9.5.1 Sump Tank Hardpoints

This entry is part 6 of 18 in the series 09 - Fuel System

According to the “flow chart” that lists the order of tasks, one of the things I have to do is build the fuel sump. In most planes I’ve flown, the fuel is fed to the engine from the two fuel tanks (one in each wing) by using a fuel selector. Low-wing airplanes require you to select either the “right” or “left” fuel tank. With these planes, you have remember to switch tanks every 30 minutes or so in order to keep the plane in balance. On the Velocity, a “sump” is used. This is a small (about 3 to 5 gallon) tank that is mounted at the bottom of the firewall. Fuel drains from both tanks into the sump which then feeds into the engine. Brilliant!

The fuel sump is pre-molded but is the front is not attached. There are a couple hardpoints for fuel lines that have to be bonded on.  So one of the tasks today is to bond these hardpoints on. This similar to the hardpoints on the floor for the seats. So they are epoxied into place and they covered with 2 layers of BID.

9.5.1 Sump Tank Hardpoints

This entry is part 7 of 18 in the series 09 - Fuel System

I’ve been working on the sump quite a bit. After getting the hardpoints installed, I’ve been sanding the inside and spreading epoxy to make a smooth surface. The inside was incredibly coarse. I probably put 3-4 coats of epoxy down (and sanding in between). But just as I think I’m ready to drill and tap for the fuel lines, I discover two “minor” problems.

The engine I’m installing would rather have 1/2″ fuel lines instead of 3/8″. The problem is the the 1/2″ fittings will interfere with the main gear when it retracts. so that means I’ll need to install two new hardpoints that won’t be in the way.

An even bigger problem is that the lower engine mounts are going to be in the same place as the tank.

Inside of firewall. The two squares with the “X” is where an engine mount bolt will be.
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Same view but with the tank in place.

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How do you get to the bolts if the tank is in the way? Or how do you mount the tank if the bolts are in the way?

A call in to the factory reveals the best way to work around the issue of where the fuel lines go in.

Instead of mounting the two feed lines on the top front of the tank, they’ll have to go on the side. Which means adding two more hardpoints for the fuel lines.

As for the engine mount bolt issue, a somewhat more… Neanderthal solution is called for… Check it out:


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Elegant, huh?

Another view.

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Won’t hold much fuel like that.

New corner installed.

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The new corners are covered with 2x BID inside and out.

Now here’s where not having built one of these airplanes before can bite you.  I’m working on this because the “Flow Chart” has it listed as task 40. Well, I’ve done 1-39 so I’m doing this. But what I learned much later is that it doesn’t have to be done now. And if I had waited, I could have made smaller, rounded clearance corners that would match up to the engine mount bolts exactly and allowed more fuel. As it is, it’s a little tight where one of the enging mount bolts is.

9.5.2 Install Sump Tank Cover

This entry is part 8 of 18 in the series 09 - Fuel System

Previously, I had chopped off the bottom corners and put on some flat pieces on fiberglass to allow for the engine mounting bolts. Then I learned that the fuel fittings that connect the main tanks to the sump will interfere with the landing gear. The fix is to move them to the side of the tank. This required installing additional hardpoints for the relocated fuel fittings.

Here is the inside of the sump tank with the hardpoints for the fuel lines installed, drilled and tapped.

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On the very bottom of the tank is where the drain line goes. This is used to check for water or contaminants. On the bottom face is where the engine supply line connects. Top right and left is for the supply lines that come from the strake (wing) tanks. And on the top center is the vent.  You can still see the two top left and right face hardpoints that I put in prior to learning that the larger fittings wouldn’t fit.

Sealing the sump tank is not required by the manual. Current aviation fuel will not breakdown the epoxy. However, it’s possible that future aviation fuels may not be… compatible with the epoxy. The reason behind not requiring the sump tank to be sealed is you could always build a new sump tank should aviation fuels change. I’d rather not build another sump tank so I decided to seal this one. I also wanted some practice with the sealing epoxy. It’s called Jeffco and it’s extremely touchy. It has a rather short pot life and can get really hot if you mix too much at once.

First I cut the back of the sump tank to the correct shape and size. Then I sanded both the tank and back.

Back of sump tank ready for Jeffco.

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I wasn’t sure how much to mix so I guessed. I guessed wrong. With Jeffco, when starts to kick, it gets hot. the hotter it gets, the faster it kicks. The faster it kicks, the hotter it gets. The more you’ve got…

Melted the cup!

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I did get the first coat on before it started to melt the cup.

Then once it got tacky, I mixed up a much smaller batch and put on the second coat.

Back cover of sealed sump tank.

