Converting the E-flite Stearman PT-17 15e ARF into a Super Stearman
In the November 2009 Ampeer (http://www.theampeer.org/ampeer/ampnov09/ampnov09.htm), I used E-flite’s PT-17 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 E-flite PT-17 was announced near the end of June 2009 and released near the beginning of August. (http://www.horizonhobby.com/Products...ProdID=EFL2950) The E-Flite Stearman PT-17 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 "full-scale.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 E-flite PT-17 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 E-Flite ARF kit.
One of the first things I did was to weigh all the components of the PT-17 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 full-scale 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 E-flite PT-17 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 E-flite PT-17 setup to be:
Motor Down Thrust: a little less than 1.5-deg
Motor Right Thrust: 2-deg
Top wing incidence: +1-deg
Bottom wing incidence: +2-deg
H-Stab/Elevator incidence: +0.5-deg (see Note1)
Decalage: Negative 1-degree
For well over 20 years, Keith Shaw has been flying a 1/6-scale, 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 0-0 to top internal longeron (on scale plane)
Top wing 0
Bottom wing +1
CG right at LE of lower wing, but could be safely pushed back some. (That's about 3.25" on the E-flite 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 0-degrees, not the +0.5-degrees 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 PT-17 ARF to be computed at 3.459” and I recommend a starting CG of 3-3/16” 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 3-5/16” 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 full-size, the model ratio is 1:8.773. The full-scale wing area is given as 297 sq.ft. The full-scale 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 E-flite Stearman PT-17 15e ARF: A few words of caution”.
2 Starting the build:
I started the actual build near the end of November 2009 by fabricating the wheel pants. They are constructed of laminations of balsa wood with 1/8” plywood used where the axle bolt goes through the wheel pant to the landing gear strut. They incorporate 1/8” dowels for keying the two halves together and two magnets in each pant to hold them together. Following Keith Shaw’s advice, I kept the axle as low in the pant as possible.
A New Motor Mount:
I wanted to use the same incidences that Keith used on his model except for incorporating his suggestion of two degrees of right thrust. To get the Scorpion SII-3026-710 motor lined up with the center of the cowl, it was necessary to drop the thrust line 0.488” from the original. The new motor mount box incorporated 1-degree of up thrust so that it matched the incidence of the upper wing. (see Note2) With the 1-degree of motor up thrust the top wing and motor are at 0 degrees in relation to the now +1-degree of incidence for the bottom wing.
See "newmtrmt.jpg' below.
Note2 Unfortunately, it wasn’t until AFTER the new motor mount had been epoxied on for several months that I realised that I had inadvertently put the sidepieces, with the new positive thrust angle, on upside-down! I couple of washers brought it to the desired angle. Luckily, I’d built the cowl to center on the motor at the desired thrust angle.
3 Building the Cowl
After designing the cowl in my CAD program, I actually wrote out some cryptic building notes.
See "cowldesign.jpg" below.
Cowl Construction notes:
FW1 (former that goes against firewall)
1. iron black Econocote on back of FW1
2. align FW1 and make pilot holes for magnets
3. drill ~1/4” in FW1 and firewall for magnets and epoxy in place
Make building jig – possibly from scrap and 3/16” balsa
Use 3-15/16” long by 3/16” sq.
Remove the “piping” and paint ring flat black
Front Cowl Ring (CR)
Remove the paper at alignment marks on CR2 and CR3.
Put template on back of CR3 and remove paper to alignment ring.
Glue 1/4” CR1 to 1/4” CR2 then CR2 to 1/8” CR3.
Glue CR3 to 1/8” CR4
Sand inside angle in cowl ring and cover inside of cowl ring with red Econokote
Assemble the cowl:
Place FW1 in jig. Set longerons in place and tape against jig triangles. Set the cowl ring assembly in place and check alignment. When okay, redo and glue assembly.
When dry, sheet cowl with 1/16” balsa
Sand front of cowl ring to shape
The dummy engine was relieved from the supplied cowl using a sanding drum in a Dremel tool. The brown “piping” was removed from the dummy engine and the inside ring painted black to more closely represent the engine in N56772.
See "cowl.jpg" below.
The cowl ring is balsa laminate. Four 3/16” stringers hold the cowl ring out from the firewall ring. The firewall ring and original firewall on the fuselage have magnets top and bottom to hold the cowl to the fuselage. This was later modified at Keith’s suggestion because he was afraid that the cowl might be sucked forward. Notes on the modification are near the end of the build.
The four hardwood blocks, used to screw the original cowl to the fuselage, were literally knocked off the original firewall.
