IMPORTANT - calling ex-Dowty Propellor personnel
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IMPORTANT - calling ex-Dowty Propellor personnel
Wanted - anyone with experience or knowledge of Dowty hydraulic constant-speed propellors, as fitted to Rolls Royce Dart engines.
In our historic aircraft collection at Speke aerodrome, we have 2 of the above propellors. We gather that these propellors use a combination of hydraulic fluid and very powerful springs to control the pitch whilst running, with the springs set to return the propellor to a feathered position which is its inert or "safe" position.
Both of our props carry stern warnings that the propellor MUST be feathered before removal from the aircraft for this reason. Unfortunately, one of them was removed whilst it was still set in a windmilling position, god knows why. However, what we understand is that this means we now have an armed and dangerous propellor sitting in a stand in our storage area with the springs under tension, that could potentially snap back to feathered position with incredible force at the slightest provocation. For this reason, nobody dares touch it and we have no idea how to go about making it safe to a) be in the vicinity of when fetching other parts and b) to reinstall on the aircraft.
I had suggested tapping the blades with a mallet at arms length to make it go, but this was vetoed as no one is quite sure how it would react, and if letting it go by itself would cause damage to the hub and pitch mechanism.
We are desperately after someone who either designed or worked on these units to please advise us on how to disarm the mechanism (or if it is indeed possible to do so without having to reconnect a hydraulic supply?).
In our historic aircraft collection at Speke aerodrome, we have 2 of the above propellors. We gather that these propellors use a combination of hydraulic fluid and very powerful springs to control the pitch whilst running, with the springs set to return the propellor to a feathered position which is its inert or "safe" position.
Both of our props carry stern warnings that the propellor MUST be feathered before removal from the aircraft for this reason. Unfortunately, one of them was removed whilst it was still set in a windmilling position, god knows why. However, what we understand is that this means we now have an armed and dangerous propellor sitting in a stand in our storage area with the springs under tension, that could potentially snap back to feathered position with incredible force at the slightest provocation. For this reason, nobody dares touch it and we have no idea how to go about making it safe to a) be in the vicinity of when fetching other parts and b) to reinstall on the aircraft.
I had suggested tapping the blades with a mallet at arms length to make it go, but this was vetoed as no one is quite sure how it would react, and if letting it go by itself would cause damage to the hub and pitch mechanism.
We are desperately after someone who either designed or worked on these units to please advise us on how to disarm the mechanism (or if it is indeed possible to do so without having to reconnect a hydraulic supply?).
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As nobody else is replying here is a Pilots who operates that type of props shall we say ideas.
The props will be on what's called the start locks which get selected by selecting reverse while the engine is winding down and the oil pressure is dropping. So they engage in a ring as they relax back towards the shaft as the rpm comes down. They are held out the way by centripetal force when the engine is up to speed.
I wouldn't say you have an armed and dangerous prop sitting there which is likely to suddenly go bang. We have had a prop that's been stuck on the locks and it was sent to the prop shop in that condition and it eventually came back. How they dealt with it I don't know but I suspect there will be an stupidly big jig which is able to hold the front casing under pressure as the retaining bolts are removed and then gradually eased off to release the spring.
The main thing to note is not to muck about with the bolts in the hub. I really don't think you can do it on site as we "had" to send it back to the prop shop they couldn't do anything with a site visit.
To get them off in the field you not only would have to provide oil pressure but you would also have to have it spinning. And if its been sitting there for years there is a reasonable chance they won't come out anyway.
The props will be on what's called the start locks which get selected by selecting reverse while the engine is winding down and the oil pressure is dropping. So they engage in a ring as they relax back towards the shaft as the rpm comes down. They are held out the way by centripetal force when the engine is up to speed.
I wouldn't say you have an armed and dangerous prop sitting there which is likely to suddenly go bang. We have had a prop that's been stuck on the locks and it was sent to the prop shop in that condition and it eventually came back. How they dealt with it I don't know but I suspect there will be an stupidly big jig which is able to hold the front casing under pressure as the retaining bolts are removed and then gradually eased off to release the spring.
The main thing to note is not to muck about with the bolts in the hub. I really don't think you can do it on site as we "had" to send it back to the prop shop they couldn't do anything with a site visit.
To get them off in the field you not only would have to provide oil pressure but you would also have to have it spinning. And if its been sitting there for years there is a reasonable chance they won't come out anyway.
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The Dart props are feathered by a pump. Look here for a guide.
dart | fuel consumption | piston valve | 1953 | 0372 | Flight Archive
dart | fuel consumption | piston valve | 1953 | 0372 | Flight Archive
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Good find I was looking for something like that.
Sounds like it has the same collet ring though just the method of operation is slightly different.
Still I don't think it will be a socket set and a load of G-clamps to sort out.
Maybe contacting Atlantic in Cov would be a good starting point. They have all manner of old stuff and hairy arsed engineers that look after it. If nothing else they will know if you have a live bomb sitting there and if its a shop job or not to sort out.
