This is a photo album of my RV - 4.

I will be putting notes and explanatory pictures as I build this air plane.

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Tuesday, November 16, 2010

Wing Incidence Drilling

 from Van's Aircraft Site

1: Don’t put in all of the wing attach bolts right away! You may have to remove
each wing a couple of times in this process. Just put in four bolts in a square
pattern for each wing. Don’t use the good bolts! Put in hardware store variety
and grease them and the holes in the spar to protect the spar and facilitate

2: Once the wings are TEMPORARILY installed, level the fuselage laterally.
You want the two wings to be level left to right. Don’t simply rely on placing a
level across the longerons (they are hand bent and may have slight errors), put a
level on the top of the main spar center section as well. If the two agree, move
on to the next step, if not, average the two. A 1⁄2” or less height error at the wing
tip is close enough. Now check the forward sweep! Hang 2 plumb bobs on each
wing, one at the tip and one at the root (tank area). Then stretch a single long
string across the front of the these four plumb bobs. If there is no or minimal (get
real, less than 1⁄2”) sweep fore or aft, then move on to #4 below. At the same
time, triangulate from each wing tip to some center point on the aft fuselage. The
two measurements should be the same (get real, within 1⁄2” or so will be fine)

3: If there is a sweep or triangulation problem STOP. The common reasons are:
         A: The rear spar structure that sticks out of the fuselage is comprised of
two pieces of 1/8” aluminum bar. One piece ends up on the front side of the wing
rear spar stub and one is on the back side (where you can see it). The forward
piece OFTEN is too long and hits the wing root rib flange and prevents sufficient
insertion of the rear spar stub into the “hand” of the fuselage (thus producing
forward sweep). If so, then remove the wing and cut off the minimum amount
required to correct for the sweep error. In general, if the tip of the wing is swept
forward 1” then trim 1/8” to 1⁄4” from the end of the bar.
         B: Sometimes the culprit is the wing rear spar stub! If it is running into the
spacer block in the “hand” of the fuselage, and that is all that is wrong, then
remove the wing and trim off as in “A” above.
         C: Sometimes it is both problems! Just be sure what the problem is
before you get too handy with the trimming!

4: If all is OK, then start the process with the bubble level of setting the wing
incidence. DON’T DRILL THE HOLE YET. There is more to this than you think.
The CRITICAL part of incidence is not so much the amount, but rather that both
wings have the same amount. Other factors to assess are the following:
        A: The edge distance of the rear spar attach bolt is PARAMOUNT. 5/8”
from the center of the hole in ALL directions in ALL of the material it goes
through is the first consideration. Take a good look.
        B: It was the hope of the designer that the flaps might fit once the wing is
installed! If you look at your flap, you will see that it has a bottom skin that
seems to want to go under the fuselage belly skin when the flap is UP. It is really
nice if this skin just kisses the bottom of the fuselage. Try putting the flaps on.
If you have edge distance met, the flaps fit well, the sweep is OK, the
triangulation is good, and incidence is set, TAKE A BREAK. Move off and away
from the airplane and take a look, you are very close to drilling a PILOT hole (say
1/8” or so) in the rear spar! IF you’re ready, go to instruction #6.

5: The only real problem you may have is that the incidence does not allow one
of the requirements above to be met. Try some of the advice below:
couple of solutions to this... one is to change the incidence of the wings (lower
the rear spar until they will). To a degree, this is OK but of course you must
retain that critical edge distance for the attach bolt. IF you can retain this
distance and not change the incidence too much (say a degree?) then what will
happen? The horizontal stabilizer may need to be repositioned. HOW, and
HOW MUCH? Clamp the wing spar in the new location, put Van’s block of wood
under the level, and raise the tail of the fuselage until the bubble centers. Now
check the horizontal for level fore and aft. Very likely you can’t measure the
change from its original position. IF you can and all else is OK then consider
changing the spacer under the front spar of the horizontal to correct it.
        B: Another solution is to simply cut off the skin that would have been
under the fuselage. Now the flap will come up in trail and you will simply have an
uncovered hole showing in the fuselage bottom where the actuator push rod will
come out (unsightly but no big deal).
BELLY. Not a big deal but if everything else looks good and you don’t want to
raise the rear spar of the wing (thus reducing the wing incidence and maybe
losing the PARAMOUNT 5/8” bolt edge distance) then you can simply bend the
lower flap skin up a little until it “kisses” the bottom of the fuselage.

6: Get ready to drill the hole! The toughest part of this is getting the hole
SQUARE to the surface of the material. Everything there in the rear spar attach
area is at a different angle to everything else. It is very hard to hold a drill
perpendicular by eye! Make a drill guide! Find a block of iron or aluminum at
least an 1” thick. and 2” square. On a drill press, drill a 1/8” hole through the
center of the block. Now you can clamp the block to the rear spar of the wing
and it will help to assure that the drill bit penetrates the surface perpendicularly.

7: Drill the hole to 1/8” diameter first! Put a cleco in and check everything
again, step back away from the plane and check again! Only then can you safely
drill the hole out to the correct size. A nice hole can be achieved by first drilling
an undersize hole and then, using a reamer, enlarge it to the final size.

8: During all of the above setup and machinations, DON’T FORGET the wing
root fairings. They can be marked and drilled during this fit up stage. The belly
skin of the fuselage can be marked and it’s hole locations identified during the
above processes as well. Also, look at and identify the transition points for the
fuel lines and vent lines from the tanks into the fuselage. This is a great time to
finish these details. How about the pitot line? Time to take a break!!

Murphy's Law

You all know about Murphy's Law right? 

It is that atavistic force that causes inanimate object to fail for no reason at the worst possible moment. 

If it can go wrong it will.

Well, not quite and, like many of these things if you drill a little deeper, the whole story is much more interesting as well as yielding some practical lessons.

It grew out of the experiences of a Colonel Stapp USAF, a pioneer in studying the effects of deceleration on humans. He was challenging the conventional wisdom that 20gs was fatal to the human body so there was no point in making the harnesses in fighter planes any stronger. Stapp begged to differ and in an heroic spirit of self experimentation put his own life where his mouth was.

After a SNAFU of the sort that any Technician or Engineer has seen many times, Stapp muttered something which eventually became Murphy's law.

To paraphrase:

If there is a way of buggering something up - eventually someone is going to do it. 
The corollary being ; If you can build that possible failure mode out or plan for it's contingency - not a bad idea to do so.

The bigger story makes for an engrossing read.