Everyone has a view point concerning cooling (at least one {:>)).  Once you get a high performance aircraft fairly clean drag-wise the largest remaining drag is generally the cooling system.

 

Ideally, you choose some regime of flight to optimize your cooling system for.  A bush pilot might opt to optimize for slow and low and high power settings.  A high altitude cruise type might optimize the cooling system for that regime.  Which ever you do its always a compromise of various factors.

 

In the many experiments with my ducting system over the past 10 years,  I believe I have arrived at (for me) fairly close to the optimum.  The reason I draw this conclusion is that at 7500 MSL and max throttle (12-14 GPH fuel flow) my cooling and oil temps increase until they are less than 5 deg below my max desired temps (200F for both). Any more power and I would encounter a cooling deficit - any more cooling and I will be carrying more drag than necessary. Now, while I do believe I am close to the optimum for my prefer operating conditions, I do acknowledge that there is a price to pay on the low end.

 

From take off roll until attaining an airspeed of 120 MPH IAS I run a cooling deficit and my coolant may hit 210-220F and oil 200F for those few minutes.  After hitting 120, I start to catch up on temperature curve and generally within 5 minutes things are stabilized around 170-185 deg for both.  I fly without a thermostat and as a consequence can see much lower temps in cruise.  In fact coming home from the Rotary Round up with OAT showing 53F and burning 8 gph my temps were below 160F.

 

My total radiator intake area is 36 sq inch with another 22 sq inch for the oil cooler.  My exit opening is rather large and I undoubtedly get a bit of drag with it – around 80 sq inches plus louvers along both sides of the cowl. 

 

I could  only do a 7 gph ground run up on a 70 F day for approx 2-3 minutes before temps start getting close to 200F.  I can taxi all day without any problem on temps. 

 

 

The flood gates are open again….

 

In response to Tracy’s question about trying the B controller, It was my experience that the B controller always worked better than the A.  By programming the B controller directly, the staging glitch could be essentially eliminated.  After programming the A controller as best as I could and copying those parameters to B, the glitch would appear in the B controller but not nearly as pronounced as when the same parameters were used with the A controller.  I could never get the glitch to persist with the B controller.  I think this makes sense considering the information provided by Tracy and Ed.  Enriching the mixture upon staging may have minimized the problem that was happening with the A controller, but mixture may still have been too rich during staging when used with the stable B controller.

 

I have attached an image of the data logged during a prolonged staging glitch.  Also included is an image of the staging region of the mixture map used by my modified system where no staging glitch occurs.  The map is unique to my system, of course, since many variables are involved in developing a good mixture map.  My system may be simpler than some in that all four injectors are the same part number and well matched.  From previous investigations, I know the limits of the linear range of the injector fuel delivery vs pulse width and I have increased the MAP where staging occurs so the injectors are operating within this linear range.  With my setup, it has been possible to compensate for operating the injectors at shorter pulse widths than the linear range to some extent by programming the mixture table, but this seems to be operating on the ragged edge of injector reliability and the effectiveness of this compensation seems variable from day to day.  In my opinion, there have been no problems associated with having the injector staging point at MAP values closer to where long term operation of the engine occurs.  I’ve collected data where the controller was changing rapidly between unstaged and staged operation and the only way of knowing this was to examine the data after the fact.  The engine ran smoothly the whole time.     

 

Based on the data collected and shown in the attached image, I’m not even sure that the glitch has anything to do with staging or even the mixture delivered to the engine.  My fuel flow indicator integrates the primary injector pulse time and doubles that when the secondaries are being used.  With 4 well matched injectors all operating in the linear region, the fuel flow indication should be reliable.  There is no fuel flow anomaly seen in the data.  The only deviations from expected behavior are the lack of rpm increase with MAP and the O2 sensors going lean (which are an indication of what is coming out of the engine- not what’s going in).  Is the whole fuel delivery system capable of following an unstable MAP indication so quickly that individual injector pulses are so lean or rich from one chamber to the next that misfiring occurs but the average fuel delivery rate is still correct?   The bottom line is that the main flaw remaining in my system is my curiosity.  Why should I care what the problem was as long as it’s gone and stays away?  I’m working on that.

