Explain Constant speed props to me

[StinkBug]

I understand the concept of variable pitch, but so far I haven't ever flown in a plane with a constant speed prop and I'm curious how you actually use it. I'm used to just giving it more power to climb, less to descend, etc. How do you work the throttle and prop for the various flight modes with a CS?

 

[write-stuff]

There is a very good description of this in Chapter 6 of the PHAK. If you don't have a copy you can download a PDF of the single chapter from:
http://www.goldsealgroundschool.com/library-phak.htm

 

[StinkBug]

I'll have to go over that chapter again. I have the book and have read it, but dont remember it explaining the actual usage of the prop control.

 

[Geico266]

Take off in fine pitch, full throttle. Pull prop back to desired RPM, adjust throttle to desired setting for cruise. No matter what you do with the throttle the RPMs stay the same.

 

[stratobee]

A constant speed prop will constantly vary its pitch to keep the same RPM. It will endeavour to maintain that whether you're climbing or descending. This is controlled by what's called a governor. The governor has small fly weights that spin, and depending on how fast they spin, they regulate valves that control the pitch throughout every revolution.

It's just like when you bicycle - you tend to keep the same cadence (RPM) on your feet and just vary the gearing according to the terrain. Or drive a manual car in hilly terrain - same principle. So a constant speed prop should perhaps more appropriately be called a constant RPM prop. It's not designed to keep the same speed through the air, or keep the same pitch of the blades - just the RPM.

 

[StinkBug]

I understand how the prop itself works, more curious about how the pilot uses the pitch control for different stages of flight.

 

[Henning]

You move the blue knob to raise and lower rpm. There are various reasons to chose an RPM, not the least of which is it's the only timing control you have which also relates to maximum inner cylinder pressure which effects your margin from detonation and CHT. Also you reduce power with a reduction of rpm as Power = Torque (inner cylinder pressure) * Time (rpm).

Since I run Wide Open Throttle/Lean of Peak, once I set my mixture I control power with RPM. For example with my 310 I had a maximum rpm of 2675 and had a sweet spot for smoothness at 2450 rpm so I used that for my level cruise rpm. If I need to climb, since I am already at WOT, I have only two choices to increase power; I can increase fuel flow which raises my ICP and CHT and brings me closer to detonation, or I can increase RPM which actually moderates ICP and CHT slightly and through the effects on flame front timing of the extra piston speed all bring me further from the point of detonation while being kinder on the exhaust valve as well.

 

[StinkBug]

So you're saying that you basically set the throttle wide open, and use the prop to select the RPM the same way I currently use the throttle to set RPM with a fixed pitch?

 

[bobmrg]

I understand how the prop itself works, more curious about how the pilot uses the pitch control for different stages of flight.

Full forward for takeoff, reduce to climb RPM (your choice or per AFM) at a safe altitude (again, your choice...many pilots leave it at the climb setting until reaching cruise altitude). At cruise altitude, reduce throttle first, then pull prop back to the setting you want per the power charts in the AFM. You can choose between speed and fuel burn...Lindbergh taught WWII pilots to use minimum RPM for best range. Still, there are no hard-and-fast rules unless the tachometer has a red "don't stay in this area" arc. When it is time to head downhill just reduce the throttle about an inch per 1000 feet and leave the prop alone until you are in the approach phase. Keep in mind that if you push the prop forward with an appreciable amount of power applied the prop tips will go bra-a-a-a-a-a-p-p-p-p and make people on the ground shake their fists at those darn little airplanes. Wait until you are on short final with power reduced.

Or....

To my mind, there is absolutely no reason to go full forward on the prop on final "in case you have to go around." Leave the prop alone until you are on the ground. This reasoning does not apply if there is a plane in the runup area, a cow beside the runway, or a kid on a bike who might decide to ride the runway just for kicks. Ninety-nine percent of the time you will be able to see anything that might call for a go-around....if that happens, prop up first followed by throttle. I habitually fly with 2300 RPM from cruise altitude to the runway surface. Many, many instructors will disagree with me.

