October, 2000 -- Aileron construction details
OK, the upper aileron spar caps are in place, and now we need to sand the outer edge to match the angle of the airfoil. The aileron spar forms the base of an isosceles triangle (the TE being the apex) so both upper and lower caps will need to be sanded to the same angle. See the black hash marks on the caps? More on that below.
The stick I'm holding is a sanding guide which will help make this hand-sanding operation fast and accurate. It's about 5/32" thick and has three strategically placed scraps of 1/16" ply attached to one side with double-sided tape (a very handy item to have around).
The guide has been clamped to the top of the TE, with the ply spacers straddling the rib/TE gussets. Now you have a flat reference surface elevated above the TE about 7/32".
I had this sanding block laying around from my last project. I have no idea what I made it for, but it is ideal for this operation. You want sandpaper on one end (probably 80 grit) and smooth wood on the other. You guessed it -- the wood rides on the reference surface while the sandpaper shapes the spar cap to the correct angle and flush with the ribs.
The black marks are put on the top surface of the cap to show your sanding progress. On the right no sanding has occured yet, while on the left the cap has been sanded down until the black lines are now dots. But before you've taken the material down that far, check the top surface with a rigid straightedge to make sure you're not sanding any concavity into the cap (which might happen if the cap was not glued on perfectly straight).
I used a Sharpie to make the marks. Be advised -- felt-tip pen ink loves to soak down into the wood fibers, which would make the marks useless as sanding guides. Quick, light strokes with the pen will prevent the ink from soaking in.
Grady and I finally got our schedules together again. We flipped the wing, and added the aileron spar caps and rib/TE gussets to the lower surface. This fellow has been doing some serious growing since March! Compare this photo with the March photo. Now Grady can easily reach the rudder pedals.
The front spar is suspended off the table top with identical-length 2x4s, and the TE is clamped to the table's straightedge with the wedge clamps. The instructions say to put the wing right-side-up before installing the second set of diagonal braces in the aileron bays, so I guess we'll be flipping the wing back over as soon as these spar caps are sanded to shape. Oh yeah... there are some plywood gussets I should install on the big anti-drag diagonals while the wing is upside-down.
Here's how I align the aileron spar caps just before going to bed. The forward edge of the caps should hold the ruler just above the rib, and when the ruler is vertical, this junction is done.
First glue tip of the month
I found these handy Epoxy Mixing Pads in the Brownells gunsmith supply catalog. They're pads of plastic-coated paper sheets that are bound at two opposing edges to make a steady mixing surface. I think they were about $7 for 3 pads, and certainly worth it. I have never seen these in aircraft supply catalogs, but they certainly should be offered there.
Sanding the angle onto the bottom aileron caps was more straightforward -- the angle is flush with the wing bottom, so no spacer is required under the sanding block. The light illuminates the gap under the straightedge when checking the alignment of the finished edge.
The way I have interpreted these spar cap angles doesn't seem to be shared by all of the AirBike builders visiting this site! (Actually, it doesn't seem to be shared by any of them so far.) The written instructions don't tell you to create this angle, but the plans certainly seem to suggest it. My explanation is too long for this page, so if you're interested go here for the rest of the story.
The lower aileron diagonals required less of an angle, and the sanding jig was a little simpler. Both ends of the stick used the same setup this time.
Sanding tip of the month
For a very handy, general-purpose, two-sided sanding pad, fold a small piece of sandpaper over double and then put a piece of double-sided tape between the two halves. I make a bunch of both 80 and 100 grit pads and leave them laying around at various places so there's always one a short reach away. These are great for rounding corners, cleaning and scuffing up glue joint areas, etc. The small boost in stiffness provided by the tape makes them just about right for most uses.
While installing compression members between the spars, I came up with this solution to an awkward clamping task. A scrap piece of 1/16" x 3/4" pine is clamped as shown with a short scrap underneath to prevent marring the workpiece. (The bottom of the clamped strip is further away from the spar than it looks here...)
Now the bottom of the clamped strip is pushed toward the spar, which causes it to bow and apply strong pressure to the workpiece. This was very effective, and allowed the pressure to be directed in any direction I chose.
Second glue tip of the month
When using these small plastic mixing cups, it may drive you crazy (like it does me) to have to use three hands -- one to hold your workpiece, one to hold the glue brush and one to keep the mixing cup from sticking to the brush and lifting off the table. Simply attach a piece of (you guessed it) double-sided tape to a block of wood and stick the cup to the tape. The added weight will keep the cup on the table, and the tape will remain useable for dozens of batches.
Also shown is my favorite brush for epoxy work. It's a garden-variety acid brush with the bristles trimmed to about 3/8" for the desired stiffness. This stiffness really helps when working epoxy into the pores of the wood.
Here's a view of the back side of the front spar at rib #3. At bottom is one of the anti-drag diagonals with its reinforcing corner blocks and plywood cap. At left are two of the compression members.
Here's the front of the rear spar at rib #7. The corner filler on top of the diagonal is my addition, which I added to all of the rear spar joints. I read about a miniMAX builder who had a very hard landing and later found that several of these joints had popped loose. (It's a credit to Wayne's wing design that this particular wing held together for one or more flights before the problem was discovered.) I will also be running a couple of bracing cables to the rear spar at rib #5 of both wings to support my upper tow line attach point above the vertical fin, so I am being extra careful to make these joints extra rugged. What you don't see in this photo is the 1/8" plywood scab that securely fastens the diagonal to the bottom edge of the spar.
This is as far as I'm gonna take the left wing at this time. The wing attach holes in my pre-welded fuselage weren't installed quite on center, so I am going to build both wings to this stage, rig them onto the fuselage and align everything perfectly before transferring the bracket holes onto the front spar. Having the leading edge in place would make this operation very unsavory, to put it mildly, so we'll save that job for another day.
The wing racks from my last project, in the ceiling directly above the work table, have been revamped to allow these much longer wings to be stored there. I'll be starting the right wing assembly right away, so I probably won't have as much to report for a couple of months.
I'm very pleased with my decision to varnish the ribs before assembly. It is an easy matter to scrape and/or sand the varnish off glue joint areas, and I know all too well the misery this will spare me from when it comes time to varnish the finished wings.
At this time I am about 236 hours into the project.
Engine news, deja vu all over again...
At this point it looks like I will be installing a Rotax 582 after all. Les Taff, my newly-found tug plane guru, threw cold water all over my plans to use the 503. She said that it would work ok (note the small "ok") as long as I was always towing out of LARGE fields, and as long as I didn't care too much about how long it took to climb to altitude with a glider in tow. Oh.
That's OK. I've been paying a lot of attention to discussions about 582s in rec.aviation.homebuilt and rec.aviation.ultralight, and I feel quite confident about running one of these puppies and getting a good reliable 400 hours out of it before a rebuild. That's a LOT of flying! According to Wayne, one of my challenges will be to find a suitable location on the AirBike for the oil reservoir, which has to be located at a prescribed height relative to the engine. I'm sure we'll be talking about that and other 582 issues a few months from now.
Break ground and fly into the wind -- not vice versa!