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

[Bill Gaylord]

This is second of two posts in a row, due to the photo limit.

The battery door has been test fitted, with a side mounted hinge and spring assisted slide latch mechanism. The hinge pin must be located such that the hinge line (hinge pin) is aligned with the center of rotation, which is the edge of the door. Slots for the hinges must be cut to accomplish the proper hinge position, and are also angled to allow for them to insert into some "meat", which was accomplished by laminating a bit of wood onto the fuse opening and the door body. After the latching pin hole is located, the surrounding area is hardened with CA, to prevent the latching pin from poking a new hole in the balsa, since it is spring assisted. I considered a small magnet, although there are some advantages to the spring assisted slide latch. The latch stays shut, doesn't require tab to grab onto, and may likely add less weight than a magnet and washer, shown by the overall weight of the finished door assembly of 1.9 grams. The plastic teardrop shaped detail part in the first photo has a mounting spring that will glue into a hole drilled into the front of the battery door. The part has a spring glued into it, to allow for shock absorption.

[davidchoate]

Looks nice. I see You got the same servois I just purchased. I am watching this thread closely as I plan to build it RC. But I believe I will Do the C-47 1st. I think it will be an easier conversion and Fly more stable once I lighten it up.Bill, Do You plan on retracts on Her?

[Bill Gaylord]

Thanks. No landing gear for this model, as even 5 grams is 5 grams too much. I thought about a plug-in gear setup, but even the mounting bases would add weight, so the idea was scrapped. The battery sits in the nose area where the front strut would sit, so there would be very little depth for gear leg insertion, making it display gear only. Granted the battery I hope to use will be adequate, it could possibly work, as the battery is installed at an angle to reduce it's height. This keeps it away from the center keel and would leave reasonable depth for a nose gear plug-in mount. Of course I'd be stuck with that package size forever. The other problem is that the nacelle bottoms would need to be cut away to allow for gear insertion. Could always leave them open and reduce weight, although it wouldn't look good. Hinging the doors and leaving them in place would add yet more weight.

[Bill Gaylord]

Pic 1. The cocpit glass is now attached, faired in with strips of 1/32" balsa, sanded flush with the edge of the plastic, followed by a coat of sanded filler. The filler tends to mush a bit when heating during covering, which is why the balsa was applied first. The balsa is also lighter, as filler weighs. The covering will be trimmed and ironed down at the edge of the plastic, being careful not to heat and warp the plastic parts. The plastic part edges must be fully glued down, or any heat applied will cause the edge to lift up in that area.

Pic 2. Don't miss any chance to shave weight, cutting away the unnecessary plastic under the top turret.

Pic 3. I came across a small 1A switching BEC on Ebay, with good reviews. I may use this part. Other thoughts are to wire the on board ESC linear BECs in parallel, with diode isolation. I could use a 4 pin plug, and wire the BECs to have the option of using either the linear BECs, or the switching BEC, using either a jumper plug for the linear BECs, or the switching BEC.

The motor wires are a bit stiff, and you want to be careful not to horse them, at the motor attaching point. I cut away a bit of the shrink tube, to allow the wires to flex closer to the motor, and fit in the nacelles. Fortunately the upper area of the nacelles is further away from the motor centers than the bottom, which will allow room for the wires.

The covered stab now weighs 5 grams, showing how covering adds weight. At this point, the fuse balances properly with an Eflite 2s-430 fully forward, and about 13 grams of weight added at the stab location. With the aft weight of the v-stabs/rudders, paint, and rear turret still to be added, that figure could be reached, although the motors/ESCs should provide more forward weight, in which case the battery can be moved rearward about 1-1/4". All in all, it appears to be pretty much on target.

[davidchoate]

I was curious to see how You were going to do the tail Rudders linkage. It'll Fly, Do the B-25 outer wing panels have antihedral, or is it an illusion? I watched the B-25 episode of Warplanes on AHC Channel, and they were not a very large aircraft at all. Cramped aqs hell inside.

