November 11, 2009
JP Instruments makes the EDM-760 engine analyzer used in the Win-A-Twin. This unit monitors each engine's exhaust gas temperatures, cylinder head temperatures, oil temperatures, fuel flows, fuel burns, bus voltage, and does much more to keep track of the airplane's essential systems. For example, the press of a button is all it takes for the analyzer to detect when a cylinder reaches peak exhaust gas temperature. Shock cooling is also annunciated, and so are the spreads between the highest and lowest cylinders' head temperatures. JP Instruments makes many other models of engine analyzers for singles and twins - each one equally packed with features essential to preserving engine health. For more information, contact them at: JP Instruments, P.O. Box 7033, Huntington Beach, California 92646; 800/345-4574; http://jpinstruments.com.
Aug. 6 - The aviation event known simply as "Oshkosh" (technically correct term: The Experimental Aviation Association's AirVenture) took place last week, and our/your Win-A-Twin Comanche took center stage at AOPA's bright yellow tent display. Literally. You enter the AirVenture's main gate, walk past four displays on the left, and there it was - AOPA's huge display area with the Twin Comanche parked out front.
We had thousands of visitors, eager to gawk and lay hands on the airplane. Many had either earned their multiengine ratings in Twin Comanches, had owned one in the past, or were current owners. In virtually every case, the crowd response was enthusiastic.
What did they like best? The instrument panel topped the list, with its Garmin AT CNX 80 GPS and MX 20 multifunction display (MFD). But another hit was the PS Engineering PAV80 entertainment system and its headrest-mounted DVD display screens. We ran Top Gun, and passersby marveled at the clarity and resolution of the system's Audiovox screens.
The Airtex interior was another big draw. Most associate Airtex with cloth interiors packaged in kits for field installation. So it came as an impressive surprise to see our/your custom-leather interior, installed by Airtex at its home office at the Trenton-Mercer County Airport in New Jersey.
By the end of the show, there were so many visitors that a grassy outline of the airplane was left behind as the airplane was towed away for safekeeping in the face of an impending thunderstorm. Where the guests had trodden, the grass was ground down by all those footsteps.
The next time the airplane goes on display will be at the International Comanche Society's annual convention at the Kansas City-Downtown airport; the convention takes place from September 14 through 19. Be sure to stop by to see the airplane as it continues its refurbishment odyssey.
Finished? No way One of the questions we kept hearing at AirVenture was about the Win-A-Twin's status. Was it done? Well, yes and no. All of the major tasks - engines, props, avionics, paint, and interior - are finished. But there's a to-do list that includes a bunch of squawks and adjustments. Here are a few:
There are some more issues, but believe me, they are small potatoes compared to the flaws revealed during that first cross-country after taking possession of the airplane last October.
Prop balancing, and more... After AirVenture, I flew N204WT to Aviation Resources at its location on the Cumberland, Wisconsin, airport (UBE). There, Jim Barker and his team set to work balancing our/your propellers. Using dynamic prop balancing procedures, it was learned that the left propeller was out of balance by a factor of .315 inches per second (IPS). In other words, the prop disc was wobbling about its axis at a rate of about a third of an inch. That's not much, but it's enough to make the engine and prop vibrate and set up unhealthy stress paths in the propeller blades and engine crankshaft. Abnormally high vibrations can also cause horsepower-robbing friction and abnormal component wear - especially of the alternator mounting brackets and exhaust stacks.
By bolting 29.6 grams of nuts, bolts, and washers to a strategic spot on the crankshaft's flywheel, the vibration level was reduced to .003 IPS. That's a 10-times reduction in vibration levels. Barker took about three hours to do the work, using a special tachometer and vibration sensor, plus proprietary computing equipment.
Then it was on to the right engine, and that's where the value of prop balancing kicked in to the max. Barker saw a .539 IPS out-of-balance condition - indicating unusually rough vibrations. And in fact, you could see the out-of-balance prop by looking at the spinner as the engine ran. The spinner tip wobbled as it spun around at 2,100 rpm during the ground runs used for balancing trials.
Barker's computer said to place a whopping (a relative term in prop balancing) 96.78 grams of weight to correct for the imbalance. From experience, Barker knew that such a large weight recommendation meant that the odds were that the prop would never balance properly. Something was wrong with the right engine's prop setup. Barker found out, and you won't believe what it was.
The prop was installed incorrectly. It was mounted 90 degrees from its proper location. Turns out, you don't just bolt a prop any old place on the crankshaft flange. You use a reference, or index, bushing to align the blades. In this case, the index bushing was aligned with the top dead center mark on the flange.
The prop was removed, reinstalled in the correct position, then dynamically balanced after three ground runs. All went well, and the right prop's final vibration reading was .040 IPS - not bad at all.
Aviation Resources also addressed some other maintenance issues - tightening the right engine's alternator belt and number-two cylinder's upper spark plug and replacing exhaust gaskets on the right engine. The oil was changed on both engines as well.
In all, it turned out to be a great maintenance stop. It was the first time in 45 hours that the cowlings had been removed and the new powerplants and props had been inspected. Guess 50-hour inspections really are good ideas.
And I could tell the vibration levels were way down on the four-hour, 30-minute nonstop flight back home to Frederick, Maryland. For the trivia-minded, I averaged true airspeeds in the 174-knot range at my 9,000-foot cruising altitude, saw sustained groundspeeds at 190 knots, burned 81.9 gallons, and logged 45 minutes of actual in some of the bumpiest clouds I've ever encountered - cumulus leavings of an aggressive cold front that pushed through the Midwest the day before.
Back to (avionics) school Modern avionics are extremely capable - and sometimes difficult to learn to operate. To help the Win-A-Twin's winner learn the CNX 80's (the principal nav unit), the MX 20's (the MFD), and the S-TEC 55X's (the autopilot) ways and use these advanced boxes to their fullest, Avionics Training Unlimited, Inc., of Collegeville, Pennsylvania, will give CNX 80 training.
The training will involve using actual CNX 80s in a structured classroom environment lasting two days. All of this under the one-on-one instruction of Master Flight Instructor Judy Cadmus. It's training that's essential for operating the airplane, and essential to the safety of flight. We'll talk more about the avionics in N204WT in future Web updates and in AOPA Pilot magazine articles.
As always, check back in to this page for more late-breaking news on the Win-A-Twin front. - Thomas A. Horne
Click here to see the milling machine in action.
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