Constan Speed Props ... a "Better" way to Teach It

J

James the MEI

Filing Flight Plan
Joined
Oct 28, 2019
Messages
13
Display Name
Display name:
James the MEI
Gents and Ladies.
After Seeing soooooo many You Tube versions of "CFIs" teaching Constant Speed Props....

and frankly only making it apparent that THEY didn't understand it... probably because THEIR TEACHERS DIDNT GET IT....

Please accept this "Intro" to Constant Speed Props
Please slice and dice it...
I am totally ok if you disagree.... it your disagreement bring positive discussion from which we all understand better.

(First a quick commentary)
THE FOLLOWING DISCUSSION DOES..... NOT.... BUILD, DESCRIBE, DISASSEMBLE THE PROP GOVERNOR OTHER THAN ONLY IN PASSING.
Further as a CFI... it is my opinion that requiring a student... or even a commercial or atp pilot to be able to draw the speeder springs, flyweights and pilot valve.... shows a continued lack of professionalism as TEACHERS.

We dont require a new student to be able to draw the insides of an alternator or voltage regulator.... not the insides of a hydraulic pump or a solid state ADHRS.....
But because it is in the PHAK or the AFH.... students are required to "D....R...A...W...." THE GOVERNOR !!!????
That is an insult....
Understand?.... Sure...
Recognized that Oil Pressure Automatically Modulated to Apply/Add/Reduce force at the prop hub.... bla bla bla....sure.
Draw the Governor?
Sure.... when the chif instructor can draw all of the above plus the inside of a carb and a fuel injector distributor.... Just because your instructor's instructor was hazed at Colorado Springs ...that doesnt mean that we can't evolve....
End of Commentary...

_______________________________________________

Constant Speed Props - The Intro you never got

Maybe let's talk about "WHY" first...
(Please copy paste this and chew it up)

let's talk about a characteristic of piston engines / reciprocating / internal combustion engines first.
a) MOST of the engines we use every day, ( car engines, motorcycle, airplane) are 4 stroke piston engines.

b) almost all of these engines regardless of their size ( Cubic inches or cubic centimeters) produce their horse power predictably... Lower RPM= lower power and as RPM increases so does power ( horse power).

c) power vs rpm can be shown on an x/y graph ( slope) such that as rpm increases, so does power... For some engines horse power increase vs rpm is a smooth linear slope.
But for other engines there is an rpm above which HP increase more rapidly due to that engines' specific traits. ( traits such as...the combination of certain induction and exhaust systems , Valve sizes and timings, ignition timing... etc... dont dwell on these traits, just recognize that "power increase curves" are not all linear)

And in General "More RPM, produces more Horse Power" And although HP does increase with RPM, there are limitations with the engine itself and additional limitations associated with the prop that ( together) will limit maximum RPM, and therefor maximum HP of a given Engine and Prop combination.

d) So Ideally for any time that you need performance like acceleration for Take Off , Climbing, Go Around ...you want the engine to to be developing near max horse power.... which means,,, YOU WANT THE ENGINE TO BE SPINNING NEAR max allowable RPM ...

But curiously a "fixed Pitch Prop" can during some phases of flight limit RPM and ( therefore) Horse Power. The next time you start your take off roll in a Fixed pitch prop airplane.... note the rpm with full throttle but BRAKES LOCKED and airplane not moving... the prop and the air are battling each other... and you should note that the RPM IS NEAR STATED "STATIC RPM"... ( 2200 - 2350... maybe 2400 ?) RPM depending on the airplane and prop... check your POH)... and then after you release brakes and start to move, notice that the RPM starts to climb.... let's repeat that... "The prop is spinning at a certain rpm when the airplane isnt moving but then when we release the brakes and the airplane starts to accelerate, the Engine and Prop Rpm increase too" !!??
So the horse power also increases once the aircraft starts to accelerate?

e) What's going on there? .... think of an engine trying to spin a prop in peanut butter. It DOES NOT MATTER THAT YOU HAVE THE THROTTLE ALL OF THE WAY IN... OR THAT THE MANIFOLD PRESSURE IS SHOWING 29".....

