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Big Sky Stearman Assembly Manual


To sell an "aircraft part" to be installed on a Type Certificated aircraft, that part must be FAA PMA'd (have FAA parts manufacturing approval), FAR 65.303A. Obviously this is impossible on most older aircraft because it is a big deal to obtain PMA. The alternative is to allow the owner/operator to produce this part and then install it on his Type Certificated aircraft. The owner may contract to have that part produced but he must be actively involved in the production of that part by providing input or assistance to the producer of the part. He could provide drawings or the original part. He could participate directly in the production of the part by personally supervising the manufactering of the part or provide quality control. The data used would have to be "approved data." Big Sky Stearman assists owners/operators of antique aircraft in the production of their owner produced part. Big Sky does not sell "aircraft parts," the owner/operator would make a log book entry that identifies the owner produced part [FAR 21.9(a)(5)], specifying his participation in the manufacturing of the part, and make a declaration that the part is airworthy. This policy is defined in FAA AGC-200 policy memorandum on the definition of an "owner produced part" dated Aug. 5, 1993. Contact Big Sky Stearman for further information.

Project Assembly Manual Revision 7

April 7, 2002
Revision No. 7


These instructions are not an official technical manual. They only provide suggestions to expedite the usual FAA/Manufacturer-approved procedures, which you must follow. A licensed A&P mechanic must supervise the repair of your aircraft and certify on a Form 337 that the aircraft is airworthy. He should inspect your repair materials before they are installed on the aircraft.

Lower and upper wing assembly instructions

1.  Unpack the owner produced parts and inventory them against the packing list. Familiarize yourself with the various ribs and other components. The kit includes an original Boeing Frame Drawing, which gives bolt sizes and part numbers for every station in the wing. Supplement that information with the original Boeing Parts Manual, AN 01-70AC-4, the original Erection and Maintenance Manual, AN 01-70AC-2, and the Structural Repair Manual, AN 01-70AC-3. <Pictures>

2.  Lay out the appropriate front and rear spars on saw horses. The plywood reinforcing plates are temporarily tacked in place with nails. Number the plates to identify their positions on the spars. Make a pencil line on the spars around the perimeter of the plates to aid in applying glue. Remove all the plates from the spars. <Picture>

3.  Identify each rib for the wing and confirm its location on the spars. Slide the ribs into their pre-marked positions. You may need to slightly rasp the spar openings in the ribs; they are a tight fit. Do not glue or nail them at this time. <Pictures>

The rib sequence is as follows:

Lower wing from the inboard to outboard:

Rib Description


  Boxed root rib


  Step ribs


  Standard long ribs


  Aileron boxed rib


  "Truss" aileron rib


  Standard aileron ribs


  Wide opening ribs


  Standard aileron ribs


  #2 tip rib


  #1 tip rib


The upper wing is much the same:

Rib Description


  Upper boxed rib


  Upper standard ribs


  Wide spar opening ribs


  Upper standard ribs


  #3 tip rib


  #2 tip rib


  #1 tip rib


4.  Apply glue to the plywood reinforcing spar plates. Using the nail tips as guides, nail the plates permanently in place. Use wood clamps to apply pressure over the wooden clamp blocks, which will squeeze against the plywood plates. After the glue has dried, remove the clamps and clamp blocks from the reinforcing plates. Use a correctly sized drill for each hole in the spars to remove dried squeeze glue from the holes and to correct any minor misalignment. <Pictures>

Ream out the oversized holes in the spars that require aluminum bushings. Bushings should fit into their holes with an easy thumb push. The bushing length should be not quite the thickness of the spar.

Make a preliminary fit of all aluminum plates that will sandwich the spars. Grind the ends of old bolts to make a "bullet", which will help locate and align fittings to the holes. If minor hole misalignment occurs, ream the holes with the appropriately sized drill. Do not remove metal from the aluminum fittings when doing this <Picture>. Apply a coat of varnish to the wood surfaces that will be covered by metal fittings and begin to install the metal hardware.

5.  Carefully lay out all parts and bolts for each station of the wing. Bolt heads may face either forward or aft, as depicted in the drawings.

The compression members have a front and rear end. Identify the rear of the compression member by its shorter diagonal member <Picture>. The length of the upper wing compression members (except for the outboard member) is 30". The length of the lower wing compression members is 30-1/2".

