Going All-Out With A Classic Balsa B-17-F

Going All-Out With A Classic Balsa B-17-F – Part 11

We finished our last building session with the basic structure of the wing, as well as the engine nacelles aft of the cowls, all built. All that was left to complete the outline was the flaps and ailerons, and I’m going to build those now. Quite a few installments back I discussed adding “scale” split flaps to the original model design and explained the provisions I made in the redesign of the wing structure to make it possible to add them when I was ready. After determining that the scale position and shape of the ailerons were correct, I pretty much “left them for later”.

Later is now, so let’s build a set of B-17 ailerons and flaps and … oh yeah … I have another “old model builder’s secret” to share with you, but you’ll have to wait for it.

B-17-11-1 As with a lot of the other structural components of the airplane which are getting modified as their turns to be built come up, the ailerons need a few changes in order to remain consistent with the standards of reasonably accurate scale appearance that I’ve set for myself. Remember the changes I made on the rudder and elevator, ‘way back? The ailerons are getting the same treatment. As they are fabric covered on the full scale B-17, the ribs are supposed to show, and it’ll matter that the rib count and spacing are right. You can see where I have drawn in pencil lines on the kit plan to represent scale rib locations. Also, like the tail surfaces, the axis/centerline of the aileron hinges is set back from the aileron leading edge. There are two reasons for this…aerodynamic efficiency and static balance. On this little model we don’t really need to worry much about the balance, but the features that contribute to the aerodynamics SHOW and deserve our attention.

B-17-11-2 I addressed that issue by designing entirely new aileron leading edges. The changes I drew in on the plan are hard to discern but you can see here what I starting. I need big fat aileron LE’s that will include/enclose the hinge axis line, and that begins as I cut out new LE blanks from a ¾” thick balsa sheet. (Yes, I’ve turned off the mini-table saw for the shot.) Hang in there and you’ll see how this will all come together.

B-17-11-3 The aileron trailing edge is getting the same custom treatment. I ended up with a TE stock dimension of 3/16” x ½”…but even with my considerable accumulated stash of model airplane building wood I couldn’t find that size…so…it was back to the table saw to cut down the 3/16” x 5/8” that WAS there. NOTE: this is not the sort of thing you need to do. I made the extra effort to make use of material I already had in my shop. Why spruce? We’ll get to that in a moment.

B-17-11-4 Now things are beginning to make sense…with the LE and TE “blanks” cut to their outside dimensions of ¾” x ¾” and 3/16” x ½” , I have them arranged in place over the plan so I can transfer the rib positions to them in preparation for cutting inset slots in both components. Why should I do this extra work? The butt-end joint of a 1/16” balsa sheet rib against the smooth face of a trailing edge creates a joint just waiting for a chance to fail…there’s very little working gluing area there. Same goes for the leading edge. Watch how I’ll deal with the issue.

B-17-11-5 There are lots of ways to cut rib slots in LE & TE stock. There are plenty of kits on the market that will have done it for you, but we are modifying this one (at the expense of a little extra weight and a lot of extra effort) so I’ll have to do it myself. Lots of care and patience with a fine blade in the scroll saw gets the job done right.

B-17-11-6 I didn’t need to have the already-cut balsa leading edge there while I notched the spruce TE, but I did it so you could compare the two parts.

B-17-11-7 The best way to make sure both ailerons come out the same is to lay out the parts of the second directly from the first. These are the ¾” sq. balsa leading edges.

B-17-11-8 Now we’re getting somewhere. Although it’s not easy to see in these photos, there is a slight taper in the aileron chord (width) from root to tip, with the result that every one of the ribs that are going to fit into all these slots will be of a different size. On a plane this small the amount of balsa I’ll waste by making a series of “rib blanks” to fit the varying lengths defined by the LE and TE as they taper closer together and then sanding them to final shape is negligible, so I chose to eliminate some needless detail work by doing it this way. You can see how this is going to work out.

