| by |
Andrew Boyd |
| This article originally appeared in the October 1996 issue of COPA MAGAZINE and appears here by permission of the author.
|
Sometime, watching a twin-engine aircraft taxi out and take off,
you've probably wondered if you could fly one. When you poke
your head inside the cockpit, it does look kinda complicated.
Lots of knobs and dials there.
But to let you in on a little secret: it's really not that tough
— most of the time. When everything is working, you just grab
two throttles instead of one, and away you go.
The systems can be a little more complex — the fuel plumbing
is a bit bizarre in some types, there's likely a gas-powered cabin
heater in the nose [a la VW bug], the props are constant speed,
and the gear retracts.
But when everything is working, it's not really a lot different
than flying a retractable single such as a Bonanza, Mooney, Comanche,
or Arrow. In fact, if you're considering getting your multi endorsement,
one idea is to first get checked out in one of these retractable-gear
singles.
The transition from a simple, fixed-gear, fixed-pitch-prop single
to a twin is probably busier than it needs to be. Make life easier
on yourself.
If you can, learn about constant speed props, cowl flaps, retractable
landing gear and other fancy frills in a cheaper single-engine
aircraft before you start shelling out the big bucks renting a
multi.
Why A Multi-Engine Aircraft?
Have you ever wondered why a manufacturer puts more engines on
an airframe? There are many people who think that it's for safety;
that a twin is safer than a single. After all, if one engine
fails, well, you just keep on flying on the remaining one, right?
No. When one engine on a twin fails, you don't lose half of your
excess thrust, you typically lose 80% to 90% of your excess thrust,
which means that if you were climbing at 1200 fpm with both engines,
if you configure and fly the aircraft perfectly after an
engine failure, you will likely see around 200 fpm, which is pretty
bad.
Most light twins, when operated anywhere near gross weight, have
very marginal single-engine performance, and are very intolerant
of pilot error in achieving a positive rate of climb. A non-turbocharged
twin will typically have a single-engine service ceiling of around
5000 foot density altitude. So, an engine failure in cruise in
summer means you're likely going to descend.
And remember, with two engines, you're twice as likely to have
an engine failure.
So why on earth would a manufacturer install two engines instead
of one? Apart from specifically-built multi time-builders and
trainers, the answer is: for more power.
If a manufacturer can't get 500 hp from one engine for a 5000
lb. aircraft, well, the answer is to put 250 hp on each side.
However, there is additional drag created by the drag of the
engine nacelle on each side.
It's interesting to contrast the performance of the Twin Comanche,
with a total of 320 hp, versus the 260 hp single Comanche.
Unless you need multi time in your logbook, I personally would
take the speedy single Comanche over the twin, especially when
you add in the extra dollars per hour of the twin.
Newer, serious light business aircraft such as the TBM-700 or
PC-12 use a single turbine engine to get the power required with
minimal drag. Also, kerosene burning turbines can fly higher
than any piston-engined aircraft, may of which are restricted
to 25,000 feet. The TBM-700 goes 300 knots TAS at 30,000 feet,
which isn't too bad for a single, eh?
There is another problem with putting two engines out in the wings.
When both engines are pulling, everything is nice and symmetrical.
But when one engine fails, it is no longer providing thrust —
the other engine is providing all the thrust, and in doing so,
it causes the aircraft to yaw towards the failed, or dead engine.
It's actually worse than that. The dead engine, in addition to
no longer providing thrust, is also creating drag, since the propeller
is windmilling, and the energy to spin the prop has to come from
somewhere.
The drag the windmilling prop creates also causes the aircraft
to yaw towards the dead engine.
Instinctively, the pilot will try to stop the yaw by stomping
on the rudder pedal on the side of the engine which is still producing
thrust.
In fact, this is how a pilot identifies the dead engine; the "dead"
foot, which no longer needs to push on the rudder pedal, is on
the side of the "dead" engine.
At this point, the pilot "feathers" the dead engine.
The prop control is pulled all the way back, beyond the feather
detent, and the prop blades rotate to maximum coarse pitch, which
minimizes drag. In fact, at this point, the propeller of the
dead engine will stop rotating. It's really weird, flying along,
looking at the stopped blades of a feathered engine.
