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Riblett Wing

By petehdgs - Posted on 23 June 2012

The letter below is a result of discussions and research on the Riblett wing as applied to the Coot amphibian.  I am trying to select a modern airfoil for use on the long coot. I now see that I overlooked that the GA30-615 was used and have therefore looked seriously at the GA37A615 instead of the GA37-315.  I will consider scaling the 615 down to 614, but I think this is the airfoil I want for the Long Coot.  There are drag and moment reductions compared to the NACA 4415 used on the Coot and drag reductions compared to the GA30-615, with about the same lift available.   

Dear Mr. Riblett

I have been reading your book GA Airfoils and was wondering if you could help me to better understand the airfoils and apply them to my application.  I am writing you because I feel like you can see the air flow on the wings, just like Dale Earnhart was able to see the air flowing around a race car.  That type of insight is rare and I like to consult with the best. 

I am working on a long version of the Coot Amphibian.  The idea is to increase the length of the airframe 25% and increase the gross weight by 50%.  That will make the airplane 2850 lbs gross and about 28 feet long, making it a four seater.  I’d like to use the rudder and stabilator from a donor PA-28 Cherokee, positioned high enough to be in the prop slip stream.  Part of the plan is to keep the steep angle of the Coot afterbody with the longer arframe to move the rudder and stabalitor into the prop’s aristream. I intend to power this aircraft with a 200HP Deltahawk diesel engine because diesel fuel is readily available at any marine fuel dock and the Delta Hawk will run on it or Jet A or JP5 if the fuel delivery system is set up properly.  The intended cruise speed is 155 MPH, or 135 kts.  The standard Coot wing does not have flaps.  I’d like to have manually operated split flaps but I am concerned about water landings on a low wing amphibian with flaps deployed. 

I have been reading all the graphs in the book and am looking at either the 37-315 or the 37-415 airfoil.  I think these are the best airfoils, but the Coot has used the 30-613.5 in the past.  As I understand it, this is a turbulent airfoil.  I also feel that the 415 will have too much trim drag, which is undesirable.  If I use the 37-315 with split flaps, or scale it down to 37-313.5 and use split flaps, that might be the ticket.  I was also considering a 37-3.513.5, but I’d need new camber ordinates for that, and it might not be necessary if I use split flaps with the 37-313.5.  

I wold love to discuss this with you in more detail. 

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