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Inside of sealed sump tank.

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After the Jeffco cured, I mixed up another batch and mixed in some cabo to make a think cake frosting like consistency. I put this mixture in a ziplock bag and clipped the corner off. Then I squeezed it out on the perimeter on the back. Then the sump tank goes on until it cures.

Once it cures, I covered the seam with two layers of bid. Then I made the mounting tabs. At this point, the sump tank is done. I still need to test it plugging the openings and submerge it in water to make sure there are no leaks.

9.0 Strakes

This entry is part 9 of 18 in the series 09 - Fuel System

It’s time to build the strakes. It’s kind of hard to describe the strakes so I’ll use visual aids.

Here’s the top view of the fuselage.
At the rear is the center spar extending out the sides. The main wings will attach to this spar.
Here it is with the wings attached.

On airplanes with aluminum wings, they’re hollow and hold fuel. Velocity wings are a solid foam core and can’t be used to hold fuel. So an interface between the leading edge of the wing and the fuselage is created that will hold the fuel.


The shaded area are called “strakes” and will hold (hopefully) about 40-45 gallons of fuel on each side. But here’s the catch: To properly build the strakes, the wings need to be mounted. But I don’t have a big enough shop. With both wings on, the shop is about 12 feet too short.

Here’s a picture with one wing on.
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I could build the strakes one side at a time but that creates some problems.

  1. It takes twice as long.
  2. Even with one wing on, it’s a pain moving around to do the work.
  3. With one wing on, the center spar is unevenly torqued. It’s possible, with supports, to relieve this stress. But it’s a bit of work to get it just right. And if it’s not, the plane will not fly true without some adjustments.
  4. Building strakes is somewhat challenging. At least the first time. I looked at going down to the factory and helping out building strakes but they didn’t have any planes down there that were at that point of construction.

So I fussed about whether to build a temporary extension so that I could fit both wings on, do it one side at a time or send it out to a facility where I could do both sides.

Because this will determine how true the plane flies, I decided that caution rather than blindly fumbling in was the best course of action.

Malcolm at Hangar 18 was busy with two builds so he wasn’t available. I met Tom Wright at Oshkosh last year. Tom owns Advanced Composites Technologies. In addition to having build numerous Velocities, he’s also builds UAV’s. Malcolm said that Tom did good work so I made arrangements to ship the plane to Tom’s shop in Friedens, PA.  Then I’ll make the short flight out in the Cessna to work under the guidance of someone who has built a few strakes so that it’s done correctly.

Getting the plane on the trailer

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Loaded up with the wings underneath, strapped down and placarded.

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The battery on the truck was dead and I couldn’t get any of our cars in position to jump.  So I pulled the 1965 International Cub Cadet out.

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Worked like a charm.

9.2.3 Lower Strake Alignment

This entry is part 10 of 18 in the series 09 - Fuel System

Once at the shop in Friedens, PA the wings (both) were attached and the plane was raised up and leveled.

Left side

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Right side

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Then it’s time to determine how far forward the strakes will join to the fuselage. This is one of the factors that determine how much fuel you’ll be able to hold. The farther forward, the more fuel. A limiting factor is the door latching mechanism. This assumes that the door is opened up where the strakes are (which they will be). Some builders have modified the door latch they can put the strakes even farther forward. I decided to skip that modification. I am hoping for 44 gallons of fuel per side. That will give me about 1,000 mile range (5 hours). Which is much longer than I can usually sit in an airplane.

One of the critical factors is getting both side IDENTICALLY positioned.

Here are the markings on the pilot (left) side.

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And on the co-pilot (right) side.

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The lower strake skins are temporary put into position and the location at the center spar is marked.

The strake skins  are manufactured at the factory. They’re made of 1/4″ sheet of Divinycel foam with fiberglass cloth on the top and bottom. Where the strake skins attach to the center spar, the foam has to be removed and the foam is beveled and then covered with a layer of fiberglass.

Here’s a strake skin with the foam and inside layer of glass removed.

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The inside of the strakes had to be sanded, filled and recovered as they were not smooth enough.

Inside of one of the strakes after sanding.     

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This is the bottom left strake skin after sanding with a layer of BID and peelply. At the bottom of the picture you can see where the foam was removed.

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The top and bottom of the center spar (which is the structural component that the wings mount to) has to be prepared.

Left side of the center spar being prepped.

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Right side of the center spar finished and ready for the strake.

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The left side lower strake skin mounted in position.

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Right side lower strake skin.

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9.3.1 Fitting Wheel Well

This entry is part 11 of 18 in the series 09 - Fuel System

The bottom skin of the strakes are now permanently attached to the spar, fuselage and door.

