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Elevator pushrod single bolt

ezetom

Member
Hi Everyone,

Im new to the RV world (actually I have an F1 Rocket) and I came across something that sort of perplexes me - apparently this arrangement is the same on most RVs so hence my post here.

Does anyone know why there is only one bolt (the lone bolt that connects the elevator pushrod to both elevators) connecting the left and right elevators together?

With only having a trim tab on one elevator, I would expect some reasonably high "torsional" loads to be carried from one elevator to the other through this one bolt.

I was thinking about drilling another set of holes in the elevator horns, and connecting together with another an3 bolt with a proper sized spacer between - this would greatly increase the torsional rigidity between the left and right elevator.

Thoughts?


Cheers

Tom
 
Whenever I feel the need to modify or reengineer something, especially a potentially critical part like this, I ask myself how many planes are falling out of the sky because they didn’t do this or that..

If you bolted the elevators with the bolt specified, then clamped one elevator and then you pressed up and down on the other, do you really feel that the bolt is the weak link? You’ll destroy both elevators before you even scratch the cad plating off the bolt.. don’t overthink this, one bolt is fine,
 
Sleep well

While it's a triple shear application, a single AN3 has a single shear minimum of over a ton. How much force do you think that long elevator push/pull can exert in compression (push, elevator up) before buckling? What about the steel rod end that is threaded into aluminum that is attached with blind rivets and a little JB Weld?

As far as the single subject bolt is concerned, you're good.
 
Failures of this mechanism is typically due to installation error putting a bending load on the rod end due to binding, or the bearing freezing up. The cyclical bending load breaks the shank of the rod end. That should be your primary concern. Make sure the rod end bearing is working properly and lubricated.
This applies for every rod end bearing in the pushrod system.

Some others have put a metal strap around the horn rod end/bolt and tied it to the pushrod as a secondary retention mechanism.
 
"While it's a triple shear application, a single AN3 has a single shear minimum of over a ton. How much force do you think that long elevator push/pull can exert in compression (push, elevator up) before buckling? What about the steel rod end that is threaded into aluminum that is attached with blind rivets and a little JB Weld?

As far as the single subject bolt is concerned, you're good."


Im not worried about the total ultimate strength, bot more the torsional stiffness between one elevator and the other.

Does anyone know why only one bolt was specified?

I cant think of any other aircraft that have the elevators only connected together through the pushrod fastener.
 
Here is a detail of the elevator "connection" on a lancair - you can also see where the pushrod rod end attaches to the horn.

I know there are issues with flutter - not so much with the F1, but generally speaking - I don't understand why the elevator half's are only connected together the way they are in the RVs... Increasing the torsional stiffness between the elevators would be a good thing in my mind..
 

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TLAR Engineering

I would be very careful using TLAR engineering on a data proven, successful design...especially when it concerns a flight control.

Unintended consequences usually do not end well...
 
Im not suggesting TLAR engineering - I would like to try to understand why van designed it this way, when every other plane I have owned, touched or studied had both elevators connected together through additional hardware. (such as the lancair example I posted)

Considering there are issues with flutter, and there is a less than rigid means of adjoining the elevators, I raised the question...
 
With the bolt properly torqued, the joint is rigid. This is all it needs.

One of the Five Rules of Engineering is "Get it good enough and move on." This is good enough.

There are any number of things that would make our airplanes stronger or stiffer. I don't know of a single one that also saves weight - they all add weight. Airplanes are a collection of compromises flying in formation. Your airplane is a particularly fortunate set of those. Best to leave it alone, without a tested (and I mean that in its most rigorous sense) reason for changing them.

Dave
 
I don't know how Vans engineer designed the control horns on my RV8 but the control horns have side flanges that contribute a lot to the bending stiffness and torsional stiffness to the parts. If you bolt together two flat plates and the bolts and the standoff inserts will increase the bending stiffness as in your picture. The control horns from the RV8 aren't flat plate so it isn't one to one comparison. You have to do the stiffness calculation for both before making the judgement of which one is structurally stiffer, and judgement to which design is less prone to flutter, and at what VNE speed. Two different airplanes were designed for two very different design points.
 

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Airplanes are a collection of compromises flying in formation.

I gotta remember this one for sure. :D

As to the Lancair being different-------well it is a different plane designed by a different person. So, a different setup is not unexpected.
 
Im not suggesting TLAR engineering - I would like to try to understand why van designed it this way, when every other plane I have owned, touched or studied had both elevators connected together through additional hardware. (such as the lancair example I posted)

Considering there are issues with flutter, and there is a less than rigid means of adjoining the elevators, I raised the question...

Then the only answer to the question you posed would come from Vans unless someone here is willing to model the aircraft and conduct an FEA on it...
 
