NACA Inlets

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Custom NACA inlets by Andy Millin

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Rich Guerra had made his the right shape and I asked him about sizing. His were noticeably smaller than the factory inlets. Rich’s are 9” wide and the factory inlets are 11.5”. Rich said his worked OK, but thought that a little bigger might be better. After hearing this from Rich, I decided to split the difference. Mine will be 10.25” wide. Picture 1

The engine cooling ducts have been cut out and the top of the fuselage is sitting on the bottom. The green glass you see at the back of the ducts is the flange from the firewall. As it turned out, I rechecked the alignment between the top and bottom fuselage pieces and the top was sitting too far forward. After this picture was taken I wound up moving the top back another 3/8”. This exposed the entire firewall flange. Hmmm….

This puts the back edge of the duct right at the forward face of the firewall. Not where I would like to have it. I understand why Velocity would have placed it there. They had no intention of building the lip on the back of the inlet. I do, and some room to create it would have been nice. If I had it to do over again I would move the inlets about 1/2” to 3/4” forward to make room for the lip. Now, I’ll just find a way to make it work. Picture 2

Using the inlets as I guide, I was able to cut the firewall openings. If you look at the larger version of this picture you’ll see that the flange was completely cut away to make room for the lip. Picture 3

I had cut the engine cooling NACA inlets some time ago. I thought I would be able to complete them before I put the top on. After cutting them I realized it would be tricky to get them to line up perfectly to the firewall openings. I left it, wisely, for later.

Now it is "later"! I want to part the cowl and get it out of the way. I plan on making the correct lip for the inlets, and that may require me to go further back into the cowl. I don’t think it is any real problem, but it means that I should finish the NACA inlets before I part the cowl. This is cool, because I know NACAs really well now and I can’t wait to see what they look like when they are done.

The inlets are formed by dropping down the outer skin of the fuselage to make the floor. The skin of the cutout is not long enough to reach back to the firewall. I made some extensions and am bonding them in place. The clecos are holding them until they cure. Picture 4

I took this picture because I thought the inlets looked cool. Picture 5

Once the extensions were bonded on it was time to attach the floors to the firewall. Picture 6

I have microglassed the extensions to the hole in the firewall. Inside there’s a radius of microglass and three plies of bid tape going from the extension down onto the firewall.

I made some sheet stock for the inlet sides a long time ago. I made a paper template of the sides and used it to cut the sheet stock to size.

In this picture I am bonding the sides in place. The dark area along the bottom is the smooshy microglass showing through form the inside.

You can also see that I have put foam over the floor extensions and covered it with bid. The bottom of the inlets is now flat. Picture 7

The view from the top. I cut these plugs to hold the sides in place while they cure. They did a really nice job. Picture 8

I'm about to put foam on the sides of the inlets. I took this shot so you could see the extra bid reinforcing tapes I put in the back. The sides are now well bonded to the firewall and the ceiling. Picture 9

In the near foreground is the template I made to cut the sides. In the middle is the template I used to cut the plugs to hold the sides.

Using both templates, I was able to cut foam strips that are nicely contoured to the sides of the inlets. Picture 10

The foam sides get bonded in with micro slurry. One of the foam pieces on the pilot side kept trying to slip down a bit. I braced it for the cure. Picture 11

Once the foam was cured on I took a little extra time to shape and contour. I wanted a nice smooth transition from bottom to top.

I laid up two plies of bid with even more reinforcement at the firewall. I was thrilled when I pulled off the peel ply.

Now to get started on the lip. Picture 12


I set up a jig behind the inlets. I made a sanding block/template to cut the lip to the correct shape. I had done the preliminary sanding on the pilot side inlet lip and was just about done with the cp side. I was getting ready to start adding foam to the lip so I could get the shape and glass it. As the back of the inlet was sanded away the block slowly came into place showing me where the lip would finally reside.

Something wasn’t right. The lip looked like it was too close to the floor of the inlet. The opening was too small. I kept sanding. “It can’t be too small, I measured it…” I looked again and it was still looking too small. I stopped sanding and got out the calipers and took a measurement.

The inlet was going to be 1.75 inches deep. It needs to be 2.3. The more I measured the more it kept coming out wrong.

Ok, it’s wrong. I screwed up. How bad is it. A quick run on the calculator says the 5/8” difference will be about 20% of the opening size. “Can I make do with 20% less cooling to the engine? No… hell no… but … the inlets are done … but it’s wrong … but they’re done … but it’s wrong.” I stopped working and went into the house.

