Variable pitch propellers
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Variable pitch propellers
Hi all. I would like to ask one of you clever cookies out there whether you can explain the mechanisms of a variable pitch propeller. I.e. how do they have the pitch vary whilst the thing is spinning. (I don't mean the aerodynamics behind it, just the mechanism) If it's an electrical motor which does the pushing and pulling (so to speak) how can something stationary i.e. the motor move something that is spinning? i.e. the prop
Thanks
oh and while i'm here, to stop me from having to start up another thread. On average, how many take offs and landings do short haul and long haul pilots make each day/week/month.
Thanks a lot
Thanks
oh and while i'm here, to stop me from having to start up another thread. On average, how many take offs and landings do short haul and long haul pilots make each day/week/month.
Thanks a lot
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Thanks for your question, I only have experience on the constant speed, variable pitch installation as fitted to the Lockheed P3 Orion.
In very simple terms:
The jet engine rotates at 13,820rpm all the time (best torque and econ rpm), with the propellor rotating at 1021rmp, all the time.
The prop control is a electronically controlled, hyraulically actuated system.
I'll stick to the inflight control system, as the system does change slightly for ground ops, (including a lower rpm setting)
From the cockpit, the FE sees rpm variations of only =/- 1% as power changes are made, because the prop control will sense any variations in prop rpm, as more fuel is added or removed from the engine fuel supply by movement of the power levers by the pilot.
For instance, if the power levers are advanced, more fuel is introduced into the engine, the prop control senses the prop starting to incr. rpm, and will immediately incr. the blade angle to absorb that energy, and thus increasing thrust, almost immediately, since no actual incr. in rpm is required.
Inside the prop control, there are three hyd. pumps, two are driven mechanically, by the actual turning of the prop, and the third is driven electically, for altering the prop blade angle when the prop is not turning.
The predominant 'dome' in the centre of the prop, contains one very large hyd. piston, with a transverse tube down the centre.
This piston is connected mechanically to all the prop blades, thereby controlling the blade angle of each.
The prop control also contains an rpm sensor, which directs hyd. pressure to move the blades to incr. or decr. the blade angle to maintain the prop at exactly 100%
There is a thing called 'ATM' or Aerodynamic Turning Moment, which tries to decr. the blade angle always, as it turns and produces lift, so effectively, the piston is only holding the blades against this effect, except, of course when very large blade angle changes are called for, when the piston will drive as required.
Also, when sudden power lever changes are commanded, the prop syncronization system has a function where it will anticipate the incoming fuelflow change (+ or -), and actually start to incr. or decr. the blade angle (in anticipation), to limit the amount of rpm swing during these turbulent times.
When the prop is feathered inflt., as the prop rpm decreases rapidly, thereby severly reducing the effective output of the two mech. pumps, the electric pump cuts in and completes the blade movement to the full 86.65 deg blade angle. very slightly to far for perfect aerodynamic feathering, and tries to drive the prop backwards, thereby mech locking the prop brake, so no prop rotation for the remainder of the flight.
This is a VERY simplified, very short explanation of a very complicated, very good system, that works very well.
I hope I covered what you were looking for,
Cheers.
In very simple terms:
The jet engine rotates at 13,820rpm all the time (best torque and econ rpm), with the propellor rotating at 1021rmp, all the time.
The prop control is a electronically controlled, hyraulically actuated system.
I'll stick to the inflight control system, as the system does change slightly for ground ops, (including a lower rpm setting)
From the cockpit, the FE sees rpm variations of only =/- 1% as power changes are made, because the prop control will sense any variations in prop rpm, as more fuel is added or removed from the engine fuel supply by movement of the power levers by the pilot.
For instance, if the power levers are advanced, more fuel is introduced into the engine, the prop control senses the prop starting to incr. rpm, and will immediately incr. the blade angle to absorb that energy, and thus increasing thrust, almost immediately, since no actual incr. in rpm is required.
Inside the prop control, there are three hyd. pumps, two are driven mechanically, by the actual turning of the prop, and the third is driven electically, for altering the prop blade angle when the prop is not turning.
The predominant 'dome' in the centre of the prop, contains one very large hyd. piston, with a transverse tube down the centre.
This piston is connected mechanically to all the prop blades, thereby controlling the blade angle of each.
The prop control also contains an rpm sensor, which directs hyd. pressure to move the blades to incr. or decr. the blade angle to maintain the prop at exactly 100%
There is a thing called 'ATM' or Aerodynamic Turning Moment, which tries to decr. the blade angle always, as it turns and produces lift, so effectively, the piston is only holding the blades against this effect, except, of course when very large blade angle changes are called for, when the piston will drive as required.