Back to opposing the yaw. We all know that as you slow down,
flight controls get sloppy — they lose effectiveness. Below
a certain speed, the rudder will not have enough authority to
oppose the yawing into the dead engine. This results in the aircraft
rolling inverted into a spin, and nearly always the deaths of
all the occupants, which creates bad press for general aviation.
What is usually recommended in this situation is to reduce the
power on the good engine, and to lower the nose to increase airspeed,
in order to maintain control. Neither of these is a particularly
desirable choice at low altitude, right after takeoff.
As a result, climbing at slow speeds is strongly frowned upon
in twins. So strongly, in fact, that this minimum yaw control
speed, known as Vmc, is painted as a red line on the airspeed
indicator, in addition to Vne. Rotation on takeoff before Vmc
is really discouraged.
Vmc
As a matter of standardization, Vmc is determined at maximum gross
weight, with the center of gravity [C of G] at the maximum aft
position, at sea level, with the flaps set to the takeoff position,
the landing gear retracted, with all engines developing maximum
power at the time the critical engine fails and windmills, with
a maximum of 5 degrees of bank into the good engine.
Whew. And hold on just a cotton-pickin' minute, you say.
What's all this rubbish about a critical engine? And 5 degrees
of bank?
Critical Engine
Let's talk about the critical engine. Look at a twin from behind.
The two normal piston engines, that the manufacturer purchased
from Lycoming or continental, will rotate clockwise when viewed
from behind, just like in a single. And remember slow flight,
from your private pilot training? Having to stand on the right
rudder, because the downgoing blade creates more [asymmetric]
thrust?
Well, exactly the same thing is at work here. If the right engine
fails, and we slow down, looking at a twin from behind, the downgoing
blade of the left engine will be inside the nacelle, closer to
the fuselage. This results in a shorter arm for the thrust, which
means less torque, or yawing.
But if the left engine fails, as we slow down, looking at the
twin from behind, the downgoing blade of the right engine is outside
the nacelle, away from the fuselage, which means more torquing
or yawing, which means you need more rudder.
All this handwaving boils down to that, as far as control goes,
it's worse when the left engine packs it in. This is why the
left engine is called the "critical" engine.
To help this, in the late 1960's and early 1970's, airframe manufacturers
started installing a counter-clockwise rotating engine on the
right side, so it wasn't as tough to stay straight when the left
engine packed it in. The downgoing blade on the right engine
is now close to the fuselage, just like on the left engine.
This why a twin with counter-rotating propellers is said to not
have a critical engine — from a control standpoint.
Banking Into The Good Engine
Now, what's this weird business about 5 degrees of bank into the
good engine? Well, remember that as we bank an aircraft, the
lift produced by the wing banks or tilts, too. We can break up
the tilted lift into two parts, the vertical and the horizontal.
After an engine fails, if we bank towards the good engine, the
horizontal component of the tilted lift opposes the yawing into
the dead engine.
If a little is good, then more must be better, right? Well, if
we increase our bank into the good engine, sure, we increase the
horizontal component of lift, and we don't need as much rudder.
But there's no such thing as a free lunch. As you bank the aircraft,
the vertical component of lift decreases, and the aircraft starts
to descend.
It's important to realize when flying multi-engine aircraft that
control and performance are totally separate issues, and in fact
are usually at odds with one another.
The 5 degrees of bank chosen for the Vmc demonstration is entirely
arbitrary, and has nothing to do with achieving maximum performance
at the higher single engine best rate of climb speed, known as
Vyse, which is painted on the ASI as a blue line.
Excessive bank at Vyse, which is nice but not needed for control,
really hurts climb performance, which is usually already hurting.
To achieve maximum [published flight manual] single-engine climb
rate, a rule of thumb is 2 degrees of bank with a non-critical
engine failure, and 3 degrees of bank with a critical engine failure.
Past these bank angles, a sideslip into the good engine will result.
And we all know how a sideslip creates drag, which is sure not
what we want during a single-engine climb.
In fact, with enough bank into the good engine, you can take your
foot right off the "good" side rudder pedal, and may
even need some rudder on the side of the dead engine to stay straight.
