Building & Flying a Murphy Rebel

Chronicles of Murphy Rebel 254R, a high wing, aluminum, two seat, tailwheel airplane.

"Everything is hard until you know how to do it"

                                                 

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DISCLAIMER: The descriptions, photos, and other information on this site contain or represent mere opinions, sometimes not even that, and refer to only part of the materials, decisions, procedures and processes used in the contruction and/or operation of this particular airplane. No warranty is made or implied as to the accuracy, completeness or wisdom of any information, or whether it may or may not apply to any other airplane or situation. The opinions expressed may and often do change, and the site may or may not be updated to reflect any changes or corrections to the information or opinions expressed. In short, the information is not advice to be followed bindly and your mileage will vary. Any use of the information on this site shall be at the sole risk of the reader.

EGT spreads solved

This post -- about some breakthrough intake manifold mods that solved the common Jabiru EGT spread problems -- is finally here: Individually, the mods are not particularly new or unique, but the story will confirm and supplement reports by others. The word "CARB" and acronym "TBI" (throttle body injector) are sometimes (rather loosely and incorrectly) used as if they were the same thing "CARB" -- but they're not! They do perform the same general functions -- metering and mixing fuel with air that then goes into the intake manifold) but they work in quite different ways. I currently use the Rotec TBI-40 (had an AeroCarb "TBI" before) and... prior to the mods below, I still had the first gen.oreiginal, stock "curving snakes" intake manifold. EGT spreads at high cruise were frequently as much as 250+ F between the hottest cylinder (usually #5) and some of of the rest, making it difficult to keep the hot one rich enough, and the rest not too rich. In general, high EGT spreads usually indicate that the hotter EGT cylinder(s) are getting a more rich mixture of fuel than the cooler ones. Before the work outlined below, I tried numerous other remedies, including carb rotation, X-vanes in ducts right before the carb, three different sizes and shapes of "plenum" boxes just upstream from the TBI (as recommended by Rotec, Ellison, etc.), checking intake tube sealing at the manifold, tube junction hoses, and cylinder flanges, swapping EGT sensors, enlarging holes on the TBI spray bar -- and other things. I was reluctant to get into modifying the manifold, but definitely should have done it earlier. Following are the recent mods: 1) INTAKE MANIFOLD: I took the manifold apart and got the inside rough milled at a local machine shop, taking the curving snake formations out, leaving a much more open, box-like space. After the rough milling, I spent 5+ hours sanding surfaces and rounding edges and corners, leaving a 120-grit surface. Credit for the porting idea and design go to Robert "originalbadbob", who was very helpful. (Thanks again!) 2) FLOW DIVERTER: I changed the original airfoil-shaped vane (or flow diverter/diffuser) in the manifold to a 1/2" round, center-drilled rod. I considered using 5/8" dia. (an option discussed in detail here) but I'm happy with the good results described below using the 1/2" dia. rod. YMMV 3) BRASS X-VANE: I soldered a piece of .010 brass shim stock to form a tube with nominal 2-1/8" diameter, about 2" long. I then soldered an X vane inside, also of brass shim stock, and epoxied the "tube-X-vane" assembly inside the downward-facing output of the std. Jabiru air mixer box. I took some time to fill and sand the inside top entry into that tube to make it even smoother and more rounded than it had been. This was the first time I had put an X-vane into the mixer box output tube, which is, relative to my other vane attempts, much farther upstream from the TBI input. The vane design was inspired by Rob "scsirob"'s nice work (see http://yhoo.it/1JD4y3a) -- though mine isn't nearly as classy. 4) LARGE DIAMETER INTAKE DUCT: At the output of the mixer box's X-vane described above, I added a 2-1/4" to 3" silicone rubber hose adapter (Vibrant Performance p/n 12710*). The top 2-1/4" dia. section overlaps the brass X-vane inside the mixer box output tube, so in essence, the X-vane empties into the expansion part of the adapter. A 2" long section of 3" round fiberglass tube provides transition between the bottom of the silicone rubber adapter and a short section of 3" dia SCAT duct. The 3" SCAT runs down to a 3" aluminum flange (ACS) with a flat .040 ring on the back, and that assembly bolts onto the TBI. Thus, the TBI is pulling from a 3" duct, which is quite a bit larger than its venturi input. 5) ROTEC IGNITION: I also installed the Rotec electric powered coil ignition, which feeds the right side distributor. (Great unit!) Between that and the more uniform mixture distribution, I'm burning at least 0.3 GPH less than before at my usual ~105 MPH TAS cruise. All the above took me a couple of months with the mill pickled, but flying this time of year in WA is somewhat limited anyway. Various photos are in the "Ron's Rebel" photo album, at http://yhoo.it/1CQ1Q84 The end result is that EGT's are now easily controllable within a 60 F -- 70 F spread throughout the entire RPM range, including WOT loads. Hurray! That's probably within the margin of sensor error and/or sensor placement variation. I really had no idea how this would turn out -- maybe better, maybe worse -- but I doubt the spreads could be improved any more, so I won't try, just fly! ;-) Any of the above steps, individually or in combination, may have contributed to improvement of the EGT spreads. Because multiple things were done at once, it's hard to know for sure, but IMHO the most important mods were porting the manifold and changing to the round diverter/diffuser. In any case, it feels like a new engine. I'm happy, the engine is happy.