Appropriate spacers, P/N 75-4513, at compression member locations will position the spars parallel to each other at 32" from the front spar center to the rear spar center. Spacer thicknesses are as follows:

















Modify a 1/2" open end wrench to hold the nuts that are captured inside the compression member end (no washers under the nut) <Picture>. Use "bullets" to locate and align hardware fittings on the spars. After the fitting is aligned, insert the appropriate new bolt. It is difficult to "torque" bolts into wood because of different densities. Tighten the bolt until the metal fitting sets itself into the wood; do not crush the wood with the fitting. All cotter pins should be vertical. Install all metal fittings at each station in the wing. When installing the spar root end aluminum main attach plates, you will have to square them to the spars both horizontally and vertically to guarantee a good fit to the fuselage frame wing attach points <Pictures>. If minor hole misalignment occurs, ream the holes with the appropriately sized drill. Do not remove metal from the aluminum fittings when doing this <Picture>. Double-check that the spar root ends are 32" from center to center. The lower and upper wing root wooden boxed rib members "float" around the compression members as they are being installed. <Picture>

6.  After you have installed all metal fittings and compression members in the wing, install the drag and anti-drag wires diagonally in the wing between their appropriate lugs. Install the fork end fittings and their locking nuts on the wires, making sure the "witness hole" in the fork end is blocked by the threaded wire. Be careful to note the appropriate length and size of the wire according to the drawings. The upper wings have two sizes of wire, a "heavy wire" and a "light wire". Be sure to have the correct wires in the each bay. On the lower wing rear spar the wire attach lugs have holes that are elevated on the lug so that the wires will not interfere with the aileron push-pull tube. Be sure to note how that affects the wire routing in the 3rd bay of the lower wing (the 1st bay being the root end). Pin the wires with a clevis pin and put slight tension on the wires. <Picture>

Place a reference pencil mark at the junction of each compression member with the front and rear spar on top of the spar in the center of the spar. Use these marks to "trammel" the wing or square each bay between compression members. If each bay is square the spars will be straight <Picture>. Apply tension to the wires and note the difference in distance between the marks. Adjust the lengths of the wires until the distances and tensions are the same. On each lower wing you should be able to sight through the four aileron hangers on the rear spar and see daylight through the bearing holes. Use a tensionmeter to tighten the wires. You can find the tensions in the Structural Repair Manual, AN 01-70AC-3. If you don't have a tensionmeter, tighten the wires to a good "B flat" twang (I didn't say that!). Use anti-chafe tape to protect all wires where they cross over each other.

7.  Set the ribs in their final positions. Make sure that the wooden diagonal braces of the ribs do not interfere with the drag wires. If they do, offset the rib slightly until it clears the wire. You do not have to glue the rib to the spar; the fit of the rib is so tight it will be hard to get glue in the joint. Nail the ribs at the spar with two nails on each vertical wooden member of the rib. Do not nail through the upper and lower rib capstrips into the spar. On the lower wing leave the two step ribs and the root rib unnailed for now.

8.  Nail and glue the wooden leading edge stringer into position. First notch the portion of the stringer at the root end to accept the step plywood. Clean out each rib leading edge notch from squeezed glue or interfering plywood. Apply glue to the notches except for the root rib and nail the stringer into position. Square each rib to the spar as you nail the stringer in place. The stringer ends outboard at the #3 tip rib.

9.  Install the aileron bay fabric tack strip stringer. Square the boxed portion of the inboard aileron bay boxed rib with the rear spar and nail the rib to the spar. Glue and screw the stringer into the trailing rib members aft of the rear spar and set the stringer into the notches provided for it on the #1 tip rib and the inboard aileron bay boxed rib. <Picture>

10.  Lay the tip bows over the tip end of the wing and start visualizing their installation <Picture>. Saw the forward end of the tip bow so that it has a clean perpendicular end ready to tack and glue into a reinforcing block on the forward end of the third rib in from the tip (#3 tip rib for the upper wing, first standard aileron rib for the lower wing). The bow will lie across the front of the #2 tip rib, on which you should sand an appropriate bevel.

As the bow lies across the tips of the spars, scribe a line across the spar tip and saw it off <Picture>. On the lower wing the trailing end of the bow will continue past the rear spar and join the #1 tip rib at its trailing end. The end of the #1 tip rib from the rear face of the rear spar should be 10-1/8" (you will check this later by hanging the aileron in place and verifying that the wing tip curve aligns with the tip of the aileron). After you fit the bow to the end of the wing, glue and screw it onto the ends of the spars. <Pictures>

On the upper wings the bow continues back over the #1, #2, and #3 tip ribs and butts into the trailing end of the fourth rib in from the tip (the first standard rib). Scribe a line on the trailing ends of the ribs where the bow crosses them and cut off the rib ends. The trailing end of the fourth rib will be 21-5/8" from the rear face of the rear spar. A triangular reinforcing block strengthens the joint of the tip bow to the trailing end of the fourth rib. Fasten the tip bow into place.

