The SparkE

SparkE is a fully aerobatic sport electric-powered model aircraft. Recommended power is an AstroFlight Sport 15 motor direct driving an 8 x 6 prop and using a 10 cell battery. Construction article and CAD-drawn plans by David Mitchell.

Introduction

The SparkE is a fully aerobatic sport electric-powered model aircraft. Recommended power is an AstroFlight Sport 15 motor direct driving an 8 x 6 prop and using a 10 cell battery. The construction is all balsa with plastic film covering. The aircraft has no landing gear and must be hand launched.

Background

I had flown an ElectroStreak (ES) for a year & decided to design my own aerobatic plane that would be powered by a direct drive AstroFlight Sport 15 with 10 cells. I also wanted to try CAD to draw the plans. I studied a number of existing designs to help me arrive at the final design. The fuselage had to be slightly wider & deeper than the ES in order to accommodate the 10-1700 cells. I also wanted the plane to balance with all motor/radio components accessible when the wing is removed (no extra hatches). The ESC was placed just behind the motor, the motor battery was under the C.G., next came the RCD 535 receiver, then the 2 HS-80 servos just in front of the wing TE. The 270mah receiver battery was placed behind the wing TE. Ailerons are controlled by a single HS-80 driving torque rods so that there were no rods coming out of the side of the fuse. Every plane has to have a name so I came up with SparkE. The English M.A.P. Sparky was my first successful RC (rudder only) plane back in the early 60s & it sounded like a good name for an electric aircraft.

This was my first attempt at designing my own plane & also using CAD. It has been a most rewarding, but time consuming, experience. To see it actually fly was a great relief.

Some extra reinforcement was added to the front of the fuselage as I found the standard ES rather weak. The plane was covered with TowerKote, a light film covering which went on well & still looks good. There is no u/c which keeps the weight & drag down.

Construction

All wood is light/medium weight balsa unless otherwise noted. I use Weldbond for most of the glue joints. Cleanup is easy with water & it dries in about an hour.

Full size plans are available through the E Zone's on-line plans service.

Fuselage

Cut the 2 fuselage sides out of 3/32" balsa. Mark the positions of formers F2 and F3 on the inside of each fuselage side. Pin both sides down flat on the building board and glue 3/8" triangle strips to the upper rear edge of each side (behind F3). Glue 3/8" triangle strips to the upper and lower nose section of each side (ahead of F2). Glue 3/16" square strips to the lower edge of each side between F2 and F3 and behind F3. Reinforce the inside of each side with 2 strips of 1/16" x 3/8" spruce or basswood. Glue these strips from the nose to 1" behind F3. Glue small pieces of 1/16 balsa between the spruce strips at the 3 former locations so that there is a flat surface to glue the edge of the formers. Glue 3/32" x 3/4" wing saddle doublers between F2 and F3. Construct formers F2 & F3 out of 3/16" balsa. Glue a 1/32" x 1/2" ply strip to the back top edge of F2 and the front top edge of F3. Cut a 3/16" slot in F2 for the wing dowel. Mark the centre of each former at the bottom edge. Glue F2 and F3 to the right fuselage side making sure they are at 90 degrees to the fuselage side. Stand the right fuselage side upright and glue the left fuselage side to F2 and F3. Check that the bottom rear of each side is the same distance above the building board before the glue dries. Glue the 3/32" fuselage floor between F2 and F3. Sand the 3/8" triangle & 3/16" strips at the fuselage rear so that the sides can be brought together. Secure the fuselage over the plans, glue the rear fuselage sides together & clamp until glue dries. Cut F1 from 1/8" ply and drill 2 4/40 clearance holes. Using a razor saw, make a series of cuts in the 3/8" triangle stock forward of F2 so that the sides will bend easily to meet F1. Glue the front fuselage sides to F1 making sure you have 2 degrees right side thrust & clamp until glue dries. Make up the fuselage top & bottom sheeting from pieces of 1/16" x 4" cross grain sheet. Glue bottom crossgrain sheeting to fuse. Cut the bottom cross member from F2. Spread a very thin coat of 30 minute epoxy over the inside of the fuselage bottom, including the 3/8" triangle stock, from the nose back to 3/4" behind F2. Lay a piece of fibreglass cloth over this area & spread another thin coat of epoxy to wet the cloth. Glue top sheeting to fuse. Make servo mounting tray for rudder & elevator servos from 3/32" balsa. Use strips of 1/8" ply for receiving the servo retaining screws. Glue the tray to the bottom of the fuselage just ahead of F3. Sand the top & bottom edges of the fuselage to achieve a pleasant rounded shape. Cut slots in the fuselage sides for the rudder & elevator pushrods. The 2 pushrods cross over each other with the rudder pushrod above the elevator pushrod. The pushrods should not touch. Spread a thin coat of epoxy over the top inside fuselage nose sheeting just ahead of F2. This is where the ESC will be attached by velcro. Spread a thin coat of epoxy over the inside fuselage bottom between F2 & the servo tray. This is where the motor battery and receiver will be attached by strips of velcro. Spread a thin coat of epoxy over the inside fuselage bottom for 3" behind F3. This is where the receiver battery will be attached by velcro. After epoxy dries, clean surface with alcohol & attach velcro strips. Make rudder & elevator pushrods out of straight 1/4" square balsa stock. Cut the 2 wing hold down triangular plates from 1/8" ply & glue with epoxy. Note that they are glued so that they are parallel to the top TE of the wing.

