Great and interesting report, Steve.  Thanks.

 

I had not thought about how different it might seem to operate near the ground at a much lower density altitude – aircraft performance on take off probably felt considerably stronger than at 7000 MLS {:>).

 

My best performance with the 2.17:1 and 68x72 prop was 196 MPH TAS at 5500 MSL (did not have my fuel flow meter at the time).

 

My best performance with the current 2.85 and 74x88 prop was 200 MPH TAS 12.3 GPH burn rate at 9500 MSL.  Pulling back to 10.3 GPH gave 190 MPH TAS.  But, I normally cruise at 7.5-8.5 GPH at 140-150 IAS  – just too cheap to go anywhere in a big hurry {:>).

 

Interesting finding on your EC2 and the manifold pressure sensors.  If Motorola indicated a filter was good thing to do, they undoubtedly had results indicating so.  I noticed that at the staging point my air/fuel indicator would run very quickly from rich to off the scale lean.  I could never figure out why it would do that.  I thought it might have to do with no fuel in the secondary runners before staging and it taking a few milliseconds to get the fuel mixture down to the combustion chamber – but the “glitch” was much longer than that would account for and my secondaries are about as close as you can get them to the ports.  I ended up enriching the mixture (considerably) around the staging point and that “solved” the problem for me.  Well, it really didn’t solve the problem since I could not figure out what it was – but it makes the response acceptable.

 

Sounds like many folks encounter that problem and you just may have found cause of the problem and a fix for it.  Thanks for the report on it.  I agree with your recommendation for folks not to go attempting to modify their EC2 – even if you are successful in installing the filter components, there is always the possibility of unintended consequences occurring else where in the system as a results.  Always a risk involved.

 

Thanks again for the very interesting report.

 

 

To anyone interested:

 

The following is a response to Doug’s recent request for updates on flying rotaries:

 

In early June, having recovered sufficient courage to try another cross country trip after feeding my prop to my tow bar (yes Ed, I still have the tow bar) in Minnesota, we finally set out on another one in our 13B powered RV-6A, this time to the west.  After squeezing between the top of the controlled airspace and a cloud layer over Great Salt Lake, negotiating a narrow corridor which happened to be  populated with scattered rain showers between restricted airspaces, and a fuel stop in Wendover, UT, we crossed Mono Lake, the Sierra Mountains, and Yosemite Park at 14500 ft with 50 knot headwinds.  Thumbing my nose at the concept of shock cooling, we descended over 11,000 ft in 25 miles and landed at our destination of Mariposa-Yosemite airport.  Other than a little initial roughness on bringing the power back up in the pattern after that descent, probably due to some lead fowling of the plugs since 100 LL was the only suitable fuel available at Wendover, the engine installation behaved flawlessly.  This is in contrast to the performance of the pilot with the different sight picture at the slower ground speeds at lower density altitude than I ever see at home in Laramie, WY, and the inclined runway surrounded by hills at MPI.  Thankfully, those details are not rotary related however, and need not be expanded upon further.

 

The friends we stayed with near Mariposa built an award-winning RV-6 with an O-360 and CS prop.  While we were there, we took a couple of side trips with the planes which gave the opportunity for some side by side comparisons.  Their take off and climb performance was much more impressive than ours which was not surprising since our Performance Propellers fixed pitch prop is set up primarily for cruise rather than climb.  On one trip, after flying a mile out over the Pacific at Monterrey Bay (just to say we did it)  I set up the plane at max power and made the flight back to MPI under these conditions.  Altitude was 5500 ft, fuel burn was 14.8-15 gal/hr, MAP was 23.8”, RPM was 5900, and IAS was 148 knots.  I don’t know what the OAT was, but it wasn’t out of the ordinary.  Our friends reported burning close to 9 gal/hr while flying alongside us during this time.  I was somewhat surprised by this and one thing in particular came to mind: parasite drag.  The attached picture was taken during this flight and two sources of drag can be readily seen: the large and sharp angled cooling air outlet, and the “bomb” which is a muffler.

 

The trip home was relatively uneventful and involved a detour over Mammoth Lakes, CA to allow more time to climb and a lower altitude for crossing the Sierras.  At the fuel stop at Wendover, I discovered a crack in one of the braces between the rear of the engine and the exhaust header, but this wasn’t a particular cause for worry since I had flown for more than 40 hrs before even installing the two of those.  Since we did not have headwinds on the return trip, the total flight time home was an hour less than the flight time out.  The landing in Laramie was in conditions not uncommon to this area, landing on runway 21 with the ASOS reporting winds from 250 degrees at 22 gusting to 30.  Thankfully, I had completed a flight review using our plane just a few days before starting the trip and crosswind technique was one of the things developed.  At least this was the last stop because the seat was definitely at risk of being soiled.

