Profile T-28 Construction Article

Build this exciting profile scale sport model by Paul Bradley. 34" wingspan model for Speed 400 power systems. A great small field flier. Full size plans available. Plans available via the E Zone's on-line plans service .

T28 from right front

MODEL DATA SHEET:
  • Wing area: 195 sq. in.
  • Span: 34 in.
  • Airfoil: 8% flat bottom with raised entry
  • Weight: 16 oz.
  • Wing Loading: 11.8 oz/ft. sq.
  • Motor used: Speed 400 7.2v or 6v
  • Number of cells: 7 (6v motor) or 8 (7.2v motor)
  • Propellor: 6x5,  or 5.5x4 (6v motor)
  • Watts/pound: 80 to 90
  • Functions: Throttle, Ailerons, Elevator

 

INTRODUCTION

What do you do when you are planning to attend your very first electric fly-in, you only have five weeks left before the event, you don’t have a plane you feel is suitable to take to the fly-in, and you are infected with a strong desire to design you own models? That was my situation prior to the 1997 DEAF annual fly-in. While contemplating this dilemma I decided to do a little reorganization of my shop to get ready to build something, my own design or not. As I was moving things from one place to another, I happened on a storage box containing several of my No-Cal free flight rubber power models. These are profile models (No-Cal meaning no calories as in profile) with a 16" wing span. The light went on. Why not scale one of those designs up to a size suitable for a Speed 400 and make it into an electric RC model?

After studying the different models, I decided to use the T-28 plan. This plan, along with seven others, were presented in the December 1996 issue of the E Zone. You can find that article by looking at the E Zone article archive (No-Cal Article). Several factors went into my decision to use the T-28 plan. First, it seems that you don’t see the T-28 modeled quite as often as other war birds. It also offered fairly simple lines and straight dihedral. I reasoned that those factors would make the construction and development of this project a bit easier given the relatively short time frame I was working with. What I ended up with is a model that is just a bit more than double the size of the No-Cal T-28. For the sake of watching weight, I limited the controls to throttle, ailerons, and elevator. Flying performance proved to be better than I had hoped for, and the Speed 400 profile T-28 is a blast to push around the sky.

The model builds fairly quickly. The only component that requires much work is the wing which uses a simple built up sheet skin structure as opposed to foam cores. It was felt the time to cut the cores and sheet them would be about the same time required to build the structure as designed. Also, I don’t have any foam cutting equipment (the real reason). Overall I found the T-28 to have a great flying enjoyment to build time ratio.

 

CONSTRUCTION

t28_plan.gif (12574 bytes)
Full size plans can be ordered from the E Zone's on-line plans service.

Getting Started - Materials

Like any electric powered model airplane project, weight is always a primary factor. This is especially true for Speed 400 powered models. The T-28 uses an all sheet wood construction, so wood selection is particularly important. My design goal was to hold the finished airframe weight to something in the 4 to 5 ounce range. This was not too hard to achieve by careful selection of the wood stock. A bill of material is presented in the following table along with the recommended weights. Generally, if you select contest grade wood, you should be able to find wood at the recommended weights, and you will have no trouble keeping the finished airframe at a weight that will give good flying performance.

Material Quantity Remarks
1/4" x 3" x 36" Balsa Sheet 2 40 gr or less per sheet
1/8" x 3" x 36" Balsa Sheet 1 22 gr or less
1/16" x 3" x 10" Balsa Sheet 1 Medium Hard
1/32" x 3" x 36" Balsa Sheet 4 5 to 7 gr per sheet
1/8" x " x 36" Balsa Strip 1 Medium Hard
1/8" x 1/4" x 36" Balsa Strip 1 Medium
1/4" x 1/4" x 36" Balsa Strip 2 Medium
1/16" plywood (2" x 3")   Not lite-ply
1/4" x 3/8" x 5" Hardwood    

Fuselage

Probably the best place to begin is the fuselage. Some of the 1/4" sheet stock that is left after cutting out the fuselage profile will be used for the wing, so it is best to start building at this step. Needless to say, there is not much to building a profile fuselage. The only real trick is to layout the 1/4" sheet stock so that enough material will be left for the wing strip ailerons, wing tips, and the wing root ribs. This is done by first glueing the two sheets together along their length. I like to do this by cleaning up the edges to be glued, and then holding the sheets together along the entire length of the joint with masking tape. The taped joint is then opened for application of the glue (Titebond in this case), and then closed. Any excess glue is wiped away and then the sheets are laid flat on the work bench and weighted down to dry. When that is done, transfer the fuselage profile to the 1/4" stock. You will find that the very top of the canopy outline will not quite fit on the sheet. Simply cut a portion of the sheet just ahead of the canopy outline and above the nose outline and glue that to the area where the top of the canopy falls. This will then allow the complete fuselage profile to be drawn. The photo shows how this is done and also shows where the wing aileron and root rib material can be taken from the remaining 1/4" sheet stock.