Stabilizer & Fin

Both the stab & the fin are built out of 3/16" x 3/8" stock. Lay wax paper over the plans & pin the LE & TE in place. Use a straight edge to make sure the TE is perfectly straight. Cut, glue & pin all the cross pieces (ribs). Sand the LE & tip to a rounded shape. Round the LE/tip corners. Build the rudder in a similar way. The elevator can be made out of 3/16" sheet balsa or built up from 3/16" strips. Join the elevator halves with a piece of 3/16" square spruce. Cut all hinge slots. I used SIG Easy Hinges cut in half so that they are 1/2" wide and 3/4" long. Sand the LE of the rudder & elevator to a sharp edge to provide clearance for control movement. Taper both the rudder and elevator using a sanding block so that the TE is 1/16" thick. Draw 2 lines on the top of the stab to mark the position of the fin. Glue the fin to the stab making sure it is at 90 degrees. Sand the stab saddle so that the stab sits parallel to a straight edge placed across the wing saddle. Glue the stab to the fuse, double check alignment & pin until dry.

Wing

Make a rib template out of Arborite/Formica or 1/16" ply. Cut 16 rib blanks out of 1/16" balsa. Pin rib blanks & template together & cut/sand to shape of template. Use a razor saw to cut out the spar & LE notches. Cut 2 wing tips out of 1/8" balsa and one 3/16" centre rib (see plan). Lay wax paper over the plans for the left wing panel & pin the 1/8" x 1/4" hard balsa bottom spar to the building board. Pin a strip of balsa to the building board to support the TE of the ribs so that the ribs lay horizontal. Align each rib over the plan & at 90 degrees to building board. Glue each rib to bottom spar with fast CA, using a little baking soda if necessary. Glue the hard balsa top spar to the ribs. Glue 1/4" square LE. Glue 1/16" x 5/8" TE strip to top of ribs. Glue 1/16" shear webs (grain vertical) to back edge of spars. Glue 1/16" LE sheeting. Turn the wing panel over and glue the bottom 1/16" TE and LE sheeting. Sand the rear edges of the TE sheeting with a long sanding block. Glue 1/4" square strip to TE. Sand LE and TE to contour of rib. Glue 1/16" x 3/16" cap strips to the outermost 5 ribs using fast CA. Glue 1/16" centre-section sheeting. Glue 1/8" wing tip to outermost rib and round the edges. Repeat above for right wing panel. Glue 3/16" centre rib to one wing panel and sand flush with sheeting. Cut opening for aileron servo. Carefully align both wing panels on flat surface and glue together. Spread a thin coat of epoxy over the center-section and wrap a 3" wide strip of fibreglass cloth from the top TE, around the LE and out to the bottom TE. Fill the weave with a thin coat of epoxy, wiping off any excess. Cut 1" wide ailerons out of 3/16" medium balsa. Drill 1/16" holes for the torque rods. Cut slots for small nylon hinges (e.g. DU-BRO #119). Do not use CA type hinges if you are planning to use only one micro-servo for the ailerons. The CA hinges are too stiff and provide too great a load for the one servo. Sand the LE of ailerons to a sharp edge to provide clearance for control movement. Taper the ailerons using a sanding block so that the TE is 1/16" thick.

Finishing and Hardware Installation

Carefully align the wing on the fuselage and secure with masking tape. Drill 2 clearance holes for the 6/32 nylon wing mounting bolts. The holes go through the wing and also the plywood mounting plates. Epoxy 6/32 blind nuts into the bottom of the plywood mounting plates. Do a final sand and then clean the model with a vacuum cleaner. Wipe down with a tack cloth and cover all surfaces with an iron-on light weight plastic film covering (I used TowerKote). Glue the 3/16" wing mounting dowel with epoxy. Mount the receiver switch on the fuselage top just behind F3. Mount small nylon control horns (e.g. DU-BRO #107) to the rudder and elevator. The elevator horn is attached on the right side on the bottom of the elevator. The rudder horn is attached on the left side of the rudder, just below the stab. Glue a torque rod into each aileron with epoxy. I used a DU-BRO #231 1/2A aileron linkage set. The wire supplied was very soft and I replaced it with stiff 1/16" piano wire. Attach all control surfaces and check for binding. The ailerons especially must move very freely. Glue the brass tube torque rod bearings to the wing TE with epoxy. Mount the Astro 15 with 2 4/40 bolts. Mount the receiver battery with velcro. Install the rudder and elevator servos and connect the pushrods (use outer holes on control horns). Mount the receiver and ESC with velcro. The antenna wire runs along the inside of the fuselage and out the air exhaust hole in the tailpost. The model should balance at the C.G. position marked on the plans. Move the motor battery to achieve the correct balance. Charge the batteries and check all works well. Install a Graupner 8 x6 folding prop. Check radio range with transmitter antenna down and motor running at half speed.

Flying

Set up your control throws as shown:

• rudder: 18 degrees L & R
• elevator: 5 degrees U & D
• aileron: 6 degrees U & 4 degrees D

The first flight went perfectly except I nearly hit myself & helper on landing. No trim changes were required. Despite the poor first landing, the plane is easy to land, is very aerobatic & has no bad habits. I normally fly at less than full throttle & can still do outside loops. Inverted flight requires very little down elevator. I'm still working on the knife-edge. Flight times are plenty long enough but I have never timed one. BTW, the secret to slow controlled landings (as taught to me) is to keep some power on until just (2 secs) before touch down. This way you can keep the nose up & still have control without stalling. It really works, I have had the plane going so slow under power that it was accused of being a helicopter. This is not a beginners plane but it is not difficult to fly. It goes where you point it and it is quite predictable. I hope you enjoy SparkE as much as I have. If you have any questions or comments please just send me an email.

David Mitchell Vancouver, B.C.
email: mitchell(at)infoserve.net