Step 6.9, Flaps

While I wait for some help on the bottom skins, I'll just move on to the flaps.  I feel a literary digression coming on so I'll offer fair warning to the easily bored.  Skip down to the pictures if you already know what a flap is for.

There are several different types of flaps, but generally, the flaps just extend downward at the trailing edge of the wing with the overall effect to increase its effective camber.  The flaps on this aircraft will be of the 'plain' type, illustrated below via the ever so convenient Google images.

So the purpose of flaps on an aircraft is to get some additional low speed performance out of your wings, albeit at the expense of a lot of extra drag.  The two phases of flight where flaps become advantageous is in take off and landing.  Having more lift during these two flight regimes allows the aircraft to use less runway.  In ascent, because the increased lift allows the aircraft to depart the runway sooner, and in descent because the aircraft can fly slower, it will contact the runway with less horizontal velocity and consequently use less runway.  

Another advantage of having flaps available for landing is that it allows the aircraft to execute a steeper approach which is safer from an "engine out, emergency glide to the runway" point of view.    Lastly, the flaps allow the descending aircraft to fly in a more nose down attitude giving the pilot better visibility of the approaching runway.

Now that we've covered flaps 101, let's see what goes inside the RV-7A flap.  I start with a quick check of the plans and then go collect the parts from storage.  After laying them out it is a fairly simple matter to understand the assembly.

Spar on the left, skins on the right, and ribs in the middle.  Yep, that's about it.  Then because one needs some boring tasks to balance out the fun stuff, one has to remove all the blue protective film and then smooth the edges of all of the parts.

After all of the edge deburring it's time to assemble the flap.  What is visible in this photo is the bottom skin and ribs.  The next thing is to fabricate the FL-706B flange.  This part transmits the load from the flap actuator to the flap.  It is a fairly beefy 1/8" thick affair and is surprisingly resistant to my  attempts to impart the specified bend.  It seems that the inboard end of the flap is 6.3 degrees off square, probably to follow the taper of the fuselage.

Whatever the cause, the plans call for the end of the FL-706B to be bent at precisely 6.3 degrees.    In my mind I could see the result of this exercise as a pile of flanges bent to 6.2 degrees and another at 6.4 degrees, with a single example of a 6.3 degree flange kind of glowing with an aura of correctness in the foreground.  I digress.  The bend is not really such a big deal except that the section to be bent is only 25/32".

If you're like me, fractionally challenged, you'll be interested to know that 25/32" is actually .78 inches.  Somehow the fraction doesn't mean anything to me until I convert it to a decimal.  Anyway, that's not much on such a thick piece of metal.  After screwing around for about an hour with several failed attempts, I finally gave in and consulted the internet.  Being the source of all knowledge, I knew I would find an answer there, but as is customary with the internet, many "solutions" are usually found.

So the problem becomes one of selecting one that sounds reasonable.  Sometimes finding a consensus of opinion online can be a challenge.  The ideas I rejected were "Use your million dollar numerically controlled 7 ton hydro static home bending brake" And "Hire Uri Geller to bend the piece with his mind."

Fortunately, most online felt that what I was supposed to do was clamp the short end in a vise and then clamp the long end between some wood to keep it straight.  Obvious isn't it?   

Well, on my next plane, I guess I can skip internet part.  And hopefully all of the screwing around wasting time part and just get right to it.  So here we are with the FL-706B  bent nicely at 6.3 degrees.

Then it's just a matter of match drilling the bent flange to the inboard rib and spar. 

Then its time to match drill the piano hinge that attaches the flap to the rest of the wing.  That's me in the photo above trying to align the hinge with some precision.  I find the magnifying visor to be more and more helpful... 

Once the match drilling is complete the flap is disassembled, the holes deburred and the ribs and skins were dimpled.

The last item before priming was the bottom of the spar is machine countersunk rather than dimpled.  Three pieces of metal are sandwiched at this point:  the bottom skin, the spar and the hinge.  Machine counter sinking the spar is possible here because it is fairly thick, unlike the skin, and this allows the dimpled skin to nest in the countersunk spar on one side, but leaves the other side of the spar flat so that the hinge does not require dimpling.

Machine counter sinking the bottom of the spar in progress.

Priming.

Then it's reassembly time.  Most of the rivets had to be bucked.  I found that it was easier to reach down inside from on top of the table. For the next flap, I think I'll put a back on the cradle so I can lay it over on its side to avoid the gymnastics.

And finally, the last two rows of rivets attach the spar and finish the flap.  The bottom row, shown above, is squeezed.  The top row had to be bucked.

One flap done.

Step 6.8, Aileron push tubes

I had planned to go straight on to riveting the bottom skins, but it is clear that I will need help to get that started.  The ribs underneath the wing walk area are just too close together for me to get my hands in there to hold the bucking bar.  Fortunately, I know a little brown woman with small arms and hands!  Although she didn't come with the kit, she has been indispensable to its construction.  If you don't already have a helper with small hands, consider adding one to your next tool order.

So I'll have to wait for Carolina to help before I can get the bottom skins on, but in the mean time I'll move on to the push tubes.

Here is the end of the first one just before the rivets went in.  Their construction is quite simple.  Just cut the tube to length and put the threaded aluminum end in place, drill and rivet.  Spacing the holes around the circumference was about the only 'excitement' in the operation.