Also, when sudden power lever changes are commanded, the prop syncronization system has a function where it will anticipate the incoming fuelflow change (+ or -), and actually start to incr. or decr. the blade angle (in anticipation), to limit the amount of rpm swing during these turbulent times.
When the prop is feathered inflt., as the prop rpm decreases rapidly, thereby severly reducing the effective output of the two mech. pumps, the electric pump cuts in and completes the blade movement to the full 86.65 deg blade angle. very slightly to far for perfect aerodynamic feathering, and tries to drive the prop backwards, thereby mech locking the prop brake, so no prop rotation for the remainder of the flight.
This is a VERY simplified, very short explanation of a very complicated, very good system, that works very well.
I hope I covered what you were looking for,
Cheers.
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Ceppo -
Check this out:
http://www.avweb.com/articles/pelperch/pelp0016.html
It is an excellent article on the subject and, although it covers more than your question, will give you a good insight into how the whole thing works.
[This message has been edited by Gerund (edited 13 June 2001).]
Check this out:
http://www.avweb.com/articles/pelperch/pelp0016.html
It is an excellent article on the subject and, although it covers more than your question, will give you a good insight into how the whole thing works.
[This message has been edited by Gerund (edited 13 June 2001).]
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Ceppo,
The Avweb site is a good one for information. Couldn't find an answer for your question about electrically actuated variable pitch props on it, so...
The entire motor is mounted in the prop hub & rotates with it. The blade's pitch change is relative to the hub so the output of the motor is connected to the mechanism that moves the blades.
Now the problem is how to get electrical power to the motor since it's rotating along with the hub & the aircraft's electrical system is not.
This is done using a brush mechanism in a similar manner to the brushes used in some electric motors/alternators etc.
All that needs to done is to mount a conducting ring on the hub that is electrically insulated from the hub. A wire connects to this ring to the electric motor to carry the current. An electrical contact from the electric system is mounted on the airframe in a such a way as that it continually brushes against this rotating surface.
Two rings & brushes are needed, one for electricity to flow in, and for it to flow back out.
This mechanism is also used for props that use electrical power to heat the blades for anti- or de-icing purposes.
[This message has been edited by Tinstaafl (edited 14 June 2001).]
The Avweb site is a good one for information. Couldn't find an answer for your question about electrically actuated variable pitch props on it, so...
The entire motor is mounted in the prop hub & rotates with it. The blade's pitch change is relative to the hub so the output of the motor is connected to the mechanism that moves the blades.
Now the problem is how to get electrical power to the motor since it's rotating along with the hub & the aircraft's electrical system is not.
This is done using a brush mechanism in a similar manner to the brushes used in some electric motors/alternators etc.
All that needs to done is to mount a conducting ring on the hub that is electrically insulated from the hub. A wire connects to this ring to the electric motor to carry the current. An electrical contact from the electric system is mounted on the airframe in a such a way as that it continually brushes against this rotating surface.
Two rings & brushes are needed, one for electricity to flow in, and for it to flow back out.
This mechanism is also used for props that use electrical power to heat the blades for anti- or de-icing purposes.
[This message has been edited by Tinstaafl (edited 14 June 2001).]
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Ceppo
The Avweb article is very good, but to add a tad of info, props that use oil pressure and a spring in opposition are known as "single acting", whereas props that use two different sources of oil pressure in opposition are known as "double acting."
e..g a light single and a light twin can both be "single acting", although as the Avweb article says the forces work in opposite axis.
The Avweb article is very good, but to add a tad of info, props that use oil pressure and a spring in opposition are known as "single acting", whereas props that use two different sources of oil pressure in opposition are known as "double acting."
e..g a light single and a light twin can both be "single acting", although as the Avweb article says the forces work in opposite axis.
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There are two basic forces acting on a prop. Aerodynamic Turning Moment (ATM) and Centrifugal Twisting Moment (CTM). ATM is due to the relative positions of the C of P and of the pivot point of the blade and will tend to coarsen the blade, CTM is due to the distribution of the mass of the blade and will tend to fine it. CTM is the stronger of the two, so left to itself a prop will fine off (and overspeed).
Most VP props use hydraulics to move the prop, combined with a mechanical governer. A double acting prop has a piston which drives the prop to coarse or fine when required. If the hydraulic supply fails, the prop will go to fine, overspeed and cause drag, so needs a protection system - usually a speed sensing system controlling a mechanical lock.
A single acting system is designed to "relax" to coarse (either by a big spring or by putting counterweights on the blades so that the CTM now drives to coarse) and is driven to fine by a piston which only pushes one way, again controlled by a mechanical governer.
