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Midniteoyl

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Hello Midniteoyl and PYAirco...


Quote:
Originally Posted by Midniteoyl
Ah just TIG the whole thing and send it out for heat-treat and forget the holes! :ban::ban:

Has this been done before for light craft? It sounds like there will be a significant linear amount of weld seam and that sounds like a lot of addittional weight. A lot. Also, how would you weld to the wing skins with all those tight nukes and crannies?
LOL... was kidding :)
 

PTAirco

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Hello Midniteoyl and PYAirco...


Hmmm...maybe I wasn't clear.Sorry. The longerons running spanwise will all be attached to the ribs above and beneath the wing. I suppose they will help to stiffen the panel spans but I would think that their primary purpose is to resist bending and torsion as you have pointed out and obviously it helps to create a path to distibute the loads to the spar.

The angles on the spar caps will also in a sense provide the same but their primary purpose is to act as spar caps to carry the loads, bending, shear etc.




I have complete confidence in the "ancient" books and I agree that anything at this point of wing structure iteration is worth investigating. Can you elaborate a little more on this? For example.

I'm not sure I understand what you mean by a free edge buckles but a tube has no free edge. I know that this is not meant to imply that a tube will not buckle. The Ixx for a circle is exactly the same in all directions therefore the largest loads must be used regarless of direction when calling out the tube so I can see why it may not buckle. This can lead to excess strength and obviously weight in directions where loads may be lighter.

With an angle, the Ixx and Iyy can be designed for the exact loads in the respective directions and those come with an edge as you pointed out. Did you mean that the tube will not buckle because the largest tube is called out for the largest load? How was weight savings then attained? I don't get it.

Also, what about attachment? Doesn't every hole drilled in a tube require some form of bushing or spacer to prevent them from crush loads when tightening things up? How were they able to accomplish it? Welded? How?

Any pictures you can share?

Thanks
Oh, I wish my scanner worked...

These test were done at The Royal Aircraft Establishment, Farnborough.

"The reasons for suggesting the use of tubular stringers were, firstly, that being free from outstanding flanges they had greater flexural and torsional stability, and, secondly,that the mechanical properties of seamless drawn tubes are generally superior to those of strip sections in the same material." (c)

There is a lot more and I will scan a lot of these articles when I get around to it and share some of this old stuff.

You pointed out "With an angle, the Ixx and Iyy can be designed for the exact loads in the respective directions..." True, but: the max. stress developed before the angle buckles will be a lot lower than for a tube of the same weight, again because there is no free edge. (With angle as thick as .125" , you may develop the almost the full compressive stress of the material, sure, but almost no aircraft uses stiffeners with this kind of thickness, typically we're talking .025-.065 at the most.)

Picture a 2" wide strip of cardboard a foot long - fold it into a 1"x1": angle and support a weight on it vertically. Now take a similar strip and roll it into a tube and it will suport a lot more weight than the angle, before it collapses. Both strips weigh the same, but the tube can develop a higher stress before buckling. (Within reason - if the diameter/thickness ratio gets too great, other stuff starts to happen, but not for the sizes you might typically use for this)

The difficulty is of course drilling these and riveting them - you need to use blind rivets and drill accurately into the tube.

If you can find a copy of M.Langley - "Metal Aircraft Construction" - it is a great books, full of gems and great illustrations, especially the later editions.
 
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Gray Out

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Hello PTAirco...

It may appear that I am being difficult, but I'm not. I just need more clarity.

"The reasons for suggesting the use of tubular stringers were, firstly, that being free from outstanding flanges they had greater flexural and torsional stability, and, secondly,that the mechanical properties of seamless drawn tubes are generally superior to those of strip sections in the same material." (c)
I'm not sure how this should be interpreted but I will provide my understanding. Anybody correct me if I'm wrong. I believe that stringers are much smaller and their primary purpose is to resist panel deflection. Also, I think that they carry a significantly smaller load to the formers than do longerons because they usually attach to the skins so there must be significantly more of them. I understand how it would have more flexural and torsional stability because of geometry, but I believe that the flanges/angle beam spar caps are meant to provide a "stiffness" factor. The radius of a circle has an automatic inherent compressive strength compared to a flat plate so the mechanical properties may be better in any material because shape is a function, but for the strength required, the tube must either be bigger or more numerous.

