Building A Wood Kit Trainer Part Three: Adding the Final Frame Touches
Building A Wood Kit Trainer Part Three: Adding The Final Frame Touches
by Frank Granelli
The rough framing was finished in Part Two of this series. The next step is to do all those final finishing touches to get the airplane ready for covering. At the end of Part Two, the airframe looked to be almost completely finished but looks can be deceiving sometimes.
Final assembly includes doing all those small jobs to get the airplane ready for covering. Part Three covers all those “minor” details such as attaching the stabilizer and vertical fin, installing the radio equipment and finishing the airframe.
Getting The Tail Ready:
Before the stabilizer is mounted, this is the time to get the elevator fitted. With the stabilizer still on the plans as shown, position the elevator in place. Most kit elevators are cut slightly longer than the stabilizer to allow for final fitting. This LT-40 is no exception to that rule. Align one end of the elevator to the stabilizer end and mark the other stabilizer end on the elevator. Then cut the elevator to fit the stabilizer as shown.
This elevator had a straight leading edge that required sanding to shape. Note the groove where the elevator meets the stabilizer in the second photo above.
Before sanding the leading edge, position the elevator against the stabilizer trailing edge and mark where the hinges will go. Use the Du-Bro hinge slot cutter to center the hinge slit in both of the mating edges. The hinges used in this airplane are the Mylar type as such hinges have proven extremely durable and trouble free in literally thousands of flights in competition airplanes that fly hundreds of flights a year for several years in their lifetimes.
This groove is actually evenly sanded on both sides as shown above. Use a 12-inch sanding block fitted with 220 grit wet/dry sandpaper for this task (I use the Great Planes series of different size aluminum sanding blocks as various photos will show). Mark the leading edge center line first, then sand to shape so that both “grooves” meet in the leading edge center. Sand slowly and gently, this is balsa wood after all and sands very quickly, requiring little force.
With the hinge slots cut in both the stabilizer and elevator, fit the elevator in place using the hinges. A good practice is to position a hinge where the elevator control horn is mounted for extra strength and durability. Move the elevator up and down keeping it tight against the stabilizer. It needs to be able to move at least 1.5 inches in both directions without pulling away from the stabilizer. If it cannot move this far, sand the elevator leading edge at a slightly greater angle. Remember to do both sides.
Once the elevator is fitted, remove it and the hinges and put them aside for now. Now is the time to fit and finish sand the stabilizer. Like many wood kit aircraft, the LT-40’s stabilizer needs to be cut at the leading edge where it joins the fuselage. Extend the cut line on the plans so it reaches past the stabilizer’s leading edge as shown. Then position the stabilizer on the plans and mark the cut line on it.
Remember all those pins used to hold the various wood pieces in place when the stabilizer was constructed? Each of those pins left a hole that must now be filled. Some of the wood pieces may not have mated completely, leaving small gaps here and there. These gaps must be filled before covering or they will show through the covering.
The holes and the small gaps can be filled using a standard light weight spackling compound. However, I prefer using Northeast Hobby Products’ Micro-Fill®. It seems to have a higher solids percentage than standard light-weight spackling compound making it easier to sand and stronger.
Apply the Micro-Fill using an old 1/2-inch chisel blade mounted in your hobby razor knife handle. Spread it smoothly and remove as much of the compound as possible while it is still wet. Then let dry.
There is another way to fill the pin holes and that is to close them using water and a hobby covering iron. Apply a drop of water, just a drop, to each hole. Wait about 30 seconds and then place the iron, set at 300 F onto the wet hole. The water immediately expands and vaporizes away, But in doing so, the steam expands the wood causing the hole to close. This does not work on actual gaps so any miss-fitting parts still need to be filled.
Once all the gaps and holes have been closed or filled, re-mount the elevator. Fit one sanding block with 220-grit and one with 400-grit wet/dry sandpaper. Sand both surfaces lightly; first with the 220 block and then follow-up with the 400 block to finish sand. The elevator is mounted to make sure that both surfaces are the same thickness after sanding.
