Chapter 9: Wheelpants

Chapter 9: Wheelpants

(Last updated 3/4/11.)

(Wheelpants installation is not addressed in the plans.)


Time to put some pants on this girl.  Wheelpants are a good investment.  They add more speed (7 to 12 knots) than any other single speed modification.  I ordered the pressure recovery wheelpants from Sam James Aircraft.  They are delivered as forward halves ("noses") and aft halves ("tails").  They are held in place by bonnets (fairings) that are built onto the gear legs.  Screws at the axles and axle bolt retaining plates replace the various internal metal brackets used on typical left-half / right-half installations. 


Use the Gary Hertzler instructions.  Sam James will send some installation instructions, too.  But don't use them.  They are intended for Vans RV aircraft and not canards.  Click here to download the canard installation instructions. (Posted with Gary Hertzler's permission.)




1. (This is not in the instructions) Widening the Wheelpants


I knew in advance that I might need to widen the wheelpants.  Other Cozy builders told me so.  But I ordered the wheelpants anyway.


It's my belief that the Sam James wheelpants are sized for the most popular customer -- Vans RV airplanes.  For them, the wheelpants fit around the tires only.  The gear legs stay outside the wheelpants.  The tires are nearly centered within the wheelpants.  The cutouts for the tires are in the traditional location -- out of sight on the bottom face of the wheelpants.


We canardians route the gear legs inside the wheelpants.  If the wheelpants are used as is, (and many have done that), the outer sides will end up being very close to the tires.  The clearances for the tire will cut into the side of the wheelpants, resulting in a scalloped shape.


If you want the tire cutouts to remain hidden on the bottom, then the wheelpants need to be widened.  I drew up some templates and made some measurements.  I estimated the wheelpants would need to be 1.25 inches wider.  So, that's what I did.  I widened the wheelpants by 1.25 inches.  After 3,000 hrs of building experience, I have no fear with hacking something up and making it work.




The photos show the widening process.  I made tick marks on the noses and tails, then split them along their seams.  I hot-wired foam spacers from blue wing foam.  The nose spacers are 1.25 inches wide full length.  The tail spacers start out as 1.25 inches wide and taper to meet the aft ends.  I made sure that the parts lined up with each other (that's what the tick marks were for), then bondoed the parts to the foam.    I sanded the excess foam flush across the parts.  I applied a little bondo to the front tip of the noses just to restore some shape.  I applied 2-BID layups over the foam, extending 1 inch onto each part.  Once that cured, I removed all the foam and bondo from the inside of the noses and tails. I finished by applying a 1-BID layup onto the inside seams.  





2. Square up the Receivers and Flanges. 

(This is not mentioned in the installation instructions.)

When you get the wheelpants, the aft edges of the noses and the flanges of the tails are not cut very precise.  In fact, mine were uneven and very wavy.  It's left to the builder to sand the edges straight and even all the way around.  I grabbed my 12-inch durablock and carefully sanded the edges and flanges until all the waviness was taken out.  A good check is to set them standing up edge down (or flange down) on the work bench.  If they wobble or if you see gaps, then you have more sanding to do. 


3. Find the Centers and Waterlines

(This is the first part of "Step 1" in the instructions.)

I used a different approach to finding the centers.  I set up two carpenter squares onto my workbench as depicted below.  With two squares, it is quick and easy to find the centers on all four sides at the same time.  Beware!  Do not assume the pant halves are symmetrical or perfectly shaped just because they came from a mold.  Little nuances are to be expected.  I rotated the noses and and tails 180 degrees and made the measurements several times to be sure.  I found the centers to be slightly different each time.  So I simply took the average between the measurements.  That was close enough for me.   



I used a similar process to find the waterlines for the noses and tails.  Again, I rotated the noses and tails 180 degrees and made the measurements several times to be sure.  Follow Gary's instructions for locating the tail waterlines.  It should be at the tail's midpoint.  This makes sense to me because the primary goal of pressure recovery wheel pants is for the airfoils to be symmetrical about the centerlines and waterlines.  Don't follow the Sam James instructions.  They give two options are specific to RV taildraggers and RV nose gear planes.



4. Pre-Drill the Cleco Holes

(This is the last part of "Step 1" in the instructions.)

The next step is to mate the noses and tails, align them to each other, then drill and cleco them together.  The cleco holes are drilled through the receivers and into the flanges.  I figured it would be easier to predrill the holes into the receivers now and use them later as pilots for drilling through the tail flanges.  The figure in Step 5 of Gary's instructions shows the approximate locations of these holes.  I deviated from that.  I drilled one hole at the center mark, one hole 4 inches above that, and one hole 4 inches below that.  I also drilled a hole on the top centerline.


