Vertical stab design

Hello all,

On my Rv-10 the vertical stab forward spar is attached by a single bolt with castle nut and cotter pin to allow rotational motion. The plans even call out for lubrication to aid the motion. I have always been curious about this design choice versus more rigidly fastening the forward spar like the rear spar. I.E. the rear spar of the vertical is rigidly mounted with several bolts. I have always theorized that the rotation allowed reduces the likelihood of flutter or perhaps it makes the assembly lighter by simply supporting the front spar in a more deterministic way and not cantilevering it like the rear spar.

I don't want to change the design, my goal is only to learn more and I have always been curious about this design choice. Can anyone explain why this design choice was made? What advantage does the single bolt design have over more rotational rigid mounting? There are so many smart people here I have to believe that someone is knowledgeable enough to actually explain it.

Thanks,

Bryan

Many years ago I saw the vert spar attach on a Cherokee. I think it was only one bolt too. To ad to that, the ‘spar’ was made up by folding the 2 vert side skins over each other… to that the front leading edge skin wrap was riveted to the sides.

The hinge means that the joint can't carry a bending moment, which makes that portion of those parts lighter. Whether it makes the whole stabilizer assembly lighter is something different - often it does but not always. It depends on how the bending moments are carried in the overall structure.

Plus, as suggested, it makes the stabilizer easier (perhaps) to analyze.

Dave

RV-8 groundloop rudder

Today I looked over an RV-8 that had ground looped. The forward vertical stab spar sheared just above the doublers where the stab mounts

Vern said:

Today I looked over an RV-8 that had ground looped. The forward vertical stab spar sheared just above the doublers where the stab mounts

Click to expand...

That is the weak spot, I have wondered if a pin would be better than having that break in flight -esp for a 7 which has had 5-6 empennage failures in flight.

Bill, if there were a single bolt mounting the vertical forward spar, than all strength to keep the vertical stab in place would have to come from the rear spar attachment to the rear most fuselage bulkhead. Take a close look at the structure in the last bay of the fuselage & gauge if there is enough rigidity to do that job.
-.025 Side skins that are comprised by the control access opening on each side
-both 3/4 x1/8 fuselage longerons are notched for the horizontal rear stabilizer spar attachment
- the .040 deck is compromised by the large hole for the elevator control arms
- yes, there is the rounded .040 bent belly plate, but is only supported by light J-strips for longitudinal rigidness
- yes, tail draggers have the steel stinger mount weldment but it offers little sideways rigidity

Vern, the 8 with vertical forward spar damage-was there evidence that the tail wheel snagged the ground during the loop and bent/twisted the rear bulkhead/ vert rr spar attach sideways? Therefore causing the forward spar attach to twist/sheer? Definitely not normal in flight loads.

Bill, do you have accident reference where the front vertical spar attachment was the cause?
I’ve heard of rudder flutter causing one - overspeed & extremely poorly balanced rudder in that case, broke the rudder spar above the top hinge & pealed the top half of the rudder right off, along with the whole trailing edge wedge.
I’ve also seen 2 other rudders (rockets) with broken rudder spars & skins folded over from flutter but fortunately planes landed safely.
Also an 8 with severely bent elevators, I assume from flutter & overspeed.
& saw pictures of a Rocket with one horizontal stabilizer side missing, I assume overloaded somehow & flutter.
Never heard of front vertical spar failure identified as first in flight failure point in an accident.