PAP Section G6-1: Dynamically balancing the propeller

After syncing the carbs I was less than pleased with the amount of vibration present, especially at low RPM. This being the case, I decided to deviate from my previous plan and do the DynaVibe deal before first flight.  I had planned to put this off until after Phase 1 flight test, as many people do, and fly the plane over to Akron, CO to a shop where others I know had done this with satisfactory results. 

The Mothership recommends doing it before first flight, however.  A member of my local EAA chapter 648, Rick Hall, owns the most expensive version of the DynaVibe balancer and offers it, along with his expertise, free of charge to chapter members (he did accept a bottle of Beefeater gin).

Using this device requires attaching three things to the engine: an accelerometer, preferably as close to the engine centerline and as near the prop as possible (shown here nearest the prop hub with the single black wire), a piece of special reflective tape to the back of one prop blade, and a laser pulse counter (the yellow device to the right).  The laser and reflective tape measure prop RPM.  The tach in the airplane shows engine RPM.

The engine is run at low RPM (2500 in the case of a Rotax 912), then up to cruise power (5200 with the airplane static, depending on how you pitched the prop) where the engine will live most of its life.  The DynaVibe then gives an angular location for the temporary stick-on balance weight and the mass of the weight.  The DynaVibe assumes the weight will be on the perimeter of the rear bulkhead for the spinner.  If for some reason the weight must be attached at a different radial location, a simple calculation allows the new mass value to be determined (r1 * m1 = r2 * m2).

I tied the tail to Rick's pickup truck since I'd had the airplane jump the chocks on a previous full-power run-up.  When that happened I feared I had set the prop pitch too fine, thereby producing too much static thrust.  My fears were allayed when I saw the correct static RPM (5200) at rull power.  On one full-power run I did throw off one of the stick-on weights, heard it hit something over the roar of the engine, but could never find it.

This process stretched over two days (due to starting mid-afternoon) but could have been done in two hours if I had been prepared.  I was not familiar with where and how the brackets for the accelerometer and laser attached to the engine.  Turns out I needed two M8 bolts which I didn't have.  A trip to the aviation aisle at a nearby Home Depot solved that problem but wasted time.

The picture at right shows the temporary stick-on weights on the aft face of the spinner bulkhead.  It was necessary to remove the paint to keep them attached at high RPM.

Van's warns against succumbing to the temptation to attach the permanent weights to the existing screws which attach the spinner to the bulkhead.  Instead, a new hole should be drilled in the bulkhead face and bolts and washers should be used to match the weight of the temporary ones.  Being an anal engineer, I calculated the weight of the aluminum removed by the drill (0.1 grams) and ground the required metal off the washers to get the matching weight.  The required weight was 13.1 grams (yeah, I know that grams is a unit of mass, not weight, but irritating though it may be they express weight in grams in Europe where the Rotax is made).

I was initially worried that the existing nut plate or rivets would interfere with the bolt head or nut of the permanent weight, but it turned out to not be a problem.

Shown first is the aft face of the spinner bulkhead, then below it the forward face.  I used large diameter washers for the AN3 bolt to keep the stack short.  Ideally, the DynaVibe would be used again to check the final balance with the permanent weights but this wasn't done.  I'm confident it's good.

PAP Section G-6 - Maintenance manual page 12-7: Carburetor Synchronization

When I sold my Honda CB-750 motorcycle I thought I'd never have to sync carbs again (that ill-handling monster had four carbs).  At least the Rotax only has two. The idea here is to ensure that the two cylinders on each side of the engine see the same throttle opening across the rev range.  This is done by starting with the throttle arm on each carb 0.004 inches from the mechanical stop using a feeler gage, then opening each carb 1.5 turns of the adjuster (clockwise).  The lock nut on each arm is then screwed down, attaching the arms to the two cables.  The two cables merge into one, which is the throttle in the cockpit.  Idle is now set for each carb and can't be adjusted individually with the throttle-stop screws once the cables are locked to the arms.  Adjusting the throttle plate opening of one carb relative to the other must be done with the ferrule adjusters on each cable.  The following didn't dawn on me initially and I learned it the hard way: the friction lock on the throttle plunger in the cockpit must be tightened down before adjustments are made to the ferrules.  This prevents relative motion between each cable and its sheath.  The ferrule only attaches to the sheath, but the cable and sheath must move as one at each carb during ferrule adjustment.  Otherwise it'll screw up the 2.75 inch plunger travel needed in the cockpit to ensure the achievement of full throttle on each carb with the plunger full forward. Francis Miller, who administered my private pilot check ride back in 1969, told me that the origin of the expression "balls to the wall" came from this: propeller, mixture and throttle "balls" all the way to the instrument panel.  I thought it meant something else.  Also, the RV-12 only has one ball.  Returning now to the topic at hand, the Rotax setup is different from anything I've seen in that the carbs are spring-loaded wide open.  The throttle cables allow the springs to open the throttles or pull them closed, a safety feature, I guess, in the event of cable breakage.

