How do aircraft engines limit revs?

[Henning]

I have not seen a carburetor that has the throttle downstream from the venturi.
The constant velocity carburetors on my Rotax 914 appear to me to have a throttle and the carburetor measures the pressure differential caused by the throttle opening to adjust the slide (variable venturi) to give a stronger signal to the main jet.It also has a tapered needle to better manage the mixture that goes up and down with the slide.

Then you have never seen 99% of the carburetors ever made. The throttle plate is always downstream (even if that means above physically in an updraft carb as is typical on most planes) from both the venturi and the main jet with the idle circuit feeding downstream of the throttle plate.

 

[warthog1984]

In my opinion in a carbureted engine the throttle limits the amount of air the engine can draw in by restricting the inlet tract.
I feel that the location upstream of the venturi does not change the function of the throttle to limit the speed of the engine.
It appears to me a venturi is shaped to speed up the air to create a low pressure to draw fuel from the float bowl in a carbureted engine.
In my Lycoming IO-320 B1A fuel injection the fuel appears to me to be injected into the ports rather than the cylinders.

I don't know how to restate this in a way you will understand, but I'll try.

In an engine, the max airflow (and hence power) is restricted by the smallest air intake cross section. For carbed engines, this is at the throat of the carb venturi. For fuel injection, this is some point of the air intake.

This is a fixed value- you cannot shove more air through the engine at any given speed without swapping components. Thus, this limits the max airflow, which controls the power and revs a given engine setup can produce.

The usable airflow at any given time is regulated (not limited) by a movable butterfly valve or similar arrangement. This valve is controlled by the throttle and is generally placed at or near the carb venturi if one exists.

clear as mud?

 

[Vance Breese]

Almost. The Throttle Plate on a carburetor limits the output flow, the Choke plate limits the input flow, the difference is the amount of vacuum registered at the main jet when the restriction is applied. The throttle will limit both fuel and air at the same mixture ratio, while the choke will simultaneously limit air and INCREASE fuel flow changing the mixture.QUOTE]

I am don’t understand why you are introducing the choke.
I don’t recall using a choke to limit the engine RPM.
In my opinion the throttle limits the amount of air to the engine and it is what I use to control engine RPM.
Thank you for trying to teach me about engines and language.
I am off to bed.

 

[Vance Breese]

I don't know how to restate this in a way you will understand, but I'll try.

In an engine, the max airflow (and hence power) is restricted by the smallest air intake cross section. For carbed engines, this is at the throat of the carb venturi. For fuel injection, this is some point of the air intake.

This is a fixed value- you cannot shove more air through the engine at any given speed without swapping components. Thus, this limits the max airflow, which controls the power and revs a given engine setup can produce.

The usable airflow at any given time is regulated (not limited) by a movable butterfly valve or similar arrangement. This valve is controlled by the throttle and is generally placed at or near the carb venturi if one exists.

clear as mud?

Thank you for your patience.
In most of the engines I am familiar without a load they will increase engine RPMs to destruction.
My Lycoming O-290 G was that way.Revolutions per minute were not limited by the venturi and if I flew fast enough in a descent they were not limited by the load of the propeller.
I used the throttle to limit engine RPMs with the O-290; not the venturi.
It is time for bed, we will have to agree to disagree.

 

[Henning]

Almost. The Throttle Plate on a carburetor limits the output flow, the Choke plate limits the input flow, the difference is the amount of vacuum registered at the main jet when the restriction is applied. The throttle will limit both fuel and air at the same mixture ratio, while the choke will simultaneously limit air and INCREASE fuel flow changing the mixture.

I am don’t understand why you are introducing the choke.
I don’t recall using a choke to limit the engine RPM.
In my opinion the throttle limits the amount of air to the engine and it is what I use to control engine RPM.
Thank you for trying to teach me about engines and language.
I am off to bed.

Because you said this:

In my opinion in a carbureted engine the throttle limits the amount of air the engine can draw in by restricting the inlet tract.
I feel that the location upstream of the venturi does not change the function of the throttle to limit the speed of the engine.

The CHOKE is upstream (before the air passes into) of the venturi, and the throttle plate is downstream (after the air exits the venturi), I was trying to get that difference across to you and why/what the difference in function is and how/why it works that way.

When the throttle is wide open, it has no limiting effect, the venturi is the restriction. The throttle only comes into play when you want to reduce the flow from the natural limit of the venturi.

