How does a CS prop work?

[N8Higgies]

I know this is probably an easy question for most of you, so I figured I'd ask the pros. I have my PPL, but only about 90 hours in 152s, 172s, and the Katana (all of which only had fixed pitch props). I'm used to the propeller spinning faster if the engine spins faster. I can't quite understand how the prop always spins at the same speed in a CS configuration if the engine RPMs increase. I know there is a prop control lever in addition to the throttle lever. Could someone take the time to explain this relationship to me a little better? What does the throttle do--increase engine RPM? What does the prop control do--change the prop pitch?

Let me also say I understand the concept of the different pitches taking different size bites of the air depending on what flying state you're in, I just don't understand how a faster engine speed doesn't translate into a faster prop speed. I have to run to a cub scout meeting with the kid real quick, but will be reading all the wise answers right when I get back. Thanks in advance.

Nate

[Mike_ExpressCT]

I'll do my best to give you the quick version of the answer, but in reality I normally take an hour to teach a student how it works.

First, remember that if you have a prop that has a "low pitch" setup, you're minimizing the forces on the prop and allow it to spin really fast while the airplane is moving very slow. This normally gives a lot of initial thrust on takeoff and really good climb preformance, but if you want a fast cruise you would have to spin the prop very very fast, which we can't really do (sort of like first gear in a car). If you have a "high pitch" prop, takeoff and climb will suffer as you're trying to make the prop take too big of a "bite" of air, but once you're going fast it's more efficient (like 5th gear in a car).

So when thinking of a constant speed prop, you want both benefits...a low pitch for takeoff and climb, and then a high pitch for cruise. Here's how it works:

You basically have a constant speed propeller hub that has a piston inside, and on single engine airplanes there's a spring behind it that's always trying to push the piston forward (towards the front end of the hub). This action forces the prop blades to a flat, or takeoff pitch (like a climb prop). However, using a governor, we can actually force high-pressure engnie oil into the top dome-shaped portion of the hub, which will push the piston back against the spring, and that will in turn force the prop blades to a higher pitch, or cruise setting.

On the engine itself, there is a prop governor that acts like a high pressure oil pump and reglator, which is hooked right to the Prop control inside the cabin (the norm is to make it a blue color), and the prop control connects in such a way that it regulates the tension on a spring. The spring's job is to allow a certain amount of movement to a series of flyweights(found inside the governor itself) that are normally connected to the cam shaft of the engine. That way, if the engine speeds up, the flyweights speed up as well.

So here's basically how it works...you're flying along at a given airspeed, and have the prop dialed in at a given RPM. You decide you want to enter a climb, so you pull back on the stick, thus slowing the airplane. In a fixed-pitched airplane, the RPMs would slowly decrease as airspeed decreases, and the prop wants to slow down in the same way in a constant speed setup. Well, those flyweights in the governor start to slow down and get pulled closer to the axis of rotation (by the spring we've set to work against the flyweights and the loss of centrifugal force), which moves a pilot valve one way and it allows oil out of the hub, thus allowing that hub spring to push the piston forward and decresing the RPMs. If you change your mind and start a descent, the flyweights will move outward as they spin faster(centfugal force), thus moving the pilot valve the other way which allows more high pressure oil to enter the hub and push against the spring, which increases prop pitch and slows down the RPMs to whatever you had set. The pilot valve is sort of a 2 into 1 valve, which will either allow oil into or out of the hub, depending on which way it goes.

continued...

[Mike_ExpressCT]

Your controls on the airplane are basically the throttle, which you use to set the amount of manifold pressure (i.e., how much air is entering the engine, thus how much horsepower the engine is producing), and you also have the prop control which again sets the spring tension in the governor and controls how much those little flyweights can move. If you allow the flyweights to move a lot, RPMs will be really high like in takeoff, but if they are restricted and can't move, that will keep the prop in a higher pitch situation for cruise. In reality, the governor is always changing the pitch of the blades anytime pitch or airspeed changes, but it's something that's done automatically for you and you don't really have to worry about it.

It's pretty simple in usage, really, and the only thing you want to avoid is getting into a low RPM, high manifold pressure situation that we refer to as "over-squaring", which is sort of like shifting a car into a high gear at a slow speed (ever try to shift from second to third and wind up in fifth? Same idea...) So it's important to remember that, before you add throttle, make sure the prop is pushed in first. That's why we normally push the prop control into a low pitch, takeoff position when we're on final approach...if we have to do a go-around, the prop is ready for the power addition. So put the pitch of the blades really low for takeoff (normally 2700 rpm), once airborne set the power for climb (throttle back to 25" of manifold pressure and then bring the rpms back to 2500), set cruise at where ever you want (20"-24" of manifold pressure and 2100-2400 RPMs), and like I said just make sure the prop control is all the way forward on short final. That's about it!