If you want more power, push power levers forward. More fuel to engine, engine speeds up, governer senses it and drives prop to coarse to maintain speed. Unless you have an Astazou engine in which case moving the power lever moves the blade first, engine slows down, governer senses and puts in more fuel to maintain speed - it's a weird system, but it works well. some a/c use this system on the ground (eg C130).
Most VP props use hydraulics to move the prop, combined with a mechanical governer. A double acting prop has a piston which drives the prop to coarse or fine when required. If the hydraulic supply fails, the prop will go to fine, overspeed and cause drag, so needs a protection system - usually a speed sensing system controlling a mechanical lock.
A single acting system is designed to "relax" to coarse (either by a big spring or by putting counterweights on the blades so that the CTM now drives to coarse) and is driven to fine by a piston which only pushes one way, again controlled by a mechanical governer.
If you want more power, push power levers forward. More fuel to engine, engine speeds up, governer senses it and drives prop to coarse to maintain speed. Unless you have an Astazou engine in which case moving the power lever moves the blade first, engine slows down, governer senses and puts in more fuel to maintain speed - it's a weird system, but it works well. some a/c use this system on the ground (eg C130).
Avoid imitations
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F3G,
If my memory serves me right, and I am fairly sure it does (I used to instruct on them) the Bulldog prop goes to coarse if left to its own devices.
One relevant incident I remember occurred to a solo student at RAF Cosford in the early 1990s. He landed long, and bounced a couple of times, making a heavy landing on the nose wheel. This caused the nose oleo to be pushed vertically upwards, damaging the constant speed unit on the back of the engine (take a look how close the CSU sits above the top of the oleo - it's very close).
This allowed the prop to go fully coarse.
He tried to go around but the engine wouldn't pull against the coarse pitch and he couldn't climb. He was very lucky to make a survivable forced landing off the airfield.
If, as you have intimated, you fly a Bulldog it is worth remembering this if you ever suspect you have dinged the nosewheel. It's much safer to close the throttle and leave it on the ground.
As I always used to teach, mainwheels are for landing on, nosewheels are for steering only.
ShyT
If my memory serves me right, and I am fairly sure it does (I used to instruct on them) the Bulldog prop goes to coarse if left to its own devices.
One relevant incident I remember occurred to a solo student at RAF Cosford in the early 1990s. He landed long, and bounced a couple of times, making a heavy landing on the nose wheel. This caused the nose oleo to be pushed vertically upwards, damaging the constant speed unit on the back of the engine (take a look how close the CSU sits above the top of the oleo - it's very close).
This allowed the prop to go fully coarse.
He tried to go around but the engine wouldn't pull against the coarse pitch and he couldn't climb. He was very lucky to make a survivable forced landing off the airfield.
If, as you have intimated, you fly a Bulldog it is worth remembering this if you ever suspect you have dinged the nosewheel. It's much safer to close the throttle and leave it on the ground.
As I always used to teach, mainwheels are for landing on, nosewheels are for steering only.
ShyT
All CSU single engine types I know default to fine pitch/high RPM and use oil pressure to drive against the strong CTM causing the fine pitch tendency. This means that a failure of the governing system still leaves full power available.
Light multi-engine a/c with feathering props use some combination of counterweights/feathering spring/pressurised gas or oil in combination with the ATM to overpower the CTM. This causes the prop. to default to coarse pitch/low RPM and eventually into feather.
A failure of the governing system/engine at least allows the failed unit to be feathered to reduce drag for the remaining engine.
There is a slight problem introduced by this feathering system in that the prop will try to go into feather when oil pressure is lost after the engine is shutdown normally. Not good for then next start!. This problem is prevented by latches that engage when RPM drops below a certain critical value (typically 800 or 1000 RPM or so). Given a real engine failure, it's important to identify if the RPM is reducing in any immediate actions so feathering can be done before the ability gets locked out.
[ 08 July 2001: Message edited by: Tinstaafl ]
Light multi-engine a/c with feathering props use some combination of counterweights/feathering spring/pressurised gas or oil in combination with the ATM to overpower the CTM. This causes the prop. to default to coarse pitch/low RPM and eventually into feather.
A failure of the governing system/engine at least allows the failed unit to be feathered to reduce drag for the remaining engine.
There is a slight problem introduced by this feathering system in that the prop will try to go into feather when oil pressure is lost after the engine is shutdown normally. Not good for then next start!. This problem is prevented by latches that engage when RPM drops below a certain critical value (typically 800 or 1000 RPM or so). Given a real engine failure, it's important to identify if the RPM is reducing in any immediate actions so feathering can be done before the ability gets locked out.