In the pic, the ferrule adjuster can seen on the cable at left, the lock nut which secures the cable to the arm on the right, and the throttle idle stop (with orange torque seal) just to the right and below.

The carbs are now mechanically synced and must be tested over the rev range, with adjustments made to the ferrules as needed.  This is accomplished by measuring the manifold vacuum on each side and making ferrule adjustments as needed to balance them.  The vacuum readings can be obtained with two individual gages or a single differential pressure gage (the bast way).

A couple of years ago when I was insanely optimistic about when I'd have the engine running I bought a kit from Aircraft Spruce which included two vacuum gages (which I later found for $20 each at Harbor Freight) and some tubing which was supposed to facilitate hooking everything up to a Rotax.  As I recall I paid a few hundred bucks for this.  Both gages were faulty, but I found them on line under a different name, still $20 bucks.  This rig would have worked, with poor accuracy, but probably good enough.

Fortunately, I was able to borrow a CarbMate, which uses an electronic differential pressure transducer (shown below).  Adjustments are made until the light is centered.  

After some puzzling attempts at a balance I started to doubt the device (couldn't be me, right?) so I rigged up a large syringe (below) hooked up to each leg of first the analog gages and then the CarbMate, exposing both legs to the same vacuum.  Both measurement methods passed the test, each analog gage showing the same reading and the CarbMate exactly in the center of its scale.  I went with the CarbMate.

I already had the syringe on hand since I had used it to test the pitot-static system earlier (discussed in a previous post).

With renewed confidence in the measurement system, I hooked it up to the Rotax.  The intake plenums on each side of the engine are connected by a balance tube which can be disconnected, allowing the two legs of the measurement device to be attached.  Although it's a bit crowded, the pic below shows the hookup.

The two black tubes in the center with brass fittings are the hookup.  The two brass fittings with the teflon thread tape go to the two legs of the CarbMate.  The initial confusing readings were due to a vacuum leak on one of the rubber hoses, easily fixed with a hose clamp.

Once all this was sorted out the process was easy.  Balance is checked at idle and at approximately 3000 rpm, this higher rpm allowing the carbs to get off the idle enrichment circuit.  Mine was perfect at idle (1800 rpm) and at high rpm but showed a slight variation (1/2 of a light on the CarbMate) part way between the two.  I called it good, figuring the engine spends most of its life either near idle or around 5000 rpm.

More Colorado wildlife: I had a few elk in my back yard.

PAP Section G-8: Fuel System Calibration (Dynon)

 In order to calibrate the fuel system in the Dynon software, data points must be taken starting with an empty tank as fuel is added in two-gallon increments until the tank is full.  The software generates a table of gallons vs.voltage output from the rotary resistor hooked to a float which was installed in the tank when it was built (as I recall, construction of the fuel tank was the first thing in the build to earn the characterization Klöster Föken).  This float-type fuel quantity sensor is not to be confused with the mechanical back-up fuel gage (also with a float).  It made sense to do this now since the tank was already empty from the weight-and-balance section.

Step one was to get 20 gallons of ethanol-free mogas in cans. Fortunately, it's available from the FBO here at my home base at KLMO.  So with four five-gallon cans of fuel, I proceeded to fill a two-gallon can and pour it in the tank until all the data points were recorded.  The Dynon looks like this as the table gets populated:

The only problem occurred when I went past 12 gallons.  The system was recording the data points but wouldn't display them.  I tried every way I could think of to scroll down but no go.  I ended up continuing the process, unable to see the voltage values corresponding to the rest of the data points.  When it's all done, you can edit the table and see everything.  The right-most knob then allows scrolling.  The data looks like this:

 

Being an engineer I couldn't help myself so I did a curve fit of the data to be sure there weren't any wonky points.  Looks good.  Amazingly so considering that I filled a two-gallon container ten times, each time eyeballing the fuel level and the two-gallon mark on the container to be sure they aligned.

Next up is the carb sync, coming soon to a browser near you.  In the bigger picture, I've secured a DAR do to the airworthiness inspection but we're dead in the water due to the government shutdown.

