By Don McKay
Vintage Diesel Combat is the perfect event for beginners to learn combat because these planes:
I chose the Iron Monger for a plane as it has a good "VooDoo" type rib/airfoil shape and looked strong. Built correctly and covered with Ultracote, my planes should last for years if there are not any major midair collisions. The newer planes I've built and used have been hit by props and crashed (breaking the prop) 20 or more times and only need patches from the prop cuts with no major structural damage. All but one diesel combat plane I've had was flyable after a crash or mid air collision and that one, was not one of the new better designs as listed below. On that one older design the mount, leading edge, trailing edge, and rear stabilizer were all broken not counting other minor damage.
To be legal in Vintage Diesel Combat in the Northwest, a plane must be built to the same dimensions and general construction as an original plane design before 1971 for a .15CI engine. See the details in Northwest rules. Until recent rule changes, a Carbon Fiber (CF) reinforced plane like the one in this article would have been illegal to use, though a fiberglass reinforced plane would have been legal. It is easier to get Carbon Fiber (CF) parts than fiberglass, and CF has a better strength to weight ratio.
Don't reinforce your planes so much that they become stiff and do not flex.
Diesel combat planes are underpowered compared with other types of combat planes. Weight is very important, but survivability is more important. Use a "food type" gram/once scale. Work in "grams" as they are a more precise measurement of 28.35 grams to an ounce. Finished planes (ready to fly) with empty tanks should weigh less than 17 ounces (482.95 Grams). I have planes that weigh as much as 18 ounces that fly well, but 16 ounces is my goal for each plane built. Balsa wood varies a lot in weight. Use light wood if you can-especially for the Leading and Trailing Edge.
Carbon Fiber (CF)
Carbon fiber usually has a mold release that hinders good glue adhesion. I always lightly sand all surfaces of CF to be glued before installing on the plane. For CF stripping (semi flexible thin material that is about 1/4" wide), I just hold the end between folded over sand paper and pull it through about 4 times. The same will work for the CF tubes and rods used on this plane design.
Diesel fuel is very stinky and tends to seep in everywhere due to the two open exhaust ports on diesel engines. This fuel residue can loosen non-Ultracote-type covering material and rot the wood. Care must be given to seal the plane correctly.
You also need a special fuel bulb and fuel tubing for "gas." Silicone fuel tubing and other parts swell when exposed to ether and kerosene in the diesel fuel.
RED MAX Plain Bearing Blend diesel fuel for break-in and practice with 25% oil is available @ $28 a gallon from Carlson, listed below.
The custom formula of diesel fuel used by my team is: 42.75% Kerosene, 30% Ether, 24% Castor Oil, 3.25% DII. Custom orders can be placed in larger quantities at Red Max Sales: (800) 742-8484. The cost for custom fuel in quarts is about $13 -- much more than using gallons of standard fuel purchased from Carlson -- see below.
The legal preferred engines are a .15 cubic inch;
It takes about one or two hours of break-in on a diesel engine before full performance can be had. Run an 8/4 or 8/5 prop for break-in and an 8/6 for best performance after that. Run for no more than 6 minutes at a time then let it cool down before restarting. The heating and cooling are important part of the break-in process. These engines prices and Red Max fuel are available at Carlson Engine Imports.
You can also get diesel engines and special custom tanks from "The King of Diesel Combat" -- Mel Lyne. Mel imports a Sharma .15 modified for combat that has a better needle value arrangement which helps prevent it from breaking during crashes or inverted landings. Mel charges about $80 plus postage for his modified Sharmas.
Glue adds weight, so don't get carried away. Thin (instant cure) CA glue makes an excellent fuel proofer that can be sanded easily. It should be used anywhere that is not protected by the covering material and placed down along the edges of the wood before covering the trailing edge (TE). CA glue can even be used to seal over fuel-contaminated material after being cleaned as much as possible with alcohol, because a "slight" oil residue acts with CA to harden it. What would take 4 coats of dope to be sealed that well can be achieved in one coat of CA glue. I spread thin CA with waxed paper to get it on smoothly and not too thick. Glues needed for construction include:
I rely heavily on a belt sander while building most planes, especially Diesel Comber Wings. This makes shaping the trailing edge, motor mounts and elevator much easier.
