Cable Brace geometry on a Sub-70kg ultralight

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Blackburn Mark

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On the off-chance that there are still some builders with cable-brace experience:

I'm looking to depart slightly from what seems to be convectional rigging geometry in order to place the load and bearing members closer together....
The build is already started so will require some re-work to implement the change.

Bracing B2.jpg
Position (A) was my original flying wire termination position and is a copy from a similar design.
Position (B) looks to my eyes to offer some load path advantages, the bulk of the mass (pilot) would not be as reliant on paths needing to be in amplified tension.

Bracing B3.jpg
Drag / Anti drag geometry seems to be a little more balanced than the original.

Despite the forward flying wires looking oddly vertical I am reasonably confident in the change and may make the swap (despite having to re-work some of the fabrication already completed) UNLESS someone on here spots an error that I am missing and can talk me out of it.

Does anyone actually build cable braced aircraft anymore?
I am assuming its an almost dead art.
 

Dana

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I would stick with A, there's a reason why it's conventional practice, it's where the strong structure including the landing gear all ties together. The location of the pilot's mass is less of an issue than the center of mass of the entire aircraft. Plus, you want the the flying wires attach point to be more or less in line with the landing wires attach point on the top of the kingpost.

Also, running the wires forward like that gives no resistance to the forward forces the wing sees at high AOA.

They're not as common any more, but people still build wire braced airplanes.
 

bifft

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Just eyeballing it, position B is pretty much in line with the front spar, so will give no antidrag resisting pull like A will.
 

Blackburn Mark

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Thanks for the reply Dana.
The question is, do you know that "reason"?
The frame is my design so the nodes or "strong points" are where I placed them.
Node A and B are equally strong.


The location of the pilot's mass is less of an issue than the center of mass of the entire aircraft.
Node A is closer to the center of mass in the vertical plane but I'm not seeing the significance.
Most of the mass will be transmitted through those eight flying wires and most of that on the front four... I cant for the life of me see the conflict between the center of mass and truss nodes.
I'm going to have to have a think unless you can enlighten me...The wing weight, for example, wont be seen by node A or B.


Plus, you want the the flying wires attach point to be more or less in line with the landing wires attach point on the top of the kingpost.
This one a funny one, to my eyes this is more a baggy quicksilver than a Sopwith Camel.... In other words, when flying, the landing wires are a tad redundant and sloppy (ish) and on the ground the flying wires are a tad sloppy (ish) and redundant (according to the wing designers notes) Bit like a hang glider.
The tail bracing wires are a little more pre-tensioned against each other for obvious reasons so I do see the distinction and your point.


Also, running the wires forward like that gives no resistance to the forward forces the wing sees at high AOA.
Bracing B4.jpg
Does this drawing change your mind?
Please say yes :)




They're not as common any more, but people still build wire braced airplanes.
I like the way they look, a little bit Steam Punk.... And its the only possible way to go sub70kg without using carbon and NASA.
 

Dana

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Does this drawing change your mind?
Please say yes
Not really. Wires from the wing to tail are sometimes used to stabilize the tail, but not support the wing anti-drag loads. The tail boom's stiffness may not be enough to resist asymmetrical anti-drag loads.

You need structure at the main landing gear to support the landing loads of the entire aircraft, so attaching the flying wires there just makes sense, you don't have to add heavy structure elsewhere. The fact that it's in a good location giving a good angle fore and aft is a happy coincidence, adding to the reasons it's conventional practice.

Have you done an analysis of the aerodynamic loads at high AOA and how they're reacted by the wires?
 

Blackburn Mark

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Just eyeballing it, position B is pretty much in line with the front spar, so will give no antidrag resisting pull like A will.
There is one of the four anti-drag wires is in my first post.... My apologies for the simplicity of my images, with all wires in, its worse, believe me :)
 

AeroER

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A free body of the wire loads will reveal which arrangement is the best compromise.
 

Blackburn Mark

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Not really. Wires from the wing to tail are sometimes used to stabilize the tail, but not support the wing anti-drag loads. The tail boom's stiffness may not be enough to resist asymmetrical anti-drag loads.
The tail has no stiffness, its in compression only, fully cable braced... Its not part of the original design.


You need structure at the main landing gear to support the landing loads of the entire aircraft, so attaching the flying wires there just makes sense
I would bias the flying strength over the landing strength every single time my good man!
I can walk away from a collapsed wheel.
Furthermore, flying wires play no part in landing loads.

the fact that it's in a good location giving a good angle fore and aft is a happy coincidence, adding to the reasons it's conventional practice.
If I was to stick to convention, I would stay on the ground for two reasons, 1) Its the conventional thing to do and 2) I couldn't make the weight requirement.