Bob Gardner

 

[Henning]

So you're saying that you basically set the throttle wide open, and use the prop to select the RPM the same way I currently use the throttle to set RPM with a fixed pitch?

Kinda, however you are neglecting to account for the mixture. I use the mixture like you use the throttle as well. This is also a major factor in avoiding detonation and carbon deposits on the exhaust valve and seat.

 

[jordane93]

Takeoff: prop full forward
Climb: reduce rpm for cruise climb
Cruise: reduce rpm for cruise
Descent: slightly reduce rpm or leave it in
Landing: full forward

 

[stratobee]

I do the same as Bob Gardner - never bring props forward during landing unless I anticipate some trouble. It's easy enough to bring them up should you need to go around. I also tend to cruise at the lowest RPM I can - just makes for a more pleasant ride with less noise. Plus I imagine the engine probably likes running less revs.

 

[Henning]

I bring the props forward after I bring the power back enough the props drop off the governor. That way there's no surge, but they're ready to go.

 

[txflyer]

Me too. I've gotten in the habit of going full prop on final because with the 180, sometimes you need little bursts of power here and there to grease it.

And it's ready if something happens. Emergency go arounds happen quick. Or a bad bounced landing you need full power quick.

I don't want to be fiddling with the blue knob.

 

[StinkBug]

Thanks everyone. Starting to make a bit more sense now.

Out of curiosity, with the prop all the way in how different is the pitch from an average fixed pitch prop? I'm guessing this varies a lot from plane to plane, but just one of those things that popped into my head. Reason being I fly 172s a fair bit, and a Cardinal was recently added to the fleet at my local club. Fairly similar planes, but the Cardinal has a CS prop. Curious about the differences.

 

[Matthew]

Thanks everyone. Starting to make a bit more sense now.

Out of curiosity, with the prop all the way in how different is the pitch from an average fixed pitch prop? I'm guessing this varies a lot from plane to plane, but just one of those things that popped into my head.

That's a good question.

I've flown fixed pitch props with climb and cruise twists. How does that compare to the high and low stops on an Arrow?

 

[FlyingTiger]

Started flying again after 21 years and now that I have passed my BFR and got some solo time in, headed back to the Arrow to get my commercial ticket. Had about 25 hours in the aircraft back in the day, but was feeling like I needed a refresher on constant speed prop aircraft operations after only flying 172Ms since my comeback. Below is an article I found that I believe gives you exactly what you are looking for. They essentially go through an entire hypothetical flight with all the various power and prop control adjustments for all phases of flight. They even include preflight procedures. Really a great read for anyone making a transition to constant speed propellers.

http://flighttraining.aopa.org/magazine/2006/January/200601_Features_Smooth_Operator.html

 

[Cpt_Kirk]

"There's a midget underneath the hood."

I'm still surprised the examiner took that as an answer on the checkride.

 

[nddons]

Started flying again after 21 years and now that I have passed my BFR and got some solo time in, headed back to the Arrow to get my commercial ticket. Had about 25 hours in the aircraft back in the day, but was feeling like I needed a refresher on constant speed prop aircraft operations after only flying 172Ms since my comeback. Below is an article I found that I believe gives you exactly what you are looking for. They essentially go through an entire hypothetical flight with all the various power and prop control adjustments for all phases of flight. They even include preflight procedures. Really a great read for anyone making a transition to constant speed propellers.



http://flighttraining.aopa.org/magazine/2006/January/200601_Features_Smooth_Operator.html

Nice article. Thanks for posting.

 

[Captain]

Thanks everyone. Starting to make a bit more sense now.

Out of curiosity, with the prop all the way in how different is the pitch from an average fixed pitch prop? I'm guessing this varies a lot from plane to plane, but just one of those things that popped into my head. Reason being I fly 172s a fair bit, and a Cardinal was recently added to the fleet at my local club. Fairly similar planes, but the Cardinal has a CS prop. Curious about the differences.

