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DH86 Refurbishment


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I’m running out of storage space. As much as I like building (and dislike repairing) the logical solution to not being able to fit another largish model into my shed, was to refurbish a seldom used one that was already in there. I’ve convinced myself that a restoration project is more interesting than repairs so with the completion of a little Aeronca Sedan (it won’t take up much space, honest) the way has been paved to bring my DH86 Express airliner up to date. Hopefully to sustain me will be the prospect of an attractive new colour scheme, but more of this later.

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The model was built in 1995/6 and whilst not flown an awful lot in that time, has logged 102 flights without any major damage. Its powered by 4 small Lasers, unfortunately glow fuel seepage and the years have taken their toll as some of the wood joints are failing, the covering (Solartex) coming away and the paint softening. In the 1990s I didn’t know of a genuinely fuel-proof paint, I would typically use modelling enamels overcoated with a clear ‘fuel-proofer’ which of course wasn’t.

On a flight in 2013 it had a double engine failure on one side and the best I could do was hold the wings as level as possible until it flopped into a muddy field adjacent to the strip. The abrupt halt bent the undercarriage, sprung the wing joiners and pulled out some rigging. I did some hasty repairs to enable it to keep flying but they were more temporary than permanent and although it had 14 further flights, I was not really happy with the structural integrity, especially as a couple of rigging anchors pulled out on disassembly after a flying session, consequently it hasn’t flown since 2015.

In truth, I’ve been saying to myself for years that the model really needed a birthday. It has ailerons on the upper and lower wings but only a servo in each lower wing driving the upper aileron via a hidden bellcrank and pushrod arrangement. This was never ideal as each upper aileron only had half the movement of the lower one. Servos have become so much cheaper, smaller and powerful in the intervening 25 years so this time it will be one servo per aileron. The original 35MHz Futaba receiver was replaced quite a few ago when I converted to Weatronic equipment, but with the demise of that company I’ve reconverted to Futaba, so again it will be an opportunity to fit an up to date 2.4 receiver, or perhaps two, renew the servo wiring and battery supplies.

The model was designed with the rigging as a functional requirement needed to hold the detachable outer sections of the wings rigidly. Some of the anchor points probably weren’t up to the strain imposed on them as I’ve had to reglue them; not a particularly successful job with them being buried in the wings. Now when all the covering is stripped off there will be the opportunity to strengthen the anchorages.

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The engines, while old and not supported by Laser Engines any more, have only been in this model so should not be at the end of their lives through wear alone. The first job is to remove them from the model, clean the carbs, make sure the bearings were free and spinning easily in fresh oil, reset the tappets, replace the glow plugs and check that they would still run ok. I had told myself that if the engines were not showing their usual zest and reliability then there wouldn’t be much point in continuing with the refurbishment. Only one engine has been run so far and its well down on compression, so that must be remedied if possible.

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The start of the restoration is in proving the engines. The outer wings have been given attention first simply as these were the easiest to extract from the shed.

One Laser 50 has been disassembled and as hoped there doesn't appear to be much wear apparent.

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The loss of compression looks to be a badly passing exhaust valve. To test, fuel was poured into the inlet and exhaust ports. While the inlet held fine, fuel quickly appeared on the other side of the exhaust valve.

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The problem now is going to be removing the collets in order to lap in the exhaust valve. I think I'm going to have to devise a better method of pressing down the valve spring retainer than fingers alone. Removing the collets will be one thing, getting them back will be another. My greatest fear here is losing a collet as that will jeopardise the whole project. Does anyone know if valve spring compressors this small are available?

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I managed to remove the valve, lap the seats and refit without loss of a a collet, it wouldn't have been successful without the plastic bag trick as the second collet sprung a few times. With my clumsy and aging hands trying to refit a valve in a 50 size engine was no easy task.

The removed valve shows dirty faces and a pitted seat

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A few minutes with some fine grinding paste followed up with Autosol polish and its starting to look better

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A re-test by pouring fuel into the exhaust port showed the seal is now good but on reassembly the compression is still down when compared with the other engine. Thinking that I had found the problem, I didn't give a close inspection of the rings, a disassembly is again required.

 

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The removed piston shows signs of gasses passing by the rings. The upper ring looks to have a smaller gap than the lower. In discussing with Jon  at Laser Engines, he suggests that the ring has lost some of its outward spring force and that a very fast run on a small prop may help rejuvenate it. 

