Leaning and Cruise Flight Efficiency
I fly behind a 3100cc Corvair engine in my highly modified KR2S, N56ML, which now has almost 800 hours on it.† Itís not the cleanest KR youíll ever see, since it doesnít even have paint on it yet, and has a lot of rough edges, but I still routinely manage to get fuel economy numbers like 48 miles per gallon while flying at 157 mph true airspeed (TAS), with a top speed of over 190 mph.† And since I burn 93 octane auto fuel almost exclusively, this means I can fly a lot cheaper than I can drive my Volkswagen.† And I donít have to worry about traffic or state troopers, the view is a whole lot nicer, I arrive about 3.5 times quicker, and I never have to stop for lunch!
Flying High - How do I get such good fuel economy?† Itís mostly due to aggressive leaning while at high altitude, and generally keeping a close watch on my air/fuel mixture from takeoff to landing.† I like to fly high on cross-country flights (usually 9500í or 10,500í) because itís smoother and about 30F degrees cooler up there, and the engine can be leaned south of peak EGTs (exhaust gas temperatures) without risking cylinder head or piston damage.† Running lean of peak at altitude lowers EGTs considerably because at altitude the cylinders donít fill as well due to the thinner air, and the inlet temperature is 30F degrees cooler, so detonation is not going to happen.† Also, I can glide about 22 miles with the engine shut down, and radio signals travel at least 200 miles at that kind of altitude.† Why fly low when you can fly high?† Donít get me wrong, flying low and fast around the patch is a real hoot, but thatís for days when Iíve only got a few minutes before sunset and need a serious flying fix.† Iíve been known to drive home with a huge KR grin on my face after one of those flights, and itís a pretty regular occurrence!
Carb Heat - Another trick to great economy is using carburetor heat to increase the carb throat temperature to about 110F degrees, which improves the fuel distribution between cylinder head banks.† Adding carb heat while watching the EGTs shows the EGT spread between cylinders becomes much closer. †As with aggressive leaning, this is not something you want to do at full power and low altitude, because the higher inlet temperatures will increase the likelihood of detonation, and at a time when you can least afford it.† Detonation is the almost instant spontaneous combustion of the air/fuel ratio in the combustion chamber due to higher temperatures, higher pressures, or lower fuel octane rating, rather than the usual controlled burn.† It is more likely at wide open throttle settings, because the cylinders get filled with the maximum amount of air and fuel, increasing the pressure and therefore, the tendency to detonate.† Thereís a pretty good ďhow toĒ on how to build a carb heat box on my website at http://www.n56ml.com/corvair/airbox/ .
Detonation is something Iíve experienced about three times now while flying.† So far Iíve been lucky, and with the help of the EIS and air/fuel meter, recognized it immediately, and in time to mitigate it before it got out of control and damaged things.† William is rightÖyou canít hear it while flying, but you can hear it (if you recognize it) on takeoff if vapor lock has set in and things are too lean to make the engine happy.† Iím familiar with detonation mainly because I keep pushing the envelope on burning 93 octane auto fuel with a high compression ratio.†
The first time I had engine detonation was on an extended ground run in my driveway, before I ever took the airplane to the airport.† For some crazy reason I thought I needed to prove that my engine would run wide open for something like a half an hour, and proceeded to do that.† I was monitoring CHTs, but had the limits set at something like 585F degrees, which is what the GM idiot light is set for (thatís why they call it the idiot light!).† When the alarm went off, it was too late, as Iíd melted the area around the head gasket on cylinder number three, which is the one that runs the hottest on my installation (as well as most others).† I could tell the engine was running rough, and turning the prop over by hand revealed no compression on that cylinder, as well as air escaping between the cylinder and head.† Time to resurface that head!† Fortunately getting the head off a Corvair is about a one hour affair, because itís all out there in the open once the cowling is off, and itís a pretty straightforward job.
