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Jul 18, 2016, 08:22 PM
If it flies I like it.
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Build Log

Pulldog Tow Plane

Pulldog Tow Plane Build

This all started when I began looking for a low budget tow plane for scale gliders that could be used on weekends when a few guys occasionally show up with their gliders and need a tow. There is a good selection of large tow planes available in kit and ARF form for 100cc and larger gasoline engines, but those offer much more performance than I needed. Not to mention the big gassers require a larger investment than my current modeling budget allows.

Some folks have had success converting models like the Telemaster and Sig Rascal 110, but to me those models have some drawbacks - such as a high mounted wing which results in mediocre/poor ground handing, especially in a cross wind; a horizontal tail which is not removable for transport; and a structure that wasn’t originally designed to accommodate a tow line attach/release. And if electric powered, significant modifications have to be made to allow battery packs to be easily removed and replaced.

Since I enjoy designing and building, I decided to construct my own tow plane to get what I wanted. A 3 View of the Pulldog is attached below, along with some photos. The Pulldog was designed from the outset for towing, and is also a good sport aerobatic model which can provide a lot of fun flying when not towing gliders. Some of the features are:
• Low wing position for improved handing, especially on the ground
• Removable wing panels with automatic servo connector hookup
• Removable horizontal tail (only one bolt) to allow transport in small cars like my Honda Fit
• Large fuselage hatch for easy battery and radio access
• Taildragger landing gear for simplicity and low maintenance
• Spoilers instead of flaps - results in easy, no-float landings
• Electric power for low vibration and low noise
• Simple structure and easy to build
• Large enough wing for reasonably good visibility at tow release altitude
• No plastic canopy, so no cracks to worry about
• Fuselage mounted Digital Readout Voltage indicator to keep track of the battery’s remaining capacity

Here are the specs:
Span 100 in
Area 1440 sq in
Length 70 in
Weight 14.6 lbs
E-Flite 160 outrunner motor
Castle Creations Phoenix Edge HV80 ESC
APC 20 X 8 electric prop
10S 5000 mah 25C LiPo battery (two 5S in series)
To date I have tested the Pulldog by towing Andy Grose’s scale Schweizer 1-26 and Scott Smith’s scale Aviation Concepts (Gunny’s) Schweizer 2-33. The day we flew the heat index was 104 degrees, so the density altitude was fairly high. All releases were at 1000 – 1100 AGL. It took just under 1 minute to get the 1-26 to 1000’; the Pulldog pulls this glider easily. 4 tows can be made before the battery pack is depleted. I have 4 battery packs, so I can have one on charge while the others are cooling off if need be.
Towing the big Gunny 2-33, which weighs 13.4 lbs, takes a lot more power, and runs the battery down a lot quicker. It took 1 minute and 20 seconds to get to 1000’. After 2 tows, I wasn’t sure if I had enough battery capacity left to make a 3rd tow. I think it would be worth experimenting with a different prop, like an 18 X 10, because the towing speed might have been just a little slow for the 2-33 using the 20 X 8 APC prop. The first tows with the 2-33 went OK, but later we had a couple of tows where the 2-33 got too far below the Pulldog and had to release, and I can’t say if it was due to pilot technique (tow pilot or glider pilot) or lack of speed. I plan to do more testing, but it’s just been too darned hot here to do any serious aerotowing, so that will have to wait a couple months.

Update on towing capability. After a couple years of towing various gliders I have a better handle on what the Pulldog can tow. Any 1/4 scale glider, except very large ones that are draggy (such as a 1/4 scale 2-33), is easy to tow, and gets to altitude quickly. Andy's Moskal 1-26 (built by SPasierb) weighs just over 11 lbs and is easy. Glass ships up to 15 lbs have been towed easily as well. The upper weight limit depends mainly upon how clean the glider is. Scott's big 2-33 taxes the Pulldog, but a higher pitch prop would probably make it easier as the 2-33 seems to want to tow a little faster. All other 1/4 scale gliders have been no problem to tow. One set of batteries (two 5S in series) is good for 4 tows with some margin left over - this is on 1/4 scale gliders, and glass ships up to 14 -15 lbs. After tow release this assumes power off until landing approach. Most folks who tow behind the Pulldog are surprised at its power, and how quickly it gets them to altitude.

