Beta Range
 

Hi everyone, could someone please explain what beta range is?

Does the beta range differ between engines and if so why?

Also how does the reverse torque (thrust) on at turboprop work to slow down the aircraft?

Thanks in advance

Reverse
 

It use's the prop blades to reverse the airflow and blow the air the otherway just like a thrust reverser

It may vary between engine models P&W or Garretts

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.

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.

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.

Also some more info from ATSB Here

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.

Newpic,

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.

Porter in descent
 

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.

Don't try this indoors. :D

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!

on the pilatus you could use beta inflight, which was as far as i remember,
(i flew the porter last time in 1987) a blade angle between +11.5°down to +4° , beta on ground was +4° to -12.5°.

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.

Feather blade angle close to 90 degrees to plane of rotation.
 

Well said, Hikoushi, I was going to say the same but then saw your comments regarding the blade angle in Feather. Nice work.

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.

There's a rather famous accident - DHC-5D Buffalo at Farnborough - in which a very steep approach (props in beta mode) resulted in a very hard landing. The airplane was rendered non-usable. :=

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.

Dis anyone notice the date that this thread was started?

...Or that the last post before it went quiet (2008) was by that Dean of PpRuNe, 411A? :uhoh:

Ok,

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.

Many many thanks.

Not knowing the PC6 in detail, but generalizing -

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.

Mm ok.. !!

In the PC-6, the power lever require to be lifted to pass over the idle gate ( to enter into reverse ).

So, there is two ranges, ALPHA is all the range the FCU, CSU etc are managing when the power lever is from IDLE to Full power, then when you go into the reverse position, it incluede the Beta range.. is it that ?

So Beta is reached by lifting the lever, and going a but aftward, not fully because it will be reverse..

All is way more clearer now !

If some PC6 pilot could come and confirm that

Correct me if I'm wrong, somebody -

But early reversible props (on most pistons & early turboprops) were NOT beta props; when reverse was selected, the prop blades went full travel to a fixed reverse-pitch stop.

This means that pulling more throttle in reverse would modulate (increase) prop rpm - just like any other fixed-pitch prop installation.

Hello guys,

I asked personally to a friend, who is CPL in a PC-6, and was ATR42 PiC before.

He told me, you obtain it ( on the PC-6, not generally spoken ) by pulling the Power Lever all the way back, against idle stops. This way the prop enters into beta mode.

The Np ( Prop RPM in % of max RPM, 100% is 2200 RPM ) is around 60%, if you activate the beta in flight, while the Prop Control is usually not full fine, but in a cruise config. You can reach around 80% by increasing this lever, to get out of vibration range. The higher the Np is, the more effiscient is the brake effect ( zero thrust become stronger, because the higher it spins, the closer it comes to a real full disc, like a round table )

Until that, I understand, but beta is NOTHING TO COMPARE WITH reverse range, wich is totally forbidden to use in flight, because so much torque and vibration would destroy all the engine mount, airframe etc, adn the slipstream around the fuselage ( so back controls surfaces like elevators and rudder ) would not be blowed at all.

Now, I would love to know how they can use beta for taxiing, or is it just for braking ? Airbrake on taxi speed are useless isn't it ? And is beta mode really a fixed and definitive pitch ? If so, I would like to know the exact beta angle.

I'll eventually ask directly to Pilatus Switzerland, but if any PC-6 pilot or mechanic see that, please help.


Sorry for my bad english I'm 17 YO, and french

In flight, Beta range is not something you select rather than something you enter to (unless you specifically move the power levers/throttles/etc below idle).

In flight, in the Twin Otter at least, you enter beta range in EVERY approach as the CSU no longer controls the prop speed, usually around 103KIAS: Flap 10 speed. After this moment, every movement on the power lever you make will see an increase or decrease of prop speed, i.e.: you're in beta range as you are controlling the prop speed with the power lever and not via the prop levers.