Install the tip bow reinforcing brace sticks. They run diagonally from the spars to the center of the tip bow <Picture>. You must custom cut and fit the joints of each stick. After you rough the bow into position and fasten it into place, sand and fair all joints so that you can lay 1/16" plywood gussets over the joints <Picture>. Install the hand hold in the lower wing tip, custom cutting and fitting the pieces into position. <Picture>

Install the leading edge plywood skins. Sand and fair the leading edge of the bow, ribs, and spar filler strips. Lay the plywood skin down into position and mark along the leading edge curve of the skin. Trim any unnecessary material off the skin. Permanently apply the lower skin first. The trick to getting the necessary compound curve into the plywood is to distribute the nailing so that no large bulges or pouches develop in the plywood. All the nail heads and glue will later be sanded off to form a smooth compound curve surface. After the lower skin is in place, varnish the interior of the area that is to be covered and apply the upper skin, having varnished its interior side as well. Do not apply any varnish where there will be a glue joint. <Picture>

11.  Begin constructing the wing walk by cutting notches in the upper capstrip and wooden support blocks of the inboard step rib <Picture>. The step support braces insert into the root rib and the outboard step rib support blocks. The braces drop into the notches you cut into the inboard step rib.

Align the step ribs on their spar location markings and nail the ribs to the spar. Permanently glue and nail the cross braces in place to the two step ribs and the root rib. Permanently nail the root rib in place. Install the filler block material on the top of the spars between the step ribs and the root rib, using 3/8" thick material on the top of the spars and 5/16" thick material on the bottom of the spars.

Place the step trailing edge into position and note that the trailing end of the step trailing edge must be 21-5/8" from the rear face of the rear spar <Pictures>. Trim off the trailing ends of the root rib and the inboard step rib to accommodate the step trailing edge. Install the step trailing edge and ensure it is in line with the trailing ends of the rest of the ribs.

Sand the entire step area to fair the surfaces that the step plywood will glue down to. Pass a straight edge over the entire step area to ensure the area is faired and even <Picture>. Raise the low areas by gluing in strips of material and sand down the high areas.

Clamp a vertical 2x4 against the outboard step rib and the rear spar. This will serve as a guide when you install the step plywood. You must steam-bend the leading edge of the step plywood so that it can follow the curve of the nose ribs. A simple way to steam the first 3 inches of the step plywood is to clamp the leading edge of the plywood to a 3" diameter pipe held in a vise. Apply hot water to a towel on the area to be steamed. Press a hot household iron against the wet towel. Keep the towel wet with hot water and produce the steam with the iron <Picture>. All that is necessary is to produce a curve that will match the first 3 inches of the nose of the step rib. Mark the rib lines and the cross braces on the plywood step so you will know where to nail it down. Generously apply glue to all of the step wooden support members and position the step plywood so that it fits in the leading edge stringer notch, with the trailing end of the plywood elevated and resting against the vertical 2x4. Nail off the front of the plywood to the notched stringer and then allow the plywood to lower down in place onto the step support frame work. <Picture>

12.  Cut a "tunnel" for the flying wire into the step over the root end of the front spar. Use triangular pieces of plywood to box the cutout. <Pictures>

13.  Varnish the interior of the underside of step area aft of the rear spar to the trailing step block and then apply the 1/16" plywood skin to cover that area. Place the drain hole provisions to the rear. <Picture>

14.  Glue and nail diagonal corner bracing between the spars in the step area. Install inter-step rib bracing sticks between the lower portions of the step ribs. Make a pattern of the sequence of the 3/8" holes drilled in the lower skins of the two step ribs. These holes let a curved needle pass through the plywood when stitching fabric to the ribs. The pattern will help you mark the fabric for the hole locations. <Picture>

15.  Install miscellaneous wood blocks at these locations: the tie-down bracket on the rear spar can be shimmed with appropriate sized blocks. Place a 5/16" x 3/4" x 4" block through the rear strut fitting on the top of the spar. Place 5/16" x 3/4" x 2" blocks on the bottom of the rear spar at the aileron boxed rib and at the tie-down location; you will attach the metal trailing edge material to these.

16.  Install the two large wing inspection doors, if desired. They are left off many sport versions because it's a good spot for weather to enter the wing.