Pushing the knob doesn't 'set a pitch' so it doesn't really compare to a fixed pitch prop.

It sets an RPM and many factors go into what RPM gets set. Another way to state it is there are a lot of factors that determine what RPM a given pitch will give. They are directly related.

Factors include DA, humidity and speed. (As the plane accelerates the pitch has to change to take a bigger bite of air in order to maintain RPM.)

 

[StinkBug]

I guess I worded that wrong. I really meant it more the way Matthew phrased it, how does the min and max pitch compare to a fixed. His example of an Arrow is a good one for me too since I mainly fly an Archer and will probably do my complex endorsement in an Arrow.

 

[StinkBug]

http://flighttraining.aopa.org/magazine/2006/January/200601_Features_Smooth_Operator.html

That's a great article that really does a good job explaining things. Thanks!

 

[Henning]

I guess I worded that wrong. I really meant it more the way Matthew phrased it, how does the min and max pitch compare to a fixed. His example of an Arrow is a good one for me too since I mainly fly an Archer and will probably do my complex endorsement in an Arrow.

The pitch stops will be flatter than a 'climb prop' and steeper than a 'cruise prop' with the ability to settle anywhere in between as conditions call for. That is the advantage of an in flight adjustable prop whether constant speed or not.

 

[Matthew]

The climb prop I'm used to is pretty darn flat. Once you get to altitude, it's like driving in first gear (it's on a tow plane). I've flown an Arrow, but it's been long enough I can't remember the climb feel all that well anymore. And at cruise, I really didn't notice any difference from any other cruise prop - other than possibly looking at airspeed vs fuel flow.

 

[coloradobluesky]

One thing you will notice that is different. When you pull the power all the way back, like getting ready for a power off landing. The way the plane feels when decelerating is different constant speed vs fixed pitch propwise. The constant speed prop goes full flat which is flatter than the fixed pitch so more drag, I think is the correct answer.

 

[Henning]

One thing you will notice that is different. When you pull the power all the way back, like getting ready for a power off landing. The way the plane feels when decelerating is different constant speed vs fixed pitch propwise. The constant speed prop goes full flat which is flatter than the fixed pitch so more drag, I think is the correct answer.

You can pull the blue handle back and alter that some.

 

[RV10flyer]

Thanks everyone. Starting to make a bit more sense now.

Out of curiosity, with the prop all the way in how different is the pitch from an average fixed pitch prop? I'm guessing this varies a lot from plane to plane, but just one of those things that popped into my head. Reason being I fly 172s a fair bit, and a Cardinal was recently added to the fleet at my local club. Fairly similar planes, but the Cardinal has a CS prop. Curious about the differences.

My RV-10/IO-540 CS prop pitch varies between 15-31 degrees. Our full flap stall speed is 54 kias and typical cruise is 160 ktas. There are a few RV-10's with fixed pitch. They will be limited on climb ability or cruise speed at a slower rpm. I would not trade my extra weight/expense/maintenance/repairs for less performance on this particular plane. The most efficient AOA for a prop is 2-4 degrees. A fixed pitch AOA in a steep Vx climb may be as high as 15 degrees. This AOA is not very efficient. A CS will increase your power off glide range too. If I pull my blue knob out at 80 kias(max glide), my rpm will drop to 650-700 rpm and will give me a 2 nm glide per 1000' agl.

 

[Jaybird180]

Takeoff: prop full forward
Climb: reduce rpm for cruise climb
Cruise: reduce rpm for cruise
Descent: slightly reduce rpm or leave it in
Landing: full forward

I believe you meant reduce throttle here. There are many scenarios where your reduction would put your out of the governing range and you still are not near enough to the ground. Also reducing below the green arc, typically will put you outside of the POH engine limitations for Internal Cylinder Pressure (ICP in Henning's post) reasons.