Several special construction pieces were used to create the cowl; a paper/cardboard template to verify the correct cowl dimensions where it mates to the fuselage (1), four triangles to jig the stringers into position with 1-degree positive from the firewall ring to the cowl ring (2), a balsa disk to place in the back of cowl for sanding the rear edge of the cowl uniformly (3) and a cowl/dummy engine centering ring to glue the dummy engine into the correct position (4).
See "special cowl parts.jpg" below.
After the inside of the front cowl ring was covered with red Econokote, the dummy engine was epoxied to the back of the cowl ring and then the stringers glued in place between the firewall ring and cowl ring.
See "cowl-construct2.jpg" below.
The 1/16” balsa sheeting was cut and glued into place. The cowl ring was sanded to shape.
Rear of cowl - see "cowlrear.jpg" below.
Front of Cowl - see "cowlfront.jpg" below.
Cowl on fuselage with no prop - see "cowlnoprop.jpg" below.
Cowl with prop - see "cowlw-prop.jpg" below.
4 The Horizontal Stabilizer/Elevator Modifications
I did the horizontal stabilizer/elevator modification next. The covering was carefully removed from both the horizontal stab and elevator and loose glue joint between the elevator trailing edge and rudder angle piece was glued. A 3/32” piano wire joiner was created. The elevator was modified to accept the joiner. There is a stress riser where the back of the horizontal stabilizer and the fuselage join caused by the up and down movement of the elevator. I used a piece of spruce to create a piece to spread the stress across a larger area of the back of the horizontal stabilizer and inlaid it into the horizontal stab. The elevator was sanded to taper it to its trailing edge. 3/32” x 1/8” diagonals were added to the horizontal stab and elevator to create triangles. Triangles are strong while rectangles are weak. The excess wood joiner was removed from the hardwood elevator joiner.
See "stab-braces.jpg" below.
5 “X-type Monokote” Hinging
“X-type Monokote” hinges, made from Cub yellow Econokote, were applied to the horizontal stabilizer and elevator.
Step 1: Apply covering to the leading edge of elevator and trailing edge of stab. Cut two strips of covering using a metal “yard stick” as the width of the strips. Remove the backing from the two strips. Turn one strip over. Overlap the two strips by 1/4” or so. Iron the 1/4” sticky side of covering to sticky side of covering on the second piece. Cut the long strips apart using the metal rule as the width to create individual hinges
Step 2: Iron hinges onto elevator.
See "elevator-hinged.jpg" below.
Step 3: Iron hinges to stabilizer.
See "stab-hinged.jpg" below.
Unit held at an angle by tape to show hinging and why it is necessary to cover the mating edges.
Step 4: Cover surfaces. Use a brand-new single edge razor blade to cut the covering between the moving surfaces. Iron the covering into the resulting slot to seal everything down. The result is a very free moving, gapless hinge.
Before Step 4 I had to fix an almost oops. I’d forgotten to add the 1/4” sq. pieces to the outside of the middle “rib” for the rigging to pass through.
See "stab-oops.jpg" below.
The elevator and horizontal stabilizer were covered.
See "stab-elev covered.jpg"
6 New Horizontal Stabilizer angle of incidence
See "H-stab templates.jpg" below.
To create the new angle of incidence for the horizontal stabilizer, I created a template and then a jig for each side of the rear of the fuselage. The jig slid into the existing horizontal stabilizer slot to cut the new stab slot further back and to set the angle of incidence to +2 degrees relative to the new motor thrust angle. Cutting the new slot was a challenge, since the rear of the fuselage, above the stabilizer, is made from balsa blocks. The new slot was then carefully sanded so that the stab aligned correctly side to side and horizontally using the lower wing as a guide.
7 Moving the servos forward
The bottom stringers were cut from the fuselage and the original pushrod tubes removed. The holes in the pushrod tube guides were opened with a 3/16” drill bit. The bit was used as a pin vise by putting a 3/16” wheel collar on it to create a gripping area to spin the bit by hand. Another servo guide crosspiece was made and was glued in across the fuselage after the servos were fitted into their new location. Balsa servo blocks were made and glued into place. The servo rails were glued into place using the servos to set the width of the rails. 1/8”x1/2” triangles were glued to the front and rear of the servo blocks and a 1/4” sq. balsa piece glued on the sides between each rail.
See "newservoposition.jpg" below.
8 The Vertical Stab and Rudder & Finishing up the H-Stab/Elevator
The rudder was stripped and 3/16”x1/4” balsa added to the vertical fin and rudder where the hinges attach.
The bottom of the horizontal stab and elevator were checkerboarded. It took approximately 5.5 hours to do the checkerboarding. The sunburst was added to the top of the horizontal stab and elevator and then the horizontal stab was glued into place. The alignment was checked again and then the glue allowed to dry.
The vertical stab and rudder were prepared for the “X-type Monokote” hinges. The rudder was covered and then hinged to the vertical stab and fuselage. The vertical stab was covered and then the vertical stab and rudder were checkerboarded; another lengthy process.
