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Author Topic: Is the Urchin suitable for 50g BMFA rubber?  (Read 510 times)
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billdennis747
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« on: January 13, 2020, 05:58:56 AM »

This may have been asked before but can't find anything. I want another class to fly preferably with the option of flying it in Classic. I have done one before (and before 50g) but lost it. The advantage is I've actually got a plan and it's pretty straightforward. The prop looks crude - is it? Will it go on 50g? I can build it lighter than the last one. I know Phil Ball flew one (and a super version) but I'm not Phil Ball.
There was discussion on the Skywalker 60 and some felt 50g would not be enough but surely it is lighter than an Urchin?
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ffkiwi
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« Reply #1 on: January 13, 2020, 06:44:23 AM »

Bill -Mike W offers a plan for the Skywalker 50....presumably a minor rehash of the SW 60 to suit the 50g motor rule.....the Urchin is a good model (just sneaks into Nos rubber under NZ rules...our FF Classic class runs from Jan 61-Dec 70) but yes- it might suffer a little on only 50g....IIRC I used 70g motors in my two....first was lost on its 2nd trimming flight, and the second languishes awaiting fuselage repair....

 ChrisM
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gman
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« Reply #2 on: January 13, 2020, 07:01:08 AM »

Hi Bill,
My Urchin certainly flew on 50g on its couple of outings. These were classic rubber events where, of course, it could have used 75g. They glide better on lighter motors of course.The only comment I would make about its suitability for the odd 50g excursion (and it will be odd because there's almost no BMFA rubber in this year's calendar) is that it's quite a build with its rounded tips, plug in fin, built up pylon etc. For 50g only I'd build a Waif, but then that's all that it can be used for.
Gavin
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cvasecuk
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« Reply #3 on: January 13, 2020, 07:29:41 AM »

A light Urchin does fly very well on 50g of rubber especially if you go to the bother of a modern prop, but as Gavin points out, there no specific comps this year for the BMFA Classes (even at the Nats).They all "Combined" so build another Urchin, lightly, and go fly on 75g!
Ron
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Soc
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« Reply #4 on: January 16, 2020, 01:31:42 AM »

Urchin prop stop

The prop stop shown on the plan is the earliest published example I know of a torque sensitive prop stop.
Moreover it works by way of a swinging arm !

However I doubt that most people equipped with just the drawing and the brief note in the text could see how it works
let alone appreciate its advantages.
It deserves a perspective drawing or an end on view and some indication of how to set up the rubber band mentioned in the text.

I wonder if there were other contemporary examples from the '60s, of similar systems in the UK ?

I recognized it for what it was because i reinvented it sometime in the '80s (see sketch)

By the way, since it should be easy to build the Urchin to 80 gm or less,
it should be really hot with 50 gm of modern rubber and modern winding methods.

Sean
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Soc
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« Reply #5 on: January 16, 2020, 04:29:04 AM »

Urchin prop stop - PS

I forgot to point out that the second drawing in the previous post shows a version of Hank Cole's prop stop
that works via a swinging arm that moves when the toque is low.
Hank pointed out that rotating parts are more reliable than the sliding pins in Montreal type prop stops.

So far as I can tell the Montreal stop appeared in the mid '60's and the Cole stop came along some years later.

Is seems Eddie black was well ahead of the field, but there are obscure prop stop details on earlier plans.
JB Knight comes to mind.

Sean
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cvasecuk
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« Reply #6 on: January 16, 2020, 06:22:16 AM »

I have built two Urchins which both flew well. (The first was stolen after a long flight!). I used an ordinary screw stop behind the nose block since I use a blast tube and the "Freewheel whilst winding" was not needed. Anyway I couldn't see how it worked because the Z shaped arm is hard against the hub and is therefore fixed. He metions an elastic band but I can't see where it would go.
As for being ahead of one's time, then how about this variable pitch Wakefield prop from  G Harris in 1944!
Ron
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billdennis747
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« Reply #7 on: January 16, 2020, 10:17:54 AM »

I just assumed it was a Garami clutch and built it as such. I ignored the loop on the noseblock as I didn't know what it was for.
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Soc
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« Reply #8 on: January 16, 2020, 09:48:45 PM »

Urchin prop stop - explanation.

To be clear about it for the benefit of passers by.

The Z shaped bit is three dimensional.
The two arms are at an angle to each-other viewed along the center arm.

When the forward arm is held against the the side of the hub
the trailing arm sticks out sideways so the hooked end clears the loop on the nose block.

When the forward arm is released the invisible rubber band pulls the trailing arm against the side of the hub
and the hook engages the loop on the nose block.

The sketch below shows a cross section of the hub and an end view of the Z shaped latch.

How the rubber band was arranged remains a mystery.

There are many ways to install a light spring to activate the mechanism.

Looking at the Feb 1960 Aeromodeller I see that there are many puzzling gadgets in a number of articles
and also Zurad's ZS-57'10 Wakefield which had a major influence on me,
so I guess the Urchin drawing didn't get much attention and
its probably not surprising that it has taken 60 years to solve the Eddie Black prop stop puzzle.

Sean
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cvasecuk
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« Reply #9 on: January 17, 2020, 04:30:36 AM »

Thank you Sean. Now I understand!
Ron
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I hate trees
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« Reply #10 on: February 04, 2020, 12:56:12 PM »

The Urchin should go ok on 50g, Phil Ball's did!
I flew mine on the max of 75g (72g dry).  I can't remember exactly what it weighed by I think 80-85g.  In 50g models I had 14str of 3/16, which worked out about 28 inches long.  I think it would be too short to go between the pegs on the original Urchin set up.  The Urchin is a really good design and can take a lot of power, my model had a 16str of 3/16.  The 50g 14str motor would be a bit less impressive.  I have always thought about building another one and going for a bit less rubber, about 60g with the cross sectional equivalent of 15str.  I think that models like the Urchin and the Senator, which are capable of flying fast at the beginning of the flight should utilize that and get high.  I was gutted to lose mine at Sculthorpe when it came down quite low after 5mins and was clocked off, only to catch a thermal and climb away for ever.
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billdennis747
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« Reply #11 on: February 04, 2020, 01:43:15 PM »

Thanks Adam. Just finishing an Aiglet, then it's an Urchin
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