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Author Topic: Avetek Pilatus PC9  (Read 9529 times)
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Work In Progress
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« Reply #50 on: January 02, 2017, 05:33:25 AM »

I'm very much looking forward to this model continuing now we're entering the New year, and I should also apologise to Gwyn for not responding earlier to his kind offer in #40 on page 2 above - email inbound!
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ZK-AUD
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« Reply #51 on: January 15, 2017, 06:25:03 PM »

Happy new year to all - after Christmas and the 10 days at Scout Jamboree and another week doing a major clean out of the shed I can report that I'm back in the saddle and that progress has resumed!

I covered and doped all of the flying surfaces yesterday and started on the fuselage.  In an early post on this thread Strat-O commented on the rather acute curvature behind the cockpit and how that might be to cover. 

Actually the one disadvantage of scalloping the formers is that it can make for some fairly complex covering challenges.  This one is nowhere near impossible however when I looked at it again I realised that while it might be an impressive modelling feat from a purely constructional standpoint it would not result in a convincing scale representation of the real aircraft.  Therefore I have filled in between the stringers of that section with some of my really really dead wood to get a nicer shape.  I'll hopefully get it far enough along to share some pictures tonight.   
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« Reply #52 on: January 15, 2017, 06:44:43 PM »

Yes... I've been caught that way before, having to fill in tissue attachment where I had nicely scalloped things. Frustrating.
Glad to hear you are back working on the model, which has inspired me to order the Porter and PT-19 to go along with my existing Auster AOP.9 and Piper Tomahawk kits
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« Reply #53 on: January 16, 2017, 04:12:18 AM »

As promised here are some photos by way of an update.  The fill in between the rear deck stringers was a satisfying little job and I think has resulted in a much better shape for virtually no additional weight.  Fuselage is now partially covered - one piece of tissue did that much, including the rear decking area, so there you go!
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« Reply #54 on: January 17, 2017, 04:33:52 AM »

Here we are - all covered and clear doped 31grams as you see it here including the prop and bearing.  Interestingly I chose to cover the fuselage in the old 'kit' tissue we used to get - principally because it has incredible wet strength.  The disadvantage of this tissue is that it is quite porous and looks like it was hewn from a block with a cross-cut saw - often remaining soft and fuzzy even after dope.  Not mine however.  The secret is 2 coats of weak shrinking dope. and 2 further coats of non-shrinking.  rubbed down carefully with 800 grit wet n dry after each coat - As smooth as Esaki.
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DavidJP
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« Reply #55 on: January 17, 2017, 07:47:06 AM »

Huh........ bit nice 'innit??  Looks as though i could be a tail dragger though??
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« Reply #56 on: January 17, 2017, 12:00:57 PM »

Yes, I had to put a block under the rear for those photos but when the prop and hook are it it sits properly on the wheels - bottom line is that it has to or the COG is wrong!
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« Reply #57 on: January 17, 2017, 03:10:30 PM »

Sorry Mike - I have just noticed the emoji I added to show I was teasing have gone missing!
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« Reply #58 on: January 17, 2017, 03:12:57 PM »

That's all right - my wife accuses me of dragging my a** too Grin!
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« Reply #59 on: January 18, 2017, 03:44:30 AM »

No pictures tonight but some good news.  We had a calm spell this evening in what has to be the windiest summer on record so I taped all the bits together and took it outside for a test glide.  Quite amazing - an absolutely rock steady floaty slow glide.  Also got the nose bearing and prop assembly sorted. 
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« Reply #60 on: January 20, 2017, 01:42:37 PM »

Encouraging! Your weight reduction efforts appear to have paid off handsomely.
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« Reply #61 on: January 20, 2017, 07:47:37 PM »

I thought it might be worth describing my process for achieving a reliable freewheeling device while using the scale spinner, which really is necessary on this model.

Earlier in this build you saw the bearing block that is an insert in the nose plug.  By filing and shimming this block as required I can fine-tune the thrustlines.  The first picture below shows the 7" plastic prop which I have lightened and balanced in the usual manner.  The clutch helix and bearing nub have also been removed.  A hole has been drilled for the new clutch dog bush, which is a firm interference fit.

Picture 2 shows the shaft assembly - we have 2 3/64 Peck bearings in the bearing block.  Working from the front of the bearing block outwards there is a bespoke washer which is the inner seat for the Jasco thrust bearing that comes next.  Then we have the prop driver plate which is soldered well to the shaft.  You can see the actual driver which is a short length of brass tube soldered on well.  Immediately after than is a short length of tube to space the prop out from the driver plate.

The next picture shows the prop and clutch dog installed and in the driving position.

To seat the spinner accurately I have glued a styrene base plate to the front of the prop.  The final photo shows my method for ensuring that this is accurately aligned in all axes.  I drilled a hole in a block of hardwood and inserted the current excess prop shaft into it to hold everything nice and square.

Next stage is to carefully solder retainers for the clutch dog and prop. and then glue the spinner in position.
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« Reply #62 on: January 20, 2017, 08:49:19 PM »

Just to finish this subject off, first up is a back view of the prop showing how the spinner plate cetres this nicely and provides a good glue surface.

Next up is the front view with prop and clutch dog retainers soldered on.

