Question from @Dream to fly @ATVpilot Have another question about propeller twisting force, one is called the aerodynamic twisting moment (ATM), another one is the centrifugal twisting moment (CTM). I heard ATM drive the propeller to a coarse pitch and CTM drive the propeller to a fine pitch. Why is that happen and how they affect the propeller in-flight? I think ATM is the dominant force of a single engine aircraft variable pitch propeller and CTM is the dominant force on the propellers of the multiengine aircraft. It is because single engine aircraft's propeller tend to stop the rotation in a fine pitch after engine shutdown to facilitate the next engine start up and multiengine aircraft's propeller tend to stop at a coarse pitch after engine shutdown to reduce the drag in case of single engine failure inflight/ during takeoff.
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@Gary Law I agree on your opinion 100% Accident wont juz happen with a single reason. Thats why we need to understand the procedure; understand the reason behind and stick with the procedure. It will minimize most of the holes in the cheese; and continous training/learning is helpful to think out of the box if needed. I shall open a new topic to discuss ADM, and also Just culture. What do u think?
I am not judging but merely stating the fact as referring something didn't happen. Accident mostly do not contribute to a single factor as in Swiss Cheese Model, and what we should always think about blocking some of the holes through reviewing the whole system and procedures. What I would suggest is that we should look into it and check what we can do systematically:
If something failed -> Why do they fail -> Is there any way can we prevent it to fail?
I have been told that every single line on the checklist and memory item is written in someone's blood. They exist because of someone has failed to do something critical during critical. It is easy to blame at a single person but this could never prevent the same thing happening again.
@Gary Law The ATR Propeller is governed by PEC, which got its logic. On takeoff/go around; ATPCS will be armed. In case of engine failure, it will command the AFU auto feathering unit to auto feather; in order the reduce drag. However, during other phrase of flight; it will follow the PEC logic and the pilot has to confirm and feather->shutdown the engine. PEC wont feather it, as it provide a chance for quick relight. However, for Tuninter 1153, while they handling the procedure for engine 2 failure(before the CL condition lever); Engine 1 also fail. Apparently, they got confused with the situation. And decided to ditch. One side note, i would like to point out. When we discuss about ACI, We are looking at the situation and try to learn something from it. Learn not to judge, if they could hv this or that. Coz we r looking at it while we are sitting calm and safe. All informations is provided to us; however, if i am in their situation; under high stress level, that might be the best decision at the moment. Juz like Captain Sully. (They will always try their best to put finger on pilots; as huge insurance coverage under human error. Someone got to paid.)
But as in the accident of Tuninter Flight 1153, the pilots failed to feather the prop after both engine failed due to fuel starvation and this created enormous drag reducing its gliding distance. Do you have any clue why ATR-72 does not have auto-feather function?
Clear and concise explanation, thank you so much! One thing would like to clrafy is about the ATM's direction in windmilling. When engine failure offer, the ATM will change to a reverse direction and put the propeller in a fine picth. Is it because of a windmilling propeller does not generate any lift and its driven by the relative wind, so the deteminative factor of the ATM become drag instead of lift?
@Dream to fly
Thanks for your question~
There are forces acting on propeller;
Thrust bending force
Thrust loads on the blades act to bend them forward.
Centrifugal twisting force
Acts to twist the blades to a low, or fine pitch angle.
Aerodynamic twisting force
As the centre of pressure of a propeller blade is forward of its centreline the blade is twisted towards a coarse pitch position.
Centrifugal force
The force felt by the blades acting to pull them away from the hub when turning.
Torque bending force
Air resistance acting against the blades, combined with inertial effects
So as mentioned, ATM drives the propeller to fine pitch(picture on the right); and CTM drives the propeller to Course pitch. (picture below)
Also, in order to increase the efficiency of the propeller design, there is variable pitch(constant speed) propeller.
Before I start to explain further ; lets learn something from the video.
The ATPL video has a great explanation on how it works!
Since I trained in the C172RG before, I will use C172RG propeller for explanation.
In the C172RG with McCauley prop, there is an propeller governor to manage the amount of oil into or escape from the propeller hub. And inside the propeller hub, there is a spring loaded diaphragm to counter-act the oil pressure supplied from the governor.
On a single engine, the propeller is spring loaded to fine pitch to counteract the oil pressure in the hub,which support coarse pitch. In case of oil pressure lose, it will stop at full fine pitch stop (It becomes a regular fixed pitch propeller)
However, in the multi engine aircraft, it designs a bit different! Oil pressure supports fine pitch, as the spring is loaded for coarse pitch (feather)
In case of engine failure( loss of oil pressure), it will automatically feather to reduce drag on the propeller, thanks to the spring tension.
Easy way to remember this~
Spring tension inside the propeller always the dominant force when loss of oil pressure; and how do you want your propeller in case that happens?
Single engine->revert like a fixed pitch blade
Multi engine-> feather; as I don't wanna deal with extra drag and handling issue as that happens.
Reply and comment below if you got confused~