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NOTE: Each of the pictures on this page is a clickable thumbnail which will open up to a much larger image for detailed viewing. In some cases you will notice differences between the photos and your plans and parts. This is due to changes made during the building of the prototype. I've tried to make note of the changes but if I missed anything please assume that the plans are correct.

Main Wing Center Section

Image 1

Image 1

1. Begin construction by making up the two dihedral braces from the four 1/8" plywood DB parts provided. Each dihedral brace consists of two DB's laminated together. You can also laminate together the two W2 center wing ribs at this time.

2. Test the fit of ribs W2 through W5 against the main spar W1 and open up the slots with a sanding stick if necessary. The fit needs to be snug but we don't want to break anything when assembling the parts. Also test fit the wing mount block WM against the slots in W2 and W3 to make sure it will slide easily into place.

3. Dry fit together W1, W4, W5, and W14. Line these up over the plan being certain that W1 is vertical and then carefully glue. Check and double check alignment while gluing to maintain accuracy. (Image 1) Please note that I did not have a full size plan when I assembled my center section and simply marked out the spar and rib locations on my building board. It is much simpler to just build over the plan!

Image 2

Image 2



4. Mark the location of the balsa filler blocks on the top of WM. Dry fit the dihedral braces into place through ribs W4 and W5 and then add W2, W3, and WM to the assembly. Carefully align everything over the plan and then glue. (Image 2)

Image 3

Image 3






5. Install the 1/4" balsa W13 as seen in Image 3. Apply some thick CA or wood glue to the top of the dihedral brace prior to installing W13 in order to bond the two firmly together.

Image 4

Image 4



6. Bevel the tops of the two W16's to match the airfoil and install. (Image 4)

Image 5

Image 5




7. Add the top two W20 pieces per Image 5.



Image 6

Image 6



8. Tri-stock the top of the wing mount block and add the 5/8" thick balsa fill blocks as shown. Sand everything flush with the tops of the ribs per Image 6.

Image 7

Image 7






9. Slide the two W6a ribs into place and glue to W1, W13, W14, and W16. Be careful to pull W6a snugly into place against W1 in order to maintain an accurate angle. (Image 7)

Image 8

Image 8



10. Place a straight edge along the top of W20 as a reference and then glue it to W6a. The ruler serves as a reference to make sure not to introduce any twist into W20. (Image 8)

Image 9

Image 9






11. Install the two SW4 sheer webs as shown in Image 9. Be sure to apply liberal amounts of epoxy or wood glue to the dihedral braces in order to ensure a good bond to SW4.

Image 10

Image 10


12. Install the two W15 1/8" balsa sub-leading edge pieces. Bevel the root ends to get a good joint where the meet in the center. (Image 10)

Image 11

Image 11







13. Next we need to block sand the entire structure to prepare it for the top sheeting. The better we prepare the structure now the less work we will have later and the surface will be much more accurate. On my Hellcat the 1/4" balsa W13 top spar was a bit lower than the surrounding structure. I capped it with soft 1/32" balsa and then sanded it to match. Image 11 shows the finished cap on the left while the right has yet to be done.

Image 12

Image 12




14. Pay careful attention to make certain W15 and W16 are sanded to match the airfoil as seen in Image 12.



Image 13

Image 13




15. Prepare two top wing skins for the center section. A short tutorial on my method of gluing up wing skins can be found on the "Tips & Tricks" page. Note that on my Hellcat I only sheeted to the center of W20 at this time. I find it much easier to get a really nice trailing edge if I sheet it seperate from the remaining structure and after pulling the center section off of the building board. I also like to trim my skins up to within 1/8" of their finished size before applying them. This allows me to access the joints around the perimeter of the sheet with CA and an applicator tip. Here is my process:
a. Apply wood glue to the tops of all internal structure except the main spar.
b. Apply medium CA to the top of the main spar and the top of W15.
c. Carefully align the sheet and stick in to position.
d. Pull sheeting tight and make sure it is bonded to W15.
e. Using thin CA glue the sheet down to W20 and to the center and tip ribs.
d. Weight the sheet with sand bags, old magazines, or whatever will conform to the curved shape. (Image 13)

Image 14

Image 14




16. Image 14 shows the first half of the top sheeting in place.

Image 15

Image 15





17. Once the top sheeting is dry you can remove the structure from the building board. Carefully break off the jig tabs and then sand as necessary to remove any small burrs. We are now ready to flip the center section over and install the remaining internal structure. A piece of foam rubber works wonders to prevent "hangar rash". First order of business is to install the remaining two W20 pieces as well as add the tri-stock to the bottom of the wing mount block. (Image 15)

Image 16

Image 16




18. Add the SW1, SW2, and SW3 sheer webs per Image 16. As with SW4, be sure to use liberal amounts of epoxy or wood glue to make sure and get a good bond between the sheer webs and the dihedral brace DB.







19. Using the pre-drilled pilot holes in the wing mount block as a guide, drill through the balsa filler blocks and up through the top wing skin.