I worked up the Ix for a tube of equal radius for the angle length I'm using with equal wall thickness. Rough estimation only for comparison purpose. The cylinder Ix was .286 and the angle was .825. As you can see the tube has an Inertia which is 1/34th of the angle. The circumference is 4.7124" for the 1.5" diameter and .125" wall, so the weight would be three times as heavy. Unless the application they discussed is different, I don't get how they were able to do it.

You pointed out "With an angle, the Ixx and Iyy can be designed for the exact loads in the respective directions..." True, but: the max. stress developed before the angle buckles will be a lot lower than for a tube of the same weight, again because there is no free edge. (With angle as thick as .125" , you may develop the almost the full compressive stress of the material, sure, but almost no aircraft uses stiffeners with this kind of thickness, typically we're talking .025-.065 at the most.)
We have to be discussing different components. Maybe we need to define it. For me, in this case, the angle will create the web and flange for a spar cap in compression and tension and will resist tension and compression loads as a primary structural element to reduce bending loads, some shear ad torsion as well although those are carried by the ribs and skin. Yes, they act as stiffners but that is not their primary purpose. For me, a stiffner does not have to be an angle (it can be a flat plate) and its purpose is to reduce panel deflection between frames (formers). They are primarily attached between the frames although they may be notched in frames (their bearing area in notch is actually small) but their primary attachment is to the skins.

I think that you may be correct about thicknesses regarding "stiffners" but those thickneses would never stand up to structural loads. I'm enclosing the loads I am working with for the Peregrine at +9/-9 so that you can get an idea of their significance. Keep in mind that these are half-span loads at 40% of length for the dimensions and weight.

Bending Moment 33,858#'s, Spar Cap Load 40,467.73#'s, Spar Web Shear at the Root 5,940#'s.


See what I mean? Compared to other ships maybe it isn't much but for a quick comparison, the root has to be able to support a military humvee to meet criteria!:shock:

The difficulty is of course drilling these and riveting them - you need to use blind rivets and drill accurately into the tube.
I have another question that comes to mind. In riveting a flat plate to a round tube, the radius of the tube will not provide as much of a surface bearing area as a flat plate. How is the riveting accomplished so that the rivet head area does not wrap and compress the skin around the tube and how is a gap between the skin and tube avoided if the skin does not wrap around the tube?



Thanks
 

addaon

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Gray Out, I'm not going to say too much here, because it's beyond my current knowledge. But I think that in interpreting the information that PTAirco found, you have to consider failure modes. Take a piece of U-channel, say 0.75" x 0.75", 20 thou thickness; and take a 0.75" diameter tube, same thickness (similar total weight). Note that I haven't done this experiment, so I'm talking out of my arse... I mean, my imagination. But I'd imagine that on the U-channel, bending towards the open side, you'd get buckling of the flanges. This is a failure mode that won't happen with the drawn tubing.
 

PTAirco

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My guess is you're using these angles in quite a different way to what I first thought;

"The same is also true for the additional longerons spaced at 15" c/c.

I suppose you're using these to increase the overall section modulus of the whole wing and not as stiffeners as typically used in a semi-monocoque structure. So the tubular stiffener idea here would probably not apply in the same way. It's only purpose was as an alternative to conventional stiffeners and perhaps the saving in weight might only apply to quite large aircraft. Still, I thought it was an interesting idea.

I am with you on the theory that a airplane can never be strong enough - my biplane is stressed to over 10G. My little single seater worries me at 8.8 ultimate....
 
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Gray Out

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Hello to all...

