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. It’s not the cleanest
KR you’ll ever see, since it doesn’t even have paint on it yet, and has a lot
of rough edges, but I still 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 don’t
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? It’s 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 9500’ or 10,500’) because it’s
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 don’t 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? Don’t get me wrong, flying low and fast
around the patch is a real hoot, but that’s for days when I’ve only got a few
minutes before sunset and need a serious flying fix. I’ve been known to drive home with a huge KR
grin on my face after one of those flights, and it’s 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. There’s a pretty good “how to” on how to
build a carb heat box on my website at http://www.n56ml.com/corvair/airbox/
.
Detonation is something I’ve experienced about three
times now while flying. So far I’ve 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 right…you
can’t hear it while flying, but you can hear it (if you recognize it) on
takeoff if vapor lock has set in and the mixture becomes too lean to make the engine
happy. I’m 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
(that’s why they call it the idiot light!).
When the alarm went off, it was too late, as I’d 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 it’s all out there in the open once the
cowling is off, and it’s a pretty straightforward job.
So the lesson learned
there was to set my CHT alarm at 450F degrees, and don’t 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 that’s probably because I have plenums that force the propwash down
over the cylinders.
The few other times
I’ve experienced detonation involving very high ambient temperatures, and delays
in getting off the ground. Because of
the cowling's 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 I’m flying on a day in the high
90’s, stop in to refuel or pick up a passenger, and shut the engine off for a
few minutes, the engine “heat soaks” while it’s sitting. So when I start the engine it’s 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 I’ve 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 I’m running a 9.4:1 compression ratio, AND only burning 93
octane auto fuel. The cure is simple
enough…either run 100LL avgas or lower the compression, or both if you like the
belt, suspenders, and parachute approach.
What I’ve learned to do is to burn a mixture of 25% avgas with auto fuel
when I know I’m 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 “mogas” 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 I’d 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 I’ve 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, I’ve not found a single drop of water in my fuel tank or filters. Call it a calculated risk, but I’m OK with
that statistical average regarding 93 octane from your favorite gas station. I’m not saying do what I’m doing, but just
making the point that if you’re 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 I’ve had no problems with either of those on my
plane, and I’ve 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 it’s 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 cockpit… it’s all out
in the wings, so I feel like that’s the mitigation factor in my case. Unfortunately my fuel IS in the stub
wings…it’d 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 detonation…and 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 can’t 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 “no problem”. 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 I’ve 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 I’ve also fire-sleeved all of my fuel and oil hoses, but this
fuel recirculation pays. There’s more on
how I made it, along with many other aspects of my fuel system on my “fuel
system” web page, at http://www.n56ml.com/fuel/.
One way to improve
fuel distribution on “slide carb” installations is to add a “Cyclone Fuel Saver”. 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, who’s 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 “slide
carb” installations, I’m not sure I’d 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 I’ve 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 don’t even bother filling in my flight log,
because I can go always go back and “recreate the crime” with EIS flight data. It’s 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, I’m landing, unless
I’ve shut it off at 12,500’ 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
“one wire” Bosch “universal” 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. That’s
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 that’s 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 that’s all.
If it’ll run 30 seconds on takeoff, I’m high enough to come back to the
field. First flights need to be done
with that attitude in mind…take 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 there’s
no substitute for running it on an engine stand with something like William
Wynne’s “ground-based” cooling shrouds that guarantee sufficient cooling while
allowing the engine to be broken in and proven while you’re 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 don’t 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 I’m lazy and don’t bother, I just don’t
run the mixture all the way in during the summer months. Well, it’s not that I have to remember that
summer/winter business…I just set the mixture wherever the meter tells me.
That’s 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 engine…when it starts to drift off
to the side of the runway, you’ll give it whatever rudder is necessary to keep
it from happening, regardless of whether it’s left or right. So essentially, there’s 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 54”x58”
prop), swap ignition systems back and forth to make sure it’ll run on either
one, check oil pressure, and go down the runway.
I can’t talk about
instrumentation without mentioning the DynaVibe. This is an inexpensive ($1500) tool that’s
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, you’re 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 I’d 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 wouldn’t
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 .
For more info on my
Corvair engine see http://www.n56ml.com/corvair
, and for more on my KR2S, visit http://www.n56ml.com/
. “Blue Skies”, as they say, but cloudy
ones work fine too…