Aluminum Built UP Spar Question

[proppastie]

For manufacturing and weight considerations, (I do not want to bend .04 2024-t3) I am considering a built up spar c-channel web with separate riveted flanges as shown in the attached file. Shown is the compression top cap, web, flange, and flat caps. Rivets are not shown. Would anyone like to comment on the stress analysis of this. We say the bending is in the caps and shear is in the web. That .016 attach angle is not considered in the web or cap calculations (or rather it is not necessary to meet strength requirements). Needless to say I am not comfortable with this but would like to see where it might fail, and be able to calculate it if possible. Any help would be appreciated. The Y should be Z sorry. Reference to chapters in Bruhn would be a great help.

 

[rb1957]

your picture doesn't look right ... the angle flanges should be at 90deg to each other, no?

analyze the spar as caps and web ... fine, look at Bruhn chaptera A5, A13, A19.
worry about the shear that the rivets are carrying (as they transfer bending from the web into the angles)

i'd worry about the loads that the 0.016" angle has to carry. sure it's conservative to neglect it in calculating the section property, but it will be "exercised' by the bending and it's good to know if it's going to yield. You can neglect it for static loads (who cares it 1% of the section yields early, doesn't carry much load) but i'd be more interested in the fatigue situation (use a 1g loading).

it's easy enough to build up the section property from considering a set of rectangles (but you're probably using CAD).

Quando Omni Flunkus Moritati

 

[proppastie]

I was concerned about the little bit (radius ) of the .016 angle sticking up. I was worried about shear of that portion. How ever sleeping on it I have come up with this modification, and perhaps an extra row of rivets minimum edge distance from the top. The skin is .016 so no matter how you look at it at some point the load transitions from .016 skin to the spar. You have a specific equation in Bruhn you might suggest? Preferably one with a sample calculation associated.

 

[rb1957]

your spar cap is "unconventional", being two plates on the web. more typical is an angle (or two). what are the skins made from ?

what does the 0.016" angle attach to ?

if you're worried about the angle picking up unwanted bending loads ... make it out of short segments rather than a single long piece.

Quando Omni Flunkus Moritati

 

[proppastie]

you bet it is unconventional, however the caps transition to one cap outboard as the bending moment decreases. This is similar to the Midget Mustang. Conventional factory built might have machined from solid caps. Every thing is aluminum 2024-T3 however the skin aft of the LE attach to the spar will be fabric. The original Carbon Dragon had wood/carbon reinforced spars, Plywood LE. Fabric aft of the LE. I am shooting for 150# empty 330# gross weight. 75-80# wing weight total. The original Carbon Dragon was 145# empty, the Sparrow Hawk is 150# empty. Until I get the stress right, and the structure sized I can not be sure where I will be

 

[proppastie]

I really like that idea. thank you very much.

"if you're worried about the angle picking up unwanted bending loads ... make it out of short segments rather than a single long piece."

 

[proppastie]

My cad will give me what ever I want, if I build from rectangles I can get "I" for each rectangle, or it will give me the total "I" for the section. I am not a stress specialist, and not sure how to check each rectangle stress, however if that is possible (MY/I) it might solve my concerns. How does one apportion the load?

"it's easy enough to build up the section property from considering a set of rectangles (but you're probably using CAD)"

 

[WKTaylor]

proppastie ...

Questions like this, especially involving homebuilt acft, may recieve more practical attention in a homebuilt aircraft engineering forum such as

http://www.homebuiltairplanes.com/forums/aircraft-design-aerodynamics-new-technology/

I personally would never wing shear skin loads to a spar web with such thin shee-metal. Why not consider a "L" or "T" extrusion riveted to the spar? taper cut-off flange material as needed for weight; and mill/sand cross-flange top-side air-foil profile.

"L" AMS10134-0501
"T" AMS10136-1201

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

Unfortunately, in science what You 'believe' is irrelevant – "Orion"

 

[proppastie]

the leading edge skin and fabric aft of that.

"what does the 0.016" angle attach to ?"

 

[proppastie]

I am looking for more specific engineering answers than I have been able to get on the other site. I have often seen specific reference to chapter numbers and equation numbers in Bruhn on this site.

I will consider anything but until I understand and am able to calculate the specific shear flows, shear, bending, buckling, I would not know how thick to make the L or T sections.


proppastie ...

Questions like this, especially involving homebuilt acft, may recieve more practical attention in a homebuilt aircraft engineering forum such as

http://www.homebuiltairplanes.com/forums/aircraft-...

I personally would never wing shear skin loads to a spar web with such thin shee-metal. Why not consider a "L" or "T" extrusion riveted to the spar? taper cut-off flange material as needed for weight; and mill/sand cross-flange top-side air-foil profile.

"L" AMS10134-0501
"T" AMS10136-1201

 

[rb1957]

i gave you a list of bruhn chapters. the homebuilt community is exactly the guys to talk to about this, and someone in your local glider community.

an 0.04" spar web doesn't sounds too odd, given this is a light weight glider (presumably).

how are you attaching the fabric skins to the structure ? presumably you attach to the spars and ribs ?? through the 0.016" angle ? with rivets ??

it is difficult to give specific answers when we're not given to complete problem. and it is difficult for you to post the complete problem ... catch22 !