Fuselage Layout

After the fuselage profile is cut from the 1/4" sheet stock, the outline is sanded to make sure it is smooth. Next the hardwood motor mounts are glued in place. I used 1/4" x 3/8" maple stock that is available in the hobby shops. I also used Titebond at this step. When the glue has dried, the fuselage is finish sanded. This involves tapering the rear of the fuselage in the stab area to 1/8". This allows the 1/4" fuselage to flair nicely into the 1/8" rudder. The edges are also rounded.

The fuselage construction is completed by cutting the motor mount from 1/16" plywood. A pattern has been provided on the plan for this. Be sure to use regular aircraft plywood as opposed to lite-ply. An alternative, which is what I used, is to use one of those great laser cut Speed 400 mounts available from Tim McDonough  (the square ones). The plan pattern is aligned with one of the diagonals between corners on the square mount, and the mount trimmed to fit. The completed motor mount and Speed 400 installation is shown in the photo.

T28 motor mount

Tail Surfaces

As long as we are cutting components from sheet stock, we might as well go ahead and cut out the fin/rudder and stabilizer parts. They are cut from the 1/8" sheet. Like the fuselage, the sheet will have to be cut and joined to allow the tail surface outlines to be completely formed. This is done by using the outline patterns as a guide in sectioning the 1/8" sheet stock. The sheet joints are made using the same technique as with the fuselage sheet joint. Please note the  grain for the dorsal fin.

Once the tail surfaces have been cut from the sheet stock, they can be final sanded Both the fin/rudder and the stabilizer are sanded to a streamlined shape. This is done more for aesthetics than for any real functional purpose. It does save a little weight, but I think I just looks better.

T28 tail surfaces

For the elevators, I chose to follow the scale separation lines. I first cut the elevators from the stab as a one piece unit. I then cut the individual elevators from the single piece following the lines in the center that allow for fuselage clearance. The center piece that is created will be glued back on to the stabilizer once the piano wire link for the elevators is in place. After the elevators have been created, the leading edges are rounded as is the trailing edge of the stabilizer. A link is bent from 1/16 piano wire per the plan. The elevators are then aligned with the stabilizer so the position of the link can be marked on each elevator. The elevators are then drilled to accept the link. Do not actually glue the elevators to the link at this time. Now take the center piece from the elevators and cut a relief in the edge that will mate with the stabilizer so the link can pass through without binding. The center piece is next glued to the stabilizer with the link in place. Put a little Vaseline, or similar product, on the link where it passes through the center piece to prevent it from being glued in place. The elevator control horn is cut from 1/16" plywood per the plan pattern.

Wing

All we have left to do is build the wing. This is pretty straight forward, and does not eat up too much time so lets get started. Begin by cutting each of the four 1/32" sheets in half ... that is into 18" long sheets. We will need to make up four sheets that are 6" wide by 18" long. This is done by joining pairs of the 3" by 18" sheets along their length. The same masking tape and glue technique used before is used here. The only difference is that I like to use CA on the thinner sheet material. I do this by laying the sheet flat on my building surface after in has been taped. I then wick thin CA along the joint. It does not take much as the CA will be drawn along the joint for a reasonable distance. You just need to hit the joint every so often along the entire length. After pulling the tape, I like to sand each side lightly with 220 grit paper. After making up each of the wing skin panels, we can move on to the actual wing assembly.

The wing is built on top of the bottom skin. Begin by transferring the outline from the plan as defined by the root line, trailing edge, tip line where the balsa block will attach, and the inside edge of the leading edge strip. In order to get the full outline at the root, you will have to cut a bit of the sheet stock off a the tip area and glue it to the root area. After each lower skin is cut out of the sheet, mark the location of the spar and each rib. Be sure to select the worst side of the skin for the surface you will be drawing the lines on. This will be inside the wing and will not matter if there are a few blemishes.

