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.

Step 6.5, Landing light install

The main activity for this posting is the landing and taxi light installation.  I have not yet decided what kind of landing and taxi lights to go with so I will just be fitting the attach brackets and lenses.  I will undoubtedly go with an LED technology because of the lower power, but I have yet to be see a landing light kit that I can get really excited about.

While I wait for the ideal LED solution to make itself apparent, I will go ahead and put in the light mounting bracket and make the cut outs in the leading edges of both wings.  I have purchased a lens and mount only kit from Duckworks Aviation.  The kit comes with all of the hardware and instructions necessary for putting a big hole in your recently completed wing and, thankfully, a much smaller hole in your wallet.

To start, use the supplied template to mark the cutout location.  Then using your choice of implements, brutalize your unsuspecting wing.  I started with the cutoff wheel and then finished the rounded corners with the saber saw.  I don't blame you if you are feeling a bit queasy at this point.  You're just experiencing sympathetic nervousness at the whole the idea of it.  Fear not, and read on.  It all works out in the end.  So here's a close up of the leading edge after the first saber saw attack.  

Some might call the previous photo gratuitous aluminum gore, but it is included here because I feel that it is important to be true to my artistic vision for this blog.  Not to mention my desire to increase the readership in the all important 18-35 age demographic. 

Once the rough cut was made, I smoothed the edges with a file and a 1" Scotch-Brite wheel in the die grinder.

Then  match drill the backing plates for the lens.  These reinforcing plates will get nutplates later on  for screws attaching the lens.

After cutting out the leading edge section, the lens itself must be cut to size which is about 3/4" larger than the hole.  The lens comes already bent to the shape of the leading edge which makes trimming the plastic somewhat more adventurous.  I used the band saw for this and had no real difficulty, but I did worry a bit.  But not too much.

I put masking tape on the lens while I was armed with sand paper.  The edges of the plastic are sanded smooth on the belt sander and then finished with a fine cut file, some sandpaper, and a trip past a buffing wheel with plastic compound.  Although I don't have a calibrated baby's bottom handy for comparison, I would say the edges of the plastic would be in the same ballpark.

Once the edges are smooth the mounting holes are match drilled through the wing skin and lens insuring a perfect fit.  Then the skin is dimpled and the holes in the lens are counter sunk.  There is a foam sealing tape included in the kit to apply to the inside edge of the cut out that will seal against the forward edge of the lens.  I put the lens in a plastic bag before installing it.  This will offer a modicum of protection against me during the balance of this aircraft's construction.

And there it is!  The right one anyway.  The left one looks just like it.  The wing is upside down on the bench so you're looking at the bottom side.  And that's all he wrote.

Step 6.4, Fitting the fuel tanks

With the fuel tanks complete, it's time to actually mount them on the wings.  First though, I primed the outside of the rear baffle and the outboard end of the tank since they will be concealed when the tank is mounted.

I didn't realize it then, but the Pro Seal fillet along the edge of the rib was about to become a problem.  I set the tank up on the wing and used some straps to provide some down force on the tank.  I began to see that it wasn't going to line up along the inboard edge where is connects to the W-423 splice plate.

Fortunately my X-ray vision really helps out in situations like these and  I was (eventually) able to figure out that it was the Pro Seal preventing a good fit.  So the tank came back off of the wing for some Pro Seal surgery.

After I removed the fillet, I smoothed down the area with an Scotch-Brite pad on the angle grinder. Then re-prime and I'm back in business.

Now the the tank fits well against the outboard leading edge skin but the gap on the top skin is not as tight as the gap on the bottom skin.  I tried to pull the tank skin down tighter with the straps, but it just wasn't having any of that...  So I decided to forgo my Oshkosh Grand Champion builder's trophy on this plane and just let it be.  There really isn't any discussion in the build manual on what the gap should be.  I just assumed that the amount was zero.  Luckily, the gap is a uniform 1/32" along the length of the tank and is not really likely to draw too much much derision from my fellow builders.  I just can't understand why the gap had to be on the top of the wing and not the bottom.

