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PTAirco

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Ok - I am willing to stand corrected: somewhere, recently, I saw a picture of some Backyard Flyer welds that looked a lot better. I still stand by my opinion that a five-sided polygon does not constitute triangulation.
 

PTAirco

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The spar design looks great. Flattened tube ends is quite common in engineering (maybe not so common in aircraft). The bracing tubes do not have to be in the same plane as long as they are kept within a diameter.
May I ask where this rule of thumb comes from?

Yes, flattened tube ends are fairly common, even in some aircraft. The Hornet Moth, for example, used bolted crossmembers in the rear fuselage, with the ends swaged in a very controlled manner, in a specially shaped die. But the Backyard Flyer way of attaching them results in theses tubes being eccentrically loaded struts. Again, yes, it probably works. Still don't like it.
 

bmcj

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a five-sided polygon does not constitute triangulation.
I agree, but I must have missed the pentagon. Was that in the internal wing photo? All of the bays in there looked triangular to me.

(Perhaps we are going back a few pages to the broomstick and witch comments... that could definitely call for a pentagram. :gig:)
 

WBNH

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Just revisited their website and a close look shows perhaps a lot of un-publicized revisions have taken place with the plane.

Back Yard Flyer Ultralight Page

Click Photo Gallery. The first fifteen photos of the newer ship they had at OSH '15. I knew that one had the 50 HP two-stroke Hirth instead of their own four-stroke conversion...and they went with the sand tires which I hope work out well...but take a look at the second pic. If that's the uncovered wing then it looks like they replaced the triangular truss spar (as seen in the photo in post 217) with a 2D truss. That looks like a significant redesign.
(EDIT: I also note in the You-Tube Video "Backyard Flyer-Sold" at the 1:30 mark you get a glimpse inside the wing where it looks like the triangular truss spar has been replaced by a 2D truss, and that vid is from Aug 2015 claiming it's a 2013 model, so the spar change must have been a while ago. But I also note a rear truss spar. In the older design with the triangular truss main spar, the rear spar was a single tube...this later example appears to have front and rear truss spars. This example also had the inverted pedals, but not the additional cockpit bracing on the left side....suggesting ongoing revisions.)

I also notice that the earlier planes had the five sided door opening on both sides, the new one appears to have additional members stiffening the cockpit on the left side and only a right side door opening (see pic 6)...I still do not see any bracing going downward to the rear landing gear attach point, but perhaps the evolution is ongoing.

They also inverted the rudder pedals and appear to have changed the brakes (also pic 6). There are also more corner reinforcements (tubular gussets?) than in some early versions as seen in pics further down in their gallery. The latest ones with far less curve to them.

Would be nice to see more pics like the first few in their gallery.
 
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PTAirco

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I agree, but I must have missed the pentagon. Was that in the internal wing photo? All of the bays in there looked triangular to me.

(Perhaps we are going back a few pages to the broomstick and witch comments... that could definitely call for a pentagram. :gig:)
No, I was referring to the forward fuselage with that. BYF_.jpg


Just a couple more "quirky" items: Bent longerons and diagonals attached in odd places.

byf2.jpg

And what does this X-thing do precisely except give you welding practice?

byf3.jpg


Those 4 longerons are basically acting on their own and all that wishful bracing in between accomplishes pretty much nothing.
 
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Tiger Tim

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Structurally speaking, at one end of the fuselage spectrum you have a boom, a monocoque structure that is rigid enough in all directions to take care of itself. On the other end, you have a truss, where the members are all very light and only really do anything in tension/compression while triangulation ensures that's all they have to do. Is there a middle ground? That seems to be what they're doing here.
 

PTAirco

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What I see in the rear fuselage is some hopeful bracing that resembles triangulation, in side view at least. However, I doubt the diagonals are large enough to really stand up to the forces. I have analyzed enough frameworks to tell you this without ever pulling out a calculator. Also - look at how the first diagonal is attached; welded to a corner stiffening piece (entirely useless and unnecessary in a proper framework), instead of intersecting at a node. It's really more like four independent tubes forming a rough pyramid (with bent tubes). Their large size makes it likely it will hold up for a while, sure. A properly designed framework wouldn't need those enormous longerons.

That X-thing in the top of the rear fuselage, all by itself, tells me one thing: the designer doesn't have a fundamental understanding of framed structures. Despite his claim to a degree in mechanical engineering. This is what really irks me about the design and why I seem a bit overly critical. I'm strictly an amateur engineer but I don't know any "real" aeronautical engineer who would look at this and say :"Yeah, that makes total sense." If you know one, enlighten me.


As far as the spar goes (and this is just idle curiosity, not further criticism): since this is a cantilever wing, the top tube of the spar framework sees mostly compression loads and the lower two tubes take the tension. Since buckling is the most likely failure mode here, would it not make more sense to have the two tubes at the top and single one at the bottom? Especially if, as I suspect, all these tubes are the same size and wall thickness. Or was it tested, like so many publicity photos in the past, by people standing on the wing and loading it up with negative Gs?
 

Matt G.

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As far as the spar goes (and this is just idle curiosity, not further criticism): since this is a cantilever wing, the top tube of the spar framework sees mostly compression loads and the lower two tubes take the tension. Since buckling is the most likely failure mode here, would it not make more sense to have the two tubes at the top and single one at the bottom? Especially if, as I suspect, all these tubes are the same size and wall thickness.
Agreed, but depending on the airfoil, the spar would have to be slightly shorter in height with the two tubes on top, since the two tubes fit nicely across the flat bottom of the airfoil, and would have to be lower as to not break the OML of the top surface of the airfoil. My $0.02, anyway...
 

PTAirco

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Agreed, but depending on the airfoil, the spar would have to be slightly shorter in height with the two tubes on top, since the two tubes fit nicely across the flat bottom of the airfoil, and would have to be lower as to not break the OML of the top surface of the airfoil. My $0.02, anyway...
Good point - that would need weighing up to see if it makes a worthwhile difference.
 

PTAirco

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Structurally speaking, at one end of the fuselage spectrum you have a boom, a monocoque structure that is rigid enough in all directions to take care of itself. On the other end, you have a truss, where the members are all very light and only really do anything in tension/compression while triangulation ensures that's all they have to do. Is there a middle ground? That seems to be what they're doing here.
I see neither a boom nor a monocoque anywhere. Sorry. Not as it's defined commonly in aeronautical engineering. It's an imperfect framework everywhere I look.

I understand what you're suggesting though. One thing you can be sure of with aircraft is that nothing is ever totally rigid. And you can't ever assume something will be. Things will deflect under load and this deflection has to be considered, not ignored by the expedient of "That looks really strong".
 

mcrae0104

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It's an imperfect framework everywhere I look.
Not that I'm defending the plane in question, but bear in mind that not every frame must be a truss (i.e. axial loading only) in order to be useful.
 

Tiger Tim

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I'm more curious as to why the recent version of the airplane does not appear to use the V-twin engine that they produce.
I wondered the same. I figure either they're testing some new engine for it, or a customer ordered a plane but provided his own engine.
 
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