But I'd imagine that on the U-channel, bending towards the open side, you'd get buckling of the flanges. This is a failure mode that won't happen with the drawn tubing.
I got it.
I just figured that the legs would be riveted so they would resist opening/closing up. But I understand that it would not be the case for the tube.


My guess is you're using these angles in quite way to what I first thought;
"The same is also true for the additional longerons spaced at 15" c/c.
PTAirco, with all of this chaos in my head you're guess is probably better than mine. Lots to see all at once from a lot of different angles.

Still, I thought it was an interesting idea.
Not was. It still is an excellent idea. That is why I am nit picking the hell out of it. I'm not ready to close the store on this just yet. What is really screwing me up is that I used half rounds (like a D shape) to run stringers in the hull of my cat because I wanted a finished interior and it worked great but I can't get the circle shape in the plane figured out.:dis:

I am with you on the theory that a airplane can never be strong enough - my biplane is stressed to over 10G. My little single seater worries me at 8.8 ultimate....
I may not be with you on this too much longer and may be reducing my expectations. I finished up all the structural components for the wings, no hardware or rivets, and that baby still came in crazy heavy for the rough calculations. Then, if that wasn't enough depression, I went out and bought a piece of aluminum plate and printed and cut out 1 rib and folded the perimeter flanges for connection. IT WAS HARD WORK and that was just a flat plate!

Sooo in my, gluttony for agony, I counted up all the required parts, imagined doing the same in a proportionate manner to their size, and then went through the whole ordeal of drilling and riveting and imagined myself discombobulating my body to get into all the little caves and crap and that put me over the edge and I pulled the trigger at the wings. Didn't even look at anything else.

This design fever is hard depressing work. This design has been compromised in every friggin sense. I'm taking a break to sort it all out. Going sailing to air it out. Maybe fishing. Don't know, but definitely going.

I was going to post some pictures of the finished wing, but screw it...no energy right now.

Thanks:tired:
 

AFAdrenaline

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hey man, i'm sorry you ran into such a roadblock... don't give up though, i think (in my completely unqualified opinion) you have an awesome design going. maybe there are other ways around your solution... maybe lower you g-limits and thus a lighter structure? not to push composites again but maybe take another look at it... a lot less sweat-equity (other than all the sanding)... but yeah i hear ya on taking a break... maybe you'll be able to take a look at it with a fresh perspective. best of luck dude!
 

orion

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But keep in mind that for a small airframe, aluminum will most likely be the most weight efficient structural approach. Even with higher end prepregs and very efficient structural concepts, a graphite based structure will be roughly equivalent to what's achievable with an aluminum one.

The frustration you're encountering is quite common, especially when working with a weight limited category such as you have with LSA. It's actually one reason my company has elected to avoid said category's developments - a cursory evaluation of even basic structural safety requirements and LSA limitations has shown us that the goals might be mutually exclusive. Since I haven't done a detailed analysis I wont go as far as saying it's impossible but I do have my reservations. And that especially applies to cases where the incoming goal is aerobatic loads.
 

AFAdrenaline

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But keep in mind that for a small airframe, aluminum will most likely be the most weight efficient structural approach.
right, i was more thinking from an 'ease of construction' point of view. unfortunately i have very little experience w/ aluminum so i can't really provide any good advice...

like i said previously... take your break and come back with a fresh point of view... things tend to work themselves out if you do...
 

orion

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I get many of my solutions and ideas usually at about 2 am, sometimes days after running into the stumbling block. Give it time - your subconscious just may work it out for you.
 

Mac790

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Gray
Gray Out said:
I'm taking a break to sort it all out. Going sailing to air it out. Maybe fishing. Don't know, but definitely going.
Gray I think it's a good idea buddy, You just need "fresh blood/energy" take a few days off, you did great job so far, hard days are typical in any design industry special if you have to cope with somekind of limitations like weight restriction, but I believe you are close to final solution, don't give up, go fishing, sailing and come back here with new ideas. Personaly it's really pi.. me off, because we have lessons with FEA Nei/Nastran at uni and we will be doing some more complicated analysis like plane structures but in next semester so I'm not able to help you at the moment.:devious:

AFAdrenaline said:
i have very little experience w/ aluminum so i can't really provide any good advice...
Me too but, I believe we have many guys here with a lot of experience and they will be more than glad to help.

