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Aeromorph 75

The power behind David Roe’s “Diva”

By Patrick Panzera, EAA 555743, ppanzera@eaa.org

Hummel Bird
David Roe’s Hummel Bird parked on the ramp at COPPERSTATE 2003

In the February issue of Experimenter we reported on David Roe’s Hummel Bird and promised that we’d report on the highly modified Volkswagen engine powering it. So in keeping with our promise, we would like to present to you the one-of-a-kind Aeromorph 75 as built by renegade machinist Doug Reid. And before you think this is just another bolt-together VW engine article, consider that the work that went into narrowing this engine to fit the round cowl of David’s Hummel Bird produced a four-cylinder VW engine which is taller than it is wide. A destroked crank, custom-shortened connecting rods, and short-skirt BMW pistons are just a few of the components and custom machine work that went into this remarkable engine.

The Aeromorph 75 started out life as a pile of aftermarket and otherwise stock VW parts, a conglomeration of motorcycle parts, as well as custom one-of-a-kind handmade components. Photo courtesy of Doug Reid.

Hummel Bird
An uncowled view of David Roe’s Hummel Bird.
The engine is the Aeromorph 75, built by Doug Reid.

Doug Reid realized a long time ago what it would take to make a compact, low frontal area, internal combustion aircraft engine. He was building a stroked VW engine and faced a situation where, at bottom dead center, the piston skirt ran into the connecting rod bearing cap of the opposite rod. He felt that something had to change, that being the use of a real narrow, slipper-skirt type piston, sometimes known as a “cutaway.” So with that in mind, Doug found a source for stroker motorcycle pistons, with the wrist pin very close to the ring lands. He made enough modifications for clearance that he felt confident to start working on the shortest stroke crank Volkswagen ever made, and developing his own custom, very short connecting rods.

All this effort and engineering worked out to create an engine with a 1,475-cc displacement, using a 92-mm bore, which he estimates makes upwards of 75 hp at 4,500 to 4,700 rpm. Doug told us that piston speed is really slow, even though rpm is high. Although the engine-operating speed seems high for a direct-drive application, the small prop being used on the Hummel Bird does well at these speeds.

VW Engine
Just for scale, here’s a photo of a stock 1,835-cc VW engine conversion. Please note the length of the pushrod tubes and the depth of the valve cover. Contrast this with the photo on the right.

VW Engine
This photo was shot from a very similar angle as the photo to the left. You can clearly see that a good 6 to 8 inches (or more) have been removed from the distance between the crank centerline and the valve cover.

The Concept
The case is highly modified and reported to require a lot of machine work. Doug’s original intention was to come up with something for which he could sell plans to potential engine builders. As he went further into the research and development, he soon realized that the average experimenter probably didn’t have the equipment nor the skill to machine some of the more complicated operations. Although he feels that the average half VW flyer would joyfully leave the half Veedub for this engine, his idea of selling plans is just not plausible; there’s almost nothing left in the engine which isn’t highly modified.

Although Doug doesn’t know if he’ll ever build another of these engines, he’d like to find someone to work with who could possibly put this engine into production. He said, “I would be interested in working with someone who wants to pursue this, somebody who already owns (and knows how to operate) the equipment, is an airplane enthusiast, and could set up all the programs to numerically make the modifications.”

The heads are basically a stock Volkswagen item, cut up and seriously modified. The entire rocker section is shaved down. Doug doesn’t remember exactly how much he cut them down, but he claims to have cut off virtually all the rocker shaft pads. He also seriously shortened the valves. “I run really short motorcycle springs,” Doug said. “I machined the heads deeper for the spring cut and modified rockers like mad with non-stock adjusters (Elephant Feet). I just got every bit of space I could get in there to make it work.”

Steel shrink-fit hub and aluminum prop spacer
Shown above is the steel shrink-fit hub and aluminum prop spacer.

The Prop Hub
In a pretty traditional VW conversion method, Doug adapted a commercially available shrink-fit hub to his highly modified crank. “Yeah it’s a long overhang,” Doug said, “but you know there’s not much torque. I don’t like a long extension on the shrink-fits. This is the second one that I’ve gone through with this conversion. The first one I bought was from Great Plains [Aircraft Supply Co.], but I didn’t like the quality of the interference fit, so I sent it back and bought this one.”

