Update on tailcone mods

Now that the tailcone is all drilled, I'm revisiting the mods I had planned to see what I need to do before primer. I made a previous post on these, but have refined a few of the details since.

I also understood why people say that making modifications takes a lot of time - I've spent a few tens of hours doing the research I'm posting here (hopefully that'll save somebody else's time later, as other builders have saved mine), and even after that I'm not 100% sure this is correct and going to work.

Static ports

I decided to use the SafeAir pitot-static kit, which comes with two nice static ports. One issue a friend recently had with his RV10's static system was water accumulation (which in his case, killed two avionics components that got flooded), so I plan to install them per instructions at the default location, then run the static lines up through the bulkhead, join them on top, then run the lines forward along one of the stiffeners all the way to the baggage bulkhead. Only at the baggage bulkhead they'll be routed down into the tunnel and all the way to the panel, where I plan to place the AHRS. With this, it will take a lot of pressure to make water get all the way to the front. I also considered adding a pitot static sump bottle to drain the water, like some Cessnas have, but it seems to be overkill.

Along the route, the lines will be attached with adel clamps, and the tee that joins the lines from the two ports will be attached to a small attachment fixture on the top of the bulkhead. This is similar to what has been done by Mouser and by Mike and Sarah, except I don't want mine to go to the back of the bulkhead (harder to inspect/maintain later), so I'll just use a nut to elevate the adel clamp over the lightening hole flanges.
Static port install (photo by Mouser)
Static line routing (photo by Mouser)
Another tee bracket style for joining the static lines (photo by Justin Twilbeck
From the bulkhead forward, I'll use adel clamps to secure the line along the top J channel. However, since I don't want to drill #10 holes into the stiffener to secure the adel clamps directly, I'll follow Justin Twilbeck's installation and use a small piece of aluminum with smaller holes to attach them:
Spacers for adel clamps that carry the static line (photo by Justin Twilbeck)
BOM for this part:

Magnetometer

I reviewed once again the installation instructions for the GMU22 magnetometer from Garmin, given that a lot of people seem to install these inside the tailcone:
Magnetometer installed inside the tailcone (photo by Terry)
Others, still, install it in the aft deck (as is the recommendation for the RV7/RV9, but not the 10).
Looking at the Garmin G3X manual, it says "In general, wing mounting of the GMU 22 magnetometer is preferred", and they provide a table of things to avoid near it:

Magnetometer disturbance avoidance from the G3X Installation manual
Another document by Garmin, "AHRS/Magnetometer Installation Considerations" shows the same table for the GMU44 (used in other packages like the G900X), making it explicit that this list is ordered by priority. Given this, and given the number of motors and antennas I'll have in the tailcone, I've decided to indeed do the wingtip mount - having it close to the landing lights is still better than having it close to all the noisy equipment. The wingtip mount should look like this:
Wingtip magnetometer mount (photo by maus92 at VAF)
Also, to be safe, since I'll be installing the NAV and marker beacon antennas on the left wingtip, the magnetometer will go on the right wingtip.

BOM: nothing at this point

Autopilot and trim servos

As mentioned in the previous post, the autopilot servos have standard mounting kits, but those expensive and can change until I actually install the avionics, so at this point, the important part is to understand what space they're expected to take, so I don't plan to put anything there.
  • The pitch autopilot servo takes up space aft and to the right of the bellcrank.
  • The pitch trim servo takes up space just forward of the aft deck (and the cables go over the aft deck itself)
  • The rudder autopilot servo takes up space further aft of the bellcrank and the pitch servo, on the center
If I add a linear actuator for the rudder trim,  it would go below the bellcrank mounting shelf, as proposed in this VAF post:
Idea for electric rudder trim (photo by Ernst Freitag from VAF)
On that installation, a flap motor was used, which is overkill - a simpler linear actuator will do just as well. I can think of two ways to connect together the rudder cable, autopilot bridle cable and the trim spring:
  • Having the spring at the bottom, the regular rudder cable in the middle, then the servo's bridle cable at the top, with them all being connected at the same cable clamps
  • Having the trim springs connect to the autopilot servo, at the same mounting screws as the bridle cables, but from the opposite direction. This requires attaching the other end of the fixed spring (on the right side) to the same screw as the pitch servo mount, and either using the standard AHRS platform or extending a small platform to mount the actuator on the left side.
The GSA28 autopilot servo can drive the trim motor, as long as it draws no more than 1A (and after checking with Garmin, it seems that a motor that drives more will not be current-limited/speed-limited - it will damage the GSA28). I've considered building a current amplifier to bridge the GSA to the servo, but then also thought it'd be overkill.

