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Why don't the wings fall off?

C

chris mitchell

Well Known Member
I hope that was an eye-catching title for this post!

This is not a building question, but I'm hoping that the engineers or at any rate the more aeronautically educated, can educate me about wings spars and spar bolts on the RV-8. I'm interested to understand something of the maths/physics of the calculations involved in the fuselage/wing joint. I've been staring at the little stub spar on my wings for awhile now. I was looking at a Spitfire rebuild a few months ago and it too has little stub spars for the wings.

As I understand it, the load on the bolts is entirely in shear - the close tolerance of the bolts and the spar/sparbox assembly ensures that the forces should be shear only with no bending possible and obviously no tension. Then its just a matter of calculating the levers involved - pivot is say the fuselage edge or the most outboard bolt, the remaining bolts are say 3 inches from the pivot, lift force is weight of the aircraft at 6g times the distance to the centre of pressure of the wing (easy to calculate with an untapered wing) and there will no doubt be an additional safety factor. Is that the gist of it? I guess there are similar calculations to make about the main spar and the stub and spar box assemblies using information about their stiffness/resistance to deformity?

I'll be interested and grateful for any information as part of my general aviation education.

Chris

RV8, empennage and wings complete, working on engine cowling
 
Hard to stomach

Chris -

I had the same negative thoughts when I looked at the spar connection on my RV7. Common sense tells you that the spar over lap should at least be to the centre of the aircraft to maximise the lapping effect and to minimise any potential vertical movement at the outboard location of the wing in respect to possible bolt tolerance. I have had this discussion off thread with other builders and the assumption is that the bolt tolerance must be so minimal (effectively nil) and the shear strength of the bolts must be so high (far in excessive of the spar strength) that it effectively becomes irrelevant.....hard to stomach the thought of that small overlap when you are pulling heavy G's. If assumption is the mother of all stuff ups then let's hope the engineers got it right !!!

Regards,
 
Structural failure

If memory serves me correctly, I think I read that structural failure is 9 g's positive.

If you pull that, you'll be passed out and won't know.
 
Webb said:
If memory serves me correctly, I think I read that structural failure is 9 g's positive.

If you pull that, you'll be passed out and won't know.
Click to expand...

Actually the wing is designed to survive momentary 9 g's loads. The -8 wing was static tested and did survive with some skin deformation. There used to be a write-up on Van's website about this testing. Not sure if it's still there.

The NAS spar bolts are very high strength, compared to even the normal AN bolts. The wing would break somewhere else before those bolts would shear... assuming there isn't a flaw in the bolt.

I'm a mechanical engineer and I haven't lost a bit of sleep over the fuselage/spar interface. Keep building, it will fly just fine and the wings won't fall off.

Karl
Now in Sandpoint, ID :)
 
I'm really not worried about the wings falling off - seems to be plenty of evidence that such a thing hardly, if ever, happens. The system is obviously at least half a century old and has been used for aircraft that likely were in serious danger of getting overstressed at some time in what was probably a short and brutal life. I assume that the wing would fail somewhere else if overstressed beyond tolerance (I'm guessing junction of inboard and mid third span). I was interested in the mechanics/maths and physics of the situation. Its not an intuitive set up - If you asked a non-flyer how to engineer such a thing they would probably suggest longer stubs that either interlocked or at least met in the middle of the fuselage.

So it was an "I'm curious" post rather than a "I'm worried" post.

Chris
 
Vans secretly has Charmin bathroom tissue engineers put toilet paper like perforations on the stub spars. As we all know nothing ever tears on the dotted line therefore gauranteeing the wings will not fall off.
 
no shear forces on the bolts

Chris,

I can't be 100% certain as I'm not currently near my parts or plans, but I don't think the bolts and pins are subject to any shear loading. The tension on the bolts should be such that the static friction between the centre spar section and the main wing spar is enough to take all the flight loads.

If the flight loads were to overcome this static friction, I'd guess that the bolts would be very quick to fail in shear.

A
 
Wing Spar Bolts

The air pressure on the wing is a distributed load, which changes with aileron and flap deflection of course. The wing spar is essentially an "I" beam which is tailored to match the bending moment produced by the wing load, under it's highest load condition.
An "I" beam reacts the bending by tension in one outer member (the lower one when you pull up) and compression in the other one. The shear web between them is there to keep the top and bottom of the "I" from moving with respect to each other. The wing bolts connect the outer sections of the "I"s from the wing spar to the center carry-through, so they are transferring the tension and compression forces through shear in the pins, and the bearing surfaces of the spar holes. The materials used are selected and sized for their shear and bearing properties, and close tolerances prevent movement which makes the joint act as 'solid' as possible. For this type of joint, the farther from the center the better because bending moment (and therefore the load to be tranferred) is highest in the center and decreases as you move toward the wingtip.
Wing spars like that on the RV12 (and many gliders), transfer the bending load in a whole different manner. They don't transfer the shear load in the spar caps, they transfer the bending load across long overlapping arms through bearing and shear (design specific). An advantage of that method is that lower tolerances can be used, which is important for removable wings.
 
