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Thread: Constant Speed Propellers, thinking out loud

  1. Default Constant Speed Propellers, thinking out loud

    Alright after a couple of years, I'm finally dedicating some time to learning the Constant Speed Propeller (CSP) operations. I've kinda veered away from these as they truly did confuse me, so I mainly stuck with fixed props (such as on the Cessna 152/172) or jet engines.

    That one extra knob throws me for a loop, although after a bit of reading, and soon to be experimentation, I'm planning on mastering that blue knob.

    In general it seems, that blue knob does nothing more than set a governor to maintain a certain speed of the propeller(s). This came about because for instance, if you dove in a fixed prop plane, without adjusting the throttle, the prop would exceed rpms limits if you weren't careful, due to the increased air speed across the prop. But, in the case of a CSP, the governor will adjust the pitch of a propeller to maintain it's set rpm. Am I good so far?

    Now according to my previous lack of knowledge, one would think that the black (throttle) lever would control rpm in a CSP plane. More throttle equals more fuel right? This can be the case, should the prop hit either mechanical stop. But in this case if we are within the stops, the black lever does nothing more than control how hard (or easy) it is for the engine to suck air into the intake. If we pull the black lever back, we restrict air flow, and so the engine has to work that much harder to pull air into the engine. To maintain the set RPM, the pitch will adjust accordingly (i'm assuming to a lower pitch), to lighten the load on the engine, to allow it to keep spinning at set rpm. Am I completely off base with this? The inverse is true, if you open up the throttle all the way, the engine will be drowning in air, and the pitch will adjust to a higher setting to induce more load on the engine.

    Manifold pressure was also a tiddly bit confusing as well. If you opened the throttle to 100%, with the engine not running, you MP should be reading whatever barometric pressure is. So if barometric pressure is 29.92 for instance, MP should read just short of 30". With the engine not running, and you close the throttle, it should also be reading 30", due to the throttle not actually creating an airtight seal. Now lets say we start the engine and put it at idle, and completely close the throttle. That little bit of airflow past the throttle valve will be enough to keep the engine running, but your MP should drop substantially. This is due to more of a vacuum in the suction chamber.

    Before I go on to pollute anymore people's idea of CSP's, I'll stop here for peer review. Am I on track so far, or should I do some more studying.

  2. Default

    You're off to a good start .. you've stayed away from the idea that the the prop-control, controls blade pitch

    If you wanna stay ahead of the learning curve, go ahead and eliminate blade-pitch all together, and use 'blade-angle-of-attack' (AoA)..

    Your throttle/MP descriptions are pretty close to accurate.. For the purposes of piloting, MP is just an indication of the how much work the engine is able to do at that time.. IoW, it's a power indication; and changes logically with throttle settings.

    The analogy that made it click for me; is to think of a CSP as a pilot.. the prop-blades as his wings.. and prop RPM (as set by the prop-control), as his target airpeed..

    Now, at any given MP, that MP can be used up in a combination of two ways..

    1) Prop airspeed (RPM)
    2) Prop forward motion (thrust)

    Just as a human pilot translates takeoff power into a desired rate-of-climb by modulating the wing AoA in order to maintain a target airspeed (ie.. Vy or Vx)... the prop pilot (CSP) modulates its wings' (prop-blades) AoA, to maintain their version of a target airspeed (RPM as set by the prop-control).

    Blade pitch is a confusing term, because different power settings will yield (all other things equal) different physical blade-pitches, but they're all the same AoA; as the CSP keeps.. "pitching for airspeed" (prop RPM). Just like a human pilot reducing power in a climb, but still maintaining Vy... the phyiscal angle of the wing to the ground decreases, but the AoA of the wing remains constant .. Same airspeed, but a dfferent rate-of-climb per the lower power setting. For a CSP, this would be lower thrust, but the same RPM.. make sense ?

    Aside from not getting caught up in the idea that the prop-control sets blade-pitch, the above narrative hopefully disuades you from the idea that pulling the prop-control out, causes the prop to "bite" more air. The only way to take a bigger bite of air, is to increase MP. By definition, a CSP is always "biting" as much as possible (within operational range).. that's what is "holding" it at the set RPM.

    The last misconception, is the "gearshift" analogy.. as you can go from a dead-stop.. accelerate.. climb, and then cruise at max speed, all with the prop-knob full forward, never "shifting gears" (and for reference, add a dive to that flight, and you've also likely put the blades through their entire range of motion.. again, never having touched the prop-control)...
    Last edited by Brett_Henderson; 03-21-2012 at 11:52 PM.

  3. Default

    At first I did think it was physically changing the pitch of the blades, but really it's changing the requested rpm, and they will automatically bite more/less air depending on where the current rpm is vs. requested rpm (or should I say rpm).

    Now I just flew around for a bit in FSX, and both the throttle and prop control levers can change speed, but throttle seems to have more of an effect. If I set prop to 2200rpm, and jam open the throttle, it's gonna give quite a boost in thrust, while the prop stays at 2200, however if I have MP at 18", and set the prop to 2700 rpm, it doesn't give quite the same boost in thrust. I'm under the impression that you set the MP to whatever (i've not figured out whatever is yet), and you use the prop control to adjust thrust.

    Really I'm trying to figure out how to manipulate it during climbs and descents. It's easy to just look at the POH, and set it to a cruise setting. But when I'm trying to approach at a specific speed, at a specific descent rate, I'm having issues.

  4. #4

    Default

    You may also consider the information on these pages:

    http://en.wikipedia.org/wiki/Constant_speed_propeller

    http://www.hariguchi.org/flying/info/prop.html

    to understand how Constant Speed Propellers work in real life.

    During takeoff and landing the prop control should be full forward (maximum rpm).