At first I tried to pick the actuator and spring by doing the proper math to set my requirements, and found that I don't have all the input data (e.g. what is the force the rudder exerts on the cable depending on speed, load factor and deflection?). I then tried to eyeball it by looking for actuators with reasonable speed, low current and reasonable torque for this, and looked at A LOT of actuators, only to find that not a single one meets all those requirements. The closest two were the LD3Q (which goes to 1.2A, but otherwise moves at ~6mm/s and has way more torque than I need at 800N) and the P16P (which goes to 1A, moves at ~9mm/s with 90N of force). While the P16P doesn't support much sideloading, it supports more than the GSA28 will normally output even if they were connected at 90 degrees with the max torque setting, so that shouldn't be an issue. The L16P also seems to be similar to the P16 while supporting higher side loads, but it looks too hobby-grade for me.

Ultimately, I don't have to decide on the exact details until I mount the autopilot servos, so while I'm deferring that for now, this has made me decide to have a tall platform on the left side and a short one on the right side - that allows me to mount the linear actuator high or low, and the avionics on the other side.

BOM: nothing for now

Avionics

I've looked at several options for mounting the avionics, and revisited the original idea of mounting them to the corrugation:
Avionics mounted to the bulkhead corrugation (photo by Elwell81 at VAF)
I realized that in the solution above is that there's no "clean" way to put nutplates on the other side, since that's on the baggage compartment.  Additionally, I measured the antenna cable runs from the avionics and realized that, at least for the Stormscope, I'd be very close to the length limit.

I'm thus switching to avionics shelves (and keeping the corrugation available for any future upgrades):
On the right side, I'll rivet two F-1037 bellcrank rib angles to the stiffeners to make rails with nutplates, which I can later attach an avionics shelf with screws to.
On the left side, I'll use a modified version of the AHRS bracket by Vans, which extends all the way to the side and attaches to the side stiffener, much like Justin Twilbeck did on his, except I'll actually install the shelf rib from Van's to support it, and another F-1037 for the side-stiffener attachment:
Idea for wider avionics shelf (mine will be on the right side - photo by Justin Twilbeck)
Attachments for avionics shelf (photo by Justin Twilbeck)
BOM:
  • Van's AHRS bracket kit
  • Van's F-1037 for the rails (2) - unfortunately F-1037A is not useful, but B and C are
  • .040 aluminum for support brackets
  • K1000-08 nutplates for the rails and brackets (21)
  • MS35206-243 screws for attaching shelves

ELT

I've also researched specific models, and so far I'm leaning towards the Kannad Integra AP, which continues to work using a second portable antenna if it's removed from the fuselage. It transmits on 121.5 for shorter than the fixed version (48h vs 100h), but if nobody has picked up your ELT after 48h, what are the odds they will in the next 48? I do plan to continue flying with my Spot Gen3 or something similar, too.

Plans on where to place the ELT remain the same. For a moment I froze when I saw this picture, which shows that the trim cables run through where I planned to place it:
Trim cables running through where I planned to place the ELT (photo by Jemo)
The panic only lasted for a few minutes, until I went back to the previous photo and saw that it actually fits even with the cables:
ELT does fit even with the trim cables (photo by Ivan & Dianne)
A secondary concern was whether the Kannad would be the same size as the AmeriKing 450 depicted - turns out the base of the Kannad is actually smaller (3.86x3.4x5.51 vs 4.27x2.95x5.64), so I expect no issues with the fitting. That said, I'm delaying the ELT purchase until later (don't want to start running the battery's useful life for no reason).

BOM: nothing for now

APU plug

I tried to order the Aircrafters' APU plug, only to find out that they went out of business :/ I bought just the plug, and will fabricate a doubler to mount it to with a cover plate, similar to the one that holds the elevator cover plates:
To easily remove the cover plate when the APU connection is needed, I'll use Camloc fasteners. In addition to Aircraft Spruce, I also found three other suppliers: Quick Release Fasteners, MilSpec and Skybolt (each one was cheaper for some of the parts). My thought is that if I need external power to start, I can live with either needing someone to fasten it after start (I'll need someone to disconnect power anyway), or just flying without the cover. I considered making a proper door for this, with a Hartwell latch and all, but in reality I'll need to open it so rarely that it's not worth the time, effort and weight. Also, what are the odds that I'll have access to an APU but won't have access to a screwdriver?

