desertjets
Posts: 7588
Joined: Fri Feb 18, 2000 3:12 pm

How Does A Constant Speed Prop Work

Thu Feb 01, 2001 1:13 am

Simple enough, how do they work, especially compared to the "adjustable" props that were available on early Beech Bonanzas.
Stop drop and roll will not save you in hell. --- seen on a church marque in rural Virginia
 
Buff
Posts: 1066
Joined: Fri Mar 16, 2007 1:29 pm

RE: How Does A Constant Speed Prop Work

Thu Feb 01, 2001 1:30 am

Governors. Old fashioned governors as seen on 19th century steam engines!

Enhanced by modern oil pressure systems and spring technologies of course.

Even the old Bonanza had a constant speed prop.

Depending on the engine type, oil pressure in the propeller dome is usually pitted against spring pressure working to move the propeller pitch in the opposite direction. On most piston driven single engine installations, the spring drives the propeller to fine pitch with oil pressure used to send it to coarse pitch.

On multi-engine and turbine powered propellers, the reverse is true, the spring tends to send the propeller to coarse pitch to the point of feather (necessary on a multi-engine aircraft to minimize drag if the engine fails) while oil pressure is used to send the prop to fine pitch and even reverse pitch, depending on the engine.

Once the pitch is established, then your question comes into importance. The governor will sense any increase or decrease in propeller RPM. Let's just use the multi/turbine propeller installation for simplicity.

As turboprop prop speed increases (momentarily for any reason), the governor senses the increase and restricts oil pressure to the prop hub. Restricting oil pressure causes an increase in prop pitch which in turn increases prop drag which slows the prop down. Remember that if the oil pressure is reduced, the spring tries to force the prop into coarser pitch. Conversely, if the prop RPM decreases (momentarily for any reason), then the governor increases oil pressure to the hub, driving the prop into finer pitch, decreasing prop drag which of course allows the prop to go faster.

The governor(s) work full time. The changes in oil pressure are miniscule but continuous. This is how the RPM is maintained at a constant.

Hope that answers some of your question!

Best Regards,

Buff
 
JETPILOT
Posts: 3094
Joined: Tue May 18, 1999 6:40 am

RE: How Does A Constant Speed Prop Work

Thu Feb 01, 2001 1:38 am

The variable incidence proppellers are controlled by a prop govener.

As propeller rpm increase the prop govener adjusts the blade angle by way of varying oil pressure in the prop hub to maintain a given RPM.

The prop govener contains flyweights and a valve that controll the entrance of engine oil into the propeller hub.

As engine RPM increases the flyweights swing out causing the valve to open increasing oil pressure in the prop hub driving the blades to a course pitch which in turns slows the engine down to maintain the set rpm. The same applies for a decreasing RPM.

Therefore as manifold pressure is increased rpm stays the same.

The reason piston aircraft don't feather after shutdown with a loss of oil pressure is because the prop govener is equipped with a feather lock preventing the blades from going to feather. A blade in feather is much harder to turn during starting due to air resistance. When the engine drops below 950 RPM the feather lock engages.

After the loss of an engine in flight the engine must be feathered immediately (before 950RPM) or the feather lock will engage preventing feathering.

If I remeber correctly the Beach props you actually had to loosen and adjust between a climb prop and a cruise prop which was fixed in flight.

JET

 
crjmech
Posts: 257
Joined: Wed Jun 07, 2000 6:31 am

RE: How Does A Constant Speed Prop Work

Thu Feb 01, 2001 2:16 am

A couple of pretty good descriptions there! I just wanted to add to the forces utilized to change blade pitch. CTM (centrifugal twisting moment) will cause the heaviest part of the blade, the leading edge, to move inline with the plane of rotation. This serves to decrease blade angle. Counterweights on the blade roots are also used to change pitch. Most counterweights will tend to move the blades to high pitch.
Thou shalt mind thine altitude,lest the ground reach up and smite thee.
 
aaron atp
Posts: 517
Joined: Sun Mar 19, 2000 1:17 pm

Comparison

Thu Feb 01, 2001 7:35 am

>>>Governors. Old fashioned governors as seen on 19th century steam engines! -Buff


Just a side note. Aircraft governor work on the same principle as other governor applications (marine diesels, lawn mowers, generators, et al): flyweights, but they regulate RPM in a different manner. Aircraft governor applications effectively change the load (wrt rotational drag) to maintain RPM with a constant throttle setting as the aircraft climbs or descends, while a marine diesel (or most any other type) governor varies the throttle position to adapt to a varying load and keep a constant RPM.

It's not much of a difference, but the concept is important. In an aircraft powerplant, it has been compared to having a infinitely variable transmission.


aaron

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