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Old 15th Oct 2012, 20:14   #41 (permalink)
 
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There is one pitch change piston inside the prop hub which is connected to all the prop blades; when its at its full extension you've got feather. When it's at it's full retraction you have max reverse. Beta is just a name for a certain range of the pitch change rod. You could just as easily call that range 'Pitch Change Rod In The Lower 40% Of Its Travel Mode'. Beta's a lot easier to say.

How do you get in it? Power from the engine has to be sufficiently low to allow the prop to slow down enough so that the PCU governor is commanding full fine. You then need the flight idle pitch stop (which is stopping the Pitch Change Piston from fining off the prop below flight idle) to be withdrawn. Usually this will be linked to the weight on wheels system. When that stop, and it is a mechanical lump of metal physically stopping the pitch change piston moving, when that is withdrawn then the pitch change piston can fine the blades through 0 degrees and into reverse. The B1900 for example has a beta valve. This creates a hydraulic lock in the pitch change piston, stopping it going beyond a certain distance which corresponds with the blade angle for flight idle.
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Old 15th Oct 2012, 20:19   #42 (permalink)
 
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Actually, I'm fairly sure there is no weight on wheels protection on the B1900. If you were to lift the power levers down through the flight idle gate, I think you'd get reverse (beta range) in the air. Maybe a B1900 pilot could try it and report back if it is protected? If not, Sky news will report back to us
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Old 15th Oct 2012, 20:22   #43 (permalink)
 
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your a naughty man
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Old 15th Oct 2012, 21:14   #44 (permalink)
 
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Meant to add, the pressurized oil that moves the pitch change piston comes from an engine driven pump; no engine rotation, no drive for the pitch change pump, doesn't matter what you do with the prop levers, blades will not change pitch.
Don't confuse this with an emergency electrical feathering pump; that supplies oil pressure to coarsen the blades to feather only.
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Old 8th Nov 2012, 12:21   #45 (permalink)
 
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Ok so, it becomes more and more clear.

For example, in my simulator, I set everything like that :

The prop angles varies from 86.1 ° ( Fully feathered ) to -11 ° reverse.

Now, when I start up the PT-6, with prop lever fully forward, when the prop slowly going out of feather ( as the engine oil press is rising, to fine the pitch of the blades against the spring and counterweights forces ), it goes to -0°30', which is the β angle.. then, when the cond. lever is at Low Idle, I get aroud 1350 RPM ( with 52% Ng ) and the prop pitch only start to coarse when the Np comes close to 2150-2200 RPM ( Max nominal rpm for that Hartzell prop, when the RPM lever is full forward ).

Some incomprehension now. When I see a PT6 engined plane starting up from the side view of the prop, I can see the blades slowly flattening while speed increases ( getting out of feather angle ) , but then it comes to a stop ( visually, the low flight pitch stop of 9°30', and it create tractive thrust ( the only flat pitch is on the extreme tip of the blades )? The drawn I posted earlier in this thread ( " Flight Idle " )could be that stop. It doesn't get lower than that, near the 0° WHY ? I can reach in the simulator those 9° to get tractive thrust, if I bring the Prop Lever back a bit ( on ground, just after start up for example )

In flight, when you are at cruise speed of 115 Knots, power 80% and Np 2200 RPM, if you fully retard Power lever, the pitch will flatten to keep those 2200 RPM, then it will come to a low pitch stop, and from this moment the Np will start to decrease, and will eventually re-increase up to 2200 RPM if you pitch nose down, and gain some airspeed ( then the pitch will coarsen again ). Now, how you can come into the flat pitch angle ( -0°30' ) while you don't have to lift the power lever to go into reverse, just by pulling it against Idle stop ?

If I set the flight idle stop to -0°30', then if my plane has low airspeed, the Np will fall below the max RPM set of 2200 RPM, and slowly reach those -0°30', and NOW the prop disc will act like a big airbrake, and I'll be able to start my impressive dive at -7000 ft/min without going over 120 IAS, but if the plane goes up to around 130 IAS, and the relative wind makes the prop gaining speed by windmilling, the pitch will coarsen again, and it will get out of Bęta range

And in flight, what is the range where the RPM Lever can actually be used to change prop speed ? I know max is 2200RPM, full low ( aft detent ) is feathered, but on the full aft stop, that is something like 1300 RPM or so ? Can't find anything on it, even with the 650+ pages manual of the PT6A-27 I have in front of me..