The peanut butter provides WAY TOO MUCH RESISTANCE... so the prop end engine cant spin up... nor will the engine ever be able to "Get in to it's power band"...nor produce it's rated horse power.

f) SOOOOOOO..... if we can get the prop to take SMALLER BITES.... it MIGHT be able to spin a little faster... and produce a little more power.... and with more power... it can spin a little faster... and increase it's rpm... and climb up the power band curve

So many instructors try to explain "Constant Speed Props" just like his instructor taught him...
They go WWAAAAYYY into the Prop Gov... but never mention that the engine NEEDS rpm to develop power ! (Too few instructors discuss recip engines and rpm vs hp.

When teaching "Constant Speed Props"... Maybe start by discussing engines, and HP ...
and that we have a type of propeller that can automatically adjust it's bite so that it can help the ENGINE get to it's HORSE POWER PRODUCING RPM RANGE.

And then .... once climb and acceleration requirements are satisfied... THEN not as much horse power, or rpm, or fuel flow are needed.... so we can help the prop help the engine maintain a different "power vs cruise vs efficiency" condition....

topics / concepts
Fine Pitch, RPM and Horse Power
Then
Coarser Pitch to help the Prop better work with the relative wind at cruise speeds
then
Then
Discuss the parts/components and Prop theory but metered specifically to the aptitude of the student.

JT
 
Last edited:
Couple of things: The hp does continue to climb as RPM increases. It's torque that starts to fall off, but since hp is a result of both torque and RPM, hp will still rise. However, the propeller is limited in its RPM; too fast and the drag mounts unacceptably as the outer portions of the blades approach the speed of sound. Too slow and it produces little thrust. And all that is aside from the terrific centrifugal forces on it.

The engine's torque falls off because the engine's design engineers designed it to produce max torque right where they wanted it. Aircraft engines are vastly different from automobile engines that way. They're designed to match the requirements and limitations of the propeller.

So the redline on the tach is set by the airframe manufacturer in conjunction with the engine and prop manufacturers, and the governor is set to control the prop pitch to keep the prop at no more than redline RPM.

The prop's pitch is adjusted to get the most efficient angle of attack for the power being put into it, with redline RPM being the final limiting factor that might require a somewhat higher AoA. That's the secret and purpose of the CS prop: turn the horsepower into thrust most efficiently. A fixed-pitch prop is like having a car stuck in third gear; the CS prop is like having a six-speed automatic.

Many engines spin at more than 2500. The IO-520 can go to 2850, which is why you hear 185s and older 206s snarl on takeoff, especially if they're using a longer seaplane prop. Even the little O-200 goes to 2750.

I have never known any exam or examiner to require drawing the workings of a governor. When I taught aircraft systems I had a cutaway governor so students could see what went on inside it but I never expected them to have a working knowledge of the thing. Shoot, some of them didn't understand carb heat and didn't want to.
 
Last edited:
Think of it as a cars transmission is how I phrase it.
 
ktup-flyer said:
You’re saying car engines make peak power at 2500rpm? Yea...no
Click to expand...

I’m saying who cares.

For someone who never touched a CS prop before it’s a easy way to understand it.

Don’t get into mind numbing detail off the bat, this is like the CFIs who get sooo into proper radio phraseology their students suck on the radio because they are inverted backwards pre analyzing every word before their shaking finger touches the button lol
 
Torque tends to peak first, then eventually power. Peak power in a typical automobile is more on the order of 4000-5000, but I'm not sure that is relevant. For your aircraft, the best poop speed is near redline - that's what you want to run for takeoff / climb.
Best economy is low rpm, high manifold pressure.
Changing prop pitch lets you get closer to where you want to be when you want to be there.

