the difference being that an aircraft engine is opti -
mized for reliability with dual ignition often installed
for each cylinder. Two-stroke engines are popular be-
cause they have fewer components than four-stroke
engines which makes them less expensive to manu -
facture, and lighter, thus increasing their power-to-
weight ratio.
Two-stroke engines require that oil be mixed into the
fuel to lubricate the engine, instead of being held in a
sump and having a separate recirculating system like
a four-stroke engine. Details on two-stroke oil mixing
are covered later under the “Lubrication” section.
One stroke as the piston moves up is intake and com-
pression, the second stroke as the piston moves down
is power and exhaust. The two-stroke engine per -
forms the same functions as a four-stroke engine in
half the strokes.
A wide range of valve systems are found on two
cycle engines, for the purpose of opening and clos -
ing ports in the cylinder to let fuel in and exhaust out
at the proper time, similar to the intake and exhaust
valves on a four-stroke engine. One-way pressure
valves, called spring, reed, or poppet valves, open
when the pressure drops within the crankcase, pull -
ing the fuel from the carburetor into the crankcase.
[Figure 4-1]
Mechanical rotary valves are driven off the engine,
rotate to provide an opening at the precise time, and
can be on the intake and exhaust ports. [Figure 4-2]
Piston porting does not use any valves. The fuel inlet
port is opened and closed by the piston position as
it moves up and down in the cylinder. This is called
a “piston ported inlet” and will be used in the Two-
Stroke Process description that follows. [Figure 4-3]
Two-Stroke Process
The two-stroke process begins with the fuel enter -
ing the engine and concludes as it exits as exhaust.
[Figure 4-3]
This chapter covers the engines found on most pow -
ered parachutes and includes the exhaust, ignition,
fuel, lubrication, cooling, propeller, gearbox, induc -
tion, charging, and fuel systems. Reciprocating en -
gine operating theory is covered, for both two-stroke
and four-stroke engines.
The powered parachute engine and propeller, often
referred to as a powerplant, work in combination to
produce thrust. The powerplant propels the powered
parachute and charges the electrical system that sup-
ports PPC operation.
The engine is one of the key components of a powered
parachute and should be maintained according to both
the engine and airframe manufacturer recommenda -
tions. Preflight information, along with maintenance
schedules and procedures, can be found in the Pilot’ s
Operating Handbook (POH) and/or maintenance ref-
erences from the manufacturers.
Engine inspections and maintenance must be per -
formed and documented in a logbook. You should
review this logbook before flying an unfamiliar pow-
ered parachute.
Reciprocating Engines
Most powered parachutes are designed with recipro-
cating engines. Two common means of classifying
reciprocating engines are:
1. By the number of piston strokes needed to
complete a cycle: two-stroke or four-stroke; and
2. By the method of cooling: liquid or air-cooled.
Refer to Chapter 5 of the Pilot’s Handbook of Aero-
nautical Knowledge for a comprehensive review of
how reciprocating four-stroke engines operate.
Two-Stroke Engines
Two-stroke engines are commonly used in powered
parachutes. Two-stroke aviation engines evolved
from two-stroke snowmobile and watercraft engines,
Crankcase Vacuum Intake Stroke—Piston
Moving up: Figure 4-3 a to b
The upward stroke of the piston [Figure 4-3a] cre-
ates a vacuum in the crankcase and pulls the fuel/
air/oil mixture into the crankcase through the intake
valve system from the carburetor. [Figure 4-3b] This
can be a pressure-actuated reed valve, a rotary valve,
or a third ported inlet system where the lower piston
skirt provides an opening for the fuel/air/oil mixture
to flow in when the piston is reaching its highest
point Top Dead Center (TDC). At this point, the
greatest portion of the fuel/air/oil mixture has filled
the crankcase.
Crankcase Compression Stroke—Piston
Moving down: Figure 4-3 b to c
During the downward stroke, the pressure valve is
forced closed by the increased crankcase pressure,
the mechanical rotary valve closes, or the piston
closes off the fuel/air oil mixture intake port. The
fuel mixture is then compressed in the crankcase
during the downward stroke of the piston.
Crankcase Transfer/Exhaust—Piston at lowest:
Figure 4-3 d
When the piston is near the bottom of its stroke, the
transfer port opening from the crankcase to the com-
bustion chamber is exposed, and the high pressure
fuel/air mixture in the crankcase transfers around the
piston into the main cylinder.
This fresh fuel/air/oil mixture pushes out the exhaust
(called scavenging) as the piston is at its lowest point
and the exhaust port is open. Some of the fresh fuel/
air/oil mixture can escape out the exhaust port result-
ing in the higher fuel use of the two stroke engine.
