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Jun 14, 2017, 10:10 AM
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Build Log

F-82 from two E-Flite UMX P-51 BL


I had two P-51's and enjoyed flying both but decided to have some fun and try my hand at making an F-82. The looks are not completely scale but it turned out pretty well. I made it so it will break down and can be transported safely in the two boxes. It flies well and looks good in the air, certainly not as nimble as the P-51 was but overall I think I'll enjoy it!

Build details below...
Last edited by mdmyers; Sep 18, 2017 at 06:58 PM.
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Jun 14, 2017, 10:13 PM
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First step is to trim off one wing from each plane (be sure to take one left and one right!). Before doing this you need to remove the silver tape covering up the wire channel. Next open up the fuselages, unplug the aileron connector for the wing you are removing and get the connector and wire pulled out. With some care, you can do this without removing the air scoop. Once you pull the connector down through the hole in the fuselage, use something poked down into the hole to get it to lay flat and then gently pull and work it out from between the scoop and the bottom of the fuselage. You can pull on the scoop gently to create a little more space to get the connector through.

Now that the wires are out of the way, measuring from the inside edge of the outermost panel line, you want to cut at about 141mm. Do your best to cut through vertically. I just used a new sharp Xacto blade and took my time. You can use the top piece of foam from the box to help mark the cut line as it is at the right distance and helps you make a straight line across the top cord of the wing. I forgot to get an initial picture of this so the picture below was taken after I had assembled my center section but you can see how the edge lines up with the joint line.
Last edited by mdmyers; Sep 18, 2017 at 06:31 PM.
Jun 14, 2017, 10:23 PM
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Due to the original dihedral you can't just join the two stubs unless you want kind of an inverted gull wing look in the center section so you will need to now cut off the remaining stubs. I cut them on the wingtip side of the inner-most panel line right at the wing root. In this case, try to cut so that the surface left on the fuselage is vertical (which means that the surface left on the wing will be angled, but we'll deal with that in a bit). There is a panel line on both the top and bottom that you can follow. I worked my way from both sides and met in the middle.
Jun 15, 2017, 09:40 AM
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To get the center section to be flat across between the two fuselages you need to make sure the ends where things meet are perpendicular to the wing surface. I used sand paper and a sanding block to do this. Work carefully and slowly to keep the surfaces flat and fit them together as you go to check your progress. The foam sands easily so don't take too many strokes between checks. I both drew the parts over a sheet of sandpaper laying on a flat surface as well as using paper wrapped around a block. I used 80 grit to get started and finished with 150. Depending on how your initial cuts worked out you may or may not need to sand the surfaces on the fuselages and the outboard end of the wings but you will definitely need to do the inner edge of the stubs. I had to do a little touch up on everything but mostly on the inner edges. On the inner edge of the stubs, because you are removing material from a joint that used to mate perfectly to remove the dihedral, you will find that the fit against the fuselage won't be perfect anymore and the stub will be slightly smaller, that's just the way it is.

When all was said and done, my stubs were about 63.5mm wide. There's nothing magic about that and it you can adjust as you like, but it gave me a couple mm overlap on the elevators when everything was assembled which was my goal.
Last edited by mdmyers; Jun 15, 2017 at 03:59 PM.
Jun 15, 2017, 10:23 AM
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Next I glued the two stubs together at the center with CA making sure they were flat, once cured this gave me a complete new center wing section. As my sanding wasn't perfect I also filled a few small gaps in the joint with foam cure afterwards.

Then I cut a piece of 3mm CF rod to 236mm and sharpened both ends on a dremel grinding disk (Note: sharper is better). There's nothing too critical about the length, it just gave me about 55mm out either side of the center section to embed into either fuselage which went the width of fuselage plus a little more into the opposite wing. I then used the sharp end to drill/poke through the center section. I used the panel line running across the sections as a guide front to back and just tried to keep it centered up and down. I actually went through halfway from one direction and halfway from the other and managed to meet in the middle. Just take your time and adjust as you go to keep it aligned.

Before I glued the spar in place I slid it to one side, positioned the center section against the fuselage and then pushed the spar into the fuselage a bit to mark the starting point. Wash, rinse, repeat to mark the other side. As mentioned previously, the center section cross-section is slightly smaller than the fuselage cross-section due to sanding so they won't match perfectly. I chose to line up the leading edge and upper surfaces as best possible leaving the differences on the bottom surface and trailing edge. You can do what you want.