If you did this as a youtube video, you could include the drawings of the prop governor. :)
 
ktup-flyer said:
You’re saying car engines make peak power at 2500rpm? Yea...no
Click to expand...
You’re saying you shift every car from first to second gear at the same rpm and the same speed?
Yea..no

the exact rpm is irrelevant to the conversation. You shift gears to put rpms where you want them for the situation. You use the prop governor to put the rpms where you want them for the situation. That is the equivalency.
 
ktup-flyer said:
You’re saying car engines make peak power at 2500rpm? Yea...no
Click to expand...
Cars aren't coupled to a propeller whose efficency drops off drastically as the blades approach the speed of sound.

You shift gears to put rpms where you want them for the situation.
Click to expand...
You need to shift gears because a piston engine puts out very little torque at low RPMs. Some oversized things like guys who put V8's in motorcycles or people with electric cars (electric have good torque across the range of speeds) don't need transmissions.
 
1) I am impressed that so many people jumped in.
2) I am also impressed that it looks like I smacked the hornets' nest
3) Dan... I hope that we can meet somewhere in the middle...
I have seen check airmen both DPEs and In-House Schools with Self Examining Authorities require the pilot to explain and draw the Prop Gov, Explain in depth the Pendulous Vanes, and explain the orientation of the shafts of the gyros in the various turn coordinator type instruments... and I cringed... yes really.

Similarly.... An in depth discussion of How Rpm, HP and Torque relate.... ok engine redline and prop tip limitations... ok....
But the reason for my keep it simple......

Not once.... have I ever... ever heard "WHY" we target an RPM.
The rest of the discussion Trq v HP v RPM... how does that help a 200 hour Piper Lance Pilot fly more safely?
My point is.... other than for designers... .. at some point Too much info can result in LESS understanding...

p.s.
Car engines are tasked with many more increase power and decrease power changes.
So of course those engines are built and tuned differently... resulting in differing horsepower and torque curves.
And while there are some engines that produce power at higher rpm.... that discussion is valid only as a "By the way"...

I maintain that a simpler description... my general description of the bell shaped curve.... provides enough foundation for enough understanding so that most pilots can then get the concepts.

Sure the next steps include efficiency.... but too much discussion on prop oao, and slip...etc... for me that's getting into weeds that dont need to be traveled by anyone other than techs.... not PPL, not even Commercial nor ATP.... these guys are pilots... not engineers... and that was my point .
Thx
 
Capt. Geoffrey Thorpe said:
.... the best poop speed is near redline - that's what you want to run ...
Click to expand...

I nominate this for best typo of the year. Had a similar experience this morning after eating some pretty strange food yesterday...

For those of us w/ little or no constant speed prop experience, the "shift gears" analogy is at least a good intro. Thinking more of shifting gears on a bicycle helps focus on the torque issue because if you're in too high of a gear on a bike, you pay for it physically immediately and understand the need to increase rpm and decrease the need for power by shifting into a lower gear.. In a car, those folks who aren't necessarily tuned to how much stress is being placed on the engine might not get the analogy as quickly.

I confess I didn't read the OP's entire post. I can slog through bad formatting, grammar, punctuation, and typos for a while (Lord knows I'm guilty of mistakes too) if the subject is interesting and I can figure out what is trying to be conveyed, but that was just too much.
 
Last edited:
Too much info.... PHD... piled Higher and Deeper...
I was advocating for "less but enough"
More and More and More.... is often less

All of this extra info can be added well after... well....after..... the student learns simply...
High RPM for High Performance Power for T/O , Climb and Go around.... then transition the prop away from performance power configuration and into Cruise and Efficiency Configuration.
 
MuseChaser said:
I nominate this for best typo of the year. Had a similar experience this morning after eating some pretty strange food yesterday...

For those of us w/ little or no constant speed prop experience, the "shift gears" analogy is at least a good intro. Thinking more of shifting gears on a bicycle helps focus on the torque issue because if you're in too low of a gear on a bike, you pay for it physically immediately and understand the need to increase rpm and decrease the need for power by shifting into a lower gear.. In a car, those folks who aren't necessarily tuned to how much stress is being placed on the engine might not get the analogy as quickly.

I confess I didn't read the OP's entire post. I can slog through bad formatting, grammar, punctuation, and typos for a while (Lord knows I'm guilty of mistakes too) if the subject is interesting and I can figure out what is trying to be conveyed, but that was just too much.
Click to expand...