reflections

Here are a few reflections on the panel and electrical design elements -- things that I might change at some point, or do differently another time around:

Endurance Bus - I had quite a lot of experience with this on the way back from S.Dakota in July, '13. Although it worked well in most respects, I would have liked the manual flap control to be available without having to turn on the Main Bus. In full manual mode, it uses no power until you need it. When you do, it's distracting to suddenly be worrying about Main Bus loads, and trying to reduce them on the fly. You're usually in the pattern after all, and need to be looking for traffic and other instruments. Other than that, it worked well as designed.

Avionics Switch - I bought into Bob Nuckolls' idea that an avionics switch just introduces another point of failure, when individual equipment can be turned ON/OFF anyway. So far so good, but there are a number of operational problems with that. First, unlike the original GNS430W, the GTN650 doesn't even have an ON/OFF switch of its own, so if the master is on, and the Main Bus, such as to start the engine, so is the GTN650, along with its relatively high load. Second, the GX330 transponder always turns on whenever the Main Bus turns on, or there's a bounce in voltage, i.e., during engine start, so I'm constantly having to turn it off manually. Third, it's best to leave things like the audio panel on, or universally switched on with radios, etc. Once I left the audio panel OFF and thought I was transmitting on the GTN650 when the audio panel, being OFF, had actually defaulted to the SL40 COM2, as wired. Yes, I should have looked at the panel's pushbutton lights, but didn't. I'm not sure where I'd put an avionics group switch at this point, and it would require another bus.

Panel Layout - I'm considering converting from carburetion to the SDS EFI system, which would entail adding some additional hardware on or around the panel. I have a guesstimate about how that might go, but....

Ignition/Mag Switching - Doing this again, I would use two toggle switches, instead of the more impressive-looking traditional keyed switch. The keyed switch doesn't really add significant security (it's easily hotwired), you have more flexibility with dual toggles and, given the checkered history of keyed switches, greater reliability. As an example, I have considered using the Rotec "electronic ignition" on one of the two sparkplug sets, but it requires a totally different switching logic -- which would be easy to implement with toggles, but not with the keyed switch.

detailing

When last we visited this journal in July, the bird was being readied for another assault on Oshkosh. The electric fuel gauges had suddenly stopped working correctly and, after lots of troubleshooting, I decided to bite the bullet and pull the left wing tank's first bay inspection port off to see if I could find out what was wrong with the sender. After believing clearing a blockage fixed the problem, and testing that theory, I buttoned it up again. Here we see the mini-paint booth created to touch up the inspection port's paint. (Unfortunately, the problems reappeared not long after we got underway.)

Nevertheless, wing strut fairings were added and after tidying up a few other minor things, the Rebel was dressed up pretty, looking ready to go.

 

faithful readers

A record downtime (hiatus herculeus) has occurred here, most assuredly through no fault of yours. Fear not, the pace of posts will soon resume.

robot liberated

So. The wonderful TruTrak Digiflight II VSGV* autopilot (A/P) has, up until a couple days ago, not been wonderful. Although the electro-mechanical installation is correct (say Halleluja!) and the servos turn in the right direction, I have been having one helluva time getting the configuration settings correctly adjusted for it to work... or so I thought. Truth is, even though it would have been great to have the A/P while flying 4000 miles (+/-) to Oshkosh 2011 and back, figuring it out hasn't seemed like a big priority, and I haven't spent all that much time troubleshooting it. Just a "something I'll get to when there's time" sort of thing.