17.  Sand and fair all the wing areas to prepare for varnish. Spray 2 heavy coats of epoxy or exterior spar varnish on the entire wing, being sure to reach all angles and crevices. Spray varnish on all the metal work also; it will provide more protection.

18.  Install the pitot and static lines in the leading edge of the left lower wing. Let the 3/16" aluminum lines form a low point at the root end of the spar and then rise as they arrive at the strut fitting at mid wing. Attach the lines with 5 adel clamps and screw to the spar.

19.  Nail the leading edge aluminum skins to the leading edges of the ribs. Make sure the wing is lying flat on the saw horses. The leading edge metal skins form a "D" spar and stiffen the wing considerably. If the wing is not level you could build a twist into the wing. Use a 6' long straight edge to make sure you have sanded all the nose ribs fair. Use three aluminum sections, 16" by 48", to form the leading edges. Trim the length to fit on the ribs. Pre-bend the aluminum edge pieces by folding them over onto themselves and releasing. They should spring back to just under a 90 degree bend. Practice on scrap aluminum first. On the lower wings, cut holes on the top of the center aluminum skin to allow the landing wire lugs and N strut fittings to protrude out of the leading edge. On the upper wings, cut the holes in the lower portion of the skin. Nail the sheet metal to one side of the spar. Wrap the sheet metal around the nose ribs, using a belt and a half-round block behind the spar to conform the metal to the nose ribs. Mark the rib locations on the aluminum skins and then nail the skins to the nose ribs and the other side of the spar. Do not nail down the length of the leading edge nose stringer. <Pictures>

20.  Form the aluminum trailing edges by bending 10' pieces in a brake (the local sheet metal shop should have one). Use a soft rivet or "pop" rivet to fasten the aluminum trailing edges to the trailing end of the ribs. The trailing edges should end 21-5/8" behind the rear face of the rear spar. The lower wing also incorporates an upper and lower trailing edge metal in the aileron bay area. <Picture>

Center Section Assembly Instructions

1.  Lay the front and rear center section spars on two saw horses. The 3/16" plywood reinforcing plates are only nailed in place. Scribe pencil marks around the plates, pry them off the spars, apply glue, and put them back on the spars permanently. Use the nail points to align them. Use wood clamps to hold pressure on the plates until the glue dries. After the glue dries, remove the clamps and ream the spar holes out with an appropriately sized drill to remove squeezed glue and correct any slight plywood plate misalignment. <Picture>

2.  Sort out the appropriate aluminum spar hole bushings by comparing the outside diameter of the spar holes with the inside diameter of the holes in the aluminum plates that sandwich the spars <Picture>. Bushings should fit into their holes with an easy thumb push, so ream the holes as necessary. The bushings should not protrude out of the holes, but should be short by about 1/16". Install the bushings and then install the appropriate aluminum plates on the spars. The outboard attach plates have a "rounded" lower edge on the front face of the front spar and on the rear face of the rear spar. These allow the diagonal drag wire attach fittings to fit correctly. Use "bullets" (ruined bolts with the tip ground to a taper) to align the plates with the spar holes. If there is minor misalignment of the spar holes with the plates, carefully ream the spar holes slightly to allow the bushings to align themselves with the plates. Permanently install the aluminum plates, leaving open the holes that will be used to attach the compression members and the fuel tank saddles to the front and rear spars. <Picture>

3.  Join together the two spars by installing the outboard compression members and the three aluminum fuel tank support saddles <Picture>. Refer to the frame drawing for the proper bolt sizes and part numbers. The bolts that attach the steel diagonal drag wire fittings to the front and rear spars are steel. The compression members are spaced at the front with a 1/4" spacer and at the rear with a 3/16" aluminum spacer. During the final assembly process of the compression members to the spars, place the right and left wooden boxed ribs into position. Install the forward landing wire steel attach strap after the boxed rib is in place. It sweeps back and would otherwise prevent the boxed rib from going into position. The boxed ribs will temporarily float in position until the wood work begins.

4.  Install the 4 wooden saddle members that attach to the front and rear spars. Be careful to determine the front and rear of the saddle by noting the angle of incidence on their uprights. Glue and nail them into their pre-marked locations <Picture>. Install the rear view mirror wooden brackets and plate on the left side of the center section <Picture>. Refer to the frame drawing for its location from the spars.

