Majestic Major (electric) build.

[Former Member]

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[Bob Cotsford]

Sullivan also do sets with metric fittings, as I recall they used to be gold adaptor/clevis for imperial threads and silver for metric. I use the Gold'n'rods a lot but I've had a few where the pins hadn't been riveted into the clevis. Easy fix, bin the clevis and buy some that have been riveted. I can't remember the details, they're on this site somewhere, but I saved a few ounces on a second-hand J60 by simply changing the servos from 148s at the rear of the radio compartment to HS225s at the front and dswapping heavy wood dowel pushrods for snakes.

Nigel, brake cables can just be fixed at one end as they are only put under tension and it's the pull that counts, not the precise position and movement at the drum end. As we are more concerned with position than with force (compared to brakes!) we need to use intermediate anchors to stop the outer flexing as the servo pulls and pushes the inner.

This was what I found:

Posted by Bob Cotsford on 10/11/2016 17:52:39:

Posted by Bob Cotsford on 02/11/2016 11:04:18:

1/2lb = 8oz=~240gm. Say a conservative 3:1 moment ratio for the effective position of the pushrods - wonder if the two 10mm pushrods plus 2mm threaded rods each end in mine weigh more or less than that 80gms? Might be I could save that much lead just by going closed loop! It will be interesting to see.

 

As near as I can make out on the kitchen scales the complete fuselage and tailplane weighs 4lb 11oz at the moment, so my target is to beat 4lb 3oz.

Edited By Bob Cotsford on 02/11/2016 11:08:14

Now down to 3lb 10oz for the fus and tailplane! I can't believe how much weight I could shed so easily - swap the pushrods for a snake on the elevator, closed loop rudder, 3*148s in a plastic mount behind the CofG replaced with HS225BBs under the tank and move the lead to the front of the cowling. The pushrods actually weighed 2 1/2oz - 75gms, the replacement Sullivan snake just 16gms, let's say 20 allowing for a balsa brace halfway up the fuselage and the closed loop for the rudder, then the standard size servos fitted behind the CofG were another 5 1/2oz in the wrong place. Those two changes have saved a lot of lead up front It will be interesting to see how it flies now.

 

Edited By Bob Cotsford on 22/10/2018 15:32:31

[Nigel R]

Bob (and Dave earlier) - thanks for explaining the brake cable / flexing thing - makes much sense.

I'm not surprised you saved a good deal of weight by getting rid of 10mm dowel pushrods - they're anything but light!

[Bob Cotsford]

That's what you get buying second-hand!

[Former Member]

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[Bob Cotsford]

Oy! I'm just big boned, my Mum told me so it must be true

[Geoff S]

Posted by David Mellor on 22/10/2018 13:55:01:

Ah.... only 3 countries in the world do not use the metric system (SI units) as their official weights and measures system.

One is Liberia.

Another is Myanmar (formerly Burma).

And the other is...... the USA.

But perhaps you could include the UK as well, since lots of Imperial things still persist here. Including 2-56 nuts!

Edited By David Mellor on 22/10/2018 13:57:47

Bu we've never used the US so-called Imperial sizes in the UK. Small sizes were mostly BA (British Association). My grandfather's watch making/repairing bench (complete with treadle operated lathe) had containers with sizes down to at least 20BA. Even then, I'm almost sure they're a metric thread and used a lot for watches - even Swiss ones. Bigger sizes were BSF (British Standard Fine) or the coarser Whitworth.

The US is slowly dragging itself into the 20th century with its weights and measures. I just wish we'd get on with it a bit quicker

Geoff

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[PatMc]

Posted by David Mellor on 22/10/2018 13:06:36:

I found out the hard way that if you don't anchor the outer tube at a few strategic points along the fuselage, the servo response is dampened considerably - even to the point of producing no deflection at the control surface for small servo movements.

What seems to happen if the outer tube isn't secured at a few points along its length is that, in response to servo movement, it finds it easier to change its shape between the two end points. So the inner (yellow) tube tightens the overall snake curve (increases radius) on "pull" and bulges it out more (decreases overall radius) on "push".

You can envisage it as the yellow (inner snake) taking the path of least resistance, which is simple bending unless constrained.

