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Side by side shoulder modification

J

JakeLewis22

Well Known Member
I'd like to hear from the owners of flying RV-6, 7 & 9's that have the shoulder room modification shown below.
shoulderroom01.jpg

The mod is also described in this link
http://www.experimentalair.com/n703rv/tips.html#shoulderroom

Several years ago I spoke to John Hughes and Ross Schlottauer, both of whom made this modification, but they no longer owned their aircraft. They sold them for different reasons but they both said that they wouldn't hesitate to do it again if they were to build another side by side RV.

I'd like to hear from the current owners of those aircraft as well as any others with this modification to see if there has been any issues as a result of notching the canopy decks.
 
REALLY BAD IDEA

Monocoque structures need longerons to carry tension and compression loads that are produced from bending and torsion of the structure. A closed monocoque structure is very stiff in torsion because of the complete shear flow path around the closed cross section.

But then, we want to cut out an area of the closed section to make a cockpit. This open section has much lower torsional stiffness. Hugely lower stiffness. As an excercise, pick up a piece of aluminum C channel and a piece of aluminum square tubing (or round tubing) of comparable size and wall thickness. See how easy it is to twist a C channel? See how impossible to twist a closed section?

So, now the designer has to find a way to get the torsional stiffness, and the bending strength of the rest of the fuselage to be preserved through the cockpit. This is done with a series of seamingly insignificant pieces that work in concert.

One example is the welded steel cross brace in the middle of the RV-8 fuselage (I'm not as familiar with the others) Note how it has small diagonal braces at the ends? Also the canopy hoop/roll-over bar, note how it has good diagonal bracing at its attachment? These convert fuselage twist into twist of the cross brace and roll bar, which are much stiffer.
Another example relevant here are the wider, stiffened channels that form rails along the cockpit sides. These convert fuselage twist into local bending - the rails act like beams that are stiff enough in bending that the fuselage can not twist.

By notching the cockpit side rails as shown here, the structure has been crippled. A weak point in bending of the side rails has been introduced that will significantly weaken the cockpit area in torsion. Really bad.

Worst case, a mod like this could change the torsional rigidity of the fuselage enough to cause an empennage flutter problem. Other likely outcomes: poor crashworthiness, possible skin fatigue cracks.

If you don't understand monocoque and shell structures, don't notch structural members!
 
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JakeLewis22 said:
I'd like to hear from the current owners of those aircraft as well as any others with this modification to see if there has been any issues as a result of notching the canopy decks.
Click to expand...

That modification greatly reduces the aircraft's crush strength if you should happen to run it into something. With that mod, it is much more likely to buckle right at that point - the passenger compartment, which is exactly where you don't want it to deform.

As others have said, bad idea.
 
To notch out a piece of primary structure without engineering concurrence is total insanity, take the power tools and step away from the airplane :eek:
 
Kyle Boatright said:
That modification greatly reduces the aircraft's crush strength if you should happen to run it into something. With that mod, it is much more likely to buckle right at that point - the passenger compartment, which is exactly where you don't want it to deform.

As others have said, bad idea.
Click to expand...

Exactly right, and if that weakened structural member buckles in a crash the upper seat belt restraints attached to the rear longerons will be able to move forward allowing the occupant to leave a nice imprint of his face in the instrument panel.

An aircraft needs to be not only airworthy but crashworthy as well.
 
Not what I was looking for

If you're broad shouldered and you've sat in your side by side RV before installing the canopy decks you'll understand why I'm considering this mod. That little bit of room makes a world of difference.

The longerons are untouched and the sliding canopy track helps bridge the gap but I do understand the concerns posted (except the ugly one, that's easy to fix).

I started this string because I have the same concerns and that is why I'm looking for real world results from the owners of aircraft with this modification.
 
While I see it is only the canopy deck that is being notched not the longeron.....the canopy deck does add some strength to this area.

I am as wide as they come and I just deal with it. Most wide people fly with someone that is not so wide so it works out. Two wide people is where it gets tight but at some point your W&B becomes the limit not the width of the airplane.

I looked at the pics in the link on your post and if that is you, you should not have any problems. Looks like you already made the notch mod to your build.
 
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Please don't screw with primary structure for all the excellent reasons given above. Lose some weight - win, win. My wife and I have both lost more than a few kilos recently because she has decided that the weight will be better used for baggage capacity :D Lots of other good reasons too, of course.....
 
bkthomps said:
I think a good ole fashion treadmill solves a lot of RV'rs problems
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Unfortunately the treadmill won't reduce bone! It's not fat that I'm dealing with, at 6'4" this is the body I'm stuck with.

I have received one Private Message from a builder with an RV-6 with this mod who wishes to remain anonymous (I can't imagine why). He says "I have experienced zero effects of this mod in 200+ hours of flying (except increased comfort)."

