The only turbprop I've travelled is the ATR 72. I've noticed that the props' rev go down (for like a second, the sound also goes down) just before engaging full throttle in order to takeoff. I've included a video (video is not mine), this thing happens at 1:40. Why the RPM drops for a sec? Or is this just the sound?
Because the pilot is taxiing the airplane with the props in the beta range (turboprops turn into landsharks if you don’t do this and you end up really riding the brakes to keep the speed down), then on takeoff, the propellers move into a maximum power setting, the engine snarls and slows a bit under the increased workload.
12$\begingroup$ I like the colorful language "landsharks", "engine snarls" $\endgroup$– SamSep 14, 2018 at 17:03
3$\begingroup$ Exactly, it's like the light turns green and you lift off the clutch -- that point where you stop slipping the clutch and it fully engages, the engine slows a bit to fully mesh with the road, but that it increases its torque, so you're not making less power but same power at less RPM, hence the growl. $\endgroup$ Sep 15, 2018 at 4:59
1$\begingroup$ @Sam As a non-pilot who visits this SE from time to time, those were the words which made me understand what was being said =) $\endgroup$ Sep 15, 2018 at 21:48
It's because he was taxiing in DISC (discing, or flat pitch making the prop like a big disc), which is in the ground beta range, then coming out of it.
Beta range is a mode where propeller blade angle is directly controlled by the power levers, and the propeller RPM is controlled by an Np governor in the fuel controller modifying gas generator (the engine core) speed to maintain rpm on a fixed schedule, depending on where the power levers are. Beta range is similar to speed governing on a turbine helicopter, where you change pitch with collective and a governor automatically controls the fuel to keep the rotor RPM constant for you.
Beta range starts just ahead of flight idle (there is no marking for this on the power lever quadrant). The range forward of idle is "flight beta" and aft of flight idle is "ground beta". On the non-FADEC PW120 series engines there is a cam called the beta cam (obviously) in the power control linkage (on the propeller control unit on the engine) that triggers the transfer of propeller control between the prop governor/prop lever setting and the power lever/fuel controller once the prop has hit its fine pitch limit and propeller governor is no longer governing.
So to be in beta mode the prop has to be "off the governor" (with no torque and the props set to MAX RPM, the prop is trying to go finer and finer to keep the rpm up, then the blades hit the full fine pitch limit, about 24 degrees, and there's nothing more it can do) and the power lever has to be back just forward of flight idle at which point the beta cam takes over to directly regulate blade angle, from +24 degress to -11 degrees blade angle. So if you are descending with the engines at or near flight idle you are in flight beta mode.
Once in beta mode the beta cam works with a beta metering valve to regulate fine pitch oil pressure to the prop such that when the power lever is moved, the cam movement displaces the metering valve, causing the prop pitch to change until the input is nullified and blade angle is fixed at a new position. The cam's profile thus gives a specific blade angle for a given power lever position.
Anyway, what's happening there is engine's torque dropping off momentarily as the engine transfers from prop rpm governing in beta (via the fuel controller Np governor controlling torque) to normal flight mode (rpm controlled by the propeller flyweight governor as set by the prop levers, with the power levers now taking over production of torque) as the Power Levers are moved past about one inch forward of Flight Idle.
$\begingroup$ Is the cam operated by that push/pull idle gate lever seen here? Also looking at that photo and reading the third paragraph can be confusing, especially stating that the beta range starts ahead of FI, did you mean aft of FI? Feel free to crop and add the image in the answer if you will be revising it, thanks @JohnK. Here's the image source you can link to. $\endgroup$– user14897Sep 17, 2018 at 12:45
$\begingroup$ I fleshed it out a bit, at the risk of making people catatonic... The beta cam is part of the prop control unit on the engine not the power levers. There are no beta markings on the power lever quadrant. Coming in and out of flight beta will be transparent to the crew since very little torque is being produced until you are moving into reverse.. $\endgroup$– John KSep 17, 2018 at 14:26
Most likely this is happening after the pilot advances the power lever -- but the power lever controls both throttle (fuel flow, in a turbine), and propeller pitch.
The propeller can change pitch pretty quickly, while the turbine takes longer to increase power (increased fuel flow must produce hotter combuster air, which must then spin up both the turbine-compressor section, and the independent shaft power turbine connected to the gear box and propeller). Usually, the engine's rotating parts will change RPM faster than the ones that spin the (relatively large) propeller, so you may hear the "whine" or "whistle" of the primary shaft spooling up before the propeller noise starts to rise, possibly even while the propeller is still slowing down from the pitch increase.
$\begingroup$ It is technically correct, but without mentioning the beta range it is misleading. In the normal range propeller pitch is controlled by the governor and that reacts to RPM, so it can't cause a temporary decrease. However the power levers do control pitch in beta and transition from it, which is where this happens. $\endgroup$ Sep 14, 2018 at 18:10
1$\begingroup$ In the video you can actually notice that as the sound goes down, the propeller becomes "thicker" — the blades are rotating to increase angle of attack. $\endgroup$– RuslanSep 14, 2018 at 19:32