 

[Vance Breese]

Almost. The Throttle Plate on a carburetor limits the output flow, the Choke plate limits the input flow, the difference is the amount of vacuum registered at the main jet when the restriction is applied. The throttle will limit both fuel and air at the same mixture ratio, while the choke will simultaneously limit air and INCREASE fuel flow changing the mixture.Because you said this:
The CHOKE is upstream (before the air passes into) of the venturi, and the throttle plate is downstream (after the air exits the venturi), I was trying to get that difference across to you and why/what the difference in function is and how/why it works that way.

When the throttle is wide open, it has no limiting effect, the venturi is the restriction. the throttle only comes into play when you want to reduce the flow from the natural limit of the venturi.

Yes that was a mistake to suggest the venturi was down stream of the throttle.
I stand corrected.
That doesn’t change my opinion that the engine is prevented from over speeding by the throttle and load; not the venturi.

 

[Henning]

Yes that was a mistake to suggest the venturi was down stream of the venturi.
I stand corrected.
That doesn’t change my opinion that the engine is prevented from over speeding by the throttle and load; not the venturi.

What happens when the throttle is wide open? All the throttle does is to limit the air that flows through the venturi, since that flow is what sucks the fuel through the main jet, that flow through the venturi is the key limiter regardless of throttle position, and that is the determinant of how much load can be accepted, and the prop pitch is the determinant of how much load is produced. That is until you get the nose down and and take the load required for whatever, lets call it 2500, rpm in level flight. With a CS prop at that point the blades will coarsen to add load until they hit the stops, typically by this point you are flirting with if not exceeding Vne. With a fixed pitch prop RPM will increase until the supersonic drag on the prop tips adds the extra load to stabilize the engine speed at the max HP it can flow fuel for. With a carb fuel flow is dictated directly by the fuel that gets sucked out of the bowl through the main jet by the mass of air that can flow through the venturi. The throttle plate is a limiting device, but it is not THE limiting device as there are further limits in the carb itself when the plate is wide open. You can remove the plate altogether and the RPM will still be limited by the mass of air that can flow through, you just won't have the ability to regulate the engine below that limit through airflow. You can however still change the RPM by changing the load through climbing or diving (or changing the prop pitch with a variable pitch prop) or reducing the fuel available through the metering block with the red knob. I rarely move the throttle on a fuel injected engine as I can get a better result restricting fuel flow than airflow. My throttles stay wide open from take off until descent typically.

 

[Vance Breese]

What happens when the throttle is wide open? All the throttle does is to limit the air that flows through the venturi, since that flow is what sucks the fuel through the main jet, that flow through the venturi is the key limiter regardless of throttle position, and that is the determinant of how much load can be accepted, and the prop pitch is the determinant of how much load is produced. That is until you get the nose down and and take the load required for whatever, lets call it 2500, rpm in level flight. With a CS prop at that point the blades will coarsen to add load until they hit the stops, typically by this point you are flirting with if not exceeding Vne. With a fixed pitch prop RPM will increase until the supersonic drag on the prop tips adds the extra load to stabilize the engine speed at the max HP it can flow fuel for. With a carb fuel flow is dictated directly by the fuel that gets sucked out of the bowl through the main jet by the mass of air that can flow through the venturi. The throttle plate is a limiting device, but it is not THE limiting device as there are further limits in the carb itself when the plate is wide open. You can remove the plate altogether and the RPM will still be limited by the mass of air that can flow through, you just won't have the ability to regulate the engine below that limit through airflow. You can however still change the RPM by changing the load through climbing or diving (or changing the prop pitch with a variable pitch prop) or reducing the fuel available through the metering block with the red knob. I rarely move the throttle on a fuel injected engine as I can get a better result restricting fuel flow than airflow. My throttles stay wide open from take off until descent typically.

I have not flown an aircraft with a constant speed propeller so I can’t speak to that.
My primary aircraft engine carburetor experience was with a Lycoming O-290 and the way we had the propeller pitched in a steep descent the engine would over speed at wide open throttle so the venturi was not what limited the engine RPMs; the throttle was.
I replaced it with the IO-320 B1A that will also over rev in a steep descent if I don’t reduce the throttle.
My Rotax 914 is the same way; in a descent I can over rev the engine if I leave the throttle open.
The original poster was asking what limits the revs in an aviation engine.
My opinion based on my limited experience is; when the load is insufficient to limit engine RPM the throttle is what I use to limit rpm, not the venturi.
You are welcome to your opinion and I do not have the skills to debate you.
Good night.