If you have this mod on your side by side RV and wish to comment, please send me a Private Message.

Thanks!
 
All those who said "don't mess with it" don't expect you'd have issues during normal flying.. it's the extremes (such as an accident) where this counts the most.. So that gent may be OK after his 200 hrs.. doesn't really diminish the importance of "don't mess with structural members."

The only person I'd trust in regards whether this is OK to do or not is the one who put that structural member there...
 
Flyfalcons said:
What would Van (or Ken, etc) say?
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I called Van's yesterday to get their opinion about this modification, I'd heard that Tom Green made this modification to his RV-6. I wanted to speak to Tom but Ken told me that he's on vacation for the next few weeks.

Ken wasn't overly concerned about the mod and he's the one that suggested I use the forum to get feedback from owners of airplanes with this modification with 500+ hours of operation. He also suggested that I ask for feedback from ONLY those with first hand knowledge of this modification but I didn't want to be rude.
 
just to reiterate

I can see that the longeron is still intact, and you might even consider a doubler to maintain/improve the buckling/crush strength.

What is still missing is the lateral bending stiffness that is required to provide torsional stiffness to the open section. To get the bending stiffness requires the width of the canopy sill/rail.

Someone who has done this and says its fine has not observed the greater torsional flexibility of the cockpit. It may cause fatigue in the skins at the corners of the cockpit or worse.

Here, lets do an experiment. Take a used paper towel roll. Feel how stiff it is in torsion. Now, cut a 1.5" wide by 2" long square hole in it. since the tube is about 1.75" wide, this hole will come fairly far down on the sides. Kind of like a cockpit.

Now, feel how much less stiff the tube is in torsion. You can even see little wrinkles form in the corners of the cut-out.

Now, find some small sticks, like chopsticks maybe, or little wood skewers for barbequing shrimp. cut 2 of them about 4 inches long. now glue the two sticks along the sides of the square cutout, with an inch of stick extending forward and aft of the cutout. Make sure the stick is well glued all along its length. 5-minute epoxy will get the job done quickly, but you could use white glue for this. Whatever, wait for the glue to dry/cure.

Now, again twist the tube. probably still not as stiff as original, but much stiffer than before the sticks were put on. You can see how the sticks get bent as the tube twists, resisting the torsion.

The cockpit side rails are providing that bending stiffness along the cockpit.
 
JakeLewis22 said:
...I'd like to hear from the current owners of those aircraft as well as any others with this modification to see if there has been any issues as a result of notching the canopy decks...
Click to expand...

As we've already seen by the flow of this thread, those people aren't necessarily the ones with the most important information about this modification. You only have to look at a couple of crashed RVs to become convinced that you don't want to make any changes that reduce the continuity or cross-section of the cockpit rail structure. By the time it becomes an issue, it will have become a major issue with major consequences.

To expand on other posters' comments, there seems to be this idea that the longerons consist solely of the extruded angles, and all the other stuff that gets riveted to them in the cabin area is inconsequential. From a structural perspective, that's not the case. All those pieces work together to add strength and stiffness to the fuselage, and do so in a way that is demonstrated to work reasonably well without undue stress concentrations.

Thanks, Bob K.
 
Perhaps a tandem such as the RV8 would suit you better. Plenty more room side to side. When you start pulling 4+ G's while having fun that will not be the time to find out if your structure is up to par. Just because other do it doesn't make it sound from an engineering point of view.
 
A friend of mine survived an accident in an RV3, and eventually succumbed to heart failure while recovering a few weeks later. The longerons fractured in many different places. The longerons broke everywhere there was a bolt attached to them on the horizontal surface of the longerons. The RV3 does not have side rails. That said I believe that the addition of any structure via rivets in the horizontal plane actually weakens the longerons.

A good friend of mine had this same mod in his RV-4...he couldn't fit even when he was pretty slim.
 
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rocketbob said:
...I believe that the addition of any structure via rivets in the horizontal plane actually weakens the longerons...
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That drilling holes in the aluminum angles weakens them shouldn't really surprise anybody. Unfortunately, in the absence of high-tech adhesive bonding or alternatives such as friction-stir welding (shout-out to Eclipse, the best job I never had), rivets and rivet holes are pretty much what we have to work with.

However, we don't really care about the strength of the aluminum angles; what we care about is the strength of the overall structure made from the angles and other bits. Yes, it takes a bit of strength out of the individual components to drill the holes for the fasteners that attach all the bits together. To compensate, you have to analyze all the joints, and add a bit more material to accommodate the expected loads with some margin for the unexpected ones. What results may have less strength in certain of the parts, but greater strength in the structure as a whole.

Thanks, Bob K.
 
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