 

[jordane93]

I believe you meant reduce throttle here. There are many scenarios where your reduction would put your out of the governing range and you still are not near enough to the ground. Also reducing below the green arc, typically will put you outside of the POH engine limitations for Internal Cylinder Pressure (ICP in Henning's post) reasons.

Yes, I meant throttle/manifold pressure not RPM

 

[Henning]

Yes, I meant throttle/manifold pressure not RPM

I reduce power for descent by reducing RPM and then I just keep adding mixture on my way down to keep the engines running, adding mixture as EGT drops off. I don't usually tough the throttles until entering the airport environment.

 

[Jaybird180]

I reduce power for descent by reducing RPM and then I just keep adding mixture on my way down to keep the engines running, adding mixture as EGT drops off. I don't usually tough the throttles until entering the airport environment.

Henning, it would be really cool if you would photoshop out the airplane in your avatar

 

[RV10flyer]

I believe you meant reduce throttle here. There are many scenarios where your reduction would put your out of the governing range and you still are not near enough to the ground. Also reducing below the green arc, typically will put you outside of the POH engine limitations for Internal Cylinder Pressure (ICP in Henning's post) reasons.

I normally run 2300-2400 in cruise. When I start my descent, rpm gets pulled back to 2100. Then I am gradually pulling throttle to maintain my 160 ktas cruise speed all the way down until 10 nm out/3000' agl or so then I pull to slow to 125 kias(Va) for bird/airplane avoidance/safety. I run a very lean mixture in descent until 10 nm out then gradually richen until full rich on final along with prop knob forward once power is below 10". After landing mix pulled 1" out until shutdown.

 

[Gsxrpilot]

Imagine driving a stick shift care with only two gears, 1st and Drive. Prop (blue knob) full forward is like being in 1st. Use it for takeoff and landing. Pull prop back a bit for cruise, is like shifting into Drive.

 

[Henning]

Imagine driving a stick shift care with only two gears, 1st and Drive. Prop (blue knob) full forward is like being in 1st. Use it for takeoff and landing. Pull prop back a bit for cruise, is like shifting into Drive.

No, it's not, there is no gear reduction function to a CS prop. If you think of it that way you are setting yourself up for detonation.

 

[StinkBug]

As a guy that builds custom vehicles for a living that also doesn't make sense because "drive" is a combination of multiple gears.

 

[Doggtyred]

I guess I worded that wrong. I really meant it more the way Matthew phrased it, how does the min and max pitch compare to a fixed. His example of an Arrow is a good one for me too since I mainly fly an Archer and will probably do my complex endorsement in an Arrow.

When you fly an Archer... for takeoff you are sitting still and you firewall the throttle... the fixed pitch prop is partially stalled/not as efficient as it could be (gross oversimplification for the purists out there). Its also not spinning at max allowable RPM... it gets you going but its not as efficient as it could be. (get to altitude faster)

If you have a climb prop, the prop is pitched to give you better performance in the low speed climbing portion of the flight, whereas if the archer has a cruise prop you lose some efficiency in climb but once you get to altitude you have more efficiency/thrust in cruise (get there faster)

The prop control is your transmission. The throttle is your gas pedal (think - standard transmission car).

For takeoff/go-arounds, the prop control is full forward - fine pitch - low gear. When you firewall the throttle on an arrow sitting stopped on the runway, the governor automatically adjusts the pitch of the blades to cause the power train to spin at max allowable RPM. At this flat pitch, less of the blade is stalled and therefore more of the blade is making thrust, so you will accelerate faster than with a fixed pitch prop set for cruise or climb.

As you go faster on the ground, if the blades stayed at that very flat pitch the motor would actually be allowed to spin too fast, risking damage as well as being VERY noisy as the prop tips approach supersonic... the governor coarsens the pitch, much like an automatic transmission would shift up to second and third gears as the vehicle accelerates..