Quando Omni Flunkus Moritati

 

[proppastie]

"your picture doesn't look right ... the angle flanges should be at 90deg to each other, no?"

you are looking at the top compression cap and flange only,



standard solid AD rivets 3/32 flat head, dimpled skin. I plan on Oratex glued on fabric, probably pop riveted to the ribs unless the glue is strong enough, thats later. The ribs are standard formed aluminum, riveted to spars web and LE. Stiffeners on spar web for shear resistant beam per Bruhn. This is a very straightforward standard structure. The thin riveted flange is different, thats what my question is about. My thinking was "its the same thickness of the skin" Also the standard spar calculations do not consider the flange, just the web and caps. If the flange is assumed the same thickness of the web, .....that is something I did not see.

there is lots in those 3 chapters but knowing "whats what" is where I need help.

Looking at my .016 flange can you explain how to separate the load or is it the same "extreme fiber" load for the whole section. A somewhat basic perhaps stupid question. Or said another way, does not the cap have to fail before the thin angle will fail.

If the load on the .016 flange is a problem, is it a problem for the skin, they are the same thickness. I have examined shear flow for the skin on the LE and there are enough rivets.

 

[rb1957]

"your picture doesn't look right ... the angle flanges should be at 90deg to each other, no?"

you are looking at the top compression cap and flange only, ...

ok, i see two flanges aligned to the web. if they are nested angles, great; if not then "odd".

some of your later questions sort of scare the cr@p out of me, depending on what you're really asking. i think the question you're asking is i have this thin gauge angle attached to the spar, can i consider it ineffective ? if you segment it into short lengths, then it will be ineffective in bending.

but see all that i can of this, i ask myself, what is that flange(angle) doing, that the spar cap couldn't ? and why don't you want to bend 0.04" 2024T3, it's about the nicest thing to bend ! maybe 6061T6 maybe a stock angle/extrusion ??

Quando Omni Flunkus Moritati

 

[proppastie]

if not then "odd".

odd

"what is that flange(angle) doing, that the spar cap couldn't ?"

attaching to the LE skin and aft skin fabric

"i think the question you're asking is i have this thin gauge angle attached to the spar, can i consider it ineffective"

yes and no, great idea short segments, when I load test my aircraft,... if the caps and web do not permanently deform... will the thin gauge angle?

that flange weights lots less than equal size of .040, I can bend it myself, no bending break, I can position it perfectly with the ribs.

see attached file for better drawing (I hope)

 

[rb1957]

i'd hope that the 0.016" would elastically buckle under test.

your drawing shows everything at 90deg ... a 0.032" Tee extrusion would work very well as a spar cap in place of the 2 0.04" plates and the 0.016" angle, and would weigh about 0.01 lbs/in (span) ... 0.1sq.in area (3*1" flanges) ... 2*0.04"*1" weighs 0.008 lbs/in + something of the angle, almost a wash.

Quando Omni Flunkus Moritati

 

[proppastie]

as you said difficult to mount the whole problem, those are (2) .093 rectangular caps, One .040 spar web, one .016 angle.

this is a cross section looking outboard from WS 2.5 I am not understanding your statement "your drawing shows everything at 90deg ." unless you mean I have not shown the slope of the ribs going to LE and TE which should definitely be curved. It was a quick sketch.

an .032 "T" section will be investigated, the .016 angle is lighter

 

[proppastie]

"i'd hope that the 0.016" would elastically buckle under test."


My fears exactly.

 

[MikeHalloran]

It might help to draw and post a complete wing section, showing the rivets, rather than partial isolated sections; our minds do not fill in the gaps in the same way that yours does.

I am not understanding how you buck the rivets in the LE/spar box, which appears to be a completely closed section.

The .093 cap is not making sense to me; it adds a lot of stiffness to the spar in a direction where the spar doesn't need more stiffness, and it doesn't add much to the spar in a direction in which the spar does need help, e.g. to resist buckling. An angle would add more buckling resistance, and also somewhat increase the I for wing bending, for less weight. Or I am completely misinterpreting something; it happens a lot.



Mike Halloran
Pembroke Pines, FL, USA

 

[berkshire]

proppastie



You say you do not like bending 2024 t3 . However there was a successful Canadian built ultralight that did just that, with a series of .032" channels sleeved tightly one inside the other and riveted through the shear web starting with about five pieces at the root dropping one every few feet until only one channel went from root to tip. The D tube was riveted to the spar at the flanges of the Channel.
B.E.


You are judged not by what you know, but by what you can do.

 

[rb1957]

Dnose probably has blind rivets.

those 0.093" plates are very heavy, even if they're several plies (which drop-off outbd). a 0.032" Tee would be much more effective, and provide flanges for skin attach.

if the angle buckles elastically then not a problem.

Quando Omni Flunkus Moritati