[Bill Gaylord]

Built with the plan shim heights, the wing has the slight anhedral angle on the outer wing panels. I'm really pleased with how the panels turned out, now that I can really see them after covering. I gave everything a few days to cover itself and it didn't happen, so I finally got back on it. With the plan height shims in place and the wing panel ends pinned flat to the board before stringing the wing panels, they end up with slight, progressive washout toward the tips. I don't know if this was intentional or not, but I couldn't ask for any better. A lot of effort went into designing this model, back in the hand drawn days. I would imagine if it had gone out of production, it would be more valuable than the Mosquito, and closer to B29 territory. It's a lot of work to build, but a beautifully detailed model.

The fuse now has one side remaining to be covered. The center wing mounting helps a bit, as you can slit the covering along the wing mounting area, and work a single piece of covering around the rear fuse, up to the cocpit area. The covering is later cut away from the wing panel attachment area, to provide a gluing surface. Without having this area to take up excess covering, it would be difficult, due to the fuselage's curved shape. It was still a bit of work as it is, but you can work wrinkles out of Microlite that would be impossible to work out of heavier coverings. The model will be finished in the scheme of the Duxford museum B25. The stab and wing panel bottoms are covered with white Microlite, which will require little painting to cover with ultra light blue-grey paint. The upper surfaces will be painted with Model Masters Blue Angle blue, already on the stab. This color also requires little painting to cover fully, being close to the color of the dark blue Microlite covering.

I test ran the ESCs individually with a diode drop, and could not heat up the linear BEC while cycling all 3 servos, which is a good sign. The BECs will be ran in parallel with diode isolation. Along with that, the motors were test ran with the wires marked for proper rotation direction. The ESCs can probably be reversed through programming, but it's easier to just tag the wires. I'll probably solder them directly, as 6 pairs of 2mm gold plated pin connectors is a good 3 grams of dead weight.

[davidchoate]

Nice. The Wing chord looks bigger than I imagined. Have You guestimated wing area yet?

[Bill Gaylord]

Wing loading calcs should always be the first thing done, giving a good idea of what you can and can't do, to achieve something that's flyable.

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.

One reason myself and others hadn't successfully flown this model in the past as a r/c conversion, is not being realistic about the wing loading and how to achieve a reasonable wing loading. I've seen some people guesstimate AUWs that are simply unrealistic, along with including unnecessary items such as landing gear, when they should be trying to shave every gram. When I looked over this model I pretty much shook my head, comparing it with other models. Being realistic, a 10oz AUW is within reason and about the lightest AUW you can really hope for, which is something you toss relatively hard and fly/land fairly fast. If it comes out lighter, then it's a bonus. Being overly optimistic about AUWs certainly doesn't help either. Building a number of various models helps, since it gives you a pretty accurate estimate of how to build the model to achieve a reasonable AUW.

[David Lewis]

Two other ways, besides reducing weight, to make a heavily loaded airplane fly better is to use a high lift coefficient airfoil and increase the wing area larger than scale. It's probably advisable to enlarge the stabilizer by the same percentage too if you go to a larger wing.

My experiments indicate you get more bang for your buck by increasing chord than span (although both are good) because airfoil Re is proportional to chord. I paint black stripes on the LE and TE to help disguise the overscale planform.

You get a more accurate picture if you go by cube loading than wing loading because you can directly compare wing loading of different size airplanes. (Cube loading = weight divided by [wing area raised to the 1.5 power])

On tail heavy airplanes, some designers increase the nose chord and/or enlarge the stabilizer to reduce the amount of nose weight required. (The larger the stabilizer, the further back you can move the CG.) The farther back the CG, the higher the maximum lift coefficient will be. It also helps to locate the heaviest items, such as battery, as far forward as possible.

[Bill Gaylord]

I thought about cheating scale on this model, but the airfoil section is out of the question, due to the molded nacelles that fit the wing. Being somewhat of a "contest" subject between builders, to actually get one of these flying, it would probably ruin it a bit, to have cheats. Theoretically high aspect ratio wings are more efficient, although at short chords lengths it's not a blanket statement, as you were indicating due to the Re. One good point about increasing chord, is that high aspect ratio planes tend to have more stability issues, if not designed properly. If any cheat would be reasonable, it would be to increase the tip chord, which I had to talk myself out of. This is one of those subjects where an extra few square inches of area toward the tips would probably really help. As for airfoil sections in general, the airfoil sections on this model are actually fairly close to the transition that I used from the inner to outer panels on the Miles Aerovan, using scale sections. It flew well, although it also has less taper toward the tips.

