Faro
Posts: 1539
Joined: Sun Aug 12, 2007 1:08 am

What is the nature and magnitude of the forces acting on wing spars in the horizontal plane?

One would imagine that front spars are in (horizontal) tension and rear spars in compression due to the force of frontal drag tending to push -or rotate- the wing back horizontally from its attachment at the root. If this is the case, how big are these horizontal forces compared to the vertical lift force they have to bear? Is it a matter of multiples or an order of magnitude difference?

Also, can flutter act in the horizontal plane as it does in the vertical?

Faro
The chalice not my son

tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

 Quoting faro (Thread starter): What is the nature and magnitude of the forces acting on wing spars in the horizontal plane?

The biggies are engine thrust and aerodynamic drag. Inertial loads play a role during acceleration but it's typically not huge. Flight control loads can also have a big horizontal component but that shows up in the drag terms.

 Quoting faro (Thread starter):One would imagine that front spars are in (horizontal) tension and rear spars in compression due to the force of frontal drag tending to push -or rotate- the wing back horizontally from its attachment at the root.

That's generally true outboard of the engines (for wing-mounted engines). Inboard of the engines it's reversed, since the engine is pulling the wing forward and the wing is pulling the fuselage forward.

 Quoting faro (Thread starter): If this is the case, how big are these horizontal forces compared to the vertical lift force they have to bear?

Roughly an order of magnitude less. Engine thrust is typically about 10%-20% of gross weight.

 Quoting faro (Thread starter):Is it a matter of multiples or an order of magnitude difference?

Somewhere in between. Closer to an order of magnitude (especially in cruise).

 Quoting faro (Thread starter):Also, can flutter act in the horizontal plane as it does in the vertical?

Yes, although I believe it's a lot less common because the gradients in the aerodynamic terms are a lot smaller for horizontal movement of the surfaces.

Tom

flipdewaf
Posts: 1690
Joined: Thu Jul 20, 2006 6:28 am

Seems logical.

How much are these loads factored into a design? Are they analysed and designed for or are they generally just "monitored" as the design evolves and tend to fall out as being ok and not a critical design point?

Fred

tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

 Quoting flipdewaf (Reply 2):How much are these loads factored into a design?

They're completely factored into the design. You have to, since some of the loads are additive (e.g. front spar compression inboard of the engine adds to front spar upper chord compression due to lift). If you don't at least check for them, the wing may be too weak.

 Quoting flipdewaf (Reply 2):Are they analysed and designed for or are they generally just "monitored" as the design evolves and tend to fall out as being ok and not a critical design point?

They analysed and designed for. In general, you determine a wide variety of load cases (particular combinations of lift, thrust, drag, weight, CG, load distribution, speed, etc.) that, if done properly, completely cover the entire operating envelope. All load cases must show positive structural margin.

Sometimes, one load case will be complete subsumed by another (the design for the lower load case "falls out as being OK" due to having to meet the higher load case) but you still check all of them. If you don't, it gets harder to certify by analysis if you change something later and you might have something sneak through the cracks (pun only slightly intended).

Tom.

chimborazo
Posts: 89
Joined: Sun Sep 25, 2011 7:51 pm

 Quoting tdscanuck (Reply 1):Inboard of the engines it's reversed, since the engine is pulling the wing forward and the wing is pulling the fuselage forward

Unless you have an engine out in, say, a twin with wing-mounted engines then it's back to:

 Quoting faro (Thread starter):tending to push -or rotate- the wing back horizontally from its attachment at the root

I'd imagine that's quite a big load on the wing actually... on a big twin with an engine out.

Tom, can you shed any light on wing horizontal loads on a T7 with an engine out, for example? Clearly the wing is designed for that type of loading, I'm now curious from reading this thread what quantity of force we're talking? Thanks.

tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

 Quoting Chimborazo (Reply 4):Tom, can you shed any light on wing horizontal loads on a T7 with an engine out, for example? Clearly the wing is designed for that type of loading, I'm now curious from reading this thread what quantity of force we're talking?

I have no idea of the actual number, but we can do a decent back-of-the-envelope calculation to get an idea.

At full thrust a 777 puts out about 230,000lbs. If we abruptly lose an engine right after takeoff it can still climb (barely). So, for a rough guide, let's assume the total drag is about 100,000 lbs at takeoff speed (a little margin leftover for the climb). At relatively low speed the fuselage isn't producing much of the drag, it's mostly from the wing, so each wing is about 50,000 lbs of drag.

There are bunch of deltas on here (you can still climb or accelerate on one engine, the rudder and horizontal stab have meaningful drag in an engine-out, etc.) but we're probably good to within about 25% to say that a 777 wing has to withstand something like a 50,000 lbs horizontal force in an engine-out scenario (compared to the engine running scenario, where it's about 65,000 lbs the the other direction).

Tom.

Faro
Posts: 1539
Joined: Sun Aug 12, 2007 1:08 am

What about CFRP spars on the 787 and the like? Wasn't CFRP supposed to be less good at compression than tension?

Faro
The chalice not my son

tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

 Quoting faro (Reply 6):What about CFRP spars on the 787 and the like?

 Quoting faro (Reply 6):Wasn't CFRP supposed to be less good at compression than tension?

Most aerospace materials are worse at compression than tension for ultimate strength; I don't think CFRP is unique in that regard. CFRP is *great* in tension because of it's fatigue resistance (you only get fatigue issues in tension). So it's true that it's better in tension than many other materials, and that it's better in tension than compression in that sense, but that doesn't means it's worse in compression than competing materials.

Compression is a problem, primarily, because of buckling. CFRP is subject to bucking just like everything else (although the really high E helps).

There are some materials that suck in tension, like concrete, but you generally don't use those in aerospace.

Tom.

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