Sounds like it has the same collet ring though just the method of operation is slightly different.
Still I don't think it will be a socket set and a load of G-clamps to sort out.
Maybe contacting Atlantic in Cov would be a good starting point. They have all manner of old stuff and hairy arsed engineers that look after it. If nothing else they will know if you have a live bomb sitting there and if its a shop job or not to sort out.
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From what I can remember. The only springs are in the locks which are withdrawn by solenoids. There is no feathering spring.
You'd have better luck trying one of the ex Emerald Airways boys.
You'd have better luck trying one of the ex Emerald Airways boys.
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Unfortunately, one of them was removed whilst it was still set in a windmilling position,
Provided you have, or can obtain, the service-documentation, it should be relatively simple to dismantle that one and thus understand how to go about rendering the other one safe.
You appear to be making an assumption that the warning is primarily about the spring......It could be, that removing wrongly, will damage / sieze closed a hydraulic valve, thereby locking it permanently in the position it is now in.
I may well be talking out of my arse here, but Iam just trying to apply lateral thought to the problem....you have an opportunity to reverse-engineer the buildup.
Someone, somewhere , will have the documentation (I know, that's why you're appealing on PPrune!
) There's no way that an expensive and complex bit of kit like that, would enter service with all the information stored only in the heads of a select coterie of maintenance engineers.
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If you go to airliners.net, tech/ops, someone called 'ozflyer333' has a set of course notes for the Viscount 720, would they help you? Someone in the US on the same site 'Access-Air' has a propeller manual for the dowty Rotol C.S. propeller fitted to the Dart. Interesting subject, am surprised a google search doesnt turn something useful up, good luck !
Last edited by bluesafari; 23rd Mar 2014 at 17:00. Reason: additional information
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prop pitch lock
The Dowty prop fitted to the Dart is a double acting prop, this means that engine oil is used at high pressure to control the prop between feather (89 degrees) and fine at 0 degrees, some props are single stop, ( HS748 and HP Herald, early F27 and Viscount) and some models two stop props ( later F27 and Viscount). the internal piston moves forward ( fine) and aft ( feather) to control the blade angles and is limited in forward movement by the pitch lock mechanism to prevent fining off in flight.
To remove the prop from the engine the blades have to be in feather to move the piston aft, the pitch lock mechanism is then removed from the hub, then the prop retaining nut can be removed to allow the prop to be removed from the splined engine shaft.
If your prop is in fine pitch there is no problem and no spring is going to cause any damage, perfectly safe to work around. My guess is that your prop is in fine ( no blade angle ) on a stand with no pitch lock fitted, if you wish to coarsen off the blades to feather you can use blade paddles to manually feather.
Other single acting props fitted to PT6 engines do have a spring to feather the blades, but not the Dart prop.
If you pm me an email address I can forward some diagrams and notes that will explain further, I hope above makes sense,
BR om15
To remove the prop from the engine the blades have to be in feather to move the piston aft, the pitch lock mechanism is then removed from the hub, then the prop retaining nut can be removed to allow the prop to be removed from the splined engine shaft.
If your prop is in fine pitch there is no problem and no spring is going to cause any damage, perfectly safe to work around. My guess is that your prop is in fine ( no blade angle ) on a stand with no pitch lock fitted, if you wish to coarsen off the blades to feather you can use blade paddles to manually feather.
Other single acting props fitted to PT6 engines do have a spring to feather the blades, but not the Dart prop.
If you pm me an email address I can forward some diagrams and notes that will explain further, I hope above makes sense,
BR om15
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Thanks gents for your advice!
There is little or no service documentation for the aircraft - it is Emerald's Avro 748 Mk1 G-BEJD that was supposed to be scrapped at Blackpool airport, but was then saved at the last minute by Speke Aviation Heritage to be restored for static display.
The other prop is indeed set safe, but we do not have any propellor experts on our team who would know how put it back together properly once dissected....
I am in touch with an ex-Emerald mech but he says it's been so long since he worked on one that he has forgotten the procedures and lost his notes. He did express interest at how it came to be removed in such a state in the first place, given the mentions on here that clearance to access the nut is only possible once the blades are feathered? One possibility that came up is that the blades had siezed in position from standing outside unprotected for many years, but then that still does not answer the previous question!
As the prop is from a 748, does that mean it is indeed single stop only, and that the stop is at the feathered end not the fine pitch end? The propellor does appear to be set to a quite fine pitch angle, but I cannot verify from memory whether or not it is zero degrees. it is on a large metal stand that supports the hub, and does not appear to have any locks on it (hence our concern about it springing back).
Mad Jock - if it does turn out to be a workshop job, who did you send yours to?
Bluesafari - thankyou for the pointer, I will check it out.