Great control, but atrocious performance, with a big negative
number guaranteed on the VSI.
You don't have to take my word on this, by the way. Tape a yaw
string on the nose of your favorite twin, and see for yourself
how many degrees of bank you really need at Vyse with one engine
out. Betcha you're slipping into the good engine at 5 degrees
of bank.
For more information about this, there's a video you can order
on this subject for $28.00 (US):
"Optimized Engine-Out Procedures for Multi-Engine Airplanes"
Embry-Riddle Aeronautical University
University Distribution Center
Daytona Beach, FL 32014
(904) 239-6484
One fascinating example from the video: a Piper Seminole at 5.6
degrees angle of bank (could you fly within 0.6 degrees angle
of bank?!) had a ZERO climb rate on 1 engine. Best climb was
a little better than 100 fpm at just over 2 degrees angle of bank.
A small note here: to achieve maximum rate of climb at higher
density altitudes, remember that Vy decreases as altitude increases,
or as weight decreases. Slight finessing of your angle of bank
and your airspeed after an engine failure could be the difference
between a climb and a descent.
Some more information on multi-engine flying is available from
AOPA's Air Safety Foundation.
The first paper is titled: "Principles to Bank On" and
is the cover story for the April 1989 issue of the ASF's Flight
Instructors' Safety Report (Vol. 15, #2).
The second is: "Engine-Out Booby Traps for Light Twin Pilots"
and is the cover story for the April 1993 issue (Vol 19, #2).
Weight
So, we can see that the angle of bank into the good engine in
addition to affecting performance, most definitely affects Vmc.
What else affects Vmc?
Well, the published Vmc figure is determined at maximum gross
weight. Does weight affect Vmc? Sure it does.
Why? Well, remember, at a heavier weight, if the aircraft is
level, the wings must be producing more lift than at a lighter
weight. Think of the lift vector being longer.
And when we bank the aircraft into the good engine, the longer
lift vector gives us a longer horizontal component, which opposes
yaw, helps the rudder, and reduces Vmc.
At least, according to conventional wisdom. But there's a catch.
At a heavier weight, the wing is at a higher angle of attack
to generate the additional lift. And as the angle of attack increases,
so does the asymmetric thrust from the downgoing blade of the
good engine, which is what we have to oppose with the rudder.
Altitude
What else can change Vmc? Well, remember that Vmc is determined
at sea level. Does altitude affect indicated Vmc? You betcha.
Why? Well, since the good engine is putting out less power at
the air higher altitude, there is less torque for the rudder to
have to overcome.
At the same indicated airspeed, the flight controls will
of course have the same effectiveness.
This little detail has caused heartburn for more than one neophyte
multi instructor, who in the interest of safety climbs to a what
he thinks is a nice, safe high altitude on a hot day for a Vmc
demo.
However, since Vmc has decreased below the stall speed, what the
new multi instructor ends up demonstrating is a full power single-engine
stall and spin. Oops.
Curiously, the official Transport Canada Instructor Guide:
Multi-Engine Class Rating [TP11575E] doesn't mention this
at all in the Vmc demo guidelines, on page 32.
Center of Gravity
Does C of G affect Vmc? Sure does. Published Vmc is determined
with the C of G at it's maximum aft location.
If the C of G is moved forward, Vmc decreases because the "arm"
of the rudder gets longer, so it can create more torque, to oppose
the yawing.
Cool Video Games
As I said before, it's not too tough to fly a multi-engine aircraft
— most of the time. But when one engine packs it in, especially
right after takeoff, there is probably no room for any error.
The pilot must do everything perfectly, quickly.
One idea for multi pilots is to get some professional simulator
training, just like airline pilots do, from FlightSafety or Simcom.
A good simulator should behave similar to the real thing, except
that crashing is less painful and expensive. It allows an instructor
to do terrible things — like failing an engine at rotation —
that no sane person would ever dream of doing in real life.
Most people that have gone through the professional simulator
twin training highly endorse it, but it's sure not cheap. If
you have Internet access, for a C-310 pilot's experience of FlightSafety
simulator training, take a peek at Mike Busch's article "Training at FlightSafety."
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