You guys have to admit it's MUCH MORE complex than the simple explanation " Bęta is flat pitch " or " It's anything that is not Alpha range "..

Hasta later

Last edited by Hueyman; 8th Nov 2012 at 12:24.
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Old 17th Nov 2012, 04:41   #46 (permalink)
 
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Now, the question is :

What kind of system, and how it happens :

That the flight idle pitch stops ( so +9,30° blade pitch ) are removed to allow blades to rests on ground low pitch stops ( Bęta, -0,30° ).

I know on some bigger planes like the Dash 8Q400 it's some kind of system related to pressure on landing gear, when the senser know you are firmly on ground, it allow the flight idle low pitch stops to be lifted, so the blade can go to Bęta range, or mostly " Ground Fine ".

On the PC-6 I don't know. If you think this is not really realted to the general Bęta range, I can start another thread especially about the Bęta used on PC-6 aircrafts.

Thanks for your help
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Old 17th Nov 2012, 10:18   #47 (permalink)
 
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i personally very much doubt beta range ( so lifting the power levers over the gate) inflight is approved for any pt6 equipped aircraft and is misunderstood with the procedure pulling back to flight idle which will result in torque dropping to zero and the beginning where the wind drives the prop in a dive. ( pt6a will allow this- on a pte331 the negative torque sensing would kick in ) . the prop governor in this situation is still active and may reduce blade angle further to prevent a prop overspeed in such a dive. the sound is noticeable when this begins.

going really into beta means the governor is ovverridden and prop pitch is controlled with the power lever ( before pulling back further and reverse kicks in- this is governor controlled again ) .

going beta inflight means beyond the really massive drag having no propspeed governor and nothing prevents you from a prop overspeed.

i do not know a porter in specifics, but like said i believe you misinterprete the sound and drag from closing power fully ( flight idle ) where the governor dictates a very flat pitch to not overspeed the prop which is driven by the wind in this situation ( the prop disc braking phenomena) and real beta range with prop governor disconnected.

you can clarify by asking a real porter pilot if he pulls back the power lever beyond flight idle by lifting it and going over the gate inflight- i do not think so.


cheers

ps : hyueman - are you from the same project like another person who contacted me regarding pt6a operation principles ?
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Old 17th Nov 2012, 14:24   #48 (permalink)
 
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Hello Aerobat77,

Thanks for answering,

No, I think the guy you mean is Marcel, another guy who is also interested in PT6A operating principle, but for another simulator ( but same purpose )...

No, as I told earlier in this thread, I can speak with a real PC-6 pilot about every weeks, but the deep Bęta use and principle is dark to many pilots, only engineers and some mechanic can clearly tell you what actually happens behind the scenes...

In the simulator, I can achieve the dive performances ( 110-115 Knots, - 6500 Ft/min, -40° pitch angle ) with a " flat " pitch of -0°30', and Np don't go beyond 2200 RPM until the plane don't go over 140 Knots ( then the governor increase pitch to reach again the governing range ). All I said is of course, assuming Prop lever is full forward.

But in this case, in short final, if you suddenly retard Power lever against idle stop, you will create so much drag that you'll stall ( in short final, 38° flaps, you want to be at 61 Kts.. ) So what is preventing ? In an old PC-6 PoH, I read that flight idle prop pitch was +9°30', and so, even when blades are resting against low pitch stops, it never create negative or zero thrust, so no worries if you retard power to idle in final, while the airspeed is low.

The question now is, what allow the prop to be at flat pitch in flight, and what DON'T allow it not to be flat in final, or other critic flight situations. Cause I'm sure there is some kind of protection to not enter in flat pitch in final approach..

BUT, there are procedures written in the manual about " Bęta approach ".. so, there is obviously a relation between that..
And here is my total lack of info

Last edited by Hueyman; 17th Nov 2012 at 14:25.
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Old 17th Nov 2012, 18:01   #49 (permalink)
 
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The overspeed governor is always there; throttle at idle with the aircraft in a dive, if the prop's speed increases beyond governered speed, the governor will increase blade pitch until speed is under control. The governor does not require throttle input to increase blade pitch.
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Old 18th Nov 2012, 11:19   #50 (permalink)
 
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I know i know i know..

But nothing about low pitch stops removals..

