Hi everyone, could someone please explain what beta range is?
Its the angle that the prop blades get moved to, it is prop dependent and goes from forward thrust to negative values for reverse thrust. Different props on the same engine have different values. To the pilot there isn't much difference because it is a range which is normally protected on the power levers and its always protected if it is fatal going into it in the air. There are some types which will allow you to selected it in air.
Does the beta range differ between engines and if so why
the beta range itself doesn't really change apart from the angles the blades work between. The differences in engines are to do with how they are designed. PT6's are a free turbine and do it differently to a Garrett which is a direct shaft engine.
Also how does the reverse torque (thrust) on at turboprop work to slow down the aircraft?
Exactly the same as normal thrust. The prop moves a mass of air towards the rear of the aircraft to produces forward thrust. And because the blade angle as been altered to push air towards the front of the aircraft it produces reverse thrust.
courtesy of: Hartzell What is beta? Some constant speed propellers are equipped for beta/reverse operation. Beta Range Beta Range is any blade angle below flight idle (Hydraulic low pitch stop). Reverse Reverse is any blade angle less than zero degrees. This blade angle produces thrust in a direction opposite to that of normal thrust. Such propellers are typically installed on aircraft with turbine engines and are used for to reduce landing roll.
Basically on a turbo-prop the propeller is spinning at a constant speed, while in flight, pulling back the power levers to flight idle will flatten out the prop blades and create a lot of drag to slow down, this flattening process still keeps the prop blades at a slightly positive pitch angle, moving power levers forward will cause the blades to take a larger bite for more thrust.
Beta-range is where power lever movements either forward or aft directly control the prop pitch angle through a mechanical linkage, usually accomplished by pulling the power levers backward over a gate mechanism (aft for decelerating on the runway after landing or for backing up). Generally speaking, beta-range is used in ground ops. only (some exceptions). I'm not an engineer as you might be able to tell, just a driver.
Roughly speaking, one can divide the range of a reversible, controllable propeller into two regions of operation; forward thrust, and reverse. Forward thrust, anything pushing or pulling the airplane in a forward direction, may be thought of as the "alpha" range, and anything that isn't alpha range may be thought of as "beta."
A propeller is an airfoil. It works by altering the pressure distribution around the propeller blade and the arc of the propeller. The propeller may be driven by the engine, and under some low power conditions when engine power is insufficient to motivate the propeller to move, it may be acted upon by another more powerful force, such as the slipstream as the airplane moves through the air; the slipstream may move the propeller. When the engine is driving the propeller this is referred to as positive torque; when the slipstream is driving the propeller this is referred to as low torque, zero torque, or in some cases, negative torque.
The alpha range of most reversible propellers allows the propeller blade to position itself to an angle which achieves what the pilot has commanded. The pilot does not set a blade angle, but tells the propeller what RPM to maintain, and the propeller does it automatically. In some engine installations such as the Garret TPE-331 or Allison T-56, the propeller turns at "100%" all the time. It's RPM is constant. The torque imparted by the engine is changed as the pilot moves the power levers; the engine temperature changes as fuel is applied or removed, and the engine tries to turn the propeller faster or slower...but the propeller won't allow it. What the propeller does do is change it's anglel in order to change the aerodynamic load it encounters, and thus keeps itself at a constant speed. When the engine temperature is increased by adding more fuel (and the engine tries to turn faster), the propeller blade angle of attack increases, and the drag on the prop also increases...the propeller RPM stays constant but thrust is increased. We call this the governing range...the range of movement of the propeller which allows it to do as commanded.
The propeller on many installations doesn't go to one speed and hold that speed. It holds whatever the pilot tells it to hold. When the propellers is going fast enough to start governing, the propeller governor will set and hold whatever speed the pilot tells it to hold. In the cockpit, the pilot moves a lever (which goes by several names depending on which aircraft and engine installation is in use) to set the propeller RPM. In so doing, the pilot is setting tension on a small spring inside the propeller governor, and the governor does the rest. It controls exactly what RPM is maintained by the propeller, and accordingly, the blade angle used to achieve that RPM.
