Author Topic: Bristol Mercury revisited  (Read 34045 times)

Offline Art K

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Re: Bristol Mercury revisited
« Reply #120 on: November 24, 2019, 09:31:21 PM »
Mike,
The thrust bearing at the base of the prop makes a lot of sense due to centrifical loading. I can imagine that without it at speed it would be impossible to rotate.
Art
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Offline Allen Smithee

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Re: Bristol Mercury revisited
« Reply #121 on: November 25, 2019, 07:44:17 AM »
I would think a pair of opposed tapered roller bearings, or perhaps a plain bearing at the "blade" end and a tapered roller bearing at the inner end would be worth considering for the combination of axial and lateral loads. One of the reasons my constant speed prop project got parked was a lack of confidence in my calculations for these stresses.

AS
Quidquid latine dictum sit altum sonatur

Online Vixen

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Re: Bristol Mercury revisited
« Reply #122 on: November 25, 2019, 10:44:15 AM »
This discussion could become interesting and perhaps controversial
If you do a full anylasis of the forces acting on the root of a folding/variable pitch prop blade you find some surprising results. The centrifugal force dominate the hydro/ aero dynamic drag and lift forces to such an extent that you can virtually ignore all forces except the centrifugal force.

This is counter intuitive but the number don't lie, you have to trust them.

As an example, you only need to look at the VP tail rotor (and some main rotors) of a model helecopter. The blades are held by a simple pin joint and are free the Flop about in a clock/anti clock direction. When they spin up the centrifugal forces swing the blades out to their correct operating position. The drag forces do not fold the blades flat.

While I was working, we designed a small folding prop blade, tube launched wire guided underwater device (not quite a torpedo) . The prop blade folded clockwise within the body diameter and deployed, into the direction of rotation when the propulsion motor spin up.

The hub of the De Havilland VP hub only has a single, flat roller thrust bearing, which is preloaded to prevent the blades flopping about when the engine is stopped. There are no plain bearings to resist bending forces. Actually the thrust bearings are basically static bearings, they only rotate the blades through 20 degrees twice per flight.

I am away from home, this week, otherwise I would post the works drawings of the hub.

I know it's counter intuitive but analysis does show the centrifugal force dominate the bending forces allowing a single bearing blade root

Mike
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Offline Allen Smithee

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Re: Bristol Mercury revisited
« Reply #123 on: November 25, 2019, 01:36:58 PM »
I'm not *that* surprised, but I do wonder if it depends on what the aeroplane is doing. The aeroplane my CS prop was intended for was likely to spend a lot of time doing flick manoeuvres, lomcevaks etc and these rapid gyrations impart significant gyroscopic stresses. I note that "agile" helicopters tend to have rather more lateral/vertical blade location in the main blade mounts, and the so-called "3d" RC helicopters have stabilised to having two axial ballraces and a thrust race holding the blade pintel shafts (or an outer ball race and an inner angular-contact bearing in one case).

But none of that is relevant to a prop for an engine that will only ever be bench run. In that case I guess you're really just after blade containment, so a hub with something like a tapered roller or thrust race to take the centrifugal loads and a simple ballracem needle roller or plain bush outboard of that as a blade steady should be enough. You could then ensure the containment of composite blades by just having a spar made from carbo tows which start at the blade tip, pass down the blade and around retaining features in the pintel shaft and then back to the tip. This is essentiall what we did with the "solid" carbon F3D props - half the toes went tip-to-tip, striaght  past the hub. the other half went from a tip, around central pin (which formed the mounting hole) and back to the same tip, alternating each blade for balance. These props would break on contact with the ground (or in mid-airs) but never shed a blade due to revs (over 31k in some cases).

AS
Quidquid latine dictum sit altum sonatur

Online Vixen

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Re: Bristol Mercury revisited
« Reply #124 on: November 25, 2019, 02:52:58 PM »
The design point I described relates to relatively steady state conditions with sufficient prop RPM's to generate the large centrifugal forces. Your extreme manoeuvres and wide rev range will not doubt create a different set of load conditions to those experience by the De Havilland VP prop.

Mike
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Online Vixen

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Re: Bristol Mercury revisited
« Reply #125 on: December 05, 2019, 04:22:23 PM »
Just returned from a very wet week in North Yorkshire to a cold and frosty south of England. However some progress has been made on the VP propeller hub.

The three stub blades were straightforward turning on the lathe, followed by drilling and tapping the M4 screw threads in the base. The full size propeller hub has three, two piece, loose roller ball thrust races, which must have been a real pain to assemble. For simplicity, I substituted a one piece brass thrust ring and a PTFE thrust bearing. Both items were milled from flat stock.




Here we can see the three stub blades resting in the Hub Barrel halves






This is the Hub Barrel loosely assembled. Amazingly the three stub blades rotate freely.