I know this is a very quick answer...if you have questions email me and I'll try to answer a little more clearly. My email is mfarley2@netjets.com

[captainron]

The prop is bolted solidly to the engine crankshaft, therefore engine and prop RPM are always the same. With the prop (RPM) control lever, you are selecting an RPM that will remain constant, within certain limitations, by varying the blade pitch to compensate for changes in throttle position or aerodynamic forces on the prop. Think of a car with a manual transmission. While you are cruising on a flat road at thirty mph, you are using a certain throttle opening to maintain that speed. When you come to a hill, to maintain the same road speed (and engine speed) you will need to use more throttle (manifold pressure) but the rpm of the engine won't change. It's just a form of engine governor.

[brayski98]

Mike,
Thanks for the explanation. I bought an RV-6A with a C/S prop and a 160 HP engine. I had some transition training that included using the C/S prop. I started thinking of it as a tranmission where it allowed you to have the right gearing for the speed you desired. And you wouldn't want to overrev by selected a low gear for the speed.

I got used to it pretty quick and I like it better. I only have about 200 hours overall and 10 in the RV

[asav8tor]

http://www.warmkessel.com/jr/flying/td/jd/16.jsp

[N8Higgies]

Okay, so I read everything I could find on the link provided as well as all the answers submitted here. I think I understand most of what goes on, but let me make sure.

The throttle lever--like the accelerator pedal in a car--controls the amount of air/fuel the engine gets, which is measured in manifold pressure. If there was no prop and the engine could spin freely, this increase in air/fuel would cause the engine RPM to rise uninhibited--like if I were to push on the gas pedal in my car while it's in neutral.

However, the prop creates an opposing force for the engine to work against--like when my car is in gear. If the pitch of the prop is low or fine (like in first gear), then the opposing force is small, which allows the engine RPMs to rise. If, however, the pitch is high or coarse, the engine would be trying to overcome a greater force (like trying to start out in 5th gear in a car).

So would it be right to say that the prop control lever is more of an adjustment on the pitch of the prop that corresponds to a certain RPM setting--assuming of course that the engine has enough power produced from the throttle to move at that RPM setting? Is that right? Please clarify if I am completely off my rocker. Thanks.

Nate

[gmcjetpilot]

The principle is the prop governor is rotated by the engine and is calibrated to send an exact amount of oil pressure to the prop based on RPM. The oil pressure (routed through a tube, bearing, hallow crank, into prop) changes the prop blade angle, from fine to course as needed. Once the pilot sets RPM, the prop governor knows if RPM increases or decreases by fly-weights. It will adjust oil pressure as needed to get maintain set fly weight position (that corresponds to that RPM). RPM will not change unless you move the prop control (with in limits).

The basic and main principle is RPM remains constant (with in limits) by varying the props blade angle or pitch. This is done automatically. If RPM is too high the pitch increases to slow the engine down by adding more load on the engine. If RPM is too low, pitch lowers to a finer setting angle to UNLOAD the engine and maintain RPM. A piston in the front of the prop is what the oil pressure works on. Usually its spring loaded to go to fine pitch (high rpm) and it takes oil pressure to make to blades go course (low rpm). Some props work different. Some props have accumulators and stops that allows the prop to feather. Some props will go to high pitch (low rpm) with out oil pressure. Regardless if you add or subtract power or increase or decrease airspeed (with elevator) you the rpm stays the same...beauty!

Operation wise its very easy, but it takes thought at first. After a few hours its second nature. The MAX rpm is limited automatically (by the prop governor) to 2.700 rpm, typical Lyc engine red line. The min RPM is limited by the high pitch stop.

Terminology
*Fine pitch, higher RPM, Prop IN, Prop control fwd (all synonyms)
*Course pitch, lower RPM, Prop out, Prop control aft (all synonyms)
*MAP (manifold pressure) affected by throttle position, idle = low MAP, full throttle = higher MAP
*MAP/RPM = power, both are needed to calculate power.
*MAP/RPM example: 25/25 = 25 in-hg MAP & 2,500 rpm
*Max MAP at sea level with wide open throttle, about 29"
*Max MAP goes down with altitude, about 1" per 1000' altitude


Typical Pilot action with Prop Control:
> Start engine (prop in, fwd)
> Run up (cycle prop control out & back in quickly, note slight
RPM drop, checks prop control & circulates warm oil to prop hub)
> Take off (Fwd - higher RPM the greater the hp)
> Initial Climb - reduce throttle first (MAP) than prop towards course
(RPM) typical "25 square" (25" & 25(00) rpm; rpm rounded to 100's)
> Enroute Climb add throttle to maintain MAP 25" (for altitude drop)
> Cruise - set throttle than RPM as desired. At high altitudes throttle
is wide open, MAP typ same/less than RPM to 2,300-2,500 rpm for efficiency.
> Descent you will typically reduce throttle (MAP) and leave prop alone
or even lower RPM slightly (for less drag). Avoid back driving engine.
> Landing - entering pattern, reduce MAP further, once slowed, go to
course pitch. RPM will not increase due to low power & speed. The prop is
"on the low pitch stop" and can't increase RPM. The reason for this step is,
if you go around, you will get full power (full RPM).
> Taxi in, do nothing with the prop should be fwd-high rpm-fine pitch.