Please stop using BETA and REVERSE as if they meant the same: They are NOT the same. BETA range, as V1...Oops said earlier, is the range when you control the prop speed via the power levers instead of the prop levers, which is ALPHA range. REVERSE range is from 0 degrees blade angle all the way to the NEGATIVE mechanical stops, which for the Twin Otter is at -15 degrees. You could say that REVERSE is "located in" the BETA range, but they are not the same.

I haven't got as far as selecting reverse in flight, I've only done up to initial beta or "null idle". Not planning on going to reverse in flight either, but at null idle in flight you get buffeting and a bit of vibration.

As for the taxiing bit, while taxiing you're always at beta range as, again, you are controlling prop speed via the power levers. The prop levers are completely useless on ground for everything other than feathering the prop. Moving the prop levers will see no variation of prop speed, but moving the power levers while taxiing WILL produce a prop speed change (you're in BETA!)

The use of null idle and reverse (i.e. anything below idle stops) while taxiing is a way to control taxi speed and in the turboprops is quite effective.

Beta is not a mode, beta is a RANGE. As a range it has points, between those points it is considered you are in beta range. As it is a range, you can move between the points which make that range, therefore, beta is NOT a fixed angle. The prop actually changes its angle between the range; the prop is in a different angle at the start of the selection of reverse than it is when it is in full reverse.

Hope it helps. It's a concept that it is hard to get your head around of at first.

Well on garettes your not controlling the prop speed your controlling the blade angle and the PPC then increases or decreases the fuel flow to maintain the taxi rpm.

So you lot might be right about free turbine machines but fixed shaft is different.

Hey thanks Escape Path !

But a point you didn't lighten up was.. the brake effect in flight.

If it's not an angle, what makes a turboprop plane able to make dive descent ( - 5500 Ft/min without going over 110 Kts ) ?

That is tricky, when I understand the thing in flight, I don't get how it works on taxi and vice versa..

Thanks

Ok now,

another interrogation :

If Bêta is the range where the power lever directly acts on the prop speed.. ok, so:

When you're applying your power on brake before take off, the prop lever is full forward for full fine, so, before the prop is reaching the 2000-2200 RPM, the blade rests on the low pitch mechanical stops ( before the CSU regulate the RPM by increasing pitch to maintain the selected RPM ). So here you are in Bêta range ?

But what about the blade pitch coming near to 0°, to produce zero thrust ?
As I wrote earlier in this thread, in the PC6 you enter into Bêta range by pulling power lever all the way back against idle stop, and you actually control the Np with the prop lever. Is this particular to PC-6, which carry PT6A-27 or -34, or it's same with your Twin Otter ?

Thanks

As you bring the power back the blade angle changes. Not directly from the throttle movement though. The constant speed unit is attempting to maintain the rpm you told it to maintain with the prop lever setting. It continuously does this as you retard the throttles until the blade angle reaches the low pitch stop. Then the CSU can no longer reduce blade angle. Then further throttle closing starts directly reducing blade angle which is now where you enter the beta range. Once the throttle is closed, you are at the minimum blade angle legally possible(on most aircraft). That is the other end of the beta range in flight. Once on the ground when reverse is selected, you go into the ground beta range.

When your blade angle reduces, there is more of it facing into the relative wind which means more drag which gives the higher descent rate. Similar can be said of an aircraft such as a Bonanza. If you have an engine failure with a windmilling engine, selecting course pitch reduces drag and increases glide distance.

Of course you could select reverse in flight and reduce blade ange even more but aircraft loss of control could result as airlow over the tail can be disrupted. On many aircraft and/or engines, damage may occur and has caused several turboprop accidents.

Some turboprops have an "approach beta" function eg DHC-6... most fixed shaft single- spool engines eg:TPE-331 have the flight idle fuel flow set high enough to prevent negative torque from occurring when the propellors reach the low pitch stop

Mmm Ok I slowly start to get it.

On the PC-6, the low pitch stop is 9°30', so it's still producing forward thrust.