Orion said:
I get many of my solutions and ideas usually at about 2 am
I have a similar "problem" for some strange reasons my brain works better in night than in day, some people call me vampire because I'm more active in night than in day.

Take care
Seb
 
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tyc

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For get small tape recorders and the like, I've long ago learned to keep a pen and notebook handy near my bed as things do go "bump in the middle of the night."

For what it's worth ...

tyc
 

Lucrum

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But keep in mind that for a small airframe, aluminum will most likely be the most weight efficient structural approach. Even with higher end prepregs and very efficient structural concepts, a graphite based structure will be roughly equivalent to what's achievable with an aluminum one....
How would wood construction, such as the Falco, compare?
 

orion

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Although wood is a much lighter material, its strength is also quite low, usually on the order of about five to ten percent of that of the typical aviation grade aluminum. Densities of common structural woods (douglas fir, spruce, etc.) are on the order of about 20% that of aluminum so the strength to weight ratio of the metal is quite superior to that of the candidate woods. And since the strength of the wood tends to correlate nearly proportionally to its material density, the ratio to aluminum tends to hold relatively consistent, regardless of wood type.
 

PTAirco

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Although wood is a much lighter material, its strength is also quite low, usually on the order of about five to ten percent of that of the typical aviation grade aluminum. Densities of common structural woods (douglas fir, spruce, etc.) are on the order of about 20% that of aluminum so the strength to weight ratio of the metal is quite superior to that of the candidate woods. And since the strength of the wood tends to correlate nearly proportionally to its material density, the ratio to aluminum tends to hold relatively consistent, regardless of wood type.
Despite that, I would argue that any competent designer can produce two equally good aircraft in either wood or aluminium. You only have to look at the Falco - a perfect example that demonstrates that wood is the equal to metal in this size of aircraft, given good design practices. How many metal aircraft can do what the Falco does and are fully aerobatic?

I would go even further and say that as aircraft get smaller and lighter the advantages of wood become more pronounced. Buckling/stability problems in thin metal being the chief reason. PLywood is still an almost unbeatable material for shear webs, for example.

I am just playing devil's advocate here since I am actually building a very light single seater in metal, but my reasons have more to do with cost, availability and being able to park it outside than any structural advantages. Which are pretty much the same reasons most commercial manufactureres use metal over wood.
 

Mac790

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Wow!!! I just googled Falco and found an amazing build log. What a great plane!! You have to check this out. just click a part of the plane in the picture for a ton of info on that build. AMAZING!!!!!!!
Have you seen empty weight for that plane. 1,212 lbs? Yes I know bigger engine, ... etc. But it doesnt change anything. It's still to heavy. I would rather compare Gray's design to Corby Starlet Welcome to the Corby Starlet Website +-4,5g roll rate 270dps. Personaly I would rather stay with aluminiun. (Gray I can give you direct link to Corby plans, PM if you want to take a look at it).

PTAirco said:
How many metal aircraft can do what the Falco does and are fully aerobatic?
I dont remember exact G-load for the Falco, but I would say Rv family, Mustang II, Harmon Rocket II, III, F1 Rocket, Nexus Mustang? and maybe a few others.

Seb
 
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orion

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Despite that, I would argue that any competent designer can produce two equally good aircraft in either wood or aluminium.
True, however the definition of "good" is quite varied and will usually depend on the person who is making the decision. If however we stick to the definition where "good" is defined by performance as a function of weight, other issues aside, the metal concept will always come out ahead and significantly so.

