B737Dude

Hello to all,

I'm stuck trying to understand how a Variable Pitch Propeller works on light single engine aircraft?

The thing that really confuses me is the difference between a CSU/PCM?

Which one will a Cessna 182 have and also can you feather a single engine a/c prop?

Also having flown the PA32.. I would take off at a fine pitch angle and after T/O I normally reduce MAP to 25 in and bring the propeller back to 2400 RPM for the climb.. once in the cruise I'll reduce the map but will stay at the same rpm until the decent. Thats what I've been taught but what happens when you bring the RPM back to 2300/2200rpm for the climb other than it taking a greater chunk out the airflow and increasing it'a drag? is there a reason to adjust the pitch of the prop at different phases of the flight or just leave it at 2400rpm?!?! No-ones unfortunately been able to explain this to me properly!

Safe Flights

B737Dude

blackhand

CSU and PCM are different terms for similar item.
I don't know of any single piston engine aircraft that can be feathered
P&W C PT6 sngle engine application have feathering ability e.g. Cessna Caravan, probably cause it is a free turbine.
I'll let a driver explain theory of props in flight.

Cheers
BH

Intruder

The engine can only produce its rated power if it is allowed to turn at the speed required for that power. If full power is at 2700 RPM, then restricting it to 2400 RPM will limit it to something less than full power.

Most light aircraft engines are not rated for continuous full power output; continuous power (>5-15 minutes, depending on the engine) may be restricted to 65-75%. Hence the reduction to 25 in/2400 RPM after gear & flaps are retracted, and at some safe altitude.

First, check your owner's manual for any restrictions. Then, within those restrictions, realize that for a given power output, an engine works most efficiently at wide open throttle (WOT) and a speed commensurate with the available MAP. So, if you have a choice of running at 24 in/2400 RPM or 25 in/2300 RPM or 26/2200, the latter is better. Rumors of engine damage due to "oversquare" MAP/RPM combinations apply ONLY to supercharged engines, and are not generally applicable to light piston engines.

Note also that engine overhaul limits/recommendations are based on "tach time," which is more a count of engine revolutions than actual clock time. So, running at lower RPM -- still within all limits -- will reduce maintenance costs somewhat.

Fakawi

There are TMGs where the Prop can be feathered. The RF 4, ASK 14, G 109, and so on...

cwatters

Think of prop pitch like the gearbox in a car.

Steep hills/climb => low gear => low pitch. For much the same reasons.

Lightning Mate

If you do that with high MAP (low rpm) you run the risk of detonation. which you will not hear because the engine is not silenced.

They are not the same. The PCM (or PCU) is in the propeller hub, but it is controlled by the CSU which is remote.

blackhand

Thanks for the correction.
Cheers

Intruder

Probably not, in a normally aspirated engine.

As long as you stay within the limits shown in the owner's manual, that should not be a problem. Light airplane engines run very rich at takeoff anyhow, which further reduces detonation potential.

Old Fella

B737Dude, the primary reason for having a variable pitch propeller is to enable the engine/propeller combination to operate at the most efficient engine RPM/Propeller Blade Angle of Attack. At low airspeed the propeller is most efficient at lower pitch settings and high RPM. As airspeed increases the propeller needs to move toward a coarser pitch to maintain the most efficient blade angle of attack, hence the Constant Speed Unit (CSU) provides a means of selecting and maintaining a constant RPM. This is achieved by setting the CSU, via the pitch control in the cockpit, to maintain the desired RPM. The propeller pitch is able to be varied by the action of the Pitch Change Mechanism (PCM). Engine power is set by adjusting the throttle to obtain the desired Manifold Air Pressure whilst at the same time being able to maintain the selected RPM. You will have noted in an aircraft fitted with a variable pitch propeller that on descent with a lower than cruise power set, RPM will remain as selected as long as airspeed is sufficient to maintain that RPM when the propeller reduces pitch (blade angle) to the fine pitch stop within the propeller. You select Fine Pitch with the Pitch Control during the approach so that full RPM, and thus full power, is available in the event of it being required e.g. in the case of a missed approach and go around.

A Squared

I'll go you one better:

Rumors of engine damage due to "oversquare" MAP/RPM combinations are horesecrap even for supercharged engines.


On the R2800's (2 speed centrifugal supercharger) on the DC-6's I used to fly, Take off power was 59.5" MAP and 2800 rpm, Climb, I think was 43" and 2400 and high power cruise was 32-34" and 2200 RPM. The only time we weren't "oversquare" was on descent.


For what it's worth, the direction the manual gives for the O-470R in my Cassna 180 is for a desired power setting, use the minimum RPM that gives smooth operation (or words to that effect) And the MAP/RPM charts therein list plenty of combinations which are "oversquare" .

Seems that Teledyne Continental Motors and Cessna never got the "oversquare" memo.