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104 inch B-17 Identification

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Jon - Laser Engines20/03/2019 14:27:05
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If you want true redundancy then the twin rx method is the best way. Dual batteries are great in theory but you still have a single point failure in the rx.

Twin rx's with a battery each is a better way to go and as Dick rightly points out you can split things. I would go right aileron left elevator, the two outboard engines and rudder, and gear on one rx and everything else on the other.

That way you have throttle control in both cases, but if you loose the inner engines and have to run them out of fuel you might need rudder to balance the outers as they have more yaw impact.

You also get undercarriage or flaps for landing.

In all honesty though i would go for a normal single rx setup and just make sure its all good stuff. Most failures come from poor maintenance and while there are unexpected failures they are rare.

Just for info, i have lost 3 models to radio gear failure in 25 years of flying. Two were swash servo failures on my heli, and one was a rx failure on a galaxy mustang. None of these were predictable and only the rx failure could have been covered off by a dual rx system.

Martin Harris20/03/2019 14:39:25
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Posted by Jon - Laser Engines on 20/03/2019 08:44:42:

In the instructions for the herc it recommended to set up the throttles on two channels so the inner and outer engines could be controlled individually. The theory was that if one engine stopped you would kill its best mate. This struck me as daft as you are turning the loss of one engine into the loss of two. If another drops you are really screwed! The only time i would consider killing the engines on a 4 engine model it would be if i lost two engines on one side. I would try and keep it flying, but i would be prepared to cut it.

Maybe they meant that you could idle the remaining engine rather than kill it, preserving balance while keeping the good engine in reserve?

I think Chris's question really pertains to the number of functions available from his transmitter. In this case, I could see a case for a co-pilot operating flaps, undercarriage, lights, bells and whistles via a separate transmitter.

From experience of twins, it is good to have isolation for the throttles for starting and ground running - you don't really want all the engines roaring away while you're making adjustments on one of them. Sadly, Chris's transmitter is unlikely to be able to accommodate that even if the non-flight functions are given to the co-pilot.

Jon - Laser Engines20/03/2019 15:43:22
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Idle or off, its still turning a 4 engine model into a twin when it could be a triple. Is just a strange thing to do. Why give the power away? All that is likely to happen is you will slow down, try to bring the sleeping engine back into play and then crash as you are too slow to control the yaw by that point. Its better to leave it.

I have never used individual channels on my twins, and i personally have never found an occasion where it would be an advantage as any problem engines get run in isolation. Im not saying its wrong to mix them, i just have not found the need.

As you rightly point out though mixing 4 is likely to be a problem!

On the Herc i started each engine and set it up. Then set up its best mate and so on. I then started all 4 and went to fly. I didnt bother checking revs and made no effort to match them in any way other than the original mechanical setup. Its just not as important as on a twin, and both the herc and B17 have massive fins/rudders so directional stability is likely to be very good.

At 100 inch i think its a bit small for copiloting to be considered. 20 feet maybe  

Edited By Jon - Laser Engines on 20/03/2019 15:44:07

Chris Walby20/03/2019 18:29:43
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Thanks Dick, that answers my question + everyone else who has contributed to the slightly wider discussion.

Benefit of dual RX, failure of one RX or failure of a servo (short circuit) shouldn't take the other RX out

Down side, more complexity with split wiring and extra pre flight checks

Everything else looks like a single point of failure will result in an inconvenience (loss of one engine) or uncontrolled loss of the model (TX failure).

This just leaves me with a personal question, would I spot two engines not responding (full, partial or dead) one aileron not responding and partial elevator control before I intervene and save it. For me I would go for single RX and just make sure everything is double checked before flying (PPPPPP wink)

Dave can have the final word as he'll get to do the maiden, just now I fully understand the rational for twin RX

Thanks for everyone's time.

PS until you can have full duplex redundancy, dual TX all the way through to a fully controllable model following any single point of failure I am not convinced doubling up some parts add benefit without complexity/greater risk of failure.

Think I'll go and have a look at the LMA's +20kg requirements and see how they deal with it, but for this its OTT.

Edited By Chris Walby on 20/03/2019 18:40:00

Jon - Laser Engines20/03/2019 21:23:12
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183 photos

I helped out with an over 20kg spitfire. Dual Rx's with a battery each was deemed totally acceptable.