The aluminum prop hub extension has a very distinct, almost airfoil shape to it. If you look at it in its current configuration, although it’s very attractive, there seems to be no rhyme nor reason to this shape. But in a previous version, Doug struggled for a long time with a cooling fan idea which he had hoped would seriously decrease cooling drag. The shape played a major part in that system.

Oil Filter
As I looked over the engine, I found a lot of very innovative pieces. Some are full-on custom pieces, some are commercially available, and some are a combination of each. I found Doug’s spin-on oil filter adapter quite appealing and could certainly tell it was a custom piece.

Steel shrink-fit hub and aluminum prop spacer
Doug’s oil filter as installed on the Aeromorph 75

When I asked Doug about it, he replied, “It’s just a standard automotive small filter, jammed between the distributor and the exhaust pipe, with just barely enough room to get it in and out. David can tell you I didn’t leave him any extra room.” (David is the owner of the plane, as well as the pilot.)

Oil filter
Dual coils joined via an MSD coil joiner; a single standard VW distributor modified to house two Compu-Fire pick-ups

The Ignition System
Although the heads have only one set of plugs, Doug considers his ignition system to be “99% dual ignition.” With his setup, Doug used two Compu-Fire VW points replacement modules and installed them inside the otherwise stock VW distributor. This arrangement is also possible with a pair of standard points, or even one set of points and one module, but Doug feels that his eyesight dictates that he no longer try to gap points. The coils are standard Japanese motorcycle epoxy-filled pieces, which receive their signal from the ignition modules.

The coils are joined by an MSD coil joiner, allowing the pilot to select between running one or the other or both. Some experts believe that the coil joiner shouldn’t be run in the “both” position. I asked Doug about his experience with this, to which he answered, “Yeah you can run them all the time. They make you think you can’t, but of course the diagram says you can.” David usually selects “both” for takeoff and for landing, but in cruise, he usually takes the secondary offline, using only the primary ignition. “That way if one does crap out, at least it’s isolated in that situation.”

“The system in general is actually reasonably simple,” Doug said. “It’s a bunch of work to weld up the stock distributor plate to drill and tap it for a second pickup or points. And you have to change the advance curve in the distributor because it has too much advance for hand propping.”

Doug believes that one should purchase the distributor with the proper advance curve and not fiddle with it like he did. “I think the distributor to stay away from is what’s normally used in a California smog legal engine,” he said. “I believe that the advance is set up for 30 degrees. You can’t hand prop at 30 degrees; it needs 8 or 10 degrees. Even at 10 degrees it can kick.”

Crankcase Ventilation
Crankcase pressure was a bit of a problem since the pistons get so close to one another at bottom dead center. With these modifications, they lost a lot of engine case volume, and David ended up having to build a breather box for it. They really didn’t want to install one, but they couldn’t keep the oil in the engine as the rpm came up. Doug believes that if one were to ever break a ring on an engine which is right on the edge of needing a breather (and one isn’t installed), the engine will lose oil pretty quickly. So although he hoped that they wouldn’t need a breather, he’s glad they have one. Doug is quite pleased with the system David built and has put a lot of hours in it.

The oil cooler isn’t located in its stock location, nor is it located in the position that standard VW conversions specify. It’s a stock Type IV Volkswagen cooler, but Doug made a bracket to locate where it is. The adapter is hidden under the shroud.

Con-Rod Ratio
“Many people are hung up on bore-stroke ratio,” Doug told us, “and often confuse it with the rod-stroke ratio. The bore-stroke ratio has very little meaning to the combustion process, so long as your rings can seal well, and unless the bore has gotten so large that the combustion chamber surface impacts the combustion process. This engine has a stroke-to-bore ratio of 0.6, or almost half. If possible, I would have made the bore even larger. The expansion of the combusting gases is virtually the same, relative to crank angle, regardless of the bore-stroke ratio. What does change the expansion characteristics is the ratio of rod length to stroke. With a long rod, high rpm doesn’t produce as much instantaneous piston acceleration (peak loads on the rod and bearings) as a short rod.”



These photos show the crank with the connecting rods and pistons in place. The photo on the left is of the assembly in approximately the top dead center position. The photo on the right would be bottom dead center. Note the handmade aluminum connecting rods and just how short the stroke of the crank worked out to be.

