Model Airplane News - - LET'S GET WET! -

When you de­cide to fly off wa­ter, you’ll need to learn a few aero­nau­ti­cal terms. The ref­er­ence point for mea­sur­ing the in­ci­dence of the wing is the “da­tum line” of the fuse­lage. The da­tum line runs hor­i­zon­tally through (or is par­al­lel to) the hor­i­zon­tal sta­bi­lizer. “Wing in­ci­dence” is the an­gle be­tween the wing chord line and the da­tum line of the fuse­lage. With flat-bot­tom air­foils, the “chord line” is not the lower sur­face of the wing; it is the straight line from the for­ward­most point of the wing’s lead­ing edge to the most rear­ward point of the trail­ing edge. In most flat-bot­tom air­foils, the lower sur­face of the wing is flat only from the main spar to the trail­ing edge, but from the spar for­ward, it sweeps up slightly. The chord line is usu­ally about 2 de­grees more than the flat sur­face aft of the spar. This an­gle of in­ci­dence is set and can­not be changed once the wing has been fit­ted to the fuse­lage. It is not to be con­fused with the wing’s an­gle of at­tack, which varies greatly dur­ing flight and de­pends mostly on fly­ing speed.

Add to the above terms two more that ap­ply to float­planes and fly­ing boats: “beam” and “keel flat.” The “beam” for a sin­gle-hull fly­ing boat is the width of the hull at its widest point, usu­ally near the step; with a float­plane, the beam will be dou­ble the width of each float. The “keel flat” is con­sid­ered to be the bot­tom sur­face of the hull or floats di­rectly un­der the cen­ter of grav­ity. It is the area from di­rectly un­der the wing’s lead­ing edge back to the step. When the model flies at high speed on the wa­ter just prior to liftoff, it is what we call “on the step,” and it rides on this small keel-flat sec­tion of the floats or hull. If the model is blocked up on the work­bench so that the fuse­lage da­tum line is par­al­lel to the bench sur­face, the only part of the floats or hull touch­ing the bench would be the bot­tom tip of the step. The keel flat should an­gle up­ward slightly, at about 2 de­grees. Some­times, how­ever, it can also be set at 0 de­grees, with the en­tire keel flat touch­ing the bench. Get­ting the keel-flat set­ting cor­rect for the floats is akin to check­ing the in­ci­dence for the wings. For­ward of the lead­ing edge of the wing, the keel gently sweeps up to­ward the nose.

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