So the lesson learned there was to set my CHT alarm at 450F degrees, and donít sit on the ground running wide open for more than a preflight check, or if longer maintain a very close watch on CHTs.† Airplanes with cowlings installed cool a LOT better flying than sitting on the ground, especially slick KRs with small cowling inlets.† My airplane with no cowling installed stays incredibly cool on the ground, but thatís probably because I have plenums that force the propwash down over the cylinders.
The few other times Iíve experienced detonation involved very high ambient temperatures, and delays in getting off the ground.† Because of cowling the small inlet size (which works fine at speed, and enables me to minimize cooling drag), cooling on the ground is marginal on a very hot day.† If Iím flying on a day in the high 90ís, stop in to refuel or pick up a passenger, and shut the engine off for a few minutes, the engine ďheat soaksĒ while itís sitting.† So when I start the engine itís already hot, then taxi out to the end of the runway, and it never really cools off.† Now wait for a plane or two to takeoff or land, and then go full throttle on takeoff, and Iíve just met all the conditions for detonation to occur.† Some details on one of these detonation incidents, see http://www.n56ml.com/flights/detonation/ .
Autofuel - Keep in mind that this is entirely self-inflicted
on my part, since Iím running a 9.4:1 compression ratio, AND only burning 93
octane auto fuel.† The cure is simple
enoughÖeither run 100LL avgas or lower the compression, or both if you like the
belt, suspenders, and parachute approach.†
What Iíve learned to do is to burn a mixture of 25% avgas with auto fuel
when I know Iím going to get into a sticky situation such as takeoff at OSH or
SNF after idling for 20 minutes), and that raises the octane significantly
enough to get me out of the detonation danger zone.† Fortunately for me, flying to
Having said that, let me mention that ďmogasĒ sold at FBOs is only required to be (and almost always is) 87 octane fuel of unknown lineage.† I never buy the stuff, for that reason and the fact that I trust the fuel from the local Raceway more than Iíd trust that 500 gallon tank at the airport that probably never sells any mogas.† 93 octane Raceway is exactly what I put in my fuel injected cars and my airplane, and Iíve never had a problem with it.† I hate to even admit this, but my plane has no means of draining water from the tanks.† I built in fuel sumps and drains, but when the vinylester cured, it somehow sealed them shut.† I tried to open one, and it stayed open and leaked, so I drained the tank and JB Welded it shut the night before my first SNF visit.†
3500 gallons of Raceway fuel later, Iíve not found a single drop of water in my fuel tank or filters.† Call it a calculated risk, but Iím OK with that statistical average regarding 93 octane from your favorite gas station.† Iím not saying do what Iím doing, but just making the point that if youíre worried about the quality of fuel from your local gas station, it might be needless fretting.
Autofuel has other disadvantages, such as nasty chemicals that can damage carburetor parts and composite fuel tanks, but Iíve had no problems with either of those on my plane, and Iíve run about 3500 gallons of auto fuel through it up to now.† I did use vinylester resin to make my fuel tanks, which was created for jobs like fuel compatibility.††
William Wynne will also tell you that another downside of autofuel is that its vapor pressure is higher than avgas, so itís easier to light off a fire with auto fuel vs avgas.† That can be a life or death difference in a crash, given the right (or wrong) circumstances.† I have no fuel in the cockpitÖ itís all out in the wings, so I feel like thatís the mitigation factor in my case.† Unfortunately my fuel IS in the stub wingsÖitíd probably be better in the outer wings from that standpoint, which one hopes would be sheared off in a really bad crash, but not necessarily.
Vapor lock is another gremlin that can sneak up on you.† The setup for vapor lock is very similar to how you set yourself up for detonationÖand vapor lock can bring on detonation easier than you might think, because the last thing you want on a hot day, heat soaked takeoff is to have the mixture lean itself out because the engine canít suck enough fuel!†
I was flying back
from Corvair Wings and Wheels one year in northern
†I started looking around for empty hangars and found one wide open with no planes in it, and a toolbox with a big fan near the workbench.† I told the kid in the FBO I was going to borrow the fan if nobody minded, and he said ďno problemĒ.† I pulled cowling off and aimed the big fan at the engine and started cooling it off.† While I had access, time, and a toolbox at my disposal, I pulled the fuel line off the carb, stuck it in a bucket (thanks dude) and it was just a trickle!