The cutout for the landing gear has to be made. This allows the gear to raise into the strakes. Basically, the gear leg is raised until it hits the strake. Mark where it hits and trim away part of the strake. The gear will now raise a little further. Keep repeating until the tire hits. Then start cutting out larger pieces until the whole leg and wheel retract into the strake.

Looking under the right strake at the fuselage.

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Towards the left edge of the picture, you can see the opening on the side of the fuselage for the main gear leg. You can also (barely) see part of the bottom of the strake where an opening has been made for the gear leg.

Right side strake.

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Gear in the up position. The gear leg cover and wheel cover is positioned.

Two things to keep in mind here. One is that fixed gear Velocities can hold a bunch more fuel. And second, I am going to pick up a couple gallons per side with a neat trick. Notice how the wheel is at an angle when it’s fully retracted? Rather than seal that whole area off, I’m going to put an angled “cap” on it and use that space. Should yield about 1.5 to 2 gallons.

Making sure there’s sufficient space all around.

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Left side wheel well and gear leg well epoxied in place. Looking from the top (rear).

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Left side wheel well and gear leg well epoxied in place. Looking from the front.

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Right side (from front). Landing gear is held up with a piece of wood.

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Something else to point out here. Notice on the right side of the picture you can see a rectangular cutout in the side of the fuselage. This is an optional feature that almost every builder does. This is next to where the rear seat is located about elbow height. Basically the side of the fuselage will be moved out creating a space. This space can be used for open storage (no door), closed storage (door) or just a big ol’ armrest area. As an open area or open storage it makes a big difference how big the cabin “feels”.

How far you move this out depends on what you want. If you want a big area for storage, you move it out about 12″. But you lose fuel capacity. Don’t make the cutout and you pickup about 5 – 7 gallons. I decided to split the difference. Mine will be moved out about 5 1/2″. I’ll lose a little fuel but gain some space. I toyed with the idea of making it angle in at the top but decided that would just add work with not much gain.

The location of the wheel and gear leg well get transferred to the top strake skin. Also the fuel cap is installed.

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I’m doing two things different here. Number one: I’m not using the fuel cap from the factory. I looked at them but didn’t like the design and construction. I found a company that makes the fuel caps for race cars and bought a couple of those.

9.3.2 Baffles and Bulkheads

This entry is part 12 of 18 in the series 09 - Fuel System

As with traditional wings, the skins require an internal structure. In the strakes, this is accomplished with bulkheads that also serve as baffles to prevent fuel from rapidly moving inside the strake. The bulkheads are 1/4″ Dyvinicel foam with a layer of BID on each side. They are then cut to fit.

Glassing in the wheel wells and gear leg wells to the lower strake.

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Holding the wells down with a heavy beam while everything sets up. Also starting to install some of the ribs. These vertical pieces are to strengthen the area and to prevent the fuel from sloshing around.

Closeup of the inside-rear area.

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Right side from the front. After everything sets up.

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Additional ribs installed.

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Notice in the corners where the ribs/baffles meet that there are small cutouts. This allows the fuel to move. The cutouts are called “mouse holes”. :-)

Tops of the wheel wells being prepared.

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Inside of the plane looking back at the right side. Fittings for the fuel supply, vent and return. On the left side of the picture you can see the storage area. The fuel line hardpoints are 1/8″ aluminum which are drilled and tapped to receive the fittings. They are then attached with epoxy and covered with 2x BID.

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Right side, looking back from the front. The temporary trial fitting of the top skin.

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9.4.2 Fitting Upper Strake

This entry is part 13 of 18 in the series 09 - Fuel System

Because there’s no way to see inside the strakes with the upper skin on, the height of the ribs/baffles is an approximation. To get a perfect fit, duct tape (is there anything it won’t do?) is applied to the inside of the upper skin where it will contact the ribs. A bead of epoxy/Cabosil is run on the tops of all the ribs. The upper skin is then lowered in to position and left overnight. The next morning, the upper skin is carefully removed.

This is the result

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After a little clean up of the spots where oozed too much, it’s time for Jeffco. As you may remember from the sump tank, Jeffco is tricky stuff! First off, it’s not needed with the type of fuel that is presently used for piston engine airplanes (100 octane, Low Lead). But if the fuel is ever changed, it could react or eat through the epoxy. So just about all builders coat their tanks with Jeffco. A very hard, resistant epoxy. The problem is the stuff has a tendency to exotherm (heat up) rapidly. When it does, the mixture will set. So it has to be mixed in small batches and applied. Oh yeah, and you have to keep a wet edge. If the area that you’re working next to sets up, you have to let it harden and then sand it. So this is definitely a good multi-person job.