Om15 - you have a pm
By what you said and my description, does this mean the prop is indeed safe to handle for the time being?
regards,
matt
There is little or no service documentation for the aircraft - it is Emerald's Avro 748 Mk1 G-BEJD that was supposed to be scrapped at Blackpool airport, but was then saved at the last minute by Speke Aviation Heritage to be restored for static display.
The other prop is indeed set safe, but we do not have any propellor experts on our team who would know how put it back together properly once dissected....
I am in touch with an ex-Emerald mech but he says it's been so long since he worked on one that he has forgotten the procedures and lost his notes. He did express interest at how it came to be removed in such a state in the first place, given the mentions on here that clearance to access the nut is only possible once the blades are feathered? One possibility that came up is that the blades had siezed in position from standing outside unprotected for many years, but then that still does not answer the previous question!
As the prop is from a 748, does that mean it is indeed single stop only, and that the stop is at the feathered end not the fine pitch end? The propellor does appear to be set to a quite fine pitch angle, but I cannot verify from memory whether or not it is zero degrees. it is on a large metal stand that supports the hub, and does not appear to have any locks on it (hence our concern about it springing back).
Mad Jock - if it does turn out to be a workshop job, who did you send yours to?
Bluesafari - thankyou for the pointer, I will check it out.
Om15 - you have a pm
By what you said and my description, does this mean the prop is indeed safe to handle for the time being?regards,
matt
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There are 4-5 company's in Europe that can do it and we use who ever has the shortest lead time if we are stuck or who ever is cheaper. Think there is a place near Stansted.
If its a dual actuator prop without a return spring It will be safe. To be honest with spring with the reverse lock in unless you start mucking about with the hub bolts which contain the spring etc its not going to do anything.
I suspect as you say the whole thing is seized and it doesn't matter what you do its going to be staying where it is. They look better for display when they are fine anyway.
If its a dual actuator prop without a return spring It will be safe. To be honest with spring with the reverse lock in unless you start mucking about with the hub bolts which contain the spring etc its not going to do anything.
I suspect as you say the whole thing is seized and it doesn't matter what you do its going to be staying where it is. They look better for display when they are fine anyway.
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By the sound of it the internal locks have not released, when removed it must have been deemed safe to do so without it being in the feather position despite the stern warnings you have mentioned, otherwise why would they remove it in that setting.
Its obviously been in that setting for many years and been quite safe, why start messing with now.
Its obviously been in that setting for many years and been quite safe, why start messing with now.
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single stop prop
The prop from G-BEJD is a single stop prop, the internal stops are at fine and at feather, the Flight fine pitch stop is not part of the prop but is incorporated into the removable pitch lock mechanism.
When electrically selected high pressure oil acts on the pitch lock and allows the prop piston to move forward to fine the blades, in order to remove the prop from the engine the pitch lock mechanism must be removed, so your prop may well be in fully fine or at the flight fine position, ( about 16 degrees blade angle at the master station).
I will dig out some single stop notes and diagrams over the next day or so and forward to you, in the meantime the prop is perfectly safe and can be handled with no problems
br om15
When electrically selected high pressure oil acts on the pitch lock and allows the prop piston to move forward to fine the blades, in order to remove the prop from the engine the pitch lock mechanism must be removed, so your prop may well be in fully fine or at the flight fine position, ( about 16 degrees blade angle at the master station).
I will dig out some single stop notes and diagrams over the next day or so and forward to you, in the meantime the prop is perfectly safe and can be handled with no problems
br om15
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Dowty Prop
Contact Frank Keeble at Fields Airmotive, Rand Airport, South Africa. He will set you right. +27-76-363-7877 or [email protected] . Dowty expert and nice guy. Many years at South African Airways doing 748 props.
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Thankyou everyone for the advice! It was a big relief to find out the prop is actually safe.
OM15 - thankyou for the email, it was most helpful and has enabled us to get to the bottom of the issue
OM15 - thankyou for the email, it was most helpful and has enabled us to get to the bottom of the issue
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I have not changed a Dart prop. since the 1980's but as I recall removing the prop not in the feathered position is not a problem with the prop itself rather that the pitch lock assembly which comes off with the cylinder head has the spring in the compressed position - it will be like that until re-installed and the prop feathered hydraulically.Be aware that the cylinder head has a left hand thread as has the inner oil transfer tube - the biggest and smallest threads on the prop assembly. The danger with removing the prop unfeathered is that if there has been a failure in the pitch lock assembly it could result in the compressed spring propelling the cylinder head at high speed when removing it which could injure personnel. The situation you have now is not dangerous as the lock unit although under spring pressure is unlikely to unlock itself - even if it did there would be little risk of injury.
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Single Stop Prop Schematic
Larger pic: mike-wsm.org.uk/zpp153L.jpg
Single Stop Prop Wiring Diagram
Larger pic: mike-wsm.org.uk/zpp154L.jpg
Two Stop Prop Schematic
Larger pic: mike-wsm.org.uk/zpp155L.jpg
Posted for om15
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