Unusual Attitude

Considerations when installing an alternative engine.


Let’s say you’re looking for an engine for your homebuilt. You are set on something new and different, and you’re willing to pioneer the installation. Good for you!

Back in the May issue, when we covered alternative engines, I warned that it’s imperative to research the engine manufacturer. Far too many people have been stung and received nothing in return for their deposit except broken promises. But let’s assume we’re past that point, the manufacturer is trustworthy, we have accurate information on weight and power specifications, and the engine seems a good match for the airframe. Now let’s consider the mechanical issues of the physical installation.


How receptive is the airframe designer to making changes to the installation? OK, this isn’t a mechanical consideration, but it’s important. Some plans providers or kit manufacturers will essentially disown you for straying from the engine they support. Still, there are many who will work with you to see that you’re successful. Choose carefully.


Is the weight a good match for the airframe? If the weight of the proposed engine is different from the one specified for your project, all is not lost. There will be some wiggle room as long as you don’t exceed the designer’s maximum structural limit for the firewall-forward installation. For example, if the specifications state that the maximum limit is 200 pounds and the recommended engine is 165 pounds, there should be no reason you can’t safely install a 200-pound FWF package, but you will need to consider how you’ll keep the empty center of gravity (CG) where it should be. If your FWF package weighs more than what’s installed in the prototype, you may have to add ballast aft of the CG or move something that’s normally located in front of the CG (such as a battery or radiator) to behind the CG.

With the cowl off, you can see that the lightweight 2200cc Jabiru had to be pushed rather far forward for CG concerns.

Can an engine be too light? Although not usually a problem, in some instances the alternative engine may be lighter than the one the plane was designed to fly with. Again, CG is important. The easiest solution is to add weight in front of the CG. But if you are going through the time and expense of installing an alternative engine that has the benefit of being lighter than what’s specified, should you throw that away by adding weight? Consider moving the engine forward. This will mean a new or modified cowl, and the appearance will be changed. Like everything else in aviation, it’s a compromise.

Thrust Line

Is your proposed engine shape similar to the prototype’s? Can it be easily installed with the thrust line in the same location? Most auto conversions use a prop-speed reduction unit (PSRU) that can affect the location of the prop’s centerline as related to the engine’s crankshaft. A horizontally opposed engine such as a VW, Corvair or Subaru will have the crank located in the same general location as a certified engine. Although the VW and Corvair don’t normally use a PSRU, the Subaru usually does. While belt drives will offset the crank centerline (typically above the crank), many geared drives are available with zero offset. And many of the geared redrives that have offset allow for installing the gearbox in an up or down position.

Thrust line variation: Can it be moved? Like the FWF weight, many designs have some wiggle room for deviation. It’s possible that your chosen airframe can handle the thrust line being moved a bit without negatively affecting the pitching moment caused by throttle changes. One way to take the guesswork out of the what-ifs is to model it in X-Plane, a dynamic, affordable and accurate computer flight simulator that allows you to design virtually any shape and size of aircraft and test-fly it (www.x-plane.com). There are physical concerns with moving the thrust line as well (particularly when moved down), and a couple of these are prop clearance from the ground or the front tire, and the overall aesthetics.

Engine Mount

It’s all about the triangles. It’s not likely the engine you’ve selected will have mounting points that match the mount supplied with your kit or outlined in the plans. So before you order your engine, talk with the supplier. Many engine companies have scaled, measured drawings referencing mounting points and prop station in three axes. If you plan to build the mount yourself, or at least make the parts and have an expert weld them, you will need to either mock the engine in front of the firewall on your actual airframe or create a stand-in firewall mockup of the airframe. Then it’s just a matter of connecting points. Take cues from the specified mount design; you want your bracing members to be in tension, not compression, if possible. Mind the prop rotation, and properly brace against torque. Consider thrust-line offset as related to the direction the engine operates. Your new engine may spin the prop opposite of the prototype, and where down and left are specified, you might have to cant it to the right. Pay attention to the nosegear strut and maingear attachments. Many engine mounts are complicated with landing-gear attachments that could become an issue. Any changes to the geometry to accommodate the engine may compromise the strength of the gear, or in the case of a nosegear strut that’s not part of the engine mount, it may simply be in the way of the engine mount tubes or an engine accessory or component.

Can the engine manufacturer help? Many engine manufacturers will assist with the design and fabrication of an engine mount, especially for popular aircraft designs. Even though this has the potential to be a symbiotic relationship, don’t expect any favors or preferential treatment. There are countless projects in the homebuilt community that got off to a good start and have yet to fly. An engine supplier can’t bank on your enthusiasm while waiting for you to finish, so be prepared to pay a reasonable fee for your engine mount.

This all-aluminum, direct-drive, small-block Ford stuffed into the rear of a Long-EZ is a one-off design built by Gary and Char Spencer.


Now the real fun begins. How does your proposed engine get cooled? If it’s air-cooled, you are 90% there (with 90% still to go). Considerations are updraft or downdraft, inlet size, shape and spacing (same for the outlet), baffles (size, shape, construction materials, pressure plenum, seals, cylinder movement, cable and wire routing) and delta P, ensuring a good pressure drop from the inlet to the outlet across the engine. If the engine is water-cooled, you have several issues, the most obvious being where to put the radiator.

So I’m a plumber now? Once you’ve considered the possible locations for the radiator, and how to pass air through it efficiently, the next consideration is routing water to and from it. But before that you really have to consider CG again, as the radiator full of coolant will have a significant weight contribution. The farther away from the water pump the radiator is located, the more issues can arise. If the radiator is inside the cowl, traditional-length radiator hoses might be used. The longer the run, however, the heavier the installation becomes, and that’s why many builders have opted to use aluminum tubing wherever possible. The lightness of the aluminum and its ability to wick away heat is a benefit, but mounting points, bends and transitions (and attachments) to flexible lines can become failure points and maintenance issues.

Oil Cooling: Another radiator? While not all alternative engines have remote oil coolers, a lot of builders add them. Like the radiator, it’s all about location. To work efficiently, the oil cooler needs a supply of cool air and a way to move it through freely. This adds another layer of complexity and additional points of failure, but an oil cooler often makes the difference in keeping operating temperatures in check. Water-to-oil heat exchangers have been used to help control the oil temps in applications where ducting cooling air to an oil cooler isn’t practical.

Exhausting Stuff

Removing the spent air/fuel mixture. The exhaust system can be nearly as difficult to create as the cooling system. There are many theories about how to manage exhaust, but most agree that straight exhaust, following the shortest path to the slipstream, is the most efficient and cost effective. Talk with the engine supplier, as there may be some easily overlooked subtleties. The Viking 110, for example, has a single opening in the bottom of the engine onto which the exhaust system attaches. The exhaust runners are cast into the head itself. Will it be in the way of your nosegear strut or bracing? The rotary engine has some of the hottest and loudest exhaust of any powerplant and may damage some exhaust systems. Turbocharged engines can be the easiest to provide an exhaust system for and usually don’t need a muffler, but the heat of the turbo itself must be managed, and getting cool air through the intercooler can be an issue.


Keeping it all under wraps. A properly fitting and functioning cowl is essential. It protects the engine from the elements, directs the airflow around and through the engine compartment and its accessories, and provides for induction air (allowing for ram air and/or alternate air and carb heat) and an exhaust system outlet. It blends the spinner to the airframe, and it must be structurally robust yet lightweight, resist harsh chemicals and heat, offer easy access to preflight items-and look good. Plus, it has to do all this without adding drag or other airflow-related anomalies.


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