Then a couple of shots with the spinner glued on and the bearing block assembly inserted into the nose block, and finally the completed item in the model. 

All works perfectly!
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« Reply #63 on: January 21, 2017, 04:20:57 AM »

No pictures tonight but some good news.  We had a calm spell this evening in what has to be the windiest summer on record so I taped all the bits together and took it outside for a test glide.  Quite amazing - an absolutely rock steady floaty slow glide.  Also got the nose bearing and prop assembly sorted. 

You jammy ********!!  Mind you being upside down does help defy gravity. Wink
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« Reply #64 on: January 21, 2017, 06:26:36 AM »

Very nice!

Scott
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« Reply #65 on: January 21, 2017, 09:25:04 AM »

I am highly impressed with your prop clutch work. It's streets ahead of anything I've ever done in that part of an aeroplane. But I think I could probably follow along using those pics as a guide. I have a current application for it, too.
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« Reply #66 on: January 21, 2017, 10:34:36 AM »

While I am impressed with the mechanics of it all, what really amazes me is how you got such a perfect fitting spinner around the prop blade. How did you measure the precise angle, thickness, and shape of the prop blade at that intersection of it and the spinner?  I have never been able to do more than hack a hole with a chain saw there and hope for the best.....
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« Reply #67 on: January 21, 2017, 01:03:06 PM »

Mike, can you explain how you engage the drive pin when it's wrapped in spinner? Do you just shake it about
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« Reply #68 on: January 21, 2017, 02:24:00 PM »

Hi and thanks for the nice comments everyone. 

WIP, I guess everyone has their little hobby horses and mine is getting the business end right on rubber models.  I believe that having a free wheel that works reliably , a decent bearing that supports the shaft properly and minimizes friction losses, a lightened and balanced prop, and the ability to reliably adjust thrustlines are collectively the heart and soul of a rubber model.  So many beautiful models I see are made less than they could be by the old paint stirrer flopping about a skinny piece of wire and retained by the 90 degree bend and helix.  Don't mean to sound strident but that's my particular schtick!

Tom, I'd love to tell you about some clever science but this is how I did it: If you look at the pics you'll se that the cutout over the leading edges is just vertical (clever would be having the spinner follow the underside of the blade too but alas...)  First important job is to make a mark exactly diametrically across so that the 2 vertical cuts are in the right place.  Get them the same depth and radius the bottom of the cuts nicely.  Lock that in and don't touch it.  Then relieve the back of the cut to match the rear camber of the prop.  Using the mark one eyecrometer get this close but ensure that you get both sides the same so that the spinner will sit level over the prop. Then, get a felt pen and run this along the join using the back of the prop as your marking gauge to get the shape perfect.  Work slowly and as the spinner fits closer and closer revise your gauged mark as required.  Hopefully this makes sense.

Bill if you look at the photo that shows the rear view of the completed unit installed in the nose plug you'll se that the mechanism is very visible.  You can just shake the thing into the right position or you can poke something small in there to flick it over - either way it's easy.  I pull the nose plug out and set the mechanism first prior to winding.   
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« Reply #69 on: January 22, 2017, 02:34:35 AM »

I had wondered whether drilling the prop to receive the clutch dog bush was asking for trouble.  When I tested the thing I dropped something I was holding which hit the prop while it was running and it snapped at that point - not surprising.  Anyway here is version 2 - a much more cunning plan indeed.  This improvement strengthens the prop hub rather than weakens it and also provides the perfect seat for the spinner - Game over!
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« Reply #70 on: January 22, 2017, 12:25:13 PM »

Whew, finally understand it!  My expectation that was leading me down the wrong path to understanding this was the locking mechanism was in front of the prop, but it's not, it's behind the prop and thus outside of the spinner.  I was also imagining that a missing spring was somehow involved but there is none required in this design. 

To maybe help others who are still struggling with how this works:  The brass tube which is on the rubber side (which is really just a peg) impinges against a swinging gate (which in this instance is called the clutch dog).  During the power phase the gate is locked against the center hub.  During the freewheeling phase the prop freewheels around and the brass peg pushes the gate aside.

What is cool about this idea is you could conceivably model a design that is spinner-less like a Thunderbolt or Hellcat.  (You would have to solve the issue of what to do about drilling the prop for the clutch dog)
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« Reply #71 on: January 22, 2017, 01:18:28 PM »

Here's a link to my Thomas Designs Bristol Scout thread which has a version of this freewheel system used on a spinner-less prop.  It was necessary to build the hub up using a method shown to me by Rich Weber   http://www.hippocketaeronautics.com/hpa_forum/index.php?topic=20305.50

I'll do a quick diagram and post it to help aid understanding
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« Reply #72 on: January 22, 2017, 01:51:49 PM »

As promised here is a rough sketch of the mechanism.  Shown in the freewheeling position
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« Reply #73 on: January 22, 2017, 06:39:24 PM »

It looks interesting Mike but I'm puzzled as to how the drive dog latches. Whoops ok it looks like it engages behind the prop shaft.
Clever and very neat.

John
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« Reply #74 on: January 22, 2017, 07:26:57 PM »

Just to complete things I thought I'd re-post the sketch of my little jig for making washers and discs the perfect size and with the perfect size hole - the washers, drive plate and the styrene spinner plates are all made this way on my little table sander
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