I was the originator I accept the criticism for my writing style.
"Stress Placed on the Engine, Mounts and Airframe".... that's worth holding onto

Thank you
 
I like to know more... If it's knowledge useful to me.

For what exam am I going to need to draw the thing?

Most of us have little desire to become astronauts and don't need to know the inner workings of a constant speed prop.

Nor do we need so much knowledge that we can reboot HAL to the "Mary had a little lamb" stage.

More and more a/c will have FADEC, autoland, and autonomous flight as the years go by. And the speed of change will be exponential as we go.

Thanks for trying to simplify the subject.
 
Hey Mr James the MEI,

You had better be teaching a heck of a lot more about CS props that you explained here.
 
...and this spiraled pretty quickly into pedantry. lol

James331 said:
a cars transmission
Click to expand...
I agree... however with so many people driving automatic cars it's sad but many (I would say most) actually have no idea what the transmission does or how it works, they just put the car in "D" and go. It's sad. I've started likening it more riding a bike and shifting gears there... if the pedaling is too hard and you are going slow you shift so your feet go faster and the bike moves faster (blue knob IN).. as you speed up when your feet go too fast and you can't go any faster you shift so your feet slow down (but not too much) and you can go faster (blue knob OUT). RPM is how fast your feet are moving.. blue knob is whatever gear you are in.. and throttle / manifold pressure is how hard you are pushing your feet

James331 said:
they are inverted backwards pre analyzing every word
Click to expand...
I never got why CFIs get so hell bent on this. I've done two TRACON tours here and asked ATC how much they care about phrasing, I even asked "do you guys really get bent out of shape if someone says "with you" and the resounding response was "as long as you don't waste air time and get your thought out clearly and do what you say you are doing we don't care"
 
my statement "Most engines produce their peak power at around 2500"....

OK... that statement was meant to start an understanding... the bell curve does apply... you can look at various power graphs for various engines and then you can shift the bell to the left or right if you need to....

Where the bell curve is shaped for newer high output engines actually does shift considerably...
An example; the 1.3 liter honda auto engine modified by Viking for aviation applications.... 130hp at 5400 rpm.
A Smaller engine... built to precision tolerances .... that can tolerate twice the rpm to which i referred.
My F150 ...4.6 liter...... manual trans... I cruise at 65 at 2000rpm in fifth gear
But if I need performance response ....I downshift, wind it up to "approximately" 2300 and go. ( the beginning of my power band)

But these discussions are exactly what a student DOES NOT... NEED TO HEAR.... until he starts smiling and says, "I got this"
 
It's like Ted Talks. 18 minutes max or it is worthless.

Learn to condense or nobody will pay any attention.
 
Clip4 said:
Hey Mr James the MEI,

You had better be teaching a heck of a lot more about CS props that you explained here.
Click to expand...
More....Yes of course...
sure
overspeed / underspeed / onspeed...check
prop gov modulates oil p per requested rpm.... got it
if the engine starts to loose oil p the prop ( Single engine piston) will move towards lower pitch..... hmmm.... why?
Sure, to reduce stress and to allow the engine to wind up into it's power band.

Also it wasn't my intention to write the whole lesson here
It WAS my intention to bring attention to the concept of "Power Band" and an alternate way to plant the seeds of understanding WHY we have CS Props and the BASIC principles of application.
I have seen WAY too many instructors start by listing a dozen terms and definitions, and then drawing the prop gov....
While the student has NO idea.... WHY this stuff exists.

Why Is often a darn good place to start.

A heck of as lot?.... Regardless of the heck of a lot that we were taught, pilots need to know what they need to know.
But Every nut and pilot valve.... why?

a HECK of a lot more...
hmmmm
 
Think there is the ops side of it, keep that transmission simple and what it does if it screws up and how to deal with it

Then there is the “look at me” systems part

There was a guy in one of my FSI recurrent classes, the is instructor was going on about some minutia systems stuff and this Englishman speaks up “why do I care”

He had a good point, from a cockpit aspect it was fun facts but operationally irrelevant
 
Clip4 said:
That’s the way most people who don’t understand CS props or don’t want to take the time to explain CS props do it.
Click to expand...

