AirWillie6475
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Reverse Thrust On Props

Wed Sep 27, 2006 1:40 pm

Is reverse thrust used inflight on props? Also do they use them to slow aircraft while taxing?? I ask because props make loud reverse like sounds while taxing.
 
cactushp
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RE: Reverse Thrust On Props

Wed Sep 27, 2006 1:49 pm

The reverse sounds would be the prop lever pulled all the way back, lowering the prop rpm. In takeoff configuration I would imagine the prop levers forward, increasing the rpm. I could be wrong though.
Sorry, I was on the landline
 
Fly2HMO
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RE: Reverse Thrust On Props

Wed Sep 27, 2006 3:20 pm

Quoting AirWillie6475 (Thread starter):
Also do they use them to slow aircraft while taxing??

On all my Dash 8 flights I've noticed that on taxiing the pilots rarely use brakes. I like to sit in row 2, which is right where the prop is so I can see it changing pitch. While taxiing I notice they put it in flat (right term?) pitch when they get a bit too fast, so the prop basically acts like a big speed brake. I believe ground idle apparently does make the Dash 8 move pretty briskly, but I could be wrong. upon landing though, the props actually reverse pitch, and make the distinct sound.

I could go into alpha and beta range theory, but thats a whole other can of worms...
 
AJ
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RE: Reverse Thrust On Props

Wed Sep 27, 2006 4:04 pm

In laymans the sound you can hear is a change in prop pitch, not rpm. The range used on the ground is 'Beta' where the prop pitch is very fine and can be in a zero thrust or even reverse pitch.

Reverse thrust is not used in flight by multiengine props as the results can be catastrophic. The stress on the blades is one thing, but the results of one prop entering reverse and not the other is the main reason! The minimum setting will be the 'Flight Idle' position. In this mode the props become big speedbrakes!
 
AirWillie6475
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RE: Reverse Thrust On Props

Wed Sep 27, 2006 4:35 pm

I see I guess they use the idle position for breaking that is why props make that continuous noise. As far as the throttle isn't it easy to put the levers into reverse since it's just one continous throttle?


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HAWK21M
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RE: Reverse Thrust On Props

Wed Sep 27, 2006 4:48 pm

If not Mistaken Don't Props have a Air-Grd sense to Avoid reverse pitch in Flight.
regds
MEL
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SNA350
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RE: Reverse Thrust On Props

Wed Sep 27, 2006 5:24 pm

Quoting CactusHP (Reply 1):

yes you're wrong  Wink
you don't get reverse thrust by lowering the RPM but by changing the blade angle, on take off the blade angle is also increased but in the other way

in both conditions the prop speed will be 100%

Quoting HAWK21M (Reply 5):

yes some have, I know the ATR has one, called the Idle Gate
it prevents you from putting the power levers in Ground idle and so you can't put it in reverse thrust
you can see it in the picture below, it's the T formed handle just under the power levers

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flyfisher1976
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RE: Reverse Thrust On Props

Wed Sep 27, 2006 6:15 pm

Beech 1900D has a gate that prevents one from going past flight idle without lifting up and pulling back. A second gate must be overcome to enter reverse.
 
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HAWK21M
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RE: Reverse Thrust On Props

Wed Sep 27, 2006 10:22 pm

Quoting SNA350 (Reply 6):
I know the ATR has one, called the Idle Gate
it prevents you from putting the power levers in Ground idle and so you can't put it in reverse thrust
you can see it in the picture below, it's the T formed handle just under the power levers



Quoting Flyfisher1976 (Reply 7):
Beech 1900D has a gate that prevents one from going past flight idle without lifting up and pulling back. A second gate must be overcome to enter reverse.

So these are Mechanical gates.Like the Thrust lever on the B737 preventing use of R/T lever if Fwd Thrust lever is not at idle & Preventing Fwd Thrust lever mvmt if R/T lever moved.