Have you done an analysis of the aerodynamic loads at high AOA and how they're reacted by the wires?
Oh my word, a magic spell...
No, not yet and I hope not to need to in the most part, If the tail can take me hanging off the end of it in both orientations, I get the feeling id fly the thing :)
 

Dana

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Furthermore, flying wires play no part in landing loads.
Of course, but the landing gear structure needs to be strong enough for landing loads, so it should be a solid place to attach the flying wires
The tail has no stiffness, its in compression only, fully cable braced...
Exactly, so it provides no resistance to asymmetric antidrag loads.
Oh my word, a magic spell...
No, not yet and I hope not to need to in the most part, If the tail can take me hanging off the end of it in both orientations, I get the feeling id fly the thing
Hanging your own weight off the tail is not a substitute for proper structural design.
 

Blackburn Mark

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Of course, but the landing gear structure needs to be strong enough for landing loads, so it should be a solid place to attach the flying wires
A and B look to be equally sturdy to me so its not a factor in this particular conundrum... I understand that it explains convention but my concerns about pilot mass have caused a second node (B) which is equally sturdy so offers up the option to grapple with and attempt to split the difference.


Exactly, so it provides no resistance to asymmetric antidrag loads.
................?
That's an odd thing to say.
The drag and anti-drag wires form a closed loop so I don't see why dropping into a spin would be vastly different than a symmetrical stall.
If one wing is pulled back, the other "must" rigidly follow it with the tail rigidly stuck between the two of them.


Hanging your own weight off the tail is not a substitute for proper structural design.
This whole endeavor is a bit of a science experiment on my part, "Sub70kg" Can it be done with a three axis? (into the unknown)
I am prepared to fail but NOT because of an inability to follow "proper" (whatever that means).... It would seem that the greatest hurdle to actually getting things done is an inability to switch from "thinking" to "doing", This site has plenty of evidence of the two practices and I am a connoisseur of the former... its a problem.
The gap between a minute, exhaustive detailed exploration and practical application can be, for all intent and purpose, infinite!
Sooooo.... where do we draw the line between the two practices?
Do we hang 100kg from a tail and guessestimate that a sub 45mph 70kg aircraft tail is unlikely to see such loads in flight and actually pick up the spanners OR do we delve into a complex fluid dynamics study that may be beyond our ability to resolve?

Its and interesting question, is it not?
Our lives are jam packed with moments where the latter wins the day and we exit in order to mitigate any and all risk.
I fear a heartbreaking tragedy like that of Franz Reichelt, I really do.... I also fear not getting to suck some of the marrow out of life before I die of natural causes.

I will round this essay of with a simple question Dana, Try not to avoid the question with evasive rationalizations.
Which one of the following is "proper":
1) An attempt under best of practical ability
Or
2) Never attempting

I fall into the latter 99% of the time.... But for some strange reason, THIS isn't one of them :)
 

Airporthound

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We've been building cable braced planes for over 45 years. Your flying wire geometry is typical.
My 1.5 cents input. A or B appears to accept the load. A is a more direct load transfer. B does offer some anti drag. With both the load goes to ?? Typical ladder structure wing ideally has internal drag / anti .....something. Diagonal struts or bay cables. Your drag wires to the nose offer little "help" in the negative load. I'll help.
 

Blackburn Mark

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Typical ladder structure wing ideally has internal drag / anti .....something. Diagonal struts or bay cables. Your drag wires to the nose offer little "help" in the negative load. I'll help.
The wing bays between the two cable termination compression struts and the wing root are indeed cable braced (not added into the pictures)


Your drag wires to the nose offer little "help" in the negative
I very much hope not to fly inverted and would hope that the conditions I typically fly in would see very little negative load :)

Are you saying you would test pilot such a scheme (B mounted wires) if all other considerations were proven perfectly safe?
 

Aesquire

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3 points.

Personally, I don't like to see flying wires or struts coming off the wheel hub. A hard landing that bends/moves the wheel support compromises the flight integrity and symmetry.

I don't understand your fuselage structure from just a side view, looks busy and heavy, can we see a front & top view?