You rotate, and pitch the aircraft for Vx or Vy or whatever you please, with the throttle still wide open... as you go faster, the prop governor coarsens the pitch to keep the RPM's where you've set them, which also happens to keep the pitch of the blades where they are most efficient in producing thrust for this airspeed. Your constant speed prop is now acting like a "climb prop" on a fixed pitch prop plane. Once you are a couple hundred feet off the ground, most POH's in constant speed prop aircraft I've flown have you pull the throttle and prop back just a bit from wide open to a specified power setting.

After you get to altitude and you lower the nose your airspeed climbs. As your speed smoothly accelerates, your prop governor smoothly coarsens the pitch of the prop blades to maintain engine RPM where you set it, and to keep the blades in their most efficient pitch range. Most of the time this is where you will pull the prop and throttle back to predetermined settings that give you that 75% or 65% power setting that most cruise settings call for.

So, from stand still to takeoff to cruse, your constant speed prop has gone from an "off the line" prop (humor me, its a term I'm adapting to the scenario), to a "climb prop" to a "cruise prop" all by itself.

A constant speed prop won't necessarily cruise faster than a fixed pitch cruise prop, nor climb any faster at a certain airspeed than a fixed pitch climb prop, but it gives you the best of both worlds.

Want to make the cruise a little quieter? pull the prop RPM back a bit, and depending on altitude you might be able to make up for it by advancing the throttle (power settings are a combination of RPM and throttle settings/fuel flow).

Manifold pressure is used to measure engine power - on cars its called a boost gauge... but it shows how much air pressure is getting into the engine to mix with fuel and go bang. Bigger MP numbers equal more power. The RPM is somewhat independent (within certain limits) of that power generation. You can have a high MP with low RPM... (bogging the engine down in low gear, upshifting from second to fifth gear and stepping on the throttle)... or you can have a low MP with high RPM (downshifting at speed from 5th to 2nd and taking your foot off the throttle).. in both instances it sounds and acts the same in cars/bikes/planes..

Read John Deakin's series on AVWEB in the archives on the issue of manifold pressure and constant speed props. I can't remember if its part of his "Fire Breathing Turbo's" 6 part series or if its independent of it, but he wrote it in the early 2000's..

 

[Doggtyred]

As a guy that builds custom vehicles for a living that also doesn't make sense because "drive" is a combination of multiple gears.

Think of it as an Automatic, Continuously Variable Transmission that allows you to set the engine RPM in the torque band and keep it there no matter how fast or slow you are going, or what position the throttle is in..

 

[Henning]

I really wish people would quit making the analogy between a CS prop and a transmission, it's nothing of the sort.

Look, these are propellers, you are pilots, you should understand them correctly.

A propeller is a screw, the steeper the pitch, the further the screw will try to pull through the medium it is in per revolution. Horsepower is a measurement of how fast you can move something in a given time. The further you moved it in a given time, the more horsepower it required.

Propellers are an airfoil, not a gear. When you increase Angle of Attack, you increase drag, when you decrease angle of attack, you decrease drag; simultaneously you coarsen or fine the pitch of the screw.

This is the basics of a Variable Pitch Prop. When you use a Constant Speed Propeller, you use this ability to vary the drag profile in order to govern the speed of the engine by using up all of the available torque.

The propellor blades are mounter so they can rotate. In the middle of the hub is a piston that drives out with oil pressure and returns either through a spring in the hub, nitrogen charge, or some counterweights on the blades. There is also a linkage between the piston and the blade to adjust the pitch. That prop hub is bolted onto the end of a crankshaft with a hollow nose and an oil port through the front main bearing to allow pressurized oil to be fed into the hub, or released from the hub.

To control this oil pressure and thereby the pitch of the blades and drag of the propellor, we employ a variation of James Watt's centrifugal governor. This consists of a set of flyweights counterpoised with a spring whose rate can be adjusted. These drive a spool valve which controls the flow of oil in and out of the hub. The flyweight shaft is driven off the crankshaft.

What you do when you move the blue handle is adjust the tension of the counterpois spring. As the engine speeds up, the flyweights want to go out and open the valve to put oil into the hub and add drag. When rpm drops, the counterpois spring brings the flyweights back in again releasing oil from the hub and reducing blade drag.