The propeller itself deserves some thought. The part of the propeller facing the back of the airplane is actually the blade "face," or the part that equates to the lower surface of a wing. The blade is really an airfoil, much like a wing, generating "lift" if you will; downwash, and thrust, as it imparts energy to the air that passes through it. The blade is allowed to change the angle it meets the oncoming air; it changes it's "angle of attack." As the angle of attack changes, so does the drag or load on the propeller blade, and so does the amount of air it's moving and the way it's moved.
The propeller is restrained from moving too far, that is, increasing or decreasing it's angle of attack too much. It's restrained by stops, or pins inside the propeller assembly that block movement of the blade as it twists to increase or decrease the angle of attack (AoA). We call that changing the pitch of the blade. As pitch is increased to "coarse" or the high pitch/low RPM position, the blade face moves from facing the back of the airplane to actually facing sideways...the blade is aligned more with the slipstream...ultimately to what we sometimes refer to as the "feather" position. Some propellers automatically feather when the engine is shut down, and you can recognize them by looking at the propeller at rest from in front or behind the airplane. The blades look thin as they're rotated so the blade face is sideways, rather than facing the back of the airplane. The blade doesn't go that far in normal operations in flight, but you can get the idea of how the blade rotates. Understanding that rotation, from minimum drag/feathered/aligned with the slipstream, to it's high RPM/low pitch position...when the blade is in just the opposite position from feathered. In it's high RPM position, the blade fact, usually painted black, if facing the back of the airplane, and it's resting against the high RPM stops.
The blade is free to operate anywhere in the range between the mechanical stops, that the propeller governor makes it go. It may be operating right at the stops (such as the low pitch stops during low power settings) or the high pitch stops during high power settings. This is all part of the "alpha" range of operations, or the normal range.
In reverse, the pitch stops are removed, and the blade is allowed to be scheduled into a different pitch range. It's important to understand what reverse or beta is not, as much as it is to understand what constitutes the beta range. Beta is everything that alpha is not. One may think of feathered, or blade aligned with the wind, as the zero degree position...though this isn't technically accurate as the blade is twisted and the angle of incidence and AoA) varies along it's length. Everything in the range of operations when the propeller governor is controlling the propeller and it's working to impart energy to the slipstream behind the propeller, is the alpha range. At feather the blade is just trying to minimize drag and is aligned with the slipstream, and passing beyond that position as the propeller blade face is rotated to actually point slightly forward, is the beta range...the reverse range.
One way to think of it is hold your hand out the car window, flat, palm down, thumb side forward, as you travel down the road. Tilting your hand to a positive angle of attack, with wind catching the palm of your hand, produces lift, and it's doing something for you to hold up your hand. That's the alpha range. Tilting your hand so there's no lift up or down, no pressure on the back of your hand or on your palm, is the feathered position; very little drag. Tilting your thumb down so the pressure is on the back of your hand, that's a little like reverse...it's doing somewhat the opposite of what you did to produce positive lift. The example, however, ends there when comparing it to the propeller.
The propeller airflow is a little more complicated, of course, because merely by spinning it produces it's own airflow, and as the aircraft moves forward another element of complexity is added; the angle of the airflow meeting the propeller blade changes with the airspeed of the aircraft. Kind of makes your head spin, doesn't it?
From the cockpit, the alpha range is everything that the propeller governor is controlling when you want forward thrust. You move the power lever, that tells the engine what to do, and it in turn tells the propeller what to do based on the RPM that you've asked of the propeller. More power wanted, bigger blade angle, etc. The beta range, however, takes the propler governor's normal functions out of the equation. You're directly controlling engine power instead of letting the engine fuel control take charge, and you're directly scheduling the propeller blade angle using the throttles (or power levers or reverse levers...depending on the airplane). When moving into the beta range the propeller stops, the mechanical devices that limit the blade angle in normal operations, are removed. The blade is free to rotate past feather and into a condition that causes a great deal of drag...and absorbs a great deal of energy from the slipstream.
Reverse thrust and the beta range isn't so much pushing air forward as it is creating a lot of drag and absorbing energy. The amount of drag it creates depends on two things, speaking from the point of view of the cockpit. It depends on airspeed; the faster you're going when you move into the beta range, the more drag is created (because drag rise increases in proportion to the square of the airspeed; double the speed and get four times the drag). The beta/reverse drag is also a function of the engine power; the more reverse applied, the faster the engine tries to turn, and the greater the negative angle commanded by the pilot (it's just the way the propeller is mechanically rigged). The more energy imparted by the engine, the greater the drag, just as increasing the blade angle into the beta range increases drag...both occur at the same time.