Andy Meade21/03/2019 13:14:18
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Posted by Chris Walby on 20/03/2019 18:29:43:

PS until you can have full duplex redundancy, dual TX all the way through to a fully controllable model following any single point of failure I am not convinced doubling up some parts add benefit without complexity/greater risk of failure.

Edited By Chris Walby on 20/03/2019 18:40:00

For sure,dual TX etc is very belt and braces, but the common points of failure are all in the air (from BMFA's own stats I think) and it's usually the battery or switch that'll go first - the cheapest components typically.

Martin Harris21/03/2019 13:27:21
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...and the cheapest and easiest to duplicate!

Jon - Laser Engines21/03/2019 14:33:14
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183 photos
Posted by Andy Meade on 21/03/2019 13:14:18:
Posted by Chris Walby on 20/03/2019 18:29:43:

PS until you can have full duplex redundancy, dual TX all the way through to a fully controllable model following any single point of failure I am not convinced doubling up some parts add benefit without complexity/greater risk of failure.

Edited By Chris Walby on 20/03/2019 18:40:00

For sure,dual TX etc is very belt and braces, but the common points of failure are all in the air (from BMFA's own stats I think) and it's usually the battery or switch that'll go first - the cheapest components typically.

And lets not forget that the most common single point of failure in model flying is the bloke holding the tranny

Tim Flyer21/03/2019 15:45:38
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From observation of failures at our club I would also concur that non pilot control error crashes are mostly caused by battery and switch related problems . It has been very rare to see receiver failure at our club . The common errors are failing to monitor or charge a battery properly, and failure of connection to the switch or receiver(sometimes due to user failure). Having two batteries and two switches reduces the chances of this. I use twin batteries where practical on my 25-30cc planes. I can fully understand those using twin receivers and twin batteries on the big models. On my 10cc sized planes I just make sure I choose good quality switches and wires and regularly check them. 

Edited By Tim Flyer on 21/03/2019 15:51:14

Chris Walby21/03/2019 22:46:25
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1024 forum posts
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The only advantage with twin RX over twin battery and smart switch I can see is if one servo shorts out and drags the RX voltage down or flattens the batteries then the other will keep going

I was advised this very rare and a suitably large RX battery would "sort it out" by just burning the servo out.

Back to my comment, and this is only directed at me! I am not sure I could fly a model with 2 of 4 engines unresponsive, one aileron, one elevator and 50/50 chance of no rudder. So my view is the same as Jon's look after the battery and switch have one RX and keep it simple.

Chris Walby21/03/2019 22:50:16
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The only advantage with twin RX over twin battery and smart switch I can see is if one servo shorts out and drags the RX voltage down or flattens the batteries then the other will keep going

I was advised this very rare and a suitably large RX battery would "sort it out" by just burning the servo out.

Back to my comment, and this is only directed at me! I am not sure I could fly a model with 2 of 4 engines unresponsive, one aileron, one elevator and 50/50 chance of no rudder. So my view is the same as Jon's look after the battery and switch have one RX and keep it simple.

Jon - Laser Engines22/03/2019 08:30:24
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183 photos

I have not flown a model with half of its controls not working so cannot comment directly on how easy it is or is not. I suspect the answer to that question will come from what position the controls on the dead rx came to rest at.

The only experiences i have had are two aileron failures and an elevator failure. The first aileron failure was when i was about 15. a simple linkage issue where a ball link had lost its grip and fell off. The 2nd aileron issue was on my 85 inch flying test bench where the hinge pins from 4 of the 5 aileron hinges on the right side all fell out. This left the aileron hanging on by the linkage and one hinge. Not very useful. I was astonished to see this as the model had been flying for years and all of the hinge pins were there before i took off. I know because i checked!

The elevator failure was a 36 inch hand launch model where vibration broke the 'high quality' hobby king rod extender. I dont use those any more.

In any case, all 3 models were landed safely and without damage. The first was easy as it just rolled slower, the 2nd needed a little more care as the wonkey aileron was making a right mess of the lift on that wing, and the elevator failure came with a pinch of luck as the model was out of trim nose up following the failure and this allowed full control using the throttle to control the pitch. it was basic but got the job done.

