Building A Wood Kit Trainer Part Four
Covering, Preflight, and Flight Trimming
by Frank Granelli
For many newer RC pilots, the transition from that pile of wood and plans to a functioning airplane is an untried challenge. More experienced modelers know the rewards of building their own aircraft. They talk about the thrill of creation, having the satisfaction of flying something they built themselves and about the fun of having an airplane with a different appearance that they designed.
But not many consider that, by building your own airplane, the pilot ends up with a vastly superior aircraft over an ARF version of the same airplane. During each construction step, this series of articles has attempted to discuss just this aspect of why building your own can be so important. The improvements made in this airplane include dual aileron servos, bolt-on wings, larger fuel tank, nose weight elimination, stronger more precise control linkages, flaperons, coupled elevator-flaps, tip weight reduction and a stronger fin joint.
The wood kit’s all plywood fuselage is also much stronger than the ARF’s balsa/ply version. The all-plywood construction is 14 ounces heavier however. But since the wood kit’s wing has more area, 921 sq. in. versus the ARF’s 900, the final wing loadings are about equal – 16.35 oz. /sq. ft. for the wood kit versus the ARF’s 16.97. If anything, the Wood kit airplane should float more and land slightly more slowly. We’ll see.
The wing is easier to cover than the fuselage so we will start there. Make sure that the servo wire exit holes have been cut into the center wing bottom and that the nylon bolts fit through the mounting tubes. Then get out your covering tools.
While there are numerous covering tools available, these are the ones that have proven most useful. Note that whichever covering iron is chosen, make sure to install a “sock” on it to prevent covering scratches. The red handled tool is the Top Flite “Woodpecker” used to make tiny invisible holes in the wood prior to covering. These tiny holes both increase the covering’s grip on the wood and help eliminate bubbles and loose areas that will later result in wrinkles during humidity changes.
Most importantly, have at least 30-40 new No. 11 razor knife blades on hand for cutting the covering. The covering wears out the blades very quickly and it is essential to use a new blade about every 10-12 cuts.
Another useful tool to have is the Coverite temperature gauge to insure that your iron’s temperature, with the sock installed, correctly matches the temperatures needed for the various covering installation steps. This tool may be hard to find these days but there is a more modern alternative in the numerous remote infrared temperature measuring tools.
One of the best sources of covering information I have found is Top Flite’s “Secrets of Great Covering with Top Flite Monokote” DVD (or tape). I learned much from this source even after 25 years of covering and still refer back to it occasionally.
As for temperatures, the best Monokote tacking temperature has proven to be around 275 degrees F. 300 degrees F is great for stretching the covering around wingtips and other curved surfaces. 325 degrees F has proven to be the best shrink temperature. Do not use this high temperature for tacking as the covering starts to crawl on the surface before the adhesive can get a good grip. Use the lower temperature first and shrink later at the higher setting.
Use 1/2 inch heat shrink tubing to secure the servo extensions to the aileron servo wires. Then final sand the wing surfaces using dry, 400 grit wet or dry sandpaper. The major shaping was done in Part Three. This sanding is just to smooth the surface. A note of experience here: I used to use up to 1200 grit paper for that extra smooth finish when painting over silk. But such an ultra smooth finish causes the heat shrink coverings to “crawl” during application at just about any temperature. All “plastic” coverings will hide any surface defects finer than 400 grit anyway so using finer sandpaper is useless and can cause problems.
Exact covering instructions are nearly impossible as all designs will vary. But the wing-covering general rules are to always install dark covers over light (put the light colors on first) and to always start on the bottom of the wing. Photo 7 shows the yellow bottom covering installed first. Since it is not a good idea to try to cover over one color with another (too many bubbles and poor adhesion) always adjust your wing color scheme so that overlapping colors meet at wing ribs or over sheeted areas.
Joint areas should always overlap by 1/2 inch at least. That is why light colors go down first so the darker color can hide the overlap. When ending at a wing rib, always overlap the lighter covering down onto the rib as shown. Note the overlap extending onto the spar that will be covered by the darker color (red). And yes, I realize that this is the top of the wing shown in photo 8, not the bottom. Having the red contrasting bottom in the photo made the yellow overlap onto the rib easier to see.
But start on the bottom anyway. Spread out the covering and tack down the center of the covering span wise. Then pull the covering along the wing span and tack the other center end. Then tack the front corners pulling as you go. Follow with the rear corners. Then pull the middle of the covering chord wise at the center and tack it in place. Now pull the center trailing edge tight and tack down.
