It isn’t a Flaw – its a Feature!

1/30 to 1/31 – 4 hours.

Laying out the flap actuation brackets (left and right) on a plate of 1/8″ 2024-T3 provided with the kit.  Notice that these are mirror images of each other.  Also, notice my crossed out lines.  This was due to an error in the plans.  Laying them out, the error was apparent.  The plans called for dropping 1 1/16 from the red line to the bottom of the bracket, but the angle so formed was visibly off.  The next crossed out set of lines were made by assuming the 1 1/16″ was meant to be measured from the upper black line, not the the red line.  Again, noticeable visual error.  Closer, but definitely no cigar!  Luckily, the plans are full size, so I just had to ignore the erroneous 1 1/16″ dimension on the drawing and draw what was shown in the plans.  Luckily, this also lines up perfectly with the flap where it is supposed to.

Cutting out the plate on the bandsaw.  There is no way the shear will go through this stock.

VERY rough band-sawed parts.  The cuts are to larger than needed dimensions, and so I’ll sand up to the lines.  Sanding aircraft aluminum is OK if you use aluminum oxide paper.  Many other abrasives are not OK, and can set up corrosion issues later on.

Here are the final rough parts.  Next, the angle gets cut into two separate spar mounts as seen below.  This is a complex cut, as there is a slight bevel across the face.

Here are the parts clamped together.

Match drilling the parts.  This is thick enough that a drill press is very wise.

Zinc Chromate (actually this is zinc phosphate – my zinc chromate is still on the way).  Now its time to drill up the hinge to fit the spar.

OOPS!  The plans call for cutting the hinges to 55 1/2″ long.  I cut them (BOTH) to 55.”  I drilled the hinge into place, and it ends 1/2″ too far outboard.  Since the inboard side of the flap is where the actuator is, this is the highest stress point of the hinge.  I don’t want to terminate it at the wrong spot.  I can choose to buy new hinges, piece in additional hinge material, shift the hinge inboard 1/2″, OR …

Note the plans that call for a AN257-P3 hinge.  They supplied an MS20257-3 hinge (which cost $2 a foot).  The MS part number superseded the AN, so no problem there.  I have more of the MS20257-3 hinge, but only 36″ lengths.  But I also have a short (24″) piece of MS20001-3 hinge ($11 a foot) on hand.  Check out the difference.  Dimensionally, they are the same.  However, the MS20001 hinge is extruded, and the MS20257-3 hinge is rolled.  Under load, the MS20257 hinge can unroll and separate.  The MS20001 hinge cannot.  I wasn’t thrilled that the flap was attached with the MS20257 hinge, but the safety track record demonstrated that it is good enough.  Except I screwed up and trimmed my weak flap hinge too short at the highest stress point.

Solution – I trimmed 3 more inches off the 1/2″ short MS20257 hinge, and add 3.5 inches or high quality MS20001 hinge.  I slid the pin out before I cut, so my pin is only 1/2 inch shorter than both pieces of hinge, and runs continuous through them.  I now have the better quality hinge at the high stress point (where the actuator works) and this will keep the flap hinge from peeling in the event of a slight overspeed.

The two hinge sections on the common pin.

Both hinges drilled and clecoed in place.  I like this better than the original, and I will do the same thing on the other side.  (I have to.  I cut them both short).

Remember the 1/2″ too short pin?  This, too, is a feature.  I centered the pin in the hinge, and drilled a tiny hole for a cotter pin – 1/4″ from each side.  This is about halfway across the end segment on each side.  I am drilling with a #55 bit.

Here is the (test) cotter pin neatly terminating my piano hinge pin.  This isn’t going to fall out.  All my piano hinge pins will be terminated similarly.  My flight instructor drilled it in to me to check these cotter pins on my Cessna Ailerons during preflight.  I have seen several flying airplanes without them.

Still more work to go on the flaps.  Now I need some 3/4″ by 3/4″ angle 1/16″ thick to finish the mounting brackets.  There are many lengths of this material in the kit box, but I am nervous about using the wrong length – one that is intended for the fuselage.  If I do that, I’ll have to replace an entire angle.  Since I have a piece of 1/8″ by 1″ 6061 T6 C-channel, I think I’ll just trim a 3/4″ by 3/4″ by 1/8″ angle out of that and use it instead.  This will add about 1 ounce to the airplane – I checked!

An extra ounce?  I know:  “take care of the ounces and the pounds will take care of themselves.”  However, this is budget building, and will require some compromise.  I also have to take care of the dollars, and the thousands will take care of themselves.


Leave a Reply

Your email address will not be published. Required fields are marked *