5. Align the Wheelpants and Cleco Them Together

(This is the last part of "Step 1" in the instructions.)

I assembled the alignment rig shown below using some cinder blocks, long sections of angle, and two metal yardsticks.  My wheelpants are 9.5 inches wide after widening.  I set the cinder blocks 9.5 inches apart.  In this manner, each wheelpant would be held snug and upright between the cinder blocks.  I made sure that the angles were parallel to one another and that the yardsticks were perpendicular to the angles.  I mated the noses and tails together, then set the first wheelpant into the alignment rig.  I put one small block under the nose and one small block under tail.  Now, it was time to herd cats.  I adjusted the blocks until the watermarks on the tip of the nose and the midpoint of the tail were the same height above the workbench.  (This assures the wheelpant is level fore to aft.)  I adjusted the nose and the tail to the left or to the right until their centerlines were at 4.75 inches, which is equidistant between the two angle sections.  (This assures the nose and tail are in line with each other.)  Now, using duct tape, I carefully applied a length of tape across the top and along each side of the wheelpant.  (Some builders have used dabs of bondo or superglue.)  The goal is to hold the nose and tail perfectly still so that they don't move as you drill the cleco holes into the flanges.  I drilled each hole one at a time and installed a cleco.  I rechecked the alignment before drilling the next hole.  I drilled the hole for the top centerline first, then drilled the center holes on each side, then drilled the others above and below.  Again, it's very important to drill one hole at a time and recheck the alignment before drilling the next hole.  I repeated the alignment and drilling on the second wheelpant.  Voila!  Perfectly-aligned wheelpants!  Cool!





6. Construct the Holding Jig

(This is "Step 2" in the instructions.)

The holding jigs are used later to hold the wheelpants level and parallel to the aircraft's centerline while you fabricate the bonnets (the "gear leg fairings").  The jig shown in the instructions assumes the tire is on the ground.  I elevated my plane off the ground to totally unload the main gear.  So I was able to build the jigs differently.  Notice that I included an adjustment feature on both ends.  Just loosen the screws a little.  Slide the bar up or down to adjust the height of the nose (or tail).  Tighten the screws to lock into place.





7. Position the Airplane 1 to 2 degrees "Nose Up"

(This is part of "Step 3" in the instructions.)

I jacked my plane off the ground, and per the instructions, positioned the fuselage to 1.75 degrees nose up.  I chose 1.75 degrees because it's a little more than half way between 1 and 2. <grin> 


You don't really have to raise the tires off the ground.  But it's really important to unload the main gear strut so that the tires are positioned like they would be when the plane is flying through the air.  Too much weight on the tires will cause the gear to spread, which will cause the tires to point differently than they would point when flying.  This will cause the wheelpants to be misaligned with the oncoming airflow.


8. Install New Tires

(This is not mentioned in the instructions.)

At this point, I put on brand new tires.  I wanted the tires to be at their maximum diameter while I installed the wheelpants.  This ensured the wheelpants will always have enough clearance when it comes time to install new tires again.  Some folks fitted the wheelpants over old, worn-out tires only to discover that the new tires rubbed against the wheelpants. 


9. Make the Cut-Out Templates

(This is part of "Step 3" in the instructions.)

The instructions have you make two templates -- one looking "head on" and one looking from the side.  You use these templates to determine the locations on the wheelpants for the tire cutouts and the cutouts for the gear leg.  You can see my head on template in place in the picture below.  I used foam board from the art department at Walmart.


I placed the template onto my work bench and placed the wheelpant nose standing up over the template.  The instructions list five positioning guidelines -- (1) keep the side of the wheelpant parallel to the side of the tire.

(2) position the axle centerline 1.5 inches forward of the vertical split line.

(3) keep 3/8-inch clearance above the tire.

(4) wheelpants are 1-2 degrees nose down relative to fuselage longerons.  (That is why you put the plane at 1-2 degrees nose up.)

(5) keep wheelpant centerlines parallel to aircraft centerline. 


 I fiddled with several placements until I met the first and third guideline.  (The others come later.)  I traced the wheelpant outline onto the template.  You can see that my wheelpant will be parallel to the tire.  I have 3/8th-inch clearance above the tire.  I have a half inch at the gear leg for the nutplate bracket that gets installed later.  And you can see that the tire cutout will be wholly on the bottom of the wheelpant!  Woohoo!