Cylinder start of Compression Stroke—Piston
initially Moving up: Figure 4-3 e
As the piston starts to move up, covering the transfer
port, the tuned exhaust bounces a pressure wave at
the precise time across the exhaust port (more on
this in the exhaust system discussion) to minimize
Figure 4-1. Reed valve is open with low pressure and closes when the pressure increases.
Figure 4-2. Intake rotary valve for a two cycle engine.
the fuel/air/oil mixture from escaping out the ex-
haust port.
Cylinder Compression Stroke—Piston Moving
Up: Figure 4-3 e to f
The piston then rises, and compresses the fuel mix-
ture in the combustion chamber. During this piston
compression process, the crankcase vacuum intake
process is happening simultaneously, as described
earlier. This is why four processes can happen in two
strokes.
Cylinder Power Stroke—Piston Moving Down:
Figure 4-3 f to g
At the top of the stroke, the spark plug ignites the
fuel mixture and drives the piston down as the power
stroke of the engine.
Cylinder Power Stroke—Piston Moving Down:
Figure 4-3 g to h
As the piston passes the exhaust port, the exhaust
starts to exit the combustion chamber. As the pis-
ton continues down, the transfer port opens and the
swirling motion of the air/fuel/oil mixture pushes the
exhaust out the exhaust port.
Piston Reverses Direction From Down Stroke to
Up Stroke: Figure 4-3 h to a
As the piston reverses direction from the down
stroke to the up stroke the process is complete. Figure 4-4. The cycles in a four-stroke engine.
Figure 4-2. Intake rotary valve for a two cycle engine.
Figure 4-3. Piston ported inlet for a two cycle engine.
Four-Stroke Engines
Four-stroke engines are very common in most air -
craft categories, and are becoming more common in
powered parachutes. [Figure 4-4] Four-stroke engines
have a number of advantages, including reliability,
fuel economy, longer engine life, and higher horse -
power ranges.
These advantages are countered by a higher acquisi -
tion cost, lower power-to-weight ratios, and a higher
overall weight. The increased weight and cost are
the result of additional components, e.g., camshaft,
valves, complex head to house the valve train, etc.,
incorporated in a four-stoke engine.
Exhaust Systems
Engine exhaust systems vent the burned combustion
gases overboard, reduce engine noise, and (in the
case of two-stroke engines) help keep the fresh fuel-
air mixture in the cylinders. An exhaust system has
exhaust piping attached to the cylinders, as well as
a muffler. The exhaust gases are pushed out of the
cylinder and through the exhaust pipe system to the
atmosphere.
Some exhaust systems have an exhaust gas tempera -
ture probe. This probe transmits an electric signal to
an instrument in front of the pilot. This instrument
reads the signal and provides the exhaust gas tempera-
ture (EGT) of the gases at the exhaust manifold. This
temperature varies with power and with the mixture
(ratio of fuel to air entering the cylinders), and is used
to make sure the fuel-air mixture is within specifica -
tions. When there is a problem with carburetion, the
EGT gauge will normally be the first notification for
a pilot.
Two-Stroke Tuned Exhaust Systems
In two-stroke engines, the exhaust system increases
the fuel economy and power of the engine. The two-
stroke exhaust system is an integral part of any two-
stroke engine design; often controlling peak power
output, the torque curve, and even the RPM limit of
the engine.
The exhaust system must be tuned to produce a back
pressure wave to act as an exhaust valve. When hot
spent gases are vented out of the exhaust port, they are
moving fast enough to set up a high-pressure wave.
The momentum of that wave down the exhaust pipe
diffuser lowers the pressure behind it. That low pres-
sure is used to help suck out all of the residual, hot,
burnt gas from the power stroke and at the same time
help pull a fresh fuel-air charge into the cylinder. This
is called scavenging and is an important function of a
tuned two-stroke exhaust system.
The design of the exhaust converging section causes
a returning pressure wave to push the fresh fuel-air
charge back into the exhaust port before the cylinder
closes off that port. That is called pulse-charging and
is another important function of the exhaust system.
Tuned exhaust systems are typically tuned to a par -
ticular RPM range. The more a certain RPM range
is emphasized, the less effective the engine will op -
erate at other RPMs. Vehicles like motorcycles take
advantage of this with the use of transmissions. Mo -
torcycle exhaust pipe builders can optimize a certain
RPM range and then the driver shifts gears to stay in
that range. Aircraft, with no transmission, do not have
this ability.
On an aircraft, an exhaust pipe has to be designed to
operate over a broad range of RPMs from idle to full
speed. This is part of the reason that simply putting a
snowmobile engine on a powered parachute doesn’t
work well.
Overall, the two-stroke exhaust system for a PPC is
a specific design and must be matched to the engine
to operate properly and obtain the rated power. It also
reduces noise and directs the exhaust to an appropri -
ate location. Exhaust silencers can be added to reduce
noise but additional weight, cost, and slight power re-
duction are the byproducts.