If I get close to 9 g's

N395V said:
Vans secretly has Charmin bathroom tissue engineers put toilet paper like perforations on the stub spars. As we all know nothing ever tears on the dotted line therefore gauranteeing the wings will not fall off.
Click to expand...

If I get close to 9 g's, I better have a change of shorts to go with that new fangled Charmin perforation line.
 
Not exactly...

Andy_RR said:
Chris,
I can't be 100% certain as I'm not currently near my parts or plans, but I don't think the bolts and pins are subject to any shear loading. The tension on the bolts should be such that the static friction between the centre spar section and the main wing spar is enough to take all the flight loads.
If the flight loads were to overcome this static friction, I'd guess that the bolts would be very quick to fail in shear.
A
Click to expand...
I can assure you that those bolts are in shear, and that friction between the spar sections is totally ancillary (and miniscule) to reacting any loads. :eek:
 
Andy_RR said:
Chris,

I can't be 100% certain as I'm not currently near my parts or plans, but I don't think the bolts and pins are subject to any shear loading. The tension on the bolts should be such that the static friction between the centre spar section and the main wing spar is enough to take all the flight loads.

If the flight loads were to overcome this static friction, I'd guess that the bolts would be very quick to fail in shear.

A
Click to expand...

Andy,

These bolts are almost 100% in shear. The tension only keeps the center section together and keeps it from twisting.

From what I found, the shear strength of the NAS bolts is 95,000psi.

Using the following you can calculate why our wings don't fall off...

2233469470042045769S600x600Q85.jpg


Single Shear..
Shear Stress = 4 . F / π. d 2 ...... Compressive Stress = F / (d . t)......Plate Shear Stress = F / (2.c.t)​
Double Shear ..
Shear Stress = 2 . F / π. d 2 ...... Compressive Stress = F / (d . t)......Plate Shear Stress = F / (2.c.t)​
 
chris mitchell said:
IAs I understand it, the load on the bolts is entirely in shear - the close tolerance of the bolts and the spar/sparbox assembly ensures that the forces should be shear only with no bending possible and obviously no tension.
Click to expand...
Certainly more than shear, the bolts are also under tension, because they're torqued to a specified amount. The tension in the bolts doesn't directly help keep the wings on, but does create high friction forces between the fuselage and wing spar contact areas. My guess is that this friction force is much higher than the shear force in the bolt, and contributes most of the force keeping the wings attached.
 
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Friction or shear?

I love this discussion. We go over this at almost every design review. Civil engineers and designers of ground hardware like friction joints. Steel building use friction joints per the AISC steel code. Flight hardware designers go pale at the thought of a friction joint. All flight hardware (shear) joints are reacted by the bolts carrying shear loads.
 
rfinch said:
Certainly more than shear, the bolts are also under tension, because they're torqued to a specified amount. The tension in the bolts doesn't directly help keep the wings on, but does create high friction forces between the fuselage and wing spar contact areas. My guess is that this friction force is much higher than the shear force in the bolt, and contributes most of the force keeping the wings attached.
Click to expand...

This too can be calculated....I will respecfully disagree with you on this comment. There is no where near enough clamping pressure and surface area to carry these loads with friction only. Also, if this was true, Van's would save allot of money not providing close tolerance bolts and ensuring that the spars are match drilled to the center section.
 
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It's the shear...

Dean Pichon said:
I love this discussion. We go over this at almost every design review. Civil engineers and designers of ground hardware like friction joints. Steel building use friction joints per the AISC steel code. Flight hardware designers go pale at the thought of a friction joint. All flight hardware (shear) joints are reacted by the bolts carrying shear loads.
Click to expand...
Absolutely! The rivets carry shear loads as well. :cool:
Friction is for brakes and tires :rolleyes:
The torque on bolts is NOT there to provide friction in the joint.
 
These bolts are certainly in shear. I can't think of much worse friction joints than two smoothly machined surfaces which are then anodized!
 
Bolt shear correct. Friction, NO!

Andy_RR said:
Chris,

I can't be 100% certain as I'm not currently near my parts or plans, but I don't think the bolts and pins are subject to any shear loading. The tension on the bolts should be such that the static friction between the centre spar section and the main wing spar is enough to take all the flight loads.

If the flight loads were to overcome this static friction, I'd guess that the bolts would be very quick to fail in shear.

A
Click to expand...

Hi,

The bolts are designed to carry all the load in "double shear" at the front face and rear face of the spar where the bolts go through the center section bars. Static Friction should not, and is not, used as part of the engineering calculation of a bolted joint strength.

Steve

RV-8QB FWF
NASA research engineer too!
 
I too see the spar attachment as a shear joint. (BTW, I suspect the small bolts bracket the big bolts to clamp the assembly and maintain true shear, ie, not allow the center section spar bars to pull the big bolts into s-curves. Think plastic deformation. Any comment from the pros?)

However, let's not entirely disclaim friction joints in aviation applications. A very large chunk of the RV fleet is flying around with a critical clamped friction joint; wood propeller hub clamping.
 