I don't know how Vans engineer designed the control horns on my RV8 but the control horns have side flanges that contribute a lot to the bending stiffness and torsional stiffness to the parts. If you bolt together two flat plates and the bolts and the standoff inserts will increase the bending stiffness as in your picture. The control horns from the RV8 aren't flat plate so it isn't one to one comparison. You have to do the stiffness calculation for both before making the judgement of which one is structurally stiffer, and judgement to which design is less prone to flutter, and at what VNE speed. Two different airplanes were designed for two very different design points.

Its not only the stiffness of the horn, but the stiffness between the elevators. For example, if flutter excites in one side first (most probably the case) the joint between the elevator halves will try to transmit all of the movement of the one fluttering elevator, to the other, the stiffer the system is, the less prone to flutter. end of story. I understand it is the way it is, and lots of planes are flying with one bolt and all is fine, however why is nearly every other aircraft I can think of, having the elevator halves fixed to each other by some means?

Here is another photo of a spam can - a bit different means of attaching the elevator control however it still shows both elevator halves being rigidly attached to each other.
 

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Show us the numbers!

If you are going to make a quantitative argument for or against a design, you need to show us the numbers. Pictures don't cut it.
 
This question got bounced around in the late '90's. I, and I'm sure some other builders, added the second bolt securing the two elevator horns together. So far after 22 years....no issues. :)
 
If I was trying to increase the overall flutter margins of some of the RVs, I'd start with the fin/rudder flutter modes, especially on the -7s with the -9/-7 rudder installed. Using a -8 rudder on the -7s would increase the flutter and strength margins.

I would also consider increasing the strength of the upper rudder hinge (or better yet, a more robust hinge design), to handle potentially high static and dynamic loads induced by the large balance weight at the tip. If the upper rudder hinge fails due to an overload, or any other reason (e.g., loose jam nuts), the rudder will most probably flutter in a mode that consists of rudder torsion (twist) coupling with upper rudder out-of-plane bending, leading to loss of control of the airplane.

The stab/elevator/aft fuselage flutter modes appear to have more than enough flutter margin.

I would posit that aircraft that have a second bolt connecting the elevator horns together have it there for redundancy or strength, and not necessarily for increased torsional stiffness of the elevator.
 
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I would posit that aircraft that have a second bolt connecting the elevator horns together have it there for redundancy or strength, and not necessarily for increased torsional stiffness of the elevator.

There probably isn't much engineering rational for the extra bolt, but our thinking back in the '90's was along the lines of redundancy. As unlikely as it would be, if the pushrod bolt fell out the extra bolt would allow the trim tab to drive both elevators and hopefully provide some degree of pitch control. I don't mind the bolt being back there. :)
 
Hi Everyone,

Im new to the RV world (actually I have an F1 Rocket) and I came across something that sort of perplexes me - apparently this arrangement is the same on most RVs so hence my post here.

Does anyone know why there is only one bolt (the lone bolt that connects the elevator pushrod to both elevators) connecting the left and right elevators together?

With only having a trim tab on one elevator, I would expect some reasonably high "torsional" loads to be carried from one elevator to the other through this one bolt.

I was thinking about drilling another set of holes in the elevator horns, and connecting together with another an3 bolt with a proper sized spacer between - this would greatly increase the torsional rigidity between the left and right elevator.

Thoughts?


Cheers

Tom

It is not just the bolt. When installed correctly, the bolt holds a variety of flat and parallel, steel components into one solid chunk, with compression created by the bolt/nut. This is a very strong arrangement and should have zero torsional movement under a lot of load, even if the heim bearing goes bad.

IMHO, this is not a place for an amatuer to redesign things. My 10 has probably twice the elev area with the same arrangement; I think it is even the same bolt size, maybe one size up.

If your setup does not have a solid stack up of spacers between the elev horns and the heim center with everything held tight by the bolt, it needs to be addressed ASAP; It is a ticking bomb.

Larry
 
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The law of unintended consequences...

"....Stiffening a rear spar is another example of how a modification intended to increase the strength of a wing can actually weaken it. Late in WW-I Fokker introduced the D-VIII monoplane fighter. The D-VIII had a series of wing failures caused by this phenomenon.

Cantilever wings were a new concept, and the importance of twisting during bending was not well understood at the time. The rear spar of the D-VIII wing was too stiff in relation to the front spar, causing the wing to wash in and fail in pullouts from high-speed dives. Fokker claimed that this was because bureaucrats from the German government had decided the rear spar looked flimsy and ordered him to strengthen it.

Fokker complied, and the wings failed in flight. The twisting effect of the reinforcement was eventually discovered by a series of careful static load tests on the wing. Production D-VIIIs reverted to Fokker’s original wing design and no further failures occurred. " Source: KitPlanes article

Conclusion: don't be the bureaucrat!

VV
 
Boy - it sure sounds like a bunch of builders/owners are questioning the design, by professional aeronautical engineers, on a series of airplanes that have never had a problem with this bolt/connection, in several thousand airplanes, which have millions of hours of of experience in the field. If you feel the need to add an extra bolt there, do so, but where does that stop? Is there another place that is under engineered? I’m not an engineer, just want to know.
 
