IMPORTANT - calling Dowty Propellor enginers/mechanics
Discussion
Wanted - anyone with experience or knowledge of Dowty hydraulic constant-speed propellors, as fitted to Rolls Royce Dart engines.
In our 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 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?).
magpie215 said:
Tbh its nearly 10 years since I last played with darts and 748s.......ditched all my training notes and tech stuff ages ago...sorry I can't be more help...just can't understand how it got removed unfeathered?
Ah fair enough, thanks for the pointer anyway as it gives us something to go on! We can't understand how or why it was done like that either. I can only surmise that, as JD sat decomposing in the far corner of Blackpool airport, either the hydraulic supply was malfunctioning and thus left the blade in the primed position or the ground crew were just unconcerned about it, thinking that the aircraft was about to be turned into tin cans rather than saved by SAHG at the last minute!Fugazi said:
Should ask Roy Coates to get in touch with Dr Mark White at the University of Liverpool's Flight Science and Technology Centre, he has a guy from Dowty come and give guest lectures to the aero-engineering students. Maybe he can find you the info you need.
Thanks! 
(Are you one of regular visitors, judging by how you know Roy?)
hidetheelephants said:
magpie215 said:
Tbh its nearly 10 years since I last played with darts and 748s.......ditched all my training notes and tech stuff ages ago...sorry I can't be more help...just can't understand how it got removed unfeathered?
Perhaps the blades are seized in the hub?
Fugazi said:
I've only been down a couple of times helping out at the open days, usually helping out with lugging the Tomahawk flight sim about. I've known Roy for a few years though within the university, (he's the goto guy in the engineering dept when a computer self destructs).
Ah I see - hopefully I'll bump into you at some point then, I spend nearly every weekend down at the collection these days!From the dim and distant memories of my Halton Apprenticeship, I recall that the Dowty Rotol prop. as fitted to the RR Dart had a number of stops in its range of movement. The terms 'Reverse Stop, Ground Fine Pitch Stop, Flight Fine Pitch Stop and Feather Stop' spring to mind. There might be another one for the maximum course pitch in the normal flight range. I can't quite remember.
The 'Turbo-Prop' phase of our course concentrated on the Dart. I probably still have my old notes at home, but I'm away at the moment. (I guess should add the words 'on business' in true PistonHeads tradition.)
In the old workshops there was one engine/propeller connected to a test rig (an electrically driven hydraulic pump and a console of about 5 or 6 switches) on which you could replicate various control settings and move the propeller throughout its range. It was possible to simulate a fault (micro-switch or stop failure, I think) which caused the propeller pitch to 'hunt' course-fine-course continuously between two positions.
We used to be grilled endlessly on all aspects of its operation by a particularly evil civilian instructor, who seemed to take great delight in failing young 'Erks' like me. It's all a long time ago now, thank goodness.
The 'Turbo-Prop' phase of our course concentrated on the Dart. I probably still have my old notes at home, but I'm away at the moment. (I guess should add the words 'on business' in true PistonHeads tradition.)
In the old workshops there was one engine/propeller connected to a test rig (an electrically driven hydraulic pump and a console of about 5 or 6 switches) on which you could replicate various control settings and move the propeller throughout its range. It was possible to simulate a fault (micro-switch or stop failure, I think) which caused the propeller pitch to 'hunt' course-fine-course continuously between two positions.
We used to be grilled endlessly on all aspects of its operation by a particularly evil civilian instructor, who seemed to take great delight in failing young 'Erks' like me. It's all a long time ago now, thank goodness.
Iirc the pitch stop mech has a warning on it stating not to be removed if the prop is not feathered. ..we used to have to motor the feathering pump to get the prop fully feathered and it used to really go with a solid ping only then could you remove the pitch stop mech to get to the prop hub nut.....retained at around 1000 ft/lbs iirc the torque wrench took 2 people to use and a third to stop the proo moving.
JoeBolt said:
From the dim and distant memories of my Halton Apprenticeship, I recall that the Dowty Rotol prop. as fitted to the RR Dart had a number of stops in its range of movement. The terms 'Reverse Stop, Ground Fine Pitch Stop, Flight Fine Pitch Stop and Feather Stop' spring to mind. There might be another one for the maximum course pitch in the normal flight range. I can't quite remember.
The 'Turbo-Prop' phase of our course concentrated on the Dart. I probably still have my old notes at home, but I'm away at the moment. (I guess should add the words 'on business' in true PistonHeads tradition.)
In the old workshops there was one engine/propeller connected to a test rig (an electrically driven hydraulic pump and a console of about 5 or 6 switches) on which you could replicate various control settings and move the propeller throughout its range. It was possible to simulate a fault (micro-switch or stop failure, I think) which caused the propeller pitch to 'hunt' course-fine-course continuously between two positions.
We used to be grilled endlessly on all aspects of its operation by a particularly evil civilian instructor, who seemed to take great delight in failing young 'Erks' like me. It's all a long time ago now, thank goodness.
It seems quite odd, with hindsight of course, that there were indeed evil civilians [we ft's had them too] that were empowered to make such a profound decision on an airmans future....The 'Turbo-Prop' phase of our course concentrated on the Dart. I probably still have my old notes at home, but I'm away at the moment. (I guess should add the words 'on business' in true PistonHeads tradition.)
In the old workshops there was one engine/propeller connected to a test rig (an electrically driven hydraulic pump and a console of about 5 or 6 switches) on which you could replicate various control settings and move the propeller throughout its range. It was possible to simulate a fault (micro-switch or stop failure, I think) which caused the propeller pitch to 'hunt' course-fine-course continuously between two positions.
