PA 44

[Raptor71]

Hi,



Thank you

[Amadis of Gaul]

Yes, the Seminole uses constant-speed props. What two types do you mean?

[A and C]

Single acting refers to the fact that the engine oil via the CSU only pushes the prop one way, this acts against a spring & nitrogen pressure to exert pressure in the opposite direction.

Double acting props use oil pressure to move the prop in both directions but you will not find these props at the light end of the GA market.

[westhawk]

What A and C said. (fundamentally) And the PA-44 uses a constant speed full feathering propeller. A full description of operation for this type of prop can be obtained from a number sources including the Hartzell props owners manual.(company website) General theory of operation for various types of constant speed propellers can be found in several FAA training publications of the 8083 series. (visit FAA.gov for free downloads)


Enjoy your studies!

westhawk

[Tinstaafl]

First, is the propeller on the PA 44 is a constant speed propeller?

Yes.

If so what is the difference between a single acting propeller and a constant speed propeller?

'Single acting' relates to the mechanism that twists the blade in the prop hub to change the blade's angle of attack. As A and C wrote, single acting uses engine oil pressure to drive the blades in one direction, and a combination of other forces to drive the blades in the opposite direction. Think of oil pressure acting only on one side of a piston in a tube. 'Double acting' uses oil pressure to drive the blades in both directions. Think of oil pressure supplied to both sides of that piston in a tube and valves to vary which side gets the most pressure.

'Constant speed' refers to a governor that senses engine (or prop RPM) and then use some mechanism to regulate that RPM. Often it is by changing the prop blade angle, which will add more or less load on the engine, causing the RPM to decrease or increase, respectively. It doesn't matter how the blade angle is changed in the hub. It could be a single acting design or a double acting design, hydraulic, electric or whatever.

The PA44 uses a single acting hydraulic design that uses the Aerodynamic Twisting Moment + oil pressure to drive the blades to fine pitch, and spring pressure + nitrogen gas pressure + Centrifugal Twisting Moment (CTM can be increased by adding additional prop hub weights at the blade root. Go have a look at some twins & you're bound to see some) to force the blades to coarse pitch and eventually feather. So, oil pressure is used to constantly fight the tendency of the blades to move into coarse pitch.

What is the purpose of the nitrogen?

See above.

Is the constant speed unit used for these two types have the same design?

The governor that senses RPM change can be broadly similar in how it senses RPM however very unlikely that it will be compatible due to how the oil pressure is routed to & from the prop hub.

[Raptor71]

Ok great, now it makes sense.

[Tinstaafl]

Are you referring to the speeder spring in the governor? Tension on that spring is set with the RPM lever, not the throttle lever. Speeder spring pressure is balanced by the rotating L shaped flyweights. There will be some RPM where the flyweight force balances the spring pressure ie oil pressure is 'locked' in the hub, preventing those other prop pitch change forces from causing the blade to move to coarse pitch. If RPM increases then the flyweight's action overcomes the speeder spring pressure, opening a valve to allow oil to flow out of the hub. If RPM reduces then the valve works in the reverse direction and allows oil under pressure to flow into the hub.

Oil is supplied by the oil pump to the governor. The governer then boosts oil pressure and sends it to the prop hub to do the desired work.

In reality, the blade angle is never 'fixed'. It is constantly changing to maintain a constant RPM