Propellor Pitch.....

[Finals19]

Just doing some online reading and was interested on any input on propellor pitch:

Props go fine at take off (assume turbo prop) because....?

- Fine pitch (low blade angle) allows maximum thrust, but limited airspeed to develop and max prop rpm. Also gives high drag

And on final approach / landing: for max available thrust if G/A is required, also can act as airbrake due to high drag

Props go to coarse pitch in cruise because....?

Produces lower rpm, improved fuel and range and lower throttle setting for given airspeed. Less drag and achieves higher cruise airspeeds.

Any more info or correction of the above would be much appreciated.

[SNS3Guppy]

- Fine pitch (low blade angle) allows maximum thrust, but limited airspeed to develop and max prop rpm. Also gives high drag


It only "gives high drag" when it's being driven by the slipstream; not when it's being driven by the engine. The difference, of course, is entirely dependent on airspeed and power setting.

[Mark 1]

Think of the propeller as a wing aerofoil that gives its best lift/drag ratio at a particular angle of attack.
This will transform to a best thrust/torque ratio.
A fixed pitch propeller will get a progressively lower angle of attack if forward speed is increased while RPM is held constant. So to absorb the full torque of the engine, the pitch must increase as forward speed is increased to maintain a constant angle of attack.

[Flight Detent]

With prop driven airplanes, there are basically two types:

* larger airplanes which are turboprop driven, which means a jet driven, constant-speed prop. The blade angle is automatically controlled by the prop governor, and the cockpit power lever(s) control the amount of fuel being fed to the engine, and thus the power output.
These sorts of setups have several safety mechanisms to stop the prop blade angle getting to low and thus having the airflow driving the prop to any large degree.
The actual blade angle is an unknown quantity, dependant on forward t/airspeed and power lever setting.
On the ground, both the blade angle and the fuel supply are directly governed by the power lever(s).
* smaller airplanes with piston driven props are quite different, their prop blade angle is directly controlled from the cockpit to utilize the limited engine power available, in the way that's mentioned, low b/angle with high rpm for takeoff and landing, for extra pulling power, and a higher b/angle and lower rpm for criusing, to get better fuel economy.

Cheers...FD...;)

[toolowtoofast]

that may be a bit simplistic FD.

there are 2 basic types of props - fixed pitch, and adjustable pitch.

fixed pitch are just that - optimised for a particular phase of flight - a fine pitch (meaning the average pitch of the blade, or its Experimental Mean Pitch) is best for climbing, short field take off, higher static RPM, and slower flight. a coarse EPM gives good cruise speed, lower static RPM, longer take off (slower to accelerate). Fixed pitch props are light, easy to maintain, and easy to manage - engine RPM = prop RPM (yes even if the prop is geared down like in a rotax).

a CSU (constant speed unit) receives an input from the pilot via the prop lever, which sets a speeder spring weight on a set of flyweights. through the magic of oil, gravity and rotational velocity, the prop will maintain the RPM set by the pilot - it will fine off in a climb (lower speed) and coarsen in a descent/cruise (higher speed), giving the advantages of multiple/indefinite variations of pitch for the airspeed required. CSU's tend to be on light twins, or higher performance singles, and are on all turboprop aircraft.

some props are essentially fixed pitch, but can be adjusted manually in flight or on the ground - these tend to be fitted to performance light/microlight aircraft. they don't have the complexity/weight of the CSU, but have some of the advantages - pitch can be set depending on the mission.

[Milt]

Simplistically variable pitch props do whatever they have to do to maintain the engine RPM that you set on the lever in the cockpit.

The props will always be at max fine pitch when they can't reach the RPM that you have requested.

That's why I do a double take on looking at the props of museum aircraft. The props should always be on the fine pitch stops unless feathered.

And that's why you exercise the CSU before take off. If you don't you will have cold oil in the prop control system giving you sluggish prop control as you feed in the power.

[SNS3Guppy]

And that's why you exercise the CSU before take off. If you don't you will have cold oil in the prop control system giving you sluggish prop control as you feed in the power.


Not necessarily.

That's why I do a double take on looking at the props of museum aircraft. The props should always be on the fine pitch stops unless feathered.


Not necessarily. Depends on the propeller system in use.