B-17-11-9 Just to be sure I get a square, tight fit at each end of those rib blanks, if there’s any doubt that my cutoff ends aren’t accurate I’ll square them off with the sanding block…

B-17-11-10 …and they’ll all fit in like this, one after another.

B-17-11-11 The next step is to use Deluxe Materials SuperPhatic glue on each of those joint. A “dry-fitted” assembly like this is the perfect application for SuperPhatic. You need to try it for yourself to see how well it penetrates each joint and locks it up tight.

B-17-11-12 All glued up and waiting to dry. Can you see the ¼” shim I’ve pinned under the trailing edge to keep its centerline height even with the much thicker leading edge?

B-17-11-13 Here’s a better look at how that works. The opposite faces of the LE and TE need to be parallel with each other. Now you can see how much structural reinforcement I’m getting from having cut all those notches.

B-17-11-14 I’ll set the ailerons aside for a moment. There’s a lot of sanding left to be done on them and I’m going to “save” it to do at the same time I finish the flaps. This is the opening I have already designed and built into the underside of the (left) wing where each split flap will go. See the small rib pieces between the nacelles? I saved those from another ‘way back session when I modified ribs W-2 through W-7 to accept the flap structure. As it turned out I won’t use them.

B-17-11-15 The split wing flap on the B-17 is basically a flat surface that retracts flush with the underside of the wing, with enough “internal” structure (on the inside) to give it strength and rigidity. It needs to be strong enough to handle some significant air loads and stay attached to the airplane. On this model I chose to use 1/32” plywood to get the best combination of those characteristics along with reasonable weight. This “blank” is going to fit neatly inside the opening in the wing.

B-17-11-16 I cut out ¼” x 3/8” spruce leading edges and 3/32” X ½” trailing edges for the flaps, glued them to what will become the upper (inside) surface of each flap with Deluxe Materials Aliphatic Resin glue, and used building weights to hold everything FLAT while it dries.

B-17-11-17 The next step was to begin tapering the assembly so it becomes thinner at the TE in order to fit inside the flap well. Here I’ve only just started.

B-17-11-18 At the same time I took the opportunity to use the sanding block to clean up the inside of the flap well where the new split flap structure will have to fit. This is the underside of the right wing.

B-17-11-19 Once again Deluxe Materials SuperPhatic is my adhesive of choice. In the case of the flaps my rib blanks are 1/8” x ¼” spruce, which I used so that their relative hardness/stiffness would be consistent with the rest of the flap structure.

B-17-11-20 Here’s one flap ready for some serious sanding to that finished taper I mentioned using another custom tool I dreamed up for this particular job. The sort-of-yellow surface closer to you is 150-grit sandpaper stuck to a long Great Planes aluminum sanding bar. The aluminum bar is locked face-up into that vise so I’ll be able to push hard against it with the flap assembly in my hand. Watch…

B-17-11-21 I have the flap lined up “the long way” with the sanding bar. With this arrangement I’m able to put a LOT of force into the piece I’m working on (the flap) and at the same time maintain complete control of it. A LOT of work (and sanding dust) got me to what’s next…

B-17-11-22 …the flap assembly with the top surface (the side that faces up inside the wing) sanded to finished shape. Notice that the 1/8” x ¼” spruce ribs have all been tapered to blend smoothly into the ¼” deep leading edge AND the sharply tapered 1/8” thick trailing edge.

B-17-11-23 The reason for that is so the flap will fit neatly into the pocket that’s already built in to receive it in the lower surface of the wing trailing edge.

B-17-11-24 Back to the ailerons! If you look closely you can see where I have added an oversize scrap of 1/8” balsa to the far end of this aileron to “make up a little shortness” that I managed to build into it. I’m using the 150-grit sanding block to face off the front surface of the leading edge…

B-17-11-25 …so the assembly will fit into the aileron well cutout. You can see where I still have some finish trimming to do on the inboard 1/8” balsa filler insert to get it properly squared off.