T28 bottom wing skins with reference lines

Make up the spars per the plan. Note that the spar begins its taper at the W2 rib location. Once the spars are ready, they can be glued to the prepared bottom skins. Use the line drawn on the skin in the spar location to locate the spars. I use CA for this joint. I just position the spar on the line and while holding it down, wick some thin CA along the joint.

T28 bottom wing skins with spar installed

We will next add the trailing edge stock and the sub-leading edge strips. These are glued to the bottom skins using the edge of the skin as a guide. I used CA for these joints as well. Again I simply held the stock down flat against the skins, and wicked thin CA along the joint. This all goes pretty quick. As you can see, the lower skin has become the plan and we are gluing directly to the skin surface.

The next step is to cut out the ribs. Cut them as one continuous rib even though the spar separates the rib into two parts. After all the ribs have been cut, we need to make up the two root ribs by gluing two pairs of the 1/4" stock W1 ribs together. I use thin CA to make this task quick and easy. Each rib is then cut and fit into position. Cut the ribs at the spar location and glue each half at the appropriate location. Please note that the leading edge of the ribs are slightly raised. I used scrap 1/32" material under the leading edge of each wing panel to properly position the skin before gluing in each rib. I used CA to glue the ribs to the spar and lower skin. After the ribs are in place, we need to prepare each wing half to receive the top skin. Sand the sub-leading edge and the trailing edge stock to be even with the ribs and to match the slope of the ribs. If you use a long T-bar type sander, this task is greatly simplified. Check to be sure the ribs are flush with the spar as well. If not, go over them with your sanding bar to clean everything up.

T28 bottom wing skins with rib and spars

Before we close up the wings with the upper skins, it is necessary to add a small amount of weight to the wing tip that is on the opposite side of the battery pack. For me this was the left wing tip as I placed the battery back on the right side of the fuselage. I found that 4 to 5 grams will provide the proper lateral balance when either an 8 cell or 7 cell pack is used.

The next step is to cut out the top skins. Make them about 1/8" to 1/4" wider along the chord than the bottom skins. This will allow for the top camber, and will allow easier positioning during assembly. Position each wing panel on your building board. Next stick a pin about " away from the wing edges at each rib location. These will be used for rubber bands that will hold the top skin in place while the glue dries. We are now ready to add the top skins. I used Titebond for this part of the assembly. Run a bead of glue along the spar, the sub-leading edge, trailing edge, and each rib. Place the panel on the board inside the previously located pins. The 1/32" scrap stock should still be under the leading edge of each panel. Place the top skin on the panel. Allow the edges to overlap just a bit. Now run rubber bands from one pin to the other at each rib location. This will bend the top skin to conform to the ribs and hold everything in place. If necessary, you can supplement the rubber bands by pinning through the top skin into the edge stock.

T28 wing on board with top skin installed
Use rubber bands when gluing on the top sheeting

After the top wing skins have dried, remove the panels from the building board. Using a long sanding bar, sand the edges to get rid of excess material, and to provide a clean straight surface. Now glue the 1/4" square leading edge stock to the leading edge of each panel. Shape the leading edge using your long sanding bar. The plan shows the ailerons to be set up for "skin" type hinges. This makes it possible to camouflage the upper surface with panel lines for the flaps and scale ailerons and hide the strip ailerons. If you use this approach, now is a good time to sand the trailing edge bevel. Do this along the entire length of the trailing edge. We will fill in the portion at the root a bit later.

Using some of the scrap 1/4" stock, make up some tip blocks by laminating two pieces together. Make two such assemblies. Cut the tip shape to the outline as shown on the wing plan. Glue the tips to each wing panel. Sand them to the contour of the tip rib. Just be careful not to cut into the wing skins. Finish off the tips by rounding them off.