10. Transfer the Cut-Out Marks onto the Wheelpants

(This is part of "Step 3" in the instructions.)

These pictures show how I transferred the markings for the tire cutout.  I drew reference marks onto the side template.  The lines show "3/8 inches above the tire", the waterline, the location of the axle, and "bottom of wheelpant".  I clamped the template to my work bench.  I set a carpenter's square with its edge 1.5 inches aft of the axle location.  (This is guideline #2.)  I placed the assembled wheelpant over the side template. I fiddled with the positioning until the nose tip was directly over the waterline and the vertical split line was in line with the carpenter's square.  You can't see it in the pictures, but I extended the waterline onto the workbench and I did align the tail's midpoint over the waterline.  In the second picture, I aligned a carpenter's square with the point on the template where the tire outline intersects the bottom of the wheelpant, then drew the line onto the wheelpant.  The two marks shown in the picture give you the fore and aft limits of the tire cut-out envelope.  In the third picture, I have changed to the head-on template.  I placed the nose (and later the tail) standing up over the template.  I placed a carpenter's square at the location on the template where the side of the tire intersects the wheelpant.  Then I drew the lines.  The picture is very dark, but you can see the envelope that contains the tire cut-out.   I repeated the process to find the cut-out envelope for the gear leg.





11. Fit the Wheelpant to the Tire and Gear Leg

(This is part of "Step 3" in the instructions.)

Okay, to me, cutting the openings for the tire and the gear leg strut are the most challenging part of the installation.    I didn't cut out all the material at once.  I cut a little and checked the fit, cut a little and checked the fit, etc, etc, etc.  I kept cutting and checking until the wheelpant orientation met all of the five guidelines mentioned above.


The instructions don't say to do this, but I used foam spacers above the tire and around the inside of the gear leg and brake caliper.  These ensured that I was maintaining clearances as I fitted the wheelpants.  I used the head on template to size the spacers.  I have a 3/8-inch thick foam spacer taped to the top of the tire, a half-inch spacer taped onto the inside of the gear leg, a 2-inch thick spacer taped across the side of the tire, and a 1/2-inch spacer over the brake caliper. 




12. Fabricate the Wheelpant Attachments

(This is "Step 4" in the instructions.)

As mentioned up front, the wheelpants are attached to their gear legs by the bonnets (fairings), outboard attachments, and inboard attachments.  The outboard and inboard attachments are made in this step.  The bonnets (fairings) are fabricated in a future step. 


For the outboard attachment, I am using wheel axle nuts sold by Van's Aircraft.  These are sleeves threaded on one end with a nutplate installed on the other end.  (Photo shamelessly stolen from Bernie Siu.)  They eliminate the need to drill and tap the axles.  I ordered them directly from the Van's Aircraft catalog for about $14 each.  Now, the Vans axle nuts are supposed to replace the Matco axle nuts.  The problem is that the hole in the Matco axle washer is larger than the axle.  The hole is larger because it fits onto a recessed groove that is machined on the end of the Matco axle nut.  The Matco nut holds the washer concentric to the axle.  Without the Matco axle nut, the washer tends to drift off to one side or the other. It ends up rubbing against the wheel casting.  That's a potential for disaster.  So long story short.  I'm using the Matco axle nut and washer to hold the wheel onto the axle.  Then I screwed the Vans nut onto the end of the axle and am using it like a jam nut against the Matco axle nut.  There  are plenty of threads on the axle to do this.  And I get the benefits of both.


For the inboard attachments, I bent up some sheet metal and formed the brackets shown in the second picture.  I drilled out the pilot holes and installed the 1/4 x 28 nutplates after the picture was taken.  While I was at it, I made another set of axle backing plates.  I never liked my first set.  I followed Bernie Siu's lead and made them thicker and circular.




13. Fabricate the Inner Doubler (Shim)

(I'm skipping forward to "Step 6" in the instructions.  Gary's instructions have you construct the outer doubler first.  I decided to switch the order to be Step 6 inboard doubler, Step 7 gear leg fairing, and Step 4 outer doubler.)


The important parts of fabricating the inner shim are determining how thick the shim needs to be, determining where to put it, and then accurately drilling the hole for the AN-4 mounting screw. 


I made sure the 3/8th-inch foam spacer was still taped securely to the top of the tire.  I put the nose of the wheelpant onto the wheel and gear leg.  I used the holding jig to hold the nose straight and level.  I measured the gap between the inner face of the wheelpant and the inner attachment bracket.  Then I marked the wheelpant with the general location of where the screw hole would be.  I cut out a 1-inch disk from 1/4-inch phenolic sheet and sanded one side of the disk a little bit at a time until it was the correct thickness.  (There was a lot of fit-checking!)