20. Next we need to sheet the bottom of the center section from the center of the main spar back to the center of W20. This will stiffen up the structure so that it does not warp while we continue to work on it. This is one of the few times that I will recommend using medium CA for attaching the sheeting. Because the sheets are relatively small, and because we cannot weight the structure while the glue dries for fear of warping it, CA is the best option. Do not sheet forward of the main spar at this time as we still have work to do up there!

Image 17

Image 17

Image 18

Image 18




21. Next add the portion of the bottom sheeting at the trailing edge. Leave these a bit long at the back so they can be tapered to form a sharp and strong trailing edge (See step 22) I use a masking tape hinge to locate these so that they can be opened up (Image 17), glue applied to the structure, and then the sheeting folded back into position without any alignment errors. (Image 18)

Image 19

Image 19





22. Use a sanding block to taper the trailing edge of the bottom sheeting to match the ribs as shown in (Image 19).

23. Add the top rear sheeting to complete the trailing edge.

TECH NOTICE 3-16-2010: 1/16" plywood doublers have been added to ribs W4 and W5 forward of the main spar. These doublers, called out as W4a and W5a, should be installed prior to proceeding with retract installation. There was some concern that the mounting structure for the retracts may not be up to the task particularly on those Hellcats which may exceed the upper end of the recommended weight range. It is essential that epoxy be used when installing the gear rails as well as the plywood doublers and that good glue joints are achieved. If your plans do not show the installation of W4a and W5a then please contact R/C FrameWorks and an addendum sheet will be emailed to you in the form of a PDF file.

NOTE: Steps 24-26 will complete the installation of the retract rails in the center section. It is a good idea with any retract, but imperatively so with the Robart 615 units, to have the gear bolted to the rails during the installation process. This serves to ensure that the mounting surfaces of the rails are perfectly aligned so as to prevent warping the frames of the retract units when they are firmly bolted in place. Failure to follow this procedure will likely result in binding issues with the Robart gear. Ask me how I know this! I did not do this on the prototype and suffered the consequences. I was able to find a solution but, trust me, it's much easier if you follow the procedure as explained below!

Image 20

Image 20



24. Temporarily fit the landing gear blocks in place. These should be a moderately loose fit in W16 and you may need to sand a bit on SW3 so that the rails will lay in place without being forced. Double check to make sure there is no glue or anything that prevents the gear rails from pulling up tight against the wing ribs as well. Place a retract unit into position on the rails and mark out the bolt pattern. (Image 20) Remove the rails from the wing and drill to suit your landing gear mounting screws. It is helpfull to mark your rails so that you know which is right and left when the time comes to re-install them.

Image 21

Image 21


25. Bolt the retract unit to the rails as shown in Image 21.

Image 22

Image 22




26. Epoxy the rails into position being careful not to glue the retract unit to the rails. It may help to loosen the mounting screws while you position the rails and then re-tighten them while the glue dries. While you've got the retract mounted in place go ahead and add the 1/8" balsa stringer just aft of the retract unit as shown in Image 22. (See plans also)



Image 23

Image 23


27. Using some scrap 1/8" balsa make up the doublers that attach to the forward portion of ribs R2 and form the pocket for the wing hold down dowel. (See wing plan) It will be necessary to relieve the doublers slightly as shown in Image 23 in order for the 5/16" diameter dowel to fit. Simply wrap some sandpaper around the dowel and sand a channel down the center of the doubler down to about half its' thickness. Temporarily fit the dowel into place when installing the doublers but be carefull NOT to glue the dowel in at this time. It is much easier to do the finish work on the wing if the dowel is left removeable.

Image 24

Image 24



28. Add some scrap balsa triangle gussets in the corners as shown in Image 24 as well as on the plans. These will strengthen the pocket created for the wing hold down dowel against any side loads that may be accidentally applied to the the dowel when installing or removing the wing. They may not be absolutely necessary but provide some cheap insurance.

Image 25

Image 25




29. Double check that all ribs as well as W15 and W16 are sanded properly to shape and then install the remainder of the bottom wing sheeting. (Image 25)

Image 26

Image 26




30. Cut, install, and sand to shape the 1/4" balsa leading edges. Temporarily install the wing dowel so that you can fit the leading edgeS around it but be carefull not to accidentally glue it in place. (Image 26)

Image 27

Image 27



31. Run a drill bit through the wing mount holes to open them up in the bottom sheeting. Next, carefully open up the holes to approximately 1/2" diameter in preparation for the installation of the wing bolt guide tubes. (Image 27)

Image 28

Image 28





32. I made my guide tubes by wrapping typing paper around a 1/2" diameter dowel that was first wrapped with wax paper and then soaking the paper with thin CA glue and kicker. You can also often find 1/2" cardboard rocket tubes at the local hobby shop which will work nicely. Install the tubes making sure they are firmly glued to WM and the wing sheeting. (Image 28)

Image 29

Image 29

Image 30

Image 30



33. Trim the guide tubes off and sand flush to the wing sheeting. Masking tape can be used to protect the surrounding sheeting from damage while you sand as seen in Image 29. The finished product can be seen in Image 30.







That completes the construction of the center section for the time being. It is recommended that you complete the outer wing panels before proceeding with flap construction if it is your intention to add the flaps.


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