I haven't figured out the right way to protect the input voltage (for one, against overvoltage, in case someone ever plugs a 28V APU into my 14V system), but I'm tempted to follow Vertical Power's suggestion:
APU port wiring diagram (from Vertical Power)
The wiring can be figured out much later, though, as well as where to attach the additional contactor - as an option, Aircraft Spruce sells a solenoid mounting bracket.

BOM:

Antennas

I had planned for many of the antennae to be on the center line of the tailcone, without realizing there's a stiffener there - those will be slightly offset to either side:
ELT antenna marking and stormscope antenna
I marked their locations on the skin:
Top skin antenna locations marked
The overall plan for the antennas remains the same, with a small correction: when I ordered my antennas, Don from Delta Pop called my attention to the fact that I want at least 1m between the ADS-B and the transponder antennas, since they're on the same frequency. This requires a change where I move the transponder and UAT antennas further apart. Given the small possibility of some day having to install a whip NAV antenna on the aft end of the tail, I decided to place the transponder forward and keep the UAT back (so that there's nothing transmitting at high power near the NAV antenna). With this, I also marked the bottom antenna candidate locations:
Transponder antenna location marked
In addition to satisfying the separation between the two, this also has the added benefit of satisfying one more Stormscope antenna requirements (being at least 48" from the transponder antenna).

Another minor detail I noticed is that the Stormscope antenna requires applying Alodine to the skin where it's installed - this will be annoying.

Additionally, even though not a tailcone mod, I also found a wingtip marker beacon kit by SteinAir.

BOM:
  • Antennas (duh)
  • .040 Aluminum for doublers
  • In the future, RG-400 cable

Wiring conduits

I'll run one conduit all the way back until just before the F-1010 bulkhead for the ELT wires as well as any future wires that may be passed through the vertical stabilizer conduit. I'll also run another conduit as far back as needed for the transponder antenna.
From the tailcone forward, I'll likely have 3-4 conduits, like this other builder (if this is your photo, let me know in the comments so I can give you proper credit!):
Conduit runs from the fuselage to the tailcone (photo from unknown source)
One of the concerns here is minimizing interference, so I'm essentially leaving all the power wires on the left side (tall shelf) and all the data wires and antenna cables on the right (rails for short shelves). Obviously some of the power wires need to cross over to the other side, so I'll install conduit cable tie mounts across the center, aft of the bellcrank, like Mouser did:
Wire tie anchors to cross the bellcrank ribs (photo by Mouser)
I'll also copy another of Mouser's ideas (hey Mouser, I think I owe you a beer :) ) and use ClickBond posts to hold the conduits going aft:
Clickbond fasteners to hold the conduits (photo by Mouser)
While I don't see a reason to raise them off the floor, having intermediate holding points without having to modify the skin or structure in any way is a big plus. Look at all the ClickBond fasteners, I saw that they have models which allow directly securing a conduit with wire ties, such as the CB4019, which seems much easier:
ClickBond CB4019 (photo by Clickbond)
Finally, I found the Wire Loom connectors, which may be interesting for connecting the conduits, if things get complex. This nice video suggests that even for a good and complete routing, I won't need those though.

BOM:

NACA Vents

This was an item I wasn't previously considering, but several reports made me want to have NACA vents on the back for overhead console ventilation:
Side skin NACA vent (photo by Bob Leffler)
For this, I can control the air flow with the Aerosport NACA valve:
Aerosport NACA valve installed with the vents (photo by Ernst Freitag at VAF)
and let the air flow into the overhead console through the bulkhead:
Flange for going from the tailcone to the overhead console (photo by Ed and Colleen)
The flange shown above does interfere with the intended routing for the static line, but I can simply place the L-shaped fitting for the line further aft.

Also, to attach the valve to the center stiffener, I'll follow Ed and Colleen's suggestion of using a F-1048D:
NACA valve with with F-1048D for attachment (photo by Ed and Colleen)
To minimize holes in the stiffeners, I'll likely also use the F-1048D bracket to secure the static line, just above the valve.

Also, as another builder suggested on VAF, I plan to add a screen to the intake side of the valve, to prevent any bugs from coming in. It will be places inbetween the hose and the valve and held in place by the the same clamp that holds the hose.

BOM:

Summary

I'll be addressing the following items now:
  • Right-side (tall) avionics shelf
  • Rails for left-side (short) avionics shelf
  • Conduits to the back
  • Static lines up to the F-1006 bulkhead top
  • NACA vents and valve
  • APU plug and cover plate
Everything else will be left for later, possibly when I buy the avionics.

1 comment:

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