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DH86 Refurbishment


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Thanks for the compliments, its well appreciated.

The little engines are bedding in nicely, even after only about 1/2 hours running, thanks to Jon Harper for getting them back to health.

Here's a shaky bit of video, in my haste I had fitted a nearly flat battery into the camera instead of a charged one, so the camera cut out after a minute of so. The sound change when the exhaust blew can be heard as it comes overhead.    

Here's one more of Mike Mennell's photos from a different angle - now I really ought to get those homemade exhausts fixed so that flying can continue.

 

MM WMFC3.jpg

 

Edited by John Rickett 102
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Hopefully, the problem with the inner engine exhausts has now been fixed. This picture shows the gap between the exhaust flexy pipe and the silencer stub; exhaust heat directly onto the silicon tube bridging the gap soon destroys it and if the flexy touches the tank, that too is melted.

 

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The simple solution was a short length of K&S thin-walled brass tubing, the silicon now forms a bit of a seal between the flexy pipe (which is slightly smaller than the silencer stub) and the brass tube but does not come into direct contact with the hot gasses. With there now being a metal bridge, there should be improved heat conduction. As an additional safety measure a piece of thin aluminium sheet has been glued under each tank.

 

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The 3rd flight was captured on video, still shaky in parts I’m afraid, but proves that the model has successfully flown again.

The day was fine enough though a little choppy leading to the model being buffeted at times. The speed was kept slightly on the high side, I’m sure this can be reduced in better conditions to create a greater sense of realism but the greater aileron movement afforded by a one-servo-per-aileron installation showed that the model can now cope with a bit of turbulence.

 

 

The little Lasers started easily and performed faultlessly, once again thanks to Jon Harper at Laser Engines for working his magic.

         

From building three other deHavilland twins and multis, I’ve found there is a slight disadvantage (for the modeller) to their designs. The adoption of a single fin/rudder puts it out of the propwash. With the DH86 the outer engines are a considerable distance from the centreline, less than perfect engine synchronisation causes a tendency to swing on the take-off run which is difficult to correct with rudder alone. To counter this, each engine servo has its own channel with the outer engines slaved to the rudder. Full rudder output will advance the opposite outer engine by about 10%, ie right rudder will open the throttle of the left outer engine. The right outer engine in this example is not reduced by 10%; modern radios make this mixing very easy as Side A and Side B, in Futaba parlance, can be programmed independently.

 

The cost of the restoration hasn’t been kept but it will have been a few hundred pounds, cheaper and quicker than starting from scratch I suppose. Cynically it could be described as mutton dressed as lamb, however I’m pleased with the final result and hope now that it has proved itself. it gets flown a bit more often than when first built.

I’ll admit that it has been a more enjoyable project than I first thought, given that I reluctantly carry out repairs and this restoration was essentially just that. In building a model from scratch most are effectively prototypes as a subsequent one doesn’t usually get built to correct the inadequacies of the original, so a first for me on that score.     

 

This was the second of a two-model refurbishment I had tasked myself to do (the other was an equally old DB SE5) to prevent unairworthy models from taking up valuable space in my shed. In truth that was only buying a bit of time as now they are complete, I feel the need to tick off another attractive subject on an ever growing list. I’d better sweep out the workshop and worry how I’m going to squeeze another project into the shed.

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Great model there John and you do yourself a disservice to say its mutton dressed as lamb. Really interesting to see the techniques used to overcome the various problems and I can only dream of ending up with a model looking as good and flies as well as that.

 

Top man and I hope to be able to see it flying some day soon.  

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Loved the video John, absolutely a delight to watch.

If I may suggest? Few layers of tightly wound glass cloth and end stuck on with silicone glue will serve as an excellent insulator in the areas where there is a risk of the bendy pipes coming in contact with the tank. You don't need to wrap the full length. Just the areas where the clearance is low and risk of touching is high. 

I am sure we will see more videos of the magnificent machine from the shows in coming days?        

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Thanks for the compliments and pleased you liked the refurbishment blog. This is the first time I've attempted to record a build or repair sequence though might try another, I see Mr Fenton's efforts are popular and if it encourages people to get building, or help to stem the decline in building which is more to the point, it can't be a bad thing.

 

Thanks for the tip regarding the use of fibreglass Manish, of course that won't burn so will be stored in the grey matter for possible use.       

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  • 2 weeks later...

I’ve had five more flights now, unfortunately the left outer has stopped on two of them. On each occasion, the throttles were closed and the model glided in.

The main needle needed opening half a turn prior to the first flight of the day, which seemed odd as I hadn’t touched the needles after the previous flights. All ok after that until the most recent flight when the left outer stopped again, and glided in narrowly missing a tree! At home, after a blast of carb cleaner, I found some debris between the main needle and the spray bar.

 

When an outer cuts, its impossible to maintain revs and turn into the cut engine, (when an inner cuts it’s not nearly as dramatic) so up to now the only course of action is to go to idle and glide down.

 

The throttles are all operated from the main throttle stick through individual servos each having their own channel. My hoped-for solution was to be able to chop both outers (so it doesn’t matter which outer stops) when necessary, leaving the inners running, I’m sure the model will maintain height on the outers alone.

 

The Futaba set I’m using is a 16SZ which has ‘conditions’ settable for various flight modes. I wanted to be able to have a condition where the left slider would activate a second condition and become the throttles for engines 1 & 4. It doesn’t appear as if that can be done, once a function is assigned to a stick or switch, it stays that way – I thought that within a condition, everything could be altered just as in setting up a new model memory.

 

I’ve now ended up with assigning an unused switch as a dual rate switch and set the alternate rate, using the ‘point’ graph in the AFR menu to bring throttles 1 & 4 to idle regardless of throttle position. This will have a similar affect as the first method but I still find it odd that not everything is re-assignable under a new condition. As an experiment, I tried altering the servo speed under the second condition – that doesn’t work either, whatever is set for the first condition stays that way…unless anyone has better knowledge of the radio? I did manage to get the conditions renamed as Normal & Idle 1&4 which will read on the main display and be switched by the dual rate switch but am a bit confused as to why only certain parameters can be changed within a flight condition.            

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For anyone who has grappled with 'conditions' in modern Futaba transmitters, the dawn has just broken for me - in any 'condition' that is going to be changed from the Normal (Condition 1) setting, the Group/Single setting has to be set to Single, the default is Group. This now make sense in that the changed condition will only be applicable to that one (single) condition.

The engine controls are now set so that the inners and outers all work together in 'Normal', or by operating a switch, will bring the outers to idle regardless of the throttle stick position. I've also set up a 3rd condition which will stop all four engines when the throttle cut switch is operated. It does away with using dual-rates to achieve the same end, there's no difference in practice but to me its a neater and more logical way of doing things.

I'm getting to be a fan of the SX radios, the conditions facility is a powerful provision.          

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