The better original planes, used in the past, were built with custom-routed and lightened leading edges that weighed about 44 Grams each, but still broke during midair collisions and hard crashes.
To build a very tough diesel combat wing you need a strong but flexible leading edge. I recommend using a light weight Sig 1" X 36" Balsa leading edge. Weight for this 36" piece should be between 35 to 40 Grams. Cut the leading edge balsa to 32.5". This piece should ideally weigh 33.0 grams or 11 to 4 grams lighter that the custom cedar type, but to this we need to add strength in the form of an aluminum, fiberglass, or carbon fiber rod.
The wing length measured from the outside of the inboard and outboard wing ribs on an Iron Monger is 31" with the outboard wing 1" shorter than the inboard wing. Each will have a 1-1/4" wide wing tip added for a total span of 33". The inside wing is 16" + 1" and outside is 15" + 1" counting Wing Tips. Three-fourths of the leading edge on each tip will be used to anchor the wingtips.
Route the center of the back of the leading edge to allow for one of these tips:
*If you use the 24" .188 OD CF Tube, or a 21" arrow shaft, then the inside 5 or 8 inches of the L/E does not need a reinforcement, because damage to the inside wing is rare except in bad midair collision where nothing will help. I used a little Dremel router table with inexpensive diamond bits to rout the place for this tube.
Glue whatever tube you use with "White Gorilla Glue" (WGG), which is flexible, fuelproof, and fills gaps best. The WGG glue adds 2.5 grams.
Do not place tube or glue in the routed area for 3/4" at each end of the leading edge as this area will be used to anchor the wing tips, which are added later. To increase strength and make the L/E fuelproof (for a long life in a diesel fuel environment), seal all sides of the leading edge, from the Center Line (C/L) out, about 3" from center, with 1 coat of thin CA (spread on with wax paper). This adds .5 gram.
The ideal total weight for a finished LE on the model pictured is 33+5.5+2.5+.5 = 41.5 grams.
Leading Edge assembled w/CF Tube, glued on, sealed with CA.
Start with a 4" X 36" X 3/16" Balsa which should weigh as close to 35 Grams as possible. Cut to 31" length and to shape with 15" span outside and 16" inside. Cut TE to shape. The width of the stab area is 9 inches where the elevator attaches. The tailing edge is 4" wide at the elevator and rounds, then tapers to 1-1/4 inches at the wingtips.
Add .025 (or thicker) CF trailing edge "rod" to the back edge, from the wingtip to the center line by cutting off the back of the TE along the TE wing taper then grooving the trailing edge and gluing the rod in with "thick" CA. I use a small rounded piece of 1/16" piano wire to make this grove. The .025 CF rod adds only 1 Gram plus 1 Gram for the glue. This CF rod keeps the TE from breaking forward during crashes. Once the CF tube is glued in place, reattach the trailing edge you cut off to install the CF rod to the center line.
Cut 1/8" off the back of the stab depth and add a 1/8"x1/8" basswood, or other wood that is harder than balsa, as a doubler on the stabilizer. This reinforces the TE and later is used to anchor the sewed-on elevator. If you don't cut off a 1/8" area, your plane can end up tailheavy.
With CA glue, add 1/8" to 1/4" wide carbon fiber strapping to top and bottom of the front edge of the Tailing Edge. The weight (before sanding) was 33 Grams (which was too tailheavy) for my plane #7, 26 Grams for #8, and 26.5 Grams for plane #9 -- shown in the picture. The CF strapping keeps the tailing edge from buckling during crashes.
1. Maple Mounts 2 each - 6" long. 2-1/4" will be protruding in front of the leading edge.
It is best to start with maple that is 3/8" wide by 9/16" deep. You can use 3/8"x1/2", which is easier to find, but if you use a Sharma engine, you'll need to add a 1/6" plywood spacer, that is cut out for the bottom of the engine, glued to the inside of the mount. This to make room to clear the bottom of the engine case. As an alternative, you can add 1/16" aluminum spacer between the engine lugs and the mount.