And yet it works just fine AS A INTRO into it prior to first flight.

Keep it simple and build onto that.
 
In teaching about constant-speed props, one absolutely has to explain AoA, just as one has to explain AoA when discussing stalls and spins and so on. Without an understanding of AoA, the purpose of a constant-speed prop is lost on the student.

With a fixed-pitch prop, the pitch is too high to let the engine reach anywhere near redline when the throttle is opened at the beginning of the takeoff roll. That means that much less than rated horsepower is being delivered to the prop, so the takeoff is sluggish, the roll is longer, and the climb rate is anemic. When we level off for cruise, we have to throttle back or we'll reach redline RPM at about the same time the airspeed maxes out, which is what the aircraft designers did on purpose when they specified the prop for the engine/airframe combination. You might cruise there (I used to do that for the last half-hour of an engine break-in flight with 172s or Citabrias) but the wear is accelerated and the fuel burn is awesome. And airspeed is pretty good, too.

And all of that has to do with AoA. Too high for takeoff and climb, too low for cruise. It's a compromise in order to save money (and weight) in the building of the airplane.


See this:

f0172-02.gif


We want that prop blade (actually, any section along its span) to be running at optimum AoA to get the best thrust for the HP, which means best efficiency and best performance. As you start moving forward, the incoming airflow accellerates and decreases the AoA. If that AoA gets low enough that thrust equals drag, acceleration stops and RPM is high. If we can increase the blade pitch we can go faster as AoA increases and more thrust is generated and RPM is kept at more efficient levels.

The engine senses AOA and its RPM will increase or decrease as AoA decreases or increases respectively. It's just a matter of the propeller loading the engine. The governor senses the RPM changes and changes the propeller pitch to keep the RPM where the pilot set it. Or if the pilot moves the throttle in or out, the governor keeps changing the pitch to keep the RPM the same (the "constant-speed" effect) but the pilot feels (and sees on the ASI) aircraft acceleration or deceleration. If he pulls the throttle back far enough the RPM will drop as HP falls far enough that even at low pitch the RPM can't be maintained.

It's all about AOA, and the lack of understanding AoA is what keeps folks from understanding not only propellers, but wings and lift, and it's why we see stupid accidents with pilots doing silly things like yanking and banking at low level. They get accelerated stalls because they asked the wing to fly at an impossible AoA.
 
Dan Thomas said:
In teaching about constant-speed props, one absolutely has to explain AoA, just as one has to explain AoA when discussing stalls and spins and so on. Without an understanding of AoA, the purpose of a constant-speed prop is lost on the student.

With a fixed-pitch prop, the pitch is too high to let the engine reach anywhere near redline when the throttle is opened at the beginning of the takeoff roll. That means that much less than rated horsepower is being delivered to the prop, so the takeoff is sluggish, the roll is longer, and the climb rate is anemic. When we level off for cruise, we have to throttle back or we'll reach redline RPM at about the same time the airspeed maxes out, which is what the aircraft designers did on purpose when they specified the prop for the engine/airframe combination. You might cruise there (I used to do that for the last half-hour of an engine break-in flight with 172s or Citabrias) but the wear is accelerated and the fuel burn is awesome. And airspeed is pretty good, too.

And all of that has to do with AoA. Too high for takeoff and climb, too low for cruise. It's a compromise in order to save money (and weight) in the building of the airplane.


See this:

f0172-02.gif


We want that prop blade (actually, any section along its span) to be running at optimum AoA to get the best thrust for the HP, which means best efficiency and best performance. As you start moving forward, the incoming airflow accellerates and decreases the AoA. If that AoA gets low enough that thrust equals drag, acceleration stops and RPM is high. If we can increase the blade pitch we can go faster as AoA increases and more thrust is generated and RPM is kept at more efficient levels.

