Mustunt IV, Part 7, Basic Building Techniques

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Its time now to carve wingtips. All of my airplanes use carved tips. These tips are approximately 1 1/2" thick. One tip will be a bit wider than the other. This is where I pick up my 1/2" asymmetry when using equal span wing panels. Tip widths are 2" plus 1/4" and 2" minus 1/4". Tips begin with two laminated layers of 1" balsa, tack glued together, and cut to the top view outline. Its easier to cut the 1" thickness of the top and bottom blocks individually and then tack them together. While the tip is 1 1/2" thick, using two 3/4" blocks leaves nothing to work with at the wing's thickest point. Starting with a 2" thick laminated blank works much better. The tip ribs of the sheeted wing aren't always perfectly square. If this condition exists, then the mating side of the laminated blocks should be beveled, as necessary, to make the laminated joint horizontal. This will give a more reliable reference line when carving. With the top view cut, a lot of work can be saved by cutting the laminated tip to an approximate airfoil on a band saw. With both the outline and rough airfoil cut to shape, the tips can be tack glued to the wing. I tack glue just a bit on the leading and trailing edge and a bit heavier on the spar ends.

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Personally, I think that the best tool for carving tips is a wood rasp. It really tears wood up quite easily when cutting cross grain, and tips are usually cross grain. Using the rasp will give a controlled, progressive shaping of the blocks. The tips can also be shaped with a Moto-Tool with a sanding drum, The Moto-Tool and sanding drum cut so fast that precise control of the shaping is difficult.

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Carving of the tips is a three step operation. First, the tip is cut parallel the wing surface, then tapered gently to reference marks and finally rounded. In the first step, I try to shape the tip block as an extension of the wing surfaces with no intentional tapering yet. This shows the tip carved to roughly parallel the wing surface.

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The wing tip is approximately 1 1/2" thick If this tip is simply rounded, with a 3/4" radius it will be very blunt and club like. The wing tip will look much better if the tip is tapered a bit and then rounded. I marked the tips 3/8" above and below the tip center with tapering reference marks. The tips were then tapered to the marks. This photo shows the tip still flat from the top to the bottom reference marks. Finally the tips were rounded. All of the carving in this photo was done with the rasp.

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After rough carving the tip with the rasp, it is sanded to near its final shape using sanding blocks of 80 grit, then 150. The tip is now close to its final shape but does not yet exactly blend with the wing surfaces. Before the final blending, the tips must first have tipweight and leadouts installed. There will inevitably be small misalignments when the tips are finally glued onto the wing. That will be the time to do the final blending of the tip to the wing.

Al

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Before the tips can be hollowed, they have to be removed from the wing. Work carefully with an Xacto knife to separate the tips. I also use debonder or nitro to soften the tack joints to make removing the tips easier. There will inevitably be splintered wood remaining on the tips and possible rib damage. Repair the ribs. Leave the splintered remains. These remains will "key" back into their original location when reinstalling the tips. For hollowing of the tip, I use a Moto-Tool and sanding drum. This combination cuts so fast that care must be exercised to avoid cutting too much. My favorite location for carving is the garage near the door where I can hold carvings directly in sunlight to gage their thickness. Hollowing tips makes a mess of the garage and me. When hollowing, the air is full of balsa dust so a dust mask should be worn. I usually do. Occasionally I forget, but regret the omission. I wonder if lungs are ever able to get rid of all that balsa.

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If the tips are hollowed from reasonably light wood, it is easy enough to thin each tip to no more than 6 - 10 grams.

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After hollowing, the tip halves can be glued together. These tips are ready for installation of tipweight and leadouts.

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Tipweight is installed in the outboard tip. The dark area inside of the tip leading edge is permanently installed tipweight to bring the finished tip up to 1 1/4 ounce. Removable tipweight is held in place by a 4-40 machine screw soldered to a blind mounting nut on the bottom of a weight platform. The weight platform assembly is epoxied to the inside of the tip and a door cut in the bottom of the tip to access the weight retaining assembly. The door, as pictured, has a 1/16" balsa tongue installed on its outboard side, More difficult to make out is the 1/16" balsa shelves on the front and rear of the tip cutout for the door to rest on. There is a plywood floor on the bottom of the door for the door retaining screw. the door retaining screw mates with a mount on the end wing rib having a 2-56 blind mounting nut. I actually screw the door to this mount separated by a piece of parchment paper before gluing the mount to the wing end. The mount is covered with epoxy and the tip glued onto the wing end with the parchment paper pulled down over the weight door to protect the door from epoxy. The epoxy covered mount fits against the tip rib and will be attached there when the epoxy cures. The parchment paper can be seen protruding from the wing/flap joint. This will be easier to visualize when I get to the cowl mount photos.

