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ENGINE.TXT
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1993-12-08
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Constant speed propeller
------------------------
About the most important changes in the way we fly will be the introduction
of a new propeller in FS5. In FS4 we were used to a 'fixed' prop and in Fs5
we will be able to use a prop with variable pitch. Sounds not so important
but just listen to this story.
When the Hurricane was introduced around 1935 it soon became the main fighter
in the RAF, but from the beginning it lacked speed at high altitudes and
climbing power. The plane was just not able to take on the new German
fighters. That was until a new American variable pitch propeller was
introduced. With this small change the plane was able to intercept and fight
the German planes. It saved the day during the battle of Britain (the
Spitfire was important but lacking in numbers!) and is well known this
important battle would not have been won without the new prop. The
manufactures of the prop told the world they saved the western world!
Now you know how important a good prop can be. But how can it make such a
difference? Well, you all know a prop is like a screw. While it turns it
'screws' itself into the air dragging the plane. But a screw does not know
slip and when it is screwed into wood it will advance at a steady rate and
can not be turned without moving backwards or forwards. A propeller can,
just step on the brakes and add power. The prop will spin faster but the
plane will not move. This effect will always happen, at any speed. The prop
will not screw into the air as much as is wanted.
An other thing to take into account is the speed at which the plane is
travelling. The blades of the prop are not only turning but also moving
forwards. And because a prop blade is a small wing it is like we want to
create lift with a wing that is being dragged around with it's top forward.
The designer has to make a prop that will works under these conditions.
The most important factor in this is the angle of the blades and the
propeller shaft. It is like the grooves in the screw. If the grooves are
close together, the screw will move only a little bit forwards with each
revolution. If the grooves are not close the screw will move much faster but
will be much harder to turn!
The propeller works the same. If the plane is standing still or is travelling
along the runway we want a prop that is designed for those speeds, it will
have to deliver power without much airspeed. The angle of that prop will have
to be small. It has to be 'in first gear'. If we are travelling at high speed
conditions are very much changed. The prop that delivered such good
performance at low speed is now almost useless. It will simply spin at high
rpm and not give much power. For these condition we want a prop that bites
into the air, has a much bigger angle to the air. With this prop we will need
a giant runway to take-off but it will give high speed, great range and a
high ceiling.
You can understand why the designers have always struggled with this problem.
It is simply not possible to design a good propeller. It will always work
best at a give speed and will always hinder the plane at other speeds.
This problem is even more important with high performance planes because the
range of possible speeds is so great. Take the Cessna 182, it lands at 55
knots, cruises at 140 knots and has a topspeed of 160 knots. The speed range
will be 160-55= 105 knots. Now take an P51 Mustang. It lands at 80 knots,
cruises at 250 knots and has been flown as fast as 450 knots. The speed range
will be 450-80=370 knots!! Just think about a prop that has to cope with
these conditions. The only way a propeller plane can be flown in this speed
range is with a variable pitch prop.
These propellers can change the angle of the blades. They can be changed
during flight to make the best prop for any speed! This is done with
oil pressure and these props are complex, expensive and heavy. Still they will
add to the performance of any plane. In FS5 you will find a lever and in one
of the drop down menus a box called PROP ADVANCE with the settings FIXED PITCH
/ MANUAL / AUTOMATIC. In real aviation a lot of planes have a automatic
system. An other advantage of such a system is that if the engine is not
running the propeller can be set in such a way that it has the smallest
possible wind resistance. This saves fuel, increases the distance the planes can
glide
and thus increases the possible survival rate if all engines brake down.
Manifold Pressure
-----------------
Thanks to Eric (and indirectly Jacob) I now have so much information
about manifold pressure even I can retyp it to help you.
"Manifold pressure gage measures absolute pressure of the fuel-air
mixture going to the cilynders and indicates this in inches of mercury"
Standard air pressure is 29.92 inches of mercury.On airplanes with
controllable speed propellors this is used in combination with the
tachometer to set up desired power from the engine.
Since a picture tells a thousands words here a few schematic drawing.