Four-Stroke Engine Exhaust Systems
Four-stroke engines are not as sensitive as two-stroke
engines because they have exhaust valves and there -
fore do not need the precision pulse tuned exhaust sys-
tem. However, directing the exhaust out appropriately
and reducing the noise are important considerations.
Again, using the manufacturer’s recommended con -
figurations is required for Special Light Sport Aircraft
(S-LSA) and recommended for Experimental Light
Sport Aircraft (E-LSA).
Two-Stroke Engine Warming
Two-stroke engines must be warmed up because met-
als expand at different rates as they heat up. If you
heat up steel and aluminum, you will find that the alu-
minum parts expand faster than the steel parts. This
becomes a problem in two different areas of many
two-stroke engines. The first place is in the cylinders
of the engine.
prevent this, slowly add power well before you get
close to the ground where you will need power. This
will give the system a chance to gradually open the
thermostat and warm up the radiator water.
Just as it takes a while for the engine crankcase and
bearings to warm up, it also takes those steel parts a
long time to cool down. If you land, refuel and want
to take off again quickly, there is no need to warm up
again for 5 minutes. The lower end of the engine will
stay warmed up after being shut down for short peri -
ods. An engine restart is an example where it would
be appropriate to warm the engine up until the gauges
reach operating temperatures. The lower end of the
engine is warm and now you only need to be con -
cerned with preventing the pistons from seizing.
Four-Stroke Engine Warming
A four-stroke engine must also be warmed up. The
four-stroke engine has a pressurized oil system that
provides more uniform engine temperatures to all its
components. You can apply takeoff power as soon
as the water, cylinder head temperature (CHT), oil
temperatures and oil pressure are within the manu -
facturer’s recommended tolerances for takeoff power
applications.
Gearboxes
Gearboxes are used on all powered parachute recip -
rocating engines to take the rotational output of an
internal combustion engine which is turning at a very
high RPM and convert it to a slower (and more useful)
RPM to turn the propeller. Gearboxes come in differ-
ent gear ratios depending on the output speed of the
engine and the needed propeller turning speeds.
A typical two-stroke RPM reduction is from 6,500
engine RPM with a 3.47 to 1 reduction, resulting in
1,873 propeller RPM. A typical four-stroke RPM re -
duction is from 5,500 engine RPM with a 2.43 to 1
reduction, resulting in 2,263 propeller RPM.
A gearbox is a simple device that bolts directly to the
engine and in turn has the propeller bolted directly
to it.
A two-cycle engine gearbox is kept lubricated with
its own built-in reservoir of heavy gearbox oil. The
reservoir is actually part of the gearbox case itself.
The gearbox oil has to be changed periodically since
the meshing of the gears will cause them to wear and
will deposit steel filings into the oil. If the oil is not
The cylinders have steel cylinder walls that expand
slowly compared to aluminum pistons that expand
quickly. If an engine is revved too quickly during
takeoff before warming up, a lot of heat is generated
on top of the piston. That quickly expands the piston,
which can then seize in the cylinder. A piston seizure
will stop the engine abruptly.
The second area of concern is lower in the engine
around the engine crankshaft. This is an area where
things may get too loose with heat, rather than seizing
up. Additionally, the crankcase has steel bearings set
into the aluminum which need to expand together or
the bearings could slip.
Many two-stroke engines have steel bearings that nor-
mally hug the walls of the aluminum engine case. The
crank spins within the donuts of those steel bearings.
If you heat up the engine two quickly, the aluminum
case will out-expand those steel bearings and the
crank will cause the bearings to start spinning along
with it. If those steel bearings start spinning, they can
ruin the soft aluminum walls of the case, which is
very expensive.
If heat is slowly added to an engine, all the parts will
expand more evenly. This is done through a proper
warm-up procedure. Many two-stroke engines are
best warmed up by running the engine at a set RPM
for a set amount of time. Follow the instructions in
your POH; however, a good rule of thumb is to ini -
tially start the engine at idle RPM, get it operating
smoothly, and then warm the engine at 3,000 RPM
for 5 minutes.
Once the engine is warmed up and the powered para-
chute is flying, it is still possible to cool down the
engine too much. This will happen when the engine is
idled back for an extended period of time. Even though
the engine is running, it is not generating as much heat
as the cooling system is efficiently dumping into the
atmosphere. An immediate power application with a
cooled engine can seize the engine just as if the engine
had not been warmed in the first place.