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Here are the parts of the sliding block adjustable leadout assembly. The guide is a piece of 1/4" x 3/8" x 2 1/4" ply having a 1/8" slot 2" long for the leadout carrier. The leadout carrier is a piece of 1/8" ply with two pieces of 1/8" brass tubing for the leadouts and a 4-40 blind mounting nut for locking the carrier in the slot. The 1/4" plywood guide is inletted into the tip then the outside carved to blend with the wing tip contours.

Al

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The question comes up from time to time about the effect of using dihedral in a stunt ship. The best answer is that if the leadouts exit the wing on the vertical CG there is no difference in the way the airplane flies as compared with stunt ships without dihedral. I see a lot of "tweaking" of the flaps on many stunt ships to get them to fly wings level. Stunt ships may normally fly with the outboard wing down a bit both upright and inverted as an effect of tipweight. I see nothing wrong with this as long as the airplane doesn't "hinge" in hard corners. Airplanes which fly in different bank attitudes upright and inverted typically have one of two problems. Either there is a warp in the wing or flaps or the leadout exit isn't on the vertical center of gravity. "Tweaking" the flaps may level the wings upright and inverted but the asymmetry of the lifting surfaces may "bite" in maneuvers when the asymmetry is aggravated by large angles of attack and "G" forces.

If an airplane is built with warp free wings, accurate alignment and a proper location of the leadout exit there should be no need for "tweaking" flaps. Personally, I never "tweak" flaps. If I ever did have a barely noticeable difference in banked attitude in level flight, I'd live with it rather than induce lift asymmetry into maneuvers.

The major point here is that the leadout exit will always align with the vertical center of gravity unless it is being held aerodynamically displaced by warps or "tweaked" flaps. If the airplane flies wings level upright and inverted it will maneuver without lift asymmetry regardless of whether the airplane has dihedral or not.

There is no reason why a properly rigged stunt ship with dihedral should fly any different from a classic configured stunt ship with a proper leadout exit.

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Where am I going with this? I did a small experiment with the new Mustunt IV. The leadouts were temporarily fixed to the center of the inboard tip rib The airplane was then hung by the leadouts with the wheels nearly touching a wall. Gravity will align CG directly under the point where the leadouts exit the wing tip. If the gears are exactly the same length and the leadout location correct, the wheels should touch the wall at the same time indicating the airplane is hanging exactly parallel to the wall. Excepting aerodynamic problems such a warps, the airplane will fly fly in the same roll attitude as it hangs. This is a practical test of the leadouts exits and vertical CG. In fact, I did lower the leadout exit by 1/16" to make a slight adjustment in their alignment with the vertical CG. This confirms the leadout exit, as designed, for this airplane and that its relation to the vertical CG is more than accurate enough even had I not made the slight adjustment. A 1/16" difference in the leadout exit would not have made a visible difference in level flight bank attitude or maneuvering flight loads.

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Here is the adjusted leadout location moved down 1/16". This is the location I will use when the leadout guide is installed 1/16" below the center of the tip. It won't show. In fact, the BBQB and BBFB Bearcats have their leadout exits 1/8" below the center of the tip rib as a result of adding 1/8" dihedral when building the first one to keep the same calculated vertical CG location as originally designed. This adjustment on the Bearcats isn't noticeable, either.

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Sometimes it seems like for every two steps completed in my building I wind up taking a step backward. This afternoon I noticed that the newly installed leadouts on the Mustunt IV looked to have a larger than normal spacing. I usually use Tom Morris leadout guides and replace the rivets he used with 1/8" brass tubing which has thicker walls. Tom's standard leadout guide has a leadout spacing of 1" but I always request that mine be shipped with a 3/4" spacing. Sure enough, I put one in with 1" spacing by accident. As long as the airplane isn't yet painted, I'm never committed to saving anything that needs changing. I cut out the new leadout guide assembly and made a new leadout holder with 3/4" spacing, glued the assembly back in and sanded it smooth. End of story?