For instance, one quick estimation of mass efficiency can be characterized by the ratio of payload (occupants, baggage and fuel) to gross weight (the ratio can also be used to quickly evaluate whether someone is too optimistic in assigning service loads or G ratings). Most conventional aircraft will shoot for about 45%, a few can come close to 50%. But if we look at the Falco's numbers, the ratio is only about 35%, which is sort of in the ballpark for most wood based airframes. Yes, there are a lot of variables that go into these characteristics so it's not a set rule, but the ratio can provide a quick glance at the airframe characteristics as a function of capability and structural materials used.

About ten years ago I recall someone taking a closer look at the Falco in order to see whether it would make sense to build it out of metal. From a weight standpoint the analysis proved to be quite enlightening in that it was determined that for the same service factors, the airplane could be about 200 pounds lighter. That would put the payload fraction at about 45.7%, much more in line with other aluminum based airframes.

But the same analysis also showed that if one were to work at producing a Falco out of metal, the beautiful shape would take a significant amount of time and effort to reproduce since virtually every piece on the airplane's body is composed of complex surface geometry. Forming the many fuselage frames, the numerous ribs and of course the body skins would be a significant undertaking. I believe the person elected to take a closer look at building it out of composites at the time but I don't know if he went forward with it or not.

You only have to look at the Falco - a perfect example that demonstrates that wood is the equal to metal in this size of aircraft, given good design practices.
Far from it and the mass fraction is really all you need to make that evaluation. Mind you I am not knocking wood and wood does have one particular advantage over metal, that being in the area of fabrication. But to equate metal capabilities to the wood shows more of an emotional argument rather than one based on actual characteristics.

Where wood shines though, as many of us know, is in its easy fabrication. The ability to be worked without the need for any extensive tools is the key to the material's popularity. That ease of fabrication and formability allows for the fabrication of more complex airframes with possibly less effort that what one might have to use with other, less flexible building products. That formability also allows one to better configure or taper the material so that the mass penalties end up being not quite as large as they might be. A good example is in the wings - it's obviously much easier to cut out varying rib shapes, something that would be quite difficult to tool up for in an aluminum wing. Furthermore, tapering the spar down at the tips is easy compared to the process that might have to be used in an aluminum structure. So yes, wood does have its advantages but mass efficiency is not one of them.

How many metal aircraft can do what the Falco does and are fully aerobatic?
Actually, quite a few. But as the material choice demonstrates, most are not as pretty or aesthetically refined as the Falco.

I would go even further and say that as aircraft get smaller and lighter the advantages of wood become more pronounced. Buckling/stability problems in thin metal being the chief reason. PLywood is still an almost unbeatable material for shear webs, for example.
Not even close - this is an argument with no merit nor basis. The scale of airframe has absolutely nothing to do with material choice and metal is still by far the most effective and efficient for all applications, including skins and webs. Each material must be used in a proper manner in order to get the necessary strength and stability and doing so, the metallic structure has the highest level of benefit for all areas, especially with small airframes where moments of inertia are limited, so high material strengths and stiffnesses will provide the most optimal benefit.

The only possible benefit is when in small airframes you hit the limit of minimum gages. This simply means that the material does not come any thinner but you don't need as much of it for a particular assembly due to the low stresses. In those application you might see some benefit from wood but these cases would represent a very small percentage and not enough to argue for wood to be the material of choice.

But again, I'm looking at structural efficiency - there are many issues of workability, material compatibility, builder preference, etc. where the choice of wood might be more appropriate than other alternatives.

I am just playing devil's advocate here since I am actually building a very light single seater in metal, but my reasons have more to do with cost, availability and being able to park it outside than any structural advantages.
Arguing structural merit has nothing to do with playing devil's advocate - the physics of structural properties and application are well defined and relatively easy to evaluate. You can argue all you want but structural efficiency can be easily defined and calculated and it really does not take much in math skills to do so. That, combined with proper engineering and design, will always result in the same answer.
 

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