Sparky 2X Speed 400

[Konrad]

That's coming on well Konrad.

I agree with your comments on the use of masking tape and sanding bars.

Masking tape must be the unsung hero of the modeller's tools. masking for paint or sanding as you are doing here, temporary support and jigs, surface for adding alignment marks etc.

I also use as sanding bar or trammel sander a lot. Sanding bars are particularly useful when working on straight tapered wings or geodetic - angled ribs. I use finished ribs at the root and tip and blanks in between. Tape over the root and tip ribs provides protection while trammel sanding the blanks to shape using the sanding bar riding on the root and tip ribs.

John

That is becoming a lost art, being able to loft the intermediate ribs between the tip and root ribs.

[Jmk89]

That's coming on well Konrad.


Masking tape must be the unsung hero of the modeller's tools. masking for paint or sanding as you are doing here, temporary support and jigs, surface for adding alignment marks etc.

Another good use is instead of double-sided tape - stick masking tape to both surfaces you want to attach temporarily; then put a few dots of CA on one of the masking tape backs and put the two pieces together with the masking tape strips back to back.  The CA glues the masking tape strips together but you can peel the tape off the pieces when you've finished.  Often much easier than using double-sided tape.  Also you can make law-tack  tape just by using low-tack masking tape!

[Konrad]

Now we come to one of the reasons I put this aside for 3 years. I found that the fin mounting was inadequate with only 1/16 pins going across the joint. I hate butt joints and try to avoid them. Also there is no purchase for the stab covering on ether side of the fin. This will make repairs on the stab difficult to recover. I have yet to redesign the 1/4 center rib and rudder structure to allow for proper mounting of the fin.

[Konrad]

Here I'm starting to build the structure that holds all the parts together, the fuselage.

This is a ladder type construction. I was shocked at just how large a building area I needed. In fact I'm build on the diagonal and still have some overhang I have to deal with. Another surprise was just how fast this fuselage is eating up the 3/16 square strips!

I'm also practicing the old technique of building the the second side over the first side. This is claimed to make identical sides, as the pins are placed in the same location for both sides. I find that in reality this is a false assumption as the stresses in the longerons vary drastically between the pieces of wood.  In the end I have to heat (steam) the sides and manually manipulate the ladders to get identical sides prior to moving the fuselage build into the 3rd dimension.

Not sure if shows up, but I'm changing the decalage as I fail to understand why the incidence of the stab (Datumed from the wing) is negative with a lifting airfoil. Here I lowered the aft end ot the stab 5mm (over 3/16").You might recall that the purpose of the stab is to provide a force counter the pitch moment of the airfoil. This lifing stab a force arrangement I don't understand. Heck most full size transport aircraft have the stab's airfoil "upside down".

With this Sparky and my Benny Boxcar a lot of this force to counter the wing is provided by a very aft CG. This results in an aircraft that is very speed sensitive in pitch.

[Konrad]

This isn't to just show my poor joinery. Rather I want to show what I do to improve the joint and at the same time improve the prep for the covering.

The first photo shows how mis-matched the side are as I pull them off the building board. Some of this is just poor craftsmanship but most of this is as a result of the different stresses in the wood.
I will be making slight adjustment to the ladder structure and steaming the ladders in an effort to make matched sides. I really have to say that the kitchen is often the best room for hobby work as it has both water and heat. (You might be hearing the wife steaming  mad in the back ground!) I've learned that an induction hot plate is near perfect for a source of steam . This is because there is little radiant heat that might burn the structure.

There is often a glue web often left behind against the parting film (Saran wrap) that offers no strength. It does have a bit of mass and can at times be seen distorting the covering. I remove this web with the tip of a number 11 blade. I then come back with some glue to make a full depth glue fillet. A proper fillet can add strength.  One needs to be careful with the fillets to make sure they don't interfere with the bulkheads to be added later.

[OZPAF]

I will be making slight adjustment to the ladder structure and steaming the ladders in an effort to make matched sides.

I have had similar issues in the past but now prefer to select matching strips for the longerons and also soak them in hot water and pin them to the curve before adding the uprights etc, leaving them to dry for at least a day.

John

[Konrad]

John, That's a good point. The longerons need to be graded prior to the builds.

I'll have to try that deep soak. I have a couple of feet of PVC pipe I can use as a long skinny tub.

[Jmk89]

It is also a good idea to strip  longerons from a sheet of the right thickness - that way they are more likely to have similar stiffness and other characteristics

[OZPAF]

It is also a good idea to strip  longerons from a sheet of the right thickness - that way they are more likely to have similar stiffness and other characteristics

Yes - I do this as a  matter of choice and necessity unless building from a kit.

John

[Konrad]

It is also a good idea to strip  longerons from a sheet of the right thickness - that way they are more likely to have similar stiffness and other characteristics

Still a good idea to grade the strips. With the quality of the balsa sheets available in the USA I often find the density of the sheet varies across the sheet.

[Konrad]

Step #2 on the Comet plans gave me issues as the upper cabin would often fall off as I was adding the cross members for the lower Wakefield type fuselage. Here I choose to add the cabin as it it was an add-on cube.  What I liked about Ed's plans was the use of an assembly aid (a line up former). Unlike Don's, Ed's has a mid spreader to support the upper longeron. I did like that Don used 2 formers to support the cabin.

This might explain why my ships are so heavy, but I like to add a lot of corner gussets. Here you can see where I'm trying to add a lot of stability. You might notice that there is a "hole" in the gusset work. This is because as an RC aircraft I will be wanting control on the ground. The "hole" is where I will be adding a plywood anchor for the upper torsion bar mount of the landing gear. (More about that later).

It is odd that the plans don't call out a sub fin. While all the sales drawings do show the sub fin.

[Konrad]

The rear cabin gave me fits! I never did come to an understanding as to why Don used 1/2" sheet. Also I had difficulty controlling the angle of incidence as I couldn't make a 3 point line for the hold down dowel defined by F7, F8 & F9 with the way I interpreted the plans. The dowel was 4mm above F7! In the end I recut F9 to be about 6mm taller than I had first cut. I now have a line from F7, F8 & F9. I had to make minor adjustments to the rear wing seat sheets.

[Konrad]

A little more on the rear cabin and that 1/2" doubler.  I think a lot of what Don is doing with that 1/2" sheet can be accomplished with radiused gussets. This should have saved some weight aft of the CG. I'm showing what I found on RCGoups that illustrates this. I still think I'll need to add these gussets.

[Konrad]

Now to the reworking of the front of the turtle deck spin mount (rear of F-9). The biggest issue was that the plans have a bit to much contact with the filler and the upper longeron. I feel this will allow the covering to distort at the longeron and F-9.  As ther is a lot of eye appeal in this area of the Sparky design I want as smooth the covering going from the rear of the cabin to the rear turtle deck. This was alwasy a problem for my in the many Comet Sparky builds of my youth. I do note that the plans show a transition on the side of F-9. I'm still keeping that and changing the bottom of the filler to being a point contact with the upper longeron.

[Konrad]

Still moving aft. Here I've added the rear spine that defines the rear turtle deck. I like that the plans call out for a laminated spine. Of note I had to lower one of the cross members to allow a smooth transition. You can see that this cross member is spanning the longeron and upright joint. To fine tune this spin's curve I'm using a wedge. I also changed the placement of the splice double to try to keep it off the covering.