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Not quite. Why all the trouble? I appended this to the dihedral explanation because of the similarity of the aerodynamic effects. Whatever is in the way between the tensioned flying wires and the CG will, in flight, line up between them, not only vertically, but horizontally as well. In inside maneuvers we pull on the up line and the up leadout exit will line up with the CG. This imparts a forward movement to the wing tip, or slight outward yaw. Outside maneuvers tension the down leadout and impart a slight rearward moment or inward yaw. Using a minimum spacing of the leadouts minimizes the amount of inward and outward yaw experienced as we maneuver the airplane. Gyroscopic precession yaws the airplane inward on outside maneuvers. If we make the rear leadout the down line by reversing the bellcrank we pick up a small correcting outward yaw as opposed to an additive inward yaw on outsides with the typical bellcrank orientation. This small compensating yaw is the reason I suggested reversing the bellcrank in the Bearcat article thirty nine years ago. Some people got the message and use the reversed bellcrank today without knowing exactly why except that it doesn't hurt.

Al

KEWL! Model airplane stuff!
Nice post Al looking forward to Part 8
regards,
Rick "catdaddy" Blankenship

Awesome work. this is really good stuff

Awesome work! This is really GREAT stuff!

Say Al.....

What's that silver plane with the round fuse & yellow Tornado prop hanging on the wall????

Can't thank you enough for sharing your ideas and techniques!

Tight lines!

Wes

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That is a genuwine Jack Sheek's AT-9. I bought it from Jack for a multi-engine trainer while finishing up the Hornet. I installed two OS .30s and did some repair work on the engine nacelles. I used left and right rotating 30s to balance any gyroscopic precession and torques. If someone made me an offer that I couldn't refuse, they could be the proud owner, Flying Models construction article included. I could also probably find a couple of the OS .30s. I gave Jack $100 for it in 1977. what do you suppose it might be worth today here in Dallas?. I have no interest is shipping it although I could probably find someone to deliver it to a contest here in the Southwest. VSC or the NATs would also be a possibilities.

Notice that the installed OS .30 has a left hand crank and a prop with reverse pitch for opposite rotation.

Al

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From here on, the engine, prop and spinner are in the way. I removed them and installed a jigging fixture which holds the airplane and allows it to swing from side to side and rotate. There are any number of ways this fixture can be held, but I simply drilled a hole in my drawing table and bolted the fixture to it. The jigging fixture came with several engine mount adapter plates. I couldn't find the one which fit the OS.46, so one was made from 1/4" plywood. This fixture is ideal for painting preparation and the actual filling, sanding and painting as well.

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The nose of the Mustunt IV, and the Mustunt II as well, don't take really well to the removable tank installation when used with an upright engine. Actually, the spinner ring is in the way of inserting and removing the tank. By cutting the spinner ring even with the top of the 1/2" fuselage side fillers, it is now possible to insert and remove the tank and tank spacers from both above and below the tank. This shot shows the flexible line from the overflow coming from the overflow vent in the bottom block. It also shows the running vent, either uni-flow or atmospheric, which penetrates the fuselage side just under the needle valve. Actually, the vent copper tubing penetrates the fuselage side through a piece of angled aluminum tubing which allows the vent tube to be removed for painting. The vent tube is a bit loose as shown and will actually point a little straighter into the wind when glued in place after painting. The location of the running vent is a bit awkward. It was a little hard to settle on an inboard location high enough to discourage tank siphoning and low enough to avoid interference with tank removal. the Mustunt II has a nearly identical nose. My Mustunt II will have to be reworked to this configuration for the reasons already stated.

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It is always a chore figuring out how to hold a cowl on. Considering the need for clearing a path for removing the tank, the best I could come up with is to put screws through the spinner ring top into mounts glued inside the lower airplane half of the spinner ring. This begins with installing 2-56 blind mounting nuts into a couple pieces of 1/8" ply. The ply is then trimmed, as necessary, to fit the available space on the lower spinner ring.

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The actual installation of the cowl retainers begins with drilling the cowl mounted spinner ring half and counter sinking the holes for flat head 2-56 screws. The mounts pictured above are then screwed to the upper cowl separated from the cowl by a piece of parchment paper. Epoxy is applied to the mounts and the cowl is taped in place with the parchment paper protruding which keeps epoxy from getting on the removable cowl. When the epoxy cures the cowl can be unscrewed and removed. In the photo, parchment paper can be seen protruding from the spinner ring split where it is protecting the upper spinner ring and cowl from epoxy while the epoxy covered mounts are adhering to the lower spinner ring.

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The same technique was used to attach a single cowl mount to the back of the cowl at the top center. Again, the cowl was separated from the mount with parchment paper which can be seen protruding from between the cowl and the fuselage. Using a single mount in this location requires some method of restraining the bottom rear of the cowling. I simply sprung the cowl to sit slightly open a bit at the bottom, and added tongues which fit inside the fuselage sides. The rear of the cowl has to be sightly compressed to fit the cowl into position. It isn't immediately apparent from the photos, but the cowl has unequal sides. This allows the outboard fuselage side to be raised to the bottom of the tongue muffler. This permits the cowl to be removed with the muffler in place. Then the muffler can be removed to get to the engine mounting screws. Then the tank can be adjusted and fuel filter cleaned. And the ankle bone is attached to the leg bone, etc.

