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BVM Super (Balsa) Bandit

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Old 11-20-2002, 02:19 AM
  #51  
Gordon Mc
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Default Fins

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.
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Old 11-20-2002, 02:21 AM
  #52  
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Default BVM Super (Balsa) Bandit

Similarly, ensure that the TE of the fin is perfectly straight when gluing the ribs to them, so that your hingeline will be perfect.
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Old 11-20-2002, 02:23 AM
  #53  
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Default BVM Super (Balsa) Bandit

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.
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Old 11-20-2002, 02:30 AM
  #54  
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Default BVM Super (Balsa) Bandit

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.
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Old 11-20-2002, 02:48 AM
  #55  
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Default Plywood strengthener ; servo mount

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.
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Old 11-20-2002, 02:51 AM
  #56  
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Default Swept fin

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.
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Old 11-20-2002, 02:53 AM
  #57  
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Default BVM Super (Balsa) Bandit

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.
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Old 11-20-2002, 02:57 AM
  #58  
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Default BVM Super (Balsa) Bandit

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)
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Old 11-20-2002, 03:02 AM
  #59  
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Default BVM Super (Balsa) Bandit

... 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.
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Old 11-20-2002, 03:16 AM
  #60  
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Default BVM Super (Balsa) Bandit

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)
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Old 11-23-2002, 02:46 PM
  #61  
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Default Fin comparison

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.
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Old 11-23-2002, 03:15 PM
  #62  
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Default Wing : gear doors

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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Old 11-23-2002, 03:23 PM
  #63  
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Default strut covers

Here you can see the strut covers fitting into the wing.

If you look carefully you will see that there has been some sheeting rework just behind the strut cover - that's because I had brain fade for a moment (okay -- maybe longer), and initially mounted the strut covers the wrong way around when I cut the openings in the wing sheeting. DUH ! Take the time to make sure you do them the right way around instead of making the same dumb mistake I did...

Next - I found that (even when I used the right orientation of strut cover) the strut cover did not lie in exactly the same plane as the wing sheeting. This was resolved very easily by sanding a very slight bevel onto the tongue of the strut cover (the top part that has the two screw holes in it for attaching to the retract). This effectively rotated the strut cover through a degree or two in order to get the correct alignment.

The comment about adding a 1/16" ply shim to the top of the strut cover (p18 & 19 of the manual) seems to no longer apply, thanks to the improved strut cover design that we discussed earlier).
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Old 11-23-2002, 03:32 PM
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Default BVM Super (Balsa) Bandit

The clamshell doors are built by layering several parts, and gluing them while inducing a curve into the door by having the outer edges of the door each supported by a 1/16" scrap ply strip. I think I may have had the strip a little further inboard than it should have been, as my doors ended up with too much of a curve on them. I have tried straightening them out a bit, but am not happy wit the result, and have ordered a new set of main gear doors so that I may try again.

In order to avoid the same problem with the next set, here's what I am going to do: Instead of building the whole clamshell door to completion before matching it to the wing, tack - glue on one part (the first 1/32 ply hinge locator) and compare the resulting curvature to the wing sheeting (just lie it on top and see how it fits); adjust the curve as necessary by undoing the tack glue joins), then only progress to the next part when the current part keeps the assembly in the correct shape.


I have discussed this with Dana at BVM, and he has indicated that it is normal for the doors to be built a bit over -curved, as they tend to flatten out a bit over time... so you may wish to stick with the manual's instructions rather than try for a smoother fit like I plan to do. Its your call...


Here's a photo showing the inside of the clamshell doors, one with only the ply & poly-ply hinge locators in place; the other completed.
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Old 11-23-2002, 03:43 PM
  #65  
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Default Gear door mistake # 2

Okay - I admit it - I wasn't having a good day when I worked on the gear doors, and simply was not paying enough attention... :cry:

The clamshell doors are not symmetrical in shape, and you need to be careful about which way around you put them in. I knew that, but still got caught out.

The correct orientation can easily be seen of you look at page 19 of the manual, and note that the hole in the door (for the screw that goes into the control horn) is forward.

I instead looked at page 20 of the manual, and because of a minor mistake in some of the photos on that page, I deduced the wrong orientation - here's why: The pics in the middle of page 20 show the hinge pin inserted the wrong way around (which I have copied in the photo in the previous post). They show the bend in the pin as being at the same end of the gear door as the hole for the control arm, then at the bottom of page 20, there is a closeup of the hinge pin inserted the correct way around, along with a comment that this is the aft end of the gear door. I put 2 and 2 together: wire bend & hole are at the same end; wire bend is at the aft end, => the hole is at the aft end. I then merrily cut a hole in the wing sheeting that matched by gear door. Ugh. Since the door is non-symmetrical, that meant that I once again had to do some sheeting rework, as can be seen in the attached photo. (Still needs tidying up, but it's better).

BVM will be correcting the photos on pg 20 that show the hinge pin in the wrong way around - in the meantime, use p19 for gear door orientation, or just realise (as I didn't until after cutting!) that its kinda obvious that the control horn side of the door must go where (a) the cylinder mount is, and (b) the cutout in R1 is for the horn to pass through.

Neither of the mistakes that I made with the doors would have happened if I had been paying more attention - so just take your time with this area, and it won't happen to you.
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Old 11-23-2002, 03:52 PM
  #66  
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Default Some minor corrections re gear doors

Regarding the manual...

On p20, there is a comment about the gear doors being oversize for a reason, and that you should not trim them to fit the opening in the wing until hinging is complete. I have to assume that this is an instruction that was copy-pasted from the manual for the composite bandit, since it makes no sense for the built-up one : we have already used the gear door to determine where to cut the sheeting, so ther is no oversized door...