All three above photos courtesy of Doug Reid

“Many engine builders are convinced that a short rod provides more torque. This is true of instantaneous torque but not average torque.In ‘stroking’ an engine, you usually end up with shorter rods to keep the piston from popping out of the top of the block, so the rod-stroke ratio is sort of tied into the bore-stroke ratio hiding the separate effects. This engine has really short rods, shorter than I would have liked. However, this is driven by the available pistons, unless special pistons are made.”

The pistons Doug used are “hypereutectic” (high silicon content alloy), which have a reputation for possessing a very low expansion rate as compared to forged. They’re still a cast piston but are reported to be stronger than standard cast pistons. The low expansion rate permits tighter cold clearances and allows for a tighter compression ring gap. Doug did his own expansion test by carefully measuring these pistons and a set of forged pistons, first at room temperature and then after heating in an oven.

Electrical System
The alternator is a typical permanent magnet unit, which normally resides inside the classic VW conversion accessory case. It was purchased from Great Plains and has been adapted to the bare engine case, as Doug installed this engine without the use of such a case. So far, the alternator has performed flawlessly. “I used the Great Plains alternator because it’s inexpensive and lightweight.

Doug’s contribution to the otherwise stock charging system was to branch off immediately and go to two regulators. He’s also fused the system to guard against a dead short that would normally ruin the alternator.

One regulator feeds the primary buss. The other regulator feeds the “essential” buss, which runs the secondary fuel pump and the secondary coil. “So if the system goes to a dead short or open circuit on the battery side, all of which could happen with crappy wiring or bad connections, we’d still be running,” Doug said. The secondary fuel pump and coil are completely separate, so if the alternator dies, the plane can run for quite a while on battery power alone. If the battery side goes, it can run indefinitely directly off the alternator. “It’s totally obvious to me. I’m an electrical retard in terms of this stuff, and yet I don’t see anybody going beyond what we’ve done.”

Click for larger image.

Intake System
The manifold itself is simply a stock dual port VW manifold, which has been modified with the use of straight aluminum tubing of the appropriate wall thickness. As you can see in the photo above, each half of a stock dual port VW manifold does a lot of bending while transitioning from two runners to one. Doug simply cut the middle bends out of an otherwise stock casting and welded in straight sections to get the shape he needed. Since the right head is forward of the left head, the right manifold bends rearward slightly, and the left manifold bends forward slightly to where the projection meets in the middle at the shared air cleaner.

The seam you see along the otherwise stock section of cast manifold was necessary to reduce the diameter of the tube stock Doug chose to use. In reality, that entire section right up to the flange was replaced with straight tubing of a larger diameter.

The carbs are standard aftermarket Zenith side draft, float-bowl units, with mixture control. They’re sold primarily to motorcycle enthusiasts. The mixture control, which is an option, adds only $10 to $15 over the fixed mixture alternative. However, in Doug’s terminology, it’s a “silly idea” since the carbs can’t be jetted.

The venturi is fixed in these carburetors, not replaceable like other brands. This being the case, Doug bored the throat and sleeved it with a smaller venturi. He also installed a smaller float needle and seat, borrowed from a standard Dell’Orto side draft.

Since this carb in a stock application is designed for gravity feed (in the motorcycle), any kind of mechanical fuel pressure at all causes it to flood helplessly, even with a regulator installed. So the fix was to install a needle and seat designed to handle a few pounds of pressure. Doug tells us that the conversion is rather easy. The original brass seat is pressed in the carb body. But one can drill out the brass seat and thread the body to receive the Dell’Orto seat. “It took a little bending and fiddling with the float, setting its level by eye, to get everything working, but this is one of the few trouble-free fixes we’ve done.”

Doug was particularly concerned with the float; when the carb is installed in place, it’s tilted a little. This situation could be worsened as the plane is a tail dragger. So when it’s time to synchronize the carburetors, Doug makes sure the plane is sitting level. But all in all, his worries are for naught. The carburetors seem to work perfectly at any angle.

The carb of choice started out life with a 40-mm bore with a 38-mm venturi (roughly 1.5 inches). Bendix/Zenith also makes another larger version, which has a 42-mm bore and a 40-mm venturi. This carb is intended for the larger displacement motorcycles. Doug likes the bigger version for motorcycles.“I thought it was great when they came out with them on the Sportster,” he said, “except it didn’t have mixture control, but it did have an accelerator pump. They proved to be kinda big for the Harleys. They didn’t work well at lower engine speeds, but they worked well for those willing to give up bottom-end power in exchange for maximum high-end performance.”