The rear of the fuselage was covered and a soldering iron used to open the holes for the outer Sullivan Golden-N-Rods (#503). The control horn was attached to the rudder, the rudder servo screwed in and the inner rod adjusted. The same process was used for the elevator. The outer rods were glued to the rod guides.
The original fuselage stringers are made from a very weak wood. One of the stringers broke during normal handling after unpacking the plane and I broke several of the stringers with my excessive handling during the modifications of the tail. Using Keith Shaw’s suggestion, I replaced the bottom wood stringers with 1/8” dowel, which closed up the bottom that I had opened to gain access the pushrod guides.
See "newbottomstringers.jpg" below.
9 New Hatch/Cockpit Assembly
I attempted to use the original fiberglass hatch/cockpit, but that did not work out. The hatch/cockpit assembly would not accept the red Econokote covering without warping. I designed and built a new hatch/cockpit using the original hatch bottom and ends that were relieved from the fiberglass assembly. The rest of the new hatch/cockpit assembly was built using balsa wood and then it was covered. This was a major undertaking and with the design and construction took over a week. The hatch/cockpit has a “screwdriver” slot on each side near the rear to help release the hatch/cockpit, which is held on by the original magnets.
See "hatch.jpg" below.
See "barelegs.jpg" below.
While I worked on building the new hatch/cockpit, and I was waiting for glue and filler to dry, I stripped the landing gear legs of paint using a razor blade and sandpaper and covered them and the wheel pants. I also covered the blue frames of the windscreens with red Econokote.
See "legs1.jpg" below.
1/32” balsa was glued aft of the hatch/cockpit because the full-scale sheeting does not end where the hatch/cockpit ends on the model.
See "bootsheeting.jpg" below.
Once the hatch/cockpit had been satisfactorily fitted to the fuselage, the sides and top of the fuselage were covered.
10 Checkerboarding the Cowl
The cowl was covered and the checkerboards added. Two cowl templates were made to help with the cowl checkerboarding. One template was used for cutting the checkerboards and the other taped onto the cowl to aid in placement of the checkerboard pieces.
See "cowltemplate.jpg" below.
See "cowlchecker1.jpg" below.
See "cowlblisters.jpg" below.
The cowl blisters were cut from a piece of aileron stock that had a piece of 1/8” balsa sheet glued to it to give the blisters the proper thickness. Each blister was cut out and then sanded to shape. The cowl blisters were then covered and glued onto the cowl. The blisters were blended into the cowl with light spackle and then the spackle covered with red Econokote stripes.
11 The Cabanes & Hatch/Cockpit Assembly
The cabanes were covered with masking tape and then Econokoted red.
See "cabanes.jpg" below.
The top of the hatch/cockpit was covered and the cockpits painted flat black with acrylic paint that I purchased at Michael’s craft store. The already assembled instrument panels were glued into place. The instrument panels were made from 1/8” thick balsa, covered with chrome Monokote trim sheet and then holes were bored for the instruments. The instruments were make from 1/8” thick pieces of dowel. They were painted black and then the instrument faces were painted on them. The instruments were glued into the panels and the panels glued into the cockpits.
See "pilots-hatch1.jpg" below.
See "headrestrough.jpg" below.
Building the very long headrest proved to be a real task. It took several designs and about one week to complete it. During this time the air scoops were made, covered, painted and attached to the cowl. I made a 1/2-inch diameter balsa cylinder and then cut it appropriately to created the exhaust. The exhaust was covered with Cub yellow Econokote, rubbed down with steel wool and then spray-painted gray with an overspray of Dulcote. The exhaust exit was painted with acrylic flat back, just as the air scoop inlets had been. The ring around the exhaust was made from 1/64” plywood and painted silver. The cockpit GPS unit was made from balsa covered with black Econokote and the “face” painted with turquoise acrylic paint. Also, while the headrest was being created, the windscreens were glued onto the hatch/cockpit assembly and black fuel line split and added around the cockpits to represent the cockpit combing.
In reality, the windscreens were actually glued on three different times over the months of building because I didn’t like their positioning.
Once the headrest was completed, it was covered and then epoxied to the hatch/cockpit assembly so that it comes off with the hatch/cockpit assembly. Toward the back of the headrest, on the bottom, is a magnet that is attracted to a wire nail head on the top of the fuselage to help hold the back of the long headrest in place.
See "finishedfuse5.jpg" below.
The transformation of the fuselage was completed on January 30 when the exhaust and exhaust ring were epoxied into place in the cowl.
The next day I cleaned up the whole workshop and the basement to get a fresh start on the wings.
See "ssprogress1.jpg" below.
Progress as of the February 2010 EFO meeting.
