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Long Coot Amphibian: Starting the wings


By petehdgs - Posted on 29 April 2013

So the garage is built, but still needs some items... The camper is on hold for now... I am proceeding to start construction on the wings.  This is where Molt Taylor suggested starting the Coot, and since the Long Coot is going to need a new wing anyway, this is a great place to start.  The wngs take up less space than the hull or the camper, and I can work on them as I finish other things necessary around the shop. 

The Coot wing chord is 5 feet and the Long Coot chord will be 25% longer, 6.25 feet long.   Using the basic idea of increasing the length by 25% and increasing the weight by 50%, puts design gross weight at 2800 and the overall length at 28 feet.  Using a designed empty weight of 1800 leaves 1000 pounds of usefull load.  Rounding weight up to 3000, gives a useful load of 1200 pounds.  So 3000 is the desiged gross weight.  The standard load test of 3.8Gs would give a test load of 11,400 pounds, and load testing a the utility category of 4.5Gs puts the test load on the completed wing assembly at 13,500 lbs.  At this rating the wing will be overbuilt by 18.4%.  A nice overbuild for safety. 

I have been re-reading all the construction notes on the Coot wing and back issues of Epoxy Works magazine looking for clues of building additional strength into the wing spars and ribs wihout adding 50% more weight.  One of the interesting things noted was adding a layer of fiberglass/epoxy cloth between two layers of plywood, or putting a layer of fiberglass over the top and bottom of wood I-Beams.  The added strength to weight increase is impressive so that will be part of my plan. 

I have done some research into the GA series of wing sections by Harry Riblett, and have selected the GA-35A415 wing section for the Long Coot Amphibian.  I selected this airfoil instead of the GA-30A615 recommended for the Coot for several reasons.  At 220 SqFt area, the wing loading will be 13.64 at 3000 pounds, up from 10.6 for the Coot.  With 200 HP and a longer sleeker amphibian, 120kts cruise should be attainable.  This is faster and with a longer wing chord has pushed the Reynolds number up to about 8 million, up considerably from 5 million at 100 kts for the Coot.  The Long Coot will thus benefit from a laminar flow wing design.  The GA-35 series wing is the first stage laminar flow design, and while not as sleek as the GA-40, it is more forgiving and still cuts drag considerably over the GA-30. 

I initially looked at the GA-35A615, but after looking at design C-Lift at cruise speeds rejected it as having too much lift to stay reliably in the laminar bucket at high cruise.  The 35A415 wing does not have as much camber as the A615 wing, so I intend to regain some of the low speed lift needed by building in Split-Flaps.  Molt's design initially did not have flaps, but he recommended their use if you were willing to spend the time to build them and design the linkage.  I plan to use a manual flap linkage salvaged from a Piper Cherokee to operate the linkage.  The flaps will be operated after the boat is on step and has accelerated to take-off speed, then pull on the flaps and leap off of the water.  On landing, touch the water with 2/3 flaps on, then dump them before slowing to prevent damage from water spray.  I have practiced these techniques in my cherokee, N5586F. 

The next step is to create an odinance table for the 6.25 ft chord and cut aluminum templates for building the ribs.  Then put the final sizing on the spars, apply the spar form from the Coot, and adjust the material for the strength desired.  Easy, right?  One miracle at a time. 

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