I removed the finest fuel filter and checked it just for grins and found zero contamination.† Then I checked the coarser filter prior to it and found no contamination.†† By the time the was all over the engine was pretty cool, thanks to the big fan blowing on it, so I buttoned it up, quickly taxied out to the runway, and took off again.† This time I watched the air/fuel meter on takeoff and it showed a much richer mixture, so off I went toward home with no other complications.† Vapor lock strikes again!† Experience has now taught me that taking off with a carb temp over about 105F is asking for detonation, at least with autofuel and a 9.4:1 compression ratio!
One thing Iíve done to mitigate vapor lock is to build a return route into my fuel system, so the fuel is constantly circulated through the engine compartment and back to the tank.† I made a small doubled ended AN-6 fitting with an orifice in it that allows the fuel to keep moving, rather than sitting in the line vaporizing due to heat.† Of course Iíve also fire-sleeved all of my fuel and oil hoses, but this fuel recirculation pays.† Thereís more on how I made it, along with many other aspects of my fuel system on my ďfuel systemĒ web page, at http://www.n56ml.com/fuel/.
One way to improve fuel distribution on ďslide carbĒ installations is to add a ďCyclone Fuel SaverĒ.† Hoaky as it sounds, the FuelSaver is a cylindrical gizmo that slides into the intake manifold just downstream from the carb, and before the split in the manifold that leads to each cylinder head bank.† It has twisted-up vanes that impart a swirl and turbulence to the fuel/air charge, thereby making the mixture more homogeneous by the time it reaches the split, so the mixture going to each bank is more uniform.† And it works!† It brought my EGT spread from about 100F degrees down to about 50 degrees.† I got the idea from Joe Horton, whoís also sold on it.† It does make a real difference in how far the engine can be leaned before the leanest cylinder starts misfiring.† †Below is a crude drawing of my fuel system, and where the bypass orifice is.
Speaking of ďslide carbĒ installations, Iím not sure Iíd buy an Ellison again, partly for the reason stated above, and partly because the mixture changes with throttle setting.† Because I can monitor the mixture so precisely with my air/fuel meter, I can detect much smaller mixture changes than most folks, without staring at a bunch of EGT bars (although I have those for reference also).† But for all I know, other carbs may be no better.
EIS (Engine Information System) - I have six EGT probes and six CHT (Cylinder Heat Temperature) probes, so I can keep close tabs on what each cylinder is doing.† These probes are all connected to a Grand Rapids EIS-6000 (Engine Information System) that updates the display and writes to a data file once per second, which is then stored on the laptop that I always fly with.† As a result, I have a digital record of almost all engine parameters for most flights that Iíve made in the N56ML. †I also monitor and record manifold air pressure, carb throat temperature, fuel flow rate, fuel pressure, under the cowling temperature, outside air temperature, engine RPM, altitude, etc. †There are other more expensive systems like the JPI that will store that same info to a Compact Flash card or something similar, but since I fly with a laptop on cross countries anyway (in order to have XM/WX weather and moving map sectional capability), the EIS works fine for me.
†I try not to fly without the laptop though, because recording every flight is very important to me.† I donít even bother filling in my flight log, because I can go always go back and ďrecreate the crimeĒ with EIS flight data. †Itís very easy to count up the number of landings for the log book, just by checking engine rpm and altitude.† If RPM drops below 1500, Iím landing, unless Iíve shut it off at 12,500í for a glide test (and then I cross-check the altitude column).† The serial output option for the EIS was $9, last time I checked, and that included the connector and wiring!