 

After returning home, a drag reduction attempt was made by cutting away much of the bottom of the lower cowling and reshaping the cooling air outlet to be much more similar to the original shape as supplied by Van’s.  This cut the outlet area approximately in half to 77 sq in which is still larger than that in the unmodified original cowling.  This configuration is shown in another attached photo.  Tuft testing showed no turbulent areas except for one tuft right behind the exhaust header outlet.  Test flights at max power were conducted with and without the “bomb”. 

     Without the muffler:  163 knot TAS at 11500 ft DA, 12.4 gal/hr fuel flow, 5800 rpm, 19.9” MAP

         With the muffler:   160 knot TAS at 11500 ft DA, 12.2 gal/hr fuel flow, 5750 rpm. 20.2” MAP

The 3 knot penalty is a small price to pay since the noise level without the muffler is unbearable for any length of time.

Cooling is still satisfactory but under cowl temperatures are higher (~160 deg F) than before as would be expected with decreased cooling air mass flow.  An enclosure was constructed around the stock Mazda ignition coils and a blast tube installed from the oil cooler air inlet to this enclosure which keeps the coils at less than 110 deg F. 

 

Several other changes have been made since first flight over two years ago:

 

One is the elimination of the cowling combustion air inlet (snorkel) and filtered air box as originally supplied by Van’s.  A NACA duct inlet and air filter was installed in the cowling side very near the throttle body.  This eliminated the over 3 foot long 3.25 inch diameter skeet duct from the air box to the throttle body.  This was done to avoid the heating of the air while inside this duct which resulted in temperatures of the air entering the throttle body in excess of 110 degrees F in flight regardless of the OAT.  Now, the air entering the throttle body remains within 2 degrees F of the OAT.  The result is a lower Density altitude seen by the engine.  

    

Another change was the elimination of the vacuum system which included the DG, AI, Mazda smog pump used as the vacuum pump, vacuum regulator, and associated tubes and filters.  In place of the smog pump, a second alternator was installed.  The second alternator led me to eliminate the second PC680 battery.  The instrumentation was replaced by a Dynon D10A EFIS.  A net weight decrease of 19 lb was realized by these changes.

    

One other change that I made before the CA trip involved problems I had with tuning the system to transition the injector staging point without hesitation or misfiring of the engine.  This had always been of only slight concern until during one flight when advancing the throttle after a stall, the engine hung up and didn’t produce full power for almost 10 seconds.  As best as I can remember since some sense of panic ensued, first backing off the throttle position significantly from full did not clear the condition, but tweaking the mixture control did.   The behavior seemed somewhat similar to SAG as described by others on this list, but I could not correlate the event to spark plug condition.  Further attempts at tuning the mixture table in the staging region were unsuccessful at eliminating this behavior.  As also described as having been tried by others on this list, I tried to pneumatically filter the manifold pressure input to the EC2 using fuel filters and various sized orifices in the lines between the plenum and EC2.  It was possible to induce surging with orifices of too small opening, but the staging difficulty was not improved with various combinations of reservoir and orifice size.  During these tests, I found that by proper (improper?) manipulation of the throttle, I could repeatedly induce the extended lack of power increase on injector staging.  Data recorded by my monitoring system clearly showed the rpm plateau persisting while changing manifold pressure with the throttle, average fuel flow consistent with the associated MAP at any given instant,  and a lean condition as shown by both O2 sensors.  Since the fuel flow was reasonable, the excess O2 could result from ineffective ignition:  fuel was there, but not being burned and using up the O2.  No faults with the ignition system could be found by tests including among others checking the ignition timing and using new spark plugs and wires.  Since the condition could be induced by changing manifold pressure, I postulated that the manifold pressure as seen by the EC2 may be unstable or unreliable.  Searching the Motorola literature on the sensors used in the EC2, I found a reference to additional components recommended to be installed on the sensor output if the output is connected to an A/D converter.  In desperation, I installed these components into my EC2 and immediately the misbehavior on staging disappeared.  I have not been able to induce it again no matter how hard I try.  My theory is that under certain conditions, the MAP sensor output goes into oscillation which may or may not persist depending on the circumstances.  I have not verified this by bringing the output signals external to the EC2 to an oscilloscope since if they are subject to instability, doing so may provide more questions than answers.  I have pretty much kept this under my hat to this point, but enough time on this change has been accumulated that I am comfortable that the change is not detrimental at least.  I certainly do NOT recommend or encourage changes to the EC2 without Tracy’s input without fully accepting the risks.  I have no proof that my interpretation of the events related above is correct, simply that it helped in my individual case.            

 

I had hoped to make it to the roundup this year, having bought the charts, planned the flight and arranging time off.  The front that Bill managed to cross was too much to try to take on from this far away.  So here I sit watching the blowing snow outside wishing that I had made it to the roundup and that the front had materialized after getting there and thus preventing my return.

 

One additional note for those who made it this far.  I’ve been spending considerable time in our local ice cream parlor in an attempt to maintain a constant system in-flight gross weight after the elimination of the hardware as described above. 

 

OOOOH the sacrifices one has to make in the name of research….  

 

Steve Boese

RV-6A, 1986 13B NA, EC2, RD1A …

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