250 lbs. (For comparision, our IO-320-B1A weighs 285 lbs.)

It is very important that an aircraft engine be compact and have a low frontal area. The Porsche fails again. Although it's a small displacement engine, the Porsche PFM is substantially bigger than the 200 hp Lycoming. Porsche seems to have no comprehension of this requirement. An induction system looms over the top, the gear transmission takes up a large amount of space in the front, overhead cams widen the engine beyond everyone's firewall width, and the exhaust system is right where the nose gear goes on most airplanes.

To be a success, an engine must be very reliable. At this point, it's too early to call. Porsche and Mooney speak very highly of the engine, but it gets 217 hp out of 193 cubic inches of displacement. That is the equivalent of getting 530 hp out of our 320-cubic-inch Lycoming. This power is attained through a higher compression ratio and higher engine speeds. History shows that geared engines, high compression ratios, high rpms, and complex mechanical design are not the hallmarks of 'bulletproof' engines.

The one absolutely horrible design feature is the fan cooling system. While it only uses a couple of horsepower to drive the fan, it totally destroys the exit velocity of the air. In normal installations, the engine acts as a radiator and the heat expansion of the air drives it out of the engine compartment. This thrust keeps the cooling drag to a minimum. One engineer estimates that the Porsche Mooney is losing 12-15 knots from the fan cooling.

Porsche claims that the engine is efficient, but this is statistical mumbo-jumbo. At best economy, Mooney's Lycoming and Continental engines are more efficient, but brake specific fuel consumption does not account for the extra drag of the cooling system and the bulbous cowling. These steal power from the installation. The faster the airplane, the greater the net power loss. The extra weight steals from the payload. What counts is moving payload efficiently. Watch the CAFE 400 for a true measure of the engine's efficiency.

It is now well understood that listening to customers and understanding their needs, desires and problems is of critical importance to the success of any commercial venture. In a Kendo blow of incompetence, arrogance and stupidity, Porsche did not talk to the major light aircraft companies before designing the engine. It's often been said that Porsche has forgotten more about engine design than aviation engine manufacturers ever knew. Porsche mistook a compliment for the truth and never bothered to find out the few things that Lycoming and Continental did know.

Had they asked, they would never have designed the silly three-point engine mounting system with one of the pickup points on top of the propeller gearbox. This makes for an awkward and complex engine mount, which on most singles also picks up the nose gear. It is difficult enough to design a tubular steel structure which will reach over the engine, but you must also provide some means of removing the engine. In the Mooney installation, the engine is trapped between the nose gear and the complex tubular structure which reaches out over the engine. You tilt the engine nose down and then take it straight forward. You can't use a block-and-tackle hoist, but Porsche has a special gizmo designed for that purpose.

Had they asked, Porsche would also have discovered the addiction to four engine mounting points-one more than you need to fly. If you lose one of the Porsche mounts, the engine is going to go bye-bye. (I have a friend who actually broke two of the four mounts on his Baron. On that airplane, the mounts are all on the bottom, so the engine continued to sit on the totally-cracked-through castings.)

The hope of a cheap engine made from stock automotive parts is dashed too. My engine friends have always said that by the time you do all of the things to an automotive engine to make it a good aircraft engine, you have redesigned the entire engine. Porsche apparently found the same thing. The engine has virtually no parts which are interchangeable with the automotive engine on which the design is based.

The engine has--techno-weenies squeal with delight!--electronic ignition. That's now a standard feature on all automobiles. By using solid state devices, all of the moving parts can be replaced with electronic devices. Once you have little silicon chips powering things, it's a natural to put the timing under the control of a microprocessor. These devices are remarkably small, and you often see photos of Oshkosh fruitcakes holding one of the tiny boxes next to a pair of magnetos while they decry the lack of progress in aviation.