 

[Tom-D]

I have not flown an aircraft with a constant speed propeller so I can’t speak to that.
My primary aircraft engine carburetor experience was with a Lycoming O-290 and the way we had the propeller pitched in a steep descent the engine would over speed at wide open throttle so the venturi was not what limited the engine RPMs; the throttle was.
I replaced it with the IO-320 B1A that will also over rev in a steep descent if I don’t reduce the throttle.
My Rotax 914 is the same way; in a descent I can over rev the engine if I leave the throttle open.
The original poster was asking what limits the revs in an aviation engine.
My opinion based on my limited experience is; when the load is insufficient to limit engine RPM the throttle is what I use to limit rpm, not the venturi.
You are welcome to your opinion and I do not have the skills to debate you.
Good night.

Of course your engine will over speed in a dive at wide open throttle. It has no load, and is being driven by the prop.

 

[Henning]

I have not flown an aircraft with a constant speed propeller so I can’t speak to that.
My primary aircraft engine carburetor experience was with a Lycoming O-290 and the way we had the propeller pitched in a steep descent the engine would over speed at wide open throttle so the venturi was not what limited the engine RPMs; the throttle was.
I replaced it with the IO-320 B1A that will also over rev in a steep descent if I don’t reduce the throttle.
My Rotax 914 is the same way; in a descent I can over rev the engine if I leave the throttle open.
The original poster was asking what limits the revs in an aviation engine.
My opinion based on my limited experience is; when the load is insufficient to limit engine RPM the throttle is what I use to limit rpm, not the venturi.
You are welcome to your opinion and I do not have the skills to debate you.
Good night.

Again, that is you restricting the RPM with the throttle, it is not an engine limit. If the throttle cable comes off, the throttle goes wide open and the limiting factor in a dive will be prop tip drag, and most likely that will have the engine turning higher than rated RPM.

The question in the title is "How do ENGINES limit RPM?", this is a design question, not operation question the way I read it. The limiting factor is drag.

 

[poadeleted20]

In a descent with my Lycoming IO-320 B1A I use the throttle to limit the RPM.
It appears to me my engine would exceed the maximum recommended RPM in a dive if I did not retard the throttle.
Do you feel I am not operating the engine correctly?
How would I limit the RPM with the venturi?

You're doing just fine - - ignore the noise.

 

[denverpilot]

LOL. Now I want to throw the hand grenade "diesel Cessna" in here and watch the fur fly.

 

[warthog1984]

Thank you for your patience.
In most of the engines I am familiar without a load they will increase engine RPMs to destruction.
My Lycoming O-290 G was that way.Revolutions per minute were not limited by the venturi and if I flew fast enough in a descent they were not limited by the load of the propeller.
I used the throttle to limit engine RPMs with the O-290; not the venturi.
It is time for bed, we will have to agree to disagree.

You are mixing two different phenomena- engine power limits and forcibly overreving the engine in a dive via the prop windmilling.

You know the throttle regulates engine rpm, but you're describing it as a design limit. This is sort of like Spinal Tap's amplifiers being "1 louder" than everybody else- the regulating control is not the power limitation.

 

[Vance Breese]

Of course your engine will over speed in a dive at wide open throttle. It has no load, and is being driven by the prop.

I am having trouble embracing your concept that the venturi is what limits engine RPM Tom.
The engine still has the same venturi and yet will over speed because the load is reduced.
The throttle is what I use to limit the engine RPM, not the venturi.
In my opinion the prop is not driving the engine even in a steep descent.
Thank you for trying to explain your point of view.
My aviation engine experience is very limited.

 

[Vance Breese]

Again, that is you restricting the RPM with the throttle, it is not an engine limit. If the throttle cable comes off, the throttle goes wide open and the limiting factor in a dive will be prop tip drag, and most likely that will have the engine turning higher than rated RPM.

The question in the title is "How do ENGINES limit RPM?", this is a design question, not operation question the way I read it. The limiting factor is drag.

It is my observation that if the rev limiter malfunctions in a modern engine with a rev limiter the engine will over speed.
I feel that with the three aircraft engines I am familiar with that I am the rev limiter and I manage it with the throttle.
I agree that what limits the RPM of most aviation engines is the load. I can see where with some engines installation that would be drag.
Thank you for your help in understanding your perspective.

 

[Vance Breese]

You are mixing two different phenomena- engine power limits and forcibly overreving the engine in a dive via the prop windmilling.