The next job is to connect up the Control Cylinder to roller bearings in the Control Arms and do whatever is necessary to get full and free propeller pitch movement through the entire 20 degree pitch range.

Mike
« Last Edit: July 29, 2021, 03:48:14 PM by Vixen »
It is the journey that matters, not the destination

Sometimes, it can be a long and winding road

Offline scc

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Re: Bristol Mercury revisited
« Reply #126 on: December 05, 2019, 09:05:36 PM »
BEAUTIFUL      :popcorn: :popcorn:          Terry

Online Vixen

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Re: Bristol Mercury revisited
« Reply #127 on: December 06, 2019, 02:39:32 PM »
Thanks Terry.

The whole De Havilland variable pitch propeller hub assembly went together with very little trouble. All that was required was a few strokes with a file to enable the hub to move to the limits of travel. Here you can see it in the course pitch position
I am now just waiting for the castile nuts and bolts, which clamp the Hub Barrel halves together, to be delivered




There is a 85 year gap between these next two photos. Which reminds me that I have still to make the two crankcase breather tubes.





Well, I'm close to crossing another part off that endless list.

That's it for now

Mike

« Last Edit: July 29, 2021, 03:49:13 PM by Vixen »
It is the journey that matters, not the destination

Sometimes, it can be a long and winding road

Offline Admiral_dk

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Re: Bristol Mercury revisited
« Reply #128 on: December 06, 2019, 08:12:45 PM »
Great work Mike  :cheers:

I must admit that in my mind the stub where a bit longer than they turned out to be - but the last picture really drives the point home to me. The biggest difference (besides scale) is the quality of the old print and the new + now you mentioned it the missing breather tubes.

Online Vixen

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Re: Bristol Mercury revisited
« Reply #129 on: December 12, 2019, 06:57:53 PM »
It's gone very quiet of the Forum lately, probably due to the winter weather. I thought it needed someone to post something for the petrol heads.

As Per pointed out, the crankcase breather pipes were missing from the previous photo. So they became the next bits to make.
The breather pipes were made form 12mm diameter thin wall tube. The upper flange was made on the lathe and then transferred to he mill, to profile the end flange and drill the four mounting holes. Forgot the photos.

The lower brackets were originally castings but, for the model, they would have to be carved out of the solid. Their awkward shape would provide an interesting exercise in work holding. The machining required the profiling to shape of three faces, the base and both sides. This would require square stock, sufficiently long for most of the machining to take place on the overhanging section beside the machine vice.

The first opp was to profile the bottom face of the lower mounting bracket




The block was then rotated to allow the first side to be profiled and the recess hollowed out with a 2.0mm diameter end mill.






A confidence check to ensure the breather pipe would fit the bracket




The block was flipped over to the opposite side of the machine vice for the other side profile and recess to be machined. I touched off the far end of the bracket and the lower face, to redefine my datum.




Here is the first bracket being parted off from the stock material.




The end result look like a pair of miniature tooth brush holders. The brackets are still in the raw, as machined, state and will need some hand work with needle files to round the edges and give them  the required 'casting' look. The mounting holes are yet to be drilled to match the fixings in the engines crankcase.




The final photo shows the two crankcase breather pipes loosely assembled to the front of the engine. I don't think they are a very pretty design, more like two rain water down pipes, almost an afterthought. However they are completely true to the original.

Another small detail crossed off the seemingly endless list.




This winter is already dragging on, and it's still only early December

Mike

« Last Edit: July 29, 2021, 03:50:58 PM by Vixen »
It is the journey that matters, not the destination

Sometimes, it can be a long and winding road

Offline mike mott

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Re: Bristol Mercury revisited
« Reply #130 on: December 12, 2019, 07:10:52 PM »
Mike the more I watch the intricate shapes that you are able to produce the more I learn about sequencing the work of making these sorts of parts.

Mike 
If you can imagine it you can build it

Offline scc

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Re: Bristol Mercury revisited
« Reply #131 on: December 12, 2019, 07:12:53 PM »
I love watching parts like this carved from solid lumps :ThumbsUp:  Inspiring        Terry

Offline Dave Otto

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Re: Bristol Mercury revisited
« Reply #132 on: December 13, 2019, 12:38:28 AM »
Very nice as usual Mike!

Dave

Offline 90LX_Notch

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Re: Bristol Mercury revisited
« Reply #133 on: December 13, 2019, 01:37:58 AM »
Outstanding work Mike.  I always look forward to your updates.

-Bob
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Offline Jasonb

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Re: Bristol Mercury revisited
« Reply #134 on: December 13, 2019, 07:13:07 AM »
Your getting there Mike, would the bracket have had a separate split cap on the original as the shape looks like it should clamp around the tube with a couple of bolts to close it up?

J

PS As I had another runner at the weekend I'll start a new IC build thread soon so it won't be so quiet for you.

 

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