I bought my A/C kit quite a few years ago from Airflow-Systems. I was shipped a so-called “Australian” evaporator, which is actually a product called a Monster Trunk System, part #685000-VUY from https://www.vintageair.com. There was a collection of metal parts and adapters in the kit, with no obvious way to set up the air flow and no instructions for this evaporator unit. The evaporator contains a 3 speed high volume scroll blower, which is ill suited to pressurizing the overhead console. Several other builders have supplemented this evaporator with an inline blower which is more capable of pressurizing the overhead console. Yet another technique has been to forget about the overhead, turn the unit around and fit enough ducting to blow air straight into the cabin – see here.
I was already committed to a conventional mounting position, having done the inlet ducts and cutout for the overhead several years ago. What I needed to do was complete the evaporator outlet ducting, including an inline blower to suitably pressurize the overhead console, cabin flood air ducting, and a means to use the rear NACA vent air, via the Aerosport products NACA vent valve. This exercise is complicated by the fact that there isn’t a single right angle anywhere in the system, and it all rapidly turned into a 3D modelling exercise. First though, here’s a description of the evaporator inlet system I put together a few years ago:
The F-1006 bulkhead attachment for air to pressurize the overhead is a difficult area. In order to get enough air volume, a significant cutout is required. This mandates a doubler plate, and there is not much room to fit one. Edge distances, strength, clearances, Aerosport overhead console flange dimensions and screw positions for the rear baggage bulkhead all come into play. I wound up using both a shim plate and a doubler, in order to tie the doubler in with the rivets which secure the top of the F-1028 baggage bulkhead channel. Rivets for the doubler are flush on the rear side (where the manufactured heads are inside the overhead console air space) and flush on the front side where the baggage bulkhead overlaps the F-1006 flanges. I lowered and moved the normal position of the baggage bulkhead top screws, in theory they should have landed right in the middle of the Aerosport overhead lower flanges, in practice they are a little above this point but still easy enough to install.
This whole area is so busy, it is difficult to find a good way to “attach” the required air duct(s) to the bulkhead. It’s also not reasonable to have hard attachment points between the bulkhead and the evaporator/shelf, due to vibration and cracking. The idea of this design is to use a 3D printed bulkhead attachment block to achieve the following:
- It can be fitted to the F-1006 bulkhead after the top skin is riveted on, sealed with some form of gasket material, and brings the air duct attachment plane clear of the bulkhead.
- Two long #6 screws act as locating pins for the flange of the main attach duct.
- A thick/soft gasket or manifold can be used between the rear of this bulkhead attachment point and the main duct, to seal airflow and provide vibration isolation.
- If (when) the evaporator arrangement changes, a new 3D duct can be printed to mate with the existing bulkhead attachment point.
I use only one hole for airflow into the overhead, the side with more area (the F-1028 is offset from the center). Manifolding air into both sides complicates things and is pointless – what matters for overhead air is pressure, not volume. I wanted electrical connections into the overhead as well, so these are on the right hand side of the bulkhead, and will be sealed off in the overhead.
For the evaporator outlet, I designed a manifold which caters for the following requirements:
- Fits onto the two irregular shaped outlets on the evaporator, with a simple rubber seal and some screws.
- Provides an outlet for the inline blower. I used a 4″ blower, because it fits. A 3″ blower would probably also be adequate.
- Provides a pair of outlets for cabin flood air. These should probably be 2.5 inches each, I used 2 inches because that’s the attachment size I have room for on the front (top) bulkhead.
- Provides a pair of inlets for the Aerosport NACA vent valve, to feed vent air from outside into the system
- Provides a place to mount a temperature probe
- Can be assembled in-place, or if necessary by lowering the rear edge of the evaporator shelf (after removing the support).
The following pictures show what I came up with. I had the prototype fabricated in tough epoxy, and it fitted fine except for an indentation on the top that I made to clear the top stiffener. For some reason my measurements were off, and the indentation missed the stiffener by 20mm. I fixed this up and made some other improvements, and just ordered the final version which should arrive here in another week or so.
The 4″ blower just fits in the required space. I’m mounting it to a metal bracket that will be riveted to the cover plate I made up for the evaporator inlet. Also mounted on this cover plate are three relays (for the scroll fan) and a pwm controller for the inline blower. A wiring harness for this can be seen in the pictures, not properly laced up or secured yet. A high side pressure sensor, and evaporator air outlet temperature sensor, are included. The system controls will be on the overhead, except for the master “A/C on” switch which is on the front panel, pilot’s side. Turning the A/C off (before rolling) will be on the pre takeoff checklist, if necessary it can be re-engaged at some point during climb out. Part of the wiring includes a connector that could be used for a micro-controller that would be capable of climate control, if the rotary switch in the overhead is set to the “auto” position.
The final duct is to go from the outlet of the axial blower to the overhead. I printed some prototypes for this on my own consumer grade 3D printer using a flexible material. Once the shape was correct, I decided to order the production part in SLS Nylon. This will be very strong, but will still have enough flex to effectively detach the evaporator/blower assembly from the airframe. Although I will be assembling all of the final components with the top skin still partly open, everything is designed to be removable and reassemble-able after the skin is in place. It won’t necessarily be pleasant working back there in the hell hole, but it can be done. For assembly, I’m going to take advantage of the skin being off and will cheat as follows:
- With everything in the tailcone finished, and with the evaporator/shelf removed, cleco the top skin on in its entirety. With Rosie outside on the rivet gun, and me inside, we’ll rivet the holes across the front and towards the rear on each of the three stiffeners.
- Remove all the remaining clecos, allowing access from each side.
- Fit the bulkhead adapter and the (flexible) duct from the inline blower outlet to the bulkhead. This can’t be done until after the (above) rivets are set.
- Install the evaporator, shelf, outlet manifold, inline blower, NACA vent valve etc, using the access from each side to make the job easier this first time.
- Fit the remaining refrigerant hoses etc. and charge the system. I plan to use an electric motor with a grooved pulley and a long serpentine belt as a means to run the compressor for this step.
- Check for leaks and proper operation.
- Cleco the skin back up, climb inside and finish riveting on the skin.