12 The wings
The covering was stripped from the lower wing panels. I tried to remove the “N” strut fixtures from the wings to make covering easier, but the first one I tried broke off. I repaired it and decided I’d cover around them.
See "wingpanelB4hinging.jpg" below.
I epoxied the attachment blocks to the aileron hatches for the Hitec HS-65 servos. Next, the Cub yellow Econokote strips were ironed to all facing surfaces where the hinges were to be applied. I noticed then that the factory had taped the wrong ailerons to the wrong panels. No harm done, but I was surprised.
See "lowerpanelshinged.jpg" below.
The “X-type Monokote” hinges were ironed on.
Once the bottom wing was hinged, the top and bottom wings were covered. The checkerboarding and sunbursts were added to the wings. While those are two quick sentences, two weeks of work was involved including a 70 mile round trip to the Prop Shop to purchase Cub Yellow Econokote, since Tower Hobbies had not shipped mine, after it had been on order over three months! It is still not here and it is the end of March as I type this.
See "checker-base.jpg" below.
A baseline of checker squares was completed.
See "checker-nextrow.jpg" below.
The baseline of checker squares and a straight edge were used to complete the checkerboarding.
See "rays1.jpg" below.
Templates were created for the top and bottom wing rays.
See "rays-lower.jpg" below.
See "rays-upper.jpg" below.
The templates were held in place with painter’s tape as well as the red rays. The template was removed and the rays ironed into place.
The N-struts were stripped and covered.
See "N-struts.jpg" below.
13 Some Clean-up
Once the wings were completed, it was time to clean up some mistakes.
The long headrest needed to be tapered to the fuselage so as not to have a straight drop, as shown in previous photos of the construction.
When I bent the new tail wheel wire, I used the supplied wire as a guide, so my tail wheel gear was way too long. I cut the wire with a Dremel cutoff wheel and attached the original tail wheel bottom bracket with Locktite Weld epoxy and the supplied setscrew.
I measured the incidence angles, again.
See "motor thrust.jpg" below.
See "incidence1.jpg" below.
The photos show the method I used, although these were taken earlier in the construction at the time when I discovered that I had inadvertently put the new motor mount sides on upside-down.
14 Creating the Graphics
I created the graphics using clear mailing labels for the yellow and red graphics for “Tillie”, N56772 and the Stearman on the rudder. I found a Stearman graphic on the Internet that I modified for the tail and used fonts that I already had to match Tillie and N56772.
See "stear_logo_blue.jpg" below.
See "stearman-logo4.jpg" below.
See "Tillie.jpg" below.
See "N-number.jpg" below.
I used the trial and error method to get the colors as close as possible using my Photoshop Elements program. It took about a full sheet of labels to get the colors close to that of the red and Cub yellow Econokote, and about a full day as well. I put Cub yellow Econocote under the clear mailing labels to get the yellow to contrast well. The Cub yellow was ironed onto the appropriate places and then the mailing label graphic placed on top of that. I removed two red squares from each side of the vertical stabilizer so that the Stearman graphic would color match. Unfortunately, iron-on covering sticks best where you want to remove it. I put a small hole in the vertical stab covering while removing the fourth and last red square. The hole was repaired with a patch. Finally the clear mailing label graphics were masked off and given several coats of clear gloss spray can enamel to bring the sheen up to that of the Econokote.
I found the graphic for the shock absorber online. I had to clean it up to look right when printed. The graphics for the shock absorbers were printed on a white mailing label, which I then over sprayed with clear gloss enamel and pressed onto the landing gear.
See "shock plate.jpg" below.
15 The Rigging
Once the graphics were in place, the rigging was completed. I used 3/16” diameter dowel to create the rigging fastener plugs for the fuselage. I wrapped 0.019-diameter jewelry wire around the 3/16” long dowel. The wire was fitted into slots on each side of the dowel “plug”. An X-acto saw made the slots. The wire was crimped into circles at each end yielding two attachment points for each plug. The outside dowel end was covered with red Econokote for the wing attachment pegs and black for the tail attachment. The attachment plugs were epoxied into 3/16” diameter holes drilled into the fuselage at the appropriate positions.
See "rigging-pegs.jpg" below.
The rigging “wires” are made from 1mm silver elastic jewelry cord that I purchased at Michaels craft store.
See "riggingcord.jpg" below.
The cord is looped and crimped with #3 jewelry crimps through the wire in the fuselage sides and run to the N-strut holders on the wing, the cabanes and the fuselage attachment point under the tail. I made the rigging spacer from 1/8” square spruce cut to length, covered with red Econokote and drilled with 1/16” holes for the rigging “wires” to pass through.
See "rigging2.jpg" below.
The “Tillie” and shock absorber strut graphic can be seen in this photo of the rigging. Not all the rigging was completed when this photo was taken.
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