With another imminent, lengthy cross-country trip ahead, the time had come to conquer these gremlins. After yet another in a long string of support sessions with the fine fellows at TruTrak, and after ruling out every possible fault with the hardware, software and installation that had ever occurred with any  autopilot, in any aeroplane, anywhere in the world -- followed by a pregnant pause -- they suggested that I try disconnecting the A/P from the main static air pressure system plumbing, and let its static air pressure input just "float" at cabin pressure which, for most purposes is just like outside static pressure. So I did and... voila!

Suddenly, instead of the usual usual topsy-turvy motion that would give even an old salt nausea, and put the Tilt-a-Whirl designer to shame, there was instant stability, with the altitude hold (V-hold) being particularly rock steady. The lateral motion still needed some minor tuning up -- something that had also not been possible before -- but that wasn't difficult and I now have a darn good robot doing the heavy lifting of steering the plane, keeping it level, and holding altitude, just like it was supposed to be doing all along!!

Having a working A/P at last is not only glorious, it's also a huge stress-reducing safety advantage. In case of inadvertent flight into IMC, the A/P can be engaged to keep the wings level and spatial disorientation at bay, then take the plane wherever it needs to go. Similarly, in case of pilot incapacitation, with just a little bit of advance training and bit more help from ATC, even a non-pilot co-pilot still has a very good chance of getting the plane down onto an airport runway and surviving, and that's another "very good thing".

__________________________

[* Back in December, 2010, this A/P was sent to TruTrak where it underwent a minor conversion -- new faceplate and a couple minor compatibility function changes -- to become an AFS-Pilot, branded by Advanced Flight Systems, makers of the EFIS's in this Rebel. However, in reality it's still a TruTrak Digiflight II through and through and TruTrak still handles all support.]

no pressure to nirvana

ATTN: To all the other gravity-feed-only-reliability-simplicity-purists, like me, especially those who may be using throttle body "carbs" (Aerocarb, Rotec TBI, etc.), also like me:

If you have grappled with "lively" EGT's on takeoff even after setting full mixture, if you have pulled throttle back on takeoff earlier than you'd really like to, if you have pulled the carb heat ON too many times to thin the air thus enriching the mixture to control EGT's at takeoff, or at cruise RPM's & up, if you have attributed your inability to get the mixture rich enough only to the effects of air turbulence at the intake, but have also tried vanes and grids, smooth ducts and large volume intake air plenums, etc. -- making that air undoubtedly as smooth as the proverbial baby's bottom -- but still had "issues" with mixture, EGT numbers and balance, if you have felt an undeniable urge to sand down or reshape the thin, high RPM end of your Aerocarb needle, or enlarge the high RPM end of your Rotec spray bar holes just a smidge with micro drills... if all this sort of foolin' around makes you wonder if the 'ole automagic mixture Bing carb might not be so bad after all, then you might want to reconsider your fuel pressure. (Not that I actually know this from personal experience, but...)
 
Of course, as those in this select group already know, both the Aerocarb and Rotec TBI throttle bodies specify a minimum 0.5 PSI fuel pressure and, if a lot of the above things have occupied too much of your time, attention (inside the window instead of out), and/or at times caused low level stress (or worse), and if you're not keen, savvy, or confident enough to actually measure such small pressures, and your EFIS-bound fuel pressure instrument refuses to be bothered with small decimals... then you might want to actually calculate the best case, theoretical head pressure between your tank outlet and the carb/throttle body inlet, before line losses -- especially if you have been making the casual but egregiously erroneous assumption that with a fire-hydrant-worthy flow rate at the carb inlet there must be adequate pressure. (Not that anyone would ever assume that, but...) Wrong! Flow and pressure are two very different parameters. Related, but different.
 
