manifold pressure

[Colonel]

Some data for you, from a recent flight of mine with a 540 and c/s prop.



Top blue line is RPM. You can see it is zero, then I start, then a quick mag
check before taxi. Runup at 12:19 and takeoff at 12:32, RPM reduction from
2700 to 2500 at 12:33

Bottom blue line is MP. Note that it starts at 30" MP (see above) and once
the engine starts, it drops to 12"MP or so. It is increased to 20"MP for the
runup - which is enough power to overcome the brakes.

Note the MP goes up to 28" or 29" MP during takeoff at 12:32. I lean the
mixture at 12:34 and you can see the (green) fuel flow (GPH) drop to 13-ish.

Fun Fact: at wide open throttle, your MP becomes an altitmeter - it will
drop as you climb :^) Note that my climb/cruise power setting is 24 inches MP
and 2500 RPM and 13 gph, because like Ricky Bobby, I like to go FAST.

[digits]

I think the car analogy although technically not completely accurate does convey some good info, it at least gets the ball rolling on relating high and low gears to fine and course props and how they affect engine loading.

Whatever helps you to understand of course.

If you can answer the following questions, you probably understand the constant speed props pretty well:

1) In cruise, what happens when you increase the throttle but don't touch the RPM lever?
2) In cruise, what will eventually happen when you decrease the throttle to idle but don't touch the RPM lever?
3) If your RPM lever has a low setting for cruise, and you decide to go full throttle at low altitude without moving the RPM lever, what will happen to the RPM?
4) Same question as 3), but this time at altitude. Will there likely be a difference? Yes or no?
5) On the ground, why does the RPM vary if you move the throttle for taxi?
6) What changes the propeller blade angle?
7) What would (likely) happen to your prop in a single engine non-aerobatic airplane if you lose oil pressure in the governor (note, might vary per airplane type)?

[TwinOtterFan]

Some data for you, from a recent flight of mine with a 540 and c/s prop.



Top blue line is RPM. You can see it is zero, then I start, then a quick mag
check before taxi. Runup at 12:19 and takeoff at 12:32, RPM reduction from
2700 to 2500 at 12:33

Bottom blue line is MP. Note that it starts at 30" MP (see above) and once
the engine starts, it drops to 12"MP or so. It is increased to 20"MP for the
runup - which is enough power to overcome the brakes.

Note the MP goes up to 28" or 29" MP during takeoff at 12:32. I lean the
mixture at 12:34 and you can see the (green) fuel flow (GPH) drop to 13-ish.

Fun Fact: at wide open throttle, your MP becomes an altitmeter - it will
drop as you climb :^) Note that my climb/cruise power setting is 24 inches MP
and 2500 RPM and 13 gph, because like Ricky Bobby, I like to go FAST.

I'm starting to get it (maybe) at 12:33 when you reduce the prop from 2700 to 2500 my understanding is I would see a slight increase in MP but you appear to have a gradual decrease?

[TwinOtterFan]

I think the car analogy although technically not completely accurate does convey some good info, it at least gets the ball rolling on relating high and low gears to fine and course props and how they affect engine loading.

Whatever helps you to understand of course.

If you can answer the following questions, you probably understand the constant speed props pretty well:

1) In cruise, what happens when you increase the throttle but don't touch the RPM lever?
2) In cruise, what will eventually happen when you decrease the throttle to idle but don't touch the RPM lever?
3) If your RPM lever has a low setting for cruise, and you decide to go full throttle at low altitude without moving the RPM lever, what will happen to the RPM?
4) Same question as 3), but this time at altitude. Will there likely be a difference? Yes or no?
5) On the ground, why does the RPM vary if you move the throttle for taxi?
6) What changes the propeller blade angle?
7) What would (likely) happen to your prop in a single engine non-aerobatic airplane if you lose oil pressure in the governor (note, might vary per airplane type)?

I'll give this a go,

1) an increase in MP as it is getting closer to ambient, with no change in RPM as it is constant? So the prop will become more course?
2) As you close the throttle you will eventually end up around 10-12 MP and the prop will continue to go "fine" trying to maintain the requested RPM until it hits the fine stops?
3) MP to ambient so 29ish if I am near SL, the engine now having lots of fuel and air would want to spin faster so would the RPM increase while the prop tries to go course and keep it at the set RPM?
4) I would assume less available power so same response but not as high?
5) on the ground at idle the prop is sitting on the fine stops, so an increase in throttle will increase the speed of the prop until it reaches a state of overspeed and adjust? essentially it is fixed pitch until fast enough?
6) engine oil through a governor using fly weights and a spring that push or allow a return of oil from the prop to change the pitch, the spring in the governor is set off of the prop lever.
7) I'm not sure, I assume what is safest? like default to the best pitch for that aircraft? like a similar fixed pitch prop?

the mechanics of the prop and governor make sense, I just see to be having trouble picturing the MP, it almost seems like sometimes it is pressure, other times its vacuum?

[Colonel]

when you reduce the prop from 2700 to 2500 my understanding is I would see a slight increase in MP

That's the theory. Practically speaking, I am configuring on a downwind departure - when
I take my left hand off the throttle to pull the prop control back to 2500, it's probably sliding
back due to a lack of friction in the lever.

Later, I've also got my left hand off the throttle, dialling back the mixture to 13 gph but the
MP seems to be pretty steady at 24 inches then.

Note the drop, then increase in MP/GPH after takeoff. I'm going full throttle because I'm
turning into my wing (poor form) and I want to keep his AOA up. You can see the MP come
back as I roll out on downwind.

[Colonel]

I just see to be having trouble picturing the MP, it almost seems like sometimes it is pressure, other times its vacuum?