Maybe the clue is in the " High or Low Idle "

At High Idle, the Cond. Lever position that should be selected during approach, Ng rises up to 78-80 %, when Power Lever is at idle.. so, Np become much higher and there is much chances that the prop blades goes out of bęta flat pitch range, to keep in the governed range.. Maybe that is the solution ?!
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Old 18th Nov 2012, 15:49   #51 (permalink)
 
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Keep in mind that not all aircraft with PT-6 engines have high and low idle. The Twin Otter did not have this feature. I believe it was around 51% Ng at ldle.
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Old 21st Nov 2012, 00:08   #52 (permalink)
 
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Mate, if the low pitch stop sets the prop at 9 degrees or whatever, it will do so if you retard the throttles whether you are maintaining altitude or in a dive a -30 degrees. The governor will command full fine all the way until the low pitch stop kicks in and the low pitch stop will set the prop at exactly the same angle everytime if it is allowed to kick in.

To be able to set the prop at an angle below the low pitch stop you will need to lift the gate or twist the power levers or whatever as if you were selecting reverse on ground. That being said, if you need to set something like 0 degree prop blade angle you will need to lift/twist/whatever the power lever. This would be the so-called "prop disc" on the Dash 8, you can't set it in flight just by retarding the power levers to idle and I don't think you can in any airplane. Well, you can, but you shouldn't!

According to your graphic, to get the "beta angle" I think you need to lift the power levers as if you were selecting reverse in ground. It cannot be done in flight by just retarding to idle
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Old 21st Nov 2012, 16:45   #53 (permalink)
 
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Mate.. " I think you need to lift the power levers as if you were selecting reverse in ground. It cannot be done in flight by just retarding to idle "

This is exactly what is crazy about that, that I cannot understand at all.. the PC-6 pilot I speak with confirms that he only retard Power lever against idle stop, but in no way lifting it toward reverse range.. this is why It's so hard to understand. Most part of PC-6 pilots don't know what is hapenning exactly when using Bęta in flight, some says the prop acts like a " wind turbine ", only slowing down the plane because it's moved by the relative wind, others said it flat pitch etc etc..

If you can reach flat pitch only by retarding power lever to full idle, that would be catastrophic in short final, if you - by mistake - retard that lever too far aftward..

So, I just thought there were kind of sensors, like the ones mounted on some regional turboprops, which allow Bęta or ground fine, or even reverse, when some pressure is sensed on the landing gear, meaning the plane is firmly on ground.. I thought on that kind of plane that would be something according to plane TAS, for example you can reach flat pitch when you're over 80 kts, but below, the low pitch flight idle stops are active again..

I hope I explain the thing clearly enough..
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Old 21st Nov 2012, 17:47   #54 (permalink)
 
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you are making some mistakes gents.

first- there are no pitch locks on a pt6a .
second - going to beta ( lifting over the gate) disconnects the governor . this is -beyond flying against a wall when being airborne - one of the big reasons beta inflight is prohibited at any aircraft and the porter is no exeption of it.

third- pulling back to flight idle with props forward on a pt6 results in torque dropping to zero and the wind driving the prop . some call is "prop disc braking" and some falsly call it beta range .

on this engine no NTS is needed like on singleshafts. pulling back an pt6a to idle with props forward gives an enormous braking effect with the steep approach capability. the higher the speed the higher the effect.

thats not only for the porter tha case . pulling back both engines to full idle stop at lets say a cheyenne and pushing the props simultany full forward will result she will drop like a rock , but its not done for passenger comfort and a stabilzed descend. the porter is just used this way in jump missions.

when it comes to land a pt6 driven airplane , lets again say a cheyenne : be at vref in 50 ft over the threshold and pull back to full idle ( up to the gate , not over the gate) and you will need a cane to remove the aircraft- she will stop in the air and fell out of the sky.

even when at slower speeds the prop disc brake effect decreases its even here very significant and you have to be careful.

don,t misunderstood beta range (lever over the gate, governor off , power lever controls prop pitch ) with prop disc braking effect ( power idle , governor on , wind drives the prop )

hope this helps !

cheers
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Old 21st Nov 2012, 18:34   #55 (permalink)
 
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mmm.. Okay... I see

So, all is suddenly clearer, taking your Cheyenne example.

But, ( yes sorry, I'm a but man.. ) why,when taxiing, the prop is making such a distinctive sound when you want to brake WITHOUT entering into reverse ( once again, the pilot told me he didn't used reverse, only power lever up to gate, and not over ).

With more or less +10°, it seems hard that the prop will have such a braking effect, that it can allow such dives, and in the PoH, it's well written the Bęta angle is -0°30', so, I assume achievable in-flight..

