Experimenter is a magazine created by EAA for people who build airplanes. We will report on amateur-built aircraft as well as ultralights and other light aircraft.
Issue link: http://experimenter.epubxp.com/i/134623
Un d e r t h e C o w l Support How open-minded is the airframe designer to making changes? Okay, so this isn't a mechanical consideration, but it's a really important factor as some plans providers or kit manufacturers will essentially disown you for straying from their design by not installing one of the engines they support. While this is the case for a few experimental aircraft offerings, there are many other designers who will actually work with you to see that you're successful, so choose your airframe carefully. Weight Is the weight a good match for the airframe? If the weight of the proposed engine differs from the one specified for your project, all is not lost. There may be some wiggle room as long as you don't exceed the designer's maximum structural limit for the firewall forward (FWF) installation. For example, if the specifications state that the maximum limit is 200 pounds, and the engine they recommend is 165 pounds, there should be no reason that you can't safely install a 200-pound FWF package. But you will have 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 to behind the CG, such as a battery or radiator. And keep in mind that the farther away from the CG that you place the weight, the less of it you have to use. Can an engine be too light? Although that's not usually a problem, in some instances the alternative engine you are considering may actually be lighter than the one the plane was designed to fly with. Again, CG being a very important issue, something will need to be done. The easiest solution is to add weight in front of the CG. The Adam A-500 carried a brick of tungsten attached to its nose gear, and I've seen a Dragonfly with a bag of tire chains stored in front of the rudder pedals. But if we are going through the time and expense of installing an alternative engine that has an added benefit of being lighter than what's specified, should we throw that benefit away by adding weight? Maybe consider moving the engine forward. This will, of course, mean that a new or modified cowl will be in order, and the appearance will be changed; but like everything else in aviation, it's just another compromise. Thrust Line Is your proposed engine shaped similarly to the prototypes? Can it be easily installed with the thrust line in the same location? Most auto conversions use a propeller Many auto conversions, like this antique Ford engine, are not supported by a manufacturer and are, for the most part, one-of creations by the builder. 28 Vol.2 N o.6 / June 2013