User Guide

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Some light twins used primarily for training have unfeather-
ing accumulators that allow you to get a prop back into its oper-
ating range simply by pushing the control back forward; other-
wise, you have to attempt to restart the engine to get oil pressure
back to the prop. In the real world, of course, any problem seri-
ous enough to warrant feathering in the first place generally
means you should leave well enough alone and get to an airport
as soon as possible.
THE NEED FOR SPEED
How can we counteract this severe yaw and turning ten-
dency when an engine fails? By the use of rudder - often just about
full rudder - against the turn. Look at most twins, and you’ll see
that they have pretty large vertical tails - significantly larger than
those of singles of similar size and weight. Why? To provide
enough “tail power” to overcome the asymmetric thrust of a sin-
gle-engine situation.
And how do they do this? Obviously, by deflecting the air
flowing over them. The faster we fly, the more effective the tail
becomes, so it’s the designer’s task to size the tail and rudder for the
worst-case situation: with the airplane flying at minimum speed
with one engine windmilling and the other at full takeoff power.
Unfortunately, this requires a fair bit of work, in its purest
physical sense. If you’ve ever tried to hand-start an airplane (Kids:
don’t try this at home without getting thorough instruction, unless
you want to end up with a nickname like “Lefty”), you’ll know that
it takes a real heave. This is because any piston engine is, in effect,
an air pump - and for a propeller to windmill, it has to turn the
attached engine over each piston’s compression stroke. Although
it’s hard to believe, at typical speeds the drag of a windmilling pro-
peller is very close to that of a solid disk of the same diameter!
The only way a twin can keep flying on one engine is to get
the failed unit to stop windmilling as soon as possible. To do this,
the blades on the constant-speed propellers used on twins are
capable of feathering, or turning completely edge-on to the wind.
Once this has been done, they’re no longer trying to turn a dead
engine, and they come to a stop with an immediate (and very wel-
come) reduction in drag.
This is so important that in the days of the great piston air-
liners, if an engine failed and its prop would not feather, the stan-
dard procedure was to shut off its oil supply in the hope that the
engine would either seize or break its propeller shaft off outright.
It’s a dangerous procedure, with a high risk of structural failure or
fire - but the drag of a windmilling prop is so great, it was con-
sidered worth the risk.
To feather a failed engine in the airplanes in this version of
FLY!, the procedure is very simple: simply pull the affected prop
control all the way back. In the actual airplanes, it has to be
pushed sideways, lifted over a gate, or pulled past extra resistance
to avoid feathering a propeller inadvertently. This opens a valve in
the prop governor that dumps all the oil pressure from the hub,
allowing springs and the blades’ centrifugal forces to swivel them
to the feathered position, if the engine is at lower power or off.
Flight Instruction
Flight Instruction
Incidentally: how come the props don’t feather when you shut
down the engine? Because starting an engine with a feathered
prop puts a huge load on it, so there are anti-feather locks that
are engaged centrifugally at and below about 700 RPM as the
engine stops. In a windmilling situation, it’ll be turning faster
than that, so it can feather. On single-engine airplanes, the
locks are simple fixed pins, since there’s no need to feather -
once an engine quits, you’re on your way to a prompt land-
ing, period! And on some types of turbine, which start quite
differently, the props do feather when you shut them down.
Got all that? There’ll be a short quiz later on…