B25 Guillow's r/c w/Flight video pg6

[Bill Gaylord]

Finally cut out and notched all the parts for this kit. Doesn't look like much after it's done, but the 3/32" wood requires a bit more cutting effort to remove from the sheets, along with the notching, which is not done on this kit. There's a few good hours required to prep the parts. I could have back sanded the sheets, but in the past it really doesn't help much, unless a considerable amount is taken off. The bottom of the notches are die-cut, but not the actual notches. It makes cutting them out a bit easier, although it also tends to cause additional wood to break off, requiring the small pieces to be glued back on. The wood is pretty decent in this kit, although the stringers are iron wood. I have some 3/32" stringers on hand, and have also cut some by hand, for the build. The weight of the kit stringers would be a killer. The same goes for the heavy wing LE stock, where I have lighter stock which will replace it.

Depending on weight, I have both 10 gram and Power Up 250 HURC outrunners. The 10 gram units are preferable, but the 17 gram 250 units are also viable. They've been a good motor in the past, where the small diameter has made them useful, for thin nacelle scale apps, and are also a bit lighter than other motors in the class size. The tail feathers will require lightening, at minimal using lighter wood. A stab thicker than 1/16" is still desirable, for both strength, and to hide functional rudder linkage inside. One encouraging thing about this build, is that every one I've seen so far as an r/c conversion could have been built lighter. They remind me of my first build of this plane, with heavy covering, wing sheeting, and using heavy kit stock stringers and LE stock. For this build, I'd prefer to use some inset planking on the fuse for appearance, before adding any sheet weight to the wing panels. In any case, light wood will be used. The engine mounts certainly don't require the added weight of a full 1/16" ply sheet, laminated to the balsa former which is used to located the nacelle covers.

[Chris A.]

Before the use of electric motors, was the idea to power your B25 model with 2 x .049 gas engines? I remember kits converted from rubber to .049s that simply vibrated apart. If you are using electrics motors can you get really aggressive lightening the wing structure along with the tail and fuselage?

[Bill Gaylord]

I could imagine what two 049s would cause, in terms of vibration. They specified 020 to 049 engines, for CL. With two 049s, the model would really weigh. For lightening, the worst contributors to weight are the heavy LE and stringer stock, as well as the plastic parts. The beautiful molded plastic parts are what make the kit, so they will stay. As for lightening the tail feathers, cutting new parts from lighter wood will be a major contributor, as well as probably reducing the depth of the LE stock on the stab and rudders. The tail feathers will not be exactly to plan, for the purpose of weight savings.

The wing panels are mostly finished, with the bottom stringing remaining. In the first photo, the pencil points to the ends of the inner stringers, which I accidentally glued a few and had to cut the glue joints, as they are not to be glued until the dihedral blocks are set in place. I had reservations about the plan construction method, which calls for stringing the top panels first, and then stringing the bottom panels, after pulling the wings from the board. Without a main spar, that could cause some warping, so I ran a single stringer under the bottom of the formers and glued it in place, during the construction on the building board. Using the plan dihedral shim block heights, the wing panels appear to be straight across the formers. One thing to note in the second photo is that the end former is pinned flat to the board, at both the front and rear. Without doing that, the wing panels would probably be warped and inconsistent.

The wood in this kit was pretty good, other than heavy LE and stringer stock. I cut a number of lighter stringers for the build, as well as having some on hand. For weight savings, the LE stock was replaced with lighter stock, pre-sculpted before gluing in place. I like to get them as close as possible without being undersized, which minimizes the shaping effort after gluing them in place. The aileron headers were replaced with lighter 1/16" sheet, as the 3/32" is a bit heavy. The kit aileron headers are also a bit short on the aileron side, as they intend for the ailerons to be thickened with cardstock covering, where these ailerons will have iron-on Microlite covering.

[David Lewis]

On the B-25, and most other 800 series kits, rubber power was never contemplated. They were intended for static display or Cox glow power control line, but I predict control line would require an experienced pilot for a model that small, heavy and fast.

