Prepping wing spars for biscuiting

Release film and other detritus inside a DaSH main wing spar tube, waiting to be cleaned out.

The last two weeks we have spent a couple of build sessions cleaning out and sanding the inside of the main wing spars and tail boom in preparation for biscuiting.  The parts come with a bit of release film still attached to the inside, plus left-over release agent (liquid that dries on top of the release film, leaving a slippery non-stick surface) and a too-smooth interior.  So we mechanically remove the left over release film (by knocking it off with a long pole, and also sometimes using an air lance).  Then we use a long pole with various amounts of Scotch Brite scouring pads taped to the end to remove any remaining release film, and lightly score the inside of the tube.  Finally, we clean out the dust and any remaining release agent using an acetone wash with a ball of rags on the end of the aluminum pole.

So far, we've mechanically removed most of the film from the inside of the tail boom, and all the main wing spars (1 center, 2 middles, 2 outers, plus a spare set of outer wing sections, a pair of longer outer wing sections, and two pair of wingtip extensions).  And we've done the majority of the sanding with the Scotch Brite, which on the tapered section requires several passes with progressively smaller diameter clumps of Scotch Brite balls at the end of the poll.  Next, we'll finish scotch briting the smallest end of the tapered tubes, and the smaller diameter ends of the longer outer wing spar and the wingtip extensions, then we will acetone wash all of them and all parts will be ready to have biscuits installed.

Another view of left over release film, with a lot of dust covering the interior of the tube

All the tubes for the main wing spars, 2 sets of extra wingtips (one longer), and 2 sets of wingtip extensions, + tailboom.  All await cleaning out the inside to prepare for gluing in anti-buckling biscuits of foam and balsa.

Using an air lance to remove some of the release film.  We found this was not too effective so we moved to using a long pole to scrape the release film off.

Scraping the excess release film (not visible -- it is in a line under the pole) off the inside of the wing spars, using the cut-off end of an long aluminum pole, and lighting the inside with a small LED flashlight near the end of the pole.

First layer of Scotch Brite, taped down well so the whole thing won't fall off.

Second layer of Scotch Brite.  (On the largest diameter 100 mm ID tubes, a third layer was also used)

Scotch Brite ball after cleaning the inside of a spar.  You can see some release film that came off during this cleaning, that had not been able to be removed previously with the aluminum pole.

Video (above) showing sanding the inside of one of the outer tapered wing spars with Scotch Brite on a long aluminum pole.  Aluminum tube is tilted slightly to get the most contact, and the wing spar is rotated around and the pole is moved back and forth inside the tube.  The tube is pushed as far down the taper as it can go.  Later, the rest of the taper is sanded by putting progressively smaller balls of Scotch Brite further down the spar, until the end is reached (in this case, it takes three different diameters of the Scotch Brite cylinder).

After each cleaning, the inside is reinspected with a flashlight on the end of a pole, and then the process is repeated until it looks like the cleaning and scoring is sufficient on the inside of the tube.  The next step (not shown), is to run acetone soaked rags inside the tube to clean off dust and any left over release agent on the non-scored areas of the tubing.

Four of the outer wing spars (and another spar in the foreground) after cleaning, with some of the left over release film detritus scattered on the floor.

The tools used to clean the spars:  An aluminum tube with a flashlight lashed to the end, to mechanically remove left over release film; and another longer tube with Scotch Brite taped to the end, used to sand the inside of the tubes.  That tube is as long as the longest wing spars (6.7 m).

Re-covering tail, Balsa prep and Fuselage tube shaping

Here's a quick post on the work we've done in the last month up to Thanksgiving.

Among other things, we've done some re-covering experiments with the tail, we've prepped and laminated all the balsa for the remaining biscuit doublers for the plane (mostly for the main wing spar and tail boom), and we've shaped and fitted most of the tubing for the fuselage and made composite sandwich gusset material for the bottom bracket in preparation for lashing together the entire fuselage frame.

Not shown in the photos below are the cleaning and prep of the main wing spars and tail boom for biscuiting (we're about half done with that), and also all the CAD work we've continued to do in order to create all the cut files for the ribs and rib doublers (we have the files for the wing root ribs done, which will let us start building the plane from the 'inside out' since the three middle and center sections of the wing all use one rib profile, but we have 32 unique rib profiles that need to be completed, along with all the doubler files, in order to build all the variations of the tapered outer wing sections).

Cutting Mylar to fit the tail surface

Trimming Mylar

Tightening the Mylar covering on the test Vertical Tail surface.  We've only used regular Mylar so far, which doesn't shrink a lot and leaves wrinkles over the leading edge.  We'll try tensilized Mylar in the next few weeks, as it seems to shrink better, in order to see if we can get a smoother surface on the leading edge.

A sample decal in place.  We'll use this same process for sponsor's decals on the fuselage.

Cutting balsa to laminate two sheets with ~20 degree angle between grain of each sheet.

Laminating two balsa sheets at ~20 degree angle to each other.  The finished sheets will be laser cut to make biscuit doublers (in a balsa/foam/balsa biscuit that goes inside the carbon fiber tubes).

Initial cutting of the miter shape into the carbon fiber tube, using a paper template.  We leave a bite extra and sand to the finished shape in an iterative process while fitting multiple times as we sand.

Underlying balsa after removing the carbon fiber.

Cutting the bulk of the excess balsa plug off prior to sanding.

Sanding the balsa plug and edges of the carbon fiber to the final miter shap (aka "fishmouth shape'), using a paper template as a guide.