To encourage building I am posting the plans here for free. Also, Manzano Laser Works has a short kit here:

That’s it for now. The next posting will start the build thread.
Al Clark
Last edited by HotdogX; Jun 26, 2019 at 03:33 PM.
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Jul 19, 2016, 10:20 AM
If it flies I like it.
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Stabilizer Construction

I started by building the stabilizer. This is built in the usual fashion. A 1/8 thick shim is required under the TE and aft ends of the ribs. Bevel the lower and upper edges of the TE to the proper angles before gluing. Make sure the block in the aft center section has vertical grain. Webbing is all vertical grain firm balsa 3/32 and 1/16 thick. Spar caps are spruce. After the false LE is glued, the frame can be unpinned and the false LE sanded down to the proper size and angles. Bottom of the D-tube is sheeted first, then pin the assembly back onto the building board, shim the TE, and glue the sheeting on top of the D-tube. After the glue dries unpin the assembly and add the TE sheeting and cap strips. Glue on the LE and sand to shape, then cut away the sheeting and LE between the S2 ribs, back to S1A. After everything is sanded the hinge slots can be cut with a #11 blade.
Elevator construction will be next.
Jul 19, 2016, 11:03 AM
Onward and Upward.
CatManDu's Avatar
Neat. This might be something to consider.

Jul 19, 2016, 02:47 PM
If it flies I like it.
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Elevator Construction

The elevator is a simple sheeted structure. Bevel the aft edge of the lower sheet, pin down, and add all the ribs. I use a straightedge at the front of the ribs to ensure proper alignment. Note the two ribs near the control horn location and the tip ribs are 1/8 thick. Bevel the 3/8 thick LE on top and bottom and install (I forgot to get a photo). Add the square balsa block between the E1 ribs and sand flush with the rib tops (sorry, forgot to take a photo). Cut the top sheet to size and glue in place. I recommend using epoxy on the TE as it provides some extra stiffness. Use a scrap balsa strip and lots of pins along the TE, and lots of pins along the LE. After the epoxy has cured unpin the elevator and clean up the edges with a sanding block. Mark a centerline on the LE, make the hinge slots, and bevel per the angles shown on the plan. Do a test fit to the stabilizer to make sure everything is OK. Round off the edges on the tips with the radius shown on the plan. Make the G-10 control horn and cut the slot (I used my scroll saw) to fit the control horn. Note the control horn slot is offset from the centerline! Glue the control horn into place.

Use a drill press if you have one to make the .193 diameter hole for the 10-32 hold-down screw. Before drilling put a shim under the TE of the stab so it is level. Countersink the hole to match the nylon screw.
Jul 19, 2016, 05:27 PM
If it flies I like it.
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Fin and Rudder

This is just a simple frame-up job. The fin post is made from two pieces of 1/8 X 1/2 spruce with 1/4 X 1/2 balsa between. This gives extra strength, is still fairly light, and allows hinge slots to be made easily. After the rudder is put together it gets sanded so the TE is 1/8 thick. The plywood reinforcing pieces for the control horn (one on each side) are inset flush with the surface by sanding some material away. Just like the elevator, draw a centerline on the rudder LE, cut the hinge slots, and bevel per the plan. Use the plan and control horn to locate, mark, and drill the holes for the control horn. Sand the fin LE to shape and round the edges on the tip of the fin and rudder.
Jul 19, 2016, 06:23 PM
If it flies I like it.
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Fuselage Sides

I thought Id add a photo first of the E-Flite Power 160 motor. Its the largest electric motor I have used and runs 2440 watts which is 3.25 HP.

Some preliminary work must be done before the fuselage sides can be built. Make some templates for the forward fuse sides by tracing or copying off the fuselage plan. Edge glue 5/16 balsa sheets together, sand, add templates with rubber cement, and cut to shape. Take care when cutting the holes so they are accurately located. Cut the scarf joint angles on the 5/16 square basswood longeron pieces, and cut the smaller basswood pieces that are located in the stab area. Also cut the 5/16 balsa gusset (one for each side) with the notch for F5. Pin the 5/16 balsa forward fuse side to the plan and add the basswood longerons and the basswood pieces that will support the stab. Glue in all the verticals and diagonals. Sand both sides flush.

The Lite Plywood doublers are in two pieces so must be glued together with epoxy before they can be installed. When cured glue each doubler to the inside of the fuse sides making sure to align them carefully. Use whatever glue you like for this. I used some large T pin guides to make sure the doublers stayed aligned as I set then onto the fuse side using CA+ glue. Using CA+ is pretty sporty and epoxy is probably a much safer choice.
Jul 19, 2016, 08:09 PM
If it flies I like it.
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Fuselage Formers and Landing Gear Mount