I came to the decision pretty quickly that I would need to tear out the inlets, re-cut the firewall opening and remake the inlets.

I was caught in this limbo. I don’t want to take the two steps back and I don’t want to accept the mistake. I spent time thinking on it trying to “clever” my way out. I couldn’t think of a way to fix it that would leave the ducts strong and looking good.

It took about five days for me to get into the shop and start tearing out the installation. It was painful. The lay-ups and foam work looked great. The only problem was it was wrong. I had to move past it and get to the demolition. The faster I get this done the sooner I’ll be moving forward. Picture 13

At the back of the inlets is the aluminum jig. I used it as a guide to sand the lip into the back of the inlet.

I have had these pictures developed for a while now. It took a bit for me to post them. This is embarrassing and just plain depressing to look at. Last night I started re-construction and I’m feeling a bit better; enough to put these up. Picture 14

Back to the inlets…

I have bonded new extensions on and bonded the floors to the holes in the firewall. The floors also received bid tape further attaching them to the firewall.

In this picture I have bonded new foam to the single skin at the back of the inlets. I just took the jigging off about a minute ago. I have to clean up a little micro ooze and sand the foam flush with the forward part of the inlet floor.

If you look closely at the forward parts of the inlets you will see some light stripes running fore and aft. These are glass strands that were pulled out as I was removing those really nice lay-ups from the first try. I will need to re-cover the bottoms during the rebuild. Picture 14

While working on the inlets I have some other lay-ups that should be done.

On the engine side of the firewall I put a fillet and two plies of bid joining the top of the fuselage to the firewall. I got a fresh supply of EZ-84 hardener at OSH and the epoxy is back to a yellowish green.

In the center-top of the firewall I laid up two plies of triax for the engine mount. Picture 15

Meanwhile, on the other side of the firewall...

You can see that I have covered the underside of the inlets with bid. I extended two plies almost the whole length and put another two plies just up past the foam.

On the sides and center I'm finishing off the engine mount reinforcements. I thought I might be different this time and show you a lay-up as I'm pulling off the peel-ply.

The center pad is four plies of triax. The side layups are two plies of triax over the hardpoint coming forward horizontally and two plies of triax over the hardpoint going up at a 45 degree angle. Notice that I had to notch the glass around the windows. Picture 16

I have already bonded the walls into the pilot side inlet. If you look closely you can see that the pilot side walls have not been trimmed flush with the fuselage yet.

The CP side walls are trimmed, sanded and are ready to be bonded. Picture 17

The walls are in and are trimmed flush with the fuselage. Picture 18

The view from the inside: I have trimmed the walls flush with the bottom of the inlets. I have sanded the sides; prepping them for the blue foam to be bonded on next.

In the very front of the inlet I had removed the inner skin and the foam. I wanted it to be flexible as I am trying to make the ramp entrance a smoother transition this time. Something needs to be better about the installation this time.

The front of the inlets was a bit of a pain. Because I had already bonded the back end to the firewall there was no play in the floor of the inlet. It made it nearly impossible to get the inlet opening to transition the way I wanted it to. In the end I put some microglass in there and will do a little sanding to get the right shape.

I should have set the transitions before I bonded the aft end to the firewall. Picture 19

The new foam sides are cut and ready to be bonded in. Picture 20

Finally, back to where I was before. The foam sides are on and are covered with two plies of bid.

Now I can get back to making the lip. Picture 21

As I mentioned earlier, the factory inlets are designed to end right at the firewall. The NACA lip is an airfoil shape that will protrude from the firewall by about ¾” and will extend behind it for about 1 ½”. The glass here is only one ply thick. I will need to add some material to make the lip out of.

I used a cardboard template to match the curvature of the fuselage and cut some foam 10 ¼” wide and 1 ¼” deep. If you look close you can see the channel I dug in the top of the pieces to accept the small piece of firewall. I’m about ready to bond these in with a wet micro slurry. Picture 22

The foam blanks are in place. When they are cured I can start sanding them to the proper shape. Picture 23

Using my aircraft grade lip sanding jig I started taking the foam down. I was patient and the pilot lip sand down perfectly. I have to admit that I’m feeling pretty good. The way the foam lip looks, I must be a genius. Picture 24

I changed the sand paper in the jig and started working on the CP side. The lip was really coming in nice. Then the sand paper inside my jig started to slide. It pulled away from the leading edge. The result was the leading edge of the lip was gouged out in two swipes of the sanding block.

