- Oct 18, 2003
- Saline Michigan
The issue in each and every case is: Is the resulting modified part strong enough and stiff enough over the long run?Ok, I'll bite at the bait...
So, a "lightening" hole, cut and flanged, will weaken the rib significantly, but a riveted aluminum "space frame" rib is not significantly weakened as designed?
I am always concerned when I find that a builder has taken upon themselves to remove material that the designer had included. When the designer finished the design, it was built and flown, and thought to be better than some reasonable level of strength, stiffness, and durability. Once material has been removed, is it still known to be good - or is the builder guessing that the designer was less than fully weight conscious or worse, incompetent? Heck, if I thought the designer was less than excellent, why would I want to build and fly the airplane anyway?
In the best of all worlds, the designer of, say, a wing, considered spar sizes, more ribs, less ribs, full ribs, skeleton ribs, cross bracing, etc, and chose the lightest design that does everything for this airplane and will be durable in service.
- If the wing in question was indeed optimized or even approached the optimal design, taking material out is likely to make the wing weaker than needed and/or softer than needed and/or less durable than needed;
- If instead, the designer was at the other end of the scale, the ribs are closer together than they need to be, and simple plywood ribs were used to speed construction, and were thicker than they need to be, then maybe you can remove a bunch of material and still be OK. Maybe;
- Or maybe a less than fully careful and competent designer ended up with some pieces that are borderline (or worse) by themselves, but give no trouble because the nearby structure is overly sturdy so the borderline part is not overstressed. Then when someone starts lowering the strength/stiffness of parts and the borderline one becomes understrength, cracks in service, and, if they are lucky, catch it and have to build a new set of wings.
Then there is the issue that many designs out there were intended for one weight and Vne that were published in the plans and other materials. Then the fleet has builders who just have to install the next bigger engine, constant speed prop, IFR panel, carpeting and side panels, spend lots of time on perfect looking paint, etc. Then the bird is heavy and cruise speed turns out to be darned close to Vne, so the airplane gets flown to higher speeds and at higher loads than the prototype was ever flown to. It might even need some more strength and/or stiffness in a bunch of places.
I prefer to start with the assumption that most folks designing and marketing an airplane successfully probably had a pretty good idea of how much material is needed on each part and the design reflects that. Removing material that the designer put in may be a risk a builder is willing to take, but I recommend against it unless a significant number of that exact airplane with that exact modification have been flown for a while.
This is all explanation of a simple philosophy for most of us: Stick to the plans unless there is a known good change, then stick to the plans for the known good change. You will know what you will get then. Removing material that the designer put in? Not unless some other smart folks have done it too, then only after those folks have demonstrated it is still a good airplane.