F1 is two pieces of 1/8 plywood laminated together. F2 and F3 are each laminated from one piece of 1/8 plywood and one piece of 1/16 plywood. The landing gear mount is laminated from two pieces of 1/8 plywood. Use whatever glue you like for the lamination. The tow release pin guide is installed at the top rear on F3. Blocks for servo mounting are installed onto the front bottom of F3. These blocks are .22 inches thick. Note the servo is installed inverted onto the front of F3. Use an aluminum arm (cut off to shorten the length) on the tow release servo and make up the linkage per the plan. Set the servo travel so the pin will protrude about 3/32 when engaged, and will drop below the fuse surface when retracted.
Jul 19, 2016, 11:38 PM
If it flies I like it.
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Fuselage Assembly

Before assembling the fuse, locate and glue the lower half of the servo rails to the doublers, and also the X battery plate rails. Note I did not install the battery floor rails at this point on the prototype because I wasnt certain of their exact location to achieve the correct CG, but the plans show the correct position so they can be installed at the same time as the lower servo rails. This is much easier than trying to mount them after the fuse sides are assembled. Install F1, F2, F3, and the triangular at F1 to the right fuse side. Glue on the left side and add the three balsa and one basswood cross pieces. Glue in the landing gear mount and the triangular pieces. Glue all the lite plywood pieces in place on the bottom. The two forward lite plywood pieces are edge glued with a joint doubler added before installation to the fuse.
Jul 20, 2016, 12:31 PM
If it flies I like it.
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Fuselage Assembly Part 2

Turn the fuse upside down and pin it to the plan top view (I used a centerline on my building board, but it’s better to use the plan). Place some 5/16 balsa scrap under the fuse at F1 so it will stay level. Drill out the two holes in F4 to 3/32 diameter. Pull the sides together at the tail and glue in F5 and the tailwheel mounting plate. Use squares to ensure the sides remain vertical. Add all the cross pieces and diagonals. Add the 3/16 square spruce (or basswood) corner braces to the tailwheel mounting plate Glue in the Phenolic wing rod tube. Install F4.

Slide the aluminum wing tube into the fuselage, place the stab on the fuse, and check that they are parallel; sand the stab mounting area if required. Make sure the stab is centered and drill through the two holes in F4, into the stab LE, with a 3/32 drill bit. Epoxy two 3/32 music wire pins into the stab. Place the stab back onto the fuse, check to make sure it is perpendicular to fuse centerline, and drill through F5 with a .193 drill bit, using the stab bolt hole as a guide. Remove the stab and re-drill the hole in F5 larger to fit a 10-32 blind nut. Install and glue the blind nut to the bottom of F5. Fit and glue the 5/16 balsa pieces that will make the slot for the fin to mount into (use the fin as a guide to get a good fit on the slot).

Cut to length and glue in the rear wing rod tube which is made from 3/16 O.D. brass tubing. I forgot to take a photo of this tube.
Last edited by HotdogX; Jul 20, 2016 at 02:34 PM.
Jul 20, 2016, 02:30 PM
If it flies I like it.
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Fuselage Assembly Part 3

Glue the two 3/16 balsa mounts to the ESC plate and install into the fuse as shown on the plan. Put a thin coating of epoxy on the plate so the Velcro will stick well.

After test running the Power 160 motor on the bench with the 20 inch prop I realized I needed to use an arming switch. This motor has a lot of power and could do major damage to persons or property with an inadvertent bump of the throttle stick. Fortunately Castle Creations provides a nice arming switch (for free, no less!) that works with a 4th wire on the HV80 ESC. Installation is simple. Bore a hole through the fuse side (location shown on the plan) that will clear the switch jack assembly. Make a 1/8 plywood disc that fits into the hole, and drill a hole in the disc for the jack to mount to. Glue the disc into the fuse flush with the outer surface and mount the jack. If all fits as expected, remove the arming switch for installation after the fuse is covered.

Cut some X spruce to length for the two servo rails. Glue them into the fuse using the upper half of the servo rail mounts to properly locate them. Install the servos in the exact positions shown on the plan.

Cut the 5/16 balsa fill-in pieces (see plan for template) and glue to the inside of the fuse sides in front of F1.
Jul 20, 2016, 02:39 PM
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awkward's Avatar
Hi Hotdog
What is the flying weight in Kg, and can you convert the motor size into equivalent CC rating
Jul 20, 2016, 06:29 PM
If it flies I like it.
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Weight is 6.6 Kg. Gas engine equivalent would be 30 - 35cc. It might be able to handle 50cc.
Jul 21, 2016, 12:29 AM
If it flies I like it.
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Fuselage Assembly Part 4

Canopy/hatch assembly is next. Edge glue 5 pieces of 5/16 thick X 4 wide X 5.375 long balsa, sand, then cut to final size which is 5.25 wide (check the fuse top width for the exact dimension) X 18.836 long. This will be the base for the canopy/hatch. Use a sharpened brass tube to make the holes in the bottom for the 1/8 thick X 3/8 diameter magnets, making sure these are accurately located.