Once the tips are in place and shaped, we can fit the ailerons. The aileron stock that was cut from the 1/4" sheet needs to be sanded to a triangular cross section to conform to the airfoil. They will be a little thicker than the wing trailing edge after the basic cross section shaping. This will allow for final fitting to the wing. Trim each end of the ailerons to fit next to the wing tips. Now using masking tape, tape each aileron in place next to the tip along the bottom of the wing trailing edge. With each wing panel laying flat on your building board, you can now final sand the top of the ailerons to blend with the wing upper surface. As before, be careful not to cut into the wing skin. Now trim any excess aileron length at the root rib. Mark the location of the aileron separation that occurs near the fuselage as shown on the plan. Remove the ailerons from the wing and cut them at the fuselage separation line. The short pieces are now glued to the wing trailing edge at the root. The gap on the bottom that is created by the tailing edge bevel is filled in with scrap balsa and sanded smooth. Also, sand the bevel on the leading edge of each aileron. If not already done, this would probably be a good time to cut out the aileron control horns from 1/16" ply using the pattern on the plan.

The final step in assembling the wing is to join each panel together. We first need to sand the dihedral angle at each root rib. I find the easiest way to do this is to first tape a sheet of medium grit sandpaper down to the work surface that aligns with the edge that will be used for the sanding guide. Place the wing panel to be sanded on the sandpaper with the root even with the surface edge. Block up the tip 1". While holding the wing panel down with moderate pressure begin to sand the root while holding the sanding block/bar vertical against the surface reference edge. By using downward strokes the wing will stay in place and hold its position. When each panel has been sanded for the dihedral angle test fit the joint. You want a good joint here since this will be the focal point of the wing bending loads while in flight. When satisfied with the joint, tape the wing halves together along the bottom surface of the dihedral joint. Open the joint and add the glue of your choice. I used Titebond for this joint. Close the joint and wipe away any excess glue. Lay one panel flat on your work surface and block the other tip up 2". Place a little weight on the panel laying on the work surface and let the joint dry. With a good joint you do not have to add any reinforcement to the joint, it will be plenty strong. That completes the basic wing construction.

T28 wing - sanding dihedral angle

Aileron Control Linkage

Since the T-28 has a fairly thin wing, I decided to mount all of the radio components on the fuselage. My trial weight and balance calculations and subsequent tests on the prototype indicated the servos would need to be placed behind the wing trailing edge. This required a little different approach to linking the aileron servo to each aileron. I’m sure there are many different ways to approach this situation, but I chose to use the design shown on the plans. It proved to be very easy to make, and works very well. The heart of the mechanism is an unused servo output whee or arm. Drill the center hole of the servo wheel to accept a brass eyelet of the type supplied with servo mounting kits. The servo wheel should be able to rotate freely on the eyelet, but not have any slop. Next sand the base of the servo wheel center hub until the brass eyelet just protrudes when inserted from the top. Use a servo mounting screw to attach the servo wheel/brass eyelet assembly to a length of 1/4" dowel. Tighten the screw so the brass eyelet is tight against the dowel, but the servo wheel can still turn freely. Depending on the configuration of your servo wheel (or arm), it may be necessary to drill new holes for the aileron linkages. You will also need to drill a hole for the servo push rod right next to the right aileron link hole. The set-up shown on the plan will produce mild throws. The plane will have good response with the mild throws, but if you want something more it is necessary to use a servo wheel (or arm) that is larger in diameter. When this assembly is finished, it is mounted in the wing center section by drilling a 1/4" hole in the wing center section at the location of the relief in the fuselage. Test fit the assembly, remove any excess length of 1/4" dowel, and cement the aileron control link in place.

T28 aileron control parts             T28 aileron control installed in wing

Finishing Details

Since the T-28 has all sheet wood surfaces, and also since it is electric powered, there are a lot of options available for the finish. The T-28 has many different color and markings available. I chose one that would be fairly easy to duplicate in terms of base color and the overall markings. My reference source is a Squadron/Signal publication which covers the T-28.

If you will be using a covering material on the surfaces, I found that applying it before final assembly made the process much easier. I chose to use tissue and dope to finish my model. This was done not out of a concern for weight, but because I had already developed templates for the No-Cal version that makes it possible for me to print all color and markings directly on white tissue using an ink jet printer. Like the model plan, I just scaled up the tissue printing templates and I was ready to go. Because I was not using a film type covering, for the aileron "skin" type hinges I used 1" wide strips of clear mylar held in place with Balsaloc. The ailerons were installed before the tissue covering so I would get a good bonding surface, and to hide the hinges under the tissue.