I removed the nose from the wheel and placed it onto my work table with the outside of the nose facing down.  I prep-sanded the inside of the wheelpant where the shim would go.  I rounded over the edges on the disk, floxed the disk onto its location on the inside of the wheelpant, then applied 2-BID to hold it in place.


After cure, I drilled the 1/4-inch hole through the shim.  You need one of those pointy things to locate the hole accurately.  I made mine by cutting the threads off an AN-4 bolt, chucking it in my drill, then spinning my drill with the bolt pressed against a grinding wheel.  After it cooled, I screwed the pointy thing into the inner attachment bracket.  I re-assembled both halves of the wheelpant onto the wheel and gear leg.  Using the holding jig, I once again made a gazillion measurements and adjustments until I was 100% positive that the wheelpant was level with ground and was pointing straight forward (parallel to fuselage centerline).


I whacked the inner side of the wheelpant.  Whack!  I disassembled the wheelpant and sure enough, the little pointy thing made a nice little mark on the inner shim.  With the nose placed back outer face down on my work bench, I drilled the hole.  I drilled in two steps.  I used a smaller drill bit and made a pilot hole first.  I placed the nose on the wheel and gear leg just to confirm that my hole through the shim was lining up with the inner attachment bracket.  (It did.)  The pilot hole also prevents a bigger drill bit from wandering off center.  I grabbed my 1/4-inch drill bit and drilled the hole to size.


Here are two pictures of the inner doubler.  It's hard to tell from the close-up photography.  The picture on the left shows the doubler floxed and glassed to the inside of the wheelpant.  The picture on the right shows the how this all looks with the wheelpant nose in place.  From left to right, it's the inner mounting screw, the inside face of the wheelpant, the inner doubler, the inner attachment bracket, axle nut backing plate, and the gear leg. 



14. Fabricate the Bonnet (Gear Leg Fairing)

(This is "Step 7" in the instructions.)


Okay, it's time to go for the gusto.  I used 6 plies on my bonnets instead of the 4 plies recommended by Gary's plans.  These wheelpants are larger than the wheelpants that the plans were intended for originally.  I figured that 2 more plies of BID would be appropriate.


I removed all the micro from the gear leg where the bonnet would be applied.  (From the top of the wheelpant to about 2-3 inches above the wheelpant.)  I prep-sanded the exposed fiberglass.  I made sure the 3/8th-inch foam spacer was still taped securely to the top of the tire.  I assembled both halves of the wheelpant onto the wheel, gear leg, and holding jig. I installed the inboard attachment screw.  I applied duct tape onto the top and inner side of the wheelpant as a mold release.  I used play-doh to form nice radiuses between the wheelpant and the gear leg.  I had an idea as to the size and shape of bonnet that I wanted.  I drew an outline onto the duct tape, then drew another outline one inch outside of first one.  Better to have a bonnet that is too big that can be trimmed down later than to have one too small and have to enlarge it.  I made sure the fuselage was still 1.7 degree nose up.  I made another gazillion measurements and adjustments until I was 100% positive that the wheelpant was level with ground and was pointing straight forward (parallel to fuselage centerline). 


Then I set about glassing the bonnet.


(Laughing) There's really no easy way to go about applying the layups.  I tried using patterns and transferring the layups from the work table to the wheelpant.  That didn't work.  Long story short, I ended up free-handing the layups -- i.e., applying  1 ply of dry BID to one half of the bonnet and wetting out the BID in place.  Attacking the outboard side first, I cut out a good sized piece of BID that I thought would cover the outer half of bonnet outline plus the outer side of the gear leg, plus one inch of overlap onto the inboard half.  I mixed some epoxy and wetted the gear leg.   I placed the BID over the general area and started wetting out the BID with my brush.  I started at the top of the gear leg, proceeding down the gear and and wetting outward to the bonnet outline.  When I got to the trailing edge of the gear leg, I simply scissor-trimmed along the trailing edge and let the BID fold down onto the inboard side of the bonnet outline.  I wet out 1 inch of "overlap" and trimmed off the excess.  When I got to the leading edge, I found that the BID would stop conforming as I tried to wrap it around the leading edge.  I forced the wrap-around as much as I could, then trimmed off the excess.  I was able to trim the BID to leave a one-inch overlap on most of the leading edge and on the wheelpant forward of the leading edge.  I trimmed the excess BID that fell outside the bonnet outline.