You know the throttle regulates engine rpm, but you're describing it as a design limit. This is sort of like Spinal Tap's amplifiers being "1 louder" than everybody else- the regulating control is not the power limitation.

In my opinion I have never had the engine driven by a wind milling propeller on any of the aircraft I have flown.
In a descent it is possible to over speed the aircraft engines I have flown with a fixed pitch propeller.
I feel all of the aircraft engines I have had experience with would over speed to destruction at wide open throttle simply by removing the load without any assist from a wind milling propeller because the venturi does not limit RPM.
I feel the throttle is not a design limit.
In my opinion the throttle is one of the ways an aircraft engine is kept from over speeding.
I brought it up because someone felt that the venturi was what limited aircraft engine RPM and I disagree.

 

[Vance Breese]

You're doing just fine - - ignore the noise.

Thank you Ron.

 

[Tom-D]

In my opinion I have never had the engine driven by a wind milling propeller on any of the aircraft I have flown.In a descent it is possible to over speed the aircraft engines I have flown with a fixed pitch propeller.

Didn't you say that your aircraft would overspeed when in a dive?

I brought it up because someone felt that the venturi was what limited aircraft engine RPM and I disagree.

Well enlighten us why does the engine do that?

 

[peerlesscowboy]

actually it is the size of the carburetor Venturi.

when it can only suck so much air it can only go so fast.

So are you saying that you could increase the RPM and thus the engines horsepower by installing a bigger carburetor?

 

[Tom-D]

So are you saying that you could increase the RPM and thus the engines horsepower by installing a bigger carburetor?

Theoretically YES.

 

[Vance Breese]

Didn't you say that your aircraft would overspeed when in a dive?

Well enlighten us why does the engine do that?

I wrote that in my experience my engine will over speed in a descent.
I feel the is because of a reduction in the power required to drive the propeller.
In my opinion a reduction in power required to drive the propeller happens at a speed well before the propeller would drive the engine.
It appears to me I don’t know enough to enlighten you.
I feel this is a simple divergence of opinion.
In my opinion the way the aircraft engines I am familiar with limit rpm is with the throttle.
Some have written that the venturi limits the engine RPM.
I suspect there is not a reason to resolve this divergence of opinion.

 

[Henning]

LOL. Now I want to throw the hand grenade "diesel Cessna" in here and watch the fur fly.

Diesels are different, they have no throttle plate, they limit RPM by fuel flow or load, governor or 'rack', whichever comes first.

 

[Henning]

It is my observation that if the rev limiter malfunctions in a modern engine with a rev limiter the engine will over speed.
I feel that with the three aircraft engines I am familiar with that I am the rev limiter and I manage it with the throttle.
I agree that what limits the RPM of most aviation engines is the load.I can see where with some engines installation that would be drag.
Thank you for your help in understanding your perspective.

You are confusing the terms limit and restriction. A limit is a design feature, a restriction is an operational feature. When you reach the maximum the machine will do, that is the limit. When you pull back from that point operationally with the throttle, you are restricting the output.

That said, what you are doing operationally to manage RPM is correct, however it is not the question that the title of this thread asks about, it is completely irrelevant to the wording of the title question.

 

[warthog1984]

So are you saying that you could increase the RPM and thus the engines horsepower by installing a bigger carburetor?

That's what the old hotrodders did- get the biggest carb you could afford to get more fuel air mix delivered to the cylinders. Limiting factors on HP then became the pistons and con rods expanding or breaking due to speed.

 

[Henning]

So are you saying that you could increase the RPM and thus the engines horsepower by installing a bigger carburetor?

It depends, but most likely yes, the carb is the first limit you'll hit, so a higher CFM carb will get you some more RPM out of it, but you would likely need to change the cam profile to make it worthwhile, and likely need to take a few inches off the prop diameter and redo the pitch profile. Once the prop tip starts exceeding .92M the drag curve starts climbing hard.

 

[brian]]

Well, this just described the "evolution" of the e series engine and the electric prop. The bigger e series continentals made more power with higher revs- going from 165hp to 225 hp. The prop shops started cutting an inch or so off the the blades to keep the tips under Mach 1. you can basically use the same case most of the way while upgrading, changing the cam and I think the crank. Obviously the fule system was upgraded to deliver more gas to the psc carb.

Definitely have to procedurally limit the Rpms as you can overspeed the prop in legal cruise. Great way to get a better than book tas... But I wouldn't know anything about that

 

[Stewartb]

The first component most airplane owners will address to improve power is the exhaust, not the carb. That ain't anything new, either.