[Bill Gaylord]

On the B-25, and most other 800 series kits, rubber power was never contemplated. They were intended for static display or Cox glow power control line, but I predict control line would require an experienced pilot for a model that small, heavy and fast.

It would be interesting in hearing from people who actually flew this model as a c/l model. I haven't found any information, although I did read about how Doug McHard supposedly had flown the model as a ff subject. I would assume that would have been with .020 engines. As for small and fast, that will probably be the same for r/c. Running some numbers, this model realistically has approximately .65 sq-ft of wing area. A 10oz AUW model will be at the high end, with a 15oz/sq-ft wing loading. I've flown the 190 at 14oz, but it has clean wings and is not a twin.

The photos below show the perfect example of how using lighter wood is probably the best lightening method. The kit wood was pretty good, but the stringers were basically hardwood. While it's difficult to lighten the design of a structure by 50%, it not difficult to find wood 50% lighter than heavy wood. An example is the 14 gram stringer sheet. The other stringer sheet weighs 12 grams. The sheet provided for building sheet tail feathers weights half of that, at 7 grams, and is decent wood like the rest of the kit. Using these stringers would have easily added 10 grams to the model if not more, based on the number of stringers used by the kit, and the weight of using lighter stringers. For this model, that's 10 grams that you can't afford.The completed wing panels with ailerons weigh 21.2 grams. They would weight considerably more, with the kit LE and stringer stock.

[Bill Gaylord]

The fuse went together well, with a hair of adjustment on the side keel slots, making a better fit for the wing panel mounting plates. The wing panel mounting plates fit well in the fuse sides, only requiring a hair of sanding to fit perfectly in the area between the lower and upper fuse keels. A test fit of the wing panels showed them to be straight, with no warps. At this point the build will become a bit more enjoyable, after getting past some of the most tedious parts, which are building a straight fuse and wing panels. The fuse as pictured with the wing panels and ailerons weighs 36.1 grams, which I can live with. The stringers that have been put aside for the fuse are a really nice grade of light wood, still reasonably strong.

[davidchoate]

AS I said before Bill, I also want to do a twin power Plane. I have the C-47 and B-25 Kits. I am leaning towards the C-47. I see the problems Dave Lewis mentioned. I saw a lot of plastic on the B-25. I would lose that,and use other methods to accomplish their purpose. I also ,if I chose the B-25 would enlarge the Plan Sheets a little. I have not gotten into any real Plans on the C-47 yet, but It appears I do not have a lot of plastic to get rid of. The formers are too wide, and the stringers are a plenty. As I said I have not a plan yet, but I could even even end up starting with a "Box Frame" and slice the formers sides to fit the sides of the "box". The P-38 ,though it has a good size, Is so loaded with plastic I am not yet skilled enough to deal with that. My 190 has that high wing loading and so a high stall speed, and fast cruise speed that I know I will have to put Her aside until next year when My piloting Skills are better. I know You know what You are doing with all the experience You have, and am glad You are doing this now as it will help Me with My twin project. I am following closely.

[Bill Gaylord]

For this model, the plastic parts will be used as the make the model, but they do weigh. That said, everything else must be lightened. The clear parts weigh less than the white ones however, with the nacelles being the worst contributors in terms of weight. The C47 would be a great subject. Just don't use sheet tail feathers as I did, or you will be adding ballast. Done again, they would have been framed. I just finished the tail feathers for this build at 4.5 grams. With the rudder linkage, they will be around 5.5 grams. I can live with that, but much more would start to require a larger batter than needed, or ballast. The tail servos will be mounted forward of the wing, for balance. There's really not that much nose on this model, given that the gun turret does not house the battery and it stops at the front former, for the J model.

The tail feathers used decent wood, with a piece used for the 3/16" thick LE that weighed only 1 gram. The CF joiner bar is not stiff enough, and the .025" joiner wire will also be used. This will be my fourth twin rudder linkage, so the bellcrank linkage scheme is pretty straight forward. The stab is not as thick as the plan callout, but thick enough to house the .020" linkage rods. The top rear plastic cover is designed for a thicker stab, so this one will require about 1/32" of filler on either side. The stab is 3/16" thick, tapering to 3/32" at the TE. The rudders are 3/32" thick.

