Denny Dart II With 7″ NP Prop Flight Test

I made another DD II fuselage for the 7″ North Pacific prop assembly.  It will use the same wings.
The prop shaft makes an angle of 5.71 degrees to the stick.  The wing, at zero geometrical incidence, has an effective incidence of 1.15 degrees.  (I am using the 1/16″ thickness divided by the average chord to estimate the tangent of the zero lift angle.  Maybe not “right”, but can’t be far off.)  That puts the wing at 6.86 degrees to the prop shaft.  Various aerodynamic data from different wind tunnels, with different aspect ratios and different Reynolds numbers, says the minimum sink for a flat plate wing is at 6 to 8 degrees.  This is about as good as it gets.
I want the zero lift line of the tailplane parallel with the prop shaft.  I estimate the upside down tailplane zero lift angle to be 1.79 degrees and I need another 3.92 to match the 5.71 of the prop shaft.  That works out to a taper of about 1/4″ in 3 5/8″ (3.95 degrees).  Close enough.  There will be some downwash on the tailplane from the wing.
Wing      1.690 gm
Fus        1.638
Prop       3.312
Total       6.940 gm
Considering the torque requirements to be similar to the Squirrel, I sized the motor as a 17″ loop of 0.083″.  It weighs 1.88 gm dry.  Total weight is 8.82 grams.  The turns table is:
127.4 tpi
% Break    Turns
100          2166
95          2058
90          1950
85          1841
80          1733
75          1625
70          1516
I put it all together and set the wing to get the same CG position as before.  By the time I was ready to go the leaves were already fluttering and flags were streaming.  But the park is surrounded by tall trees that partly block the wind.  The field is approximately 350 feet on a side square.  There was a kids soccer practice taking up most of the field.
I put in 1,500 turns (69%), just a bit more than enough for the cruise.  It went up nicely over my head and drifted down field, bobbling and meandering around in the turbulent air, recovering well from the disturbances, with a slight preference for turning right.  It came down just inside the fence, telling me that more turns would not be a good idea.  I found that the prop hanger was bent to the right.  After bending it to the left, the plane made consistent left circles.
No videos, no times.  It is evident that this will be a great flyer on more turns.  Probably will fly too well for these little local parks.  I will need a much bigger field to wring it out.  If I can get up early enough to beat the wind, I may get some longer flights in tomorrow morning.
The wing warped in the hot car on the way home.  I blocked it on my table and it seems to be straight now.
Gary Hinze

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