---------------------------------
air/fuel / engine
---------------------------------
^
In this tube the fuel/air mixture is guided to the cilynders. The valve
determines how much mixture enters the engine and how much power it
delivers to the propellor. The / is the valve, its now half open.
At the ^ sign the manifold pressure is messured.
When the engine is off the manifold presure will be the outside pressure!
In Fs5 this is standard about 28 in. When you start the enigine but keep it
idle running this will happen;
---------------------------------
air/fuel | engine
---------------------------------
^
The | indicates the valve is almost closed. Just a bit of fuel air is let into
the engine. Now the manifold pressure will drop to under 10! That is because
the engine is acting like a pump. It will never drop far under 10 because
that wouls mean the valve is closed completely and the cilynders would not
get the fuel and air they need. The engine would be dead.
Now let's open the throttle wide;
---------------------------------
air/fuel -- engine
---------------------------------
^
Now the manifold pressure will rise to just under outside pressure. In an
not charged engine the pressure can never get above airpressure! The
difference will always be 1 or 2 in. The maximum indication you can expect
is 28 to 29 in. on takeoff.
In an supercharged (!) the engine has compressors that bring the fuel/air
mixture to a higher pressure before it enters the manifold. This makes
it possible to register more than the outside air pressure. This type
of charge is done by an integral part of the enige and pressure is built
after the mixture goes through carburator.
A turbo-supercharger is exhaust-driven and compresses the air before it
is mixed with the fuel. Here also the pressure can be higher than outside
pressure.
EGT (Exhaust Gas Temperature).
-----------------------------
Still with me dear friends? Here we go with another lesson. This time
about EGT, mixture control and leaning of the engine.
First the basic rules;
"As an all-round figure, gasoline engines mistures for combustion
engines are abouyt 1:15, that is 1 lb fule to every 15 lb of air, or
about 7% fuel and 93 % airby weight. For richer mixtures an 8% to 10%
fule-air ratio is found"
For takeoff a mixture setting of rich is used. This assures the best
combination of power and cooling. As you climb air pressure becomes
less and the mixture will get richer and richer. The engine might
even run rough. So you need to lean the mixture. Leaning will also
ensure the engine delivers max power at all power settings. Leaning
is simply making the mixture contain more air and less fuel.
Leaning can be done with the mixture control. The basic rule here is
to lean the engine until it starts to run rough and than ease it
forwards to bit until the engine runs smooth. The engine will deliver
max power at the throttle seeting just before the engine was getting
rough. A good guide for leaning the engine is the exhaust gas
temperature (EGT) gage. This gage shows the temperature of the
exhaust gasses. Now listen carfully because here's how the EGT works
and why it is usefull.
Since exess fuel or exess air in the mixture produce a cooling effect
on the exhaust gasses, the EGT gage may be used to find the optimum
for cruise and other conditions.
Above I have written to rich the mixture a bit when it gets rough.
With the EGT this 'a bit' can be measured. It should be about 25
degrees F. This puts the engine on the rich side of highest EGT. The
25 degree drop is for best fuel economy. A 100 degree drop (on the
rich side) would be best for best power.
In climbs use full rich mixture until reaching 5000 ft then lean for
added-power and smoothness. Other planes have a lower limit of 3000
feet for lean in the climb. I am not sure how our friendly Cessna is
in this stack. I would say more on the 3000 than the 5000 mark.
It might be better to be on the rich side because the engine has a
much easier time. Temperature is lower, pressures on all parts is
lower and more lubricant is available. In cruise with fixed pitch
just lean to max rpm and than richen the mixture to be kind to the
engine. In controllable prop machines lean until roughness is
encountered, then richen slightly to eliminate roughness for smooth
operation.
If you start from a field at high altitudes (or race in I6) you might
want to lean the engine a bit on takeoff.
During descents the mixture you will have to richen the mixture
because the mixture will get to lean. Since power is not very
important at descents you might even push the mixture controll all
the way in (to full rich) and not worry about it again. At landing
the mixture must always be full rich to ensure smooth and reliable
running.
That's all folks. Hope you have learned something.
Greetings, Mathijs Kok (you flight instructor)