[ 08 July 2001: Message edited by: Tinstaafl ]
The Bulldog prop is controlled by engine oil. The CTM (modified by the counterweights which are at right-angles to the prop mass distribution) drives to coarse and the oil pressure drives to fine. If the engine fails, there's no oil P and you can do what you like with the RPM/pitch lever! You certainly put the lever to max RPM(fine pitch) for PFLs for the subsequent overshoot.An engine underspeed is an indication of an oil problem and potentially serious!
The Jetstream (Astazou engine) goes fine with hydraulic P and coarse with a big f*** off spring in the prop control assembly. On shut down, if the start latches don't engage, the prop shuts down feathered due to the spring. As does the Tucano, and presumably all those small t-props which don't seem to have start latches and start perfectly well from the feathered position. (As does the Jetstream, if truth be told, because once the engine starts turning, the pressure builds quickly and drives the prop fine wards to the correct position).
Hope all this helps - glad to discuss further.
DH
The Jetstream (Astazou engine) goes fine with hydraulic P and coarse with a big f*** off spring in the prop control assembly. On shut down, if the start latches don't engage, the prop shuts down feathered due to the spring. As does the Tucano, and presumably all those small t-props which don't seem to have start latches and start perfectly well from the feathered position. (As does the Jetstream, if truth be told, because once the engine starts turning, the pressure builds quickly and drives the prop fine wards to the correct position).
Hope all this helps - glad to discuss further.
DH
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The avweb article didn't mention the bracket type constant speed propellor or the IOC (Integral Oil Control) props.
Bracket type a la Havard (and apparently Bulldog) use external counterweights to drive the prop coarse with engine oil driving to fine.These do run to coarse as the engine shuts down and can often be manually moved back to fine when stopped.
IOC props (on some of the big radials)Have their own oil system and do do use engine oil.
Although the mechanics are the same there is a very basic difference between the piston engine and turboprop. The difference is that most turboprops link the Prop controls to the power lever (interconnect) while on the pistons engines they are completely separate. It is possible to damage a piston engine by overboosting ( oversquare).
Not mentioned in the comments was that on big radial engines the feathering is actually accomplished with a separate feathering pump that drives the prop into feathered postion, normally with a separate compartment in the oil tank (feathering reserve) and a separate line to the CSU.
Feathering pumps develope around 600 psi but even that is not enough to overcome a runaway prop !
During the war the Germans actually put feathering props on some of the single engined fighters.
Bracket type a la Havard (and apparently Bulldog) use external counterweights to drive the prop coarse with engine oil driving to fine.These do run to coarse as the engine shuts down and can often be manually moved back to fine when stopped.
IOC props (on some of the big radials)Have their own oil system and do do use engine oil.
Although the mechanics are the same there is a very basic difference between the piston engine and turboprop. The difference is that most turboprops link the Prop controls to the power lever (interconnect) while on the pistons engines they are completely separate. It is possible to damage a piston engine by overboosting ( oversquare).
Not mentioned in the comments was that on big radial engines the feathering is actually accomplished with a separate feathering pump that drives the prop into feathered postion, normally with a separate compartment in the oil tank (feathering reserve) and a separate line to the CSU.
Feathering pumps develope around 600 psi but even that is not enough to overcome a runaway prop !
During the war the Germans actually put feathering props on some of the single engined fighters.
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Deltahotel/Shytorque/Tinstaafl
Thanks for sharing your knowledge; I do sometimes fly a Bulldog, to keep current on CS props, as my own a/c is a Pup.
The POH for the Bulldog does contain a line item in the forced landing checklist saying "propellor full increase", so maybe it is to encourage safe practice for simulated exercises.
In the event of a real engine failure it would seem to make more sense to glide with coarse pitch I would have thought, as their would be less windmill drag.
If the prop lever setting doesnt make a difference, then I can understand why it is safe to teach "fine" pitch
[ 12 July 2001: Message edited by: Final 3 Greens ]
Thanks for sharing your knowledge; I do sometimes fly a Bulldog, to keep current on CS props, as my own a/c is a Pup.
The POH for the Bulldog does contain a line item in the forced landing checklist saying "propellor full increase", so maybe it is to encourage safe practice for simulated exercises.
In the event of a real engine failure it would seem to make more sense to glide with coarse pitch I would have thought, as their would be less windmill drag.
If the prop lever setting doesnt make a difference, then I can understand why it is safe to teach "fine" pitch
[ 12 July 2001: Message edited by: Final 3 Greens ]