Final sanding of edge during fitting

Fitting a tube miter on the Fuselage Frame (Chain Tube to Top Tube)

Fuselage Frame with 4 of the 5 main tubes fitted and ready for tacking and lashing

Carbon fiber-Nomex honeycomb composite panels under vacuum (one is 1/16" thick, the other 1/4" thick -- these are to be used as gusseting material for the bottom bracket attachment to the fuselage frame at the Chain Tube).

Completed CF/Nomex panels

Hot wired root airfoil sections, ready for rib cap laminations

3D Print in ABS of the hook on the Lift Wire Clamp, to test sizing and fit with the sailing thimble used at the end of the lift wire.  (actual part will be machined out of aluminum and will include 'wings' that get lashed onto the main wing spar).

Tail & Controls testing

Testing the tail sections, tail mounts and control system

On Monday we finished putting together the test rig for the tail surfaces and control system, mounted it all on an SUV and did the first tests of the setup at 15 mph to verify that everything worked.  This helped us verify the geometry and location of the servos, control rods et cetera and verified that the servo and control horn setup worked, along with the V-bracket mounts for the horizontal and vertical tails.

The next test during the daytime will be to do a higher speed test to verify that these components still work properly and hold up under the full force expected with full-sized tail surfaces at flight speed.  (These test tail surfaces are 1 m span for both horizontal and vertical tail sections, but the real tail surfaces will be 4.5 m and 3 m long, respectively.  So the force will 4.5 and 3 X what these test sections see at flight speed -- thus we will increase the speed to give roughly equivalent force so the mounts, servos and control rods will get tested under real flight loads -- meanwhile the test tail surfaces themselves will get a workout at much higher than normal load per meter...).  We don't anticipate problems since these components are used on model aircraft that fly much faster and at much higher loads, but it's always good to test to make sure.

On these first tests, everything performed quite well.  We didn't see any problems.  Since we didn't have the right connectors to run everything off of the batteries that we will use in flight, we used an inverter on the car battery, to a transformer with 6 V DC output, wired to the controller.

Programming the Servos (for speed, max angle etc.)

Trying everything out on the bench before installing

Gluing the horizontal tail servo box in place

Attaching the control rod to the horizontal tail control horn

Tightening the rod end

Closeup of control rod ball end attached to control rod (through Mylar)

Control rod hook up complete

Setting up the vertical tail control rod/rod ends (everything is on its side during the installation)

The final setup (with controller in place, power cord not shown)

Test rig on the truck, hooking up the power

Prepping the racks to hold the test rig

Our power supply (12 V DC to 120 V AC inverter using car battery).  Joystick and phone cord visible on left.

Test setup almost ready to go -- final clamping and testing the joystick

The test rig setup with power and joystick connection

The view from inside the truck

Testing the tail surfaces, mounts, servos and control rod setup

Video of first tests of the tail surfaces & control system, 10/21/13

Moving the test rig back inside after the test (a tight squeeze)

Tail test sections

Ted holds the completed Horizontal Tail test section

In September and early October, we built all the rib sections for the Horizontal and Vertical tail, then assembled a 1 m span test section for each tail using an aluminum spar.  These test sections will be used to test covering techniques, decide on leading edge construction for the tail, and to develop the servo control system.

Hot wire cutting tail rib foam sections

Tail rib caps in clamps

Vacuum bagging the tail rib sections

Completed rib sections

Cutting the ribs

Tail ribs for test section with doublers

Gluing full face doublers on end ribs

Prepping the tail end ribs for spar caps

Getting ready to vacuum bag the end ribs

Vacuum bagging end ribs

Repairing end rib cap delamination

 

Making leading edge for V-Tail

 

Test fitting leading edge to V-Tail

H-Tail and V-Tail test sections, not yet glued

H-Tail and V-Tail test sections, not yet glued

Tail control horns and bearing brackets

Completed ribs with foam removed for trailing edge

All the parts for H-Tail and V-Tail test sections save leading edges

Marking the spars

H-Tail parts

V-Tail parts

 

Gluiing brackets and control horns on H-Tail

Assembled H-Tail w/ glue drying

Glued V-Tail test section

Heat activated gluing tests w/ various contact cements and Mylar

Mylar on various test glue samples

We determined that most contact cements (like UHU Por, Foam Tac etc.) worked pretty well as heat activated glue for Mylar, though they might have stuck a bit TOO much.  Plliobond also made an excellent glue, and was a bit easier to peel the Mylar off from and reattach.  Thinned Pliobond worked best, but with the replacement solvent for MEK (Ethyl Acetate and petroleum spirits), you must add the solvent very slowly to the Pliobond, or else it turns into an unusable gooey mess.

Applying Mylar to a test frame (using thinned Pliobond glue)

Applying the heat gun

Most wrinkles gone.  The tensilized Mylar worked better than regular in this regard.

 

Applying ammonia to 1/32"balsa for 1mm jog leading edge on H-Tail

Completed H-Tail w/ leading edge

Making clearance notches for the V-Tail Depron leading edge

One Half of leading edge in place

Completed V-Tail test section with leading edge

Clearance for control horn on V-Tail test section

Damage to foam ribs from solvent for Pliobond -- need to use care to keep only on tops of rib caps

Making clearance holes in Mylar for control horn and bearing brackets using Mylar Tape

Clearance holds in place

Applying and tightening Mylar on H-Tail w/ covering iron

Completed H-Tail test section with covering material (Tensilized Mylar)

Closeup of clearance holes after shrinking covering

The tough wrinkles at the trailing edge we couldn't get rid of

 

Waviness of balsa leading edge is evident here (2 mm Depron LE on V-Tail is much better)