For this model I did not use the super light Japanese tissue typically used on free flights, but rather "art store" tissue, It is a little bit heaver, but has more body and is less translucent which gives better color density when printed. To apply the tissue, all surfaces were given four coats of clear dope that was thinned 50% with thinner. Each coat of dope was sanded with 400 grit paper. The tissue panels were then laid dry over the appropriate surface. Thinner was brushed through the tissue to soften the dope. With just a little rubbing with my finger, the tissue adhered to the surface very nicely. This method makes positioning of the pre-printed tissue panels very easy. When all the tissue was in place and trimmed, the model was assembled. Two top coats of the same 50/50 clear dope were then applied to complete the finishing process. Each coat was lightly sanded with 400 grit paper.

If you are considering building this model and would like to try the printed tissue covering, I can provide a complete set of 8 tissue panels with the markings you see in the photos for $10 plus shipping (to be determined by your location). My mailing address and e-mail address are shown at the end of this article, so just let me know if  I need to fire up the old ink jet printer.

T28 from left front

Equipment Set-up

Needless to say, the installation of the equipment will depend on the specific items you will be using. I have shown on the plans the basic location of the components I used. These consisted of an FMA 2000 receiver, an 8 cell battery pack, an FMA S80 servo for the elevator, and an FMA S100 (Voltz) servo for the ailerons. I also used a Sprite 20 for the ESC. The ESC is not shown on the plan since there can be so many different possible configurations. The real joy of a model of this type is the complete flexibility on locating the equipment to hit the desired CG location. The battery pack can be moved fore and aft over a large range to handle just about any balancing need. To attach the receiver and battery pack, I used a system that was learned long ago when flying control line models. In those days in order to secure the fuel tank on profile models, we used rubber bands wrapped around a simple wire hook that passed through the fuselage. That is the method of attachment shown on the plan for the battery pack and the receiver. The battery pack gets two sets of hooks and the receiver one. It is real easy to move the heavy battery pack and lock it down with this arrangement. It is also very secure during flight maneuvers. The ESC is held in place with a Velcro patch. Depending on the wiring arrangement for your ESC, it may be necessary to cut a pass-through hole in the fuselage for the receiver lead.

One additional equipment installation note: to deal with the usual problem of the receiver antenna location, I clipped the factory antenna 4" from the receiver. I soldered a female plug from a Radio Shack DB-9 computer cable connector kit on the end of the 4" antenna wire. I then cut a piece of magnet wire to the length of the antenna that was cut off. A matching male plug from the computer cable pin kit was soldered on one end. With the receiver installed, the magnet wire antenna was plugged in. The loose magnet wire was then routed along the bottom of the left wing surface near the outer edges. It was then glued in place with CA. The male plug was also glued to a small standoff block so it would not flex and fatigue the joint. This made the antenna nearly invisible, allowed the receiver to be removed at will, and did not affect the receivers range performance.

T-28 battery pack installation    T-28 receiver installation

FLYING

I have set up the T-28 with two different Speed 400 motor configurations. My preferred set-up is the 7.2v motor, 8 cells, and a 6x5 prop. The model was also flown with a 6v motor turning a 5.5x4 prop on 7 cells. The latter prop was selected to give about the same current draw as the 7.2v motor prop/cell configuration. Both power arrangements give good results, but the 7.2v motor set-up is just a bit better for the current draw. In that configuration using 8 500AR cells, I am able to get 3.5 minutes at full throttle.

The model is easy to hand launch. The fuselage area just below the wing provides a nice grip. The T-28 is neutrally stable. That means you do need to fly it all the time. It will go where you point the nose except for sustained vertical. The model will loop easily from level flight. Rolls are fairly quick and do require just a bit of down elevator to hold the line. Inverted flight is easy with just a little down elevator. It lacks a rudder, so some maneuvers are limited. Once you notice the power going away, you should have plenty of time to set up your landing.  The T28 has a nice medium glide with the power off. This makes it easy to set up your landing approach with little or no power. Just keep the nose down a bit during the approach to avoid stalling and the model  will settle in for a nice smooth landing on the fuselage bottom.

I have really enjoyed flying my Speed 400 profile T-28. If you build one, I hope you will be equally rewarded.

T-28 in flight

 

Paul Bradley
BradleyP(at)ix.netcom.com
4603 Westerdale
Fulshear, Texas 77441
281-346-1874
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