Sig Chipmunk Kit

[Mark Radcliff]

I am starting to build a Sig Super Chipmunk kit. In looking over the wing plan it appears you build it flat on the bench. The ribs are tapered so if you build it flat on the top then the bottom will have some dihedral. Is this correct? Also, I am making it electric so if anyone can advise the motor and battery you use it would be appreciated.
Thanks.

[Dave_Trible]

I am starting to build a Sig Super Chipmunk kit. In looking over the wing plan it appears you build it flat on the bench. The ribs are tapered so if you build it flat on the top then the bottom will have some dihedral. Is this correct? Also, I am making it electric so if anyone can advise the motor and battery you use it would be appreciated.
Thanks.

We recently built one in our shop here.  The flat board method works OK since only about the last four inches of cord actually are in contact and ends up straight.  We propped up the wing forward to help it stay straight.  It might be slightly better if you built more sophisticated jigging but their method has worked out ok for a long time.  I have one of the original Jim Van Loo style kits which I will build on the board.  We apply the top side sheeting and cap strips,  Then flip it over and nail it down again before sheeting and capping the bottom side.  When finished it should be straight.

Dave

[wwwarbird]

 Dave,

 Does that answer his diehedral question? I'd have to dig deep in the shop to check but I'm thinking the wing L/E and T/E should be straight, no diehedral.

[Dave_Trible]

Wayne when all said and done they will be straight and parallel.  The last few inches of the ribs, 1-12 or whatever are the same.  That is all that actually touches the board.  Note the leading edge WILL NOT be parallel to the board at this point, but will be co-planer with the trailing edge albeit at an upward angle from the table and will be a little higher in the center because of the wing taper.  The leading edge is 1/4" square and is pinned into a V notch at the front of the ribs.  It will be sort of floating in space just as it would if you used a two-rod wing jig as will the spars.  That made me a little uneasy so I propped up the spars and leading edge in spots along the span as required before I applied the sheeting to help hold shape when I applied pressure for pinning the sheeting.  It isn't as bad as it sounds.  I seem to remember this was also how the Nobler and a few others were assembled.  Goldberg put some to-be-cut -off feet or sections on root and tip ribs to help support them while being built on the board.  For my own designs I build a jig system supporting leading and trailing edges parallel to the wing board.   That jigging takes about as long to build as the wing does.  They didn't usually do this in the 'old days'  but got away with it because the wings were smaller and you still have careful thought given as you work.  Some built them totally "in the air" without nailing down to a board at all and the better builders could get away with it......I'm sure I wouldn't.  I am actually about to put teabag paper on my new Charles Mackey Lark for Classic and it also was built this way.  Came out straight as an arrow.


Dave

[wwwarbird]

 Kind of confusing but, ok.

 In this case I'd shim and fix/pin both the L/E and T/E however necessary to stay parallel to the board, the center rib(s) to be level with it, and then carefully work from there. Just me.

 Whatever the case, there shouldn't be any diehedral in this Chipmunk wing.

[Ken Culbertson]

They didn't usually do this in the 'old days'  but got away with it because the wings were smaller and you still have careful thought given as you work.  Some built them totally "in the air" without nailing down to a board at all and the better builders could get away with it....

Even the not so better builders got away with it, everybody did!  This is a Nobler kit wing being built using a chair for a "jig" somewhere around 1974.  Came out straight.

Ken

[Dan McEntee]

  When in doubt, just follow the instructions! Countless Chipmunk wings have been built this way, if you don't want to have to use some sort of jig, which will require some extra work to get set up if you don't have one, even a simple rod jig, and you will have to learn how to use that also.  I have built several Chip wings this way and the important thing is to have a nice, flat surface from the trailing edge to the line where the riser blocks for the spar get pinned down, and make sure the riser blocks are all the same size cut from the same piece of balsa with the same side pointing up. Run a Sharpie down the length of the side of the piece that you want pointing up.
  Type at you later,
   Dan McEntee

[Steve Thompson]

I built one using an A-justo-jig.  I highly recommend it if you can find one. 

Does anyone know of a similar or better two rod jig system that is available?

[Steve Thompson]

Found some more information on the A-justo-jig on a Stunt Hangar thread titled:

 
adjusto jig- locating jig holes

[fred cesquim]

hello Mark,
i would support the L/E and T/E paralell to the building board using some shims of wood or L shaped alluminium stock, this will set all of the ribs with the centerline paralell to the board and prevent any dihedral or twists.
the picture shows my "fancy" jig but you don´t need to go that far. just make sure L/E and t/e are paralell and you´re good to go.
the picture is just to show how the wing look while on that support.