So, when entering into Bêta, the power lever becomes a correlation of blade pitch and Ng speed, exactly like the reverse, but with a minimised min pitch ( on the PC-6 I've read that -0°30' is the lowest angle ( Bêta angle ) that can be reached without entering into reverse.

Are there any experienced PC-6 Pilots there to confirm or correct what I said ?

Thanks so far guys

Beta
 

Hueyman,

The simple answer to question is that in aircraft propellor design the symbol alpha is used for a positive angle of attack and beta for a negative one. Therefore when the blade has moved from positive to negative angles relative to the airflow the guys who do big sums use the symbol beta to show the angle on their diagrams.

MM

I would be careful about interpeting the above statement as having to do with beta range. It is just the throttle controlling blade angle, and I suppose gas generator speed(N2/Ng) on the PT-6.

I should think that one can be producing forward thrust while in beta. Of course, at what propeller station the blade angle is measured at would affect this as well.

A good example of drag being used from a propeller if fine(or flat) pitch is the Rolls Royce Dart engine on aircraft types such as the HS748 and the F-27. They have no reverse. On short final when the throttles are closed, the propeller blades are at the flattest pitch that they can be while in flight as they are on their low pitch stops. At a typical landing speed they are now creating some drag but not too much at their positive blade angle.

Once the aircraft touches down, a handle is pulled removing the Flight Fine Pitch stop(FFPS) and the blades immediately go to a much flatter pitch but not into reverse. Engines remain at idle. This much flatter pitch creates a significant increase in drag and helps slow the aircraft down. You can also hear the different sound. As speed slows, the effect becomes less and less which of course is worriesome on a very slippery runway.

Pull the handle in the air and you will get a big surprise if you are more than a few feet high. You know they went to fine on landing by lights in the cockpit and I suppose directional problems if one didn't move. And if one doesn't go into ground fine, do not open the throttle after landing or it will get really hot(the engine that is).

So the slower you are, the less effect fine pitch has on deceleration. The Allison 501(at least some of them) had something called beta follow up. As the power was increased, blade ange coarsened to give increased thrust although rpm stayed the same. A pitch stop mechanism followed up behind the propeller based on throttle position. Therefore, if an engine flamed out on takeoff roll, the pitch would be stopped from reducing too much. Otherwise immediately after the failure, you have an engine at a relatively high ground speed in flat pitch causing serious directional control issues. With the beta follow up keeping the pitch coarser, you still had directional control problems but not as bad. Therefore you will have a lower VMCG and have less controllability issues immediately after the failure below V1 but prior to the other throttles being closed.

Its also to note that that torque setting produces more drag than feathured, most experenced crew want the flight idle tourque set as low as possible about 2% is good for me. Inexperenced crew hate it low and prefer it up at 8 percent.

If its goes below 0 and there isn't a NTS system fitted the plane basically stop in the sky. 250knts down to stalling in a Nm in level flight. And can get very interesting especially if your in a turn and the inside engine is the one set to low. You get a huge leans effect and it feels as if the wing has grounded on something and the whole aircraft is rolled and yawed into that side.

Also as well the props that are usually fitted the hub is controlled by oil pressure and a big spring. As big as that spring is if you get into beta range its not strong enough to return the props into Alpha in flight due to the aerodynamic forces. Its happened a few times over the years and the plane has been turned into a brick with everyone killed.

Ok guys,

But the PT6A Free turbines and the Fixed shaft Garett ones didn't use the same mechanism/principle I guess.

If I want to truly understand how it works, it's to reproduce it in a simulator. Because of that I have to really know about it.

Thanks

same basic idea for both types, prop governing range (forward thrust) power lever controls fuel and rpm is controlled by prop governor, beta range, power lever directly controls blade angle, rpm is controlled by fuel flow governor

This is in contradiction to what the PC-6 pilot I had the chance to ask told me..

