6.11.0 Overhead Fresh Air Duct (and Flipping the Fuselage!)

For those of you interested, click on the link to watch the video to see how we flipped it. The process took less than a half hour.


At this point the airplane is still rightside up. I purchased a couple of 12 foot 2x10’s to use as the lift point in the back. I bolted these to the main spar where the wings normally get attached. It was a bit tricky drilling the holes because of the angles involved. The span of the main spar, as installed, is just a bit less than 12 feet long so those 12 footers worked perfectly. I also purchased a 1” ID, 12 “ long iron pipe along with a couple of caps. This pipe is pre-threaded for caps or other fittings. One cap helps to hold the pipe in place on the board while the other helps to keep the strap from slipping off. The outside diameter of that pipe is 1.33” so I needed to purchase a 1-5/16” hole saw - an odd size for sure - but easily available from one of my favorite aviation suppliers, Amazon! I made a couple of five foot outriggers, shown here sticking up, that will eventually be used to support the aft end once it is flipped. I did have to trim these boards some to get the fuselage somewhat level (although when flipped, level isn’t that important).

So that the two 2x10’s wouldn’t twist apart, I added screws every couple of feet to lock the two boards together. I probably could have gotten by with just one board, but I was afraid that it might twist under the weight. These two boards didn’t twist at all. I also used a 2” strap to help lift it (the pipe acted as the rotation point and lift point). I initally tried lifting it with a 1” strap but I could tell immediately that it wouldn’t be strong enough. The 2” worked perfectly.

Fuselage - The Big Flip

Prepping everything for "the big flip". The airplane needs to be flipped for work on the bottom and the inside of the fuselage.

And here it is, upside down! The shop crane (from my other favorite aviation supplier, Harbor Freight) did a great job of lifting the aft end. We used about four guys in the front to hold the nose up during the flip - one on each door and one on each side of the nose. We added another guy on each strake end to help stabilize the aft end. Another guy operated the crane. The outrigger boards were trimmed by about 6” to get the fuselage somewhat level. I also ended up adding some extra supports on the bottom to keep the boards from wobbling back and forth. These not only need to support the fuselage, but me as well since I’ll be crawling around inside quite a bit. In the end, I found this setup to be very stable.

Fuselage - The Big Flip 1

Job complete - it took about 20 minutes along with a bunch of guys to flip it.

The photo below shows the extra support that I added to the outriggers to help stabilize them.

Fuselage - The Big Flip 4

Rear support - the 2 x 10's are bolted to the center spar along with some 2 x 6 outriggers.

I don’t need these steel saw horses often, but when I do, they come in very handy. I’m only using one of the pair, but in this case, that’s all I need to support the nose using the canard bulkhead as the support point. This saw horse has adjustable legs that extend it up to about 40”. The legs also lock in place tightly especially with weight on them, so that saw horse isn’t going anywhere. I also added a couple of tie wraps, looping them around the saw horse and through the canard attach bushings.

Fuselage - The Big Flip 2

Using a steel saw horse to support the nose, using the canard bulkhead as the support point.

Fuselage - The Big Flip 3

Another view of the steel saw horse holding up the nose.

We did the original upside down flip back at the end of September in 2020. It’s now the end of September, 2021, and the airplane is back to being right-side up again! Yay. Even though I didn’t think I’d have it upside down for that long, it actually worked out well. There was no rush getting it back to normal and I was able to get a lot of work done during that time.

So, in this first photo, I’m prepping it for the reverse “big flip”. It’s pretty much the same procedure using the same guys as before.

IMG 2426

Getting ready for the 2nd "big flip" - this time right-side up!

IMG 2427

And here it is, back on its wheels again!

And here we are, back to normal. Thanks to this great group of guys from EAA Chapter 810 here at the Payson Airport!!! Woohoo!

Here are a few more photos of the guys at work! All told the flip took about ten minutes!

Now, onto the overhead fresh air scoop and duct. The NACA scoop’s hole was actually created in Section 11.1.5, along with the big NACA scoops used for cooling the engine. I waited until the fuselage was flipped to install this scoop though. Since the hole was already there, all I needed to do was some trimming after which I laid down some BID to lock it into place. It’s the scoop in the middle in the photo below.

Fuselage -  Engine NACA scoop layups. 1

The NACA scoop layups are all finished.

Now that that is finished, it’s time to start working on the air duct, air vents and cabin lights. The air duct is fed air from the scoop and acts as a plenum. Therefore, the air vents need to be mounted in the duct itself. I purchased four of these really expensive vents (about $110 each!!!). Their build quality is excellent. Other builders have purchased less expensive (in some cases plastic) vents that either didn’t last long or leaked air. I only want to build this once so I opted for more expensive vents. (And even these were cheaper than some I saw that cost well over $200 each!!!) I also purchased four LED cabin light fixtures (much less expensive than the vents) that are actually designed for RV trailers. They are also well built and very bright! The trouble is, they are a bit too large to fit inside of the plenum, and besides, water can get inside that plenum which I’d rather not get into the light fixtures. So, I decided to build some outboard supports for these light fixtures.

Fuselage - Cabin - Lights and Air Vents

Test layout for cabin lights and air vents.

Using some leftover foam, along with a bunch of duct tape, I created molds for the light fixtures. It’s hard to tell from the photo below, but basically, the two lights and two vents will line up as shown in the photo above.

Fuselage - Cabin - Lights and Air Vents 1

Making some "wings" to hold the cabin light fixtures.

Here’s what the extension pieces look like on the inside. Two of the foam pieces have been removed, but I left the other two in so you could see what they look like.

Fuselage - Cabin - Lights and Air Vents

Photos showing the foam inserts used to create the bump outs.

