Saturday, 18 February 2012

Time flies!

Well, it would appear it has been a while since I last posted.
This is not, as some have suggested :) due to a lack of progress, but I have been in that stage of building where a huge amount of work produces almost nothing to look at.

My main progress of late has been the ailerons. It turned out that building the ailerons is almost as big a job as building the wings.
The original aircraft ailerons had a steel tube spar, (similar to the Ryan STA method) with aluminum nose ribs, aluminum main ribs, aluminum rear spar, and trailing edge.
The hinges are permanently attached to the tube, and held in place by steel brackets.

First step in building the ailerons was to work out the actual size and articulation. Since I know the airfoil and chord, as well the hinge points are fixed, I could work out the size of the aileron. I have no idea what the original travel was, but based it on basic practice, max 25 degrees up and down, with no bias as of now.
EAA replica showing the uncovered aileron
I had a good idea of how the original was built, based on the EAA replica photos, so this was my guidline, but I did not really know what they were like under the leading edges, so this is more of a guess.
 I started by making the aluminum rear ribs. These take a number of fixtures, one to router the blanks, another to router the lightening hole, another to drill the loactor holes, another to bend the LH and RH flanges, and another to bend the attach flange.
Once I had the rear ribs done, I started on the small leading edge attach ribs. They are also aluminum, and attach to the steel brackets which are welded to the spar. The process for these was the same as the rear ribs, router and forming fixtures. I determined that I would need 12 nose ribs per aileron, 7 LH and 5 RH, so it involved alot of cutting, drilling and forming.

The white material shown is teflon. I was able to get a large section of 1/ 1/4 inch thick teflon block. This stuff drills and machines really well, and holds up well to the forming process. I understand it is extremely expensive, but luckily I did not have to find out!

Once all the aluminum rib sections had been formed, I started on the steel brackets which would mount to the 4130 steel tube spar. I decided the best way to mount the brackets accurately would be via a tube section which slid over the spar, and I would rosette weld the tubes with the bracket attached, as this would minimise heating the ailerons spar, (I was concerned about distortion over the 8 foot length, as it has 3 hinge attach points.

To make the brackets, I cut the tube sections, and also the steel plates. I cut multiple steel plates from .040 4130 sheet by making a cutting block I could run an air nibbler around.
Once the tubes and plates had all been made, for a total of 48 brackets (more on why so many later) I made up a fixture to weld them in LH and RH pairs.

Once all the LH and right hand brackets had been welded, and cleaned up, I could start assembling the ailerons.
I then needed to permanently locate the hinge locations to the rear spar, and I had been concerned about drilling holes directly through the wooden blocks which glue behind each hinge point.
I decided to try and mount the hinges via a "floating bushing" arrangement, which would hopefully self align the attach bolts.
I made up bushings which fit within the hinge, and over the attach bolts. I then milled a square cut through the wooden blocks which was about 1/16th wider and deeper than the bushing.
I cut grooves into the bushing (065 wall) for better glue adhesion, (but realised later where could the bushing possibly go anyway!!)
The theory is that by glueing the block over the bushing and allowing the glue to set with the spar and hinges bolted in position, there will be no misalignment of the three hinge positions. The difficult part is not permanently glueing the aileron to the wing in the process.

But thankfully once the glue dried the bolts slid out (pre waxed) and the hinges released nicely.
The spar now moved freely on the wing, with no friction or binding. I polished the spar tube under each hinge location, but it`s a shame the original did not allow for some sort of bearing, rather than tube on tube.

Next step was to rosette weld the LH and RH attach brackets in place on the spar, which locates the leading edge aluminum brackets, and also locate the rear ribs on an aluminum U channel which I bent up from 032 2024 T3 aluminum.
I cut down some standard trailing edge stock to more closely match the original width.
I ran the trailing edge through a shrinker to shape the section which matches to the wing tip bow.

I made up a separate steel attach bracket, as well as smaller nose and main tip ribs, to match the tip bow area. The aileron tapers in this area both in width and depth. More router jigs, form blocks, drill blocks.

I then fit the 5/8th aluminum tube through the ribs and drilled it to secure.

The next step was to form and mount the leading edge sections. I started by rolling some sections of 020 2024 T3 sheet to get the basic shape, the built a fixture to mount them to the leading edge brackets.
By moving the uprights to different locations I was able to drill and mount each section.
Once all the leading edge sections had been drilled, I removed them and trimmed each to the final size, and refit the aileron to the wing for a final check.

To complete the ailerons to pre cover stage the following needs to be done:
Paint the steel aileron spar black.
Form the curved section of 5/8th aluminum tube at the tip.
Replace all clecos with screws, bolts, and rivets as required.
Add a counterweight in the leading edge section.
*I suspect the original did not have counter balanced ailerons, and with so much weight behind the hinge point why did it not flutter at speed? well the EAA replica may show why. When I moved the aileron on the EAA version it was almost solid, now I imagine that 20 years sitting in a museum does not help, but I noticed the ailerons were mounted directly via bolts through the spar, and this obviously created a great deal of binding friction. If the original was done the same way, then it would have been very hard to move the ailerons, and this may have actually helped to counter act flutter.
Then again, Gen Doolittle had so many other issues associated with flying and racing the SS, maybe he forgot to mention it also had the odd indication of flutter!
My ailerons are almost friction free at this point, so I think it would be critical to balance them at minimum 10% nose down, (110% total weight forward of the hinge point)
I have temporarily hung a steel weight to give an idea of balance, but without all the correct fasteners in place and the clecos removed, as well the fabric and paint estimated, I cannot determine the required weight yet.
Due to the extremely short area ahead of the hinge, it will take a substantial weight to balance them.
I have allowed attach points between the steel leading edge brackets to attach a balance weight tube, and this will be completely hidden.

I will also need to leave the leading edges un rivetted at this stage to allow for the Canadian stage inspection.

The RH aileron is also all ready to assemble, with all parts completed, so this should go together quite quickly compared to the first one!


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