Experimenter

June 2013

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.

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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 Volkswagen, 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, but not always. While belt drives will offset the crank centerline (typically above the crank), many geared drives are available that have zero offset. And many of the geared redrives that have offset allow for installing the gearbox in an "up" or "down" position. Can the thrust line be moved? Like the FWF weight, many designs have some wiggle room for deviation. It's entirely possible that your chosen airframe can handle the thrust line being moved a little without negatively affecting the pitching moment caused by throttle changes. One way to take the guesswork out of the what-ifs of moving the thrust line 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, netting accurate results. Visit www.X-Plane.com. Above—Tis Dragonfy appears to have an extended nose but it's so well blended it's difcult to tell for sure. Below—With the cowl of, we can see that the very lightweight 2200-cc Jabiru had to be pushed rather far forward for CG concerns. Odds are good that your chosen airframe has already been modeled, so all you have to do is tweak it a little and check the results. But there are physical concerns with moving the thrust line as well, particularly when it is moved down, and that includes propeller clearance from the ground or the front tire, as well as the overall aesthetics. Consider the interference during a hard landing or a flat tire, or even while rolling over uneven ground. And of course, consider aesthetics. A spinner located too high or too low as related to the cowl can wreck the lines of an otherwise beautiful aircraft. Engine Mounts Engine mounts are all about triangles. It's not very likely the engine you've selected will have mounting points that will match the engine mount that came with your kit or is outlined in your 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-axis. If you're prepared to build your mount yourself, or at least make the parts and have an expert weld them together for you, you will need to mock the engine either in front of the firewall or firewall mock-up. 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 it can be helped. Don't forget about propeller rotation, and be sure to 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, Photography by Pat Panzera and where down and left may be specified, you might have to cant it to the right. Pay careful attention to the nose gear strut (if it's a nose dragger) and main gear attachments if so included. Many engine mounts are complicated with landing gear attachments that may 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 nose gear 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 provider help with the mount? Many engine manufacturers will assist with the design and fabrication of an engine mount for your airframe, especially if it's a popular design. They will have a vested interest in your EAA Experimenter 29

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