Certified Flyers II
Duchess Multi-Engine Study Guide
2
Section 1:
Engine-out Aerodynamics
TURNING TENDENCIES
The turning tendencies that affect a single engine aircraft (p-factor, torque, spiraling slipstream,
gyroscopic precession) will also affect a multi-engine aircraft. Because a multi-engine aircraft
has two engines many of these turning tendencies increase.
A twin engine aircraft where both engines are rotating the same direction is called a
conventional twin
. To combat p-factor and torque, aircraft with
counter-rotating
propellers
have been developed (the BE76 Duchess has counter-rotating propellers). The p-factor and
torque from counter-rotating propellers cancel each other out which results in less rudder needed
to oppose their turning tendencies.
CRITICAL ENGINE
A
critical engine
is the engine which, if lost, will most adversely affect the performance and
handling characteristics of the aircraft. The affect of the critical engine is most significant when
the aircraft is operating at low airspeed with a high power setting (thus more p-factor and
torque).
On a
conventional twin
with propellers rotating clockwise, the critical engine is the
left
engine.
On an aircraft such as the BE76 Duchess with
counter-rotating
propellers there is
not a critical
engine
because the yawing and rolling caused from losing either engine is identical.
There are four factors which determine if an engine is critical:
1.
P-Factor
2.
Accelerated Slipstream
3.
Spiraling Slipstream
4.
Torque
WHAT HAPPENS WHEN AN ENGINE FAILS?
Two things happen when an engine fails: Yaw and Roll towards the dead engine because now
lift, thrust, and drag act on your aircraft asymmetrically.
1.
Yaw
- Asymmetrical thrust will cause a yawing motion around the C.G. toward the
inoperative engine.
2.
Roll
- Induced flow (
Accelerated Slipstream
- extra lift created by accelerated air over the
wing) from the operating engine and lack of induced flow from the inoperative engine
causes asymmetric lift on wings. This results in a rolling moment around the C.G.
towards the inoperative engine.
3.
Roll
- Yawing moment from the asymmetric thrust will cause the operating engine to
move faster through the air as the aircraft yaws. This causes faster velocity air to flow