Shiny fuel tanks!

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The last picture shows the wheel well. Remember how I said that I was going to use the the angled area for additional fuel? That area needs a bit more work. Openings so the fuel can flow in and out have to be cut and then the Jeffco can be applied.

Wheel well jeffco’d:

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Here’s the outside edge of the left strake looking back from the front. On the right, you can see two of the bolts that hold the wing in place. The hole in the bottom provides access to the bolts so the wing can be removed.

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9.4.3 Upper Strake Installation

This entry is part 14 of 18 in the series 09 - Fuel System

Inside of the plane looking at the right side. The rectangular opening is the area that extends into the strake. The plans say make it 11 inches deep but mine is only about 5 inches.

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 Another modification. As you have seen, when the landing gear is retracted, the wheel is at an angle. To allow it to retract further, a “divot” is cut into the inside of the top skin of the strake. Then it is covered with two layers of BID.

It’s kind of hard to see it in this picture, but the lighter colored area is the divot.

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The inside of the top skins coated with Jeffco . You can see the area of the wheel well that won’t be holding any fuel and the divot is a little easier to see.

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The inside of the top skin has to be sanded where it will adhere to the bottom strake and ribs.

Here’s the inside of the top skin after sanding.

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Finally it’s time to bond the top strake skin on. An epoxy/cabosil mix is applied to the top of the outside edge and ribs.

Strakes ready for the top.

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Once the top is on, everything is weighted down to hold it in place.

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After everything has cured, it’s time to test for leaks. Since how well the fuel caps seal is an unknown, they’re covered with plastic and taped.

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Many builders attach a pressure gauge to one of the fittings and then pump some air in the tank and monitor the pressure overnight. Tom has a slightly different approach.

He uses an altimeter which is connected to a fuel fitting. The a vacuum pump is used to create negative pressure in the tank until the altimeter reads 1,000 feet. Then everything is left overnight. If there are no leaks, the next morning the altimeter will still read 1,000 feet. If it reads zero, then there’s a leak.

Fuel tank test underway.

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Everything is done and now the airplane comes home!

A word or two on the idea of getting outside help.  As I mentioned before, the strakes become a structural part of the airframe.  It’s what keeps the center spar from twisting.  As such, doing it right is absolutely critical. In addition, to that, even after doing everything just right, there can be leaks.  There are “tricks” that are used to find and fix these leaks.  And finally, manpower.  I am building 100% solo.  I have zero help. As a result, I have had to come up with some inventive solutions building so far. But with strakes, there are a couple of situations where quite literally, the more people the better.  I just don’t have access to that resource. So those are just three of the things that resulted in me getting some outside help.

At the end of all this, it took a little less than 3 months start-to-finish.  It would have been less had I been able to get to PA more often. At the end of the process, I’m very happy with the results.  But even with the very experienced help of Tom Wright, there are a few things that if I had to do it again, I would have done differently. For example, I would have 1) Slanted the baggage opening of the strake cutout in at the top. This would have resulted in more fuel capacity without a loss of much usable space. 2) Really lowered the gear leg tunnel to maximize the fuel capacity. 3) Made the fuel caps flush and moved them inboard a bit. And there probably more. I can’t imagine how long this would have taken had I tried to do it solo.

Thanks for the help Tom!

9.5.5 Sump Tank Installation

This entry is part 15 of 18 in the series 09 - Fuel System

When I went to mount the sump tank, I had another surprise. This one was somewhat self inflicted. When I created the mounting tabs on the sump tank (according to the manual), I made them flush with the back of the tank.

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The problem is that the surface where the tank mounts is not flat. Part of the transverse bulkhead layup extends out to where the tabs are. In this picture one edge of the sump tank is just to the right of the center of the picture. I could have left the original tabs on, but then the tank would be sticking out about a quarter inch from the firewall. So I ground off the old tabs.
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And made some offset tabs. So I got my handy sheet of “Layup glass” (A Hangar 18 term for a big sheet of glass that you wax up) and made some risers for the mounting pads.
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Then I put the glass and epoxy down and let it cure.
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Finished sump tank with offset mounting tabs
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To mount it you’re supposed to drill through the firewall and use a four bolts and nuts. But then you need two people to loosen or tighten the bolts. So I used a Hangar 18 trick. After I drilled the holes, I enlarged them on the firewall side so the head of the bolt would be lower than the surface. Then I put the bolts in and covered the heads with epoxy/micro.
Firewall side showing two countersunk bolts with epoxy.
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Inside left view with the two bolts.
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Sump tank installed and bolted in position.
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