Mixture control arms
The mixture control arms can be seen attached to the mixture control screws. Although the installation of these arms seems like an elegant way to add in-flight mixture capability, the actual flight experience is less than satisfying.

Doug opted to go with a carb which has mixture control, but getting it to be in-flight adjustable, to use Doug’s words, was a failure. “I tried to get it to work, but there’s not enough range.” With the levers Doug added (shown in the photo above), the throw of the arms is only 90 degrees, which works out to be a quarter turn of the needle with the 32 thread pitch. There just wasn’t enough movement of the needle. “I tried finding needles with a steeper taper at the bottom,” he said. “I wrecked a couple needles widening them out.” In Doug’s opinion, the only option is to go with a set of little sprockets and a urethane chain with a stainless steel cable. Hopefully the loads will be light enough that this system can work out. “If we can get a full turn on the needle with 1¼-inch sprockets, that should work.”

Fuel Pumps
The fuel system consists of dual electric fuel pumps, plumbed in parallel for redundancy. They’re a product marketed by CB Performance of Farmersville, California. The pumps are rated at 3.5 psi (pounds per square inch) and run on 12 volts. They’re of the “pusher” variety of inline rotary pumps and should be mounted as close to the fuel tank as possible. An internal pressure regulator supplies a constant 3.5 pounds per square inch of uninterrupted fuel pressure at up to 30 gallons per hour. The cost is approximately $65. They weigh less than a pound each, so running two of them isn’t a serious weight penalty.

Click for larger image.

The low pressure fuel pumps (mounted in parallel) can be seen nested against the firewall in the above photo.

Both pumps are fed via a single filtered line from the fuel tank, which is split in two by a Holley Performance Earl’s fuel distribution block, which has been cut down some in size and was once larger than shown in the photos. After the fuel is drawn through the pumps, it’s filtered again by a pair of Earl’s filters, before the parallel flows are joined together by another modified Earl’s fuel distribution block. From there, the rejoined fuel supply is routed to each carb, teeing off first at the closest one and then terminating at the one furthest down the line. Between the last fuel distribution block and before the first carb is a fuel temperature sender. In addition to fuel temperature and all the usual suspects, Doug and Dave have installed cylinder head temperature monitoring for all four heads, as well as exhaust gas temperature monitoring for all four exhaust ports.

So What Does It Weigh?
“The stupid dry weight is 120 pounds,” Doug told us. “That’s with no exhaust system, no propeller, no prop extension nor bolts, no fuel pump, and no aluminum shrouding, as well as no engine oil, but it does include the entire induction system, both the oil filter and the air filter, both ignition coils, as well as the full ignition system, including wires and the MSD coil joiner. I call it stupid because so many engine specifications show weights with a half-stripped engine. I guess that this full firewall-forward is about 160 pounds. David’s guess is a little higher – he says 165.”

At one point, the guys actually weighed the firewall-forward package, but that was back when they had the cooling fan in place. Several modifications have taken place since then, and there has never been a reweighing.

The Engine Mount
Doug is proud of David’s display of craftsmanship, as manifested in the work done to produce the baffling. He’s sure it doesn’t add much to the firewall-forward weight. The engine mount is another issue. Doug said, “The engine mount is a little overkill. I’m using industrial quality, electrical motor rubber vibration isolators.”

Engine mount
By creating their own hard points on the engine case, they eliminated the traditional “accessory case” used in most VW conversions. Starting with a clean slate like this and utilizing the stock VW transmission mounting holes, Doug was able to get creative and incorporate a rugged $6.50, off-the-shelf electric motor rubber “isolator” into the engine mount.

The Exhaust System
A fully independent, four-pipe tuned exhaust with an augmenting outlet was designed and built by Doug. “We battled with the sizes and shapes and all that, and we are still not done, but it’s getting closer,” he said. The headache of the exhaust augmentation being implemented is that it has a self-destructive nature. “We’re willing to live with some destruction but not constant.”

Augmenter system
Looking under the plane with the cowl off, you can see all four pipes entering the mixing section of the augmenter system.

Doug didn’t start out designing the cooling system to utilize exhaust augmentation. He designed the exhaust system around full-length tuned pipes which naturally fell into the cooling outlet. Experimentation with the system has shown that they’re not only benefiting from the tuned exhaust system, but that the augmented cooling air extraction is definitely working for them.

