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Joined: 1/30/2002 From: San Jose, CA, Status: offline
Well, that's the fuselage about as far as it can go until I get the fuel cells, engine installation kit etc from BVM (supposed to ship this week), so I've gone back to built-up parts. Let's take a look at the fins.
There are two fin shapes available with the Super Bandit, one is "swept", the other is "straight". I'm building my kit with the straight fin, but will be doing the swept fin for a buddy's Super Bandit that I'm building for him - so we can look at both.
I would recommend that whichever fin you build, you read the instructions for both. At this stage of the build process, BVM assumes that you are familiar with their construction techniques now (because you've done the wings & stabs), so the instructions become a bit sparser. Reading both sets of instructions can help ensure that you don't miss anything.
With the old-style Bandit (not the Super), only the swept fin was available as a build-up structure - the straight fin was available only as a composite structure. Consequently, the straight fin is an all new piece of kit, while the swept is basically unchanged from the original Bandit.
There are minor differences between the two construction-wise, reflecting the more modern techniques that BVM is adopting now - for instance, the straight fin uses lite-ply for the LE and TE, while the swept fin uses balsa for these parts. Either will be plenty strong enough though, so don't worry about not having ply in the swept version.
Let's look at the straight fin first.
Like the wings and stab, the fin is built on top of the plans by gluing the ribs to the board, through some diamond-shaped cut-outs in the plans. On the swept fin (see later), there are tick-marks on the ribs and plans to aid in alignment. The straight fin does not have this little refinement, so take the the time to align everything in the same way as we did with the stab parts earlier - using 1-2-3 blocks etc to project the plan lines upwards and ensure alignment.
Here you can see me aligning the ribs, and using a straight-edge to ensure that the thin plywood rib R1 stays perfectly straight when gluing it to the spars etc - if this rib bows (which it can easily do due to it being so thin), the fin will not seat properly on the fuselage, so take the time to get it right.
< Message edited by Gordon Mc -- Nov 20 2002 4:24PM >
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Joined: 1/30/2002 From: San Jose, CA, Status: offline
Here's why it is important to use a straight-edge on the TE - one of the ribs (R2) is slightly undersized (BVM has been notified of this, BTW)... if the TE was snugged up to the end of each rib, it would consequently bow inwards.
< Message edited by Gordon Mc -- Nov 20 2002 3:30AM >
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Joined: 1/30/2002 From: San Jose, CA, Status: offline
The rod that is used to secure the fin into the fuselage is made up of two parts - a steel outer pipe, and an aluminum inner rod which you must glue inside the steel pipe. Since the aluminum rod is slightly shorter than the pipe, ensure that they match at the bottom .
The rod is held inside the fuselage with a sturdy aluminum bracket that has a grub screw tightened up against the rod. Inside the fin, the rod is secured by not only gluing it to the ribs, but also sandwiching it between a couple of plywood support plates and balsa sticks as is shown below:
You may notice from the photo that the shear webs go up only as far as R4, and also that the R2 <-> R3 shear web leaves a slight gap since it is about 1/8" smaller than the gap between the ribs.
< Message edited by Gordon Mc -- Nov 20 2002 4:25PM >
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Joined: 1/30/2002 From: San Jose, CA, Status: offline
With the swept fin, BVM supplies a small plywood stiffener for the area where the servo arm will exit the side of the fin (See the photo in post #60 further below). No such part is provided with the straight fin, but I feel that it would be a good idea to add it. I discussed this with BV earlier this week, and they may add the part in future kits. For now, I simply used some of the scrap 1/32" ply to make one up. Sorry - I don't have a photo of this, but you can see what the part should look like by checking the photos of the swept-fin construction (p29 of the manual), or post #60 below.
Here you can see how the servo fits into the bottom of the (now sheeted) straight fin. This makes for very good access to the servo, while keeping the rudder linkage as short as possible.
This shot also shows the pre-drilled hardwood dowel that goes at the front of the fin; a 4-40 bolt is inserted through the top of the fin, passes through this dowel, and then screws into a blind nut mounted inside the fuselage.
< Message edited by Gordon Mc -- Nov 20 2002 4:27PM >
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Joined: 1/30/2002 From: San Jose, CA, Status: offline
As mentioned earlier, the swept fin has a slight refinement that the straight one does not - tic marks on the ribs and the plan in order to aid alignment.
Posts: 6747
Joined: 1/30/2002 From: San Jose, CA, Status: offline
As with the straight fin, I'm taking extra care to ensure straightness of various parts, especially the bottom rib (R1).
Please note: if you decide to use the method I'm using here (clamping a straight-edge to the parts), then the weight of the straight-edge and clamps can twist the rib / LE / TE unless you provide additional support.
