



                     TWIN ENGINE FLIGHT PRINCIPLES
                          by Steve Boyer

Introduction:

The purpose of this text is to give the basics of twin
engine flight for the purpose of applying it to model aviation.  It is
based on the principles used to teach multi-engine flying to full
sized aircraft pilots.
Hopefully, applying these same basics to model flight can help avoid
frustration with a venture into multi flight..

The text will be in four parts.  The first will be definitions to add
some multi terms to your vocabulary and help make the rest readable.
The second will be differences  from single engine flight.  The third
I'll call "When one quits", and the forth will be a few procedures to
help single engine flight.


1.
Definitions:

Vmc    This is the biggie.  It stands for "Velocity, minimum control".
It's simply the minimum airspeed that a twin can fly  with one engine
out, and the other producing full  power.  You need airflow over the
rudder and aileron to counter the running engine trying to yaw the
airplane right or left.
 The full power on the engine remains constant.  So in a slowing
airspeed situation, you'll come to a point that you  will run out of
rudder and aileron throw, and control of the aircraft will be lost.

P-factor   When an airplane is flying through the air at an angle, as
in a climb, the propeller is pulling harder from one side.  This
sounds confusing.  The best way to "see" it is to put a weight on the
tail of a tricycle plane or use a taildragger, and then stand directly
over the propeller that has been placed horizontal.   If you look
straight down at the prop, you will see one side is at a greater angle
then the other and therefore taking a bigger bite.  Propeller
factor (P-factor) is the main reason (along with torque) you need
alittle right rudder in all climbs. It's  even more
important with twins because it makes one engine more critical to lose
than the other, as is explained later...


Drag   This is an easy one.  I just want to differentiate
the two types.  PARASITIC drag is caused by all the junk in the
airstream, like landing gear, antenna, etc.  INDUCED
drag is the result of the wing producing lift.  At high
airspeeds, more of the total drag is parasitic.  At low airspeeds,
most of the drag is induced drag, much less parasitic.
 At low airspeeds, you are squeezing as much lift as possible out of a
wing, and it's costly in drag..

Climb   This seems simple enough, but maybe not.  What is a
climb?  Why does an airplane climb.  An airplane climbs ONLY because
it has EXCESS power for a given airspeed/altitude.  If you do not have
excess power, you cannot SUSTAIN a climb.
The best you can do is use up some of the power you stored in
airspeed, and trade it for climb.  Sooner or later you run out of
airspeed and now you'll be trading altitude power back into airspeed.
Only EXCESS power, supplied by an engine, can prevent the necessary
trades.  Altitude and airspeed are nothing more than stored engine
power!!!



2. Differences from singles.

When both engines are running the is little difference from singles.
 The main benefit to full size airplanes is not the extra power (you
lose A LOT of that power just to keep all that extra weight in the
air!!!).  The main benefit is dual systems.  Two alternaters, two
vacuum systems, etc, PLUS the chance to limp to an airport and not
land in the mountains at night, in the rain,
in low visibility, with no instruments....  To model aviators, none of
this applies. We fly over our airport all day.  We never fly by
instruments that need powered aboard the airplane, and we
don't need two systems when we have none!  If we want extra power, we
strap a .60 onto a .40 sized model.  Who would be
crazy enough to put two engines onto a model airplane and have no
benefits of twin flight, but ALL of the dangers??  Well, scale for
one, and the bored for the other.  You can't build a scale B-25 with
one engine.   Twins are tricky  when one quits.  Single engine piston
airplanes have a much better safety record than twins!  With a single,
you have no control problems when one (the only one) quits.  A twin
has big control problems, plus TWICE the likelihood of one quitting!!!


3. "When one quits":

When one quits, the other engine will be trying it's best to fly the
plane to the scene of the accident. The plane will yaw quickly into
the dead engine, with an accompanied roll.  The stopped, or
windmilling prop will be increasing the parasitic drag on the dead
engine side, causing more yaw/roll...  The down aileron (if you have
started to counter the roll with aileron only) on the dead engine side
will increase  induced drag (the down aileron is trying to create more
lift) causing more yaw/roll potential.  The parasitic drag all over
the plane has increased dramatically with aileron, and rudder
deflection, and a grossly untrimmed flying machine, which causes the
airplane to slow down, increasing the need for MORE rudder and
aileron which increases all of the above.  This all
increases the need for MORE!? power from the running engine which is
causing this chain of events in the first place!!
This all happens in the span of a few seconds..

The most startling thing is that instead of losing 50% of your ability
to climb, you have lost 80%-90%!!(This 10-20% is still much better
than full size twins because of model planes huge power to weight
ratios, but still substantial)..  Remember the definition of a climb?
You have lost 50% of your available power, but MOST of your excess
power...  What little excess power your running engine has (over and
above what's need to sustain the plane in it's current state) may be
taken up with increased drag.  Improper control input will overwelm
what little excess power the engine has and you will start trading
airspeed power to maintain the stored altitude power at the current
height.  The aircraft WILL slow to Vmc and control will be lost.
Again, all in a few seconds.


4. Procedures:

All this does sound hopeless, but it can be managed.  First,
understanding whats actually happening is half the battle.
Hopefully, the above text has done that.

Now, what to do.  The simplest procedure is to chop power to the good
engine. Although the airplane will still be alittle out of balance
(one engine idling, one engine dead), it should be fully controllable.
As I stated before, we fly mostly over our airport.  Even if you are
out over the bushes, you are better off going into the bushes/weeds
controlled than a spectacular spiral dive into the center of
the runway.  We don't have that great of need for sustained single
engine flight like  full sized planes, so why do it?  IT IS IMPERATIVE
TO START THIS COURSE OF ACTION EARLY, BEFORE CONTROL LOSS.  This might
be the hardest of all to do, that is RECOGNIZING that an engine has
quit in time to take action.  The most realiable is sound.... When one
goes, the props go out of sync and a sickening snarl developes.
If a roll quickly developes, chop  power and dead stick....end of
procedure.  If you  tend to fly way, way out, you may 1. not hear the
model in time, 2. not see it well enough to recognize when its doing
something unexpected...some things to consider...