Once I had things marked to my satisfaction, I re-centered the spar and then dripped some CA into the joint from both ends letting it wick in and cure.
Last edited by mdmyers; Jun 15, 2017 at 09:52 PM.
Jun 15, 2017, 04:21 PM
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The next step was to carefully drill the ends of the spar through each fuselage where you marked them making sure that the center wing section lines up where you want it on each fuselage. In my case the spar came through right in the back of the battery compartment. I then glued the center section permanently to the left fuselage after squirting some foam tac into the holes for the spar and using CA where the foam met. The only reason I chose the left was because the mating surfaces on the right side were a little better looking fit and I wanted to use them as the break down joint.

Note: If you don't care about being able to break your F-82 into two halves, you can just glue both halves together and skip ahead to the elevator section below. If you want to be able to break it down, read on...

I then cut two more short sections of the 3mm CF rod to use as locating pins on the break down joint. They were each about 25mm long and I drilled/poked them into the center section half their length deep in the locations shown in the photos. There was nothing special about their location other than I just wanted one toward the leading edge and one towards the trailing. I then took those rods out and made one other one that was just long enough for the point to stick out of the holes a few mm. With the short rod in one of the locating pin holes, I inserted the main spar into it's hole and after making sure things were lined up to my liking, press the fuselage and center section together to mark the locating pin spot on the fuselage and then repeated for the other locating pin position. I then stuck the two original locating pins into the center section and glued them in place with some CA, eye-balling them to be sticking straight out the edge. When the glue set, I then assembled the two halves again, inserting the main spar first and then lining up the alignment pins on the marks made earlier and pushed things together so that the alignment pins penetrated all the way in.
Last edited by mdmyers; Jun 15, 2017 at 09:53 PM.
Jun 15, 2017, 04:44 PM
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Rather than just using holes in the foam as receptacles for the spar and locating pins on the break down joint, I decided I wanted to insert some kind of tubing in the holes that the CF rod could slip into as I felt it would be a more rigid and durable joint. I found some CF tube at the LHS that was nominally 5mm OD x 3.1 mm ID. My 3mm rod slipped rather tightly into that so I drilled the ID out with a 1/8" bit and that proved to be a good slip fit. I cut 4 sections of the drilled out tubing about 12mm long to use as the sleeves. I enlarged the main spar holes on either side of the battery compartment and the holes for the alignment pins with a hand held drill bit so that I could slide the sleeves in (I used a short sleeve on either side of the battery compartment rather than one long one figuring it would be easier to get the two short sections aligned). After some dry fitting and adjustments to the holes to make sure things lined up as desired when pressed together, I put some foam tac on the outsides of the alignment tubes, put them their holes and then slid the two halves almost all the way together so the main spar and alignment pins would hold the tubes in correct alignment while the glue dried. I used foam tac rather than CA to extend the working time and I left a couple mm gap between the two halves while glue dried so any excess would not stick them together accidentally. Also be careful not to get any glue inside the tubes!
Last edited by mdmyers; Jun 15, 2017 at 09:55 PM.
Jun 15, 2017, 07:20 PM
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To hold the two halves together my brother suggested rare earth magnets. We had concerns about being able to separate the halves if I used a couple larger 3/8" disks I had so I initially tried three sets of 3mm disk magnets as seen in the earlier pictures. After installing those I wasn't quite confident in the hold so I added a set of slightly larger 4mm square magnets at the front and that felt better to me. After taxi tests and a couple flights I've see no sign of any separation. To install them I carved a hole using an Xacto, carefully laid the magnets in there and aligned them flush with the surface using a toothpick and then put a few drops of CA around the edges to wick in and hold them. Once cured, I stuck the mating magnets on, assembled the two halves and pressed them together to create an impression around the magnets in the other half. After carving out and installing these other magnets, I covered all of them with some scotch tape and was done with the wing!

One other good suggestion from my brother regarding the magnet install: After installing them on the first half and sticking a mate to them, take a sharpie and put a dot on the exposed face of each of the mating magnets and be sure to install that dot down into the holes you carve in the other half. That way you know you've got them attracting and not accidentally repelling.
Last edited by mdmyers; Jan 27, 2018 at 11:43 PM.
Jun 15, 2017, 07:46 PM
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Now on to the elevator: My elevators ended up overlapping by a couple mm but they were misaligned front to back, I guess because they were just glued at slightly different rotations in the factory. I worked an Xacto blade between the fuselage and elevator surface on both top and bottom of one of them so that I could rotate it. I then aligned the hinge lines (rather than the front or back edges) and wicked some CA back into the gap to hold it in place. I marked a line in the middle of the overlap at the tips and then cut off each elevator at that point. With mine that was at a point 86mm out from the rudder measured along the hinge line but that could vary depending on the dimensions of the center section. I also glued one of the 3mm magnets to each half on the bottom side to help keep things aligned. Not sure if that's completely necessary and they don't necessarily keep the edges tightly together all the time but they do seem to help keep the halves aligned vertically.