I'm sure there is a specification ONLY for the Porter, and this is why we can't get this fully explained..

Thanks much though !

PoH I speak of is : http://dl.avsim.su/dl/ticket/ZmlkPTM...tus-Porter.zip

Go page 33-34 of 176
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Old 21st Nov 2012, 19:26   #56 (permalink)
 
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Quote:
why,when taxiing, the prop is making such a distinctive sound when you want to brake WITHOUT entering into reverse ( once again, the pilot told me he didn't used reverse, only power lever up to gate, and not over ).
some people call "reverse" anything below flight idle , some call "beta" anything below flight idle and this mixes it sometimes up

.but there is beta and reverse and both are different things.

technically ,when you taxi, and are in flight idle the turbine is running idle speed and the prop is pitched to flight idle pitch .going over the
gate engages beta but initially not reverse.

in beta the turbine is still fixed on idle speed but the pitch starts to go below flight idle pitch . forward thrust and power absorption reduce , with the same output from the idling turbine the prop speed start to rise. the rising speed of the blade tips as well interferences from the low pitch of the blades generate the sound you talk about.

pulling back further engages reverse. the main difference is that the governor kicks in again ( reverse is instead of beta governor controlled !) and the turbine spools up , power output increases but with thrust back since the blades are now negative.

braking "without reverse" means a taxi in beta but before the turbine spools up again.

Quote:
With more or less +10°, it seems hard that the prop will have such a braking effect, that it can allow such dives, and in the PoH, it's well written the Bęta angle is -0°30', so, I assume achievable in-flight..
due to its design nature a turboprop generates thrust by moving a large amount of air with relative small velocity. thrust occours when there is a speed difference between air pushed by the props and the airspeed of the aircraft.

so in taxi, with an airspeed of virtually zero the thrust generated by the props is pretty high even in idle- thats the reason we need beta at all for taxi. in flight idle especially higher powered twins would need a constant brake input without beta.

in a dive with a high airspeed and pulled back power the speed of the air surrounding the aircraft starts to be greater than the speed of the air pushed by the props of an idling turbine , the thrust goes to zero and the props , now driven by the wind, start to act as an massive airbrake .

cheers
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Old 21st Nov 2012, 20:03   #57 (permalink)
 
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Ok thanks Aerobat.. so , in a dive, the blades are never coming near flat pitch.. but keep it's low pitch stop 9°30' until Np goes higher than 2200 RPM for example ( assuming prop lever is firewalled of course ) and only that is sufficient to brake enough

Okok.. I'll try that in the sim and will report results back to you.

Thanks a lot !
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Old 21st Nov 2012, 20:31   #58 (permalink)
 
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give better a good whiskey than reports from the sim :-)

i cannot say how the simulator manages this, but heared that especially turboprops in the microsoft sim are hard to simulate correctly.

nevertheless- going over the gate ( so in true beta) disengages the prop speed governor on ANY pt6.

doing this in a dive is asking for bursting the gearbox by propoverspeed .

the free turbine design pt6a without a automatic spool up to prevent negative torque on the prop shaft and so allowing the prop disc braking - pulled back to full idle- will brake as hell in a dive without the "need" of going into beta due to the described above.

that reflects the impressive dive on a porter and correct reports of your pilot that he goes NOT over the gate inflight.

enjoy !
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Old 21st Nov 2012, 20:36   #59 (permalink)
 
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Okay dude ;-) So your lasts statements explain it ALL !

Thanks

Not Microsoft Flight Sim... someone is already doing one for it. Mine is for X-Plane ( Pilatus PC-6.. A bit of history, and firsts WiPs ! - hueyman.overblog.com )

Thanks you very much Aerobat, and also thanks to all of you.
Valentin
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Old 21st Nov 2012, 21:53   #60 (permalink)
 
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PC-6 B range

Dear Aerobat77 despite your vast experience regarding the PT-6 family i have to notice that you' re misinformed regarding PC-6 operations.
PC-6 are indeed cleared to enter the B range in flight, as you can see in any PC-6 POH, the bit of information that' s probably missing is the fact that the PCL hard stop is set so that a movement of said lever toward idle brings it in the B range without the need to lift up the lever.
Just to clarify even more lifitng up of the PCL to go toward the Reverse range in flight is forbidden on PC-6 too.
I tried to keep it simple and concise, hope it helped.

Regards
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