    Ask a flight instructor has some answers on proper use of a CSP:

    http://www.askacfi.com/4303/how-to-u...speed-prop.htm
    - Richard
    I'm running FSX within a Phenom II X4 965 (3.4GHz) using 8GB of DDR3 RAM and an EVGA GeForce GTX550Ti over Windows 7 Home Premium 64-bit. Scenery is Tileproxy over higher res mesh, with Wx by REX

  5. Default

    At first I did think it was physically changing the pitch of the blades, but really it's changing the requested rpm, and they will automatically bite more/less air depending on where the current rpm is vs. requested rpm (or should I say rpm).
    You said the dreaded "bite" word.. ! A "bite" would be a function of both blade angle, AND RPM... Since the prop is always "biting" as much as it can, a physical change in blade-pitch HAS to slow it down (reduce RPM).

    Think of the CSP as a pilot again, where his wings are the prop blades. If he is asked to maintain a different airspeed(RPM), he pitches (changes AoA) for that different blade airspeed (RPM). Just like the human pilot changes wing AoA to maintain a different airspeed.

    (That Wiki link is full of many, common misconceptions)

    This is why RPM changes have much less effect on airspeed. Power is set by throttle. A common, light GA engine's actual power peak is well above 3000 RPM, but props become less efficient at those RPMs.. as the blade-tips can become super-sonic.. So.... (aside from anomolies) that's why max thrust is at max RPM.. you've yet to reach the engine's potential HP peak.

    A reduced RPM for a given MP, is simply an RPM where less HP is generated for that given MP. That's why takeoffs are done at max RPM, and an airplane's max airspeed is at max RPM.. and why you "could" fly at max RPM all of the time, except that the engine wear/tear and fuel-economy are not proportional to the increased HP.

    To help visualize this, and see one of the biggest advantages for using a CSP.. ponder an airplane at 8000msl. At 8000 feet a wide-open throttle (normally aspirated) will net something like 22" of MP. With a fixed-prop, you cannot use all 22", as it will push RPMs tooo high,, but with a CSP, you can have the throttle wide open, utilizing all 22", and the CSP maintains an efficient RPM. In fact, in something like a Cessna182, when you plan to climb directly to cruise altitude much above 5000msl.. you apply full-throttle at takeoff, and will not touch the throttle again, until some point during your descent.

    Another useful visualization, is an airplane pitch-trimmed for level cruise. At that point, power changes alone, will not change airspeed, but will translate into climbs/descents.. ie.. a Cessna 172 trimmed for hands off cruise at 100kias, will stay at 100kias as you increase power, but the airplane will begin to climb. The CSP equivalent is a prop buzzing along at 2500RPM.. An increase in power will result in more thust, but the RPMs stay the same..
    Last edited by Brett_Henderson; 03-22-2012 at 08:55 AM.

  6. #6
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    Another way to look at it (simpler for me) is that as the propeller goes to a more coarse pitch, the air resistance to its rotation increases, and to a more fine pitch the air resistance decreases, so the governor changes blade angle as it senses an RPM change to vary the resistance in order to keep the RPM where it belongs (within its operation range, of course).

    A common, light GA engine's actual power peak is well above 3000 RPM,
    I'm curious, Brett, how an engine with a redline of 2400-2700 RPM has a power peak above 3000 RPM. I assume you weren't speaking of geared engines.

    Larry N.

  7. Default

    I'm curious, Brett, how an engine with a redline of 2400-2700 RPM has a power peak above 3000 RPM. I assume you weren't speaking of geared engines.
    The red-line is aviation-related (partly engine-life and reliability, but mostly prop efficiency/diameter... you can stick a Lycoming O-360 in a car and operated it at something close to automotive RPM ranges..

    In some circles, CFIs would sign off High-performance endorsements (greater than 200hp), using 200hp airplanes (ala some Cardinals, Mooneys, Arrows) whith the CSP max-RPM set 100RPM higher than the 200HP certification (do not try this at home..lol)

    IoW.. put just the engine on a dyno, and you'll find a peak in the power-curve, well above 3000RPM..
    Last edited by Brett_Henderson; 03-22-2012 at 11:35 AM.

  8. Default

    Another way to look at it (simpler for me) is that as the propeller goes to a more coarse pitch, the air resistance to its rotation increases, and to a more fine pitch the air resistance decreases, so the governor changes blade angle as it senses an RPM change to vary the resistance in order to keep the RPM where it belongs (within its operation range, of course).
    Again.. the coarse/fine pitch anaolgy causes confusion.. you can get the exact same pitch in degrees, at any number of power/airspeed/RPM combinations..
    Last edited by Brett_Henderson; 03-22-2012 at 11:50 AM.

  9. Default

    Terms like "bite", "pitch" and "angle" can also cause confusion about what exactly is meant.
    It could mean the angle of the blade's chord relative to the prop's plane of motion but also the angle between the blade and the the airflow relative to the blade.
    This relative airflow is a combination of the blade's rotational speed (which increases with the radius) and the forward speed of the aircraft.

  10. #10

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    Quote Originally Posted by Brett_Henderson View Post
    Again.. the coarse/fine pitch anaolgy causes confusion.. you can get the exact same pitch in degrees, at any number of power/airspeed/RPM combinations..

    Apparently.


    I suggest you take a look at the FAA's Pilot's Handbook of Aeronautical Knowledge, chapter 6, page 6, subsection "Adjustable-Pitch Propellers".

    http://www.faa.gov/library/manuals/a...apter%2006.pdf

    Also, the page I cited earlier, http://www.hariguchi.org/flying/info/prop.html shows how the mechanisms work.


    -Richard
    Commercial, Instrument, Certified Flight Instructor, ASEL

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