Now, do the same with the vertical fin and rudder. After final sanding, both surfaces should be smooth have no pinholes or gaps. The control surfaces, rudder and elevator, should match their respective trailing edges exactly. I have found that 400-grit sandpaper is about ideal for finish sanding. Remove any sanding dust with a tack cloth.
I used to employ 600 grit and then 1200 grit sandpaper for this task. But then I learned a few facts. Most iron-on coverings will “hide” any surface irregularities smaller than 400-grit. More importantly, the covering can “crawl” on surfaces that are over-smooth, such as those sanded with 1200 or 1600 papers. That makes covering more difficult.
If you intend to cover your airplane with silk span and dope such as is still popular among Control Line professionals, then go out to 1600-grit or even beyond. But this airplane will be covered using plastic iron-on film so I stopped at 400-grit paper.
All except the trailing edges must be rounded off. First, draw a center line on all the leading edges and the side edges of the stabilizer. Draw the centerline lightly using a dullish pencil and try not to indent the wood. The edge above was inked and indented just for the photo. It was later filled to hide the dent and line. You don’t have to do it that way.
Then use a sanding block with 220-grit sandpaper to round off the edges. Use the centerline as a guide and keep the rounded edges centered and straight. While not extremely critical, having straight leading edges on the tail surfaces does make the airplane fly better. Once the leading edges have been rounded, use 400-grit sandpaper to finish sand them to smoothness.
Why all the sanding blocks? Wouldn’t hand sanding be easier and faster? Yes to both but no one can hand sand balsa wood without cavetating it no matter how good a sand demon you might think you are.
Here is one weight savings not available in any ARF or RTF airplane. I am concerned about tail weight in the LT-40 because I plan to use the O.S. Max .46 AX engine. This engine is very powerful and reliable but is also much lighter than many similar engines. It is definitely lighter than any 40-sized engine that was available when the LT-40 Kadet was designed so many years ago.
Keep the tail as light as possible by cutting lightening holes in the rudder and elevator. I used one of the ply fuselage cutouts as a template. Then use a No. 616 razor knife blade to cut around the template. This blade only has the cutting edge at the very vbottom. Place the dull section against the template and the cutting edge against the balsa. The knife blade will follow the template without cutting into it but will cut the balsa.
The weight savings may not look like much. As the scale shows, only 0.1 oz. was saved. However, that 0.1 ounce is all the way back there. It is, in fact, 35 inches to the rear of the proposed Center of Gravity (CG). That equates to a 3.5 inch ounce weight savings. Performing the same operation on the elevator yields a 0.2 ounce weight savings providing another 7.6 inch ounce weight savings (the elevator is 38 inches behind the CG). The total weight savings is 11.1 inch ounces.
The engine will be only 13 inches forward of the CG. Therefore, saving just 0.3 ounces at the rear accounts for almost a full ounce of nose weight that would have been needed for balance. Remember to keep enough wood in the control surfaces for strength once they are covered.
At first, I was not going to mention this next building tip. But it is extremely useful even if the airplane is already covered and receives a few dents under the covering. So, here it is but you must do this legally. The next time you are at your doctor’s office, ask for a diabetic syringe. Request just one, no more. Explain what you want to use it for. Most likely the doctor will give you several once you explain.
Explain what you may ask? Notice the wood is dented in the bottom of the rudder (Ok, I dropped something on it). You need the syringe to fix dents in your balsa wood airplanes! How? Simple, fill the syringe with water and inject some water into the dented area. Wait a few minutes and then apply a modeling covering iron set at 400 F directly to the dented area. The escaping steam will expand the wood and the dent will disappear.
This works even if the wood has covering over it and the tiny hole left by the needle in the covering is invisible. But make sure you acquire the needle from your doctor as it is a prescription item. Always keep it covered and out of sight/reach of any children and pets. Treat it like you do your very sharp hobby razor knives; keep it away from doing any possible harm to those not able to understand that such tools are dangerous at times.