Luckily, I just happened to remember my old 6th grade teacher from almost forty years ago: "Some day you'll have to evenly space six rivets around the end of an RV7A aileron push tube."  And she went on, "The required spacing will be .524 x D.  Remember this and you'll thank me later."  I can't believe I ever doubted her.  

So I quickly mark out the spacing for six rivets on a piece of paper that is then wrapped around the tube.  The final result is six perfectly spaced holes thanks to Mrs. Sharp, with a little help from Archimedes et al.

Next, the ball end bearings are screwed in and the end-to-end length is adjusted per plan.  I temporarily installed the tube so I could play with the bellcrank and get the photo below.

Those familiar with RV anatomy, may notice that the other smaller push tube is also installed, but I didn't get any photos of its construction.  The smaller tube is made from steel and is fairly heavy.  I'm not sure why Van has elected to use a steel tube here;  Good thing its not very long.  He probably didn't want to open up a larger hole in the rear spar that an aluminum push tube would  necessarily require.

Moving on the the bottom skins now -- really!

Step 6.7, Aileron bellcrank

A bellcrank is a mechanical contrivance the changes the direction of travel in some kind of rigid linkage.  It is analogous to how a pulley can change the direction of travel for a rope.  Bellcranks are often used in aircraft to link the various push tubes that transmit the force from the control stick in the cockpit to the control surfaces on the wings or empennage.  In the RV series of aircraft, each wing contains a bellcrank the turns the linkage connecting the ailerons.

So this is a bellcrank, I realize it would be pretty much impossible to deduce its function from this photo, but I didn't stop to take many photos during the assembly, so here it is.  What I am actually working on though, is a part of the autopilot.  The aileron roll servo control arm.  In the photo above, the aluminum tube with the ball-end bearings allows the roll servo to move the bellcrank.  Which is fascinating of course, but just how does the autopilot control an airplane? 

Perhaps you are thinking that an autopilot is just a computer and so is my Iphone.  And further, you are 85% certain that your phone can't change the direction of an airplane.  True enough.  But an Iphone could make an effective autopilot if it could transmit its computed flying solution via servos to the airplane's control surfaces.  A servo is just a special motor that can convert a number from a computer, into a force that can move the aileron.

In the right wing, the aileron servo is attached by  the control arm to the servo.  The installation manual specifies 5.00" as the nominal length for the adjustable arm.   As you can see, I've clearly blown it as the length measured 5.002".  Oh well, I guess its not too late to start over on the airplane.

And at the lower right is the servo itself.  It looks pretty much like a servo for a RC model airplane, this is just a bit larger and a lot more expensive.

And here is the bellcrank and roll servo assembly.  The push tubes are not yet attached, but the servo control arm is clearly visible in the forefront.  In this view, the push tube from the cockpit would come in from the left and the push tube connecting the aileron would exit at the bottom.

There is still some plumbing to run through the wing to the Pito tube and some wiring for the roll servo, Pito heater, and for the stall warning device.  Then it will be time to close up the wings.

Step 6.6, Pito prep

There are several tasks which must be finished off before closing the wing with the bottom skins.  The Pito mast connects the Pito tube to the underside of the wing. 

What is a Pito tube?  Glad you asked.  The Pito tube in its simplest form is just a small pipe stuck out into the wind.  The air that is forced into the end of the pipe forms a pressure that is proportional to the wind speed.  Or in our frame of reference, the speed of the airplane though the air.  The Pito tube was invented by Henri Pito about two hundred years ago. Unfortunately for old Henri, his invention had to sit on the shelf for nearly a century before the powered airplane was invented.

The Pito mast (blue) is attached to the main spar and to the bottom skin at one end, and of course, the Pito tube (silver) on the other.  To get the mast through the skin, we simply cut away all the aluminum that would prevent us from doing so.  Easy.  I start with four 1/4" holes that I can further enlarge with a uni-bit so that the holes just touch the line.  Then file and sand  to smooth the edge.

And check the fit with the mast.

Good.  Now this Pito tube also happens to be a heated Pito tube.  The purpose of the heat is to discourage ice formation that could block the tube.  The heater has a controller to regulate the temperature and this controller must be mounted near the Pito tube.

And so it is.  Out of the way, yet close to the Pito tube's attachment location and to the bell crank inspection plate.  The last thing to do is to prepare the plumbing that is coming out of the Pito tube.  

The keen observer notes that there are two tubes emerging from the Pito mast.  One is for the Pito function and the other is a second Pito tube whose end meets the air stream at an angle relative to the first.  A computer in the cockpit can measure the difference in pressure between the two tubes and calculate the Angle of Attack of the aircraft.

Those not familiar with aircraft terminology will be relieved to know that the AOA meter in the cockpit does not suggest the angle at which we should meet the enemy.  Instead, the AOA meter indicates the angle of the wing relative to the air stream impinging on it.  All wings will stall at a given angle which is unique to that wing.  Therefore, knowing the angle at which the wing is flying, gives us pilots a leg up on preventing a stall which, surprisingly, many passengers will find objectionable.

To connect to the plumbing which eventually makes its way though the wing and into the cockpit, the the tubes coming out of the Pito need AN fittings.   The AN fittings in this case are just anodized aluminum couplers that fit the aircraft standard of 37 degree flared ends.  So I put the flares on the two ends of the Pito and presto, we're ready to install.

Well not quite ready.  The actual installation will have to wait until the bottom skin is being riveted on as the three rivets along the forward edge of the base plate also go through the skin and spar.

Next, I will battle the aileron bellcranks and the autopilot roll servo installation.