What im trying to point out is that models remain quite controllable even with damage/failures. The trick is to not worry about it and just fly what you have got. In the case of the B17, say you lost the two inboard engines, an elevator, an aileron and the gear you can fly around quite happily with the outboard engines throttled back while you wait for the inners to run out of fuel. Once they do, its a simple matter of landing as normal.

Having a plan already in your head to cover these eventualities is key to success. If it takes you by surprise you may waste time in a panic trying to work out what is going on and what to do about it. In that time flying the model may become a back burner project in your head as your focus is on fault finding. That is where things start to get out of hand. Forget the fault finding, just fly the plane.

With all that said, i would still just go for a single battery/rx setup. I work on the basis that something as small as a fun fighter spitfire can easily kill someone so a model like this is just as dangerous. i always put the same level of care into the setup of a small model as i do a big one.

Edited By Jon - Laser Engines on 22/03/2019 08:38:18

Chris Walby24/03/2019 20:55:57
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1024 forum posts
237 photos

I got a strange look this morning after tidying the kitchen table ad then heading off to see a club mate, especially as I took her car!

20190324_130552.jpg

All went well extracting the fuselage from the top of the garage roof and loading it up. Nice gentle drive home and a bit of staggering around and its home safe and sound although it was going to be tricky hiding it in the kitchen!

Couldn’t resist having a look in the bomb bay and just checking the wings fit. Then more staging around getting it back through the back door and into the garage.

wink

20190324_141053.jpg

Dave has a couple 8 channel RX's and the conscious is that we may need a bit of weight up "front" so that's looking like the plan at this time.

Best I get a move on and glass/paint those wings so we can see where the C of G is.

Chris Walby13/04/2019 22:03:45
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1024 forum posts
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Not ready for "Forum Members new models "let see them" as we managed C of G measurement and engine testing today between the rain, sun, wind and hale!

Good news is it goes through the clubhouse door to keep the rain off, bad news you can't turn it over as the wingspan is greater than the ceiling height!

20190413_113052.jpg

Interesting the only prop interaction is between No3 and the side of the fuselage as its closest.

Rather long list of things to sort, but only to be expected as Dave and myself didn't build it

  • Three blade props as 11 ii the max diameter that can be fitted
  • RX battery bay cut door and wire back to bomb bay
  • Find and fix air leak
  • Relocate retract valve and replace servo
  • Replace throttle servo with stripped gears
  • Reset all the throttle end points
  • Tidy up wiring in bomb bay
  • Re-pin the elevators with an additional hinge
  • Paint cowls

Overall a good day and hopefully nothing too major to do before more engine tuning and some taxi tests.

Then we just need either a southerly or easterly and Dave said he'll go for it !

Martin Harris24/04/2019 13:09:38
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8945 forum posts
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If its any use, I have the authoritive information from the pen keyboard of the late owner, found in an old club newsletter:

So what next? let’s go for a multi engine model, as everyone builds a Lancaster let’s go for something different why not a B17 Flying Fortress then the problems started. I could not find a kit in this country so I bought a set of plans but after discussions with Mick Exxxx we came to the conclusion that the model could not be built.

I then consulted the internet and found a number of companies in the states that have the kits available, one such was Wingspan Models in Atlanta, at first I looked at the plans which were on 7 large sheets. At first I thought I would buy the plans and cut the parts out myself but as this model has 103 inch wingspan I found that you needed 436 parts to complete the model, and I would find that a bit difficult as all parts are made from lite ply. So I ordered a short kit from the states

As a bit of background to the last part, the builder, Brian, did have slightly more challenges to face than us "normal" modellers - he lost both a leg and the use of his right (dominant) arm in a motorcycle accident in the late 50s/early 60s when he was 19! Brian started RC aeromodelling in his 70s, learnt to fly with a modified single handed radio and was a prolific builder, spending as much time making jigs to help him build than actually sticking things together. Although there were a few specialist jobs that some of us assisted him with, his models were kit and plan built almost entirely by himself.

One interesting thing is that Wingspan don't list the 103" version - has it been superseded or did they only offer the plan and part kit on demand?  It might be worth an email to find out more?

Edited By Martin Harris on 24/04/2019 13:26:20

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