溫故知新系列之 Propeller System

呢個禮拜就睇下Propeller System

Fixed-Pitch Propeller

Low Pitch (high rpm) for climb

High Pitch (low rpm) for cruise

The climb propeller has a lower pitch, therefore less drag. Less drag results in higher rpm and more horsepower capability, which increases performance during takeoffs and climbs but decreases performance during cruising flight.

The cruise propeller has a higher pitch, therefore more drag. More drag results in lower rpm and less horsepower capability, which decreases performance during takeoffs and climbs but increases efficiency during cruising flight.

Constant Speed Prop

How airspeed or throttle setting affect the blade angle?

Once a specific rpm is selected, a governor automatically adjusts the propeller blade angle as necessary to maintain the selected rpm.

For example, after setting the desired rpm during cruising flight, an increase in airspeed or decrease in propeller load causes the propeller blade angle to increase as necessary to maintain the selected rpm. A reduction in airspeed or increase in propeller load causes the propeller blade angle to decrease.

DA42 Propeller System

Why the blade is twisted?

Fine, Course, Feather? 

• Feathering propellers must be used on multi-engine aircraft (with constant-speed propeller) to reduce propeller drag to a minimum under one or more engine failure conditions.
• the propeller blade angle parallel to the line of flight (approximately 90°)
• With the blades feathered, the propeller stops turning and minimum windmilling
• Feathering is important because of the change in parasite drag with propeller blade angle.

Important Concept: single-engine airplanes v.s multiengine airplanes
(Ref : Airplane Flying Handbook (FAA-H-8083-3C) Chapter 13)

As a review, the constant-speed propellers on almost all single-engine airplanes (Hartzell propellers) are of the non-feathering, oil-pressure-to-increase pitch design. In this design, increased oil pressure from the propeller governor drives the blade angle towards high pitch, low rpm.

In contrast, the constant-speed propellers installed on most multiengine airplanes are full feathering, counterweighted, oil-pressure-to decrease-pitch designs. In this design, increased oil pressure from the propeller governor drives the blade angle toward low pitch, high rpm—away from the feather blade angle. In effect, the only thing that keeps these propellers from feathering is a constant supply of high-pressure engine oil. This is a necessity to enable propeller feathering in the event of a loss of oil pressure or a propeller governor failure.

當兩個引擎既飛機，失去一邊引擎時，吾feather 就姐係在windmill 情況 (low pitch, high rpm)…反而會create 好大既drag, 所以設計上要做到用engine 轉先會有油去set the blade to low pitch (high rpm) – 即無油變feather (high pitch, low rpm, reduce drag).

單引擎用constant speed propeller (e.g Hartzell propellers)，佢既運作同大多數雙引擎既constant speed propeller 運作原理相反!!!
當Enginge 轉就會有oil, 反而是用oil to set the blade to high pitch angle (low rpm, reduce drag) – e.g during cruise…呢類propeller 係無油時候反而會去windmilling 狀態，即 low pitch (high rpm), 雖然好大drag, 但間接用drag (aerodynamic force) 去做類似hand prop既動作, 睇下可吾可以restart 個engine, 如果吾得某程度上就因為design 會做成好大既drag!

經常都講…揸constand speed prop 既機好講究rpm lever 先定 throttle lever 先…網上就教 “Keep the Prop on Top” Principle

所以Increasing power 就 右至左 ; 反之 Decreasing power 就左到右 — 咁就會做到 rpm control on top

至於點解…就係我永遠想做到係用engine 推個prop 而不是反轉黎…

https://krepelka.com/fsweb/lessons/commercial/commerciallessons01.htm

When both manifold pressure and rpm need to be changed, avoid engine overstress by making power adjustments in the proper order:

• When power settings are being increased —increase rpm first, then manifold pressure.
• When power settings are being decreased, reduce manifold pressure before reducing rpm.
  (If rpm is reduced before manifold pressure, manifold pressure automatically increases, possibly exceeding the manufacturer’s tolerances.)
• To prevent damage to radial engines, minimize operating time at maximum rpm and manifold pressure, and avoid operation at maximum rpm and low manifold pressure.