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Definitions
- Direct drive means that the propeller is bolted to, and turns at the same speed as,
the crankshaft. No reduction gearing is used.
- Normally aspirated means that the engine has no supercharger or turbocharger to
maintain sea level atmospheric pressure at higher altitudes and, therefore, that its
maximum available power decreases with altitude.
Theory
Fuel flow through either a carbureted or fuel injected induction system must be
adjusted manually, in almost all instances, to provide for the most efficient fuel
to air ratio for efficient combustion within the cylinders. Given certain fuel to
air mixtures, it is possible to have a situation where the engine will run rough or
will not run at all. Since air density varies with temperature and altitude, it is
important to understand when and how to adjust the mixture control to obtain the
best performance, fuel economy and maximum life from your engine.
A key point to understand is that here is an optimum fuel to air mixture setting at
which to achieve either "best economy" cruise or "maximum power."
Why Lean?
You should lean the mixture for:
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Improved engine efficiency.
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Greater fuel economy (i.e., minimum specific fuel consumption) and longer
range.
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Smoother engine operation - saves engine accessories and engine mounts.
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Longer spark plug life, less fouling.
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Reduced maintenance costs.
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Reduced operating costs.
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More desirable engine temperatures while operating at cruise altitudes.
When to Lean
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Lean anytime the power setting is 75 percent or less at any altitude.
(Full throttle or climb power through 5000 feet density altitude usually means
mixture full rich.)
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At high altitude airports, lean for taxi, take-off, traffic pattern entry and landing.
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For landings at airports below 5000 feet density altitude, adjust the mixture for descent,
but only as required. You can't go wrong if you keep the engine running smoothly!
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Before entering the traffic pattern, go to full rich.
Always consult the Pilot's Operating Handbook for your specific aircraft for
the proper leaning procedures.
How to Lean
- Tachometer Method (for use with fixed or variable pitch propellers):
Set the controls for the desired cruise power setting as shown in the POH. Then gradually
lean the mixture from full rich until the tachometer reading peaks. In smooth air, you
should also notice a slight increase in aircraft speed. At peak RPM, the engine is
operating within the maximum power range. For best economy operation, the mixture is
first leaned from full rich to maximum power, then the leaning process is slowly
continued until the engine starts to run rough. Then, enrich the mixture sufficiently
to obtain a smooth firing engine. Obviously, some engine power and airspeed is sacrificed
when operating at best economy. What you gain, however, is increased endurance.
- Engine "Rough" Method (for use with fixed or variable pitch propellers and
engines equipped with float-type carburetors only): With this method, first
set the throttle to the appropriate power setting, (75 percent power or less). Lean
the engine gradually until the engine starts to run rough; then enrichen the mixture
slightly until the engine is again running smoothly. You will then be operating near
the "best economy" mixture setting.
- Fuel Flow Indicator Method (for use with any type propeller):
The POH for aircraft equipped with fuel flow gauges contains appropriate fuel
flow settings or, alternatively, the fuel flow gauge may be marked for correct
flow at each power setting. You need only lean the mixture to the published or
marked fuel flow values to achieve the correct mixture.
- Exhaust Gas Temperature (EGT) Method (for use with any type propeller):
Peak EGT occurs essentially at the rich edge of the best economy mixture range.
Operation at peak EGT not only provides essentially minimum specific fuel consumption
but also 95 to 96 percent of the engine's maximum power capabilities for a given engine
speed and manifold pressure. In addition, engine operation is very smooth at peak EGT.
In comparison, a very noticeable power loss or roughness will occur when the engine
is operated at the lean side of the best economy range.
High Altitude Operations:
At high altitude airports (5000 feet density altitude and above), lean for taxi,
takeoff, descent and landing. Use the following procedures:
- Startup and Taxi:
Lean at 1000 RPM (all propeller combinations) until RPM peaks, then enrichen slightly.
- Before Takeoff:
Go to full throttle and lean mixture. With a fixed pitch prop,lean to maximum RPM and then
enrichen slightly. With a variable pitch prop on carbureted engines, lean to engine
smoothness. If you have an EGT gauge, lean to +100 degrees F. on the rich side of peak.
With fuel-injected engine, lean to the correct fuel flow setting according to the POH
for your specific airplane.
- Traffic Pattern Ops:
Always lean at traffic pattern altitude for landing at high altitude airports, but only after
you have established maximum power. This practice will ensure maximum available power in the
event that you need to make a go-around.
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