An't there Electrical Air-Grd locks on Turboprops.
regds
MEL
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LimaFoxTango
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RE: Reverse Thrust On Props

Wed Sep 27, 2006 10:46 pm

Quoting FLY2HMO (Reply 2):
On all my Dash 8 flights I've noticed that on taxiing the pilots rarely use brakes. I like to sit in row 2, which is right where the prop is so I can see it changing pitch. While taxiing I notice they put it in flat (right term?) pitch when they get a bit too fast, so the prop basically acts like a big speed brake.

This position is called "Discing". The props pretty much acts like a flat disc against the oncoming air. More often that not on landing, the loud noise you hear is the props going into discing and not necesarily into reverse.

Quoting Flyfisher1976 (Reply 7):
Beech 1900D has a gate that prevents one from going past flight idle without lifting up and pulling back. A second gate must be overcome to enter reverse.

This is pretty much the same in the Dash 8, however there are two triggers under the power levers which have to be lifted up before you can select discing or reverse. These triggers are also connected to a microswitch which will cause a horn go off if these triggers are lifted up inflight.
You are said to be a good pilot when your take-off's equal your landings.
 
DashTrash
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RE: Reverse Thrust On Props

Thu Sep 28, 2006 12:13 am

The system in the Dash 8 not only sets a loud chirping horn off, but it also tells the engine computers to do the opposite of what you're asking. If you select disc or reverse in flight, the engines will do the opposite. It will also "lock out" both computers. Maintenance must come out and reset the system, meaning the company will know you just did a bad thing.

Lifting the trigger sets off the horn, pulling the power levers back locks out the computers.
 
Fly2HMO
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RE: Reverse Thrust On Props

Thu Sep 28, 2006 6:44 am

Quoting LimaFoxTango (Reply 9):
This position is called "Discing". The props pretty much acts like a flat disc against the oncoming air. More often that not on landing, the loud noise you hear is the props going into discing and not necesarily into reverse.

Ah yes the term I was looking for. I've actually seen the props go into reverse only a handful of times, seems like discing is usually more than enough to brake.
 
PlainSmart
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RE: Reverse Thrust On Props

Thu Sep 28, 2006 6:48 am

I know the Cessna caravan can be but into reverse in the air. When they landed the C-130 on the aircraft carrier they used reverse before touching down.
 
cactushp
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RE: Reverse Thrust On Props

Thu Sep 28, 2006 7:45 am

So it would be an optical allusion that the props are spinning faster in the "forward" position then in the "Discing" position?
Sorry, I was on the landline
 
DashTrash
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RE: Reverse Thrust On Props

Thu Sep 28, 2006 8:29 am

Quoting Cactushp (Reply 13):
So it would be an optical allusion that the props are spinning faster in the "forward" position then in the "Discing" position?

Yes. The prop RPM varies little on the ground with the condition levers in MIN. Usually 785 - 820 RPM, and most are a lot tighter than that.
 
AT502B
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RE: Reverse Thrust On Props

Thu Sep 28, 2006 9:57 am

The airplane I fly is equipped with a Single PT6 Turboprop with a reversing propeller. I use reverse often on final to slow down or to perform a very short landing. As soon as you enter Beta the slowdown is immediate and you'd better be ready on the rudder as the air flow over the tail is lost. It's great fun for landing short though.


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411A
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RE: Reverse Thrust On Props

Thu Sep 28, 2006 10:35 am

Upon undergoing training in the DHC-6 in 1967, the deHavilland pilots demonstrated the following...

Over the threshold of the runway, at 1500 AGL, flaps 40 (can't remember the IAS, but it was slow).
Beta/reverse selected.
Very rapid descent followed, with the nose of the airplane WAY down, speed very slow.
Exited reverse/beta at 100 AGL, a large roundout/flare, landed, and turned off the runway at the 1000 foot point.

With practise, this short landing, over the 1500 foot 'obstacle' could be accomplished in 800 feet.

An interesting maneuver, to say the least.

Later on, of course, inflight use of beta or reverse, was dis-allowed, by an AFM ammendment, IIRC.
 
