
Apr 04, 2010, 12:04 PM  

Build Log
Converting the Eflite Stearman PT17 15e ARF into a Super Stearman
Background:
In the November 2009 Ampeer (http://www.theampeer.org/ampeer/ampnov09/ampnov09.htm), I used Eflite’s PT17 ARF as an example for selecting a power system for a biplane. I have not had a biplane since my glow days in the late 1970’s and early 1980's. I used to fly an Airtronics kitted Acro Star bipe in IMAC competition and for fun, and I loved it. Why I had not added a bipe to my electric fleet, I do not know. The Eflite PT17 was announced near the end of June 2009 and released near the beginning of August. (http://www.horizonhobby.com/Products...ProdID=EFL2950) The EFlite Stearman PT17 15e ARF piqued my interest in having a bipe again. I began to closely follow the thread about it on RC Groups. (http://www.rcgroups.com/forums/showthread.php?t=1070110) As I was poking around the Internet looking for Stearman information, I ran across a photo of “Tillie”, aka N56772. It is a 450hp Super Stearman in the “right” colors, so I just had to model it. See "fullscale.jpg" below. I spent time collecting as many photos of N56772 as I could find on the Internet. Once I had some documentation, I was ready to start the conversion. I purchased the Eflite PT17 kit in October 2009. Since the flying season was over for me, and the building season couldn’t start until my shoulder healed, I had a lot of time to learn a lot about the EFlite ARF kit. One of the first things I did was to weigh all the components of the PT17 ARF kit. I weighed the ARF kit’s contents and found that the assembled airframe would weigh 999.35g/35.25 oz. See "CAD.jpg" below. Since I planned to modify the Stearman into a Super Stearman, just as has been done with the fullscale plane to create a Super Stearman, I carefully started a CAD drawing of this model. I accurately measured the various components of the kit and entered them into my CAD drawing. A couple of minor problems surfaced concerning some of the initial flights of the Eflite PT17 in the RC Groups thread during this time. Some folks, especially low time pilots, had a problem with the plane climbing too steeply on takeoff when too much power was applied. There was discussion in the thread about the Center of Gravity (CG), angles of incidence of the wings and horizontal stabilizer/elevator, motor down thrust and decalage. I learned that the datum line was the top of the fuselage hatch. Using my Robart incidence meter and a piece of paper and ruler for measuring for down thrust and right thrust, I found the stock Eflite PT17 setup to be: Motor Down Thrust: a little less than 1.5deg Motor Right Thrust: 2deg Top wing incidence: +1deg Bottom wing incidence: +2deg HStab/Elevator incidence: +0.5deg (see Note1) Decalage: Negative 1degree For well over 20 years, Keith Shaw has been flying a 1/6scale, 1200 sq.in. model of a 450hp Super Stearman. It is modeled after Bill Barber's "Black Baron". These are the incidences that he used on his great flying model. Thrust line 00 to top internal longeron (on scale plane) Top wing 0 Bottom wing +1 Stab +2 CG right at LE of lower wing, but could be safely pushed back some. (That's about 3.25" on the Eflite model. KM) A little right thrust might be nice for sport flying, 2 deg or less. (Just a note via email to me about what I might want to do for right thrust. KM) Note1 When the covering was removed, it reveled that the horizontal stabilizer sits flat on top of the “crutch” that the hatch/cockpit cover rests on. Therefore, the horizontal tail incidence is 0degrees, not the +0.5degrees that I’d measured with the incidence meter. There was concern expressed on RC Groups about the Center of Gravity (CG) position as given in the manual. I used Martin Irvin’s method for finding the CG. His method was presented in his Quiet Scale column in the April 2001 of Electric Flight International. Using Martin’s method, I found the CG for the stock PT17 ARF to be computed at 3.459” and I recommend a starting CG of 33/16” back from the leading edge of the top wing. Since I moved the horizontal stabilizer rearward on my Super Stearman, I recalculated the CG at 3.584” and my starting CG is at 35/16” from the leading edge of the top wing. To use Martin’s formula, I had to know the wing area. I carefully measured the wing area several times. I found that the top wing has 277.6 sq.in. of wing area when the center cutout is NOT included in the calculations and that the bottom wing has 279.8 sq.in. of wing area when the area that does not exist inside the fuselage IS included (lost span), as is a typical practice on models. My measured total wing area for this model is 557.4 sq.in., not the advertised 608 sq.in. Using the top wingspan to scale the model to the fullsize, the model ratio is 1:8.773. The fullscale wing area is given as 297 sq.ft. The fullscale wing area, when scaled to 8.773 is 3.859 sq.ft. or 555.7 sq.in., which confirms my wing area measurement, if the model is somewhat scale. I used a lot of the data that I had collected from the RC Groups thread and my calculations for the article that I wrote for the December 2009 Ampeer (http://www.theampeer.org/ampeer/ampdec09/ampdec09.htm), “The Eflite Stearman PT17 15e ARF: A few words of caution”. 

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