Although you can't see it in most of the photos, I always have something (e.g. scrap balsa blocks)underneath the straight-edge to take the weight and prevent any such twisting.
< Message edited by Gordon Mc -- Nov 20 2002 4:31PM >
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Joined: 1/30/2002 From: San Jose, CA, Status: offline
As with the straight fin, the shear webs that go between the 2nd and 3rd rib bays are slightly shorter than the gap. Whereas the location of the shear webs within the gap was unimportant with the straight fin, the swept fin requires a little more care due to the fact that the metal rod will end up going through the pre-cut holes in the shear webs... in my case I found that the best result came from having the rear shear web lined up with the lower rib, and the forward shear web lined up with the upper rib.
IMPORTANT :
The kit for the swept fin is retained from the original Bandit, it's not a new design. However, the original Bandit used a smaller servo than the 9411 that the Super Bandit calls for. Consequently there are some slight changes that you need to do. First, the opening in the servo-mount partial-rib must be opened up a bit - only about 1/32" in length, but about 1/8" in width.
Furthermore, the greater width of the newly recommended servo also affects R1, as the servo must be inserted through the gap in R1 in order to be mounted. You should open the servo mount up as necessary before gluing it it, but R1 should be opened up after sheeting has been completed, because otherwise it might not retain its shape correctly during sanding or sheeting. After the structure has been sheeted, the sheeting itself lends enough strength that you can cut away much of R1 without adversely affecting the strength of the completed part. (This has been verified by Dana ta BVM)
< Message edited by Gordon Mc -- Nov 20 2002 4:34PM >
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... and since the rod does not have the same kind of support platform as the straight fin does, take care when gluing the rod in that the root rib still stays perfectly straight. Here I'm just tack-gluing the rod ; after this, we will again use plywood & balsa supports with Aeropoxy to hold the rod firmly in place.
< Message edited by Gordon Mc -- Nov 20 2002 4:36PM >
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Joined: 1/30/2002 From: San Jose, CA, Status: offline
and here you can see the completed internals of the swept fin, ready for the second side to be sheeted.
NOTE - as mentioned earlier, the instructions are considerably abbreviated at this point of construction... make sure that you consult the plans and photos, not just the written instructions , otherwise you may do something like close the fin out without putting the hinge support blocks in place.
In this photo you can also see the small plywood stiffener where the servo-arm goes through the skin. This is the part that I described earlier when I suggested that you might want to add one of these to the straight fin.
Please ignore the small balsa blocks that are just above the servo mount - these are not part of the kit, but I just decided to add them to give a little more support to the servo tray (especially at the spar side)
< Message edited by Gordon Mc -- Nov 20 2002 4:36PM >
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Joined: 1/30/2002 From: San Jose, CA, Status: offline
Here's a photo of the two available fin types, shown next to each other for comparison. At this point, they are ready to be glassed.
Note that you must select a fin type when ordering your Super Bandit - you don't get both. However... so far I can see no reason why you couldn't order your kit with one fin type and purchase a separate fin kit for the other type, in order to be able to experiment with both and compare the flight characteristics, or simply give yourself some variety in the aircraft's look...
Both fins use the exact same F7 position for the central aluminum block that the fin's rod mounts into ... they have different positions for the forward 4-40 bolt, but it seems perfectly reasonable for you to put in two blind nuts into the fuz-mounted nut plate instead of just one.
Note - in both fins, there is a small aluminum tube supplied - this is a guide tube for the 4-40 bolt that holds the front of the fin in position. The method used for cutting the tubes at the factory seems to fold the tube over inside itself slightly at the end, making it look as if the tube has a thick wall and the 4-40 bolt head will not fit. Simply use an Exacto blade to clean the end out, and you will see that the bolt fits just fine. I chose to fit the tube after all construction was done, so that it does not interfere with the process of shaping the LE.
< Message edited by Gordon Mc -- Jan 24 2003 11:25PM >
Posts: 6747
Joined: 1/30/2002 From: San Jose, CA, Status: offline
Since the fins are done (apart from glassing + adding the control rod), I'm going to jump back to the wing. I previously stopped at the point where I needed the missing retract cover plates in order to work on the gear doors. BVM has since sent these parts not just to me, but to all Balsa Super Bandit customers.
The enclosed photo shows the gear installed (minus wheel), with the strut cover on, and the retract cover is sitting upside down to show the ply spacers that adjust the height of the ply part to match the basla wing sheeting. You can also see the reinforcement in the wheel well area - BV suggests 6oz cloth as an option here, but I didn't have any lying around, so I used some 1/2 oz CF with 3 oz fibreglass layered on top of it.
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