Couple things to think about here.

First - where is the evidence of the PROBLEM that you are trying to solve - how many failures of this connection have been documented? There's no point in offering a solution in search of a problem. Leave that to the politicians, they are plenty good at it.

Second - accepting at blind face value that the joint would be "stiffer" for a moment - why is that a good thing? What engineering study has been done for this "fix" that indicates that this is actually the weakest link in the chain for the elevator/drive system, AND that we need to improve it? If this bolt connection is NOT the weakest link in the system, then changing it offers zero improvement, yet adds complexity, cost, and weight.

The OP keeps asking why other aircraft manufacturers do it differently - I would suggest you ask THEM why that is. Vans seems to be vastly more popular than any of them at the moment, I'm not sure the Vans design is the problem here.
 
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The law of unintended consequences...

"....Stiffening a rear spar is another example of how a modification intended to increase the strength of a wing can actually weaken it. Late in WW-I Fokker introduced the D-VIII monoplane fighter. The D-VIII had a series of wing failures caused by this phenomenon.

Cantilever wings were a new concept, and the importance of twisting during bending was not well understood at the time. The rear spar of the D-VIII wing was too stiff in relation to the front spar, causing the wing to wash in and fail in pullouts from high-speed dives. Fokker claimed that this was because bureaucrats from the German government had decided the rear spar looked flimsy and ordered him to strengthen it.

Fokker complied, and the wings failed in flight. The twisting effect of the reinforcement was eventually discovered by a series of careful static load tests on the wing. Production D-VIIIs reverted to Fokker’s original wing design and no further failures occurred. " Source: KitPlanes article

Conclusion: don't be the bureaucrat!

VV

What happened with the first fokker monoplane had nothing to do with stiffness - it had to do with shear center of the structure. I am actually a trained structural Engineer, and this was an interesting topic that we studied...
 
So as it turns out, F1 did add a service recommendation to add not only another fastener as I mentioned, but as well, size up the rod end to 1/4" and fastener to connect to the horns...
 

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Discussion always good

Thanks for posting the question. Since I’m at the stage of installing control services and looking at that task the discussion in the thread is good to read.

While I Have not questioned the single bolt, reading the discussion brought several things to think about while doing the installation.

No reason to attack the persons question or reason for posting. The information given in response is Helpful even if to confirm vans original design. I love the points, counter points In thought process as people explained, things I wouldn’t have thought about if the original member hadn’t posed the question.
 
So as it turns out, F1 did add a service recommendation to add not only another fastener as I mentioned, but as well, size up the rod end to 1/4" and fastener to connect to the horns...

As a trained structural engineer, you no doubt read the note stating that there is no confirmed data on that mod, only that it shouldn’t do any harm.

Maybe this would be a good project to actually model and do an FEA on; then there would be data…
 
Been there; Done that - it ain’t cheap

Fellas:
The testing on the F1 elevator mod has been approved by other fellas that have plenty of the alphabet behind their names. The testing has gone thru more than one engineering group, and plenty of $$$.

The owner of the ship is very happy with it now - he has dive tested the ship to an area I would not go, but I can say the 240KT TAS Vne is solid after the testing and flight testing. I would say that 260KT COULD be a safe next step for ships that receive the horn mod.

Yes - the pilot/owner did the flight testing.

The engineers did mention that the elevator horns and tubes could use the next thickness of material to raise the Vne to 300KT, tho the parts that mimic the RV4 horns did hold together.

I had a few gents look at the aft section of the F1 fuselage after the Mk.3 Empennage was developed, and that attach area was also beefed up (0.50 thicknesss) after seeing cracks etc at the aft deck around the elev horn cutout, and some additional parts added there and at the horizontal stabilizer rear spar attach.

Some of you F1 fellas might want to apply the aft mods, tho if you can control your TAS in a descent, you’ll be fine at the std 240KT TAS numbering smooth air.

Please read: I did not make the drawing above. The drawing does not show the additional 1/4” bolt + spacers that should be put in the aft area of the horns to help reduce horn twisting - which is the basis for the mod that Tom laid out. I’ll see if I can figure out who did that drawing and put my name on it, and get the drawing updated or removed.

As I mentioned before, any stock F1 should be OK with the 240KT TAS number. If you want to go faster, you’ll need to do some work and spend some $$$ before I would approve 241KT TAS.

Best,
Mark
 
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Strictly an FYI, the horn assembly as installed in a Wolf Pitts built for the late Greg Connell.

Pull for up elevator, one reason (but not the sole reason) the push-pull tube can be a smaller diameter.

Upsized rod end, with lube fitting.

Tie bolt, with aluminum spacer between the horns.

Gusseted horn to elevator spar junction.

Aft support for the center hinge.

Education and recreation, illustration of concepts. This is an airshow Pitts expected to have a very hard life. I am not advocating modification of a Vans elevator horn assembly.
.
 

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