Lower Wings, Ailerons and Flaps Construction Details

New left lower wing 1936

The left lower wing was replaced in early 1936. A new wing with aileron cost $515 with the N-number painted on. The damaged wing was returned to the factory for a refund of $88.

Spars for lower right wing

The lower wing panels are shorter than the upper wings. The span of the upper and lower wings are the same except the lower wing panels attach to the fuselage outboard of the landing gears. On the lower wings the front spar is 1/2" thick and the rear is 5/8". On the upper wings the thicker spars are the front spars. The thicker spars are the ones where the flying and landing wires attach. At this point in the construction, none of the plywood reinforcement plates are glued on. They are just tacked on by a few nails so they can be drilled with a drill press. They will be pulled of in order to slide the ribs on then glued in place.

Root rib

The root rib is reinforced with two diagonal braces as support against the tension of the fabric covering. When a fuel tank is installed in this bay, the root rib "leans" again the tank for this support. My Staggerwing originally only had the standard tanks in the upper wing and an optional fuselage tank under the baggage compartment. At some point in its history, a fourth tank was added in the left wing. I am omitting this tank in my restoration.

Inner aileron hinge brace member

This member is designed to brace the rear spar against the twisting load from the aileron attachment. The original right lower wing had been dissembled when I got the project. However, I can tell from the spars that this brace was not on my original wing. I copied it off another B17L wing (I think this wing was from serial number 25). When a fuel tank is installed in this bay, the first compression member is modified to resist the twisting load.

First compression member

The first compression member is simply a 7/8" steel tube. When a fuel tank is installed in the lower wings, this compression member would have been modified

Aileron bell crank and cable brace member

This member provides reinforcement against twisting of the rear spar caused by the aileron. It is also a compression member against the tension in the aileron control cables.

Second compression member

This compression member is located where the interplane strut and landing wires attach. The tie down fitting goes around the landing wire trunion. My aircraft did not originally have this fitting. I copied it off of one seen on serial number 25.

Third compression member

The third and last compression member is a 5/8" steel tube, just like the forth compression member in the upper wings.

Metal aileron fittings ready to prime. The hinges are being fitted to the aileron spar.

Aileron ribs

All of the aileron ribs were built by the previous owner of my project, Sam Johnson.

Aileron lower leading edge plywood ready to install.

The lower plywood cover is ready to glue and nail on. It is attached to the wing to insure no misalignment can creep in.

In the second image my helper, Suann, is at work. A entire army of tiny nails were beaten in with peewee hammers.

Aileron root rib reinforcement

Reinforcements are added to keep the tension in the fabric from deforming the root rib.

First aileron hinge

The four main aileron hinges are made from the same casting. All four are machined differently. The large hole about midway in this image is for the flap actuator rod. On the second and third hinge the smaller hole is machined for the aileron bell crank.

In this photo and some of the following, only the bottom part of the plywood aileron leading edge has been installed. The curved top plywood will be added later.

2nd and 3rd aileron hinge

The aileron bell crank is mounted between the second and third aileron hinge. Note that second hinge has been modified at the top attach point. Also the large hole for the flap actuator rod on the second hinge is a little larger than the correspoing hole in the first and third hinge. This simplifies the alignment of these hinges. A few shims were necessary between the hinge and the spar in order to obtain good alignment of the hinges (in addition to some colorful language... )

All of the moving parts of the aileron system use the same bearings. The barings are used in pairs one pressed into each side. There is a total of 16 per wing.

Forth aileron hinge

The un-machined hole near the center of this hinge will be used for the flap actuator rod return spring.

Fifth aileron hinge

The last hinge is a welded steel structure. This hinge was added to the design after the ailerons were lengthened on the prototype serial number 3 B17L. Comparing my original 1934 right wing and the 1936 left wing, I noticed that there was an added flange to attach the hinge to the wood tacking strip. I added this modification to the hinge.

Aileron tip

The curved part of the aileron will be constructed from flattened 1/2" aluminum tube. This part will be built at the same time as the wing tip in order to obtain a perfect match of the curvature, or at least, the perception of perfection.

Aileron bell crank

The cable is attached to the bell crank by clamping it between two short bushing and a 1/4" bolt.

Change in aileron bell crank after serial number 14

The aileron bell crank was changed after serial number 14. The first image is cropped from Beech drawing B17730. As the note indicates, this design was used on the prototype serial number 3 and models after serial number 14. Comparing this drawing to the parts in the above section, you can see that the radius of the attach point of the link is different. It is 2" on my serial number 12 aircraft and 1 5/16" after serial number 14 according to the drawing. The second image shows three links that go from the bell crank to the aileron. The first one is an original skinny link from my left wing. The second one is a robust home made link found on my right wing. The third one is an example of a longer link used with the bell crank in the drawing.