 

[Ted]

OEM exhausts are notoriously bad. Ideal would be actual tuned headers. This would be easy to do since you have such a small RPM band. Iid optimize for cruise RPM.

 

[Vance Breese]

You are confusing the terms limit and restriction. A limit is a design feature, a restriction is an operational feature. When you reach the maximum the machine will do, that is the limit. When you pull back from that point operationally with the throttle, you are restricting the output.

That said, what you are doing operationally to manage RPM is correct, however it is not the question that the title of this thread asks about, it is completely irrelevant to the wording of the title question.

I have tried to embrace your use of the language and I have been unsuccessful.
I am often confused and endeavor to be confused on a higher level.
I see from your many posts that the specific meaning of words is important to you.
I am probably more communication oriented specifically writing to the reader.
On my convoluted life’s path one of the things I did as a designer was to be involved in the engineering meetings for producing a 100 cubic inch twin cylinder motorcycle engine.
I realize that this has very different parameters compared to an engine for aviation.
One of the common needs was to limit engine RPM.
There were lengthy discussions about how to limit the rpm because we wanted to have the engine last and high rpm would shorten its life considerable.
Some thought that simply having a tachometer would allow the rider a choice between engine longevity and performance and he could effectively be the rpm limiter.
After running the proof of concept engine on a dynamometer we discovered that the engine didn’t breathe at higher RPM and the torque was falling off so quickly as RPMs rose there was no reason to give people that choice.We put a rev limiter on it.
Based on my experience I feel using the operator as a rev limiter is an engineering choice and goes specifically to the question originally posted.
I was not trying to answer the original question; I was pushing back on the idea that the venturi was used to limit RPM in aviation engines.
Thank you for helping me to be more careful in my choice of words.

 

[Henning]

I have tried to embrace your use of the language and I have been unsuccessful.
I am often confused and endeavor to be confused on a higher level.
I see from your many posts that the specific meaning of words is important to you.
I am probably more communication oriented specifically writing to the reader.
On my convoluted life’s path one of the things I did as a designer was to be involved in the engineering meetings for producing a 100 cubic inch twin cylinder motorcycle engine.
I realize that this has very different parameters compared to an engine for aviation.
One of the common needs was to limit engine RPM.
There were lengthy discussions about how to limit the rpm because we wanted to have the engine last and high rpm would shorten its life considerable.
Some thought that simply having a tachometer would allow the rider a choice between engine longevity and performance and he could effectively be the rpm limiter.
After running the proof of concept engine on a dynamometer we discovered that the engine didn’t breathe at higher RPM and the torque was falling off so quickly as RPMs rose there was no reason to give people that choice.We put a rev limiter on it.
Based on my experience I feel using the operator as a rev limiter is an engineering choice and goes specifically to the question originally posted.
I was not trying to answer the original question; I was pushing back on the idea that the venturi was used to limit RPM in aviation engines.
Thank you for helping me to be more careful in my choice of words.

1lim·it

noun \ˈli-mət\ : a point beyond which it is not possible to go

If you can push the throttle in further, you have not hit the limit. Yes, the proper use of words is important, that is why we have different words to represent differences in concept, it allows for clear and concise communications of varying ideas. Of course this only works when everyone involved understands how to use the words correctly, otherwise it just drives further confusion, that's why they make dictionaries.

 

[Tom-D]

I wrote that in my experience my engine will over speed in a descent.
I feel the is because of a reduction in the power required to drive the propeller.
In my opinion a reduction in power required to drive the propeller happens at a speed well before the propeller would drive the engine.
It appears to me I don’t know enough to enlighten you.
I feel this is a simple divergence of opinion.
In my opinion the way the aircraft engines I am familiar with limit rpm is with the throttle.
Some have written that the venturi limits the engine RPM.
I suspect there is not a reason to resolve this divergence of opinion.

In your way of thinking the load on the prop is the limiting factor, the static RPM which the engine can make will be determined by load. simply because the engine is making as much power as it can, the throttle plate is wide open, the Venturi is a set size, so adding or removing load. this is proven by the cruise or climb prop theory.

BUT

Remove all the load place a weight equal to the prop for flywheel, and the engine will go to a RPM that is allowed by the diameter of the Venturi, and the MAP created by the airflow thru it.

 

[poadeleted20]

So are you saying that you could increase the RPM and thus the engines horsepower by installing a bigger carburetor?

