|BlueSky Business Aviation News|
Since this is how itís usually planned, letís look the process from back to front. I.e., let us take the desired end point as our given and work backward from there. This is how I learned it in the Army, as well as engineering school: figure out what you want, then put in all the steps necessary to get there. I know. I wish it were more complicated and sexy, but really, thatís the basic process most people use. For better or worse, itís also appropriate for most situations.
I thought I might take something I had mentioned in a previous column as an example. What I would like to have as an endpoint is a fully certified horizontal stabilizer for a Gulfstream G450 made out of a sustainable, non-toxic, wood or wood-based material using all non-toxic processes and materials. I picked the G450ís horizontal tail because I am familiar with it, and I picked a new, eco-friendly (and people friendly) material because I think this would qualify as a huge innovation and one with powerful symbolism. Moreover, I think it is very achievable.
The first thing I would want to do is determine the market potential, of course. The G450 is a classic aircraft. It entered service in 1987 as the GIV, and over 500 have been produced, and it is still in production today (though perhaps for not a lot longer as Gulfstream moves upward to the G650). Five hundred possible airplanes doesnít sound like a huge amount, but the GIII tail isnít much different, nor is the GII. Comes to that, the GV tail isnít all that different, either, so conceivably, with some intelligent planning and good design work, we could make a horizontal stabilizer that with minor tweaks could serve the entire line of Gulfstream jets. In fact, with a certified, eco-friendly horizontal stabilizer in hand, we might very well be able to sell a few as a factory option, if we can work with the folks at Gulfstream. The management there are exceptionally good, so I would think we could work something out, but thatís still a small part of the plan.
What does ďcertifiedĒ mean?
A certified horizontal stabilizer. What does that mean, really? Iíll say it means an STC. That is, a Supplemental Type Certificate from the FAA. This would be the appropriate document because we donít want a new type certificate for the airplane, we just want to change one part of it, and a TSO, or Technical Standard Order, isnít designed or intended to cover major structure. Since this is for a Gulfstream product, we would probably want to start with the Atlanta FSDO or Flight Safety District Office. There are an enormous amount of highly technical details associated with getting through the certification process and there are people with the skill and experience of making that sort of thing happen. Generally speaking, we call them ďcertification engineers.Ē I know a couple. We would hire at least one for this effort.
Several things would have to happen before we can get our STC. We have to flight test the new tail on a test aircraft showing that it performs the same as the existing all aluminum tail in every respect. That is, it wonít flutter early and provides all the same amount of longitudinal authority that the original stabilizer did. We would also want to clear it for operation in known icing conditions. The FSDO will have the final say in exactly what tests they want to see before granting the STC, but those are the basics.
Prior to our test flight program, we will need to show that the replacement stabilizer is as strong structurally as the original stabilizer. Since this is an entirely new material, both the FSDO and any rational chief engineer would want to break one, i.e., load it up to ultimate and make sure it does not break prematurely while measuring exactly where it does break. The FSDO wants to make absolutely certain that the structural strength is what we claim it is, and the chief engineer wants the data to adjust or calibrate his structural model so we can do it again in less time for less money.
The standard industry process would be to design the structure using one of a number of different suites of tools. Essentially, we have to be able to draw it and take that drawing and turn it into a 3-D model. The CAD program does that for us. There are both engineers and drafters who are very competent with various CAD packages. CATIA is what I used when we were designing what has become the 747-8. There are other programs on the market, obviously, but CATIA is what Iím most familiar with.
An iterative process
Once we have a model, then we add material properties and design loads. This will likely be an iterative process. That is, we will design up a first cut at the structure, then run it through a number of different scenarios to find our critical case and make sure that the model says we have sufficient strength. If we donít, then we redesign the parts that we think will fail in that case and retry. Once everything shows good we check the weight. If itís a little heavy, we start looking at the parts that are a bit over strength to see if we can lighten them up some. Often we do trade studies on the weight differences between various combinations of rib pitch and skin gage. There are a lot of variables here and no one has any experience with the new material, so the first design cycles are likely to take a longer than one might expect if one were doing the same thing in aluminum.