To go into reverse, the power levers are retarded to idle, and in most cases, the propeller levers are lifted up and over a mechanical gate on the power quadrant in the cockpit. A small valve assembly in the propeller and governor (beta valve) either opens to allow controlling oil and fluid in the governor to bypass the governor, or in some cases closes to prevent the normal function of the governor. As the power levers (now called reverse levers...sometimes an extra set of levers riding "piggyback" on the pain power levers, sometimes the same set of levers used to control power in the alpha range) are moved aft and pulled "deeper" into reverse, the pilot is actually scheduling the blade angle manually. This is really what constitutes the beta range.
In the alpha range, the propeller governor schedules the blade angle based on what the pilot has requested. The pilot doesn't really control the propeller or the power; he just tells the engine what he wants by positioning the levers in the cockpit and watching the engine instruments, and the engine and propeller do the rest. In the beta range, however, the pilot schedules everything himself, and rudimentary limiting devices are put in place to make sure he doesn't give it too much blade angle, or too much power. In alpha, the propeller self governs, in beta, the pilot directly controls the prop and power.
As you can see, there are several ways to look at what constitues Beta and reverse. In the simplest sense, everything that isn't alpha range (or forward thrust) is beta. It's just not that simple. Many pilots think of normal range, beta, feather, and reverse as entirely different things...and think of beta as the region of prop control "that makes that funny sound" when taxiing. This is incorrect, but it's also a more common-use way of thinking of beta and reverse.
Another incorrect way of thinking about it is reverse thrust actually pushing air foward. This happens to some small degree (enough that some aircraft can be backed up slowly using reverse thrust), but it's not air being pushed forward that accounts for the effects of reverse thrust when landing. It's a massive increase in drag (a spinning propeller being driven by the slipstream experiences a greater drag rise than if you attached a big plywood disc out there, the same diameter as the propeller arc), and one in reverse experiences a dramatic drag rise above that. A better way to think about it is that because there's only so much drag to be had out of the slipstream and the propeller, the pilot makes more drag by altering the blade angle and then applying power.
What angles and ranges actually constitute "beta" differ slightly in definition (with respect to degrees, control positioning etc) by name between different manufacturers, but in general you can still think of everything that the propeller governor controls, which is producing positive thrust, as the alpha range, and everything else as the beta range. There are significant differences in the way different turboprop engines work, and the philosophy that drives them, and accordingly the way the propellers and their controls work, but if you think in terms of alpha/forward thrust, beta/everything else, you'll have the proper framework to address most any turboprop or piston engine with beta and reverse capability.
So, I've seen fantastic video of Pilatus Porters in vertical dives following free falling skydivers at the same rate of descent. The plane appears to be hanging motionless behind the skydiver. Is the pilot using beta pitch to arrest the rate of descent in this case?
I would agree with pretty much all guppy's summary apart from the fact that reverse is purly a drag thing.
The garretts can throw serious amounts of air forward. Enough to throw a ground handler on his arse when taking out the prop locks. And when power back they struggle to remain up right. The book figure which is advised for landing which is worked out without reverse thrust can be halved in practise. In fact it has saved my bum with a brake failure in PLY, by the time I had gone for the brakes we were only halfway down the strip down hill and I could have quite happily reversed up the top with a landing restricted load.
One other point that seems to be confused in an otherwise thorough post:
Feather should not be thought of as the "Zero Degree" position; remembering that the "Blade Angle" is relative to the direction of prop rotation and not the airplane's forward motion, feather is actually closer to 90 degrees. The propeller does NOT pass through a feathered position to reach beta angles; feather is the MAXIMUM (mechanically limited) angle the blades can acheive. As you enter Beta range, the prop will have gone the other way, through fine pitch. Logic says that to reach a maximum-drag blade angle (Reverse), one would not want to pass through the MINIMUM drag configuration (Feather).