We used to be grilled endlessly on all aspects of its operation by a particularly evil civilian instructor, who seemed to take great delight in failing young 'Erks' like me. It's all a long time ago now, thank goodness.
Mojocvh said:
It seems quite odd, with hindsight of course, that there were indeed evil civilians [we ft's had them too] that were empowered to make such a profound decision on an airmans future....
There were some good civvy trainers (the grandfather figures) but I found the nasty ones far worse than any service instructor. As you probably know, most were ex-SNCOs, now holding 'Officer' status and coming to the end of their working lives. They took a dim view of the new generation of RAF serviceman and were out to prove a point.I remember one stating that he would refuse point blank to ever again fly on an RAF aircraft, given the current crop of trainees. Now there's a morale boost!
magpie215 said:
Iirc the pitch stop mech has a warning on it stating not to be removed if the prop is not feathered. ..we used to have to motor the feathering pump to get the prop fully feathered and it used to really go with a solid ping only then could you remove the pitch stop mech to get to the prop hub nut.....retained at around 1000 ft/lbs iirc the torque wrench took 2 people to use and a third to stop the proo moving.
So, it does beg the question of how they managed to get it off without supposedly being able to get to the nut due to the pitch pistons?Anyway, I have managed to get a lead on some dowty hub maintenance diagrams, so fingers crossed!
This thread has lit a spark of nostalgic interest, so I've been searching the internet for further information.
There were many variations of the Dart/Dowty engine and propeller. Not all props were reversing so the blade range of movement for these simpler models went from Ground Fine all the way to Feather, with sprung collets in the hub forming the Flight Fine Pitch Stop. The models I trained on were reversing, so had the additional Ground Fine Pitch Stop.
You say your propeller is in the 'Windmill' position, so it seems it's on the Flight Fine or Ground Fine Pitch Stop (if it has the latter). Or it might simply be at the Ground Fine limit of movement (for a non-reversing prop).
The pitch change piston in the propeller hub is double acting. Oil pressure moves it forward to turn the blades to fine pitch (assisted by the blade centrifugal turning moment). Oil pressure moves the piston rear to turn the blades to course pitch.
Personally I think your propeller is safe to handle, although I wouldn't try dismantling the pitch change mechanism without expert knowledge/assistance. The sprung collets would need to be released before the blades can be moved. On the aircraft the collet lock is removed by oil pressure. If the collets can be released somehow by hand, you would still need oil pressure to move the pitch change piston and the blades. You might also be able to turn the blades by hand.
I hope this information has been useful but it's certainly not exhaustive and might not even be accurate for your exact model of propeller. So please continue to be careful and get some help! SAFETY FIRST!
There were many variations of the Dart/Dowty engine and propeller. Not all props were reversing so the blade range of movement for these simpler models went from Ground Fine all the way to Feather, with sprung collets in the hub forming the Flight Fine Pitch Stop. The models I trained on were reversing, so had the additional Ground Fine Pitch Stop.
You say your propeller is in the 'Windmill' position, so it seems it's on the Flight Fine or Ground Fine Pitch Stop (if it has the latter). Or it might simply be at the Ground Fine limit of movement (for a non-reversing prop).
The pitch change piston in the propeller hub is double acting. Oil pressure moves it forward to turn the blades to fine pitch (assisted by the blade centrifugal turning moment). Oil pressure moves the piston rear to turn the blades to course pitch.
Personally I think your propeller is safe to handle, although I wouldn't try dismantling the pitch change mechanism without expert knowledge/assistance. The sprung collets would need to be released before the blades can be moved. On the aircraft the collet lock is removed by oil pressure. If the collets can be released somehow by hand, you would still need oil pressure to move the pitch change piston and the blades. You might also be able to turn the blades by hand.
I hope this information has been useful but it's certainly not exhaustive and might not even be accurate for your exact model of propeller. So please continue to be careful and get some help! SAFETY FIRST!
JoeBolt said:
Mojocvh said:
It seems quite odd, with hindsight of course, that there were indeed evil civilians [we ft's had them too] that were empowered to make such a profound decision on an airmans future....
There were some good civvy trainers (the grandfather figures) but I found the nasty ones far worse than any service instructor. As you probably know, most were ex-SNCOs, They took a dim view of the new generation of RAF serviceman and were out to prove a point.Not, NOT the kids fault, I emphasise, rather the once magnificent training given to the RAF's aircraft engineering tradesmen had been destroyed by senior officers meeting political budgetary markers...imo 'natch.
I was harking back to the early 80's before the SAC(T) grade had been dreamed up. However, I suppose it's only natural for the 'old sweats' to view the up-and-coming generation with criticism. I'm sure my instructors, some of whom were old enough to have started their training in the 1930's, were on the receiving end of similar derision, if not worse.
There is no big spring in the Dart propeller pitch lock. No danger that something will "snap" The reason for feather is that otherwise the spinner cannot be removed. If spinner is removed, just use the proper tools to remove pitch lock. Once that has been removed , you can unscrew the locknut with special tool, so the propeller can be removed from the engine shaft. Success.
Robheineken said:
There is no big spring in the Dart propeller pitch lock. No danger that something will "snap" The reason for feather is that otherwise the spinner cannot be removed. If spinner is removed, just use the proper tools to remove pitch lock. Once that has been removed , you can unscrew the locknut with special tool, so the propeller can be removed from the engine shaft. Success.
Thank you, but you realise this was originally posted over a decade ago?I suspect the problem has been long solved.. or the prop scrapped since then..
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