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DB Sport and Scale Auster J1 Autocrat


Danny Fenton

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I can see absolutely no reason why you cannot cut the shaft down a bit but note:


1. Yes, it will devalue the engine if you ever wanted to sell it.

2. Be very careful not to bend anything when you cut it - make sure everything is well supported.

3. Whatever you use to cut it (I would use a small cut-off disc in a Dremel type tool), there will be a lot of swarf / filings / abrasive dust flying around so make sure that you cover up absolutely everything iin bags and tape and then do it again and thoroughly clean everything after the cut before removing any bags etc. The smallest bit of crap can get into your engine and ruin it.

4. Eye protection is an absolute must.

 

Good luck

Simon

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If the engine is only going to be used for this model then cut the shaft off, usual thing nut on first then (irrespective of what others have said) disc cutter to cut then file to clean then nut run off to clean threads. Obviously protect engine from crap. Sure if it’s perfection I wanted and it was to be cut to within a thou’ then I’d strip the engine then stick the crank in the lathe and machine it down, but…………..

Edited by Ron Gray
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I would definitely wait until you’ve fully flight proven the model Danny.

 

If it turned out that you needed a different propeller, especially one of higher pitch, you might end up with insufficient thread if you have already cut the crankshaft. Not all props of a particular size have the same thickness of boss.

 

In the meantime, you could make a thick washer to mount in front of the prop. so that you could still use your spinner once it has been made.

 

Brian.

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Thanks chaps, I doubt this engine will ever want more than an 8 inch pitch prop. People keep forgetting that you can use a threaded sleeve if you run out of thread. So that the thread goes inside the prop hub.

Anyway as you suggest Brian I wont do anything until after its flown.

 

844.thumb.jpg.ffbe5d69ad91d5b5a795e18e0937a506.jpg

 

I tested my starter, works really well ?

Cheers

Danny

 

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  • 1 month later...

So did some more running in of the Saito today, starts very easily using the bounce back technique so the starter was left at home, unfortunately the stainless bendy pipe fractured. I was only using one mount for the scale pipe instead of two, easily sorted.

 

If any of you can help me source some tubing? Somebody suggested using some hi temp silicone tube between the sections. I am looking for some 16mm ID but the model shops all have zero stock, is regular 200C stuff good enough?

 

Cheers

Danny

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Thx Cymaz, just before you posted I googled the part number I found on the Nexus site, "LOGIC Hi-Temp Silicone 15mm ID x 150mm 350øC L-LST15HT" They have zero stock. But I got a hit on Ebay, one is ordered for just under £6, free post.

 

Thanks for you help chaps.

 

If you are curious to hear that home spun exhaust, well before it broke lol, today's session is here:

 

Any advice on tuning the Saito would be gratefully accepted. High end is fairly easy, highest revs with a tacho then richen a quarter turn to play safe, but how do you set low end?

 

two 16oz tank-fuls so far

 

Cheers

Danny

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Here's what I do...

 

Set top end

Let engine idle for a few secs

Open throttle briskly

If the engine pops and splutters, it's too rich - close the low end slightly (these changes need to be really small)

If the engine sort of revs but gets there slowly, it's too lean - open the bottom end slightly

Repeat until happy

If it transitions rapidly, no struggling, no pops and splutters - congrats!.

 

If it's fresh out of the box, or you've done the 'blow down some fuel tube' exercise, it won't be far off.

 

If it's a new engine, be prepared to repeat after a few flights.

 

Other views and methods are available.

 

P.S. I'm assuming you know the location of the screw to adjust.....

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Thanks Cymaz, very helpful 🙂 I have been using a bounce/rebound start and its been working very well. In fact I read on the Saito manual you should never flick through compression. So seeing this guy bouncing the prop confirmed that.

 

I have heard I should run 2 or 3 gallons through the engine, sounds like a lot to me, thoughts?

 

Cheers

Danny

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Danny, In hindsight it looks like the flexy pipe which was rigidly anchored at both ends, may have contributed to its early failure.

If you make up a new pipe which is rigidly fixed at the engine end and only pushes over the stub on the exhaust manifold (with a short length of silicon on top to act as a seal) you may find that the flexy pipe does not have to work so hard.

With the fracture occurring only a couple of inches from the engine, the exhaust is still going to be hot at that spot and I doubt silicon will last very long, possibly two flights!

You could try inserting a brass sleeve inside the flexy before sliding on the silicon, this will prevent the hot gasses from directly impinging on the silicon, conduct the heat away from the upstream bit of flexy and will also act as a support. Without a support, the silicon is going to take all the load just where its hottest so I'm sceptical that it will afford any kind of long term solution.

Unfortunately while silicon will stand up to 2 strokes, a 4 stroke exhaust is much hotter, although it cools quickly along the length of the pipe, but where yours has broken, the heat is possibly at the limit of what silicon can withstand.

Yes, put a tiny drop of threadlock on the screws, but don't over do it if you want to remove the same screws in the future.

One final thought, a wooden prop will dampen engine vibration better than a glass filled one.         

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Thanks John, wise words indeed. I will cut the pipe closer to the scale exhaust, and use a new longer piece from the engine, and a silicone joiner. That way I don't need to do any silver soldering.

Yes I am using a wooden prop, I think the low end mixture was too rich, causing the excessive vibrations. Looking at videos of other 4 strokes running they aren't as violent as this.

Being a novice at these things doesn't help. I am quite looking forward to another go. Will have to subject the neighbours to ten minutes 🙂

 

Cheers

Danny

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  • 2 weeks later...

Okay its been three weeks and the silicone tube still hasn't arrived. Most shops had no stock, the one I ordered from clearly didn't have what they stated, or couldn't find it.

Anyway got fed up with waiting and silver soldered the pipe back together. This time I added two rigid mounting plates, these are bolted to the bulkhead and an additional ply plate. I hope this will be sufficient.

 

852.thumb.jpg.a96e685b44260fe5c24ce2c7ad07b7d1.jpg

 

854.thumb.jpg.31b43cf981a2dd54ede8a1ca534f6911.jpg

 

Want to get on and fly it!

 

Cheers

Danny

 

 

Edited by Danny Fenton
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