hey guys what's up John here from www.fly8ma.com and today we're
going to cover in the shortest and simplest way possible
what a constant speed propeller is, what it does, why we have them, and what you
need to know for your commercial or CFI check ride let's go ahead and dive right
in first what is a constant speed prop well a constant speed prop is a
propeller that has blades that twist rather than just stay fixed to the hub
so they actually change their angle of attack and change their pitch via
control in the cockpit now what this looks like we can easily tell walking up
to any airplane by simply looking at the propeller hub so when you look at
something like an arrow or a 172 RG it has a constant speed propeller we can
see these propeller blades that go into a hub and it's round at the base the
bladed twists and that twists via oil pressure we'll talk about that more in a
second here compared to these fixed-pitch propellers we can see here
like you'd find on a 172 or a regular 152/172 where it's going
to be a fixed pitch propeller where it's all one solid piece of metal and the
blades don't twist at all per the control in the cockpits they
have that natural twist built into them but they're not twisting in the sense that
we're talking about where they're actually changing their angle of attack
changing their pitch relative to the hub other clues you have when you walk up to
New York Lane they have a constant speed propeller is you have a blue lever or an
additional lever on the flight control so you have your throttle your carb heat
your mixture but there's an extra lever there and it's much like a mixture
control often times where you can screw it in and out you can make finite
adjustments to it that's often a prop lever or prop knob oftentimes it's blue
and some of the older airplanes that might be black or a different color but
any sort of blue knob is a very big hint that you have a constant speed propeller
that you're going to control and set the RPM it is possible however on very new
airplanes like some of the series's or some other airplanes that have fade act
like a DA 42 to have constants be propellers that do change their pitch do
change their angle but it's all computer controlled so you just are left with one
single throttle lever rather than throttle to control manifold pressure
and you are actual blue lever to control the propeller rpm now what does a
constant speed propeller do well it changes its angle of attack it changes
the pitch of the blades to maintain a constant rpm hence constant speed speed
to being the rpm of the engine so regardless how much power the engine is
putting out or airspeed increasing or decreasing the propeller rpm will stay
the same where you know from flying your Piper arrow or warrior or Cherokee or
your Cessna 152 or 172 when you accelerate or decelerate in air speed
you see changes in rpm because there's more or less drag on the propeller in
the case of a constant speed propeller airplane that will not occur so as you
increase or decrease air speed the RPM stays the same what will change is the
twist of the blades on the propeller the angle of attack as they twist to take a
bigger bite of air as their speed increases maintaining the same load on
the inch of maintaining the same our engine rpm for that given amount of
power that the engine is producing now why do we have constant speed propellers
on airplanes why not just have regular fixed-pitch propellers like some of the
basic trainers do well think about being on a bicycle do you want to ride a
single speed buy school and have to go uphill and downhill or do you want to
have a bicycle that's 28 speed or 24 speed where you can actually change the
gears for when you're pedaling uphill downhill or on flat ground because guess
what our airplanes climb going uphill go straight level ground and they
descend they go downhill so if you're trying to make the most efficient
propeller for all those different speeds well it's really hard to do that's
always a compromise on a fixed pitch propeller whereas with a constant speed
propeller you can vary the gearing it's like shifting gears in your car or
shifting gears on a bicycle to match the load on the engine with the speed that
the airplanes traveling at so slower and climb speeds faster and cruise speeds it
makes the engine more efficient we can go a little faster burn a little less
fuel climb a little better all those sorts of things
that's why we have them now let's go ahead and dive into what you need to
know for your commercial or CFI check ride so let's first step into the
cockpit here and take a look at our basic controls you're used to seeing a
tachometer and we still have the tachometer but we also have a manifold
pressure gauge manifold pressure gauge is power it's how much you shove that
throttle forward tachometer is simply how fast the propeller spinning it could
spin very fast and take a very small bite of air and give you really not a
lot of thrust or a lot of power to pull the airplane forward through the air or
it could turn at low rpm take a big bite of air think about being a low speed on
your bicycle you turn your feet very slowly but you spin the back wheel very
quickly so you can go very fast same thing with the airplane you could
actually be at a low rpm setting take a big weight of air and develop more
thrust there's always going to be an off angle of attack on the propeller for a
given airspeed and when I saying will attack on a propeller it's exactly the
same is on a wing however it's inverted 90 degrees so instead of the air flowing
sideways as we're used to straight from the nose to tail across the wing that's
angled attack for our wing the angle attack for the propeller is in the
vertical plane rather than the horizontal plane because those blades
think about them just slicing up and down they're moving so quickly it's not
really moving forward I'll just think about slicing up and down and moving
forward very slightly and we can look at this diagram here that shows us when the
airplanes sitting there on the runway ready for takeoff and you hold the
brakes and you apply full power and you have the blades pitch to say maybe an
arbitrary number like 8 degrees angle attack so that propellers taking an
eighth degree angle attack bite of air it's taking that full bite eight degrees
now as the airplane accelerates air begins to flow through the propeller in
the horizontal plane and that causes a lower angle of attack so all of a sudden
now the propeller has lower angle of attack which has a little bit less drag
then so as you accelerate down the air runway in your 152 or in your Piper
Cherokee you see the RPM slightly increase as air speed increases our PM
increases but with the constant speed propeller it's constantly taking a
bigger and bigger and bigger bite of air to maintain that set rpm and maintain
that set angle of attack the angle of attack that the manufacturer designed it
to be most efficient for because with any wing or any propeller in a propeller
is just a wing spinning in the vertical plane any wing or propeller has an
optimum angle of attack it is designed for and any more any less is going to be
more drag just like we know our maximum lift over drag speed on our airplane our
best glide speed those all come from a precise angle of attack that we want to
fly on the wing where the wing is produces the most lift for the least
amount of drag same thing with the propeller there's going to be an RPM and
a speed at which it produces the most lift or forward thrust the thrust
pulling the airplane forward or power where it can produce its most thrust
pulling you through the air for a given rpm in a given speed and ultimately give
an angle of attack now to sum that up this is the end of part 1 so let me sum
it up for you here in the simplest terms a constant speed propeller is simply a
transmission or gear shifting on a bicycle or in your car
you can have high rpm or low rpm you can have high power settings or low power
settings and you can vary the amount of power separately from the RPM why is
that good well we can go high rpm high power for climb we can go high power low
rpm for cruise play as we accelerate we have them for efficiency they run off
oil pressure from the engine that's how they control their angle and to control
the amount of oil pressure being fed to them we have a propeller governor we're
going to talk about that in part two that prop governor is what's connected
to your blue lever in the cockpit so there's a cable that runs from that blue
lever to the prop governor it senses the RPM of the engine and then varies the
amount of oil pressure going to the propeller to control the blade angle
which controls the RPM by controlling the load so if the RPM needs to high it
increases the load lowers the RPM back down does a very good job of modulating
that rpm that's the basics that's what I really want you to take away from this
video part 2 has a lot more detail in it and a lot more examples of actually
using one of these on the airplane to kind of correlate what's going to happen
with manifold pressure rpm go ahead and check that out part 2 is included in our
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