Then use the rule of halves to tack down the leading edge first and then the corresponding trailing edge covering until the covering is fairly tight. Wrap the covering onto the trailing edge as shown. Use the iron set hotter, 300 degrees F, and seal all the edges. Use the heat gun, starting from the center and working outwards to shrink the covering in place. After the covering is tight, use the iron set at ~325 degrees F to permanently adhere the covering to all the wood areas. Keep the iron in contact with each surface for about 3 seconds.
While it is possible to cover the bottom of the curved wingtips with the same piece of covering from the wing, it is very difficult. So I recommend stretching a separate piece of covering onto the tips first, overlapping at the outermost rib. Then cover that overlap with the bottom covering.
Once the bottom is covered, do the top. Installation is the same but overlap the top covering around the leading edge and onto the bottom covering by 1/2 inch. When overlapping colors at the ribs, make sure to have at least a 1/4 inch of overlap area.
Cover the ailerons before installation if you are using the slim synthetic hinges. These have proven to be great hinges despite their fragile appearance. Some of my Pattern airplanes have reached about 1,000 flights and these hinges have performed flawlessly. Make sure that the design on the ailerons matches the wing covering.
Match the hinge slots. Then drill a 1/8 inch hole in the center of all the slots. This hole channels the thin CAA into the innermost sections of the hinge for maximum grip.
When installing the ailerons, place a card between the aileron (covering removed for clarity) and the wing’s inner trailing edge to insure there is proper clearance between the two surfaces.
For now, leave the top wingtips uncovered. You may find it necessary to install a tip weight there for proper lateral balance. Cover the rest of the wing.
You may wish to install some contrasting trim strips where the colors overlap. First, it looks better if you do. But using trim strips also means that overlaps can be slightly off and still look great. Think of trim strips as molding in a house. Molding looks good but also hides a host of drywall sins.
Use trim solvent to very slightly moisten the adhesive side of the trim piece. Then position and apply the trim piece. Finally, use the iron to permanently affix the trim strip.
If you plan to install adhesive decals of any size, first moisten the area with a few drops of dishwashing detergent in water. The slide the decals in place and position. The “wet” water allows the decals to be moved into position without tearing. Then squeeze out the water using a playing card. For complete instructions on this, see the Sport Aviator “Black Horse Liberty Review” article.
Just as with the wing, start covering the fuselage at the bottom. Since the Kadet’s fuselage tapers, lay a piece of covering over it as shown and mark the dimensions using a thin felt-tip pen. The ink can be removed later using rubbing alcohol. Cut the covering along the line and overlap it 1/4 inch up the sides. Install as was done with the wing; center tacking first, then edges and finally all the sides. Leave a very small loose area at both ends. Iron the covering down from the center towards both ends. The heated air will escape out the small “hole” at each end. When done, seal the holes tight. Then apply each side and finally the top covering.
Where the covering meets the stabilizer and vertical fin, overlap the covering by 1/8 inch. If the colors match, overlap the flat covering onto the vertical as shown in photo 23. If the colors are different, install a 1/2 inch filet of the lightest color first, then cover the flat area first. With every joint, try to picture which way the burnt fuel will flow.
Fuel residue on the vertical fin will flow down towards the stabilizer. Therefore, make sure that the fin covering is on top of the stabilizer covering so the residue flows over the joint, not into it. This is why the sides overlap the bottom, residue flows down and away while the top covering overlaps the sides; the residue flows over the joint and down the sides.
When covering sharply curved surfaces, cut the covering into thin slits as shown. Then iron in place. When covering the engine compartment, keep exhaust residue flow in mind again. Note how a single solid piece will cover all the 1/2 inch overlaps and allow the residue to flow down and out without soaking into the joints.
A good idea here is to coat all the covering joints in the engine compartment with 12-minute epoxy brushed on. Try thinning the epoxy with denatured alcohol about 5% if you find the straight epoxy too thick to brush on.
Brush on a coat of the epoxy inside the fuel tank compartment as well. Make sure not to clog any of the firewall bolt holes.
Use a straight edge to center the cut over the main landing gear groove. Adhere the cut covering in place with the iron. Then install a small piece of covering over the groove as shown. This area takes a lot of stress so an extra covering piece would not be a terrible idea.
Once everything is covered, it is time to reinstall all the hardware. The original installations were detailed in Parts 2 and 3 of this series. But always remember to use thread locking compound for the final installations.