Readying The Fuselage:
The fuselage servo tray is placed over the landing gear blocks so install the main landing gear block and its vertical pieces first. Use epoxy for this and make sure all surfaces are in full contact before applying the adhesive.
Test fit the servo tray in place as shown. It should be 2 inches rearward of the fuselage former just under the wing’s leading edge. Use 5-minute epoxy to insure a very firm joint. Position a few weights with the fuselage on a flat surface to insure a strong bond. If the tray comes loose in flight, elevator control becomes impossible and the result obvious.
Position the fuselage on a flat surface. Make sure it is sitting level and square as shown. Insert the plywood stabilizer mounting plate and clamp in place. Check that the plate is exactly parallel with the wing saddle and level with the rear fuselage sides. Then remove it, apply 12-minute epoxy and re-install the plate so it is exactly parallel with the wing saddle and in position. This is critical since a stabilizer that is tilted from the wing will apply a rolling moment with elevator input. Sand the rear fuselage sides if necessary but get the stabilizer parallel.
Put the “windshield” in place and then glue it with regular wood working glue rather than CAA or epoxy. Clamp in place until the adhesive is dry. Epoxy or CAA is hard to sand and the edges of this windshield must be rounded off to the fuselage. Wood glue sands easily yet forms a strong bond.
The forward hatch is held in the rear by a plywood plate that must be exactly centered on the hatch. The plate not only holds the hatch cover down but also centers it making a rear mounting screw unnecessary. But it itself must be centered for proper positioning. Use thick CAA for this task. Once mounted, drill the screw hole in the top of the firewall. Mount the hatch cover using a screw.
Once all the adhesives have dried, remove the clamps, check everything over and get ready to sand. First, fill in all the pinholes (the water trick won’t work on plywood) and any gaps around the joints. Then rough sand with 220-grit until everything is smooth and flows together. Make sure that the landing gear block is sanded flush to the fuselage bottom and any gaps are filled.
Use the sanding block to rough sand all the exterior fuselage surfaces. This includes the windshield and the hatch. If you used the bolt-on wing mounting system previously outlined, then cut very short pieces of the wing hold down dowels, insert and glue them into the fuselage holes that they would have filled if using the rubber band wing mount system. Cut the dowels just “short” to completely fill the hole and use filler to level the “repair” with the fuselage sides. Filler is much easier to sand that is a hard wood dowel!
After sanding the fuselage smooth, round off the edges with the sanding block. Do not forget the hatch cover edges.
Once all the edges have been rounded off using the hard block, use a soft sanding block as shown to make sure that the rounded edges are true. After all the sanding is done, switch to 400-grit sandpaper and give the entire fuselage a final finish sanding. Had this been a balsa wood fuselage, the final finish sanding would have come last. But this fuselage is basically a plywood baseball bat; strong, light, hard and very difficult to dent unless you hit it with a real bat.
Mounting The Stabilizer:
The Sport Aviator article, “Building an ARF Trainer Part II” has a complete description of attaching the stabilizer to the fuselage Just consider this stabilizer as an ARF that they forgot to cover. Measure from the stabilizer tips to a center point on the front of the firewall and side to side from the rear fuselage. All measurements must be equal. But above all else, make sure the stabilizer is in the same plane as is the wing.
The LT-40 Kadet uses a tube within a tube control system. While this works, it has always proven to be temperature sensitive and sometimes, unreliable. The temperature sensitivity means that the elevator and rudder trims constantly change throughout the course of a flying day. This can be annoying and does make things difficult for the new pilot.
The system requires that metal rods be glued into each end of the inner control rods. Usually, this is OK but sometimes, those tubes work loose. This can cause a loss of elevator control which is always attention getting.
The answer is to use the outer tubes provided but to substitute a metal pushrod for the inner tube. This airplane uses the 36-inch Sullivan Push rods No. 511. The Du-Bro 10B pushrods will also work.