 

In response to Tracy’s comments on cooling drag and airspeeds:

 

During the last month, I changed part of the fuel system and eliminated the fuel transfer system with its associated switches, timer and electric fuel pump.  That was replaced by a duplex fuel valve that I made (I couldn’t handle the price of Andair’s duplex valve).  Now I can just operate the valve and take fuel from either tank and return it to the same tank with no electrical requirement.  The change in operational procedure is minimal: change the valve position periodically instead of pushing a button, but decreased parts count and failure modes are an improvement, I hope.  Ed’s recent experience reinforces my inclination toward two independent fuel sources.  At least it’s no longer possible to transfer fuel out the right tank’s vent, not that I’ve ever done that, of course.

 

I exercised the fuel valve in the shop (the screen, pumps and filters all easily come out of the plane by taking out 4 bolts and disconnecting the fuel lines and electrical wires) and verified its operation even under conditions that should never happen in the plane.  The system passed all the tests I could think of.  After reinstalling the fuel module, I tied the plane down and made a ground run for 40 minutes at low cruise power settings (7gal/hr fuel flow), changing the valve position every 5 minutes.  Again everything worked fine. 

 

The ground run was also a test of alternator out flight endurance.  With the alternators off, the battery voltage was 10 volts after 40 minutes and the engine was still running fine.  This was with only the engine essential electrical systems powered up.  I should have made that test a long time ago instead of worrying every time I flew, not knowing how long I could maintain altitude in the event of alternator failure.  The recent discovery of Steve Fossett’s final destination, and realizing that on our return trip from CA we flew within a few miles of that place if not directly over it, finally prompted me do this endurance test.  I’m quite sure that the time, effort, and expense of the search for Steve Fossett would not have been expended looking for Steve Boese.

 

What does this have to do cooling drag and with airspeed?   I’ve really never had problems from insufficient cooling.  If I can operate my plane tied down on the ground for 40 minutes with an OAT of close to 70 deg F while burning 7 gal/hr of fuel and not overheat, it seems quite possible that the cooling system still may not be optimized.  I’ve always had a thermostat installed in the water pump housing which prevents the coolant temperatures from going too low. 

In flight at higher power settings and moderate temperatures, the thermostat is fully open, however, so I am not sure the airflow through the cowling is excessive.  The inlets to the coolant radiators are unmodified from the original cowling configuration as supplied by Van’s.  My instrumentation shows that I am recovering over 90% of pitot tube pressure at the coolant heat exchanger faces.  The inlet to the oil cooler is the same area as one of the original Van’s inlets and the new duct to the oil cooler is long enough and shaped such that it seems likely that internal pressure recovery is occurring in it.  The coolant radiators are so close to the air inlets that the duct shapes are definitely not desirable and internal pressure recovery in them is unlikely.  Since I have good pressure recovery, this must be happening in front of the inlet opening.  Is this what is meant by external diffusion?  Is a bubble of high pressure air sitting at the front of the cowling while in flight?  If so, then the sensation of coming up against a wall while accelerating in flight is more accurate than I thought.  The wall is just transparent. 

 

The change in the cowling shape and reduction of the outlet area resulted in maybe over 5 knots increase in IAS but not more than 10.  That increase might be substantial at higher airspeeds, but where I am stuck- probably not.  Also, the fact that I can remove the huge “bomb” and only gain 3 knots at most seems to indicate a high degree of drag elsewhere and not just in the region of the cooling air outlet.  Other than having an oil cooler inlet slightly larger than the original Van’s snorkel, the increased outlet area, and the ‘bomb”, the  external configuration of my plane is standard RV-6A and I have been told that my construction is as good as many others.  The concept of reaching a critical point in drag reduction is a new one to me.  In that context, my efforts and lack of positive results start to make some sense. 

 

Tracy, your analysis is more than likely correct (as usual)  and I am thankful for your input.

 

Steve

      

 

-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Tracy Crook
Sent: Thursday, October 23, 2008 10:00 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: N613BX update

 

 

__________ Information from ESET NOD32 Antivirus, version of virus signature database 3267 (20080714) __________

The message was checked by ESET NOD32 Antivirus.

http://www.eset.com