According to authoritative-looking places on the Splinternet, it takes a 27.72" high water column (WC) to create 1 PSI at the bottom. However, gasoline has only 73.9% of the density of water. (Avgas may be a smidge different for all I know, but we're just looking at the big picture here.) Now if you put your plane in an up attitude which approximates a decent climb rate (say 10-11% for a lot of aeroplanes, and a taildragger's normal ground attitude) and you measure the actual height between your tank outlet and the carb inlet -- shooting a laser level line across a horizontal reference level that's under the tank outlet and under the carb to measure from works well enough -- the ridiculously simple formula for fuel pressure at the carb goes like this, where "H" is the height of the tank outlet above carb inlet:
 
(H in. /  27.72 in.)  x  0.739 = your theoretical, best case PSI at the carb [before line losses].
 
Example: If your H is only 17 in. (it can happen if your inlet is oriented on top) you divide that by 27.72 and get  0.613, which would be the theoretical PSI at the carb inlet if you had water in your tank. (Let's not go there.) Then you multiply that by 0.739 and you get 0.453 PSI of gas at your inlet -- before any line losses. As previously noted, that's not even supposed to be enough for the aforementioned carbs to work properly. Now, I'm no good at figuring out pressure losses in the real world of real plumbing, but if you want to allow, say, at least a 20% margin for line losses, higher flight attitudes, and rounding errors, you'd need close to 23" of head pressure -- at all flight attitudes, including slow flight -- to be sure the carb was getting 0.5 PSI from gravity alone. Of course, slow flight could entail a 15-18 deg. up attitude and much lower head pressure, not 10-11 deg., so even a 20% margin may not be enough.
 
If that example is close to your real world situation, and you add a Facet 40104 2.5 - 4.0 PSI solid state, low current, electric "cube" pump, you will reach mixture control nirvana in no time. No more early throttling back on takeoff, no more carb heat to manage EGT's, average ~50F EGT spreads at cruise, etc. It happens, and when it does, it's "a very good thing". If you were perspicacious enough to have planned for the possibility of such a pump in the first place, and left space for it, maybe even pre-drilled mounting holes for it, then the pump installation will go relatively smoothly (i.e., require only a couple of days of work, spread over 2-3 separate UPS deliveries) and might end up looking something like this:
 
 
[No one will ever suspect this installation is in 254R, thank goodness, because of the obvious use of an airframe ground -- a never-before-done absolute no-no. I have standards. Except it turned out to be really easy and, because the forest of ground tabs is less than 2 ft. away, the airframe ground had an even lower resistance than an appropriately sized wire.]
 
If you also had the foresight to leave a tiny dimple in the metal of your controls sub-panel at precisely the mid-point of the last open space for a switch, and can thus easily drill it out to mount a protected-action switch to control the pump, right where such a switch should be, you might drill that dimple out to switch size after first protecting the paint with tape:
 
 
 
If all goes well, according to your ancient, almost forgotten, so-called plan, you will also be nearly ecstatic to find that the addition of the pump hardware doesn't significantly reduce flow rates and, in the unlikely event of a pump failure, you can still manage the engine to get where you need to go, just as you did in the past. Woot!
__________________________

ADDENDUM: Much of the above recitation of trials and tribulations has been somewhat exaggerated for dramatic effect and, if you've diligently managed the EGT temps on the engine all the while (even if "while" includes criss-crossing the continent) then the engine, though sometimes thirsty, has probably not been abused, much less damaged. The only real downsides have been that A) some of the time you've been operating with 10-15% less power than you could have (If you'd always used cooler, non-carb heat outside intake air, for example), B) you've spent more time looking at the panel than you should, and C) you've maybe felt some of that low-level stress from time to time. And though you will be forever chagrined, dismayed, embarassed and humbled that you didn't figure this out earlier, at last you shall have complete mixture control throughout the whole operating range of the engine, just like it's supposed to work. Other than that, no biggie.

electricals redux

Every once in awhile, I am reminded of things that are probably more important than others, which sometimes leads to updates of previously posted information here. I came across one of those recently, an entry in the Library section of the site regarding Bob Nuckolls' book and philosophies of aircraft electrical design, and felt compelled to update it, for emphasis. If you don't know Bob Nuckolls' work backward and forward before designing or building your airplane's electrical system -- or even before reviewing or understanding the work of someone else you've hired to do it for you -- that would be a mistake, perhaps seen later in hindsight as a serious one. Read more, here.

