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  • Turboprop questions...

    My first question has to do with a cable that I often see on turboprops that runs from the tail to the main fuselage-what is that for?? (I've seen it on other types of aircraft.

    My second question is about the propeller speeds, or RPMs-how can the speed of propellers on a turboprop be reduced without reducing the speed of the airplane??

  • #2
    I can only answer your second question as I haven't seen the first myself. A picture maybe?

    anyways. I think you reduce the propeller speeds by feathering the engine. A turboprop is basically a jet engine that uses it's blast to spin the prop. There may be a way to make it so the engine uses the jet more than the prop. Idk, I'm not a bombardier technician and I have no idea who is on these forums
    I'm the guy... Porter Guy

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    • #3
      Originally posted by UALdave View Post
      My first question has to do with a cable that I often see on turboprops that runs from the tail to the main fuselage-what is that for?? (I've seen it on other types of aircraft.

      My second question is about the propeller speeds, or RPMs-how can the speed of propellers on a turboprop be reduced without reducing the speed of the airplane??
      Old fashion radio antenna
      Older generation a/c have them.
      Good example is the Antonov 12.
      [photoid=7326974]
      “The only time you have too much fuel is when you’re on fire.”

      Erwin

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      • #4
        Originally posted by UALdave View Post
        ........................................
        My second question is about the propeller speeds, or RPMs-how can the speed of propellers on a turboprop be reduced without reducing the speed of the airplane??
        Turboprop propulsion usually consists of a jet turbine running at constant speed and connected to a variable pitch propeller by a reduction gear. Aircraft speed is varied by adjusting the propeller pitch. The turbine exhaust thrust is minimal compared to the prop thrust.

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        • #5
          Originally posted by Highkeas View Post
          Turboprop propulsion usually consists of a jet turbine running at constant speed and connected to a variable pitch propeller by a reduction gear. Aircraft speed is varied by adjusting the propeller pitch. The turbine exhaust thrust is minimal compared to the prop thrust.
          Ding,
          (cue old P3 FE voice)

          The Turbo prop is designed to run at 100% at all times. The thrust is achieved by make the prop pitch change in response to throttle lever position. More pitch = bigger bite of air = more speed. There are speed sensors combined with throttle lever position that will command the fuel control to add more fuel to keep the engine and prop RPM at 100%. If you reduce throttle less gas is needed to keep the RPM up so it will reduce fuel flow. Always working in combo to keep engine and prop RPM 100%

          On the P3 engine RPM was 13,820 and the prop was 1080 or so.

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          • #6
            Originally posted by ErwinS View Post
            Old fashion radio antenna
            Older generation a/c have them.
            Good example is the Antonov 12.
            [photoid=7326974]
            It's one of the two antennas used by the ADF navigation system.

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            • #7
              Originally posted by UALdave View Post
              My first question has to do with a cable that I often see on turboprops that runs from the tail to the main fuselage-what is that for?? (I've seen it on other types of aircraft.

              My second question is about the propeller speeds, or RPMs-how can the speed of propellers on a turboprop be reduced without reducing the speed of the airplane??
              1st: HF Antena. Also found in jets like the F-28.