To finish the elevator and make it more scale I trimmed both outer halves down to 12mm measured from the rudder along the hinge line. The real F-82 had no elevator outboard of the rudders but to duplicate that would have meant cutting off the control horn portion of the right side fuselage and then needing to tie both elevator halves together to function from the left fuselage control horn. This would have complicated the ability to break things down into two pieces and I had concerns about the elevator stiffness on the far end from a single control horn so I chose to leave both horns and accept the scale discrepancy. Also, as with the elevator, the center wing section is not scale because on the real F-82 it was straight across the leading and trailing edges rather than "V'ing" in as they do in my conversion.
Last edited by mdmyers; Jun 30, 2017 at 03:14 PM.
Jun 15, 2017, 08:18 PM
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The RX in each fuselage has been separately bound to the same memory location in my TX. Travel speed and distance seems to match perfectly between the two RX's and the AS3X required no adjustments but certainly double check yours before flight.

As far as CG, I initially had the batteries all the way forward and added a weight on each nose to put it at the 40mm recommended for the P-51. My taxi test method was to start a take off run getting the tail up and increasing speed to the point where I could pull back the stick and get it a few inches off the ground and then reduce throttle and land again. With both added weights in place, I both had problems with some nose overs and felt like it was going to take full elevator just to get it started into the air so that seemed nose heavy. I tried removing one weight but it still felt nose heavy on test runs so I removed the other weight and it felt good. I maidened it that way the next day and it flew well but was still slightly nose heavy. On my second flight I moved the batteries to the middle of the battery compartment and it required a few clicks of up elevator but it's pretty close now and measures about 45mm back from joint where the wing meets the fuselage. After a few more flights I've moved the batteries back a bit more and the CG is now at 46mm and it flies very nicely with neutral surfaces.

As far as general flight characteristics, you do need to keep some up elevator on the entire take off run or it can nose over. Once in the air it can certainly do loops and rolls but the rolls in particular are somewhat ponderous. I suspect that the aileron surface area is rather small for the now larger wingspan and performance could be improved by extending the ailerons inwards which I may try at some point. Looking at pictures of actual F-82's it does appear that the ailerons are probably a greater percentage of the wing width (see this image).

Finally the half of the plane with the center section glued to it required some modifications to the bottom piece of foam in the box to fit but nothing too major. Keep the plane toward the front of the box when marking the areas to cut, in the pictures below, you can see I took out more than necessary at the back of mine because I didn't.
Last edited by mdmyers; Jun 29, 2017 at 10:01 PM.
Jun 29, 2017, 10:27 PM
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I have been successful hand launching this but the method is a bit unorthodox because the two fuselages make things difficult to hold. I reach my arm under the stabilizer and grab the wing center section with my thumb on top and fingers on the bottom and give it a good shove straight out. On mine, I've found that when just holding it and running up the throttle, there is a significant rotation of the nose downward. The first time I tried hand launching with full throttle, it just nosed down and ploinked right in. With a little more playing, I have the best success at about 55-60% throttle and holding full back on the elevator (at high rates). With the original elevator it would still nose down a bit but then would level off and begin to climb and you could add power. I'm not enough of an aerodynamics expert to explain why but the nose down tendency goes away when actually in flight although if you slow way down and then slam the throttle to full, you can see the nose dip a bit.

I did decide to add some surface area to the active part of the elevator both to help with hand launching and because I just felt the elevator authority was lacking a bit. This probably makes sense since by rough estimation, the modification probably reduced the wing area to 3/4 of it original but reduced the elevator to 1/2. I found a piece of plastic that was 0.25mm thick (I think it was the screen protector piece out of an old Otterbox phone case) and cut 4 trapezoidal pieces as seen in the picture that were 21mm wide at one end, 11mm wide at the other and 84mm long (the same length as my elevators). I then scuffed up the facing surfaces with sandpaper and glued them together in pairs at the back edge and let them dry. Once dry, I spread the front edge and slipped one over each of the elevator surfaces so that the back was 27mm from the hinge line. I taped them there temporarily with some painters tape and flew a couple test flights and found it did help with hand launching (you still need full up on launch but now it will fly out level or even pitch up a bit). I then went back and glued them in place and replaced the painters tape with scotch tape both to double down on them staying in place and to more invisibly blend the leading edges of the plastic strips a bit in terms of airflow. I may get around to painting things to match and I may also play around a bit with my DX8 to see if I can set up a flight launch mode to help with the elevator so I don't just have to remember to hold up elevator in on launch.
Last edited by mdmyers; Apr 26, 2019 at 05:50 PM.


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