I've now tried this and indeed the compression is now much improved, unfortunately I couldn't get the engine to lean out properly, it was popping as if too rich but screwing in the main needle would make the engine cut out without getting rid of the 8 stroking, so the mystery continues.    

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The engine has been run again on a 12 x 6 GRP prop , previously on an 11 x 5 wood and both times the engine peaks at 7700, not good. I've also discovered that there is a clicking noise in the engine which occurs at exactly the same times as the rich popping sound. This may be the rings being pressurised by the combustion gases and expanding to the sides of the liner. 

There is an internet seller advertising reproduction rings for a Laser 50, so I've ordered two, they can't be any worse than the ones here, luckily the old rings came out without breaking so all is not lost!

As I can't do any more with the engine until the new rings arrive, I thought I'd start to strip the covering from the outer wings. Here's what a covered and and uncovered wing looks like. The complicated bellcrank arrangement was to drive both upper and lower alierons from a servo mounted in the outer engine nacelle. Installing bellcranks like this seems laughable now but was a common method of connecting a servo to a control surface in the mid 90s. Small servos installed in the same location on the upper and lower wings should be a fairly simple job.

More by luck than foresight, the hinges have removeable pins so detaching the aileron was straightforward.       

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7700 is dead slow, but the misfiring and inability to lean the engine fully is the likely cause and their may not be a mechanical problem. 

Bad fuel and/or plug can cause these misfiring issues and i have seen it myself many times when my testing glow plugs are finished. Its most apparent on the 70 and 120 as these have the lowest compression ratio of our range, the 50 is lower still so i would probably treat it to a new plug if you have not done so already. 

The clicking sound can be caused by the cam going over centre and is nothing to worry about, but a clacking sound as a valve opens can point to excessive clearance. It would have to be really bad for it to rev that slow though and both are only audible when the engine is turned slowly by hand. If the clicking sound comes when the engine is running and is more of a harsh crackle that is likely to be detonation and again points to plug or fuel. 

Have you/will you change the bearings? if not i recommend you do as castor wont have done them any favours and they may fail a little way down the line. Also if you did do the bearings were the cams touched at all?

Given the relatively light use the engines have seen they do look a little deep fried. I suspect they have run pretty hot so i would recommend some time is spent looking at the cooling to see if it can be improved. 

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Jon,

Thanks for the assistance. The clicking sound is heard when the engine is running and only occurs when the engine 'pops' as happens if the engine is too rich and only at the top end. I'll change the plug and try again. I don't not suspect the fuel, the other engine ran a couple of days ago on the same fuel and seemed fine, that one peaked at 8800 on a 12 x 6.

No, I've haven't changed the bearings. are these available for the 50s?

As part of the rebuild, I can certainly baffle the engines, they sit under fibreglass cowls with quite a small inlet (de Havilland look) but with a sizeable outlet, however making and fitting a fibreglass shroud to enclose the cylinder and head is a good idea.

    

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that outlet might be acting as an inlet looking at its position. To check, look at one of the cowls head on as the air would see it. If you can see that opening on the under side its working as an inlet and the air is stagnating in the cowl. A small lip on the hole will help sort it out. 

bearings are standard sizes. 608-2rs with rear seal removed for the front, 6001 open for main, 608 open or 2z for pinion drive. If you want to replace them on a budget the simplybearings.co.uk cheapo bearings work fine. They wont last as long as a branded bearing, but as most model engine bearings rust out or gum up before they wear its not really a concern

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I don't think the engines are quite as cooked as the earlier pictures suggest. The previous pictures were taken in my workroom which has 'warm white' lighting,  this has perhaps given the distinct brown appearance. The picture below was taken outside, in natural light and the staining doesn't look quite so dramatic.

I take the point that even though the engines generally run at half throttle, there could be a insufficient flow through the cowls so I'll look at fitting dams at the front of the outlet holes as well as shrouds for the cylinders.

I agree also that it would be prudent to replace the bearings, they have had to sit in contaminated oil for over 25 years now, castor and synthetic, and while being aware that Laser make fine engines but perhaps this is asking a bit much.......how long was the guarantee!   

       

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they do look a little better in that light i must say, the exhausts look pretty fried though. 

