Constant speed propellers?

[David MacRay]

So how do you work them?

[Colonel]

Easy. Throw money at them. Money is what makes them work.

There's a lot of complicated gibber-jabber, bit it's actually pretty easy.

You know there's a throttle that moves in and out. There's a prop control
that moves in and out. All the way in, is first gear. All the way out, is top
gear. You will probably never use top gear.

You take off with the throttle / prop / mixture all the way forward.

The simple way to fly it is to reduce power from left to right:
throttle back then
prop back then
mixture back.

The simple way to increase power from right to left:
mixture forward then
prop forward then
throttle forward.

Prop all the way forward gives you max RPM. Low gear for accelerating off the line.
This is the position for takeoff and landing.

Prop all the way back will give you min RPM. High gear for highway cruising. You
will never pull the prop all the way back in normal flight. Ever.

It does this by continuously adjusting the AOA of the prop blades in flight,
as you speed up and slow down, to try to maintain the selected RPM
within the pitch stop limits of the blades.

Example: on downwind you slowly reduce the throttle until the RPM starts
to fall. That means your prop is at full fine pitch, and when you push the
prop control forward for landing, nothing will happen, because the prop
blades are already at full fine pitch (max RPM). At that point, you really
are flying a fixed-pitch prop, albeit a very fine one (climb prop).

There's a lot of BS about oil pressure and counterweights that you really
don't need to know. Most pilots don't. Most AME's don't.

Try to change RPM as slowly as possible.

[David MacRay]

Thanks.

I presume all the way back is feathered and would only be used to reduce drag if the engine quit. Close?

[Nark]

Yup, but generally only in twins.

A basic breakdown: engine oil is used to hold pressure against fly weights keeping the propeller pitch at the setting you choose, ie RPM setting.
If the engine were to fail, and lose oil pressure the fly weight would over come the propeller pitch due to centrifugal force and they would feather. Ideally to zero drag created by the prop being driven by the relative wind.

Singles generally don't have the pitch range for full feathering.

[Nark]

[David MacRay]

I probably should have flown something with one by now but never got to it.

[Colonel]

Useless detail, unknown to most pilots and mechanics:

There are actually two entirely different kinds of c/s props.

The ones you normally find on light singles, go to fine pitch when they lose oil pressure.

The ones you normally find on twins and aerobatic aircraft, go to coarse pitch when they
lose oil pressure. These are counter-weighted props.

This tiny detail is normally totally irrelevant, but ...

One day, my friend Kevin - a retired military pilot and test pilot at the NRC in Ottawa -
was in a dive in his RV-8 at 200 knots. Don't ask why. That's just what retired military
and test pilots do, ok?

But the Lycoming IO-360 in his RV-8 has a very wide oil sump, and if you don't run it
full - which you wouldn't want to, without a slobber pot - and has a nasty habit of
unporting in attitudes that you would not consider unusual. Ask Peter.

So, at 200 knots, Kevin goes to zero oil pressure in his RV-8. That's actually not as
catastrophic for the engine as you might think - bearings and such. See the AEIO
Lycoming notes for some eye-opening comments on that.

No, the problem was when he went to zero oil pressure at 200 knots, his prop went
full fine, and he went through 4,000 RPM.

Not a typo. 4,000 RPM. His prop and bottom end were instantly junk, but rather
interestingly, held together for the remainder of the flight, which I suppose is
educational for the rest of us. A "destructive test" in engineering jargon with
a sample size of one, which is somewhat statistically inhibiting.

As usual, I'm sure I said the wrong thing. I always do, you will know if you know me.
I gushed, "I wish I could have heard it!!" which makes sense if you know of my love
of great music:



Kevin is a great guy, and he got his engine overhauled and bought a new, counter-weighted
prop like all us aerobatic guys have, so when we lose oil pressure, the RPM drops and
we don't kill the engine. I was after him to install the half-Raven with the slobber pot
so he could run full oil without dumping it all out. You pay a little bit in horsepower for
the windage, but IMHO it's worth it to keep the metal parts wet.

Actually, I lie. The first few S-2C's from Aviat had a non-counter-weighted claw which
required the use of an accumulator to avoid problems in downlines. Ask Bill Finegan,
who did some wild vertical dive tests. Later, Hartzell brought out a counter-weighted
claw so the accumulator was no longer required. My friend Freddy had Hartzell try
to convert his early S-2C claw to counter-weighted, IIRC they fucked it up. Tricky.

Kevin was awesome beyond belief. Quiet little guy. One day at the airport, an old
military buddy of his flew in, doing civilian charter. Asked for Kevin, he was not there,
I told his buddy.