[NcGunny]

Bill,
They may have used a bunch of old Comet wood..lol!! I came across a old 109 kit from Guillows that was all hard wood. I cut out 2 parts and boxed it up for another day and probably will just scan the sheets and reprint onto some light stock. Looking good so far!!

[davidchoate]

Thanks for the tip.

[Bill Gaylord]

The wood wasn't bad, just the stringers and hard stock being typical Guillows. They're usually either too heavy, or occasionally far too light. The stringers seem to be either grained at some highly weakening angle, or well grained but too heavy. These stringers would be good for a larger wing build with shear web main spar construction. I did build a Sterling kit recently, where the wood was heavier and stronger than basswood. Had to cut most of the parts by hand.

The rudder linkage is now in place. The .020" pushrods exit the stab bottom and are bent rearward around 5/16" from the rudder hinge line. If the rudder linkage connection point is closer to the rudder hinge lines, then the rudders are floppy. Bend the wires much further rearward, and the wires become floppy, due to the bend angle. The rudder pushrod linkage ends are bent 90 degrees downward, where the control horns install under them. The pushrods naturally exit the stab at a downward angle, due to rubbing against the stab bottom surface if they are rotated upward. This works well, as it allows for down elevator, and keeps them from moving upward and dislodging from the control horn linkage points. Slots are cut into the rudders for the sheet plastic control horns. The horns are adjusted into position to center the rudders, and are then glued into place. There is no adjustment afterward, short of removing and reinstalling a control horn, so you want to center the rudders well. The front of the stab has a slot cut into the center, which allows for the servo driven pushrod to insert into the stab, with a 90 degree bend on the end. The stab has gusseting above and below this pushrod entry slot, to reinforce the main spar joint in the center. The pushrods all enter the bellcrank from the bottom and are held up by the sheet in the bottom center of the stab, so they cannot dislodge from the bellcrank. This arrangement eliminates the need for s-bends, which would make installing the bellcrank more difficult.

Given the relatively small ailerons on this model, a slight rudder mix would probably be a good thing. I did that with the Miles M38 and Aerovan twin rudder aircraft, which handle well.

[Bill Gaylord]

The aileron "cable" pushrods are now installed, using .015" music wire. There are only a few points of short length tubing sheathing, as full sheathing is unnecessary and weighs. Note the short lengths of hard balsa stringers used as wire guides along the bend, where the cable would buck back and forth changing length, without them. With the thin wire the friction is low and acceptable, and is reduced further once the pushrod wire ends are installed in the aileron control horns. The control horns will be located to flex the wire inward toward the fuse, which both holds the 90 degree bends into the aileron horns, as well as reducing pushrod wire friction. After test fitting the pushrod wires into the single aileron servo E-Z link, the wire ends exiting the wing at the fuse attachment side will have temporary 90 degree bends placed on them. The wires were cut longer than necessary, as these 90 degree bends will be cut off, after installing the ailerons and aileron horns. The temporary bends will keep the wire from falling into the wing after covered, which would be a problem.

The last photo shows the approximate location of the three servos in the fuse, viewed from the bottom. They will be screwed in place on mounting blocks, not yet installed. The servos fit perfectly between the two fuse side keels, which is what is temporarily holding them in place for the photo. A thin shaving will be removed from the area of the fuse keels that is now tight against the tail servo control horn centers. While I prefer to keep battery doors as small as possible, the lower hatch will probably run across the first stringers below the lower fuse side keels. This will provide access to all three servos, as well as the E-Z link on the aileron servo, which will require holding firm with tweezers, while tightening the set screw.

As of now, the Eflite 2s 800 mAh appears to be easily capable of balancing the model in the front of the fuse, considering the yet to be added aft weight of stringers and covering. If possible, I'll downsize to a smaller lipo still capable of powering the two 10 gram outrnners, to reduce weight further. I really don't care for 1/1-2 oz of battery weight, if the battery can be downsized a bit.