Once the epoxy is fully cured it’s time to measure the mounting flanges for the screws that hold them to the air duct. This is fairly simple - just measure and drill the appropriate holes for the screws! In this case I’m using stainless 8-32 aircraft grade screws.

Fuselage - Cabin - Lights and Air Vents 1

Prepping the duct for nutplate installation.

Here’s the panel screwed down to the duct along with the a test fitting of the light fixtures.

Fuselage - Cabin - Lights and Air Vents 2

Test fitting a pair of the cabin lights.

This is what the panel looks like without the lights installed. Once again I’m using nut plates for mounting the lights.

Fuselage - Cabin - Lights and Air Vents 3

This photo shows the cutouts for the light fixtures along with the installed nutplates used to hold them in place.

Next it’s time to drill out the holes for the air vents. The air duct is open to the NACA scoop in the back which pushed air into the duct. When the eyball vents are opened there should be plenty of fresh air for the passengers and crew. The light fixtures are outside of the duct.

Fuselage - Cabin - Lights and Air Vents 4

The holes are now drilled for the air vents.

And finally, here are the eyeball air vents temporarily installed in the panel. It all looks good to me!

Fuselage - Cabin - Lights and Air Vents 5

Both the light fixtures and the air vents are now temporarily installed. Everything fits and looks good!

I mentioned that the duct feeds air from the NACA scoop on the outside of the fuselage to the air vents. The duct, as received from the factory, is supposed to be permanently installed. I’ll talk about that later. Because I’m not doing that, I need to close off the end of the duct to keep air and moisture from spilling out the back. Therefore, I created a couple of “plugs” (basically a couple of small pieces of foam that I then covered with a couple of layers of BID) to close off the aft end. You can see the result of that in the photo below.

Fuselage - Cabin Overhead Air Duct

The end of the air duct gets sealed to block the air flow.

Because this duct is basically open to outside air, moisture can also get in if it’s raining, or humid, or whatever. Because of that a drain line needs to be installed in the duct. The factory supplies a hard point for that. Unfortunately they didn’t pre-tap it, but that’s ok because everyone uses a different size fitting. In my case, I’m using a standard AN fitting similar to the other fuel tank vent lines and fuel lines. 

Fuselage - Cabin Overhead Air Duct 1

Hard point installed for AN fitting used to drain any water that gets into the duct.

I mentioned above that I decided not to permanently install the duct. There will be a lot of wires running through the duct to the overhead switch panel, as well as wiring to the LED light fixtures mounted in the sub-panels. Because of that I want to be able to easily access the inside of the duct for maintenance purposes. That means it needs to be easily removable. I stole this idea from another builder who made rails out of angle aluminum. Since this isn’t structural I’m using aluminum stock from Home Depot, which actually works better because aircraft grade stock has a radius on the inside corner. For this application I do not want that radius because this rail has to bend a bit to fit the contour of the ceiling. You’ll notice that I cut some slots every four inches in the vertical sections to help let the aluminum bend. That worked well. These rails are permanently installed to the ceiling with structural epoxy and rivets. You can see the rivet holes on the bottom side of the rail. I also installed nut plates on the vertical sides which are used for attaching the duct to the rail.

Fuselage - Cabin Overhead Air Duct 2

This rail gets installed on the cabin roof which then holds the cabin air duct in place.

Here are the four rails temporarily screwed to the ceiling for a test fit. Once it’s glued down it’s too late to adjust! You can see the NACA scoop towards the top of the photo below that feeds air into the duct from the top of the fuselage.

This photo shows the four rails temporarily in place to test fit the overhead air duct.

The rails are installed and the duct fits perfectly. Eventually the duct gets covered in upholstery to make it look good. (Of course, the ceiling will also get covered.)

It looks like it fits!

There’s an overhead switch panel that also needs to be installed. It bumps up against the air duct, as shown in the photo below. The switches get mounted in the sloped center part of the assembly. Here I’ve marked and drilled the mounting holes for the panel. Basically it just wraps around the carbon fiber beams that strengthen the door openings. Nut plates get installed in the beams.

This panel will eventually hold a bunch of switches used for various aircraft functions.

And here’s the finished product. I had to do some trimming to get it to fit around the carbon fiber beams.

This panel will eventually hold a bunch of switches used for various aircraft functions.

So, how do I get wires into this switch panel? Some builders run a conduit through the carbon fiber beams and then into the long conduits that are used for hot oil lines and battery cables. There’s also no easy access into those conduits from inside the cockpit - the long conduits are meant to carry “stuff” from the front of the airplane to the back. Some builders cut a channel in the ceiling between the air duct and the switch panel, which is also strengthened with carbon fiber layups, but in my opinion that then weakens the structure at that location. So, after noodling it out for awhile I decided to create my own small surface-mount conduits. I took some scrap foam, as shown below, to create a channel for the wires. These two conduits will run between the switch panel and my air duct overlays (that hold the light fixtures). That way I can easily run wires from the duct to the switch panel.

Fuselage - Cabin Overhead Switch panel 1

I need some way to get wires from the air duct into the switch panel, so I'm making a couple of conduits out of foam.

Each foam piece gets one layer of BID on the top and inside the channel. I’ll also add a layer of BID on each end.

Fuselage - Cabin Overhead Switch panel 2

One layer of BID on the outside of the conduit. I'll also put a layer of BID on the inside as well as the ends.

So, let’s skip ahead a few months. The conduits were installed and are actually now covered with upholstery. You can see them at the bottom of the photo below. Those conduits are being used as wiring ducts from the overhead panel, through the lamp and air vent fixture, then into the overhead duct.

This photo was taken when the fuselage was still upside down.

ui© John Trautschold 2018