B-17-11-26 Now it’s right and I have taped the aileron into its cutout (this is the left wing) so I can mark exactly where the still slightly-too-thick leading edge needs to be trimmed a bit thinner to match the wing surface ahead of it.

B-17-11-27 I do that part like this, back on the vise-gripped sanding bar. It happens that I have already sanded away the pencil reference line that showed about 1/16” of extra depth at one end of the aileron.

B-17-11-28 Here’s that aileron again, with the correct thickness sanded into the leading edge and a smooth taper of the upper surface all the way to the TE. (The opposite side looks just the same.) If you look carefully you’ll see that the taper in the aileron surface stops about ½” back from the front/upper “corner” of the leading edge.

B-17-11-29 There’s a reason for that…we still have to “add the roundness” to the leading edge cross section. Look back at image B-17-11-9…left of center at the bottom of the part of the plan you can see there’s a small penciled-in circular diagram that defines the finished cross section of the aileron LE. Putting this in is the final bit of shaping I have to do on the ailerons.

B-17-11-30 Can you see that curvature starting to form? I have to rotate the aileron up and down around the hinge axis/centerline while I sand away balsa to sneak up on the radius I’m looking for.

B-17-11-31 When it’s done it looks like this. Here I’m hand-holding a fine (320-grit) sanding block for some final smoothing of the aileron LE.

B-17-11-32 All shaped, the left aileron looks like this when I slip it into the left wing aileron well and tape it in place. What’s happening here is my preparation for adding aileron hinges. A check of your own B-17 documentation (like my 3-view) under the wing) will show that there are four hinges on each aileron, set back from the from the leading edge in narrow slots that are visible when the aileron is mounted in place. Here I’m transferring reference marks for those hinge slot locations to the wing/aileron junction.

B-17-11-33 Extrapolating from my drawings I determined that each aileron hinge slot needs to extend back into the leading edge 3/8”. (I checked this out in advance in order to know how to dimension those LE blocks deep enough to contain the slots without falling apart when I cut them). The good old Dremel scroll saw is the perfect tool for doing the actual cutting of the slots.

B-17-11-34 I’m using medium size Robart Hinge Points again, partly because they are easy to adapt to the “off center” mounting this aileron configuration requires and because the portion of each hinge that will remain exposed will “look scale”. Here I have drilled an appropriate hole in one of those leading edge slots and slip-fitted one hinge into place.

B-17-11-35 With all four hinges temporarily installed I was able to cross-check their actual fit against the reference marks on the wing I made in step B-17-11-32 and then drill pilot holes for the front end of each of them.

B-17-11-36 With that done, the hinges fit like this.(Again, if you look closely you’ll see another of those “little adjustments” I had to make. The rear end of each hinge (the part that fits inside the aileron) is just a bit too long not to hit the trailing edge…so…I clipped each one off by about ¼”. NOTE: I’ll deal with the issue of “exposed hinge” that’s not securing anything later in the project. It’s OK for now.

B-17-11-37 Dry-fitted exactly into place the left aileron looks like this. I’ll come back to these assemblies later to work on things like control horn mountings.

B-17-11-38 Do you remember that “model builder’s secret” I promised to let you in on? Here it comes. My original plan was to end this session with both the ailerons and the flaps dry-mounted in place as a neat finish before I go on next time to start building the fuselage. What you see here is the finished left flap hand-held in place next to the flap well in the wing. I have made some pencil line reference marks where the flap leading edge meets the lower wing surface where I was considering using more Robart Hinge Points to mount the flap…BUT…at the last moment I was not comfortable that this was the best way to do that particular job. The secret is this. When I discovered that I wasn’t satisfied with the way I’d originally planned to do the job, I STOPPED. Deciding on the best way to hinge the flaps can wait. While I start work on the fuselage I’ll have plenty of time to figure out the right approach and go back to it. In other words, I chose not to allow an artificial deadline to pressure me into doing bad work.

See the entire build series: Building the Balsa B-17-F