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#101
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effect of changed thrust line.
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#102
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effect of changed thrust line.
On Wed, 19 Nov 2008 13:15:02 -0600, Jim Logajan
wrote: wrote: Jim Logajan wrote: wrote: OK - got some more info. The center of mass is something like 34 inches behind the firewall and roughly 7 inches above the top engine mount point on the firewall. so roughly speeking 13 inches above the prop centerline. What is that the center of mass of, exactly? The empty airframe only (i.e. no engine)? Or is it the center of mass of a complete airplane (sans fuel, pilot, passenger, and baggage) with the engine mounted at the original point? I'm also curious as to how you determined the location in two dimensions. I can think of a few ways of determining it, but they all involve a bunch of effort. Center of mass for the completed plane when built to specs. Provided by the designer. I don't suppose the designer was or is willing to provide general advice or guidance on mounting your engine? Sure beats asking total strangers on the net. :-) If I were in your position these are a few of the things I'd keep in mind: * The center of mass (CoM) of an empty airplane and full airplane are in different locations in three dimensions. In the two dimensions of interest, depending on how fuel, passengers, and baggage are loaded, it moves around in those two dimensions such that it is geometrically impossible to move engine "down" and find a single angle that yields the original moment arms for all load configurations. The one configuration I don't think I would use to compute engine mounting angle would be the empty configuration. * Moving the engine down relative to the original design specs also moves the empty airplane CoM down. Assuming the Corvair and O-200 are both the same weight of about 200 lbs, and assuming the empty weight of the plane is 815 lbs, then if the engine is moved D inches down, the CoM moves down D*200/815 inches. If D = 4, CoM moves down about 1 inch. * I don't know where the CoM of passengers and baggage would be, but they probably move CoM down (and presumably fuel moves it up). As an example, if all that usable load is about 600 lbs, then if that load's CoM is L inches below the empty airplane's CoM, the CoM of the whole airplane moves down L*600/(815+600). So if, for example, L = 5, then CoM moves down about 2 inches. So between lowering the engine and throwing in stuff, the CoM of the airplane in this configuration differs by about 3 inches from that used in the original empty airplane spec. But the load probably moved the CoM forward or backward too, affecting another variable in the equations. Fun, huh? * How much is the horizontal tail in the propwash? That and a different angle of incidence of the propwash on the fuselage might give me more down pitch than I calculated by assuming the only factor affected by the engine move and rotation was the thrust moment arm. I might also risk losing thrust efficiency because of increasing fuselage interference in the propwash. * At some point I'd realize that the CoM is variable enough that for the small amount I'd be moving the engine, it would probably be best to mount the engine so that the engine thrust line is aligned with the most natural fuselage longitudinal. Then make trim adjustments during the test flights. Thanks Jim. I just talted to a P.Eng who has a LOT of aeronautic experience and he said I likely would not notice any change, but he would cut the recommended 1.5 degrees roughly in half for my install... I don't think there will be any detrimental effect as far as propwash is concerned - might even improve very marginally. |
#104
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ClareSquared , was effect of changed thrust line.
On Sun, 23 Nov 2008 17:51:18 -0500, wrote:
OK guys, the pictures of the mount are up on my website at: http://www.pegazair.on-the-net.ca/Cl...ared/Mount.htm Drop by and take a gander. You can get to the rest of the project from there as well. that is either a bloody big aeroplane or the wife is the cutsiest little thing. her washing line is half the height of the rudder!! ......yeah I know. it was taken from the second floor.... nice workmanship. thank god you're not a simmer! Stealth Pilot |
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