Gosh, no one will deny that solid state ignition is a better system, but no one has yet offered such a system as a reliable stand-alone package like a magneto. They all depend on the aircraft electrical system. In the case of the Porsche, dual electronic ignition requires two complete electrical systems. The extra alternator, battery, associated wires, switches, connectors and circuit breakers weigh much more than a pair of magnetos.

Magnetos may be old fashioned, but they are reliable devices. While the electronic ignition would be more reliable, the electrical systems will introduce a host of new failure modes for the ignition system. A better solution is the electronic magneto, in which the breakers are replaced by solid state devices. These work well, and Bendix has been making them for oil pipeline pump motors for years. They aren't offered on aircraft because of the expense of certification and because the breakers in magnetos have been acceptably reliable.

The single power lever system is one of the more appealing features of the engine design. It greatly simplifies the operation of the engine since it changes the propeller speed with the throttle setting, and the mixture is set automatically. On the high-revving Porsche, the vibration level is very low, so this scheme works. You wouldn't want such as system on a Lycoming since you would prefer to set the prop for the lowest vibration level. Remember that the most efficient power setting for an engine depends on much more than the characteristics of the engine. The speed of the airplane, the drag characteristics, the propeller design are also part of the equation. If you want to cruise at maximum efficiency, you must be able to set the manifold pressure and propeller rpm yourself.

We seem to be suckers for "new technology". I have just returned from Disney World and Epcot Center, where there were many dilettante displays of new technologies which promise a "better life"-and yet a glance at the crowd would indicate that a more sensible diet and a bit of exercise would be of greater benefit.

I don't believe in sudden leaps in technology for the sake of technology. It just is not reasonable to adopt all of this new technology and expect it to work without trouble-after all, by redesigning virtually every part, Porsche has already stated that the automotive engine would not work. Take the Malibu engine as an example. It turns out that the problems that recently grounded the fleet were caused by a tiny change in the piston pin design and a change in thread lube on one step of the engine assembly.

The piston pins were of a new brass-plugged type whereas the old reliable pins had a solid aluminum center-who knows what the reason for the design change was, but this tiny improvement wasn't. For years Continental engines have been assembled with ordinary mineral oil as thread lube on the crankcase through-bolts, but they changed to castor oil because it doesn't show up in a black-light check for oil leakage. Unfortunately, the poor extreme-pressure lubrication of this oil caused the bolts to reach the final torque before the proper bearing pinch was attained. So an entire fleet of airplanes was grounded because of a slight change in the design of the piston pin and an oil change in one tiny step in the engine assembly.

This is a good time to take quiet notice that in competition aerobatics-aviation's most brutal test of technology-the hottest airplane today is the German Walter Extra 230. Although titanium, carbon fiber, Kevlar, and all of the exotic materials have been used in other airplanes, today's best is a modified Stephen's Acro-fabric covered steel tubing for the fuselage and wire-braced tail group, a wood wing and a Lycoming up front. The 230 hp engine is simply a ported-and-polished 200 hp IO-360. The propeller is a lightweight wooden Muhlbauer constant speed prop-less efficient than our Hartzell but the light weight is easier on the crankshaft in violent aerobatics.

But the most important feature of the Extra 230 is the design of the wing. Construction of the wing is Polish pine-similar to spruce but denser and stronger-and covered with birch plywood. (The wing, but the way, is built by Sportavia at a price of $25,000 each). The symmetrical airfoil is a slightly squashed radius which extends back to about 10% of the chord and then extends in a perfectly straight line to the trailing edge, which is 15mm thick. The ailerons are about 60% of the wing span and in cross-section are essentially the same as the Falco's rudder, except that they are about 3mm higher than the surface of the wing, and the leading edge stands proud by about 10mm at full aileron deflection. The rate of roll is 320° per second. The airfoil was developed during WWII by the Luftwaffe in one of their zany experiments. It was never used on an airplane, since it's a lousy airfoil for anything but competition acrobatics and had escaped the notice of almost everyone.

The Malibu engine and the Extra 230 illustrate the more natural progression of progress-small incremental changes taken one at a time. If you want some technology, go buy a loran.