plenitude

All carburetors need a smooth airflow coming in, and (though technically not carburetors) the throttle body injectors are no exception. With both throttle bodies used on 254R -- the AeroCarb and the Rotec TBI-40S -- various techniques have been tried, with varying degrees of success, to reduce or eliminate turbulence at the throttle body intake. The principal method of achieving this is to provide a large volume airspace upstream from the throttle body, and avoid tight-turning tubing, rough edges on flow path transitions, etc. Though not perfect, the fiberglass "blob" plenum served reasonably well upstream of the TBI-40S for quite awhile, including to Oshkosh '11 and back. Fiberglass was chosen to create a free-form shape to make the most of the available space & volume between the TBI and firewall. Its large but odd shape worked better than low volume round SCAT ducting, but who knows what kind of odd vortexes and standing waves were created inside such a blob? When the engine mount was changed, the blob no longer fit, and couldn't be easily modified enough.

Next, crossing "X" vanes were used inside a standard silicone duct, with initially great results. Unfortunately, that was short-lived, because atomized fuel distribution in the downstream intake manifold is highly dependent on the radial position of the TBI's spray bar, and the effect of minute changes there is complicated by slight changes in the radial position of the upstream vanes. The odds of getting and keeping them both in an optimal orientation for operation across the engine's power band are slim. In short, it wasn't long before they got out of whack with the result being the subtle rough running at higher RPM that is symptomatic of airflow turbulence in the throat of the throttle body. Such a sound is particularly grating when coming from an inherently smooth-running 6-cyl. engine. So what to do?

Make another plenum. This time, an aluminum conical shape. The new plenum project, which took several days overall, began with mass production of small angle clips to attach the flat ends of the cone, with Whitney punch at the ready. Even though it looks simple, it is possible to screw this up several different ways. (Don't ask.) I wanted the round end plates to go inside the cone to maintain its circularity.

The business end of the conical plenum is the small-end plate which bolts onto the throttle body. Layout started at the center point of the 2.5" cutout, and positioning of the four bolt holes on 2.5" centers. The outer circumference was drawn with a 5" diameter -- roughly the largest circle that would fit between engine mount legs at the entry plane of the throttle body. After cutting the center and bolt holes, the outside circle was cut last. In spite of best attempts to standardize the tabs, in the absence of some high falutin' CNC machine, they do differ and have to be carefully marked, matched, and their positions tracked and maintained, early and often. Of course, the corners of all those angle clips have been rounded smooth for stress relief, like any other part in the plane.

After forming the cone with an 8" diameter bottom (rear) plate, work began on the mold for a fiberglass collar to hold the 2.25" x .049" intake tubing stub. I used common modeling clay to both hold the tubing and shape the transition, as seen in the next two photos. It still took me nearly two hours just to form the collar the way I wanted it. Those kindergarten skills are rusty.

After coating the tubing, clay, and cone surface with mold release agent, fiberglass layup began.

As the fiberglass cured, the rough edges were cut back, and the new plenum started to take its final shape.

The next task was to make an access port to reach the bolts holding the small end onto the TBI. That started with a doubler, which was used to mark the hole for the port from the inside circle, and the cover plate or door, from the outside circle. (Eventually, this shape was enlarged more than seen in this first edition.)

The next two photos of the interior show priming of mating surfaces on the tabs, the smooth fairing of the input tube stub entry, 3M firewall sealant around the small end plate and tabs, and nutplates for the access port.

Eventually, machining, forming, and assembly were complete.

The first access port proved to be too small. After enlarging the access port by scooping out the rear edge, the exterior was coated with Everbrite, and the interior swabbed with Corrosion-X. The completed conical plenum is shown below bolted to the TBI-40S, with only a gasket between, from upper and lower right side views.

First ground runs indicate this may be the best anti-turbulence mod yet. In ground tests, the mill runs smoothly and EGT spreads are a mere 50°F, at least up through 1600 RPM. That's awesome. Flight tests will follow soon.

As usual, every piece and process had to be done more than once, and this project took several full days. Really. So there wasn't much point in putting tools, materials & supplies away until it was actually done. (That's my theory and I'm stickin' to it.) All the rivets, screws, nibblers, coatings, sandpaper, etc., are right there... somewhere.

 

locking knob & latch exposed

At the request of another builder, I've put together a page with a full series of new annotated photos showing the design and installation of the left side locking knob door latch system. (Of course, earlier photos can be found using the Search function to locate items with "latch" etc. in the title or description.)

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