Sand/Taper to streamline and lighten each maple mount from 3" back (from the front) to the back end on both the top and bottom pieces. You will also Sand/Taper the front of the mount from 1" back to the nose. The goal here is to lighten and streamline the mount.
2. Center Rib Stringers 2 each
The center Rib Stringers are made from bass or pine wood that is 1/8" thick by 3/8" wide. Taper 2.5 inches of inside ends of the stringers, that attached to back of the TE. This gives the airfoil a nice taper at the center of the plane.
Glue both of the center stringers to inside of the mounts starting 2-1/4" back from the front of the mount, using "Strong" CA Glue. Clamp in place while the glue is drying. These stringers run from the front of the leading edge all the way back to the back edge of the Trailing Edge. Later the 1" wide leading edge will need to be notched 3/32" on the top and bottom to clear the center rib stringers since the LE is 1" thick and there is only going to be a 13/16" clearance with proper spacing for a Sharma engine.
3. Mount Spacer 1 each 3/8" X 13/16" X 2.75"
The mount spacer uses 3/8" to 5/16" thick "hard" balsa with the grain running up/down between the maple nounts and center rib stringers. There should be 1-1/16" total space between the inside of the maple mounts for a "Sharma" engine. Other engines this may vary. Because you have two 1/8" thick "center ribs stringers" added between the mounts and the mount space, the thickness of this spacer should be 13/16" thick. Glue the mount spacer to center rib stringer with "strong" CA glue, clamping in place.
4. 1/16th Plywood Mount Strengthener 1 each
With "strong" CA glue, attach a 1/16 ply doubler, from behind where the leading edge will be, to the outside of the maple mount, center rib stringers, and balsa spacer to help hold it together during crashes. Some people drill a couple of holes through the mount and mount spacer and add dowels or bolts also, but I don't believe it is necessary with the 1/16" doubler added.
Drill the mounts for the engine and secure with bind nuts. For a Sharma type engine, the back of engine crank case should be placed 3/32" to 1/8" in front of where the front leading edge will be, which is 2-1/4" back from the front of the mount. This allows for head clearance with the leading edge when the engine is offset and gives the proper center of gravity (C/G). The engine can be moved further back into a slightly "notched" leading edge, for those who like a more "tailheavy" airplane; but remember the smooth maneuvers that keep up the air speed are more important than tight turns in underpowered planes like Vintage Diesel Combat planes. Also it is easy to add a little "tail weight" later for correct balance.
Rough shape (streamline and lighten) the mounts before installing them in the wing, because it's easier to sand before installation. Also round off the outside edges slightly except where the engine mounts. This makes the mount stronger (rounded edges are stronger than sharp edges), lighter (saves up to 2 grams), and causes less turbulence/drag as sharp edges increase drag dramatically.
Installing the center Rib/Mount
Install the mount before adding the Mount Doubler (below). Install at a slight angle to give the engine a 2-degree engine outthrust, but make sure you don't get any up-or-down thrust angle. Use WGG to attach the mount to the LE. Glue the tapered ends of the Center Rib Stringers to the back of the TE using CA. With the proper taper this should be an area if 2 to 2 21/2 inches. Add shims between the center rib stringers and the trailing edge so the full width of the TE is glued to the stringers. This helps to keep the TE from spitting during crashes.
5. Mount Doubler 1 each (Add this after the mount is attached to the leading and trailing edges.)
Add 1/16" plywood doubler to the back of the mount over the bind nuts for the engine running all the way back at least to the inside edge of the leading edge. Add a 1/8" to 3/16" Balsa doubler over the plywood for better streamlining and strength.
6. Nose filler/faring 1 each
Add a 1/2" square by 1-1/16" nose piece between the mounts in the front to allow for streamlining. You need to cut out this nose piece to clear the front of the case with the crankshaft.