Anti drag structure not seen? There should be a cable from leading to trailing edge at a diagonal. ( or a strut ) The tail bracing may attach at the anti drag wire location but doesn't do the same function and typically is dependent upon the rigidity of the wing. ( Edit written before above post )

And because I don't know the structure, moving the wires under the seat may make entry and egress tougher? Butt pinch? ??

The UPSIDE of not bracing to the wheel hub is you can have suspension/wheel travel without compromising structure. Possibly saving a bunch of tubes?
 

Blackburn Mark

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The UPSIDE of not bracing to the wheel hub is you can have suspension/wheel travel without compromising structure. Possibly saving a bunch of tubes?
I cant afford the weight, its one of the reasons it has such a large wheel, an attempt to make up for the lack of suspension.
I was looking forward to attempting to build a lightweight gas spring but that would need a weighty swinging arm.
Even elastic cord would need a sturdy swinging arm.... I even looked at GRP rods, it all cost too much weight so we have a single fat wheel.

Frame:
Bracing B7.jpg
Bracing B8.jpg
Bracing B9.jpg
Bracing B5.jpg
 

Lucky Dog

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If the wings have sufficient internal anti-drag structure, (Phantom, Quicksilver) then plan B will be sufficient because the flying wires (beyond carrying the lift loads) only need to prevent the wing from twisting, which they do in their triangle arrangement. (Quicksilver) The tail and fuselage "X" bracing is necessary to keep a lightweight fuselage perpendicular to the wings (Airdrome Dream Classic), so that box is also checked. Every aspect of plan B has been and is being used in tried and tested UL aircraft. Unlike most aircraft, however, the pilot of a UL is nearly 50% of its mass, so placing the flying wires under the pilot is good practice (Phantom, Hurricane). Also, unless you use a single wheel, your flying wires will be attached to an axle that is cantilevered 3 to 5 inches away from its attachment point, which has been a weak point for TEAM minimax aircraft and the one I am pictured with in my avatar image. IMO, Plan B is a good alternative.
 

TFF

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I think the advantage of the wheel is that the axel is strong enough to be the carry through for the cables. Two jobs in one. Move in he cables forward and the carry through at the cables have to be beefed up and you still have a heavy axel. With such a minimal aircraft, you can’t afford to have two of anything.
 

AeroER

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I cant afford the weight, its one of the reasons it has such a large wheel, an attempt to make up for the lack of suspension.
I was looking forward to attempting to build a lightweight gas spring but that would need a weighty swinging arm.
Even elastic cord would need a sturdy swinging arm.... I even looked at GRP rods, it all cost too much weight so we have a single fat wheel.

Frame:
View attachment 120208
View attachment 120209
View attachment 120210
View attachment 120211
The aft support tube will collapse at some point. Either due to under strength or fatigue cracks. Cracking is about equally likely at the ends or the unsupported bends.

If the bends have sufficient static strength, you're adding unnecessary weight.

You have a nice 3D drawing. I'm curious about the size of the design man you're using, and whether there is enough headroom and legroom for you, or full size pilots 75 inches and taller.

A mockup to check fit us easy enough.
 

Blackburn Mark

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If the wings have sufficient internal anti-drag structure, (Phantom, Quicksilver) then plan B will be sufficient
Its a little more Quicksilver in strength than Phantom/Hurricane... Its more a Phantom Light :)



I think the advantage of the wheel is that the axel is strong enough to be the carry through for the cables. Two jobs in one. Move in he cables forward and the carry through at the cables have to be beefed up and you still have a heavy axel. With such a minimal aircraft, you can’t afford to have two of anything.
I am looking at ways to avoid two heavy shafts, I'm considering a form of dyneema Spanish windlass for the wheel axel if I move to using position B
I have built the hubs with interlocking spacers running on a 25mm tube for the wheel so a good portion of the weight will be contained within the interlocking spacers in shear as long as they are under tension.


The aft support tube will collapse at some point. Either due to under strength or fatigue cracks. Cracking is about equally likely at the ends or the unsupported bends.

If the bends have sufficient static strength, you're adding unnecessary weight.
If you are talking about the rear fork legs, there is a strut missing in the drawings that bridges just below the bends.
 

Blackburn Mark

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I'm curious about the size of the design man you're using, and whether there is enough headroom and legroom for you, or full size pilots 75 inches and taller.
Its so long since I picked up that body model I cant remember but I do remember getting the tape measure out for some rough comparisons and I am 5' 8"
I am hoping with seat cushion and knee height, ill be able to accommodate any small leg errors :)
Ill just have to have tight belts if the headroom is a little small.
 
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