Once the periphery of the tank is screwed down, the AN3 bolts that attach to the back of the tank via the Z brackets are applied.  This is where I thought the real difficulty would occur.  The bolts have to mate with the nutplates on the Z brackets through holes drilled through the main spar web. The slightest misalignment could prevent the bolt from threading into the Z bracket.  Here are the AN3 bolts and torque wrench ready for action.

The view inside the wing.  Getting ready to tighten these three down.  There are a total of 21 bolts for each tank.  To my surprise and great delight, all of the bolts in both wings slid right in.  It seems oddly unlikely and somehow inappropriate;  Like I was using someone else's luck.  I'm used to things being far more difficult than I expect and this just seemed wrong.

The last Z bracket is accessed from the forward side of the spar.  Note that I am working around that stupid balloon.  I've been so impressed that my tanks are actually holding air that I haven't wanted to disconnect the balloon (nearly a week at the time of this photo).

And that is it for the tank attach.  Up next,  landing and taxing light install.

6.3 Fuel tank leak test

After a very long, tedious, and messy job of finishing the fuel tanks, one may wish to know whether or not they will leak.  I know I did, but I'm just kind of curious in that way.  There's a fuel tank test kit from Van's which I dutifully purchased.  The contents were a Schrader valve, a cap for the fuel line and a page of xeroxed instructions for testing the tank.  Not too bad for six bucks, I guess.  For those not familiar, August Schrader is the guy who invented the valve we all use to keep the air in our tires.

The Schrader valve is placed into the fuel drain fitting and the AN cap over the fuel line, and a balloon over the vent line.  The balloon was not included in the kit.  The instructions were simple: just tape over the fuel cap (I guess the fuel filler caps are not air tight).  Then put the balloon on. Then fill the tank with air.

I attached a short section of aluminum tube to the vent fitting to make a right angle turn providing more room for the balloon.  Also, using a surprising amount of electrical tape, I made the end of the tube larger to better fit the balloon.  Then using safety wire around the neck of the balloon it is fastened to the vent pipe.  

It is at this point that the tanks are ready to fill with air.  The purpose of the balloons is to protect the tanks from over pressurizing and to provide a visual indicator of success!  If the tanks are overfilled, the balloons will just burst before the tanks are damaged.  Which is exactly what happened to my first few balloons.  The air filling the balloons is definitely a lagging indicator because of the small diameter vent line.  Eventually I learned to stop filling the tank when the balloon was only 2/3 full and it will coast on up to full as the pressure in the tank and the balloon equalize.

Once the balloons are full, each rivet and seam is tested with soapy water.  Initially, I had some leakage around the access plate on both tanks.  Tightening the screws did the trick.  All that is left to do is  wait...

Waited long enough?  

Well, I waited 3 days with no discernible loss of pressure.  That is really all I would expect the balloons to last anyway.  If I had been a little smarter, I think I would have inflated a few 'control' balloons as a test of their ability to just hold air.  I'm hoping that if air can't find its way out in 72 hours, then aviation fuel won't either.  Fingers crossed.  

Incidentally, I took the tape off of my fuel filler caps to test their leakage.  They are not leaking, so it seems that the tape was unnecessary.  That must be the deluxe part of my "Deluxe locking fuel cap."

Time to move on.

Step 6.2 Fuel tank assembly

To begin the assembly of the fuel tanks the first thing to do is get the ribs in place.  The area under each rib is thoroughly cleaned with a Scotch Brite pad and then with MEK.  The edges of the ribs get the same treatment.  The is where the Rick Galati fay sealing method begins.  Fay sealing just means that the two surfaces to be joined are both 'buttered' up before assembly such that the sealant is trapped in place by the fasteners (rivets).  What makes the Galati method different than that described in the build manual is that he does not immediately rivet the wet sealant.  Instead, using the Galati method, one clecos the parts together and allows the sealant to cure for some time before riveting.  The cure time is what makes this  method more attractive as the final result is less messy.  Trying to rivet the nose ribs is tough enough without having to do the rivet wrestle with wet sealant oozing everywhere.