When you move the blue handle and adjust the counterpois spring, you adjust what RPM the spool valve opens and closes at. Here is the really important part to understand: When you select a lower RPM, you are selecting to increase load on the engine, that is why you see a rise in MP when you pull back the prop.

You can get yourself into trouble with detonation if you are making too much power when you pull back. Pulling back and reducing RPM also changes the timing of the flame front vs Top Dead Center which will also move you closer to detonation.

This is managed by running high RPM when in high power modes, and reducing torque (Internal Cylinder Pressure) to operate in low RPM modes. You can reduce torque one of two ways, restrict airflow with the throttle or restrict fuel flow with the mixture, I prefer to use the mixture.

Also, since Power=Torque*Time, an engine turning more RPM will be able to make more power, so when climbing is necessary, I increase RPM rather than increasing torque with mixture or throttle.

 

[Jaybird180]

I really wish people would quit making the analogy between a CS prop and a transmission, it's nothing of the sort.

Look, these are propellers, you are pilots, you should understand them correctly.

A propeller is a screw, the steeper the pitch, the further the screw will try to pull through the medium it is in per revolution. Horsepower is a measurement of how fast you can move something in a given time. The further you moved it in a given time, the more horsepower it required.

Propellers are an airfoil, not a gear. When you increase Angle of Attack, you increase drag, when you decrease angle of attack, you decrease drag; simultaneously you coarsen or fine the pitch of the screw.

This is the basics of a Variable Pitch Prop. When you use a Constant Speed Propeller, you use this ability to vary the drag profile in order to govern the speed of the engine by using up all of the available torque.

The propellor blades are mounter so they can rotate. In the middle of the hub is a piston that drives out with oil pressure and returns either through a spring in the hub, nitrogen charge, or some counterweights on the blades. There is also a linkage between the piston and the blade to adjust the pitch. That prop hub is bolted onto the end of a crankshaft with a hollow nose and an oil port through the front main bearing to allow pressurized oil to be fed into the hub, or released from the hub.

To control this oil pressure and thereby the pitch of the blades and drag of the propellor, we employ a variation of James Watt's centrifugal governor. This consists of a set of flyweights counterpoised with a spring whose rate can be adjusted. These drive a spool valve which controls the flow of oil in and out of the hub. The flyweight shaft is driven off the crankshaft.

What you do when you move the blue handle is adjust the tension of the counterpois spring. As the engine speeds up, the flyweights want to go out and open the valve to put oil into the hub and add drag. When rpm drops, the counterpois spring brings the flyweights back in again releasing oil from the hub and reducing blade drag.

When you move the blue handle and adjust the counterpois spring, you adjust what RPM the spool valve opens and closes at. Here is the really important part to understand: When you select a lower RPM, you are selecting to increase load on the engine, that is why you see a rise in MP when you pull back the prop.

You can get yourself into trouble with detonation if you are making too much power when you pull back. Pulling back and reducing RPM also changes the timing of the flame front vs Top Dead Center which will also move you closer to detonation.

This is managed by running high RPM when in high power modes, and reducing torque (Internal Cylinder Pressure) to operate in low RPM modes. You can reduce torque one of two ways, restrict airflow with the throttle or restrict fuel flow with the mixture, I prefer to use the mixture.

Also, since Power=Torque*Time, an engine turning more RPM will be able to make more power, so when climbing is necessary, I increase RPM rather than increasing torque with mixture or throttle.

Good explanation there.

 

[mikea]

What will blow your mind is what limits the RPM is not a speed control. The RPM is the fastest rate that the engine can spin a prop that is taking that much of a bite into the air. What's limiting the RPM is the power limit of the engine.

On my 235 a flat prop spins at about 2550 RPM always. I control power in the pattern with MP. Reduce to 20" on crosswind, 16" for slow flight, 11-12" to descend.