I was a little surprised. I remember thinking that I should be about ready to blow my stack with these inlets. Instead, it was what it was and I should just take it in stride. I made a simple dam and put some pour foam across the leading edge. Ten minutes later I had it trimmed to size and was back to sanding. The CP lip had a great contour to it but because of the blue foam/tan pour form mix, it wasn’t as pretty at the P side. But … WHO CARES!!!! It is going to get covered with bid here pretty quickly.

I decided to show you the finished pilot side lip just because it looks better. :) Picture 25

I covered the lips with four plies of bid. They are leading edges and will have to take rain and bugs. I staggered the lay-ups by ½” so I didn’t have a four ply drop off on the top. It will make filling and sanding easier later.

The bottoms didn’t want to stay up. After I covered the lay-up with peel ply and plastic wrap I slid some foam inside the inlets. It looks like it should cure just the way I want it. Picture 26

I know, I should have taken a picture of the lip after I removed the peel ply. I'll try to remember to get one later.

This is the back side of the inlet. I had made the walls and floor extra long so they protruded through the firewall. This was to accommodate the lip coming past the firewall. I was going to wait to finish dressing this up until after the cowl was parted. But … I can’t part the cowl with all that stuff in the way. It needs to be done first. I cut it all flush; square with the inlet floor. The back of the lip extends 1 ¼” past the firewall. The floor of the inlet extends just an inch past the firewall.

In this picture I have just finished dressing the foam. I’m about to apply one ply of fine bid to close it out. Picture 27

Boom Boom NACA NACA Boom. We cool the Lycosaur.

The next step is to fill and sand. It will be easier to access if I get the top cowl off of there first. Picture 28

I have cut most of the way around the cowl line. The cut line forward is covered with duct tape. The lay-up goes from the cowl onto the duct tape.

You'll love this. The peel-ply I'm using now came from Wal-Mart and cost $1.00 per yard. Much better than the $4.50 that Wicks wants and it works great. Picture 29

The cowl is now parted. I set to cowl on the floor so you could see the flange. Also notice the bump out I made on the cowl line to accommodate the NACAs coming back further.

The cutting went pretty well, but the lines are a bit wavy. I won't be done with the cowl until they are straight.

The cowl is now parted. I set to cowl on the floor so you could see the flange. Also notice the bump out I made on the cowl line to accommodate the NACAs coming back further.

The cutting went pretty well, but the lines are a bit wavy. I won't be done with the cowl until they are straight. Picture 30

I took the Permagrit and straightened the forward edge of the cowl line. I covered it with duct tape and clecoed the cowl back on. You can see in the picture that I filled the gaps with microglass. I didn’t want to use micro because I thought this edge might get abused and should have good chip resistance. Picture 31

The cost of doing it my way. Since I deviated from the factory and made my own NACA inlets, I expected the plenum runners in the engine install kit would not fit. The lip at the back of the inlets also pushed the inlets down the firewall. I managed to compound the uniqueness by making the flanged, piano hinge cowl attachment. All of this means I need to come up with my own way of getting the air from the inlets back to the engine.

It won’t be that bad. The factory runners are close and can be modified. I have to make the flanges at the firewall first. Then I can work on joining the two.

I made molds from blue foam. I rough cut plugs on the band saw then file, fit, file, fit. You know that old chestnut. In the picture they are covered with Aircraft Grade Adhesive Composite Release Film.

As much as I loved seeing the engine on the plane, it was going to be much easier to lay-up the glass with it out of the way. At this point it was a matter of hooking it up to the Cherry Picker and remove the bolts. I’ll probably lay-up the glass tomorrow night. Picture 32

The glass has been laid. The corners were aggressive for three bid. I let the epoxy setup for two hours before I placed it. Still, it did not want to adhere to the underside. Once I had the glass in place I covered the lay-ups with plastic wrap. It allowed me to work with it and mold it using my hands. I checked and re-set it every hour for about five hours. It was Saturday, so I had plenty of time.

I took the picture after I removed plastic wrap. It is Sunday morning and I was anxious to see how it turned out. I am pleased. These should work nicely. I think I will let them cure for another day before I try taking them off. Like any piece, I want to start trimming things up. It is pretty now in that the lay-up is done and “I made this.” Other than that, it has a face only a mother could love. :) Picture 33

The plenum flanges have been trimmed, sanded, drilled and bolted. Picture 34

Mounting the runners will be a treat. I made my bed, now I get to lie in it. The outlets from my NACA inlets are a different shape and location from the factory. I have a preliminary trim on the runners. Just about nothing lines up.

