Head wing cantilever.

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stanislavz

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I think the forward sweep of the P130 (like the similar Cygnet) changes the situation, when compared to a straight wing. Or, maybe you've already accounted for that.
Post 25 and 34. Both airplanes have spar at 30% of chord, and it is forward swept at 3.5 degree.

1635349806982.png
Human bag ends at 40% of wing span. Will have to measure my best seat position in a car.
 
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stanislavz

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high wing with heads after spar..

1635362055464.png

1635362077106.png

50mm extra height in this setup.. (1150 vs 1100 ) meat bags are 200 mm moved to tail..
 

stanislavz

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The main spar(only spar) of the me109 sweeps forward though. Maybe that's why it didn't flutter and maybe the OP could do that as well with his . . . ?
Maybe.. but our speeds are in rv-12 range. Which is straight.
 

wsimpso1

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Placing the main spar aft and/or sweeping the wing forward can produce divergence in deflection - deflection under AOA and load that tends to produce more AOA and thus more load. The big loads are lift and pitching moment, and pitching moment reacted to the fuselage thus increases. Easy enough to estimate and include in our analysis. Since it its done in so many airplanes, including some sailplanes, I can not imagine it is one of those big effects. It should be included in the analysis and may drive a modest bump in skin stiffness.

Do the analysis, find the deflections in bending and in torsion, feed the numbers back through until they quit changing. Large deflection option ON in your FEA. If deflection converges, good, check that the structure is OK, no bending or buckling, etc. If it diverges, it needs to be stiffer... My suspicion is we are usually pretty overbuilt for skin stiffness and thus in resistance to torsion, and so are probably OK. You still have to do the analysis.

We have so many birds out there with modest amounts of negative sweep, it probably is a pretty small actor.

Billski
 

ragflyer

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Hi! Billski! Just curious ...I know in the past you have done mostly traditional (non FEA) analysis for your bird. More recently I see references to FEA. Have you started doing FEA on your design? If so what drove the need? Thanks!
 

wsimpso1

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I have been doing FEA (Nastran) since 1986 and SolidWorks since about 1999.

For my airplane, I did not need any of that. My wing is straight on the main spar at 37.5%, so my wing is a little swept. The tailplanes are straight on the hingelines at 60% so they are swept too. The skins are beefy with a light forward spar and drag spar. Deflections are tiny, no worries over divergence. Straight up composite plate theory works fine and does shear, bending and torsion at each station just fine.

Now some folks are building with forward sweep, aft spars, cradles under engines, etc. And folks are playing with FEA. Large deflection FEA is a neat way to do this. So I mention it.

You could do it with closed form plate theory and iterate until it settles down, then revise design as needed.

Both require some serious analysis skills. It is still engineering, not child's play...

Billski
 

stanislavz

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Now some folks are building with forward sweep, aft spars, cradles under engines, etc. And folks are playing with FEA. Large deflection FEA is a neat way to do this. So I mention it.
Thank you. I would like to add one more step - one need to test FEA itself on used materials and technology.

Make half wing, quater wing to test you technology and skills. And load same as fea and compare deflection. In more than one point.

Yes it is easy yo press buttons, but you really need to get you hands dirty..
 

ragflyer

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Thanks Billski! I assumed you indeed used FEA in your professional life. Totally agree, when it comes to aero-elastic issues and even elastic (buckling) issues FEA is indispensable. Best for amateurs to stay away by designing it out.

I know you have found good use of composite plate theory useful in your design. Personally though I find simple assumptions are good enough to calculate strengths of typical composite structures particularly given that most composite designs including Rutan's designs) , I find buckling more often the critical condition. I find a combination of closed form solutions and historic data sufficient in most cases to design out buckling. FEA of course could help as well.
 

stanislavz

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I find a combination of closed form solutions and historic data sufficient in most cases to design out buckling.
I could be too much cautious - but i also did found this used by some bigger companies consultants ehich i were able to access - use that standart layout as in our previous design, but add this extra ply here, because it look right.
I do not feel comfortable with this one approach. But FEA does show some neat and nice tricks, where rotating a one plier at 15 degree gives you interesting results.

On the other hands - as per BoKu real life example you need highly qualified workers to achieve same result in real life implementation.