5.  Install the drag and anti-drag wires, routing them through the aluminum saddles, wooden saddles, and the rear view mirror bracket. Put chafe tape on the drag wires at the locations where they pass through the metal members and where they cross over each other. The drag wires are attached to the diagonal steel fittings and you should apply slight tension to them. Turn the center section upside down and place trammel marks in the center of the spars at the junction of the 2 outboard compression members and the center saddle with the spars. Alternately tighten the wires until each "bay", left and right, is square. Set the tension on the wires at 650 lbs. minimum, 700 lbs. nominal, 750 lbs. maximum. Lock offthe fork ends and cotter pin the clevis bolts. <Pictures>

6.  Construct the leading edge by slipping each of the 8 nose ribs onto the leading edge stringer, gluing and nailing them into position to match the marks for the spar nose rib locations. Apply the assembly to the front face of the front spar. Glue and nail each rib to its position on the spar. Install the left and right boxed ribs on the spars. Position the ribs 1/2" inboard of the spar ends. Glue and nail them in place with filler blocks under the 1/16" webs, front and rear, upper and lower. <Picture>

7.  Locate the rear spar trailing ribs by their part numbers and center them on the rear spar up and down. Glue and nail them into place with triangular glue corner blocks on their pre-marked locations <Picture>. Refer to the frame drawing for clarity. Create the hand hold openings by using a plywood backing plate in conjunction with ribs 1308 and 1352 <Picture>. Refer to the illustrated sketch detailing that assembly. To rough in the trailing edge bow, start by stiffening the boxed rib trailing ends by clamping a 2x4 along the length of the boxed rib. Lower the center section laminated bow into place from above, resting it on the top of the ribs' trailing ends. Note how much each rib needs to be trimmed to allow the bow to come down into place. Once the bow fits snugly against all of the trailing ribs, glue and nail it into place with the triangular glue blocks. The bow is considerably oversized and you will need to grind it down for a final fairing before applying the trailing edge plywood. Strengthen the attachment of the boxed rib trailing ends and the trailing ends of the bow with triangular glue blocks. The final length of the center section bow tips should be 21-5/8" from the rear face of the rear spar. Allow a 1-1/4" wide trailing end for the wing fairing to be able to grab.

8.  Apply the plywood skins to the leading and trailing edges. Glue and nail the front and rear spar filler material spanwise to the top and bottom of each spar, filling in the areas between ribs. Sand and fair the leading edge ribs, trailing edge ribs, and bow so that the skins will evenly fall onto the structure. The leading edge skin is in two lengths. Scarf-join them over one of the leading edge ribs. To create a 1/2" scarf on the edge of each plywood piece, place the edge of the plywood on a sharp edge of a table and sand successive layers back on the plywood. The laminations will form lines that should be even and straight. Precurve the plywood so that it will bend around the leading edge without cracking. Fold it in the middle and tie string around it at 4 locations. Pour boiling water into the fold and allow the hot water to soften the plywood at the fold. Progressively tighten the strings until the fold is quite sharp. Allow the plywood to set until dry. Release the strings and the plywood should have a 90 degree fold to it. Cut the cut-outs for the front spar lift fittings and strut fittings. Glue and nail the plywood to the lower portion of the leading edge.

Varnish the interior of the plywood and the front face of the spar. Use a belt and a half round block behind the spar to form the upper portion of the plywood to the ribs and glue and nail.

The trailing edge plywood is applied in five pieces on top and five on the bottom. Scarf-join the outboard pieces over the ribs. Glue and nail the center piece to the spar, ribs, and bow. Apply the lower skins first, varnish the interior of the trailing edge, then apply the top skins. After the skins have been roughed in and the glue has dried, grind and sand the skins and bow smooth. Be sure to provide some strategic drain holes in the lower skins. <Picture>

9.  After the center section has been sanded and prepared for varnish, pre-fit it to the upper wings to determine if the leading and trailing edges are in proper alignment. Grind and sand the center section trailing tips to align with the upper wing trailing edges.

Varnish the center section with two coats over everything. Fit the rear mirror nut plates. Prepare the center section for fabric cover and cover as desired. After pressure testing the fuel tank, glue in felt chafe strips, install the fuel tank permanently. There are different methods of dealing with the top of the fuel tank in the center section. It was originally left exposed. If you install it that way it has to be removed at every annual to inspect for wood deterioration per AD 50-06-02. One way to cover it is to apply some felt over the top of the tank (to hide the ugly upper skin) and glue a covering of fabric around the perimeter of the tank to the spars and boxed ribs. Using 1/4" wide aluminum fabric tack strips, screw down at one inch intervals around the perimeter. The fabric covering rescinds the AD and hides the fuel tank.


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