Wing Structure/Stress question. 1

[proppastie]

I am attempting to convert the Carbon Dragon Ultra-lite Sailplane to aluminum structure. There is a spec. in Basic Glider Criteria (FAA) for a 150# limit load at the tip. Depending on the assumptions made with the wing structure the axial loads on the rear spar are from 3000# to 1500#. The latter is calculating a Phantom beam at the Leading edge, and the former is for no inclusion of the Leading edge in the calculation. How do experienced aircraft designers stress a D box leading edge structure,... fabric aft of the rear spar. Do you position a phantom beam at the centroid or some other place for the calculation of the stress????

 

[rb1957]

i'm assuming your load is up (or down). if the rear spar is the primary loadpath out of the wing, then you should use it alone. if the D-nose is a substantial structure and well attached to the fuselage then you could count on it too. if the rear shroud is cloth (or similar) ... sounds pretty ineffective.

i'd expect that the spar/fuselage attachment is designed to take all the lift moment, and that torque would come out of the wing as a shear, and the drg moment might be a couple between some fwd attmat (D-nose?) and the rear spar.

Quando Omni Flunkus Moritati

 

[Sparweb]

Names can be deceiving. This is the Dragon made from "natural" carbon, not synthetic carbon.

If your aluminum changes lead to increased structural weight, then the loads on the airframe will increase in proportion, and your stress analysis/tests will be affected.

I don't remember hearing of a "phantom" beam method. It's been a long time since I looked at the VLA or LSA requirements. If I read your question correctly, I believe you are referring to a wing drag case. If you could tell us where you are on the V-N diagram then we would be sure we understand your question clearly (rb1957 and I seem to be disagreeing so far).

The 150# is applied horizontally at the wing tip, and then applies a compressive force on the rear spar. Correct so far? Where does the specific 150 pounds come from? The reason for considering drag like that is that the attachment must not buckle or wrinkle when subjected to this compressive load. All members of the attachment and the rear spar itself must withstand this - in COMBINATION with the lift loads that will also be experienced at the same time.

These loads should be related to maximum speed X safety factor combined with an appropriate lift acceleration load factor (say, 2g, check this) and then transition to the maneuving condition where the speed is reduced but the lift load factor is increased to the maximum (what is it for the Dragon - 5g?).

The normal aircraft category regulations known as FAR 23 have an excellent explanation of these terms and the V-N diagram. Once you become familiar with that diagram you can specify which load condition is your concern.

STF

 

[rb1957]

agree with comments re load and VN diagram ... i assumed the load was a quick and simple alternative to detail loads.

if the 150 lbs is horizontal then there'll be a horizontal couple at the wing root. the entire rear spar will be in tension (or compression) and the other 1/2 of the couple will be somewhere else.

150 lbs is a lot of drag from a wing (even if it is a "ground handling" case.

Quando Omni Flunkus Moritati

 

[proppastie]

This load case is "hard landing drag a wing tip" "Wing Tip Landing". Page 39 and 40 ibid. It is not on the V-N diagram. I have not been able to find an example of the calculation for this case, so I invented the "phantom beam" to plug into my truss calculation. The load is shown on my post in the attachment file and yes it is actually 225# ultimate 150# limit in the horizontal "X" direction, shown "-X" in my picture. The load comes from the FAA publication, and I would guess is as good an assumed load as any might be. My assumption is the fittings will be strong enough when I get to their design, however I am checking the Buckling of the rear spar, and the difference in results is the difference between .025 and .040 web thickness. My assumption is the Leading Edge is a substantial structural component. We know that cord-wise moment/torque is handled by the leading edge and I have samples of those calculations.

 

[rb1957]

so you've got a large drag moment to react on the Dnose and the rear spar ?

couple between RS and centroid of Dnose. if the load acts only aft, then the rear spar will be in compression and the Dnose in tension.

do we need FE to solve this ?

Quando Omni Flunkus Moritati

 

[proppastie]

The attachment is the centroid of the "Dnose" if that is reasonable location then I am done.

 

[rb1957]

attachment ?

Quando Omni Flunkus Moritati

 

[proppastie]

see post # 1, and thank you all. As to the FE I use Grape for the truss loading and then Bruhn C7.7 for the calculation. Are you offering to check me with FE.

 

[rb1957]

ok, your pic isn't stunningly obvious to me ...

rear spar is node 39 - node 20 ?

Dnose is node 17 ??

node 16 is ? wait, there are two 16s ??

and what's that blue diagonal line ??