[Bill Gaylord]

Enough with the inset planking, as the fuse with battery door, gear, and pushrods is now 41.5 grams. Still not bad, but I need to keep this build as light as possible, while avoiding the "covered frame" appearance in the frontal area. The entire weight of the fuse, wing panels, and tail feathers is now around 70 grams. The receiver is located on a plate that runs across the cocpit area. The battery door fabrication first required a former to be added across the fuse bottom, to create a 90 degree surface to the fuse keels, as the second fuse former sits at an angle and would not allow a door to open. The access door formers were first traced against the fuse, and then undersized for 1/32" sheeting. The 3/32" stringers on the access door were installed, with the formers inside the opening area. The framed door was then removed from the fuselage and sheeted, and then lightly sanded while held in the fuse, to perfectly blend with the fuselage.

[Bill Gaylord]

Always thinking ahead, I started test fitting some of the plastic parts. This is one of those projects where just in general, you always have to be thinking ahead. The canopy, front turret and tail piece are fitting well now. The side gunner turret windows have a large area of plastic that runs from former to former, which adds weight. All you really need is the turret bodies, with small sections of balsa added between the stringers to attach the turrets to. Since the turrets are offset and in different locations on either side of the fuse, only one piece of balsa is required to be added to the right side of the fuse (see photo). With this mounting arrangement, a good bit of the plastic weight will be cut away, leaving just a small flange on either side of the turrets for glue attachment.

The motor mounts are in place at about 18mm height, which will get me close. Better to add an additional plate/washers, versus being out too far. The plan is to orient the motor mount bases when viewed from the front in the "X" orientation, versus the "+" orientation, which minimizes the size of the mounting base required. The actual motor mount base plates are made using 3/32" hard balsa, for secure screw attachment. The remainder of the mounting "boxes" is made from light 3/32" sheet stock, with the "dead weight" cut away from behind the mounts, after installing them. The engine mount bases are tapered a bit, to remove the outward facing engine mounting angles, designed for CL. The speed controls will mount above the motor mount boxes, with a small hole drilled for the wires to pass through the wing, and into the fuselage. The aileron cable wire is at the very top of the wing, not interfering with these wires. The wires entering the fuse need to be kept away from the aileron cable wires, using wire guides glued onto the fuse former located near the wing leading edge, where the wires enter into the battery/gear bay. These were fabricated as "C" shaped pieces from 1/32" ply, glued onto the former as wire guides.

[Bill Gaylord]

Ready to start covering parts now, after finishing a number of small details. The ready to cover wing panels now weigh a combined weight of exactly 1oz, which is as good as I could expect. The small scale inlet vents were trimmed and sanded down to about 3/32" in depth, and glued into notches cut in the wing LE. This is pretty much scale, as the large molded plastic piece would also be too large, add weight, and disrupt the LE airflow. Done again, I may have just cut the vents into the wing LE. The stringers and formers were notched for the rear vent covers, which are molded on a plastic plate that fits across two formers and stringers. The notching was done so that the top surfaces of the cover plates are now flush with the wing surface, which will eliminate a visible raised edge in the covering. Non functional scale landing lights were also added to the wing LEs.

The front and rear gun barrel mounts are made using rubber servo bushings, inserted into holes drilled in the plastic parts. They stay permanently in the plastic parts as they are flanged which locks them in place after insertion, but allow for plug-in gun barrels, which are both flexible due to the rubber to prevent damage, and allow for removing the gun barrels for flight, if desired. The brass bushings glued onto the gun barrels come with the rubber servo mount bushings. The fuselage side guns appear to just barely clear the 5" propeller tips, with a test fit. They were sanded down about 1mm, after trimming from the plastic parts sheet.

The top fuselage antenna has a spring glued into it, which will be glued into the fuse, to allow for a flexible antenna mount, to prevent damage. Balsa was added to the fuse top center keel, to provide additional surface around the drilled mounting hole for covering attachment and to strengthen the area around the hole. The scale antenna wires will probably be added also, using CL string.