The next is for those diehards that want to fly the airplane around in
the pattern, on one engine, to a landing. Oh boy..  The MOST CRITICAL
time is takeoff.  Power is high, airspeed is low, and  altitude is
non-existant.  This is where most twin-engine fatalities occur in full
size aircraft...on takeoff..  Everything is against you at this point.
After all, WHEN ARE GLOW ENGINES MOST SUSEPTABLE TO RUNNING PROBLEMS?

You have to be thinking  "engine failure".  What would I do
if......You won't have time while it's happening. The most important
thing is to USE THE RUDDER. When one quits, you have a big time yaw
problem (which results in a roll).  Yaw is the realm of the rudder.
You need to push back with rudder.  If you instictivly only use
aileron, you will probably crash.  What to do with the power setting
is a sticky problem.  Theoretically, you should go to full
power because you want whatever excess power is available, just
remember this also goes to "full problems"..  If you go to partial
power for a while, the plane will be far more controllable, but may
not sustain altitude and airspeed. The partial power Vmc  will be
propotionally slower to full power Vmc, so you can fly slower
(although probably not hold altitude) on partial power.  This can be a
trap.  If you are too slow, and add full power, instant Vmc will be
achieved.
It's also important to know which engine quit.  Because of P-factor,
one engine will  have a greater yawing arm then the other. On standard
engines, the right engine will have more leverage (which is bad) than
the left.  So, it would be more critical to lose the left engine.
Some aileron  will be needed to get a small bank (around 5 deg.) into
the good engine because the new working center of the airplane is no
longer along the fuselage, but now out on the wing toward the working
engine. Again you need to know which engine is running.  The plane is
actually in balanced flight with this small bank.

How can you quickly tell which engine quit?  If you are
holding a hefty amount of rudder, the stick will be pointing to the
GOOD engine.  Working stick, working engine.  It might be easier to
think of stick pointing away from the dead engine, dead stick, dead
engine.  It's also important to know which engine has quit because
when you drag it around on full power and position it for final, you
are going to have to pull off some power.  Again, maybe dead sticking
is the answer.  BUT, realize that if the right engine is running, you
will be holding alot of rudder/aileron into that engine,,, as you pull
power off, rudder/aileron will have to slide out also.  This is also
true for the left.  It's important to understand beforehand which
engine is running before power changes. This is a good time to bring
up power changes.  KEEP THEM TO A MINIMUM!
Every power change in single engine flight changes ALL other factors,
especially rudder and aileron.  One power setting for the pattern
(possibly full) and then one power setting for final (possibly dead
stick).  These are my suggestions.  Scale model pilots have to
consider that they have a scale rudder fighting a very non-scale
engine.  The power to weight ratios  with models is out of sight
compared to full sized.  The actual rudder needed may be 3 times the
size of "scale".  Something else to consider. Also, having one engine
going in and out of 4 cycle (2 cyc.  eng.) may cause as much of a
handfull as one flat quitting. Chopping to a dead stick may be the
only answer in that case.  I don't know.  There are alot of
variables.

I do not want to sound like a master of R/C twin flight.  I am not.  I
fly R/C, but not much over a beginners level.  I am, though, a
certified flight instructor, and an airline transport pilot.  I've
probably taught 25 students multi-engine flight, mostly for the
commercial license. It's the most DANGEROUS flying I've ever done. In
that type of training, you take the aircraft to the edge, and past,
over and over on one engine.  Multi-engine training has
one of the worst accident records in aviation because of this.
I fly for airlines now, but value greatly what I learned teaching
others...Just wanted to pass some along.

To recap......Concerning twins:

1. Don't do it.  Just hang a bigger engine on.
2. Okay, do it, but chop power and dead stick at the first sign of
trouble.
3. Okay, try to fly it, but remember the 3 most important rules of
aviation.  They are 1-Airspeed, 2-Airspeed, 3-Airspeed---Keep thy
airspeed, less the ground come up and smite thee...
4. So you've flow multi, lost and engine, crashed spectacularly and
are re-reading this to get a  hint where you might
have gone wrong....See #1. <g>.

**********************************************************************
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The following is an article written by Stan Andrews and published in
the
January newsletter of the R/C Sportflyers Club in Kansas City
Missouri.

TRANSMITTER MODIFICATION FOR TWO THROTTLE CONTROL

If two separate throttle servos are needed to operate your twin engine
plane,  this may be an idea that you are looking for.  This set-up
requires at least a six or seven channel radio receiver and
transmitter of the newer
variety.
This modification allows you to start one of the engines and adjust it
independently,  then set it at whatever idle speed you desire.  Then
get the other engine started and running without upsetting the engine
already running.  Once both engines are running,  by flicking a switch
they both will operate together from the throttle control in the usual
manner.
Those of you interested in trying to make this modification should
contact me since I don't plan to draw the scheme to outline it.  It
takes a switch that you can get at Radio Shack and maybe a resistor or
two depending on how closely your sixth or seventh channel corresponds
to your throttle control.  Basically,  you plug in one servo to the
sixth channel on the receiver and the other to the throttle control.
After you have the engines started and running,  you flip the switch
and they are both on the throttle control working together.
This is not a mixing of two channels in that you can control one
channel on one stick movement and one from another.  It works great!
 












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