In water-cooled engines, on a long descent at idle, the
coolant cools until the thermostat closes and the en -
gine is not circulating the radiator fluid through the
engine. The engine temperature remains at this ther -
mostat closed temperature while the radiator coolant
continues to cool further. If full throttle is applied, the
thermostat can open allowing a blast of coolant into
the warm engine. The piston is expanding because of
the added heat and the cylinder is cooling with the
cold radiator water resulting in a piston seizure. To
changed, the filings themselves are abrasive and will
cause even more wear.
Some gearboxes have the electric starter motor built
into it. When activated, the motor turns the gearing,
which in turn cranks the engine itself.
Four-stroke propeller reduction gearboxes use oil
from the engine oil system for lubrication.
Centrifugal Clutch
Some gearboxes come with a built-in centrifugal
clutch, and others have allowances for installation. A
centrifugal clutch is very useful in a two-stroke engine
because it allows the engine to idle at a lower speed
without the load of the propeller. Otherwise, two-
strokes can generate a lot of vibration at low RPM
when loaded. As the engine speeds up, the centrifugal
clutch engages the rest of the gearbox and smooth -
ly starts the propeller spinning. When the engine is
brought back to idle, the clutch disengages and allows
the engine to again idle smoothly; the propeller stops
when on the ground and windmills when flying.
Propeller
The propeller provides the necessary thrust to push
the powered parachute through the air. The engine
power is used to rotate the propeller, which in turn
generates thrust very similar to the manner in which
a wing produces lift. The amount of thrust produced
depends on the airfoil shape, the propeller blade angle
of attack, and the engine RPM. [Figure 4-5] Powered
parachutes are equipped with either a fixed-pitch or
ground adjustable pitch propeller.
Fixed-Pitch Propeller
The pitch of this propeller is set by the manufactur -
er and cannot be changed. Refer to Chapter 5 of the
Pilot’s Handbook of Aeronautical Knowledge for ba-
sic propeller principles.
Ground Adjustable-Pitch Propeller
Adjustable-pitch propellers for PPCs can only be ad -
justed on the ground with hand tools. If an engine is
over-revving, more pitch can be added to the propeller.
If the engine is not developing the full recommended
RPM during flight, then some pitch can be taken out
of the blades. This should be done per the PPC’s POH
and by a qualified technician.
Induction Systems
The induction system brings air in from the atmo -
sphere, mixes it with fuel, and delivers the fuel-air
mixture to the cylinder where combustion occurs.
Outside air enters the induction system through an air
filter on the engine. The air filter inhibits the entry of
dust and other foreign objects. Two types of induction
systems are used in powered parachute engines:
1. The carburetor system is most common; it mixes
the fuel and air in the carburetor before this
mixture enters the engine intake, and
2. The fuel injection system, which injects the fuel
into the air just before entry into each cylinder.
Carburetor Systems
PPCs use float-type carburetors. Reference the Pilot’ s
Operating Handbook of Aeronautical Knowledge for
basics on float carburetor operation.
Modern two- and four-stroke carburetors operate with
one of three jetting systems, depending on engine
power. [Figure 4-6]
When the throttle is closed, for engine idling, the throt-
tle valve is closed and the fuel is supplied through the
idle (pilot) jet and idle (pilot) air passage. The fuel/
Figure 4-5. Engine RPM is indicated on the gauge.
Use Fig4-3bnew.jpg
air/oil mixture is supplied to the cylinders through the
bypass hole. [Figure 4-7]
As the throttle is advanced and the throttle valve is
raised, the fuel is sucked up through the main jet but
is controlled by the opening and taper of the jet nee -
dle and needle jet. This is effective throughout most
of the mid range operation. About half throttle, the
main jet size starts to influence the amount of fuel
mixed with the air and this effect continues until it
is the main influence at the highest throttle settings.
[Figure 4-8]
Two-Stroke Carburetor Jetting
Carburetors are normally set at sea-level pressure,
with the jets and settings determined by the manufac-
turer. [Figure 4-9] However, as altitude increases, the
density of air entering the carburetor decreases, while
the density of the fuel remains the same. This creates
a progressively richer mixture, same fuel but less air,
which can result in engine roughness and an appre -
ciable loss of power. The roughness is usually due to
spark plug fouling from excessive carbon buildup on
the plugs. Carbon buildup occurs because the exces -
sively rich mixture lowers the temperature inside the
cylinder, inhibiting complete combustion of the fuel.
This condition may occur at high-elevation airports and
during climbs or cruise flight at high altitudes. To main-
tain the correct fuel-air mixture, you can change the
main jets and the midrange jets setting for base opera-
tions at a high density altitude airport. Operating from
low altitude airports and climbing to altitude where the
mixture becomes rich for short periods is OK.
Figure 4-7. Pilot or Idle jet system.
Figure 4-8. Jet needle/needle jet and main jet system.
Figure 4-6. Throttle position and jetting system used.
Figure 4-9. Typical 2-stroke carburetor.