So in a typical flight you might move the prop control as little as 4 or 5 times, including run-up.

General golden rule
When you decrease power you lead with throttle (MAP) than prop (RPM)
When you increase power you lead with prop (RPM) and than throttle (MAP)

There are no mysteries. It's like riding a bike, you can explain it all you want, but to learn you have to actually do it.

The advantage of constant speed props is you SET the RPM. With FIXED PITCH, RPM is indirectly related to airspeed and throttle position. With C/S you can set the most efficient RPM for takeoff, climb, cruise and even descent. Going to low RPM in a descent lowers drag, like up shifting on a down hill in a car. The opposite is also true, in the pattern a C/S prop can provide more drag in fine pitch/high RPM. This is also an advantage. You can make steeper approaches & land shorter with out slipping with a C/S prop verses fixed. Also its great for Acro. Set RPM and throttle and leave it alone. With fixed pitch prop you have to add power on the up-lines and pull power back when nose down to keep RPM below engine red line.

Typical RPM is from engine red line of 2,700, down to 2,300 or less. High RPM is not efficient in high speed cruise but great for take off. High rpm can make more HP, but also makes more noise and burns more fuel. Super low RPM may be restricted by prop/engine limitations for extended operations due to harmonic vibrations of prop/engine combo.

Power setting is not MAP/RPM
On take-off everything is FWD (throttle, prop, mixture) and you have about 29"/27; At 1000 feet you reduce to climb power 25/25; Cruise can be almost any combo of MAP/RPM or "square settings" such as: 25/25, 24/24 or 23/23. It does NOT have to be square, but more on that later. As you climb you can't maintain MAP. You lose about 1" MAP per 1000 feet due to lower air density, so by 8,500' you're going to be down to about 21" MAP. At 21" you can run at any RPM you want, but lower RPMs are more efficient. You do lose a little speed with lower RPM's, but in general gain MPG. So anything from 21/27 to 21/20 would be fine, if prop limits allow. Some props have min RPM limitations with different MAP's.

At lower altitudes where MAP is higher you can run wide open throttle (WOT) all day at +25/+25, but unless you're printing money, you want to get below 75% power so you can lean the heck out of the engine. You must be at 75% or less the lean. With practice you memorize a handful of typical MAP/RPM combos to set 75% or less power. A little laminated table of map/rpm is handy.

You could leave the prop full FWD all day long and fly it like a fixed prop plane, but that's like riding a 21-speed bike in 1st gear.

You can run OVER square where MAP is more than RPM. Not only can you do this, its a good idea with LIMITS. About 1" or 2" MAP over RPM is allowed. If MAP is real high and RPM low, its equivelant to LUGGING your car in high gear and slow speed. 24/23 would be fine, about 75% power at 1,500' std day. Where 23/24 is little less power and more noise. Above 5,000 feet, MAP is low, so generally you will not be able to run over square. There are some practical min RPM's allowed with props. You can drop down to below 2100 rpm, as low as 1900 or 1800 RPM and save gas while still flying fairly fast. Again prop limitations or MAP/RPM combo limits must be observed. In general the lower the RPM the better the prop efficiency. Efficiency does not mean speed, just better miles per gal. It also depends on altitude and airframe. The RV's max/best range speed is around TAS 130-140 mph, not too slow.

Bottom line, that blue knob gives you more control over the power settings and saves gas, while making takeoffs and landings shorter and climbs faster. With that said a Sensenich fixed prop is pretty awesome, and with the RV's excess performance you may not miss the difference if you are use to low performance planes. The Sensenich is set up pretty nicely for cruise so its efficient, but only at one condition and limited altitude range. The C/S prop again gives more range and control of conditions you can fly while keeping efficient.

[JHines]

Originally Posted by N8Higgies

<SNIP>

So would it be right to say that the prop control lever is more of an adjustment on the pitch of the prop that corresponds to a certain RPM setting--assuming of course that the engine has enough power produced from the throttle to move at that RPM setting? Is that right? Please clarify if I am completely off my rocker. Thanks.

Nate

That's exactly correct. There also is (was) such a thing as a variable-pitch prop that does not have a governor - so the cockpit lever directly controlled pitch, with the work of maintaining a specific RPM left up to the pilot; and some "two-speed" props that had only two pitch positions.

[bigbill25]

Originally Posted by gmcjetpilot

> Landing - entering pattern, reduce MAP further, once slowed, go to
course pitch. RPM will not increase due to low power & speed. The prop is
"on the low pitch stop" and can't increase RPM. The reason for this step is,
if you go around, you will get full power (full RPM).

I think you mean "go to fine pitch".

--Bill