Once the outer tubes are installed through the fuselage former holes, one problem becomes very evident. The elevator pushrod is higher than the servo’s output arm. The Kadet was originally designed when the servos where larger and higher than is now the case. While the pushrod could be bent downwards to meet the servo arm, it is better to raise the servo 1/4-inch using 1/2 x 1/4 spruce blocks as shown. The elevator rod now runs straight back to the control horn without bends for the tightest control possible.
It is a good idea to use scrap balsa from the kit to make internal rod braces at each former as shown. This stiffens the pushrods even more for very precise control. After the braces are installed, apply some medium CAA where the tube enters each brace.
After installing both servos, insert the pushrods into their respective outer tubes. Attach the elevator to the mounted stabilizer. Attach a clevis and a control horn to the rear threaded end. Use the pushrod assembly to position the elevator control horn on the mounted elevator. Make sure that the control horn is in the middle of the fuselage to insure that the control rod has a perfectly straight run to the control horn and that there is no friction when the rod is moved.
Normally, the stabilizer is covered later along with the fuselage and vertical fin. But the Kadet is a little bit different. The vertical fin is mounted almost completely on top of the exposed stabilizer. The fuselage does not cover the stabilizer top at all. If the stabilizer were covered at the normal time, you would have to either cut a slit in the top covering where the vertical fin is mounted before mounting the fin or use two covering pieces which would show.
Now is the time to pick your fuselage colors. This airplane will be covered in Cub Yellow and Missile Red Monokote® with black trim. The colors are in honor of a late friend of mine, Al Bossler, who taught me much about RC flying in the 1970’s and whose airplanes were always in yellow and red. Now that is something you just can’t do with an ARF!
Covering an airplane will be handled mostly in Part Four of this series. But here is a quick version of covering the stabilizer. First, set the iron temperature to 275 F. While the iron is heating up, cut a 1/2-inch wide piece of either the fuselage covering or the stabilizer covering whichever is the lightest color. Make it long enough to cover the stabilizer’s chord where it meets the bottom of the fuselage. Fold the strip in half and then iron the crease into the corner formed by the stabilizer and the fuselage.
Cut a piece of the stabilizer covering large enough to completely cover one stabilizer half with at least an inch overhang on each side. Place the covering over the small piece already ironed in place on the fuselage/stabilizer joint so that it is flush against the fuselage and covers the small fillet piece already in place. Apply heat only to the center as shown. Do not apply any heat except to the center 1/4-inch of the covering at the fuselage joint.
Pull the covering tight at the center of the stabilizer tip and tack that down with the iron. Then pull the covering taught at a 45 degree angle frontwards and outwards at the front of the stabilizer at the fuselage joint and tack it in place. Do the same for the tip front. Repeat for the rear tacks.
Grab the center front of the covering, pull it forward and tack it in place at the leading edge. Now do the same for the trailing edge. The covering should be very tight at this point. Now use the Rule of Halves. Go halfway between each tack point, pull the covering taught and tack in place. Continue until the covering is tight and the tack points are less than an inch apart.
Then iron down all four edges completely. All three edges should be completely covered and ironed firmly in place before trying to shrink the covering. Trim off the excess covering with a very sharp razor hobby knife. Tack down the trimmed edges. Then use the iron to shrink the covering. If the wrinkles do not disappear completely, that means an edge is crawling or loose. Re-iron all the edges. Do not try to remove stubborn wrinkles as that could ruin the covering. Make sure the edges are firm before final shrinking. Repeat for the other stabilizer half’s underside.
Do the same for the stabilizer top using one piece of covering instead of two. Make sure the covering overlaps the bottom leading edge by 1/4 inch. The covering should extend over the fuselage top a little as shown. After the covering is tightly shrunk, position the vertical fin over the covered stabilizer, making sure it is straight.
Mark the middle of the fuselage at the front of the vertical fin. Align the center of the front vertical fin with this mark as shown. Mark the center of the stabilizer at the fin’s rear and align the rear of the fin centered on this mark. Trace the outline of the fin onto the covering.
