Propellor Pitch.....
Thread Starter
Join Date: Mar 2003
Location: UK
Posts: 437
Likes: 0
Received 0 Likes
on
0 Posts
Propellor Pitch.....
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.
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.
Join Date: Oct 2005
Location: USA
Posts: 3,218
Likes: 0
Received 0 Likes
on
0 Posts
- Fine pitch (low blade angle) allows maximum thrust, but limited airspeed to develop and max prop rpm. Also gives high drag
Join Date: Aug 2000
Location: S Warwickshire
Posts: 1,214
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Jan 2002
Location: Brisbane, Australia
Posts: 960
Likes: 0
Received 0 Likes
on
0 Posts
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...
* 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...
Join Date: Aug 2007
Location: Godzone
Posts: 391
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Oct 2003
Location: Canberra Australia
Posts: 1,300
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Oct 2005
Location: USA
Posts: 3,218
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Simplistically variable pitch props do whatever they have to do to maintain the engine RPM that you set on the lever in the cockpit.
...through the magic of oil, gravity and rotational velocity, the prop will maintain the RPM set by the pilot...
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.
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.
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).
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.
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.
Join Date: Feb 2005
Location: flyover country USA
Age: 82
Posts: 4,579
Likes: 0
Received 0 Likes
on
0 Posts
T
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.
he props should always be on the fine pitch stops unless feathered.
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.
Join Date: May 2006
Location: Londonish
Posts: 779
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Moderator
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 ...
Only experience is with the Callair .. it was so slow to change pitch that a quick circuit overtook the pitch control for landing ...