Actually, next time you run the engine if its still rubbish take the exhaust off and try it again. If its blocked with castor it can cripple the performance with the excessive backpressure. 

When it comes to cooling, strangely an engine toodling around at half throttle can over heat more readily than a one zipping about doing aerobatics at high power. A friend lost his piper cub to this as his low/slow wafting around at 1/3 to 1/2 throttle gave a constant heat output from the engine but little in the way of cooling airflow. His mustang on the other hand (with the same transplanted engine) although running high power has ample airflow due to its speed and due to its aerobatic nature cools well on loop recovery when the throttle is back but the speed is still very high. 

I notice this myself on my La7 as it never over heats in normal warbird aerobatic flight but can start to suffer at shows when i am more limited to flat/constant throttle flying as im trying to find space within the gaggle of aircraft in the slot with me. 

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In anticipation that the engines can be sorted out in the near future with new bearings and rings, efforts have been redirected at the outer wings.

All the Solartex has been stripped off to inspect the airframe. There are a few patches of cracked sheeting and broken glue joints though they should be simple to rectify. The three main tasks for each wing are:

  • Install a servo bay
  • replace the rigging supports
  • Replace the wing jointing tubes

The easiest job will be installing servo bays so I've started on that. First thoughts were that the snake outers would make handy tubes for the servo extension leads but the the diameter is too small, so its out with the outers. Using a piece of sharpened brass tube which would slide over the outer it was a quick job to cut though the balsa and old glue to leave a straight row of holes for the cable.  

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It turned out quite fiddly to fit in the servo but its done now so just have to copy the procedure for the other three wings. The hatch, which is a piece of 1/16 sheet glued to 1/64 ply, will be covered in 50 or 80gms fibreglass cloth to strengthen, stabilise and provide a flat finish for painting.  

For reasons which don't stand scrutiny I'm prejudiced against servos which don't begin with Futaba, Hitec or Savox. The single advantage for me of Hitec servos is that they can be easily reversed (with a programmer) so there are no worries about how to hook them up. This model will probably have all the aileron servos driven from one channel so reversing, if necessary, can't be done in the transmitter alone.  

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Danny, thanks for the kind words.

Any of the Hitec digital servos have the same programmable features, among the things that can be set within each servo are: reversing, end point, centre, failsafe position, servo speed. These things can be carried out with modern transmitters for the respective channels but if using multiple servos connected to one channel, then being able to set them within a servo can prove especially useful. You do need to buy a programmer though, I've had mine (HFP-20) for probably 10 years now. The current model is the HFP-30 which, as an example, is being sold by Steve Webb models for £72. There is a cheaper Hitec programmer, about £20, its not as straightforward to use but achieves the same results.    

 

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All 4 servo bays have now been completed and so its onto the next jobs - sorting out the wing joining and the rigging supports. Both jobs need to be carried out in unison with the fuselage so as it was a decent day after a cold and snowy week it was time to get to the darkest corner of the shed.

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The fuselage, grateful I'm sure for being brought into the warmth, doesn't look too bad from this angle although there is much paint softening in the engine bays.

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The covering is coming away and a crease appears right in the centre when the fuselage is picked up by the wing stubs. I bet the main spar is cracked, all will be revealed once the covering is removed.

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The tail looks ok so might get away with a good clean, rub down and repaint.....we'll see.

 Before getting on with the airframe, the very first job is to remove the engines and make sure everything can be freed, right now they are locked up!

     

Edited by John Rickett 102
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Are those inner engines 62's John? They look like they could be 70's as they have vertical plugs. 

If they are 70's i can service them here but if they are 62's i cant...

*Removes Laser engines work hat and grabs weekend modeller hat*

however, i could look at the 50's and 62's myself if you like? I dont normally offer as experience has told me that people do not see me doing the work at the weekend as something different to me doing the work at Laser during the week. it then ends up being an argument if things go wrong or it takes an age for me to get round to it as i repair/work on engines all week so any weekend jobs are done when they suit me. 

Provided you are happy with the above and that you are dealing with me as a fellow modeller and not Laser directly then i am happy to take a look at the engines as its a great looking project and i dont mind lending a hand on this occasion to get the model flying again. 