If some french people see this, I post the original message :

Sur le plan de l'emploi de la Beta, je l'obtiens donc au sol au roulage en plaçant la manette des gaz en butée (bruit trè s caractéristique), et en vol, également en la plaçant sur Idle. Une fois que j'ai obtenu ce frein hélice, la vitesse hélice a tendance à tomber vers 60% Np. Je peux toutefois réaugmenter le nombre de tours hélice en réavançant cette même commande, pour sortir de la plage de vibrations. Je peux rester en Beta jusqu'à environ 85% de Np. Bien sûr plus j'augmente la vitesse hélice, Np, plus l'avion est bruyant, mais j'augmente aussi l'efficacité de mon frein hélice.


Now I'll try to translate it as best as I can :

About the operating of the Bêta, I obtain it on the ground by retarding Power Lever against Idle Stop ( very characteristic sound ), and in flight also by retarding that lever to Idle. When I obtained that " prop brake ", prop speed tend to slow down to 60% Np. I can increase that prop speed by increasing the Prop Lever, to get out from vibration/buffeting range. I can stay in Bêta until it reaches around 85% Np. Obviously, the faster my prop, the louder the plane... but my prop brake effect is also more efficient.

This is what is happening in the PC-6 B2H2 aircraft, surely not comparable to how it's used on a King Air, or Twin Otter, or any other planes. If needed , I'll eventually start a dedicated thread for the Beta Range ON the PC-6 plane, I'm sure many Drop Zone pilots are roaming around..

look at your type not the general. Anything off a forum take with a pinch of salt.

read the POM and stick to it.

The Propeller Constant Speed Unit is usually a centrifugal governor supplying or restricting oil pressure to the pitch change piston that is in turn connected to the blades. The PCU maintains the selected RPM by varying blade pitch, i.e. increasing pitch as the engine generates more power or decreasing pitch as engine generates less power to maintain selected RPM.
Throttles control the amount of power the propeller absorbs, not the speed of the propeller.

As engine power is reduced, the PCU will continue fining off the blades to try to maintain RPM. But there will come a time when the blades are at maximum fine, the PCU has no way of knowing this, it'll just keep fining off the blades to maintain RPM, all the way to max reverse if you let it.

So there will be a Beta valve of some sorts fitted, to prevent the PCU fining off the blades below a certain positive angle, the 'Flight Idle' stop, which may be a mechanical lump of metal or a hydraulic lock.

As the prop slows below governed RPM, any increase or decrease of engine power will directly affect prop RPM (by shoving more hot gas through the prop's power turbine) i.e. the throttles are now controlling prop RPM, you're now in Beta range. Selecting reverse will usually bring another governor into play to prevent the main PCU increasing pitch.

Ram the throttles forward, the prop speeds up until it reaches governed speed, the PCU will now start to coarsen the blades to slow the prop down and maintain selected RPM; you're out of Beta range again. throttles no longer control RPM, they're back to controlling power.

Man, thanks but all that is the basic of Constant Speed prop operation

I understand very well that below the governed range, when the blades are resting against the Flight Idle stops, the Power Lever now directly affect Np

Now, how you enter into Bêta mode, or if you prefer, the one used to brake.
Because on the PC-6 the Flight Idle max fine pitch is 9°30' at 30 inches station ( position of the blade that the measure of the angle was taken ), I can tell you there is no brake effect, or at least not the one we're lookin' for.

So, what allow the blades to come at around -0°30'..

There is a good scheme here :

http://aerosolution.ca/inside/courscaravan/PT6-3.jpg

But, if it's work like that, when you start your PT-6, pushing all the way forward the Prop Lever after start, as the Power Lever is fully retarded, the prop should be in Beta Range, AND Beta angle, so, viewing from the sides, the blades would be really flat, having the flat part in the center.

To show you what angles I mean, I used this PC-6 prop I modelled :

http://imageshack.us/a/img577/1852/sssos.png

Uploaded with ImageShack.us