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FanFacts H


ello again, I hope that in the last month your workshop runneth over with the leftovers from con- struction.


In the previous column, I discussed c.g. calculation using a simple schematic for non-complex airframes. I then stated that I didn’t have a clue as to how airframes with chines/LEX’s were calculated, and having seen a pencil sketch of a really cool looking airframe with both an LEX and canards, I figured I needed an answer. So off went an e-mail to Bob Parks (an aero-engineer with a modeling resumé as well as a full-scale re- sumé) and posed the question: How? I have in the past challenged him with my math/equations (and common sense?) inept- ness as we attempted to figure the c.g. on a three-wing (canard, wing and tail) plane that I was asdoing a proof of concept build, so I am acquainted with the AVL program (and its associated drawings), and some of the ability of engineers to calculate things … but I am ahead of myself. The first thing Bob mentioned was to look


at the landing gear. Huh, you say? If you measure just about any tricycle-geared plane built non-fly-by-wire, you will find the c.g. is 12–15 degrees in front of the landing gear where the wheel touches the ground, and Bob pointed out that the US military has a requirement that the c.g. be at least 15 degrees in front of the main gear. Well and fine I hear you saying, so how does that re- late to the drawing from last month. If you take a wing for a P-39 or F-86,


where you have a more “normal” wing span versus length, and a generous wing area, you will find that the 25% MAC in the wing is somewhat similar to the drawing. But what about a teeny-tiny little wing on an F- 104? Doing a quick measurement with my daughter’s left-over elementary school pro- tractor, shows the 15 degrees in front of the


by greg moore You can reach Greg Moore via e-mail at jetflyr@comcast.net


PHOTO: ANDY ZANER A 144-inch long T-38 takes up lots of room and really looks impressive!


main gear line goes right through the mid- dle of the leading edge flap (at the root), and using the drawing method, it is aft of the L.E. flap hinge line, just barely in front of the tire. So my favored graphical method (the drawing) is only an aid, and in this case is inadequate, but it is still close for most of the things we build. Interestingly enough, a modeler in Germany used the protractor method to figure the c.g. for his very large, scratchbuilt F-100, at the 20-degree line. The problem is that a lot of modern jets


(F-16 and later) are designed to be unstable, because that can enhance their maneuver- ability. The c.g. is still at least 15 degrees in front of the main gear, but you don’t want to try a model with that c.g. location! The c.g. will need to be well in front of there for a model, so the landing gear method won’t help you (and if you use a scale landing gear location, you are going to have a model that’s going to need a lot of up elevator and/or airspeed to get off the ground!). This reading of Bob’s e-mail, and then measuring, reminded me of several sessions with Ivan doing the same measurements to


ensure that the c.g.-to-wing graphical mea- surement also coincided with the c.g. to land- ing gear measurement. I always presumed we were just ensuring that the landing gear was placed correctly; I never thought about using the landing gear placement as a pri- mary means for determining the c.g.! Okay, we’ve determined that we want to


design an edf for a super/blows-hard fan unit and it has a rounded fuselage that we want to account for as well as wing downwash on the horizontal stabilizer placement. Now what do we do? Over the last 10 years, Bob has mentioned Blaine Rawdon’s Plane Geom- etry spreadsheet http://tinyurl.com/797s249 in several e-mails and discussions on several of the forums, time and time again. So for around $20.00, you can calculate al-


most everything (except for canards) using essentially the same equations and methods as the designers of WW2 airplanes did, and achieve a huge level of accuracy. Plane Geometry comes with some pretty good in- structions and explanations of why you are doing things, but it helps if you are familiar with some basic aerodynamic concepts like lift coefficients and so on.


A B


Looking at a thread on the Internet linked to a thread three times removed, a very large F-100 was being built, and the builder posted a photo (at left) showing the c.g. of the aircraft. His 20-degree line is shown, and the aft most c.g. is at about 12 degrees. Interesting how commonalities sometimes show up! Three-view (above) of an F-104 from the Internet demonstrates the shortness of the wing and the two calculated c.g. positions: a) 15 degrees in front of the main gear and b) calculated graphically.


44 MAY 2012


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