Props, Fixed, Constant speed, Variable speed

[Orf]

Could anyone tell me the performance difference between fitting a fixed, constant speed or a variable speed prop?

 

 

[Matt]

There's too many variables to give a complete answer...but will clarify - you probably mean "variable pitch" and not "variable speed"...as they're all variable speed.

 

Basically the three types are:

 

* Fixed pitch

 

* In-flight Adjustable (manual - either electric or hydraulic)

 

* Constant Speed

 

Fixed Pitch is a compromise approach, depending on pitch setting, it will either give very good takeoff performance OR very good cruise performance, not both. A common setup would provide the best compromise between takeoff and cruise.

 

In-flight Adjustable provides a variety of pitch angels to suit the flight regime i.e. finer pitch for takeoff/landing and coarser pitch for cruise - this is controlled by the pilot and similar concept to a manual gearbox in a car. It requires the pilot to monitor manifold pressure gauge and RPM and to adjust the pitch to meet optimum combination.

 

Constant Speed effectively provides an "automatic" version of the in-flight adjustable prop. An RPM/Power is set for each stage of flight and requires little/no input after being set.

 

The "performance" of each will vary depending on aircraft/conditions etc.

 

 

[Orf]

Thanks Matt, my mistake I did mean pitch.

 

Am I correct in saying that a variable pitch will reduce fuel consumption and increase cruise speed over a fixed pitch prop? I am talking about cruising at altitude over long distances.

 

I gather that the constant speed prop is a bit of a compromise and would not be as good as a variable pitch prop. Obviously the constant speed would not take any pilot management as would the variable.

 

I am thinking of buying a LSA where there are options of prop to fit to a Rotax 912 100HP engine.

 

 

[GraemeK]

Constant speed is not really a compromise - in fact, it is the next level of complexity from the in-flight adjustable.

 

Thus, with in-flight adjustable you might set a particular pitch - say coarse for cruise. If you then start to climb, the RPM's will fall since the prop is working harder (just like a fixed pitch prop) - so you will have to manually adjust it towards fine to optimise the performance.

 

Whereas with the constant speed, the prop will automatically move towards fine to maintain RPM - which is what you want it to do to keep thrust as high as possible.

 

The constant speed units often have a manual setting, so you can set pitch directly.

 

 

[Tomo]

I'm not sure if you can get a CSU prop for a Rotax? (constant speed unit). I could be wrong though.

 

I know you can get variable pitch, ground adjustable, and fixed pitch though.

 

 

[Matt]

There are a couple of electronic Constant Speed props available for the Rotax - Airmaster and Woodcomp have them.

 

 

[jetjr]

Is there a in flight adjustable (auto or manual) to suit Jab engines?

 

 

[facthunter]

variable pitch props.

 

The Rotax (having a hollow drive shaft) can utilise a direct acting in-flight adjustable prop. I personally would be reluctant to fit a propeller to a Jab. motor other than the maker recommended one... Nev

 

 

[Guest Maj Millard]

I have seen the Indavario (Italian) in-flight adj fitted to some aircraft. It is a very nice three-blade unit that is easily fitted to the 912 hollow-shaft prophub. It is cockpit controlled with a large screw-in knob that hydrylically adjusts the prop pitch. About 2.5 turns fully controls the blades from fine to course. Does require some carefull setting up, and bleeding of the system prior to operation. Has a fine pitch adjustment on the hub. Haven't heard much of them lately, and they were quite expensive to import as I remember. They are very nice to fly with, and half a spin of the knob right after take-off really improves things. They are a big blade and wouldn't suit a Jab. One comment from an owner after I had done some 'high-speed' runs in a Storch was, " I've never seen a Storch go so fast" !!

 

The internals of the Airmaster hub (constantspeed/in-flight adj) is very similiar to the Indavario one.........................................................................................Maj..

 

 

 

 

 

 

[phvdw]

I've fitted this type of prop

 

correct name is "idrovario" by alisport(italian); possible in 2 blades high speed or 3; 4 blades.

 

some remarks; if used with rotax only with 912uls 3; who has hollow shaft and gearbox adapted to mount a CSU, we've choosen for the jihostroj.

 

Works very good on our aircraft.

 

Take of short, good climbing caracteristics approx 1100ft/m at 450kg and reasonnable cruise speed (127kts).

 

 

[facthunter]

Variable pitch Props

 

Rough rule of thumb, you don't need a variable pitch prop under 100-110 knots cruise speed.

 

Draggy aircraft are better fitted with a larger diameter prop. This usually needs a reduction gear and appropriate undercarriage geometry.

 

The factors against CSU's or variable pitch are cost complexity weight and reliability. Propellers should be matched to the engine and torsional vibrations accounted for. These are the big unknown factor in prop life and safety. The safest prop is a FIXED pitch two blade made of wood, (or carbon fibre), as they are more capable of coping with the vibrations. These are not the type of vibrations that you feel when the engine is running. You are unaware of them till a prop blade falls off. Nev

 

 

[HEON]

I must disagree with the statement that in flight adjustable props are not rearly needed under 100-110kt.

 

In any amphibian or seaplane they are very valuable as they allow an increase in pitch after a water take off.

 

Water (especially still water) requires more power (finer pitch) to become unstuck than land.

 

The only one I can quote is a Super Petrel: a inflight adjustable prop changes it from a 85kt to a 95kt cruise aircraft. To me that is worth the effort as it is quite a high percentage increase in speed.

 

 

[phvdw]

To jetjr,

 

for jabiru engines GT propellers is proposing a electrical constant speed propeller, till today no experience with this propeller

 

 

[facthunter]

Response.

 

To Heon. The difficulty of getting a floatplane on the step is a special case, especially in smooth water. As I grew up opposite Rathmines I am well aware of this as the Catalina's sometimes had to rough-up the water, before commending the take-off run. Sometimes they went for ages before unsticking and overheated motors are a constant worry.

 

I did say "rough rule of thumb" and it is valid, in that context.

 

You can easily fall into the trap of seeking performance that is not going to happen.

 

The terminal speed in level flight is a function of total drag and available thrust. This is complex as propeller efficiency, engine torque and engine RPM are factors in the thrust side.

 

A variable pitch propeller is a compromise in itself as the "twist" of the blades only suits one particular pitch setting.

 

For me, the advantage ot CSU is in the take-off performance, and the availability of a cruise at lower engine revs. My concern is that these props are truly in the "experimental" category. It should be noted that the Schneider Trophy racers were all fixed pitch props designed to be "correct" geometry at the speeds attained. 'They were notoriously dangerous in the take-off phase for the tendency to "bury" one float

 

 

[Russ]

"Ivo" make several versions of "inflite adjustable" props. From low HP engines, up to around 200HP engines. Electrically controlled, and can be set to a constant speed setting if asked by pilot.

 

Know several guys useing them............great reports. Only prob, they are not cheap.

 

hope this helps..............