Ideal tank weight (like Mel Lyne provides) = 28 Grams (1 oz) or less. Used in my plane #4, 5, & 8. I built the tanks used in planes #6, 7, and 9. The tank should hold between 1 7/8 to 2 ounces. The Dimensions should be 7/8" Max thickness, 2-1/2 inches long and 2-1/8 inches wide. They need to be "uniflow" type where both the pickup and vent are located in the same area separated by 1/4 inch with the pickup tube the longest. The fill/vent tube should exit the tank in the top, inside front corner. The pickup tube should exit at the front, bottom, center. There can be no leaks whatsoever. If you make your tank, start with a 4 ounce stunt tank and cut it in half as this will cut down on the amount of soldering you need to do. Use only silver solder with acid flux, to be strong enough to hold up in crashes. I use a torch to heat the parts during soldering. If you don't know how to solder well, don't even try. Go out and buy a tank. A "Mel" tank cost $20 and he may not have any, but a lot of fliers have these tanks in old planes that are no longer being used that you can get for $20. Glue the tank in place with WGG along the leading edge and against the plywood engine mount strengthener. Over the tank you will add planking later.
I have had problems with tanks I made being dirty inside due to solder residue. This is best cleaned out with acetone. Mel tests his tanks after kicking them around on a hard floor for a while. After cleaning (and kicking), I test the tanks for leaks by blowing up a surgical-tubing pressure tank with air and attaching it to the fill tube with the overflow tube closed off. If, after 30 minutes, it has not leaked any noticeable air, then it is sealed-for now. I retest all tanks before each contest. You also need to test tanks after crashes because the tanks may spring a small leak. A small leak is all it takes for the uniflow tank not to work properly and cause fuel to leak into your plane, which is very bad. If you do get a leak that contaminates the plane with diesel fuel, then -- as soon as possible:
I have had to do this twice. You can see why the tank needs to be good from the beginning.
Once you start using your tank, always keep it sealed (with "Gas"-type fuel tubing) when the plane is being stored to prevent internal corrosion and keep the castor oil residue from turning to sludge.
The ribs are shaped like VooDoo ribs (only thicker), but instead of a 1/6th sheeting on top and bottom on the VooDoo tailing edge, it used a 3/16 balsa sheet up the center. The general rib dimensions are 1" thick at the front, 1.5" thick at center of the cored, which peaks 1.5" back from the from of the rib. The front of the tailing is 5-3/4" back from the front of the ribs. To get the proper rib shape is use the Sig Leading edge and follow the curve up and over the wing cord 1.5" back from the front of the rib.
The Rib length varies. The 1/4" rib to the left and right of center rib/mount are 8-1/2" long. These get covered with CF strips. The next two ribs to the inside and outside of the ones above are 7-1/2" long. All the other ribs are 7" long.
Out of light wood, I cut 1/8 ribs for most of them with 1/4'" ribs for the ribs of either side of the center rib/mount and the outboard wing tip. Care should be given that the ribs are symmetrical and the cutouts for the 3/16" trailing edge are straight.
Once the ribs are installed, add reinforcing to key areas. I cover the top and bottom of the two inside 1/4" ribs with CF strapping, overlapping for 1/2" on the leading edge and running all the way to the back of the trailing edge. I also put on 1/2 inch wide 1/16" thick balsa, cross-grain doublers on each rib in front of the TE to hold the ribs from splitting during crashes.
Wingtips are cut from 1/4" wood and fit in the groove cut for the CF Tube cut in the leading edge. The inside wingtip should be made from "light" balsa. The outside tip can be made from basswood or hard heavy balsa for more strength where it counts, since you need to add some wingtip weight anyway.
Add 1-1/2" triangular balsa gussets made of light 3/16" balsa from the wingtip to both the LE and TE. For maximum strength make sure the wood grain on all gussets runs in the direction of the angle.
On the inboard wingtip, drill through the 1/4" wing ip, two 1/8th inch holes for aluminum tubing glued in place for the leadouts. Place the two lead-outs exactly 4" (for the up line) and 5" back (for the down line) from the "front" of the LE. That is a lot of line rake, but needed on these low performance planes.