Using the Dash-8 as an example, the Beta Metering Valve activates with the Power Levers just slightly above Flight Idle, maintaining the blade angle at 17.5; this actually occurs in flight. On the ground, pulling the throttles past the Flight Idle gate will continue to push the prop farther into Beta, REDUCING the blade angle more and more until it reaches the "Disc" position, which has the blades at 1.5 degrees, almost flat relative to the motion of the airplane. During this stage the engine is idling, only putting out enough power to keep the RPM from decaying into the restricted low-RPM range. This "Discing" is the big drag referred to above, and in this position the prop actually throws some air SIDEWAYS (go stand next to one in Disc sometime if you don't believe me!). Passing another stop into the "Reverse" range, the prop angle goes negative, decreasing all the way to -12 degrees; simultaneously the ECUs will increase engine power, which DEFINITELY blows A LOT of air forward (again, go stand in front of one sometime!).
System-wise, the Beta valve actually controls regulated bleed-off of oil pressure in response to a blade-angle feedback signal from the prop, doing so to maintain a specific blade angle commanded by a linkage to the Power Lever. In the extreme lower range of Beta an additional valve (Reverse Valve) opens to allow unmetered oil pressure into the fine pitch side of the prop pitch change mechanism, which is then regulated by the Beta Metering Valve as described above.
Operationally, in the "Alpha" or Governing range of the prop, the PCU or governor varies blade angle to maintain set RPM; in the "Beta" range, the governor is bypassed and the Power Lever directly controls blade angle to adjust drag for ground (or sometimes flight) operations.
Again, the example above is for the PW 121 in the DHC-8, which is a free-turbine engine. Similar principles apply to most others. Hope this helps!
It should be remembered that reversable propellers are not just reserved for turbopropeller aircraft, many large piston airliners of yesteryear had them as well. Types I've personally flown, DC-6, DC-7, L1649A, B377.
In simple terms, beta range occurs anytime the constant speed unit of the prop governor does not control prop blade angle. In beta, prop speed is controlled via the power control levers. Every movement of the power lever will equate to a different prop speed because the prop will be at a fixed pitch, the blades sitting on the flight idle torque / primary low pitch stop.
In simple terms, beta range occurs anytime the constant speed unit of the prop governor does not control prop blade angle.
The above is the most succinct, most accurate, and most truthful answer I have seen in the whole thread.
All I can add to it is this - If your aircraft is equipped with a reversing propeller, you can determine whether it is in beta range as follows:
1) If the propeller speed that you have set with the propeller speed levers equals the rotational speed of the propeller that you observe on the Np gauge, you are NOT in beta range (you are in constant speed range).
2) If the propeller speed that you have set with the propeller speed levers is LESS than the speed you see on the Np gauge, then you ARE in beta range (beta reverse valve is controlling the propeller).
Don't let anyone else confuse you with more elaborate explanations, or baffle you with BS - the quote cited above and the two points I have added are the simplest possible way of defining it.
In the case of a Pilatus PC-6 B2H2, powered by a P&WC PT6A-34, driving the all aluminium, fully featherable, and reversible four bladed Hartzell HC-D4N-3P Propeller...
There are three controls, from left to right :
- Propeller Control : Normal operation and FEATHER
- Power Lever : IDLE STOP and REVERSE
- Idle Control : HIGH IDLE, LOW IDLE and CUT OFF
My question is, how to reach the Beta ? Do you have to lift up the Power Lever, entering into REVERSE range ? How should the Prop Control Lever be ? ( If I undertood, excepet if it's in feather position, the Beta mode bypass it and don't care of the Prop Control position ? )
If you control only blade angle in the beta between +4° and -12.5°, with which control ? So what is the Ng in beta, it is near to idle or near to full speed, does it depends in HIGH-LOW idle settings ?
I would LOVE someone could clearly answer those questions.
You select beta range with the power lever (aka throttle) by moving it below Flight Idle. In this range the throttle directly controls not only engine power/torque/fuel flow, but ALSO prop blade angle. That's the engineering definition of beta control mode.
This means the throttle is linked to two control cams, engine power (Ng/core speed on the PT6) and prop angle, since in beta the prop governor is disengaged. The two cams may not be perfectly matched, so prop rpm may not be a specific value, but "close enough for government work" as my dad used to say.
And of course, most a/c types have beta mode inhibited/blocked in flight.