April 28, 2013

Scorpion Mk2 assembly, day 2.

Today I will detail the installation of the optional speed brake.

This comes as an option and it gives a really nice look to the plane and helps in slowing it down. I highly recommend it.

Here is the content of the speed brake kit along with the fuselage access hatch.

The first part of the job is to scuff the back of the hach to eanble a good gluing of the components on the speed brake. I use grid 60 3M sandbooster hooks and loops pad on a dremel rubber wheel.

After sanding, I degrease/ de-dust the part with acetone.

I then cut the speed brake off the hatch using a thin Dremel diamond cutting disc. The disc is exactly the width of the CNC pre-cutting on the hatch and makes a perfect job.

I then place the speed brake in the open position and mark the position of the hatch lip for future cutting. The cutting should be close to 30 degrees inwards. It might need adjustment when the speed brake is operational and checked for cinematics later on.

I then scuff the aluminium tubes with a coarse Permagrit file. I also scuff the inside of the tube with a M3.5 jobber. Don't forget to degrease with tube with acetone after scuffing. The Robart hinges need to be scuffed and their axle waxed as well. I use Meguiar's wax for this job.

Once their axles waxed, the hinges can be glued into the aluminium tubes. I use 5 min epoxy for this job. The hinge will stop short of the axle area since the tube diameter is slightly shorter than the root of the hinge .

Tack glue the tube on the speed brake using a few drops of CA gel. Put the hinge at 90 degrees up and set the speed brake flat on your table. You can easily adjust the tube angle on the horizontal and vertical planes that way before spraying CA kicker.

Do the other tube/ hinge the same way. Both hinge should be parallel in all positions.

The next step involves gluing the actuator and the control horn. First drill the plywood control horn with a 1.5 mm jobber in the center of the semi circular end. Then shave off this area so that it is just thick enough to accommodate the M2 control link. harden this part with thin CA. Drill at 1.5 mm again. That way the hole area will be optimally stiff.

Install the M2 control link on the horn, then insert a section of Tygon fuel tube on the link to keep it fastened, screw the short section of M2 threaded rod into the link and the actuator push rod. Also consider adding a bit of thread lock on this side.

Position the assembly on the speed brake so that the control horn sits at the front of the panel. The link hole should be located at xxx mm from the front of the hatch. Tack glue the horn to the speed brake with CA gel.

The next step consists in gluing the front of the hinges. I recommend to drill the front of the hatch to allow the hinges to be glued through the composite lip. This will make the speed brake attachment considerably stronger. Drilling should be done with a 4.5 mm jobber. Drill at slow speed to avoid damaging the thin upper hatch skin.

Once drilling is done, glue the hinges with 5 min epoxy through the hatch lip and trim the tip of the hinges flush.

The speed brake must be maintained in the closed position with some pieces of tape while the glue is curing.

Insert the M3 threaded rod at the middle of the Robart actuator ( the hinging hole might get re-drilledif a bit tight. Optimal jobber size for this is M2.9 ). Install the two nuts on either side of the actuator, leaving a 0.5 mm clearance. Use thread lock on the nuts.

Install the threaded rod on the hatch horizontal lip so that it is a 21 cm from the front of the hatch. Adjust the M2 threaded rod so that this is achieve when the actuator is in the retracted position.

Check the extended position. The panel should be extended at 30 to 45 degrees. This angle can be changed by ungluing and changing the position of the control horn. Putting it closer to the rotation point will get the speed brake more extended. Once the cinematics of the speed brake is satisfactory, glue the M3 threaded rod inside the hatch lip with 5 minutes Epoxy.