Simplistically variable pitch props do whatever they have to do to maintain the engine RPM that you set on the lever in the cockpit.


Not necessarily. Constant speed propellers do that. Not all variable pitch propellers are constant speed.

...through the magic of oil, gravity and rotational velocity, the prop will maintain the RPM set by the pilot...


Gravity plays no part in the process. Oil pressure, spring tension and compression, aerdodynamic twisting force, and centripetal force, however, do.

The blade angle is automatically controlled by the prop governor, and the cockpit power lever(s) control the amount of fuel being fed to the engine, and thus the power output.
These sorts of setups have several safety mechanisms to stop the prop blade angle getting to low and thus having the airflow driving the prop to any large degree.


In some engines that's so, but others not at all. You're describing a negative torque sensing system which helps prevent the prop from absorbing slipstream energy and therefore imparting the resulting drag to the airframe. This certaily isn't the case with all turboprops, and not at all with large piston mounted propellers such as on radial engines. In fact, in many propeller installations, regardless of piston or turbopropeller, when the power is retarded enough that the engine is no longer driving the propeller, the propeller will be driven by the slipstream, and RPM becomes a function of airspeed and governor setting, rather than governor setting and throttle/power-lever position.

Try seeing if a Hamilton Standard hydromatic propeller modulates to reduce drag in a negative torque sensing mode some time. It doesn't. Doesn't even feather on it's own...it must be driven there by a dedicated pump.

On the ground, both the blade angle and the fuel supply are directly governed by the power lever(s).


Not necessarily.

Think of the propeller as a wing aerofoil that gives its best lift/drag ratio at a particular angle of attack.
This will transform to a best thrust/torque ratio.
A fixed pitch propeller will get a progressively lower angle of attack if forward speed is increased while RPM is held constant. So to absorb the full torque of the engine, the pitch must increase as forward speed is increased to maintain a constant angle of attack.


Not necessarily. As airspeed increases under constant power, pitch increases and for a given section of blade angle of attack increases to some degree, not decreases; both to take advantage of the power absorbed by the engine while maintaing a constant RPM, and as airspeed increases the blade angle must also increase to prevent an overspeed condition.

[barit1]

The props should always be on the fine pitch stops unless feathered.

On most twin-spool turboprops (PT6, PW100 etc, CT7, ...) the prop will go to feather when shutting down - that's how the linkage is set up. When starting the engine, the core (gas generator) simply doesn't care whether the prop is feathered or not.

But a fixed-shaft (single-shaft) engine like the T56 etc. MUST be in fine pitch for starting - too much drag otherwise.

And whatever you think of en.wikipedia, their pages on controllable and constant-speed props are well done.

[Mark1234]

Simplistically it's analogous to gears in a car:
1st gives great acceleration, but poor top speed
5th gives quiet economic cruise, but weak acceleration, and not top speed
4th probably gives best top speed
3rd is a good balance between 1st and 5th

A fixed pitch aircraft is stuck in 3rd.. A variable pitch prop allows you to configure to fit your requirements

Variable props come in several flavours:
Ground adjustable (essentially fixed)
In flight adjustable, non regulated
In flight, single lever automatically set blade angles (e.g. Cirrus)
Constant Speed Unit - prop angle is automatically adjusted to maintain RPM within the limits of the coarse/fine pitch stops.

CSU is the more common implementation, even on GA aircraft. A lot of the ultralight/Light sport aircraft coming along seem to have variable pitch props however.

Generally CSU singles are riged to fail to fine pitch with a loss of oil pressure as it's the most safe condition for a single (no assymetric considerations). Some aerobatic aircraft (e.g. pitts) fail coarse to avoid overspeed conditions.

Multi engine a/c generally try to fail coarse/feather to reduce the drag on the dead donkey and limit the assymetric problem.

Negative torque systems stop the prop driving the engine by moving toward coarse/feather when unloaded. As far as I'm aware, they are only common on turboprops.

I was under the impression that gravity plays a part in making the speeder weights drop as rpm slows in the csu. However, that may be a simplification - I'm having a problem working out how that gells with extreme pos/neg G in a csu aerobat like the pitts/extra etc.

[john_tullamarine]

In flight adjustable, non regulated

Only experience is with the Callair .. it was so slow to change pitch that a quick circuit overtook the pitch control for landing ...