His buddy gave me the scoop. Kevin had been selected for test pilot training, which
is quite an honour in the RCAF. There are several test pilot schools in the world, and
he was scheduled to go to the one in France. Very snobby and posh.

Kevin, of course, asks the RCAF, Can I bring my motorcycle to France? The RCAF brass
checked, and said no, you can't. So Kevin says, I don't think I will go to test pilot school.

Now, this is exactly the kind of attitude that the RCAF brass was looking for, in a young
pilot. After a while, they figured out how Kevin could take his 1000cc Suzuki to France,
and off he went to test pilot school, driving around in France with Canadian tags on his
bike.

The RCAF brass was so pleased with Kevin, when he got back, he was put on Trackers,
which he told me did not roll very well.

What a great guy. I remember one day, he gakked the spark plug thread on a cylinder
on his RV-8 so he pulled the cylinder off, and he needed help putting it back on, because
the local AME had locked himself in his hangar and wouldn't talk to anyone. Not even the
chicks. I am not making this up. The local AME is not mentally well. Surprising no one,
he used to be a TC Inspector.

So I help Kevin put his cylinder back on his RV-8. It's really not very hard. It's all the
crud around it that takes the time, and I left him to do that himself. The trick is to
compress the rings on the piston and insert it hanging out of the bottom of the cylinder
with just enough room to slide the wrist pin in, after you line it up with the small end
of the connecting rod. Push it in, put a nut on one of the top studs, and go get the
torque wrench. I have no idea how people do it the other way - with the piston already
on the rod.

I really liked Kevin. He retired from the NRC. I have no idea where he is now.

Here's a picture of Kevin flying #3 on me. Cookie is #2, flying Dr Phil's airplane.



Cookie is dead now, of course, but he wrote quite a book about his exploits.

https://www.google.com/books/edition/Vi ... CAAJ?hl=en

Dr Phil hated his engine, as far as I could tell. 2,000 RPM right after start. Has
a PhD, works for RCMP or JTF or CSIS or someone like that. All black hats and
cloak and dagger, and metal in the oil filter.

[David MacRay]

Obviously he needed to dive because it wouldn't go 200 knots in level flight. Even a simpleton like me can figure that out.

[digits]

Easy. Throw money at them. Money is what makes them work.

There's a lot of complicated gibber-jabber, bit it's actually pretty easy.

You know there's a throttle that moves in and out. There's a prop control
that moves in and out. All the way in, is first gear. All the way out, is top
gear. You will probably never use top gear.

It's that gear analogy that confused me for the longest time, because it isn't correct. Well, sort of. It's like sayin the throttle controls the speed. Which isn't correct, sort of.

If you want to compare the constant speed prop to the car, I think it would help by explaining it differently. When instructors mentioned "gear" to me as a student, the gear of a car would pop into mind. It confused me as to how changing gears would give you a constant RPM. It didn't make sense.
Using the car analogy, it would be better to describe it as "Imagine a car with infinite gears. So you can have gear 1, 2, 3, but also 1.1, 1.3, etc. You set the RPM you want the engine to operate at, you set a certain amount of fuel flow with the pedal and then the car will figure out the highest gear it can go to which will give you the maximum speed for that fuel flow/RPM combo".

In a fixed prop, you set fuel flow with your throttle. The engine then accelerates/increases RPM as much as possible with the given fuel flow.
In a constant speed prop/variable pitch, you still set fuel flow with throttle, but you also decide what RPM you want to operate at. The engine will then change the blade of the propeller as coarse as possible, to give you the best possible speed.

Then there is weird stuff that could happen if you ask it something it can not do. For example go high rpm with zero fuel flow, which doesn't hurt the engine, or go low rpm with full fuel flow, which isn't too good for the engine.

[Colonel]

A c/s prop is a lot like a CVT transmission in a car. Not everyone has
CVT, though.

Throttle doesn't change behavior regardless of what prop or transmission
you connect to it. It opens, it closes. Manifold pressure increases and
decreases and along with it, torque.

It confused me as to how changing gears would give you a constant RPM

Not many people drive a manual transmission any more, but I will try to explain.

Let's say you are in your car cruising at 2000 RPM, and you want to speed up. You could
cruise at 4000 RPM but that would be kind of noisy. So, you shift up to a higher gear,
and the RPM drops down to 2000 RPM at the faster speed. Changing gears allows you
to keep a constant RPM.

The fundamental problem is that people don't understand engines any more. They think
there are magic fairies under the hood or something.

Ask a pilot the difference between torque and horsepower, and I guarantee a blank look.

Hell, ask him to write the lift equation on the board. He makes his living with it, and
he doesn't have a clue.

Gotta keep things simple, and sometimes simple analogies aren't perfect.