              2nd: I'm not sure what you mean, or how this applies specifically to turboprops.
              To begin with, the airplane (any airplane) is a energy trading machine. An easy way to increase the speed without increasing power would be lowering the nose! thus trading altitude (potential gravitational energy) for speed (kinetic energy). This works even if the engines are off or non-existant (glider).
              Leaving that trivial case aside, let's think of an airplane that is flying straight and level at constant speed and then manages to increase the speed and lower the prop RPM at the same time. How is that possible?
              This answers applies to any airplane with continous speed / variable pitch proppelers (be it a turboprop or a piston engine).
              Well, the key is in the effective power, which is the power that is acutally used to propel the plane.
              To fly straight and level at a fixed speed, the plane needs one given ammout of effective power, which equals the power the plane consumes in terms of drag.
              If the prop starts to give more effective power than that, something must happen with the excess energy: It will convert either in kinetic energy (the plane will increase its speed) or in potential gravitational energy (the plane will increase its altitude).
              Since we've propoesed that the plane was flying straight and level and would keep at constant altitude, the excess enenrgy goes to speed.
              So the original question turns to this one: How can the speed of the proppelers be reduced and the effective power increased at the same time?
              Well, the prop doesn't make energy. It just takes energy from the engine and turns it into thrust. The part of the energy given by the prop that is used to produce thrust is, as we've said, the effective power. The energy taken by the prop from the engine is the shaft power, which equals torque times RPM. The ratio beteen the two is the prop efficiency, which of course is always less than 100% because the prop cannt convert into thrust more energy than it is taking from the engine, but it will convert less because part of that energy will be lost in the drag that the prop makes as it rotates. Typical efficiencies are about 80%, and tend to be somehow robust around the design point of operation, meaning that small changes in shaft power, airpseed, RPM or pitch will not affect the efficiency a lot (note: these variables are not independent: if you change any one of them, at least some of the others must change too, but you can manage to change any of them and leave at some -not all- of the others unchanged). Let's use this robustness in the prop efficiency and simplify the exercise assuming that it will not change during the experiment.
              So now we can reduce the original and the second question to this third one: How can we reduce the engine RPM and increase the engine power (shaft power) at the same time?
              Let's see. Say that you are in a car with manual transmission. You are at constant speed in the first shift at 3000 RMP. The gas pedal is only slightly pressed. At this point you put the second shift and press the gas all the way down. What will happen to the RPM? What will happen to the speed? The RPM will suffer and instantaneous and very significant reduction, and the speed will start to increase. Why? Because now, with the gas pressed all the way down, the engine is burning much more fuel, which means that much more energy is available. The shaft power, which is again torque times RPM, will increase a lot even if the RPM is reduced because the torque will increase much more than that. So there you have: RPM reduced, speed increased. How does this translates to a constant speed / variable pitch prop?
              These props work as if it was a continously variable transmission (like if the car had infite shifts): Yous set one given RPM, and every time you press the gas instead of icnreasing the RPM the transmision changes in very little steps so as to keep the engine RPM constant (while the wheels RPM and hence the speed of the car increases).
              In airplanes equiped with this kind of props, when you move the throttle forward more fuel goes to the engine and the engine tends to increase its RPM, however there is a controller that will increase the pitch of the prop so as to make more "force" (in fact torque) to keep the RPM constant. The end result: more torque and the same RPM = more shaft power = more effective power = more energy than needed available = either more speed or more altitude (if the altitude is constant, then it's more speed).
              Now, the RPM of the prop is set with another lever which is called RPM control (also pitch control, but it's important to understand that it doesn't directly control the pitch, you set a coarser pitch for a lower RPM and a finner pitch for a faster RPM, but once set the ptich will "control itself" to keep that RPM value constant).
              So what happens if you add throttle but at the same time set a coarser pitch (lower the RPM)? Well, it depends. If you reduce the RPM a lot the "RPM" part in "torque times RPM" will win and you'll lose power (and speed), but if the "torque" winds (due to the advancing of the throttle) then the power will increase and you'll gain speed.

              Now, this is not something that is very commonly done.
              Typically the prop is set at max RPM for take-off (where it has the ability to produce more thrust even if with a lower efficiency), then set at the optimum setting for climb and cruise (where it produces a given thrust with max efficiency and hence less fuel burn), and finally set again to high RPM before landing (to have max thrust available again in case uf a go-around).
              When have you seen a turboprop doing that? (incrase the speed while reducing the prop RPM)?

              --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
              --- Defend what you say with arguments, not by imposing your credentials ---

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              • #8
                Originally posted by Gabriel View Post
                1st: HF Antena. Also found in jets like the F-28.
                With due respect to Gabriel, I'm going to disagree and say that the wire antenna you see on most turboprops here in the United States is the ADF sense antenna (as opposed to the directional antenna located elsewhere). Most turboprops in the U.S. and Canada don't get out of range of VHF radio and thus don't have HF installed. That being said, they may have different equipment in other parts of the world where it's not possible or economical to put ground based VHF transmitters.
                The "keep my tail out of trouble" disclaimer: Though I work in the airline industry, anything I post on here is my own speculation or opinion. Nothing I post is to be construed as "official" information from any air carrier or any other entity.