The bell crank on the left 1936 wing had been modified from the short arm to the long arm.

Aileron torsion box

There is a large gap in the top part of the leading edge plywood at this point. In order to provide some torsion stiffness across this gap, this box is built on the back side of the spar. The top and bottom will be covered with 1/8" plywood.

Aileron upper leading edge plywood.

The plywood strips for the upper aileron skins were boiled then clamped in a form.

The form I used was a 1 1/2" dowel bending the plywood 90^o.

There is always some spring back when the plywood is taken out of the form. This time the fit was very good.

Basswood formers

The openings in the top skin is reinforced with basswood formers.

Curved Part of Aileron Trailing Edge

The curved part of the trailing edge is made from partially flatten 1/2" aluminum tubing.

Early Wing Flutter Problems and Aileron Mass Balance

From April 1936 to May 1940 there were six fatal Staggerwing accidents which were caused by wing structural failure. The aircraft involved in these accidents were: one B17L, one B17R, one D17S, and two F17D. One other F17D, serial number 330 NC2663, experienced wing flutter, but was able to land safely. The wings off of this aircraft were shipped to the factory for study. As a result of these accidents, CAA issued Airworthness Maintenance Bulletin No.26 on June 10 1940. This bulletin restricted the never exceed speed for all models to 160 mph and 140 mph in rough air. Also instrument flight was prohibited.

This image is cropped from Beech Service Bulletin 76.

After completion of an investigation by the Material Division at Wright Field, Dayton, OH Airworthiness Maintenance Bulletin No. 35 was issued on Sept 26 1940. This bulletin describes modifications to correct the flutter problem and remove the restrictions described in Airworthiness Maintenance Bulletin No.26. For the B17 and C17 models Service Bulletin 76 was issued by Beech. This service bulletin covers serial numbers up to and including 135. It describes adding lead counterweights to the leading edge of the aileron. This bulletin also required the reduction of the never exceed speed from 241 mph to 225 mph. For E17 and F17 models Beech Service Bulletin No. 74 was issued and bulletin No. 75 for the D17 models. This modification have apparently fixed the flutter problem as there has not been and Airworthness Directives issued on the B17L in the past 79 years.

More details can be found in the "Flutter Report" from the Staggerwing Club. (Note: There is a typo in this report. It list the N-number on F17D sn 330 as NC2863. The correct N-number is NC2663.)

For the purpose of comparison, the aileron on the right was built as it would have been built in 1934 with the counterweight yet to be added. The one on the left has the weights added in accordance with Service Bulletin No. 76. The counterweights extends from the tip of the aileron to the hinges at the pushrod attach point near the center of the aileron.

The aileron on the right has been modified according to Service Bulletin No. 76 with the added spruce fairing and the bottom rounded to match the 5/8" radius.

The lead counterweights are attached to the spruce fairing with numerous #4 machine screws. Excess screw length will be cut off before covering.

Ailerons Finished and Varnished

All B and C models use a split flap, referred to as "decelerator" in vintage brochures for this aircraft. The B17L model have the flaps on the lower wing. Other models which have flaps on the lower wing are the B17B, C17L and C17B. Some models have them on the upper wings; such as the B17E, B17R, C17E or C17R. The upper wing flaps are longer than the lower wing flaps. The shortening of the flaps on the lower wing may have been done because the lower wing flaps reduce the effectiveness of the ailerons, which are just behind the flaps.

The flaps are actuated by a control cable attached to a leaver on the cockpit's floorboard. The cable pulls a slide tube inboard. Two diagonal turnbuckles push the flap down (up in these images) since the wing is upside-down. A 16" spring holds the slide tube outboard and keeps tension on the control cable. The longer flaps on the B17E, B17R, C17E or C17R models have three of these turnbuckles.

Springs hold the flap closed. They are temporarily attached to the flap in this photo. The flaps have to come off for wing covering, because the fabric goes under the flap's hinge and up the spar a few inches.

The machine screws in this photo are the close stops which rest against the aileron hinge brackets. The slide tubes are tinned.

The old turnbuckles were replaced with a slightly larger size. One of the originals were slightly bent. The old ones were an odd size, 13/64-30 thread size. I replaced then with 1/4 turnbuckles. They are safetied in a different manner. A drilled jam nut is safety wired to the turnbuckle barrel.

Tinning the slide tube.

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