On P20, as mentioned in the last post, the photos in the middle show the wrong orientation of the hinge pin - the 90 degree bend goes AFT, i.e on the opposite side from the control horn hole.

On P21 there is an instruction about trimming the balsa sheeting to 1/4" width, then making the recesses for the hinges. This instruction needs to be moved to an earlier stage - in its current position it is after we have already glued the hinges in place. (Always read ahead in any instructions!)
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Old 12-07-2002, 02:43 AM
  #67  
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Default Clamshell gear doors

Okay - I got the second set of gear door parts earlier this week, so have now built a second set of the clamshell doors, this time using 1/32" per side for generating the curve, rather than 1/16". I am much happier with this result, and will be using these doors (rather than the originals built per the manual).

Just to repeat what was stated earlier though - BVM does not consider the over-curving of the door to be an error - that is the way they believe the doors should be built. My decision to make the doors flatter is a personal choice, not a recommendation that you ignore the BVM instructions.

Okay - on with the build. Once the doors have been built and the hinges installed onto the doors, its time to glue the mating hinge halves inside the wing. Rather than try to hold everything in place by hand while gluing the hinges in place, I elected to clamp the hinges, check for binding etc., and then glue the hinges. (I also supported the door with a small "lip" on the inside of the wheel-well, as will be seen in the next post.)

Here's a view of how I clamped it using two mini "quick-grip" clamps. It takes a bit of maneuvering, but you can get both through the holes in the root rib:
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Old 12-07-2002, 02:45 AM
  #68  
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Default BVM Super (Balsa) Bandit

Then I checked to ensure that the door would open cleanly, and that the wire hinge pin could be removed. After a couple of iterations involving minor adjustments to get the desired operation, I then glued the edges of the hinges from inside the wheel well, then removed the clamps to complete the gluing.

You may also notice in this photo, that I have glued in a few small pieces of wood just under the wingskin, protruding out into the wheelwell. These served as a "lip" that helped align the clamshell door proerly in the opening, before the hinges were glued in place.
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Old 12-07-2002, 03:06 AM
  #69  
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Default Trim the strut

BTW, the clamshell door will not close fully when the strut is installed and retracted, until you trim some metal from the strut.
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Old 12-07-2002, 03:09 AM
  #70  
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Default Gear door horn

The horn that activates the clamshell gear door is actually a cut-down offset hinge.

There is a slight oversight on the plans that may catch you out...

The attached image is made up of three parts. At the top you will see the drawing on the plan, showing how to trim the hinge. This is wrong - there are two different parts that make up an offset hinge, and BVM has accidentally shown the wrong one.

If you try to make the part that is supplied look like the one in the plan (as I initially did - see the middle photo in the attached pic), then you will end up with the wrong geometry for the mechanism.

The pic at the bottom shows how the horn should look.

Note that the pics in the manual actually show the correct shape for the oart - it's just the wing detail plan that has the wrong outline.
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Old 12-07-2002, 03:13 AM
  #71  
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Default Flap hinge

The flap hinge is done similar to the BVM Bobcat's aileron hinge - two-piece Du-bro hinges, with a long, single, removable hinge pin being used. To allow for the enormous amount of travel that the flap allows though, the hinge is done at the bottom edge.
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Old 12-12-2002, 02:00 AM
  #72  
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Default BVM Super (Balsa) Bandit

Here both the flap and the aileron are hinged. I actually completed this stage before I finish-glued the wingtips on (they were only tack-glued until this point)....

I did this simply because I wanted to be able to mount the wings, and check the alignment of the flap, aileron etc, and then use the extended TE of these parts as a quick double-check that the wingtip was glued on exactly at the right angle. It turned out to be fine, with no adjustment necessary.
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Old 12-12-2002, 03:17 AM
  #73  
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Default Servos, pushrods etc.

The cutouts on the underside of the wing, for the servo access hatches, are made by transferring the servop location from the wing detail plan to the wing, then cutting a small hole in the middle and gradually enlarging the hole until a servo will fit cleanly into it.

For the flap, note that the servo is supposed to fit fairly tightly up against the rear spar. I found that the large rubber grommets on the servo stick up rather a long way, and interfered with the fit, so trimmed the rubber very slightly (just the part that sticks upwards).

Here you can see the servo hatches cut in the wing. The manual shows the servo hatches both being rectangular with rounded corners, but I made the flap ones a slightly different shape - following the line of the rear spar at the front of the hatch, just to give me a little more space to work in when routing the servo wires during installation.
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Old 12-12-2002, 03:22 AM
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Default Servo mount blocks.

The servos for the flaps and ailerons are each mounted by having two hardwood blocks glued onto the plywood servo mount base, and having the black plastic L's screw onto these.

On the side of the servo that the servo wire comes out of, it is necessary to relieve the block to allow for the servo wire. You could simply cut the block narrower and maintain it's rectangular shape (removing the ability to use the second screw hole in that side of the mount). I instead choose to shape the block as shown below.

Remember that the servo will be pulled upwards during removal, so make sure that there is sufficient leeway for the wire to pull past the edge of the hardwood block without imposing any strain on the wire, or having any sharp edges rub against it.
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Old 12-12-2002, 03:32 AM
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Default BVM Super (Balsa) Bandit

Also, since the screws that hold the servo to the black plastic part of the mount protrude slightly through the plastic part, they can catch slighty on the block during removal.

This is a truly minor detail, not an actual problem, but here's the approach I've used on all of the Bobcats I've built, as well as now using on the Super Bandit... simply use a 1/16" cutting bit to route out a small groove at the point where the block and those screws would meet - now the ends of the screws slide up the groove during servo removal or insertion.
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