DC3CV3407AC727
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RE: Reverse Thrust On Props

Sun Oct 01, 2006 8:36 pm

used to back into parking spots using reverse with the CV-340,when I was flying her over 100hrs per month,and quite comfortable in type.
the rumble of round engines is like music to me,likewise the thunder of thr JT8D
 
Woodreau
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RE: Reverse Thrust On Props

Sun Oct 01, 2006 10:24 pm

Quoting 411A (Reply 16):
Very rapid descent followed, with the nose of the airplane WAY down, speed very slow.

Thats what happened when the sim instructor demonstrated, with the attendant "Don't do this in real life" admonishment - lifting the power levers above the idle detent and pulling them from idle to fine. We went from 18000 to 8000 pretty quickly. The nose went from level to 45-50 degrees nose down. I have no idea if that's what would happen in real life.

When I taxied around in a 1900, I rarely used the brakes, the power levers were in ground idle on the straightaways which will still accelerate the aircraft to a taxi speed over 40kts if you don't watch your speed. control the acceleration and speed by manipulating the levers between the two detents -ground idle and ground fine (on the 1900) or "discing" in the D8.

When you're stuck in the conga line #38 for takeoff, you can't taxi on one engine on a 1900, but you can still shut one down to save fuel for the one-hour taxi to takeoff and leave the other engine on ground idle. From a stop, the aircraft will slowly accelerate to about the pace of the crawl towards the runway.
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usnseallt82
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RE: Reverse Thrust On Props

Sun Oct 01, 2006 10:54 pm

Quoting AirWillie6475 (Thread starter):
Is reverse thrust used inflight on props?

On most flights, save a few experimental souls who want to try stuff out, you will never see beta being used during flight. It is a major safety of flight issue that could bring down an aircraft, much like a deployed thrust reverser in flight. You could play around with it in smaller aircraft, but reversing your thrust during flight is never a good idea unless you're in an aircraft designed to do so.

Quoting HAWK21M (Reply 5):
If not Mistaken Don't Props have a Air-Grd sense to Avoid reverse pitch in Flight.

Many also have a WOW switch, or weight on wheels, that prevents beta from being used until the aircraft is on the ground.
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JAGflyer
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RE: Reverse Thrust On Props

Mon Oct 02, 2006 5:21 am

I work at a GA airport and we get many prop aircraft. The biggest we see is a B1900 once every week or two. The large aircraft with props have controls to change the pitch of the prop which as I understand moves the prop into a position where it doesn't make thrust. A motor turns the actual blades so they dont push the plane forward. As for taxi I am not sure if they use it during taxi but I know the ones with back mounted jets do sometimes open the clamshells for a second while taxiing to help slow the plane down as it taxies
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Ralgha
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RE: Reverse Thrust On Props

Mon Oct 02, 2006 5:48 am

Prop RPM will change very little while in beta mode The goal of beta mode is to maintain a constant RPM and vary blade pitch/engine power to provide thrust. Enter the reverse region, and the prop RPM will increase as the levers go farther into reverse. Likewise with alpha mode, the prop RPM will increase as the power is increased until it hits the governed RPM set by the condition levers.

I pulled one power lever below flight idle in the sim once. The airplane flipped upside down in the blink of an eye, and we lost 13,000 feet in short order.
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flymatt2bermud
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RE: Reverse Thrust On Props

Mon Oct 02, 2006 6:22 am

As mentioned most turboprops must have weight on wheels before taking the props into reverse, it is possible to feather the props (zero thrust) in flight. I can't imagine ever needing to slow down in flight with faster than what gear down, full flaps and props feathered will give you.
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DashTrash
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RE: Reverse Thrust On Props

Mon Oct 02, 2006 7:32 am

Quoting FlyMatt2Bermud (Reply 22):
As mentioned most turboprops must have weight on wheels before taking the props into reverse, it is possible to feather the props (zero thrust) in flight. I can't imagine ever needing to slow down in flight with faster than what gear down, full flaps and props feathered will give you.

The only time you would feather a prop in flight is with an engine failure, or prop governor failure that did not require the engine to be shut down. Flight idle at max prop RPM is where you get the "speedbrake" like drag from a prop.

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