I followed this same approach for the inboard half.  For each successive layup, outboard and inboard, I started 1/8th-inch lower on the gear leg to avoid a huge bump.


After cure, I cut and trimmed the bonnet to shape.  The big questions are, to what size? To what shape?  Gary's instructions don't offer any guidance.  Builder responses ranged from one inch off the gear leg to 3 inches off the gear leg.  I went with my best guess.  Wider is better than narrow since this fairing IS a STRUCTURAL item.  But wider doesn't look sleek and sexy.  My fairing is 3 inches forward of the gear leg, 2.5 inches at the split line, and tapering to 4.5 inches aft of the gear leg.  I basically made the fairing look like the wheelpant.


I made a template for this and traced the outline onto the bonnet.  I decided where to locate the mounting screws.  I can't say if I got this correct either since there is no guidance as to how many screws.  So I just guessed.  All screws were placed 3/4-inch inside the bonnet outline.  I drilled cleco holes through the bonnet and the wheelpant halves.  You must drill these holes before removing the wheelpant halves!!


I then struggled for a few minutes to break the wheelpant halves free from the bonnet.  I cut the bonnet along the outline and sanded the edges nice and smooth.  I reassembled the wheelpants with clecos.


Here are a few pictures!!




15. Fabricate the Outer Doubler (Shim)

(I'm backing up to Step 4 in Gary's instructions to construct the outer doubler.)


Just as with the inner shim, the important parts of fabricating the outer shim are determining how thick the shim needs to be, determining where to put it, and then accurately drilling the hole for the AN-4 mounting screw. 


I made another pointy thing, albiet a lot longer, so that the pointy thing could almost touch the outer face of the wheelpant.  Again, you use the pointy thing to accurately locate the hole for the outer mounting screw.  I screwed the pointy thing into the nutplate on the end of the Vans axle nut.  I re-installed the nose to the bonnet using clecos and I installed the inboard mounting screw.  (You no longer need the holding jig at this point.  You now have the bonnet and inboard mounting screw holding the nose straight and level.)  I gently whacked the outboard face so that the pointy thing could leave its mark.  Whack!


I removed the nose and removed the pointy thingy.  I drilled the hole for the AN-4 outer mounting screw and prep-sanded the inside surface around the hole.   Unlike the inner doubler, the instruction have you drill the hole now.  You use the hole later to help keep the outer shim aligned as it cures in place.  I re-installed the nose onto the wheel.  I measured the gap between the outer face of the wheelpant and the end of the Vans axle nut.  I made more disks, this time to a wider diameter of  being 1.25 inches.  It took two disks and a washer to fill the gap.  I mixed up a small batch of 5-minute glue with flox added in.  I quickly buttered up each disk and washer with the 5-minute glue.  Without delay, I stacked the disks and washer onto a spare AN-4 bolt.  The bolt keeps the holes in the disks aligned as the glue cures.  I mixed another small batch of 5-minute glue and flox.  At the wheelpant, I buttered up the side of the disk stack that would attach to the wheelpant.  I inserted the stack between the wheelpant and the Vans axle nut.  I threaded the AN-4 outer mounting screw through the wheelpant, through the disk stack, and into the Vans axle nut.  I waited until the glue hardened.  


After the glue hardened, I removed the nose from the wheel and set it onto my work bench.  I applied a flox radius around the stack and covered it with 1 BID.  After cure, I redrilled the hole for the AN-4 mounting screw.


Here are two pictures of the outer doubler.  The picture on the left shows the doubler floxed and glassed (1-BID) to the inside of the wheelpant.  The picture on the right shows the how this all looks with the wheelpant nose in place.  From right to left, it's the outer mounting screw, the outside face of the wheelpant, the outer doubler, the Vans axle nut, and the Matco axle nut and washer.  Please note that for right now, I used a really long AN-4 bolt and spacers as my outer mounting screw.  I need to order screws that are the correct length. 




Second Wheelpant

As predicted, it took less than half the time to install the second wheelpant.  I checked the alignment between the wheelpants.  Theoretically, the distance from nose center to nose center and the distance from tail center to tail center are supposed to be the same.  Mine are pretty darned close, off by a scant 0.05 inches.  I'm well-pleased with that!


Future Steps

Well, that's the progress as of February 14, 2011.  All that's left to do is install the nutplates and mounting screws, then do the cosmetic filling and sanding.  I'm very happy to have the wheelpants installed and done.  The wheelpants installation took a lot longer than I excepted. 


Now it's time to give this girl some horsepower and a propeller.  You'll find me in Chapter 23 soonest.

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