But... If you take that minimalstic layout to withstand hangar rush (200 gsm cf on pvc foam / 400-450 single skin) - it is overbuild for air loads for most home build. So here you can tailor you stiffeners placement to avoid buckling.
 

raymondbird

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Placing the main spar aft and/or sweeping the wing forward can produce divergence in deflection - deflection under AOA and load that tends to produce more AOA and thus more load. The big loads are lift and pitching moment, and pitching moment reacted to the fuselage thus increases. Easy enough to estimate and include in our analysis. Since it its done in so many airplanes, including some sailplanes, I can not imagine it is one of those big effects. It should be included in the analysis and may drive a modest bump in skin stiffness.

Do the analysis, find the deflections in bending and in torsion, feed the numbers back through until they quit changing. Large deflection option ON in your FEA. If deflection converges, good, check that the structure is OK, no bending or buckling, etc. If it diverges, it needs to be stiffer... My suspicion is we are usually pretty overbuilt for skin stiffness and thus in resistance to torsion, and so are probably OK. You still have to do the analysis.

We have so many birds out there with modest amounts of negative sweep, it probably is a pretty small actor.

Billski
I've read you and autoreply talk about orienting the skin fibers like this x29 but would it also help with a rearward spar in a straight wing?
1635680585426.png
 

stanislavz

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but would it also help with a rearward spar in a straight wing?
As far as i tried to run some numbers - on wing root you have same/similar force. If you planform CP in in same % of wing chord. But rear pin will be less loaded in forward swept wing.

Visited aircraft musem in Krakow with Henryk. Some engine consideration by chief engineer.
 

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wsimpso1

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I've read you and autoreply talk about orienting the skin fibers like this x29 but would it also help with a rearward spar in a straight wing?
View attachment 117447
The X-29 did some fiber orientation to get favorable bending-torsional coupling from the wing skins. When I talked with two different professors knowledgeable on the topic (while in grad school) they said that you can not do enough with skins alone - mostly the X-29 avoided excess divergence with a lot of bending and torsional stiffness aided by favorable bending-torsional coupling.

Could you do this on the spar? The same problem as the with the X-29: The spar set by its mission is much stronger and stiffer than the skin set in shear and spanwise bending, but not usually much for torsion. Wing skins are way capable in torsion, to the point where spar torsional stiffness in most airplanes can be just about forgotten about. The light way is to react torsion near the leading and trailing edges.

To make the spar work, you would need to put on a lot of plies oriented for bending-torsion coupling. You could probably make the caps this way, but by the time they are stout enough to carry all the bending with what is an unfavorable orientation for that purpose, the weight would be way high. And you would still need to do all of this with the skins too.

Anyone really interested needs to get a copy of Tsai and Hahn or Jones, learn the math, and start playing with wing skins and spars in it. They might discover a part of the design space that let's you get good coupling and light weight.

Billski
 

raymondbird

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The X-29 did some fiber orientation to get favorable bending-torsional coupling from the wing skins. When I talked with two different professors knowledgeable on the topic (while in grad school) they said that you can not do enough with skins alone - mostly the X-29 avoided excess divergence with a lot of bending and torsional stiffness aided by favorable bending-torsional coupling.

Could you do this on the spar? The same problem as the with the X-29: The spar set by its mission is much stronger and stiffer than the skin set in shear and spanwise bending, but not usually much for torsion. Wing skins are way capable in torsion, to the point where spar torsional stiffness in most airplanes can be just about forgotten about. The light way is to react torsion near the leading and trailing edges.

To make the spar work, you would need to put on a lot of plies oriented for bending-torsion coupling. You could probably make the caps this way, but by the time they are stout enough to carry all the bending with what is an unfavorable orientation for that purpose, the weight would be way high. And you would still need to do all of this with the skins too.

Anyone really interested needs to get a copy of Tsai and Hahn or Jones, learn the math, and start playing with wing skins and spars in it. They might discover a part of the design space that let's you get good coupling and light weight.

Billski
Howdy Bill!

So, a glass skinned design with a rearward spar, do you see any advantage at all to orienting the skin plies like this X29? I think I read that with your bird, (slight rearward spar too I believe) you calculated a few degrees of twist up at full G load and you built in wash out to compensate. A better way I guess but yours has extremely stiff skins. What if they were just plywood?

Thanks
 
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