Remove the vertical fin. Cut down the center of the covering and pull back both sides of the covering that is under the fin’s position on the stabilizer. These will form the stabilizer to fin covering fillets like those on the bottom side. The fillets seal the wood against any fuel residue that might eventually creep under the covering at these joints.
Re-position the vertical fin onto the fuselage and stabilizer. Make sure it is perfectly straight using the marks you previously made. Look down the fuselage to insure it is straight. This is critical so get it right or your airplane will need to fly with an offset rudder. Yuck!
Use 12-minute epoxy for strength and working time. Glue the fin in place and again make sure it is straight. Clamp in place as shown. Use the metal triangles to insure the fin is 90 degrees to the stabilizer.
Install the rudder and run the pushrod back to it as was done with the elevator. Position the control horn, mark it and mount it in place on the rudder. The rudder and elevator will be covered in the Part Four. For now, the fuselage is done. Installing the fuel tank, throttle control rod, remounting the engine and putting the gear in place are all best done after the fuselage is covered.
Finishing The Wing:
The wing was almost completed in the previous 2 parts. However, it still needs to be finished sanded and covered. But before that happens, the center fiberglass reinforcing tape must be applied. Most ARF and RTF aircraft manufactured today use aluminum or steel tubing to reinforce the wing joints. But this is heavy and not needed when the airplane is constructed by the pilot.
Instead, fiberglass tape is used and then the wing is covered. The stock kit uses a 1-inch piece of fiberglass tape for this vital task. However, this wing is a bolt-on and that means that there are no rubber bands to reinforce the center joint as was originally intended. It just might be possible that the bolt-on wing would need more strength in the center to compensate for the lack of rubber bands.
Before applying the tape, fill in any pin holes and gaps using the wood filler. Rough sand the wing center section. If all is well, finish sand the area that will be under the tape and at least an inch past its position using the 400-grit sandpaper block. Finishing sanding is necessary as the tape forms the final undersurface for the covering so it must be smooth and dent free.
I choose my ole’ reliable; the 3-inch fiberglass reinforcing tape for this airplane. I know this system works. It has proven itself over the last 40 years. Put the tape in the center of the wing. Spread the tape outwards from the center as shown.
Use thin CAA to adhere the spread tape to just the wing’s trailing edge. Then pull the tape over the top of the wing, under it and then back to the trailing edge from the underside. Pull it tightly so that the fiberglass conforms to the wing’s surface without wrinkles. Apply the thin CAA to adhere the tape back onto itself on the trailing edge.
Now you know why the front wing dowel was not permanently glued in place in the previous two parts. Place pieces of low-tack masking tape just at the tape edges to protect the wood from the epoxy.
Mix up some 30-minute epoxy in a cup or mixing cup. Then apply the epoxy to the tape brushing outwards. This removes any final small wrinkles. Apply a liberal amount of epoxy but not enough to drip off the edges or flow onto the protecting tape.
Give the epoxy 2-3 minutes to soak into the tape and the wood. But don’t wait too long. Use some old playing cards to scrap off most of the epoxy. The correct amount of epoxy just fills the cloth’s weave without excessive buildup.
Once the scraping is done, remove the protecting tape. This allows the epoxy at the very edge to flow slightly towards the wood making the tape joint difficult to detect once covered.
Now turn the wing over and do the same to the underside while the top section is still wet. Pull it tightly against the wood and spread the epoxy outwards to eliminate wrinkles. Don’t worry about covering the bolt or front dowel holes as those will be cut out later.
The aileron servos are installed after the bottom of the wing is covered. That way, the wires can be pulled through the wing front the still-open top. The ailerons are also installed after the wing is covered just as in building an ARF airplane.
Covering is the next step in Part Four. Part Four will also cover mounting all the hardware, simple as that was done during building, installing the fuel system and control rods. Part Four will also cover the initial test flights and trimming. Stay Tuned.
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