Aircraft Grade

Greetings Aeronauts,

This week has been interesting. In Monday’s post I talked about getting a copy of the VP-2 plans and the idea of building both the VP-1 and the VP-2 in 1:4 scale to compare the plans. Looks like I will begin working on an airframe sometime next week. I am really looking forward to start actually building. This weekend is jammed with Grave Mistake workshops and the Step Out: Walk to fight diabetes 5k that my wife and I participate in every year along with some friends.

Throughout the week I have been interacting with the great people in the Volksplane Yahoo Group and the fine folks at Oshkosh 365. I got several replies when I asked for some advice on adhesives for wood aircraft. The plans call for Aerolite which does not appear to be readily available. For the most part I got a lot of thumbs up for T-88. Other suggestions included, Aerodux which is a Resorcinol Formaldehyde Adhesive like Cascophen, and West System Epoxy. I am not sure which one I will go with but I am leaning toward the T-88. I need to get the MSDS for each of them as well as take a look at volume, weight, coverage, and relative strength comparisons, and yes, the cost comparison, too.

Another question I had was about Certified Aircraft Grade Lumber. It looks like there ain’t no such animal. In short, your aircraft lumber supplier checks the material to make sure it meets the standards of Aircraft Grade, those standards being MIL-S-6073 Military Specification, Spruce, Aircraft, MIL-P-6070B Military Specification, Plywood and Veneer Aircraft Flat Panel, ANC-18 Design of Wood Aircraft Structures, and of course the standards set in AC-43.13 1B Maintenance & 2B Alterations Aircraft Inspection, Repair & Alterations. They inspect the material, stamp it, and price it accordingly. It is not a federally recognized certification, it is a voluntary compliance to Military Specifications set back when the military used wood aircraft, with a little updating now and then. Don’t take that to mean that I think it’s all hokum; I don’t. The standards are there because they are appropriate to the application. If you are not sure how to grade lumber or are not sure if you can determine compliance reliably, buy lumber from someone who can make those assurances!

While Sitka Spruce is the de facto aircraft lumber, it is not the only species used. The main factor in choosing wood other than structural strength is grain, vertical grain (VG) to be precise. A VG Clear Douglas Fir (DF) is one of the closest quality woods to Sitka Spruce, in fact it is stronger than the spruce in most respects. The drawback with DF is its weight; DF runs about 26% higher in weight than Sitka. In this aircraft, a majority of the structural weight is in the plywood and not the lumber, so the impact on gross weight using DF instead if Sitka should not be that much. Without an electrical system, using minimal instruments, and other weight saving details like single occupant controls, I think the trade-off in favor of structural strength is reasonable, even wise, not to mention that as a Pocket-Change Builder I can’t overlook the cost of DF being significantly lower than Sitka. Having a supplier less than 20 minutes away doesn’t hurt.

Most likely I will end up going with the VP-2 airframe. It is intended for a higher gross weight and a higher useful weight. With two occupants taking up 170 pounds each, that gives me 340 pounds to work with as a single pilot. While I won’t need all of that, I sure as heck am not under 200, nor have I been since 6th grade. I wrestled in the 220 class in Jr High. The extra airframe strength and the VP-2 load capacity provide a significant safety margin over the VP-1 design which is why I am leaning in that direction.

Something fun thing to add to this week was my first EAA Chapter meeting. There are two chapters here in San Jose with a couple of others less than an hour away giving me a lot to chose from as far as finding a good fit. Thursday night was Chapter 62’s monthly meeting. I am not sure if I am going to join 62 yet, but I did have a good time there. The chapter is more of a flying and activities chapter than a building chapter. As it turned out, last night was a great night for me to go to a meeting there because the speaker was Zeke Smith, author of Advanced Composite Techniques.

Zeke demonstrated his process for vacuum forming a leading edge to be used on an an ultralite project. The end product has a thin outer skin and foam strengthening with plenty of room for similarly light ribs. I was amazed at the strength of the leading edge without any support and the incredibly light weight. If I was working on a composite project I would be buying this book right now. I may still get it for this project to use his techniques to form the engine cowling and turtle deck of the VP. That might even give me more weight advantage for the DF…

Only slightly off topic, I can easily see myself building a composite VP-2. In fact I can see some of the layups for Zeke’s vacuum process in my head now. To quote Red Leader Stay on target.

For anyone interested in helping out the FlyBoy, I need to get a print copy of AC-43.13 1B/2B. Contributions to the Pocket-Change Builder’s Fund are always greatly appreciated.

Until next time, blue skies and tailwinds,