Al

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Installing a cockpit can be a fairly intricate job on a Stunt ship. This one is sorta middling. It begins with taping the canopy in place and adding strips of narrow masking tape to simulate the outlines of the canopy framing. The narrow masking tape strips are cut with an Xacto knife. I stick a 3/4" piece of masking tape onto my wife's glass top coffee table, align a two foot steel ruler, and slice away. Its my experience that an Xacto knife won't scratch glass. I cut strips about 1/8" wide and use a lot of them. Even the narrow canopy frames are usually made up of at least two strips.

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Esthetically, it isn't easy to decide where to put the frames, and the fact that the Bearcat canopy was originally made for a larger airplane doesn't help. My first try at taping a frame which, on a real airplane would have been the front of the sliding canopy, was about 2" from the turtledeck forward face and had nearly vertical sides. This made the canopy bubble too short and the forward window panel too tall and narrow. Well, it was only masking tape. I tried four different positions for that first frame, each time moving the tape forward. Each relocation enlarged the bubble canopy and shortened the forward window panel. The last movement looked best with the frame tilted slightly forward. When satisfied with all of the canopy frames the canopy can be removed for painting.

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I paint the inside of the canopy, free handing the paint, to try to match the tape canopy frames on the outside with paint inside. Why bother? If the appropriate areas of the inside of the canopy aren't painted, the glass strips and epoxy used to hold the canopy on will be visible from the outside looking through the canopy at the opposite side. Free handing paint isn't easy, but then there is no great need for great accuracy. A fairly good job will look as good as an excellent job with the airplane is finished.

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If a canopy is going to be used, this usually indicates the need for a pilot. I've already addressed the structural modifications necessary to keep a Williams Bros. pilot in his seat. Now it has to be painted with paints for plastic models. I start by painting the headgear and coat brown, followed by white on the shoulder harness and scarf. There needs to be some effort to do the eyes and eyebrows and a light beard usually adds something. Some of the finer markings were made with a Sharpie extra fine point pen. The beard is a bit of black smeared around with a thinner brush until it is nearly gone. If the goggles are used, the lenses should be masked, as the entire pilot will be sprayed with a flat lacquer paint to blend all of the paint and kill the gloss. I use Model Master Lusterless (flat) Lacquer Overcoat.

The instrument panel is painted black and instruments are simulated with silver circles made by dipping a piece of aluminum tubing into silver and pressing it onto the instrument panel.

Screw the pilot to the cockpit floor and run some CA around it. The screw alone shouldn't be counted on to secure the pilot figure. Install the cockpit floor in the airplane and be very careful to see that absolutely every joint has at least one layer of glue, preferably two to avoid dust on the inside of the canopy. Glue the pilot's head to the head rest to provide still more resistance to vibration. Esthetically, mounting the pilot further forward would have looked better, but the chance to strengthen the mounting of the pilot was considered more important. I used TightBond to glue the cockpit floor in place. Touch up the paint as necessary and mask the entire airplane leaving only the cockpit exposed. I don't want to contend with paint contamination when painting the rest of the airplane. Once again, the entire cockpit, including the pilot are sprayed with the flat lacquer. This blends the entire cockpit much as spraying just the pilot blended the color paints into a semi-believable appearance. When the masking of the pilot's goggles is removed, its time to install the canopy.

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Before the canopy is installed it must be cleaned. There is nothing which spoils my day more than removing the canopy masking on a new paint job and finding a finger print on the inside of the canopy. I attach the canopy to the fuselage with Zap finishing resin used full strength. This is only the first bonding of the canopy. Later, it will be more firmly attached with many layers of glass cloth strips and thinned epoxy. For now, I just want to hold it on. Don't use CA. Ca may smoke the inside of the canopy as it fires off. The canopy is fitted to the fuselage and held down with masking tape fore and aft. The canopy sides are pushed into the inletted areas, wet with glue, and held there with T-pins. Tomorrow, I'll remove the tape and T-pins and add the glass and epoxy which makes up the real structural joint which retains the canopy.

When painting the airplane as a whole, the canopy is masked with vinyl tape covering all of the areas to be left clear. The paper masking tape is removed, exposing bare canopy plastic, so that the canopy frames will receive paint on the outside closely matching the inside painted cockpit frames.

Al

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Sorry, Just copying some information.

Al