PAP Section G7: Weight and balance

In order to perform the initial weight and balance calculations, the airplane must be weighed containing oil and coolant but no fuel.  Since several procedures which came before this required fuel in the tank, the tank must now be drained (an odious task). The build manual suggests removing the gascolator bowl and running the electric fuel pump until all gas is evacuated.  This would be a major PITA, requiring  removal of the lower cowl and four safety-wired bolts.  I opted instead to once again remove the fitting on the gascolator outlet and attaching the device I previously made to facilitate measurement of the fuel flow rate from the electric pump (see previous post).  Worked great with just the upper cowl off.

The build manual says to place 2" blocks under the mains to facilitate leveling of the fuselage, then to adjust tire pressures to fine tune the leveling process.  For the purpose of weighing the three wheels, the blocks make a negligible difference.  On the above pic, LF is the nose wheel and LR and RR are the mains.  I was pleased with the total weight of 735 lbf considering that I have the optional landing light and autopilot servos.  The expected range of values seems to be 735 lbf - 800 lbf.  However, mine is without wheel pants, paint and an interior, other than seat cushions.  I'll certainly add wheel pants and wrap (no paint) later.

For the measurement of the moment arms, however, it's worth leveling.  I used 2x4s (1.5" thick) which was actually a bit too much, requiring a bit of tire pressure adjustment.  I dropped a plumb bob from the wing leading edges just outside of the mains, snapped a carpenter's chalk line, then made the required measurements from that.  Van's datum is arbitrarily 70" forward of the wing leading edge and all moment calculations are relative to this datum.  The empty CG is then easily calculated by recalling something we learned in the first week of Engineering Statics: For a non-accelerating object, the sum of the moments about any point must equal zero.  With my wheel weights (shown on the pic) and arm measurements, my CG is 81.18 inches aft of the datum for the empty airplane.

With the moment arms supplied by Van's for pilot, passenger, baggage and fuel, my no-fuel CG with me in the airplane looks like this:

The CG location is 80.76 inches, well within the allowable range of 80.49 - 84.39 (shown wrong in the notes on my spreadsheet).  As I imagine most people do, I put this in an Excel spreadsheet to make it easy to play around with various fuel and bagage loads.  With 50 lbf of baggage, a 180 lbf passenger and full fuel, I'm behind the aft limit.  I put this spreadsheet on my iPhone and iPad for easy use.

More Colorado Wildlife:  In a first for me, I discovered a half-eaten rattlesnake in my back yard.  The front third and rear third were gone, leaving what I would think would be the best part of the snake for eating, the fat middle third.  I was unaware that any animals dined on rattlesnake, bet it turns out several do.  Number one on the list is coyotes, which I frequently see (and hear at night).  Number two is mountain lions which I've had visit my yard several times (see earlier post).  Third was bobcats, which I've gotten on the trail cam in my yard twice.  Bon Appétit.

I also had a momma bear and cub on my cam behind my house.

PAP Section G6: First engine start

Before the first engine start, a procedure must be followed which purges air bubbles from nooks and crannies, and most notably the valve lifters, throughout the engine.  This is accomplished by removing the spark plugs to allow easy engine rotation, removing the oil return line at the oil tank and providing a clean container to collect any oil which makes it that far, and turning the prop vigorously until a 40 psi reading shows on the pressure readout in the cockpit.  This causes the oil pump to move oil from the tank throughout the engine and back to the now-disconnected tank return line (maybe).  The instructions say that this may take 40 - 60 revolutions of the prop.  As with all things these days, many Ewe-Tube (they're a bunch of sheep, but that's another story) videos exist showing this process.  The best I found is here.

This process can be sped up by capping the oil tank overflow line and pressurizing the oil tank to 10 psi with an air source before turning the prop.  It is claimed that this step is optional and simply speeds things up.  My friend and ace Light Sport mechanic Bill Snodgrass had done this procedure before and had fabricated a rig to facilitate this.  We first tried it without the air pressure and couldn't produce any reading on the oil pressure gage by turning the prop.  With the air pressure, however, we quickly saw an oil pressure of 55 psi and declared it done.  No oil made it to the catch container but Bill said this is normal in his experience.

After the purge, I did a normal "burp" of the system, which is done before every engine start with a Rotax.  This being a dry sump oiling system, oil which leaves the crank shaft, rods and rockers and accumulates in the crank case must be returned to the oil tank.  This is done in a novel way: rather than using a pump the way race cars do, blow-by from the piston rings pressurizes the oil, forcing it back to the oil tank.  After the engine is run and shut down, oil is left in the crank case, making it impossible to check the oil level in the oil tank.  With the engine off and the cap off the oil tank, the prop is rotated in the normal direction until the distinct sound of a flushing toilet is heard, indicating that the oil is now back in the tank and ready to be checked.