But what do you get with Porsche's Dilettante's Delight? Just take a cold, hard look at the Mooney Porsche. Even though the airplane has more power than the Mooney 201, it is five knots slower at altitude. The difference is apparently in the extra cooling drag of the fan-cooling system and bulbous cowling. And for this decrease in performance, you pay $100,000 more than a 201. (Mooney should have known better, but part of the high cost is attributable to the frightfully expensive engine instruments-for an engine that virtually renders instrumentation obsolete.) The plane is $12,000 more than the turbocharged, intercooled Mooney 252. This is progress? The plane simply demonstrates what wonderful aircraft the 201 and 252 are.

The Porsche engine, by the way, was originally rated at 200 hp, but the rated power was increased to 217 hp only after the dismal performance of the Mooney was evident. The essential problem with the Porsche engine, one expert told me, is that Porsche never understood the interface and interaction of engine, propeller and airframe. They thought delivering power to the propeller was enough.

The Porsche aero engine is the brainchild of Porsche chairman Peter Schutz, a dynamic man whose decision-making process might be described as "bold" or "reckless" depending on your point of view. Porsche has sunk a lot of money in this engine-I've heard estimates from 8 to 20 million-and it is now obvious that Porsche will never recoup its investment. As a result, Peter Schutz has recently been... well, the official version is that he resigned. My guess is that the project will be abandoned within the year. Oh well, Porsche can at least be grateful it didn't put its money on the Starship!

 Road & Track:
"The new chairman has a reputation as a no-nonsense finance man and may trim un-profitable projects, such as the 911-based aircraft engine.

"At this time, Porsche may do well to remember the words of the company's founder: 'Shoemaker, stick to your last.."

Does this mean that aviation has lost its one best hope for new engine technology? Will we forever be condemned to yesterday's technology? Of course not. It was a Camelot engine, yet another example of a large corporation exercising monumental stupidity. Porsche learns most of what it knows of engine design from the millions it spends on racing. Porsche's public relations people can easily paint the picture that the aero engine was just such an experience and the lessons learned will be valuable for the company. Everyone will believe it, and they'll all go home happy.

It is a beautiful, well-made engine that does have its place. It's the great engine for a blimp or a SeaBee amphibian. And Roy Lopresti could stick a Porsche and a nose wheel on a Swift and never have to move the main landing gear!

What could Porsche have done instead? For the Falco, they could start with our four-cylinder Lycoming and then simplify and add lightness. Replace the heavy Prestolite starter with a lighter B&K starter-or better yet combine the alternator and starter into one device like they do on jets. Install a couple of lightweight electronic magnetos. Make the prop governor an integral part of the accessory case. (Since the engine already has an oil pump, why do you have to pump it again to operate the prop?) Optimize the combustion chamber. Balance the engine. Flow-balance the induction. Redesign the engine mounting system so the Lord mounts fit into rings in the engine, not in the engine mount. Put the oil pressure port in a more convenient location.

Our new Porsche IO-320-B1A would then put out 185 hp and weigh 15 to 20 lbs less. The engine would cost $25,000 to $30,000 new and almost all of us would say it was too expensive and go back to buying overhauled Lycomings, but airframe manufacturers would buy them-if anyone was still buying their airplanes. Porsche could have done these things. Instead it produced a heavy, complex, bulky engine which does not produce the power or efficiency to justify breaking all the cardinal rules of aircraft design. It's a pity.

My views are obviously in the minority. Sherman McCoy has a Porsche Mooney and loves it. Teterboro-to-Southampton is an easy commute of 30 minutes.

From time to time, aviation goes a little nuts and everyone starts talking about some great new thing as the savior of aviation. This was the case in 1988 when everyone seemed to be enthralled with the Porsche engine. This article was the first one criticizing the engine. After this, Kas Thomas wrote a similar article in Light Plane Maintenance, and in time the engine's faults became widely known. As you may know, the engine was a failure and is no longer manufactured.