VP and Constant speed

[davidatter708]

Can someone explain to me the difference between these types of prop?

[Malcolm G O Payne]

I may be wrong, but as a simplifiedanswer, I believe that a VP prop can have its pitch varied by the pilot to a pitch angle that they hope suits the airframe speed, but no more. For instance, a fine pitch can be selected for take-off and a coarse pitch for the cruise. A constant speed prop has the addition of a governor that maintains a constant engine rpm, regardless of throttle position. There are limits, of course, depending on the handling of the aircraft

[barit1]

One common (older) type of variable-pitch prop was the two-position prop developed by Frank Caldwell of Hamilton Standard. It used hydraulic pressure to drive it to low pitch for TO, and when the pressure is released, centrifugal counterweights brought the blades back to high pitch for cruise. It's called a single-acting hydraulic/counterweight prop.

Woodward Governor Company later provided a separate governor unit that could modulate the hydraulic pressure to set the blades at any intermediate angle to hold a constant RPM, regardless of throttle or airspeed (climb or dive) changes. :cool:

Most modern props use refinements on these basics. :8

More history - see: http://www.memagazine.org/supparch/flight03/goldage/goldage.html
and scroll down halfway.

[FlyingForFun]

Agree with both answers so far.

But.... variable pitch refers to any method of varying the pitch, and that includes constant speed props. Constant speed props are a sub-set of variable pitch props. The purpose of a constant speed prop, and the general way in which the settings are used, is the same as any other variable pitch prop - it's just the exact method of adjusting the pitch is different, with some of the pitch changes happening automatically.

FFF
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[dublinpilot]

Agree 100% with FFF.

Nowadays the terms tend to be used interchangably.

[Rocket2]

Many 2nd generation motor gliders such as the Dimona & Grob 109 series use 3 position props (fine, coarse & feathered) but they are not constant speed. Nowadays manufacturers have moved to constant speed - even on the new vererable Scheibe Falke 2000 series.

[Zulu Alpha]

David,
Variable pitch props are just that, the pitch is variable. One type of Variable Pitch prop has a constant speed mechanism as well, this is the constant speed type. This is probably the most common, particularly in small to medium Pipers/Cessnas/Maules etc.
There are other types, ground adjustable, two and three position, and just variable from the cockpit.

The advantage of constant speed props is that the automatic system keeps the prop speed contant by automatically varying the pitch. This is typically done by controlling the oil pressure feed into the propeller hub. There is a governor which adjusts this oil pressure (and hence the pitch) to keep the speed to that set by a lever in the cockpit.
The pilot therefore just sets the required engine RPM on a third control (Throttle, mixture and now prop speed ) which is usually blue. Opening and closing the throttle does not change the prop speed. To measure the power, a manifold pressure gauge is used and this indicates how wide the throttle is open.
This automatic speed control mechanism means that the engine is running at the optimum speed at all times, takeoff, climb, cruise and descent.
This gives great get up and go on takeoff and a great climb rate whilst maximising the cruise speed.
One other big advantage is in the descent. With a constant speed prop you just point the nose down and leave everything set. The engine cannot overspeed as the prop governor prevents it by coarsening the pitch. Because you don't throttle back you can usefully convert all your height to speed and there is no shock cooling (as the throttle is still open).
The downside is that there is a higher maintenance cost for CS props.

There are many subtleties with different types so please consider the above a general simplified answer which will apply to the majority of small singles with VP props but there are less common versions which will differ.

[IO540]

One other big advantage is in the descent. With a constant speed prop you just point the nose down and leave everything set. The engine cannot overspeed as the prop governor prevents it by coarsening the pitch.

What seems to have been forgotten is that a CS prop gives you superior pitch stability.

With a fixed pitch prop, if you point the nose down, the plane speeds up, the airflow through the prop goes up, the engine revs up like crazy (going from say 2300 to 2500 RPM in a few seconds) and (given that prop thrust is proportional to the cube of the RPM) this greatly increases the thrust, which causes the plane to speed up, which causes the wing lift to go up, which causes the plane to porpoise upwards. So holding altitude, or holding the glideslope on an ILS, can be fun.

With a VP prop, pitch changes done with the yoke don't have this effect of exxagerating the phugoid oscillation, resulting in much better pitch stability.

Because you don't throttle back you can usefully convert all your height to speed and there is no shock cooling (as the throttle is still open).

Not entirely correct; if you point the nose down, the airflow through the engine goes up, and the engine gets cooled more. Especially if done together with reducing engine power, this can excessively cool the engine. In fact probably the biggest engine management issues are in planes with the bigger engines (like an IO540 and upwards) which have CS props almost universally.

There is very little extra maintenance. The direct operating cost goes up by £2-3 per hour, which is about 1/20 of the fuel cost alone.

[Zulu Alpha]

Not entirely correct; if you point the nose down, the airflow through the engine goes up, and the engine gets cooled more. Especially if done together with reducing engine power, this can excessively cool the engine.
Thats why I said
With a constant speed prop you just point the nose down and leave everything set. The engine cannot overspeed as the prop governor prevents it by coarsening the pitch. Because you don't throttle back you can usefully convert all your height to speed and there is no shock cooling (as the throttle is still open).
With the throttle open you will not get shock cooling from the extra airspeed

As for the pitch stability, I have not noticed this but I usually trim for the descent anyway so this may mask the effect.

£2-3 per hour is a good estimate if you don't have anything go wrong and fly a lot of hours. It can be more than this is you fly lower hours as the 3 yr overhaul tends to be a fixed cost and therefore the hourly rate depends on the hrs flown. If you need a new prop then a 3 bladed for a Piper Arrow is about £8000. We needed a new one because the hub was said to have corrosion, although we couldn't see it. I don't think fixed pitch props generally get hub corrosion.
You are correct that the benefits generally outweigh the costs but I was just pointing out that there is a small negative to offset the advantages.