Van's over-engineering compared to certified aircraft

[sprucemoose]

If there was ever an example of "over engineering" by Vans (defined here as a part stronger than it needs to be) it is the RV-6 spar carry through. They took the -3/ -4 style of spar attach and just extruded it out a few feet. Made for a clumsy, hard to install and probably over-strong spar center section. Look at a 6 and 7 side by side and you'll see what I mean.

This may or may not have anything to do with the higher rate of in-flight failures of 7s vs 6s, even though they are in all other ways very similar.

[hevansrv7a]

While I was building the tail on my 7A a very experienced AI visited me and immediately saw a crack in a part.

I called Van's and got it replaced. My AI helper pointed out that the part's alignment was not correct as to the "grain" in the sheet from which is was cut. He had been my instructor in a metal 101 class at a local aero school so I knew what he was saying. Cracking like that can result from the alignment issue.

I discussed that with the Van's tech and he brushed it off by saying they are not falling out of the sky. There cannot be any question that properly aligning the intended load on the part with the grain in the metal is how you do it best. But that doesn't mean it is necessarily faulty if you don't. It depends on what loads it is intended for and how well you can predict its strength if you don't align. Van's is getting as many parts as they can from a sheet of aluminum without, apparently, regard for grain in this context.

Add that the the well known problems with the 7A nose gear.

I love my 7A but I don't think that "over engineered" is a particularly accurate assessment.

As for Cessna landing gear, I've abused some of them pretty awfully and with no obvious damage. I would not want to do the same to my 7A, especially with regard to the nose gear. Yes, I did make the SB mod AND added the anti-splat device from a 3rd party. But I stopped using grass strips, too.

[FloMo14Builder]

In my profession, accusing someone of over-engineering is, at best, accusing them of neglecting their professional responsibility (which is against the law for a Professional Engineer in most states). That term is highly offensive to a structural engineer. The primary goal of structural engineering is to arrive at a design that can resist the required loads with an optimal structural system. What defines "optimal" can vary (least cost, fastest speed of construction, lowest weight...)

I do not think the OP was in any way intending to denigrate the Engineers at Vans. I believe he used the term over-engineered in reference to the perceived robustness of the design. I think many of the replies have very aptly discussed the dangers of exaggerating the capabilities of any design. My whole point in posting this is so that people without this perspective may gain some insight into why there have been some testy responses to the topic.

There are many places in an RV structure where having stress risers (nicks, scratches, burs, etc..) will not cause any problem over the life of the aircraft. However there is not a reasonable way of evaluating and communicating which ones are and are not a problem to such a vast manufacturing base. I would assume, in an area with any significant shear flow in a Cessna you will find nicely deburred and smooth surfaces.

I have no problem shooting for the level of quality that Vans describes in the first five chapters. I just hope I don't hold myself to such a standard that I never finish...

[mbauer]

Really enjoyed reading through this thread.

Interesting points to think about.

Worked several years as an AutoCAD technician for a group of Civil Engineers.

To me "Over Engineered" means a group of engineers trying to accomplish what one could do in less time.....

Best regards,
Mike Bauer

[rocketman1988]

"...I would assume, in an area with any significant shear flow in a Cessna you will find nicely deburred and smooth surfaces..."

...and that would be a faulty assumption. Having owned many different Cessnas and Pipers over the years, I never found that to be the case...

[BillL]

Originally Posted by David Paule

"Over-engineering" is a rather vague term. To some it means over-strength and to others, it approaches the infamous "paralysis by analysis," excessive use of engineering time.

Are the RVs over-strength? Apparently not, from all I've read. They seem to be adequately designed for their mission, without a whole lot of margin. Remember, their mission includes being built by amateurs.

Has Van's wasted excessive engineering on them? Probably not, because they are still in business.

Dave

My vote: Best answer.

[FloMo14Builder]

Well I am glad I stated it as an assumption. I have not had the opportunity to see under the skin of light aircraft. For what it?s worth, the certified aircraft I see in production have extremely high standards for edge preparation, but those are all made for operations in the flight levels. Thanks for setting me straight Bob.

Of course, if the stresses are sufficiently low, the stress risers are a non issue.

[rocketman1988]

...and those old cessnas weren?t primed either!

[Snowflake]

I remember seeing a horizontal stabilizer from a Luscombe that was de-skinned for restoration. The ribs looked like bananas, none had been fluted. The AME said that Luscombe didn't flute the ribs, they just bent them into place and riveted them.

Regrading the difference between a -6 and a -7, the -6 was designed as someone said "with a slide rule" which may be an overstatement, there were probably hand (electronic) calculators involved as well. But more calculation was done longhand, i'm sure, and those calculations would be done at discrete points in the structure. In between those points, the loading and structure sizes would be some kind of average. That could lead to slightly oversized parts in places.

On the other hand, the -7 was designed with a computer, and as such could be optimized at every point on the aiframe to meet the mission, with safety factors, and no more. There are practical considerations as well, such as reinforcing for wing-walks, that probably aren't factored into the (flight) structural loading, but are necessary for human interaction.

[rocketman1988]

I wouldn’t knock that slide rule...look through aviation’s past and you will find some really impressive design work that was done...you guessed it...on a slide rule...