Mount the bell crank center-hole about 4 inches back from the front of the LE by drilling a hole through the center rib/mount assembly. I used a 4/40 counter-sunk bolt into a blind nut and a 2" nylon bell crank spaced in the middle of the wing thickness. I cut out the back of the mount spacer a little to clear the bellcrank. I mount the bellcrank so the pushrod exits on the outside of the center rib/mount. I use old .021 lines as leadouts. I add the pushrod after covering the airplane by splitting the covering and using a pushrod with a "Z" bend where it attaches to the bellcrank.
On plane #9 I added several layers of 1/16 balsa cap stripping on the top and bottom of the center rip after installing the bell crank. This I shaped to give the center rib the same airfoil as the rest of the plane. This makes the center rib stronger and looks better. On all the other planes I've built, the center rib was flat and that did not seem to matter in the plane handling.
The planking helps secure the motor mounts and prevent the LE from breaking on impact in the center of the plane, plus provides a place to connect the covering to the plane. I use light 3/16 triangular planking that is thinned out to clear the tank at the LE. This planking runs diagonally from 1 inch in back of the motor mount out to the ribs on either side of the center ribs. Use "thick" CA glue to seal well where the tank vents exit the planking. The grain on this planking should run parallel with the angle for maximum strength.
I cover all my diesel planes with regular Ultracote which weighs 31-33 Grams on an Iron Monger if put on in one sheet lapping over only on the trailing edge. Regular Ultracote weighs .30 oz per square foot. That is about 35% heavier than some other types, but has many advantages including:
If your planes come out heavy, then covering with a Core House covering would weigh only 20 grams or 37% lighter.
There is a light weight version, called Ultracote Lite, but I've never used it and don't know how much weight is saved or how tough it is, but I know the Free Fight guys use it. It might be worth a try if your plane is coming out heavy.
I use a Hanger 9 "electronic" heat-controlled iron as it has much better temperature control than the less expensive rheostat types that get too hot at times while resting idle.
If you can cover in just one piece of material instead of the usual 4 pieces, it saves about 4 grams of weight by having minimum material overlap. With one-piece covering, your seams should be only around the engine mount, wingtips, and TE.
It also helps to add additional CA glue around the mount and tank vents to help prevent fuel contamination. If you use Ultracote, use any color except carbon/kevlar since this is "MY" color.
The elevator can be made several ways:
Note: 1/16" piano wire or thicker aluminum welding-rod-type pushrods both weigh about 4 grams. Thicker aluminum rods flex less and are easier to bend, but fatigue if bent too much. Use a large but lightweight nylon control horn and set it up with about 20 degrees of throw, in both up and down positions using the hole next to the top hole. This allows for more or less control to be added by changing the connection holes at the field.
I sew the elevator to the stab with waxed dental floss using a figure-8 stitch in six places through 1/16" diameter holes drilled 1/8" in on both the elevator and stab, and use CA to fill the holes after being sewed. CA glue does not stick to the waxed surface, so the hinges remain flexible, and diesel fuel does not affect the waxed hinges much in a season. I do replace the hinges after two years. Other methods work also. Some use nylon fishing line, which lasts longer, but I think it is too stiff.
Both the PAW and Sharma engines are tough, but the needle valves need to be protected from damage when crashing or inverted landings. Replacement needle valve bodies cost $20 and you will lose the match on air time if yours breaks. I use 1/16" thick aluminum sheet attached to the two top engine mounting bolts and bent with a hole drilled that the needle valve goes through. This supports the needle valve from being bent backwards during crashes.
The original 4 wrecked planes that were given to me were repaired and used for practice and teaching my son to fly. Only one of these was destroyed in a contest. No. 4 is still used in contests as needed. All these planes fly stable and turn good. Weight RTF:
I could have saved 13 Grams or 1/2 ounce by using Core House clear covering on each of these planes, but felt the extra strength and fuel resistance of Ultracote was worth the extra weight.
To trim your plane, see my article "Trimming a control-line combat plane"
At right is a photo of one of the original Iron Mongers from the seventies:
Note: The two inside ribs and planking are now spaced wider and the bellcrank exits the outboard wing rather than as shown in the picture of the original planes blow.
This page was upated Feb. 25, 2009