DanH

DanH

Legacy Member
Mentor
Need some feedback regarding a nasty subject.

The RV4 roll bar has no bracing; one can imagine an unfortunate incident where the airplane gets upside down while sliding and folds the roll bar tubes forward or aft:



Here's the deal; a tipover canopy mod for an RV-8 usually incorporates a similar single-plane roll structure. One qualified builder is currently doing a structural analysis regarding how well it might hold up in the event of a flipped airplane. Such analysis requires some assumptions about the forces involved; always good to get a reality check about assumptions.

The nasty question: Does anyone recall a flipover accident in which the RV4 roll bar folded?
 
Yes....

Hi Dan,
I looked at a -4 in Atlanta that had flipped at a pretty high speed, in a pasture after an engine out (purge valve activation). The guy had a teenager in the back, hit a berm and flipped hard....so hard, in fact, that the stab had dihedral!! It slid along upside down and the rollbar folded some but still held them off the ground. Surprisingly strong.

Regards,
 
Bracing

I'm no engineer, but this is a picture of my "4" rollbar. The tops been modified, but what is so unbraced about it?...just my $ .02


 
Thanks Pierre. Did you notice any significant deformation around the top longeron area, where the roll structure is attached?

Bill, "has no bracing" was a poor choice of words on my part. Obviously it has two brace struts. Assume a load case where the airplane is upside down and sliding along it's longitudinal axis. The highest material stress would be found in the main hoop, in the area where the struts attach.

Right now the engineer running numbers on the hoop for the RV8 is assuming a combined 3G down and 1.5G fore-aft load. That may be excessive or may not be enough. Even anecdotal evidence about RV4 flips can be useful in determining if the assumption is realistic.

Is your assembly as supplied by Van's, except for the replacement of the tip? If so, do you happen to know the main tube diameter and wall thickness?
 
Interesting question Dan.

My first reaction when I saw this thread was to wonder if the "inverted sliding" case is actually very realistic. I can see a nose-over flip of course, but there would be little forward motion once you got inverted. I'm trying to imagine a scenario where you get inverted and are sliding at a significant velocity to give you the forces. You might very well have a realistic case - I may be suffering from a lack of imagination....

Paul

Paul
 
Paul/Dan
My 6A flip over had suffucient fwd velocity to shear the two 3/16 bolts holding the front rollbar.The rollbar pulled away from the longeron at the rear bolt with bending of the small web at the front of the attach point..One longeron bent and the other tore the aluminium angle of the longeron.[slider 6A]

Paul-[7A just inspected and about to organise initial flight.]
 
Here (Norway) a roll bar has to withstand a 9g vertical thrust, theoretically.

Added:
I have to add that officially theoretical calculations is needed to be able to get approval to fly. I don't know when that rule came into effect, and how detailed the calculations was (if they were ever done).
 
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Yes

DanH said:
Thanks Pierre. Did you notice any significant deformation around the top longeron area, where the roll structure is attached?

BillQUOTE]

Yep, the top longerons were badly deformed but the entire cockpit area was intact with no protruding sharp pieces of longeron or bracing to injure the occupants. The fuselage was basically totalled since almost every diagonal brace and skin was deformed and the top longerons were buckled quite a bit at the roll bar attach points. It appeared to be as stock built as could be. Overall, I was very impressed with its crashworthiness. It did the job it was desinged to do but if you really need more peace of mind, the forward braces could be cut and move further forward and re-attached higher up the main roll bar but then you start getting int o ingress/egress issues. The you need to stiffen the support structure under the forward roll bar bracing, etc...a lot of work. If I can find the pictures, I'll post them.

Regards,
Click to expand...
 
flip over

I have an rv-4 and I am more worried about getting the canopy open/getting out, if I end up on my back.

With these small wheels and tires I think any off airport landing on soft ground has a good chance of ending with a nose over.

Chris Murphy
 
roll over

Even if the canopy could be broken away is there enough room to crawl out of the front cockpit? Sure hope to never have to try.

Chris Murphy
 
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