And before i am swamped with requests to rebuild dead laser engines the answer, im afraid, is no. I am offering my help here as the condition of the engines makes it look like they are actually worth trying to save (most are utterly ruined). I imagine the cost of replacing all 4 engines is prohibitive and as the project looks like it hinges on the engines im happy to help on this occasion to try a make them reliable, even if they have lost their edge when it comes to power. As mentioned before, i do this stuff all week long and a queue of customers waiting for me at home is not my idea of a fun weekend. A few one off's are ok, but i dont need another full time job. I hope you guys can understand. 

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21 minutes ago, Jon - Laser Engines said:

 

Provided you are happy with the above and that you are dealing with me as a fellow modeller and not Laser directly then i am happy to take a look at the engines as its a great looking project and i dont mind lending a hand on this occasion to get the model flying again. 

 

You Jon, are what forums are all about! Well played sir. 

I agree, it's a fabulous model and a very worthy cause. It would be a shame for it to be written off.

Now, anyone want to fix my seized OS15 max so I can resurrect some oil-soaked wreck with no historical or modelling significance?

Thought not...

?

 

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9 minutes ago, Graham Davies 3 said:

You Jon, are what forums are all about! Well played sir. 

I agree, it's a fabulous model and a very worthy cause. It would be a shame for it to be written off.

Now, anyone want to fix my seized OS15 max so I can resurrect some oil-soaked wreck with no historical or modelling significance?

Thought not...

?

 

What happened to the 15? i might have a 15fp i could part with if you cant fine any spares. 

And thanks for the comments. I had to ponder for a while but Im happy to help John out with this as it just looks awesome and from the calls we have had it was clear the engines were causing him a little concern so its the easiest way to sort it all out and let him focus on the rebuild. 

Edited by Jon - Laser Engines
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1 hour ago, Jon - Laser Engines said:

What happened to the 15? i might have a 15fp i could part with if you cant fine any spares. 

And thanks for the comments. I had to ponder for a while but Im happy to help John out with this as it just looks awesome and from the calls we have had it was clear the engines were causing him a little concern so its the easiest way to sort it all out and let him focus on the rebuild. 

I was joking Jon! Kind of you to offer but it's perfectly preserved by the plug of mud that surrounds it since I dug it out of the hole it made!

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Jon, Thanks for the kind offer to see what you can do with the engines. I will accept (I'd be daft not to), you are the expert in the field of Lasers so will know what should be done and what spares are available.

The inner engines are 62s. Its been so long that I looked at them that I had forgotten they had vertical plugs (and the clamp arrangement for the inlets and exhausts). I bought all 4 engines at the same time, or very nearly, especially for this project - it was about 1994 when I was planning it, so I can't explain why the 50s are the earlier version although I seem to remember that the 62s had just replaced the 61s so perhaps these were the first to be upgraded.

All of that is of no consequence, I'm only concerned that the engines run reliably. The model was a delight to fly and needed little power, in fact apart from full power checks prior to take off, I don't think the engines have ever been run flat out in the air, the model simply didn't need it. All up weight when first completed was 24lbs, it may have gained a bit since then, however the engines even if not giving of their best will still be more than adequate (as Mr Rolls apparently used to say about his cars).

The engines have now been removed and freed off. A combination of carb cleaner, after-run oil and fuel quickly released the stuck cranks and carbs and there is even some compression. I won't do anything further with them apart from packing them up and sending them to you - before you retract the offer.             

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No problem John, get them away whenever you can.

The chronology is interesting though as...ok this is going to get complicated. 

The 61 was the first engine we made in 1982/3 ish. 

in 1984 the 75 was introduced using the stroke of the 61 with a new larger bore

In 1986 the 45 was introduced with a totally new bore and stroke in a smaller crankcase. 

in 1988 the 45 was replaced by the 50 using the stroke of the 45 and the bore of the 61 in the 45 crankcase

in 1989 the 62 was introduced using the stroke of the 45 and the bore of the 75 in the 45 crankcase

in 1990 the 50 was discontinued 

in 1992 the 62 was discontinued and replaced by the 70 which used the stroke of the 45 and a new bore size that we still use today, in the 45 crankcase. 

 

So, if your engines were bought in 1994 they should, in theory, all be 70's with vertical plugs but clearly they are not. Did Neil put together a bunch of specials for you? some of the very late 62's did have vertical plugs but not many and they were all gone by 94. 

I feel like an archaeologist going through history, and we havent even got to unearthing GD3's OS15 yet!

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