If both ends of the outer are securely fixed the only unwanted movement of the inner can be within the constraints of the tolerance between the inner & outer. It doesn't matter whether the outer is anchored at any points in between or if it's left to flop about, the lost movement between servo & control surface is identical.

Personaly I wouldn't use a plastic snake in a vintage model, they always look out of place. My preference would be pull - pull wires & if possible make one or both linkages internal, or at least as unobtrusive as possible. If I do use snake(s) I use this type of bowden cable.

[Former Member]

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[brokenenglish]

David, there's some misunderstanding here.

Reading the posts, I thought that I agreed with Pat and, having watched your video, I still think I agree with Pat.

It seems that, in the video, your outer isn't attached anywhere (not at either end). At the servo end, the outer is just waving about, which obviously prevents any of the desired proper movement at the other end.

Both ends of the outer must be firmly attached, so that they can't move either longitudinally or transversally, which was the essential point of Pat's post, and that will give control accuracy that will be sufficient for any vintage model.

If you redo your video, with both ends of the outer attached firmly, then either the clevis will move your fingers or you'll stall the servo (which would happen irrespective of the type of linkage).

[Former Member]

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[eflightray]

At least you didn't make the mistake I did.

Place a servo at the tail

Luckily I found out early on that the amount of lead needed in the nose to compensate wouldn't leave room for the motor. The elevator servo got move forward to the more logical place, next to the rudder servo that used pull-pull wires. The elevator used a balsa pushrod , (doubled up 1/4" square if I remember right.

Keep the tail end light, or better still even lighter.

Ray.

Not here.

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[eflightray]

Snake flexing ?

On my now defunct Lancaster, I used snakes, but taped them to a long strip of balsa as it was a straight run.

Run.

[kc]

As always with David's threads it's fascinating stuff and his ideas are worth thinking about and using on our own models.

I always ask David an awkward question and get a worthwhile reply........so today i am going to ask if his 'exotic' balsa laminates have been tested and proven better than solid. By tested i mean samples tested for deflection and probably to destruction rather than proven in a model.

I remember an article in RCMW where samples of balsa truss work were tested for deflection ( by Alistair Sutherland I think ) with interesting results - Warren truss was a good as anything I think. Basically he fixed a sample at one end and hung weights on the other then gradually increasing weight until it bent and then failed.

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[PatMc]

Posted by David Mellor on 24/10/2018 15:20:55:

I would say that you and Pat are right and that the important conclusion iff the second experiment is that - as you say - there would be sufficient control accuracy for any vintage model with outer tube restrained only at the ends. But not enough for models with bigger control surfaces working against bigger forces which are still sufficient to cause some flexing and reduced control.

David, any model that is suitable to be controled using a snake system only needs the two ends to be securely anchored. Intermediate anchors don't change the degree of lost movement one iota no matter how great the forces.
All flexing between the two anchor points is equal to both the outer cable and the enclosed portion of inner cable.
Therefore the total projecting portions of outer remains the same whether the snake is flexing or not.

It doesn't need a video to demonstrate this logic.

I've been using "snakes" from bowden cable (similar to the type I linked previously) successfuly for around 40 years in wooden, fibreglass & blown plastic fuselages. It would rarely be practical to make intermediate fixing points inside the latter two types of fuselages.

The models have ranged between slow flying vintage to fast, relatively heavy, competition gliders requiring precise control surface movement.

[PatMc]

Posted by David Mellor on 24/10/2018 18:21:07:

Once again......my thanks to Pat, Brian and Ray for their comments. I'm currently dithering a bit on the tailplane construction and so I'm grateful for all ideas at this stage

If the dithering is due to weight/cg considerations, don't get too hung up. The Jnr 60 (& presumably the MM) has a large area tailplane in proportion to wing area as well as a reasonable distance between the two surfaces. This amounts to a model with safe cg limits over a fairly wide range. The Flair plan for my Jnr 60 shows the cg at 28% my model flies with the cg at 40%. The rearward cg has no noticeable effect on the model's docile handling qualities but both the speed range & glide are improved.

I've never seen either of the original KK version plans but I suspect that the 1949 version may well have had a very rearward cg as was common back then.