This will be a very interesting exercise since there are no published design allowables for this new material. So, before we can run the models of our structure, we have to figure out what our allowables are. This is actually a fairly simple process if we assume that the design will be ďorganic aluminumĒ and that the structural design will look exactly like the aluminum version, but with different thicknesses to reflect the nature of the new composite material. If it were my money, this is the way I would go.
While it would be even more innovative to create a structural design that more fully takes advantage of the differing nature of the new material, that can wait a bit, I think. First, we would want a pretty conventional looking structure so everyone feels comfortable with it. Moreover, changing only a few variables at a time allows us to feel our way forward without over-committing to something that we might not be able to do with the funding available. Unless you are a defense contractor, there is no such thing as unlimited funding. We want a new, breakthrough material in use on actual aircraft. We can put off the perfectionist tendencies for a couple of design cycles.
That said, a new material with new behaviors opens up the possibility of truly innovative structural designs. If one of those new possibilities is blindingly obvious and low risk, we would include it and just do the extra work to get it certified. Remember, the point here is innovation, but we must balance that with fiscal clarity. Good design is hard, but itís a lot harder without money.
There are fundamentally two possible materials that we are considering for this design. One is a cellulose-based composite material. It uses cellulose fibers in a conventional resin matrix. The fibers are available commercially from Lenzing, an Austrian textile company, as either lyocell or Tencel. By now, there may well be other sources. The other material option is a sort of ďplywood.Ē It is actually much more sophisticated than what you can buy down at the Home Depot. Back in the 1920s and 1930s a good bit of work was done on this sort of material. One German investigator used birch plies .004 and .002 thick--less than half that of todayís woven carbon fiber materials--laying up as many as 200 plies in his test pieces. An American inventor used steam heated dies to apply heat and pressure to the layup and could produce a full set of wings and a fuselage in two hours using a phenolic resin to bond plies of wood. While not as strong in tension as aluminum, the low density of wood more than made up for it with superior buckling performance. None of the metals do as well in buckling as laid up wood plies.
Which flavor of win?
Personally, I favor the laid up wood plies because I think it would have less embodied energy than the cellulose fiber composite. Moreover, if we want to move completely away from toxic materials and processes, we would probably want to avoid conventional resins. Even with conventional resins, however, a cellulose-fiber composite material would be a huge win. There is no lose, here, just different flavors of win.
Basic materials testing
Before we can characterize our basic material to ascertain the design allowables, we have to do the fundamental testing to see what material performs best for our purposes and derive a standard. Is it a .004 thick ply or .006? Is a 6 oz cellulose ply better than an 8 oz ply? Are we better served by a wood ply or a cellulose fiber ply? What are the manufacturing process implications? Who will be our suppliers? How will they be certified? We will have to spend some money here to develop the data necessary to make the basic decisions about material selection, and no, NASA wonít help.
From the other direction
The more conventional way to express the flow would be to say we have to spend a lot of money to characterize materials, develop the allowables, create a good design, prove the model, demonstrate that the structure is meets all the requirements and that the flight characteristics are the same as the original. And then sell lots of them. But thatís not really how we think about these things. We start with our destination, and then we figure out to get there from here.
Terry Drinkard is a Contract Structural Engineer based in Jacksonville, Florida whose interests and desire are being involved in cool developments around airplanes and in the aviation industry. He has held senior positions with Boeing and Gulfstream Aerospace and has years of experience at MROs designing structural repairs. Terryís areas of specialty are aircraft design, development, manufacturing, maintenance, and modification; lean manufacturing; Six-sigma; worker-directed teams; project management; organization development and start-ups.
Terry welcomes your comments, questions or feedback. You may contact him via firstname.lastname@example.org
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