[Dave_Trible]

The problem with doing the Chippy wing and others like it is the design of the TE itself.  There almost isn't one.  It is a very small strip or cap (1/8" x 3/16" or 1/4") applied AFTER the wing is built.  The trailing edge really consists of the sheeting top and bottom with an open gap between them until you put the cap on.  This way leaves you no rigid edge to key from or mount in a jig.  This works only if built on the board.  These were common in the day of the cloth hinge.  It stinks in the time of nylon hinges.  You must either stuff some balsa blocks in behind the cap or change the whole way it is done back there to give enough 'meat' to sink a hinge into.  I chose the blocks on the Lark.  On my designs I use a 1/4 square TE which is what I jig,  then lay sheet on top and bottom.   This leaves a TE thickness of 3/8" which work peachy with 5/16" or 3/8" flaps.  Some of the old designs used 3/16" or 1/4" flaps which made the thinner wing TE more desirable.  The Magician used stiff 1/8" flaps- near impossible for slotted hinges.

[wwwarbird]

The problem with doing the Chippy wing and others like it is the design of the TE itself.  There almost isn't one.  It is a very small strip or cap (1/8" x 3/16" or 1/4") applied AFTER the wing is built.  The trailing edge really consists of the sheeting top and bottom with an open gap between them until you put the cap on.  This way leaves you no rigid edge to key from or mount in a jig.  This works only if built on the board.  These were common in the day of the cloth hinge.  It stinks in the time of nylon hinges.  You must either stuff some balsa blocks in behind the cap or change the whole way it is done back there to give enough 'meat' to sink a hinge into.  I chose the blocks on the Lark.  On my designs I use a 1/4 square TE which is what I jig,  then lay sheet on top and bottom.   This leaves a TE thickness of 3/8" which work peachy with 5/16" or 3/8" flaps.  Some of the old designs used 3/16" or 1/4" flaps which made the thinner wing TE more desirable.  The Magician used stiff 1/8" flaps- near impossible for slotted hinges.

 Ahh, arrgh, it's that type of T/E. Yeah, they're a bit more of a pain.

 With them, I've still fixed the L/E parallel to the board as described earlier. (Actually, and ideally, I'd start by pinning a spar down to the board before setting the L/E if possible, depending on the design) If it's "fish mouth" ribs you can start with the spar as mentioned then go right to my next step below**, waiting to add the L/E last once you're finished with assembling the T/E. That scenario negates the need to even fix the L/E to the board.
 
 **Regarding the T/E assembly, (assuming the ribs have no "feet") I've pinned down a piece of square or rectangle stock to the board, full length spanwise, to use as a reference for setting the rearward portion of the ribs on. Fasten this full length "reference" piece parallel to the T/E just far enough ahead of where the "sheet" for the T/E will be to get the centerline of the rib airfoil to sit parallel with the board. (you're simply supporting the rear portion of all the ribs with this piece) As you progress this full length "reference" piece then serves two purposes, it allows you to assemble the wing straight by keeping the ribs against it, and also gives you a handy spot to pin that part of each rib to.

 This routine isn't the easiest thing to explain in type,  and I'm not saying this is the way it all has to go, I'm just saying I've done it this way many times with success. To each his own... 

[Chris McMillin]

I am starting to build a Sig Super Chipmunk kit. In looking over the wing plan it appears you build it flat on the bench. The ribs are tapered so if you build it flat on the top then the bottom will have some dihedral. Is this correct? Also, I am making it electric so if anyone can advise the motor and battery you use it would be appreciated.
Thanks.

Hi Mark,
Did you get an idea of how the Chipmunk wing is built? Dan McEntee has built a couple that I know of, one he flew at  a Lawrenceville Nats in the 90's. It's been a while since I looked at the plans but Dan helped me when I last built an open wing, a Southwick Skylark. (I built it in halves and joined it in the middle IIRC.)
The one piece construction isnt too hard, like Dave Trible and some other guys posted, use bench blocks to keep the TE and spar parallel in planform and head-on, no dihedral.
Hope it's going well, long time since we've spoken! Miss the AMA Conventions...
Chris...