In the picture I have partially completed the flanging lay-ups on the lower portion. I elected to not put glass on the inside face of the runners at this time. First, it does not line up with the bump-out on the plenum. Second, this lay-up is complicated enough without trying to reach over it and place glass in a awkward, tight spot. Third, two plies of bid moves fairly well. These lay-ups are three bid and it is noticeably stiffer. It really didn’t like the sharp corners. My plan is to let this cure, drill and cleco, and then work on the inner face with the plenum off the engine. Picture 35

The flange lay-up turned out pretty well. I need to lay-up flanges on the inboard sides of the bases. The picture is of the inboard base of the co-pilot runner. As you can see there is quite a gap between the runner and the plenum. The wet glass can bridge a gap, but one this large really needs to be supported.

The other problem is the flange on the cooling chamber. The factory cut this corner pretty close. For a normal Velocity, this probably isn’t an issue. With the angle on my runners, this is not going to leave much room for contact. Picture 35

The top and bottom of the runners were pretty close to the desired shape. The sides were just not going to work. I thought about this for a bit and decided to just remove the first few inches and rebuild them. Picture 36

This is one of those times that I wish I had more experience with composites. I know how to use the tools in my tool box, but I’m guess there might have been a better one out there. I needed to have something that would hold the right shape. I needed to get it in place while the runners are being held in the right position.

I thought the pour foam was a bit risky. On first thought, I dismissed it. If the dams aren’t made correctly, it will just run to the bottom; potential big mess. I couldn’t come up with anything better, so I gave it a try. Funny, I spent a couple days thinking on how I would do this. When I finally pulled the trigger on the pour foam, I had the buns curing in place in about and hour and a half. Picture 37

The shapes are coming in nicely. Picture 38

The other problem was trying to lay the glass. The outside lay-ups were fine, it was the inside lay-ups that were going to be a challenge. The inside of the runners are very close to the engine mount; awkward location and tight space. I finally decided to remove the engine again. There was no other way to get the runners off.

The glass is laid and I think we are looking good. Picture 39

The lay-ups turned out great. I’m very happy. The runners now match up to the flanges. The next step is to make the slip joint between the two. Since they were already held in alignment, this should be pretty straight forward. If you look closely, you can see that I put some more sticks inside to make sure it doesn’t move. Once the inside was reinforced, I was able to remove the stuff from the outside.

After this last lay-up I’m running low on epoxy. I ordered some more, but it hasn’t arrived yet. I spent the time tonight doing prep work for the slip joint lay-up. When the epoxy arrives, I’ll be ready to pour. Picture 40

I got the idea for the Slip Joint from Rick at Velocity. The idea is to lay a band of glass that laps onto both sides of the joint. The glass is laid on duct tape so it will not bond to anything. The outboard side is overlapped. I left a piece of 4 mil plastic sheet between the layers on the left so they do not bond.

You know, for as much as I spend time on prep, and as careful as I am while I’m laying this stuff up, there is still that moment of truth when the epoxy is cured and it needs to come apart. Picture 41

Just like magic, it comes apart. Always a nice surprise. You can see the glass underneath the overlap.

Before I take it all the way off, I'll drill holes for screws on the left and right sides.Picture 42

Here they are; ready to mount up. It took a little time to get all the jigging and pour foam out of there. I wanted the inside of the runners smooth, so I spent a few hours cleaning them up. I also installed nut plates on the inside, so the slip joint could be screwed down.

Up next is the oil filler neck. Picture 43

You might have noticed before that I had some extra holes in my runners. This is another byproduct of doing the NACA inlets my way. The exit of the inlets is down and aft from the factory design. When I installed the runners, one of the first things I found was interference with the engine mount. Since I didn’t know what else I would have to do to the runners, I just opened a hole and moved on. I knew I would have to close the holes up eventually. It is time. Picture 44

I’m either going to be very clever here, or waste a bunch of time and some glass. I waited to do this lay-up on the weekend. It was going to take a lot of time. I have laid glass over the openings. I left the glass over the openings nice and loose. In the picture, the glass has been curing for 5 ½ hours. I put plastic wrap around the edges so I could massage it and make it lay-down for me.

I took some double sided foam insulation tape (the kind used for sealing up doors and windows) and put an 1/8” pad on the interfering points on the engine mount. This should create some clearance for me.

I think it is time to put them on the engine and hope I have the right amount of slack in the glass. I also hope it has cured enough to not pull away from the runner, yet flexible enough to mold to the engine mount. Fingers crossed. Picture 45

The "Bump-Ins" on the runners.Picture 46