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..
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.
Location: 5° above the Equator, 75° left of Greenwich
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.
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 ?
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 ) ?
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
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 ?
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.
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.
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
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.
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..
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'..
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 :