Quando Omni Flunkus Moritati

 

[proppastie]

rear spar attach node 17 main spar node 20 the Phantom beam at centroid of the LE transitions to tip of root rib at node 16. the other 16 is element 16. Element 19 the diagonal line is the drag spar. This is the structure geometry (except for the Phantom Beam at the centroid of the LE) of the carbon dragon. The wing is shown backwards to correlate with the wing drawing for the Carbon Dragon. the tip load is going in the -X direction at node 39

 

[rb1957]

if there's a main spar and a rear spar, isn't that the natural loadpath for drag moments ? but then it looks as though there's a "drag" spar increasing the effective section.

the effectivity of the Dnose depends on how its attached to the fuselage.

why call your effective Dnose a "phantom beam" ?

as i understand your sketch, i'd use the main spar and the rear spar inbd to where the drag spar picks up and the rear spar to drag spar inbd of that. and you have to check the effects of the kick load from the drag spar ...

Quando Omni Flunkus Moritati

 

[Sparweb]

Proppastie,

I downloaded the plans from IHPA.ie and studied the wing drawing. Your FEA model is wrong.
I think you should set aside Grape for a while (though it is an excellent FEA program) because you have made some errors that will not be clear unless you deliberately draw by hand free-body diagrams of the wing structural attachment. In this case, the wing is clearly designed to avoid the need for any FEA in the design or analysis of the structure.

FEA is great for solving problems where there are redundant pathways for loads to transfer from one structure to another. Even then, if you can't draw a FBD you will quickly get into trouble.


STF

 

[proppastie]

ouch... could you be more specific, maybe a sketch of your FBD

 

[rb1957]

no, you should be able to draw a FBD ... show us what how you think the loads are being reacted.

Quando Omni Flunkus Moritati

 

[proppastie]

what is drawn is the rear spar, main spar, ribs, drag spar, phantom spar at centroid of LE. horizontal load at tip
I also have drawn only the solid ribs, main spar, drag spar, rear spar. horizontal load at tip.
I also have drawn only the solid ribs, main spar, drag spar, rear spar. horizontal load at tip. and phantom spar at the LE. Each gave different axial loading for the Rear spar as should be expected.
I have successfully done many school problems truss analysis with answers with Grape.
The assumptions that are made as to the loading and geometry certainly will affect the answer.

All of the above are drawn, which would you like to see.

 

[proppastie]

1.)so you've got a large drag moment to react on the Dnose and the rear spar ?
couple between RS and centroid of Dnose. if the load acts only aft, then the rear spar will be in compression and the Dnose in tension.

2.)if there's a main spar and a rear spar, isn't that the natural loadpath for drag moments ? but then it looks as though there's a "drag" spar increasing the effective section.
the effectivity of the Dnose depends on how its attached to the fuselage.

3.)as i understand your sketch, i'd use the main spar and the rear spar inbd to where the drag spar picks up and the rear spar to drag spar inbd of that. and you have to check the effects of the kick load from the drag spar ...

why call your effective Dnose a "phantom beam" ?

Ok I see 3 different answer above, I do not know the correct answer which is why I asked the question.

I call it a phantom beam because it is a load path of the centroid of the LE and there is not really a beam there.

I am converting the structure to aluminum, I am not necessarily copying the wood structure in aluminum. I am starting out that way to understand the process but I have thoughts of changing the structure when I understand how to.

 

[berkshire]

You can see a study set of plans at this link.
The original aircraft was wooden construction with carbon fiber spars.
B.E.

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

 

[rb1957]

"I call it a phantom beam because it is a load path of the centroid of the LE and there is not really a beam there." ... if the Dnose is effecctive then it isn't "phantom". it is effective if nailed into the fuselage, you haven't said how this is attached. if there is a rear spar and a "drag" spar then this is probably the right base for drag moments (i suspect that is why there's a drag spar).


Quando Omni Flunkus Moritati

 

[proppastie]

"you haven't said how this is attached."
Planed on solid root rib and bolt and fitting at LE....Note this is not per carbon dragon plan, and would cause different results from same.

attached are the different diagrams I have looked at. My fittings are assumed fixed, Rear spar attach, Main Spar attach, LE fitting. Main spar fitting is in the center of the aircraft.

I am only investigating Rear Spar Buckle from only one load case the tip load of 225 ultimate "Wing Tip Landing".

" if there is a rear spar and a "drag" spar then this is probably the right base for drag moments (i suspect that is why there's a drag spar)."

I was using the LE for structure, however case 1 shows just main spar,.. rear spar,... drag spar, and solid ribs as per CD drawing. This loading had the highest axial load as would be expected. I was hopping for something better.