Now it's show time.  With my ex-fireman friend Chad Rennicke, complete with fireman's hat, manning the fire extinguisher, I inclined my head a few degrees and said a silent prayer that I had hooked everything up right -- all the wiring, gas line fittings, oil line fittings -- then turned the ignition key for the first time.  It cranked immediately, oil pressure came up, gages looked good, relief flooded over me, then it quit!  Instantly I knew I had forgotten to turn the fuel valve on.  This done, it fired back up and ran great.  No fire, no smoke, no funny sounds.

Incredibly, after working on this airplane since 2011, I feel for the first time that I'm within sight of the end of the build.  I'm ready to fly.

PAP Section G4: Measuring fuel flow from the electric pump

It was with fear and trepidation that I poured four gallons of fuel into the tank for the first time, exposing the entire system -- all the fittings from the tank to the engine and back to the tank -- to gasoline.  I had tested the tank itself when I first built it back in North Carolina (see earlier post detailing that particular Klöster Föken), but the rest of system had never seen fuel, much less pressurized fuel.  When I first added the fuel, I did so with the fuel valve in the cockpit closed, limiting the gasoline to about half the fittings with gravity. providing the only pressure (~0.5 psi).  All appeared well.  I then opened the cockpit valve.  Seemed OK. When I switched on the pump, however, I immediately had a leak which I eventually traced to the fitting going to the fuel flow meter.  Tightening this seems to have fixed it.

For the actual measurement of the flow rate, the manual says to remove the fitting at the gascolator outlet, slip a 5/16-inch fuel hose over the fitting, turn on the pump and measure the time required to pump one gallon into a gas can at waist level.  The time is not to exceed 180 seconds.  

The first problem with this scheme is that without completely removing the bottom cowl, which involves detaching the oil heat exchanger from the cowl (a major PITA), the fuel fitting must be accessed from above.  I did remove the piano hinge wires from the lower cowl, allowing it to swing down a few inches with the oil cooler still attached.  With the new RV-12s, the oil cooler is no longer attached to the cowl, eliminating this headache.

The second problem is that a 5/16-inch fuel line doesn't come close to fitting.  The male threads on the fitting measure about 0.55 inch, so a 1/2-inch line fits well.  I used a short segment of 1/2-inch, a right-angle fitting, then the rest 5/16-inch.  The right-angle fitting made it possible to hook it up from above.

The time required to pump one gallon was 170 seconds.  Next up is doing the purge process to get the air out of the oil lines and lifters, burp it, and start the engine!

I recently returned from my 36th pilgrimage to Oshkosh.  The big news at the show was the final publication of MOSAIC, the long-awaited update to the rules governing Light Sport aircraft and Sport Pilots.  It's simple now: any aircraft with a clean stall speed, Vs1, of 59 knots or less can be flown by a person holding a Sport Pilot certificate.  All of Van's airplanes with the exception of the RV-10 qualify.  No medical.  Retractable gear with a variable-pitch prop: check.  Wanna fly a 182 or a Stearman?  You're good.

SB-00102 Control Stick Pushrod Inspection

The purpose of this post is twofold: to document that I have performed this Service Bulletin and to prove to my friends following this blog that I'm still alive and still working on the airplane.  The Service Bulletin was brought about by a fatal accident involving an RV-12 which resulted from improper installation of the rod ends on the aileron push tubes.  Over 600 RV-12s are flying so I guess over 599 builders did it right, but Van's says that "out of an abundance of caution" (seems like I heard that phrase a few thousand times back in 2020) the SB was necessary.

The diagram on the left shows the proper installation and the pic at right shows the improper installation on the accident airplane.  What I don't understand is this:  In the accident airplane, the stick on the right was functioning.  It's not much of a reach to put your hand on it from the pilot's seat.

To verify that did it correctly I snaked my borescope in through the cutout for the right stick and took a pic.

The alternative to this is to remove the floor pan, an odious task which involves removing hundreds of Phillips-head machine screws, half of which have the heads boogered up (that's the official machine-shop term) from five or six previous removals.

Back in 1948, Edward Murphy said "If a thing can be done two ways, one of which results in disaster, someone will eventually do it that way." 

Off Topic:

I'm getting ready to move my antique car from North Carolina to Colorado, causing me to look longingly at some pics of it.  What you see is an example of what can happen when your engineering students have too much access to your car.  It can grow teeth.