Glue on the two 3/16 balsa canopy sides in the exact position shown on the plan. Add the 5/16 square balsa cross members. Edge glue six 1/8 X 3 X 5.375 balsa pieces together and sand the surfaces smooth. Bevel the front edge to match the plan and trim and bevel the aft edge so it exactly meets the 5/16 base sheet, then glue onto the 3/16 balsa canopy sides and cross bracing. Sand the edges flush with the 3/16 balsa canopy side pieces.

Check the fit of the 1/16 plywood tongue it should fit snugly between the fuse doublers at the front of the canopy/hatch. Glue the tongue to the canopy/hatch base. Cut a piece of 5/16 square basswood to fit snugly between the fuse doublers near F3, and glue to the bottom of the canopy/hatch base where shown on the plan. A good way to glue these pieces is to place them between the fuse doublers in their exact positions, add three small spots of CA+ to each piece, then carefully set the canopy/hatch base on top of the fuse making sure the edges are aligned with the fuse. Wait a couple minutes, pull the canopy/hatch off and finish gluing the pieces with CA.

Add the two 5/16 square basswood stiffeners to bottom front of the canopy/hatch, making sure they are located to fit just inside the fuse doublers. These stiffeners will need to be notched at the front to clear the 1/16 plywood tongue. Rough up the edge and one face of each magnet and glue them in place with epoxy. Check the fit of F3A to the fuse and glue in the two magnets, making sure they are oriented properly to attract the canopy/hatch magnets. Then glue F3A into the fuse.

Put the canopy/hatch in place on the fuse. Cut three pieces of 5/16 balsa to cover the top front of the fuse. There should be a gap of about 1/64 inch at the front and rear of the canopy/hatch. Edge glue the 5/16 balsa pieces, cut to length, and then glue them to the fuse. If you plan to use a digital readout voltage meter, cut a hole for it, located about over the top of the ESC mount. Dont forget to glue on the two scrap balsa fill-in pieces to the fuse sides at the top front.
Jul 21, 2016, 04:18 PM
If it flies I like it.
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Fuselage Assembly Part 5

Study the tailwheel mounting plate on the plan and then cut the 1/8 O.D. nylon tube slots. Check the fit of the nylon tube before gluing for the correct angle, then glue the tubes into the tailwheel mounting plate. The front end of the tubes will be glued to the fuse later after the cables are installed.

Cut the slot on top of the fuse for the rudder pushrod (see plan for location). Glue the fin into the fin slot (you can attach the stab temporarily to check for perpendicularity). Looking from underneath you will see a gap between the fin post and F4. Cut a piece of wide spruce to fit the gap and glue in place. Check fin perpendicularity again before the glue sets.

Make the rudder pushrod per the details on the plan. Cut down a large red servo arm to match the one shown on the plan. Install the pushrod, add the clevis and horn at the rudder and check the fit. Some slight bending of the aft 4-40 rod might be required for smooth operation.

Attach the tailwheel assembly to the fuse. Make the tailwheel steering cables per the plan details and attach so the tailwheel steering springs are stretched a bit. Now the front of the 1/8 nylon tubes can be glued to the fuselage frame so they align with the steering cables. Once everything is correct the rudder, rudder pushrod, and tailwheel assembly can be removed (leave the cables attached to the rudder servo arm and let the ends hang loose near the tailwheel).
Last edited by HotdogX; Jul 22, 2016 at 10:58 AM.
Jul 22, 2016, 11:59 AM
If it flies I like it.
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Fuselage Assembly Part 6

Make up the elevator pushrod per the plan details. Use a heavy duty servo arm on the elevator servo as shown on the plan. Use a Sullivan S591 4-40 aluminum ball connector on the aft end. Install the 4-40 ball onto the elevator control horn (ball goes on port side of horn). Attach the horizontal tail, hook up the elevator pushrod, and then adjust the length. I made up a simple tool that makes connecting the Sullivan aluminum ball connector a bit easier it is shown at bottom right on the fuse plan.

Make the tow release plate from 1/8 aluminum as shown on the plan. Cut a piece of 3/16 plywood to 5/16 X 1, and glue to the fuse top (see fuse side view and F3 details for position). Move the 3/32 diameter tow release rod to its extended position, set the tow release plate onto the rod, center the rod in the hole on the aluminum plate, and tack the aluminum plate to the 3/16 ply block with CA. Drill and tap the two holes for 4-40 screws, install the screws, and check for free movement of the tow release rod. Remove the aluminum plate before covering the fuse, and re-install later.

This is a good point to round off all the edges (except under the stab) on the fuselage to a radius of 3/32 1/8 inch.

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