This subject has proven easy to balance, even with a relatively small 2s-430 mAh lipo, which is probably about as small as I care to go and have an ample rating. I cringe at the size of the batteries people use in ARFs these days. My philosophy is no ballast, and use the lightest battery possible. With lipos, generally the lightest battery that will supply the required power will still provide a reasonable flight time of 5-8 minutes. I think one of the issues that other builders encountered was not using light wood in the tail section, along with lightening the tail feathers a bit, which caused them to need a heavier battery and/or ballast. As for cubic loading, there's no substitute for having 100+ builds of diversified subjects from 16"-52" span to develop working specs from. At the sub 30" span (also depending on area) wing loading becomes increasingly critical for every inch of decreased span.

[mustangman]

bill - I see your super cub made it on the guillows modelers photo page!! awesome!! mark

[Chris A.]

With regard to weight saving, Tom Arnold designed and successfully flies a Heinkel He 219 A-6 twin engine night fighter on rubber. The He 219 is configured much like a B 25; mid-wing, radial shaped nacelles, slab side fuselage, moderate aspect ratio wings and twin tail. However the Heinkel fuselage is slimmer, but deeper and the horizontal stabilizer has dihedral. Your comment about wing stringers, leading and trailing edges is what caught my attention. Arnold used carbon fiber strings to reinforce very slim wing spars. The fuselage is also box and former. HIs plans are available and show two wing designs, a short wing with short outer wing panels and the prototype long nacelle version with the extended long aspect ratio outer panels, both using carbon fiber. As for stringers, how much weight savings would there be if you went to 1/20th stringers? As for the leading edges, did Micro Tools ever make a modeler's miniature router that you could make the leading edges "U" shaped by routing out the center??

I read a comparison between the B 25 and the B 26. The B 26 originally started with a very high wing loading but later wing design resulted in a larger area wing. The B 25 had better take off performance, but both had a landing speed between 115-125 MPH. You had to have some muscle to fly either of them as both lacked boosted controls.

[woundedbear]

Hay! Bill that's one more nice build! Are you going to post some video of it in flight? Love the paint scheme you chose for this airplane Too. Guillows isn't thinking about stopping production of this kit are they ? I hope not, I live for the day when all of Guillows current kits are all laser cut kits, and they start adding new subjects to build.

[davidchoate]

I am ,as I said before, Going to do My 1st multi engine build very soon. I was set on the C-47, but staring at the plan , It has snall wings. I want to figure out the area, but forget how to find the area of a triangle. If anyone knows please tell me. I understand that at full scale this wing is great. I get the "more bang for Your buck" thing, and if they only made the C-47 like 1 ft. larger I think it would be fine. Also Do You count the nacelle, and fuselage parts of the wing (C-47) in the wing area. I have the B-25 kit, and a nice size Dornier 335 Push Pull twin engine kit which is complex, but light, and has a variant of the night fighter version that had two cockpits. I guess they needed another man to operate the radar. The night fighter had a larger wing, and the kit supplies plans and LC parts for both. I don't know yet. I 'll wait and see how Bill's Flies, and may do the B-25 if His results are good.

[Mitch]

It does not make sense to me, but I have been told you count the fuselage portion and any nacelles. so just look at the wing on the plan. For a rough measurement I just take the wingspan, and look at the cord, I choose what I believe is the average. Then it is just Length x Width, or Wingspan x Average Cord.

A more accurate way that works I have been told is to trace out the wing on cardboard. Weigh the cardboard as the entire rectangle first, then cut out the shape of the wing, weigh the cardboard again. Multiply this percent of the original weight by the area of the rectangle. This is very accurate if you have an accurate scale and an odd shaped wing.

Example:
Size of cardboard is 10 inches x 40 inches = 400 square inches. weight is 10.00 grams
Cut out wing and weigh again... say it is 8.36 grams
400 X .836 = 334.4

I have never done this but I have read an article describing exactly this in the FAC Newsletter!