Here Carol is putting the final touches on the stiffeners by covering the rivet tails with a blob of Pro Seal.

Each riveted hole in the tank is sealed in three ways.  The rivet head to the wing skin outside surface is sealed before the rivet is driven by placing a blob of sealant in the dimple.  The mating surfaces between the inside skin and the rib or stiffener is sealed when the pieces are brought together. And finally, the edges of each piece (again, rib or stiffener) are sealed by applying a fillet of sealer around the perimeter of the piece.

The ribs are first attached with cleco through the nose so the the rib can be placed without rubbing off the sealant by trying to slide the rib in place. 

In the two photos that follow the process begins.  The top and bottom of each rib has sealant as does the inside of the skin where they meet.

When the inside ribs have been fastened through the leading edge, the skin can be closed around the ribs using the wing fixture to hold the skin closed.

The rear most clecos are put in first in order to pull in the skin tight around the rib.  This takes a bit a force as the skin's lifelong preference has been to do otherwise.

Once the rear most cleco is in place, all of the other holes along the rib will be aligned. The rest of each rib continues without much drama. Occasionally a hole may not line up enough to get the cleco in. A dental pick our other sharp tool will usually to the trick. At this point the dimpled holes really want to just snap into place.

In the photos below, the rib under this line of rivets has already been glued with Pro Seal using the Galati method. The the dimple is then lightly counter sunk to clean the dimple of semi-dried Pro Seal. This allows the rivet to be set at the correct height with the addition of Pro Seal under the head.

Next a dab of Pro Seal is applied to the dimple.

Then the rivet is set in place.  The excess Pro Seal is pushed out at both the top and bottom, insuring a good seal.  Then the rivet is driven normally.

After riveting the skin to the internal ribs there were a still few details I had to complete before putting on the end ribs.  

In the photo above the "anti-hangup"  bracket is installed across the access plate in the tank that has a flop tube fuel pickup.  The idea here is that the flexible nature of the flop tube could somehow allow it to get hung up on the access plate, so this bracket is attached across its diameter.  I curved the center of the bracket to give some additional rigidity to the structure.

Here is another anti hangup detail.  The diagonal brace keeps the flop tube out of trouble on the other side of its bay.  In the right wing, the standard fuel pickup tube is fabricated and attached to the access plate.

This is the bulkhead end of the flop tube just before installation.  Note that the safety wire prevents the flop tube from unscrewing itself from the right angle bulkhead fitting.  If this were to happen, it might be the pilot and passenger that were screwed.

Don't panic, the tail end of the safety wire was neatly trimmed and curled over before the flop tube was installed.

Fuel tank vent line.

This is inside showing the vent line connection to the inboard rib and the doubler plate at the leading edge.   In this photo the tank is upside down.  When this photo was taken, the inside edge fillet had not yet been completed.  Note the difference in the fillet between the inboard rib at right and the other rib at left.

Capacitive fuel level sending plate is installed.

Finally the fuel tank attach flange is riveted in place.

By this point the dogs have had enough and, no doubt, so has my gentle reader.  The little dog takes the big bed.

And Carol was heading out as well.  But not before we had attached the rear baffle.

The next day we riveted the skin to the rear baffle.  All that remained for the left tank was to Pro Seal in the shop heads on the rivets we just drove and pray that it doesn't leak.

That's it for the left tank.  And as for the right tank, you may recall the immortal words of Herman's Hermit's 'Henry the eighth':

"Second verse, same as the first"

So, I'll be kind and spare you the Pro Sealed messy details.