...or slapping on a turbocharger. Anything which increases the maximum mass of fuel/air mixture to the engine will increase the potential RPM and power available. And in that regard, the induction system design is a limiting factor in engine power (and thus RPM, as long as the prop remains the same), which is why I get a bit more full throttle power/RPM out of my Tiger with its LoPresti cowl/induction system over the stock induction system -- increased airflow and cooler (and thus denser) inlet air.

At the end of the day, what limits RPM's in piston aircraft engine is the equilibrium between prop drag and engine torque. Many factors determine how much drag the prop creates, and how much power (torque) the engine creates, and I think all the significant ones have been covered here. Which one is limiting at ay one time changes with circumstances and system design, so you can't really say any one element is the limiting factor.

 

[Silvaire]

...At the end of the day, what limits RPM's in piston aircraft engine is the equilibrium between prop drag and engine torque...

At the end of the day we see how a POA thread can go completely off the rails as the OP was simply asking about rev limiters that purposely prevent the engine from exceeding its designed redline RPM.

Again - there is no such thing on your typical GA engine.

 

[Tom-D]

...or slapping on a turbocharger. Anything which increases the maximum mass of fuel/air mixture to the engine will increase the potential RPM and power available. And in that regard, the induction system design is a limiting factor in engine power (and thus RPM, as long as the prop remains the same), which is why I get a bit more full throttle power/RPM out of my Tiger with its LoPresti cowl/induction system over the stock induction system -- increased airflow and cooler (and thus denser) inlet air.

At the end of the day, what limits RPM's in piston aircraft engine is the equilibrium between prop drag and engine torque. Many factors determine how much drag the prop creates, and how much power (torque) the engine creates, and I think all the significant ones have been covered here. Which one is limiting at ay one time changes with circumstances and system design, so you can't really say any one element is the limiting factor.

Exactly...... there is no one thing.

 

[Henning]

So, lets just take level cruise as an example. As Ron said, it's an equilibrium. In order to increase performance, you have to increase torque. In order to increase torque you have to increase fuel flow, in order to increase fuel flow you have to increase airflow.

In order of quantity of improvement per dollar spent is first put on a tuned exhaust system in a Tri-y format where opposite firing cylinders are tied in a Y then those 2 together on a 4 cyl. With a 6 you tie the 120°s into a collector then those 2 into a Y. For the extractor pipe trim length the best thing I have found is put on a 3' long tube, then draw down its length with a yellow lumber keel (Crayon would likely work as well) then go run it hard. Where the keel burn off ends and line remains, that is where you cut the pipe.

Next is to add carburetor or FI capacity, this would include turbo charging/normalizing.

Next is to change pistons and add some compression.

Camshafts would normally come before pistons, but in certified aircraft the options on this are a bit limited, though it can be done if you go Experimental R&D and have the cam reground then fly it around to develop the data for an STC (there are planes out there flying under these auspices for decades unrestricted after the first 40 hrs).

In the middle of all this, one has to modify a fixed pitch propellor to make use of the extra torque developed and to absorb it limiting the level cruise RPM, re-establishing the equilibrium at a slightly higher airspeed (the main benefit of adding power is in increased rate of climb).

 

[Tom-D]

In order to increase torque you have to increase fuel flow,

I believe you have the cart before the horse here.

Torque is, the resistance to twist. To increase torque you increase load, to maintain speed, you add power.

 

[coloradobluesky]

Power = Torque x Speed. Speed in this case is rpm's, not airspeed.

 

[Henning]

I believe you have the cart before the horse here.

Torque is, the resistance to twist. To increase torque you increase load, to maintain speed, you add power.

It's a chicken or egg relationship. You can increase fuel flow which will increase RPM then re pitch the prop to increase torque by reducing RPM for the same fuel flow.

 

[coloradobluesky]

Has anyone ever seen a pressure curve for the inside of a cylinder based on changing the prop pitch and/or the manifold pressure. I assume pressures are higher for higher manifold pressure etc. Not really sure.

One thing is true (I think), given two power settings that result in the same airspeed, the one with the lower rpms will use the least fuel.

 

[Tom-D]

It's a chicken or egg relationship. You can increase fuel flow which will increase RPM then re pitch the prop to increase torque by reducing RPM for the same fuel flow.

the formula is a triangle at which point do you want to start the change. Torque/RPM/Power

I can lighten the load, RPM will increase. Torque may not.

any time you change one point it will effect one of the others but maybe not both.