The exhaust system is an area on which Doug and David are still working. Of concern to them is eliminating the self-destruct mode of the system in general and insulating the cockpit from the heat radiating from the pipes. It can get quite warm with the engine running with no forward movement. The aluminum is very efficient at transferring heat. David tells us a little more about the exhaust system in the article we published on his plane in February 2010.

For more information on exhaust augmentation, we published a great article by Dan Nicoson in issue 66 of CONTACT! Magazine. Reprints of that issue are still available, as are all issues of CONTACT! Visit www.ContactMagazine.com for a full list.


The Propeller
Reading everything about propeller design that he could get his hands on and after making several other successful propellers, Doug decided to create what he considers to be the ultimate propeller for this engine and airframe combination. To him, this meant getting as much sweep back as he could, with the hopes of reducing the susceptibility to drag. So by using carbon fiber reinforcement extensively throughout the radically swept tips, Doug built the propeller seen above.

“I’m dying to know just how much I won or lost, but I’d have to build a straight propeller to compare or I won’t know,” Doug told us.

Although the propeller appears to be exclusively wood, the carbon fiber begins where the white paint starts, just out at the tips, the outer third. Doug believes that quartersawn oak is the best material for the base structure of his propeller, so that’s the wood he used. “It’s the stiffest thing I could get that was affordable enough to use.” Although maple is the wood of choice for many modern propeller makers, oak was used extensively in the earlier years of aviation and is still used by many prop makers today.

Tree species

Average specific gravity, oven dry sample

Static bending modulus of elasticity 

Impact bending, height of drop causing failure

Compression parallel to grain, max crushing strength

Compression perpendicular to grain, fiber stress at prop limit

Shear parallel to grain, max shear strength



106 psi





Southern Red







Maple, Sugar







Maple, Silver







Oak, Scarlet







The table above provides laboratory values for several properties of wood that are associated with wood strength. Note that due to inadequacies of samples, these values may not necessarily represent average characteristics.

When I asked what airfoil profile was used, I was told, “It’s a flat bottom propeller airfoil. It’s a ‘Doug Reid’ airfoil. When you get real thin, there really isn’t much airfoil there. I don’t hold back trying to make the leading edge fat, because David flies off the pavement. And even the first propeller I built for him, which never had much in the way of leading edge protection except a little fiberglass, he never picked up a chip or a ding in any way. He had 700 hours on that prop and it’s still good.” Doug reports that the specs on the propeller are 46.5-inch diameter with a 46-inch pitch.


This propeller offers the best of both worlds. Typically VW cranks, especially those with the standard shrink-fit prop hub attachment, can’t utilize composite propellers without suffering some form of crank failure. Doug’s prop, however, seems to have found a good balance with his use of wood inboard mated to carbon fiber outboard. It’s not only a good use of materials; it’s an economic way to get the benefits of a composite propeller at wood propeller prices.

Doug Reid

Usually when we do an interview or publish an article written by another, we like to print a brief biography at the beginning of the piece. Since Doug’s background isn’t as straightforward as most, I’ve decided this time to dedicate a little space at the end of this article.

Doug has a background in working on cars and motorcycles for a living. “I’ve flown a little but never had the money to be much of a pilot,” he said. “I learned to fly in high school, but I never did get a license.” Doug told us that he almost got a license about 25 years ago when a friend had a plane and flying was cheap.
Doug is self-taught in machine shop practices. He spent a little time working for an old-timer when he was a young man. “He was an old German machinist who learned all the ‘old school’ ways. He started working at the age of 9, making washers by drilling holes in pieces of scrap metal. It was certainly old school.” Doug told us that the machine shop he learned in was carpeted, and only classical music was ever played on the sound system.

As for Doug’s shop, “Yeah I have a bunch of old junk, but I’m very proud of it. My lathe is from the 1800s. The newest patent date is 1886. The other one is early 1880s.” When I facetiously asked him if it was still hooked up to a water wheel, he replied, “It had an overhead belt rig which I converted to be run by an electric motor. As I like to tell people, it was worn out by the end of World War I, but it doesn’t matter too much to me. However, it would certainly be easier on me to have newer equipment.”

Doug Reid

Doug standing proudly next to his Aeromorph 75, prior to installing it in David Roe’s beautiful Hummel Bird. Photo courtesy of David Roe.


I’ve seen his work, and I can tell you that it doesn’t take good equipment to make a great product. ~Pat


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