The discussion of shear forces in wing fittings reminds me of a tragic incidence that happened many years ago when a Vari-Eze lost a wing in flight. Correctly installed both the upper and lower wing glass spar caps are sandwiched between two 1/8" aluminum plates about 6" x 2" each. Eight 1/4" bolts are installed through the sandwiched assembly. The bolts are in shear and basically prevent the glass spar cap from "pulling out" from between the aluminum plates. The builder forgot to install the 16 bolts (8 upper and 8 lower) on one wing. Interestingly the aircraft flew for a number of hours with just the epoxy bond to the 4 aluminum plates holding the wing on in shear. It took a high speed low runway pass to pull the wing off.:eek:

Fin
9A
 
Friends don't let friends fly plastic airplanes.

JUST SAY NO!!!

:p

Karl
Now in Sandpoint, ID :)
 
Once you stab the wings you'll be convinced them suckers arn't about to come out. The bolts are just there to make the pax feel better.
 
DanH said:
(BTW, I suspect the small bolts bracket the big bolts to clamp the assembly and maintain true shear, ie, not allow the center section spar bars to pull the big bolts into s-curves. Think plastic deformation. Any comment from the pros?)
Click to expand...
Those parts are thick enough that true shear is never in question. The clamping doesn't hurt, but the main reason for putting nuts on is to keep the bolts from moving and working, which could open up the holes in the aluminum over time. The small bolts help transition the shear load in stages, rather than immediately dumping all the load right into the big bolts, kinda like transitioning the thickness of the spar cap in stages rather one big step.
 
Zip ties

I was an Aero major and we had a lab where we had to calculate the forces on the spar/botls/etc, design a suitable replacement...and for extra credit, design one that would never actually be used, but was fun to think about...so I proved that it was possible to actually attach the wings of the T-34 with zip ties!!!!:eek:
 
Smoke and bolts!

Computers work on smoke, if you let all the smoke out of your computer it wont work anymore.

Plane wings are held on by bolts, if you take the bolts out, you can't fly anymore.

Shear or Tension, any significant friction, is a byproduct not a requirement.:rolleyes:
 
no friction?

If shear and not friction is holding the wings on an RV* then:

a) why is there a pretty significant torque specification for the wing bolts?

and

b) why does the RV-12 have such huge wing attach pins? (which, BTW react the wing bending moment across the width of the entire fuselage and not just the depth of the spar as in RV-8's et al)

A

(* excluding the RV-12, obviously)
 
Zip Ties ?

LAMPSguy said:
I was an Aero major and we had a lab where we had to calculate the forces on the spar/botls/etc, design a suitable replacement...and for extra credit, design one that would never actually be used, but was fun to think about...so I proved that it was possible to actually attach the wings of the T-34 with zip ties!!!!:eek:
Click to expand...

:confused: But you didn't say how MANY zip ties! ;)
 
This discussion is...

.......getting very similar to the theory that..."The prop is there to cool the pilot; thrust is only a beneficial byproduct" arena.

BTW, an -8 lost its wings at the end of the fuel tanks several years ago, after the pilot pulled in excess of 9 G's. Yes, shear.

Regards,
 
Van who???....

This is almost as bad as the 'conveyor belt' argument! You folks who discount shear in favor of friction, could just ask Van, unless you've attained the level of expertize where you wouldn't believe him either. Want to really scare yourselves? Check out the wing attachment system on a King-Air sometime.
 
Andy_RR said:
If shear and not friction is holding the wings on an RV* then:
a) why is there a pretty significant torque specification for the wing bolts?
and
b) why does the RV-12 have such huge wing attach pins? (which, BTW react the wing bending moment across the width of the entire fuselage and not just the depth of the spar as in RV-8's et al)
A
(* excluding the RV-12, obviously)
Click to expand...
The torque is standard for that size bolt. Torque is to preload the bolt (for tension joints really), not provide friction. As Ross noted, smooth aluminum surfaces make lousy friction pads. Wing bolt torque is not critical on RVs.

I haven't studied the RV12 wing attach, but as I mentioned earlier, that type of attachment works entirely differently than the RV6-7-8 attachments. Those attach pins are also in shear. The pins are big most likely to get more bearing area in the holes in the spars.
 
Build it per the plans!

Bottom line is.......build it per the plans and fly it with the design limitations and the wings WILL NOT FALL OFF!
 
<<The small bolts help transition the shear load in stages, rather than immediately dumping all the load right into the big bolts,>>

Ahhhhh. Like this Fin?

 
This reminds me of a conversation I had with an engineer a long time ago. My first job right out of high school was at P.A.C. (Pacific Airmotive Corp.) in Burbank California. They rebuilt jet engines mostly for airlines. I was amazed by how few bolts were used to hold the engines on the mounts in the test cell. In the case of an RB211 that would be four bolts. I asked an engineer (my desk was adjacent to the engineering department) why that would work. His reply seemed simple enough. It only has to keep the engine from moving the first tiny bit. If the engine were to move at all most likely it would keep moving. He explained to me that the close tolerance bolts used to bolt the engine in the cell did that job by fitting closely enough (emphases on closely) to not allow any movement to occur. The bolts of course have to be of a quality to resist the shear also. My own take away message from this is don't abuse the provided bolt holes while your building your plane.
 
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