Heh. Without a turbo/supercharger pumping air into the engine, MP is always
at or less than ambient pressure, which always decreases as you climb.

30 inches MP is pretty much all you're ever going to see, without some kind of
blower. I used to have an M20J with this weird ram air intake flapper behind
the prop, which was supposed to add an inch, but ...

[Colonel]

7) What would (likely) happen to your prop in a single engine non-aerobatic airplane if you lose oil pressure in the governor (note, might vary per airplane type)?

This is a great question ... most singles when they lose oil pressure, the
prop goes full fine - same as if you pushed the blue knob all the way in.

However, aerobatic singles and twins always have counter-weighted props
which go full coarse when they lose oil pressure.

Why is that?

Neat tip: in a C182 after an engine failure, you can extend your glide if you
pull the blue knob all the way out.

Note that in a single, you want to descend with minimum RPM for maximum
speed. Do NOT increase the RPM before descent if you want the Tower to
request a speed reduction from you, following jet traffic.

[David MacRay]

This is some good stuff. Thanks guys.

I was about to guess wrong on the loss of oil pressure question.

[digits]

I think the car analogy although technically not completely accurate does convey some good info, it at least gets the ball rolling on relating high and low gears to fine and course props and how they affect engine loading.

Whatever helps you to understand of course.

If you can answer the following questions, you probably understand the constant speed props pretty well:

1) In cruise, what happens when you increase the throttle but don't touch the RPM lever?
2) In cruise, what will eventually happen when you decrease the throttle to idle but don't touch the RPM lever?
3) If your RPM lever has a low setting for cruise, and you decide to go full throttle at low altitude without moving the RPM lever, what will happen to the RPM?
4) Same question as 3), but this time at altitude. Will there likely be a difference? Yes or no?
5) On the ground, why does the RPM vary if you move the throttle for taxi?
6) What changes the propeller blade angle?
7) What would (likely) happen to your prop in a single engine non-aerobatic airplane if you lose oil pressure in the governor (note, might vary per airplane type)?

I'll give this a go,

1) an increase in MP as it is getting closer to ambient, with no change in RPM as it is constant? So the prop will become more course?
2) As you close the throttle you will eventually end up around 10-12 MP and the prop will continue to go "fine" trying to maintain the requested RPM until it hits the fine stops?
3) MP to ambient so 29ish if I am near SL, the engine now having lots of fuel and air would want to spin faster so would the RPM increase while the prop tries to go course and keep it at the set RPM?
4) I would assume less available power so same response but not as high?
5) on the ground at idle the prop is sitting on the fine stops, so an increase in throttle will increase the speed of the prop until it reaches a state of overspeed and adjust? essentially it is fixed pitch until fast enough?
6) engine oil through a governor using fly weights and a spring that push or allow a return of oil from the prop to change the pitch, the spring in the governor is set off of the prop lever.
7) I'm not sure, I assume what is safest? like default to the best pitch for that aircraft? like a similar fixed pitch prop?

the mechanics of the prop and governor make sense, I just see to be having trouble picturing the MP, it almost seems like sometimes it is pressure, other times its vacuum?

1. Correct
2. Yes, the prop will go fully fine trying to maintain the requested RPM. At a certain point, you will not be able to maintain the rpm (when the prop is full fine) and the rm will starts to decrease. Just like taxiing on the ground. You're basically flying a fixed pitch prop at that point with a very fine setting (= good for climbing, slow for cruise).
This is normal behaviour, and does not harm the engine/prop.
3. The prop goes to maximum coarse angle (without feathering, that's a special case), and then the rpm will increase as the extra power coming from the engine has to go somewhere. This is hard on the engine, and is to be avoided in most (all?) airplanes. In this situation, the correct sequence of events is to increase the rpm first ,before adding more throttle.
4. Correct, you will end up with less power, so less chance that you would get to full coarse. Just because you can go full throttle at 2300 rpm at 10 000 ft, doesn't mean you can do that at 2000 ft.
5. Pretty much. It will keep accelerating until it reaches the requested rpm set by the rpm lever. On the ground that is usually full forward, which means you will/should get maximum engine rpm before the prop starts moving to coarse (during the takeoff roll)
6. Yes, it's basically a balance between aerodynamic forces and oil pressure
7. Whatever way your aerodynamic force is working, is the way the prop will turn. You will have lost your oil pressure. So generally it's either full fine or (close to) feather. Twins are built to go to full coarse and expect you to shut the engine down if necessary, as you have a second engine. Singles usually go full fine. You'd have to throttle back to prevent an overspeed, but at least your engine still works. Aerobatic engines tend to go to full coarse as well, otherwise if you go full fine at extreme speeds, the engine would get damaged due to the crazy rpms.

[anofly]

the manifold pressure gauge on a normally aspirated engines, if they are running , is a vacuum gauge if you interpret it. Atmospheric is say 30.00 inches and you are running at say 24.0 inches so you are at 6" of mercury vacuum (with the reference being atmospheric pressure). The Manifold pressure gauge is an "absolute" inches of Mercury" gauge. The reference is a "perfect" vacuum. If the engine is running ,it will never quite hit atmospheric pressure (30 inches in our case) due to intake geometry, filters, throttle obstructions etc. in addition any time the velocity of a gas increases the pressure tends to drop..like in the venturi of a carb. to make sure things are running right on a constant speed on takeoff, you need to make sure the rpm is up near max AND the manifold pressure is up close to ambient pressure. Some folks make note of ambient and look for something say an inch or thereabouts less for takeoff. I look for something above 29 or so, it will fly with that If the mice made a nest in the intake or similar, you will get much less than ambient and you best stop and go back to the barn and check things out..