When assembling the nose gear, remember to install an additional 5/32 wheel collar as shown. The wheel collar helps absorb landing shocks so that the steering arm does not bear the full brunt of the “collapsing” force of the touchdowns. Since the ground stance of any nose wheel airplane is important, make sure that the nose wheel spring is only a card stock thickness away from the fuselage bottom.
Since the battery is mounted under the fuel tank in the forward compartment, it is subject to vibration. Wrap as much soft foam around it as will fit under the removable tank floor. Secure the connection with the on/off switch with 1/2 inch heat shrink tubing as was done with the aileron extensions.
When installing the servos, life is easier if you have a 6-8 inch long 1/16 in. drill bit. If you don’t, you may have to use a pin vise (small hand drill) to drill the servo mounting holes. When all the controls are connected, the fuselage servo compartment should resemble photo 33. Note the ugly angle in the nose steering connecting rod. This angle helps absorb steering and landing shocks to protect the servo.
Many builders actually install a small “V” in this rod for the same purpose. That works fine but I find it hard to do without some special tools. The bend shown has worked for 40 years so I think you will be safe trying it instead of the “V”.
While all the above covers just the highlights of covering and final assembly, the rest of the airplane covers and assembles the same way. After everything is completed, everything should be covered and installed except the two top wingtips.
The Center of Gravity (CG) is all important for any aircraft. “An airplane that is too nose heavy may not fly all that well, but one that is tail heavy flies only once.” While not totally true for 3-D aircraft, that old saying is very true for all RC sport and trainer aircraft.
Once everything is installed, mark the airplane’s CG of 3.5 inches back from the wing’s leading edge. This is a good starting point for most beginner-piloted Kadets. This particular airplane was balanced at the 3.75 inch mark as I wanted a little more roll and elevator response on my aircraft. The most rearward CG point for the Kadet is 4.25 inches but that is for very experienced pilots and would have to be moved further forward before using the airplane for training.
Unless you have a balancer (Great Planes makes a good one), make the CG mark on the bottom of each wing’s outer rib. Have an assistant place their finger on the mark as you do also. Then lift. If the airplane’s nose drops to about a 15 degree down angle, the CG is correct. If not, add nose or tail weight until it does assume that position. Balancing is performed with an empty fuel tank.
Because of the elevator and rudder cutouts, the lighter hinges and the carbon fiber control rods plus a lot of sanding of the rear fuselage, this Kadet balanced exactly at the 3.75 inch mark without adding additional weights.
But setting the correct CG is not the end of balancing. If you want your airplane to make concentric loops, roll evenly in either direction while tracking in a straight line, then it must be balanced laterally. If one wing is “heavy” the airplane will always drop that wing in maneuvers causing it to deviate from the straight flightline the pilot has chosen. Trying to learn how to fly using an airplane that drifts constantly to one side makes things very difficult.
Use nylon fishing line placed under the engine’s trust washer and also under the middle rudder hinge as shown in photo 34. There was absolutely no wind when we balanced this Park Pilot magazine Spitfire outside for the photo. But the procedure is best done indoors when a picture is not needed.
When you laterally balance your Kadet, the most likely result will be that the right, muffler side, wing will drop. It did on this Kadet. However, remember back in Part One when each rib was weighed and the heavier ones were positioned on the left side? That extra bit of work paid off as laterally balancing this giant-winged airplane required only 9 grams! Basically, that meant a washer and a hold-down bolt.
Epoxy the birch block and the “T” or “blind” nut in place. Screw in the bolt and washer from the outside. This system will allow any weight adjustment proved necessary during the first trim flights. If you color scheme is on the dark side, you can turn the washer black using cold gun bluing solution to help hide it. If your wingtips are light, then leave the washer in its natural silver color. It will be nearly invisible.
Recheck the balance and then install the top wing tip covering. Recheck after covering but the balance should not have changed as those small pieces of very light covering weigh almost nothing and should have been equal anyway.
The final “balancing” task is to adjust the airplane’s ground angle. Place the fuselage wheels down on a flat, level surface. Place a small level on the wing saddle lengthwise. The bubble should center on that second, rearward line. In case you have wondered why a level has that second line, it is the one degree mark. When the bubble’s centers on the mark, then the surface being leveled has a one degree slant (used when installing piping).
It is important that the fuselage have that slightly negative attitude. This prevents premature takeoffs while making it easier to land without bouncing as the main gear will always touch first even if the flair is not perfect.