Air/fuel meter - I lean my engine by using an air/fuel ratio meter that I paid about $40 for several years ago.† The oxygen sensor I use is an inexpensive ďone wireĒ Bosch ďuniversalĒ O2 sensor, that costs about $20, and lasts one or two hundred hours on average, since I burn mostly auto fuel.† 100LL works for maybe 50-100 hours before the sensor needs replacement.† Thatís insignificant compared to the price of fuel I run through the engine during that time.† The meter is composed of a 10 segment bar LED thatís broken down into green (proper mixture), yellow (a little rich or a little lean), and red (for way off).†
†I find this inexpensive air/fuel ratio indication to be indispensable, and an invaluable safety warning device.† If I do something stupid with the mixture knob, or something like vapor lock deprives the engine of fuel, the A/F meter is the first thing to notify me.† Reaction time on it is about one second, and one glance at it tells me if the mixture is too rich, too lean, or just right.† It also aids me in testing because it gives me a known mixture point that I can use at different RPMs, altitudes, and speeds to compare factors such as the effects of carb heat on fuel economy.† For example, I can have the mixture leaned all the way out so that the very bottom red LED on the meter is just flashing on and off, and when pulling out the carb heat knob, the mixture goes up past stoichiometric (perfect 14.7:1 mixture) and well into the rich area.† The RPMs drop a little also, but I can give it a little more throttle, then lean it back out, and eventually end up getting better fuel economy, and mixture distribution between cylinders, while burning less fuel, but still turning the same RPMs as before adding carb heat.† For more on this air/fuel meter setup, see http://www.n56ml.com/corvair/o2meter .
Ground runs - Call me crazy, but when I rebuild an engine, I might run it for a minute or two wide open during the early morning or when temps are cool, but thatís all.† If itíll run 30 seconds on takeoff, Iím high enough to come back to the field.† First flights need to be done with that attitude in mindÖtake off, climb around the pattern, and keep the airport under you until a few minutes of climbing at wide open throttle convinces you that your engine rebuild was successful.
Of course thereís no substitute for running it on an engine stand with something like William Wynneís ďground-basedĒ cooling shrouds that guarantee sufficient cooling while allowing the engine to be broken in and proven while youíre standing safely on the ground.
Takeoff - I rarely take off with the carb set on full rich, except in the wintertime.† The air fuel ratio meter tells me that in the winter the Ellison carb is set just right for the cold temperatures of wintertime.† In the summer, if I donít tweak the mixture screw down on the carb about an eighth of a turn, the carb will run too rich at wide open throttle.† Since Iím lazy and donít bother, I just donít run the mixture all the way in during the summer months.† Well, itís not that I have to remember that summer/winter businessÖI just set the mixture wherever the meter tells me.†
Thatís a bit like the folks that worry about whether or not they can handle a counterclockwise rotating engine on takeoff as opposed to a clockwise engineÖwhen it starts to drift off to the side of the runway, youíll give it whatever rudder is necessary to keep it from happening, regardless of whether itís left or right.† So essentially, thereís no difference.
My runup is a pretty simple matter.† I just wait until the oil temp is warm enough to fly (my personal limit is about 80F, but most would consider that too low), and I run it up to full throttle for a second or two to make sure I can get max rpm (right at 3000 rpm with my current Sensenich 54Ēx58Ē prop), swap ignition systems back and forth to make sure itíll run on either one, check oil pressure, and go down the runway.
I canít talk about instrumentation without mentioning the DynaVibe.† This is an inexpensive ($1500) tool thatís used to balance the engine/prop/spinner combination.† Unless you are a very lucky person, by the time you finish bolting all of that stuff together, youíre going to have an engine imbalance the first time you fire it up.†
The DynaVibe will tell you exactly where to place a small stack of washers on your spinner to make it even smoother than it was before (and Corvairs are as smooth as it gets, short of turbines).† To have your engine balanced can cost anywhere from $150 to $400, and takes about two hours.† I change my engine/prop combination so often Iíd go broke in a hurry doing that, so I bought my own.† Four or five guys (or an EAA chapter) could buy one of these and charge $75 per balance, and it wouldnít take long to get in the green.† More details are at http://www.n56ml.com/dynavibe/ .
Hopefully these experiences will give you a head start when you fly your CorvAircraft for the first time.† To learn more about building and flying Corvair† engines, join the CorvAircraft email list by sending a message to CorvAircraft@mylist.net .