The engine senses AOA and its RPM will increase or decrease as AoA decreases or increases respectively. It's just a matter of the propeller loading the engine. The governor senses the RPM changes and changes the propeller pitch to keep the RPM where the pilot set it. Or if the pilot moves the throttle in or out, the governor keeps changing the pitch to keep the RPM the same (the "constant-speed" effect) but the pilot feels (and sees on the ASI) aircraft acceleration or deceleration. If he pulls the throttle back far enough the RPM will drop as HP falls far enough that even at low pitch the RPM can't be maintained.

It's all about AOA, and the lack of understanding AoA is what keeps folks from understanding not only propellers, but wings and lift, and it's why we see stupid accidents with pilots doing silly things like yanking and banking at low level. They get accelerated stalls because they asked the wing to fly at an impossible AoA.
Click to expand...

Aoa?

This is fun to know stuff and systems info, ops wise that’s just going to make things harder on the student.
 
The independent instructors on my field, and there are a fair number of them, do their ground school in the FBO lobby. Once or twice a day, I sit in there to have a cup of coffee or a Coke and relax a bit. I always enjoy the blind teaching the blind that occasionally pops up in those discussions.
 
Last edited:
James331 said:
Aoa?

This is fun to know stuff and systems info, ops wise that’s just going to make things harder on the student.
Click to expand...

If the student doesn't understand it, the learning becomes rote. And I know that many, many students haven't the frames of reference to understand it. My generation was about the last generation that had significant numbers of people that understood what went on under the hood or inside the cowling. One motor vehicles were largely automated, all the driver needed to know was what pedals to push and which way to twist the steering wheel, and all the knowledge one used to need to operate a car wasn't necessary anymore. So we get students that have no idea why the engine makes the noises it does, and don't want to know. They just want that pilot license as cheaply and quickly as possible.

I hope the young doctors that look after me haven't learned their profession by rote. If they have, we're all in trouble.
 
James331 said:
And yet it works just fine AS A INTRO into it prior to first flight.

Keep it simple and build onto that.
Click to expand...

if you are going to explain it as a transmission, you should explain it as a continuously variable transmission, which most people don’t understand anymore than a CS prop.
 
Man, no wonder people get things so wrong and often over complicated with instructors making it so needlessly hard to understand. This isn’t really that complicated. I learned it in the air (along with the rest of the things you learn for complex endorsement) in a few hours of practical training, and a couple more hours reading on my own.
 
Salty said:
Man, no wonder people get things so wrong and often over complicated with instructors making it so needlessly hard to understand. This isn’t really that complicated. I learned it in the air in a few hours of practical training and a couple more reading on my own.
Click to expand...
Anything more than a jacknife is complicated for a lot of folks now.

On the other hand, computer problems are usually way over my head and I sometimes have to ask someone young to fix it.
 
I’m getting flash backs to trying to teach 1s and 0s thinking engineers how to fly, give me a surfer any day over that mental mess.

If you understand how to drive stick, or ride motorcycles, operate a tractor etc, a CS prop is quick and easy to learn.
 
99% of the audience for this lesson only need it to navigate the FAA's annoying flip-floppery in using high/low rpm vs high/low pitch to trick them on written and oral exams.
 
Those unable to drive a manual transmission car, or competently fly a constant speed prop airplane, should stop driving and flying completely.
 
Those unfortunate to drive a Nissan or Subaru with the CVT transmission will be able to transfer that understanding to a CS propeller.

For the rest of us with REAL transmissions, a CVT (Constantly Variable Transmission) drivetrain will maintain the engine in the power band, depending on the load required. A CS prop works the same way, through totally different mechanical means. That's it for details!

For the OP: Tighten up your spell check, grammar and punctuation. Horsepower is one word and "it's" is not "its." I lose interest in TL/DR posts when they are not written in English... ;)
 
Dan Thomas said:
I hope the young doctors that look after me haven't learned their profession by rote. If they have, we're all in trouble.
Click to expand...

MUCH worse than you think is coming your way soon ... the above will be the LEAST of your problems ... have had several patients find out about rationing and denials already the hard way ...
 
You must log in or register to reply here.
Back
Top