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Advice for Converting FF to RC
  • I have a set of plans for the Blackhawk.  This airplane was originally a 72 inch wingspan free flight model powered by a 1/5 hp gasoline engine.  The kit/plans were produced by the Peerless Model Airplane Company in 1938.  The design incorporates a fully flying horizontal stabilizer that incorporates dihedral and twin vertical tails.  I would like to convert it to electric RC using elevator, with rudders or ailerons or both.  Where I need help is deciding if both rudders and ailerons are required to fly the plane.  It seems like just rudders would be more in the "spirit" of the plane, but I don't know if they would be enough to turn it.  There is a single motor up front; the rudders will not be directly in line with the propeller wash.  The span of the horizontal stabilizer is 25 inches. Ailerons would be far easier, assuming they would be sufficient.  The wing has 5 inches of dihedral as well.  I performed a similar conversion with an 80 inch Sal Taibi Powerhouse, and rudder/elevator worked great.  Included is a small picture of the plane.  Please let me know if I can provide more information.  Any help or advice is much appreciated!

  • That's a looker!

    A well-engineered free flight design must incorporate spiral stability. This is done by having excellent directional stability. This is far less important in an RC design. My hunch is that the vertical stabilizers are big enough. I would leave out the ailerons.

    Note that the rudders would not be in the propwash during straight flight, but they would likely be as soon as you start turning.

    If you do decide to add ailerons, you would need to decrease the amount of dihedral. You have ten degrees right now, which is a lot for a high wing airplane. I would cut it in half, more or less.

    As I am sure you know, it will come out tail heavy unless you take some steps to compensate.

  • Thanks Carlos!  Rudders only it is.  I'll circle back in a month (or two or three) and let you know how it worked out.
  • Another question: the saddle for the horizontal stabilizer offers the option of having 1-1/2 degrees of negative incidence, or alternatively, 1/2 degree degree of negative incidence.  Which do you think would be better, and why?

    Thanks again!
  • I would go with 1.5 degrees. Old style free flight models liked to use lifting horizontal stabilizers. They would really be flying around like a canard, which introduces a different set of problems. Modern model airplanes all have a downforce on their horizontal stabilizers.

    So 1.5 degrees seems like it would be a better choice. Frankly, I do not think that either choice will cause problems. You always need to fine tune it with the elevator, anyway.

  • Well, I've finally finished the airplane.  All up weight is 4.0 pounds.  It has a 550 watt motor, so I think it'll have plenty of power.  It actually came out a bit nose heavy, so I moved the battery back a bit and all was well.  It is covered in coverlite covering, with black paint and red monokote trim.  I had to use my imagination on the side art.  The maiden won't be until April when my father visits again.  Thanks for the advice!
  • I am glad I was able to help. I assume that the maiden flight already occurred?


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