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                • #9
                  Originally posted by snydersnapshots View Post
                  With due respect to Gabriel, I'm going to disagree and say that the wire antenna you see on most turboprops here in the United States is the ADF sense antenna (as opposed to the directional antenna located elsewhere). Most turboprops in the U.S. and Canada don't get out of range of VHF radio and thus don't have HF installed. That being said, they may have different equipment in other parts of the world where it's not possible or economical to put ground based VHF transmitters.
                  You're right. It's ADF, not HF.
                  The new (if something about ADF can be said to be new) ADF antennas combine the sense and loop antennas in one compact package and hence get rid of that long wire.

                  --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                  --- Defend what you say with arguments, not by imposing your credentials ---

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                  • #10
                    It is my understanding that you keep the propeller RPM constant as well as the turbine RPM during cruise, but not 100% of the time, and that you do need to make adjustments to the turbine RPMs depending on flight regime and conditions.

                    One thing to keep in mind is that the velocity of the propellers needs to be below M1.0, as you get to M1.0 you will start to have shocks forming on the propeller tips, which is not good for efficiency (and many other factors).

                    I have yet to hear of a case where you use more the exhaust jet than the propeller (other than a turbofan with a BPR of less than 1).
                    [SIGNATURE GOES HERE]

                    Felipe Garcia

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                    • #11
                      OK, here's an example of the cables I was talking about-you can see them at 1:47 in this video, and later on in the video as well. It's an RAF BAe-146:

                      British Aerospace BAe-146 CC2 Take off RW27 Gloucestershire airport :EGBJ: on April 3rd 2012, I OWN ALL OF THIS VIDEO COPYRIGHT RESERVED

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                      • #12
                        There are a lot of disturbing comments on propellor pitch.

                        (and a shorter version than Gabriel)

                        100% all the time (no)
                        The pitch determines the speed of the aircraft (no).

                        Like a typical engine, there's an RPM (usually higher) that has "the most" power and an RPM where you want to just cruise along (usually lower).

                        During takeoff and early climb (and just before landing), you operate at the "100% RPM"

                        For most other flight modes, you slow the propellor down a bit (typcially 80 to 95% RPM)

                        Now, that being said, an automatic governor system is constantly adjusting the propellor pitch- while holdilng RPMs essentially constant.

                        When you first start the takeoff, the pitch is very shallow because you are going slow and a "big bite" would slow the engine down.

                        Then as you speed up, the pitch becomes steeper. But get this right- the speed determines the pitch- NOT the pitch determines the speed!

                        The other thing the fancy governor system does- If you are flying along at full power, the propellor is operating at moderately steep pitch. If you pull the power back, the pitch becomes flatter (allows the engine to still spin at the same speed, but generate less thrust if you will), but the RPM stays pretty much the same.

                        Thus they are called "constant speed propellors" (perhaps even moreso than "variable pitch propellors"). In fact, this is nothing more than a transmission like on a ground vehicle- it lets the engine spin fast at high HP RPMS when you are going slow, and then when the engine "begins to overspeed" it shifts up to "slow the engine back to the max HP speed" You continue through several gears until you hit cruise, you go to an even higher gear (often called overdrive) and slow the engine down a bit for more quiet and less wear and tear.

                        There were a few early propellors where the pitch control was more manual and not linked to a governor.

                        One other thing- there are some complex things called beta mode and reverse (related to each other). It's a way you trick the governor to get the prop pitch to do strange things including reverse thrust.

                        and, well one other other thing. There can also be an oddball ground-idle mode where the pitch is extremely steep, but the RPM is very slow. At this point the engine is producing no real useful power per se, and would be terribly ineffecient. Not sure why that mode exists- but it might be simple safety to help the props spin down faster on shut down, etc.
                        Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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                        • #13
                          It's a military aircraft so it could be both for HF and ADF but just looking at it, it seems that the main purpose of those wires is to accumulate ice during icy conditions.

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