Over the past month I’ve had to deal with something that was always going to stop the project dead in its tracks – sorting out how to mount an Airflow Systems A/C compressor inside a Showplanes cowl. The main problems are:
- The A/C compressor doesn’t fit – it smashes into the cowl
- The A/C compressor occupies almost all of the space normally used for the left hand air inlet plenum
I’ve seen other build efforts that range from leaving out the left hand induction air plenum entirely, to an A/C compressor mount position that occupies a good part of the area of the left hand inlet duct – in turn compromising the cooling air for cylinders #2, 4 and 6.
Recent efforts in Australia, such as this one, use a longer arm on the A/C mount to drop the compressor down low, and make a bump on the bottom cowling to suit. Armed with the latest mount kit from AFS, I proceeded to test mount the A/C compressor, and immediately ran into quite a bit more trouble than I had expected.
Tensioner wheel, bracket
The supplied drive system simply didn’t work. The tensioner (idler) wheel, using the supplied bracket, hits the starter motor. I can see that with the previous “short” arm, the system would swing up higher and that would free up enough room for the idler wheel to swing past the starter motor. I sent feedback to AFS about this but received no reply. After consulting with the Australian builder referenced above, it turns out the solution was to:
- Add spacers, for a total of 7/16″ of spacing, to the engine mount. This moved the entire compressor assembly “away” from the starter motor by 7/16″.
- Machine a replacement idler pulley, which was the same size as the AFS supplied pulley, but without the ridges on each edge – saving around 1.5mm of edge distance.
- Make a longer arm, even longer than the “long” one AFS supply
I started making up additional spacers (AFS supplies three 0,063″ spacers), using 0.063″ Alclad, but I was only able to add two of these before I ran out of threads on the engine studs used for the A/C mount. I didn’t want to replace these studs, and I didn’t want to switch over to low profile lock nuts – the A/C compressor is quite heavy. So, my spacing limit without resorting to these measures was 0.3″.
The guy who did the VH-BKK installation referenced above graciously had another idler pulley made, and sent it down to me. Armed with this, I still needed to change the position of the idler mount, which means I have to build a new idler mount bracket. I’ve ordered some 6061-T6 plate to do this, in the meantime I drilled a new hole in the existing bracket (rendering it structurally unsound) just to prove I have the hole position correct. This is shown in the following picture, along with the new idler pulley. The clearances everywhere around this are very small, but this is typical and they will be adequate. The idler pulley can now swing up past the starter motor with about 1.5mm of clearance.
I also had to change the Serpentine belt, for this arrangement I fitted a 4PK1130 belt, rather than the 4PK1113 belt provided with the kit – just 17mm longer.
It is necessary to cut out a section on the front of the Showplanes cowl and fiberglass a bump in place to provide adequate clearance for the front of the A/C compressor. There’s nothing hard about this, and one good thing about the Showplanes cowl installation is that the Serpentine belt does not overlap the inlet at all (unlike with the Van’s cowl) – so there is no air leakage associated with treatment to clear the Serpentine belt, and no compromise to cooling air inlet area.
LHS Inlet Plenum
I had expected to solve this problem with various cuts and balloons etc. to the existing inlet plenum. After looking at the problem, I didn’t even attempt it because (a) this was going to beyond my fiberglass skills, and (b) this is probably beyond anyone’s fiberglass skills. There simply isn’t a clear enough path through the maze to make an adequate shape, and unless I could turn the plane upside down, gravity was always going to work against me.
After some soul searching, which included consideration of leaving the A/C system out, I decided to go ahead and create a 3D model of the entire system as a means to 3D print a replacement inlet plenum. The top end is identical to the Showplanes system, using the lower 1/3 of the circular inlet hole for induction air. The plenum has to step down and change shape, wrap around the bottom of the A/C compressor while clearing the bottom cowling, and then join in to a replacement shroud around the existing Showplanes air filter system. Here are some screenshots which include various early prototypes and the final model, which includes an air takeoff to account for the standard Van’s 2″ scat tube outlet for air to a heat muff.
The inlet plenum as shown can be 3D printed in one piece. It is secured at the bottom by three screws in the LH air filter shroud, using nutplates, same as the standard Showplanes arrangement. Around the compressor, two places mount to the A/C compressor lugs using Buna-N rubber spacers, AN3 bolts and washers. The vibration-damping spacers are rated to 200 degrees F which should be adequate. At the top, a flat Alclad plate which extends down from the existing #2 cylinder mounting flange is secured across the heat muff air inlet area, and at the very front of the inlet, using #6 countersink screws with nutplates on the underside of the inlet plenum. One of the front positions uses a low profile nut, embedded in a hex shaped hole, instead of a nutplate.
For the plenum material, the choices are currently between UV resistant ASA, or the more expensive UV resistant tough resin. Both have high glass transition temperatures and adequate chemical resistance. The plenum does not extend back too close to cylinder #2.
To prototype the system, I printed the plenum in sections on my cheap consumer grade 3D printer, using PLA, and taped the parts together. It takes about 100 hours of printing time to do the complete model, and I’ve made several revisions to refine the design and fix up the various maths mistakes I made along the way. It’s been a laborious process but I’m happy enough with the end result. It takes a high end desktop platform 15 hours to render the model in suitably fine detail, and a bit of repair processing to derive the final STL file. I uploaded the (64MB) STL file to http://www.craftcloud3d.com and was quoted just over U$200 to print the model in Black UV resistant ASA, with 40% infill.
Just today I pulled the trigger and ordered the final article, it’s being printed by a vendor in Sweden, and will be delivered here within two weeks. There is still a chance that the final piece will require some small adjustments. There’s a limit to how far I can go with taped together sections, and how many times I can re-measure everything. The final model is way more complex than I had expected, and has consumed far too much time, I’m tired of fretting about it so it’s a relief to get it off my desk.
In the meantime, there are a few other, much simpler pieces I can design to help with the baffling around the governor etc. I’ll complete these, but mostly I’ll be getting back to actual, real, physical work on the project – there is a lot still to do.