Reverse Thrust on turboprops

[fonawah]

To all turboprop drivers out there.....Do turboprops have reverse thrusters? If they do, how do they work? Is it something to do with changing the angle of attack of the blades?
Thanks in advance

[KoruLounge]

If the propeller blades are turned through the fine stop to a blade angle of about minus 20 degrees and power is applied, reverse thrust is obtained.

[fonawah]

Thanks KoruLounge

[PAXboy]

There was a thread on this subject just a few weeks ago, "Turning Props backwards" in Tech Log. Search or select threads for last 20 days to locate.

As a non-pilot, I know that turboprops use reverse thrust for both braking and for 'power back', as opposed to 'push back'.

[saudipc-9]

In the CF we called it "Beta". It was sometime ago but I don't think we had to go as far as 20 degrees. I think it was in the order of 12-14 degrees on the Twin Otter. Very easy to get. First idle power then a twist of the throttle grips and pull the power levers back and "Hey Presto" you have Beta.
Very good for STOL landings, IF you get it right. If wrong you would get Beta in the air (about 3-4 feet AGL) and the result was a very heavy landing:rolleyes:

[KoruLounge]

I'm not going to get type specific, so whether it is minus 12 or 20 degree blade angle it doesn't really matter as long as the propeller blade angle passes through the arc where blade forces causes the propeller to be in windmilling mode.

If the power is applied when the propeller blade angle is in this arc, propeller overspeed will be likely since propeller torque will be acting in the same direction as engine torque, ie. propeller will be driving the engine.

In this case the prop governor will prevent prop overspeed.

[pigboat]

Some older turboprops that don't have beta mode have something called ground fine pitch. All ground operations are carried out in this position. After landing, when the weight of the aircraft has settled onto the wheels, you can select the propeller flight fine pitch locks to be hydraulically removed, thus allowing the prop blade angle to decrease to zero degrees.

[Basil]

Such as, e.g., the RR Dart.
It wasn't unknown for the Argosy to be marshalled into a position from which a power-back was required by station crew who were unaware that it couldn't :(

[saudipc-9]

KoruLounge,
Not that I doubt what you wrote but say again??????????????

[411A]

Ah, the very reliable RR Dart.
If ground fine not selected (or available) after landing, the engine had to be shut down because it would severly overheat otherwise.

Reversable propellors are nothing new, DC-6's and Connie's had 'em a long time ago.
On the first flight of the DC-6, 'ole Ben Howard (Douglas chief test pilot) backed it up for the audience.
Donald Douglas (Sr.) was not amused, so i'm told.

[Ref + 10]

All the reverse thrust stuff has been covered but one other point. Don't use reverse thrust on unsealed airstrips. Forcing the airflow forwards causes reingestion of air into the engine. Dust, stones and other stuff going through the engine isn't exactly desriable.

Ref

[pigboat]

411A, knew a guy that was trying to beat a thunderstorm into KPIT one day, and landed the F-27 with the HP cocks in the fuel on position instead of cruise lock out. What he didn't notice was that both props had hung on the cruise pitch locks. The TGT's got up over 1000 degrees, and both engines seized. You could chin yourself on the props.:eek:

[south coast]

ref +10...

we fly into a lot of dirt strips in africa and reverse is very important...because as you can imagine, if the strip is dirt it normally means that it is fairly short so reverse is very much needed...we normally take it out of reverse at 60kts.

[saudipc-9]

south coast,
What are you flying? On the Twin Otter we had intake deflectors/doors which would prevent getting dirt/rocks/ice etc in the engine on dirt strips. Worked great.

[DummyRun]

Ref+10 - Reverse thrust always used on the mighty Lockheed stratopig on dirt strips just cancel it before you get enveloped by the dust cloud also helps to see where you going rather than going imc in dust at 50 kts.

KoruLounge/Saudi pc, - the Hamilton Std/Allison combo definitely gives +ve torque, ie engine driving prop, in reverse just airflow reversed. -ve torque could occur in flt but there is a system which prevents that,however don't propose to go into that now unless you're really bored.

[KoruLounge]

I probably didn't express myself clearly,

There is positive torque (ie. engine driving the prop) at reverse thrust setting (ie. negative blade angle). I was just pointing out that there can be negative torque if the negative blade angle is too small, but usually the governor will prevent any application of power while the blades are transiting through to reverse thrust blade angle.

On the Metro, negative torque is prevented by a system called NTS (Negative Torque Sensing).

[Northern Chique]

I have from experience come to the conclusion that if on the metro, one side works and operates the blade into beta and negative angle and the other doesnt, it turns to poo very quickly!

Apparently if the NTS fails, and the prop drives the gearbox, its the gearbox which suffers really quite badly... the other consideration is the metro is a shaft drive not a free turbine. Any abnormal activity from the prop, engine and gearbox is bound to be expensive.