If the pitch is too negative, lengthen the nose gear. If too positive, lower the nose gear when possible. It was not possible on this airplane so another remedy needed to be employed. The main gear can be slightly bent inwards to raise the fuselage rear. If you do this, remember to re-bend the axles to be level.
But this airplane’s nose was just barely too high. Since it flies from grass fields that can be rather long, the 3-inch main wheels were replaced with 3.25-inch ones. Problem solved. The nose now sat in the perfect one degree negative position.
Well, there she is. My own unique SIG Kadet. Don’t think there is another like it anywhere in the world. This one is all mine! It has my modifications and embodies all that I have learned building and flying the 219 airplanes that preceded it. (Yes, that is what its “220” number means.)
The preceding does sound a bit possessive, doesn’t it? Guess that is what happens when you build your own airplane from a pile of wood. It somehow becomes more “yours” than and ARF or RTF airplane does. Even were it built “stock” with no changes, the airplane you build is somehow more yours than one you assemble.
But how will it fly? This Kadet should be able to do some things that stock Kadets can not. The most obvious advantage will come from employing the flaperons. The Spektrum DX-7 transmitter also allows the flaps to be electronically coupled to the elevator much as are Control Line flaps. That should also be interesting.
Using the bolt-on wing mounting system reduces weight and drag from all those rubber bands. The lower drag helps this Kadet maintain fairly steep climb angles at takeoff. Of course, that powerful O.S. Max 46 AX might have a little something to do with the airplanes excellent climb rate as well.
Liftoff was fairly quick after just a 50-foot ground run with no headwind. While flying conditions for the Kadet’s first test flights were on the windy side, the 20-30 mph winds were exactly across the runway equaling no headwind. Still, the airplane handled extremely well. For a trainer, its handling was magnificent. (Too prideful and possessive?)
This article is not meant to be a review of the Kadett’s performance as that has been discussed in the Kadet LT-40 ARF review. But parts of the airplane’s performance related to its building should be discussed.
Because the control systems employ carbon fiber control rods, controls are precise with no play at all and center perfectly each time. The airplane’s proper lateral balance lowers the pilot workload involved in maintaining slip angles during crosswind landings. These factors make the airplane easier to control in high winds.
But as the photos bear out, the Kadett’s maximum demonstrated crosswind component is well below 30 mph! Despite holding an almost full slip into the wind and cheating a little by heading into the wind as much as the runway allowed, the Kadett was still drifting sideways during the landing approach and touchdown. But it did land safely each time that day.
Checking out the flaperons had to wait for another, calmer day. That day came at a Vintage Radio Control Society Fly-in. Yes, the Kadett is VR/CS eligible. Deploying the flaperons for landings reduced the already short ground roll to almost nothing. Landing speeds were reduced to about 10 mph or less. Takeoffs with flaperons deployed were almost immediate requiring very short ground runs; no more than 25 feet.
With flaperons deployed, the Kadett could be held to zero ground speed in a moderate 10 mph wind. I tried that the first day also by heading into the 20-30 mph wind and the airplane flew backwards relative to the ground!
Coupling the flaperons to the elevator made for awesome 20-foot square loops and inverted spins. The airplane’s “snap rolls” even approached something recognizable as a snap roll using the coupled flaps (stock Kadets don’t inside snap very well). With coupled flaperons, the Kadett would even fly outside loops; something few flat-bottom airfoil trainers can do.
A final trim note: the Kadet needed no elevator or rudder trim to fly straight lines and concentric loops. However, a tiny amount of right aileron trim (one beep only) was needed. It could be that the left wing was a tiny fraction longer or vice-versa. No matter the reason, removing the washer part of the tip weight allowed the airplane to fly level with no aileron trim. Maybe I didn’t do a good enough job laterally balancing it? Regardless, the final tip weight needed was just 5 grams. Weighing ribs can pay off.
This is the first airplane kit I have built since building the Top Flite RC Nobler for Model Aviation magazine back in 2001. I had almost forgotten just how satisfying and rewarding kit building can be.
I urge you to give building a try while you are learning to fly using your ARF or RTF trainer. You might just feel about your creation as you watch it take wing as I did and do about this Kadet.
As for me, I still have about 50-60 wood kits in storage. And I think I hear the Mark’s model SBD Dauntless Master Craft kit and the Bridi XLT Pattern airplane calling my name. Then there is that 1/4 scale DH-4 kit from Balsa USA plus their Fokker DR-1 Triplane. I wonder which will be the next building project for me? And I almost forgot about that 10-foot span B-36 from KYO or their B-29. I better start building right away.
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