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pmnewlon

Well Known Member
As has been said multiple times: some in the EAA and the kit manufacturers appear to be covering themselves for future litigation. I wouldn't know where to begin to see that changed. Congress? Hardly as most are attorneys and are interested in seeing the propagation of their species rather than sanity.

I've been reminded twice in the last couple months about this:

- I needed an 'aircraft grade' 5/16-18x1" bolt for the starter/alternator bracket on my RV4. I thought I'd buy four and have a few spares so when I was at Lane Aviation I asked for them. "We have to order those. Are you sure you need four?" Well, I only need one but thought I'd get extras. Why? "At $28 each I am thinking you might just order the one that you need." The way they explained it to me was that compliance with the 'Aircraft Safety Act of 2000' ( http://www.govtrack.us/congress/billtext.xpd?bill=h106-3862 ) has driven the cost of low-demand parts out the roof.

- I am dabbling with a modified IF1 design (I own plans for two and want to meld the good parts of both) and have been told by two aircraft designers that they wouldn't touch my project for less than $10K just because of the liability. For starters I 'simply' want to make the cockpit four inches longer. Before you jump all over me that any such design change, even though relatively small, ripples through the whole airframe: I understand that. I am guessing though that for a good engineer (as I assume both are given their credentials) the time spent to validate the changes would not exceed a week's full-time work. Most likely far less. The only thing either would commit to outside of that was to tell me 'buy my book, read it, and you will know where to start'.

So where is a guy like Greg Hale to get such analysis done for the mods he wants to make to the RV10 kit? Or should he just not push the envelope with 'his' airplane mods? If one doesn't have the skill-set to handle the requisite calculations and feels the 5-digit CYA fees are excessive, she should just not build? or just build exactly per a kit maker's plans? To that point, how do I know that the plans I own, drawn oh so many years ago have had the necessary analysis done? If I embarked on building 'per the plans' I would see a lot of head-nodding since I wasn't deviating from said plans.

Turns out that one of the designs was done by the engineer/draftsman simply taking measurements from an existing airplane that didn't have drawings available any more, sitting down at his drafting table and producing them. Since it was flying it didn't need analysis, right? When I talked to a guy that actually did just that he told me he got the BaJeezus scared out of him the first few flights before he realized he needed to analyze things like horizontal stab incidence, etc. himself.

In the meantime most people I talk to say 'find and build something that looks like what you want and doesn't need modifications to the plans'. I have the mathematical skills, the ability to learn, and the desire to succeed at my dream but sure would rather spend the time making parts than doing the math or earning the extra money needed to make someone comfortable with the liability exposure of doing the calculations for me.

I see the other side of the coin but have to say I side with the camp that says Greg has taken enough flogging; I applaud him for his daring. I dream about being independently wealthy and pursuing a law degree and a mechanical/aero degree. I would provide engineering services to the experimental enthusiasts, and give away legal services to dilute the value placed on them :D

"Ugh" I say as Atlas Shrugs.


Phil
 
pmnewlon said:
...I needed an 'aircraft grade' 5/16-18x1" bolt for the starter/alternator bracket on my RV4...
Click to expand...

Heh, I bet it came head-stamped with the asterisk that means garden-variety Grade 8.

Anyhow, plenty of perfectly nice airplanes have been designed and developed by people who are not engineers. For example, I'm currently developing a high-performance racing sailplane for kit construction, and I've had almost no formal engineering training. I'm thinking the extensive informal training I've had, and occasional guidance from real aero engineers, will be sufficient.

If you want to see an example of an airplane designed and developed by ordinary folks, have a look at the HummelBird. Morry Hummel was a sheetmetal guy who got engineer and designer Dick Schreder to double-check his work. After he started publishing plans, they were later improved by Bill Spring. And after that, pretty much everybody who built one stretched it out an inch or two here or there to fit themselves into it. I like to think of it as Rocket science, but not exactly rocket science.

Thanks, Bob K.
 
Engineering Analysis

Phil,
When I went looking for a kitplane these were my priorities:
1. Was the airplane properly engineered and tested?
2. Does the airplane have good performance and handling qualities?
3. Is the kit manufacturer likely to remain in business during the time it takes to build one of his airplanes?

I have seen lots and lots of poor preliminary design and lots and lots of poor detail design in commercially available kits. How do I know? Well after a 30+ year career in mechanical design, in many cases all I have to do is look at a part, estimate the loads, and visualize the load path. So a lot of airplane kits were disqualified after screening them with my first three priorities. I chose an RV-9A and am looking forward to finishing it so I can confirm my choice was the right one for my mission.

But while I have not made any structural modifications, I have made some minor ones in systems. I think this is inevitable in custom built airplanes. And I recognize that under the law (there's those pesky lawyers again) I am the manufacturer and ultimately responsible for the safety of any changes I make. The problem as I see it is that people make changes without adequate knowledge or appreciation of the compromises that have already gone into the aircraft design.

Getting to one of your questions, there is more to engineering than making a few calculations to prove the design. Generally engineering is a synthesis of preliminary design, detail design and testing. In the early years of airplane design, lots of testing was done to compensate for the lack of theory and design methodology. Sometimes the flight test proved the inadequacy of the design with dire consequences. The disadvantage of modifying an existing design (after you have confirmed that it was properly designed and tested) is you don't know all of the assumptions the original designer made. However, a study of certified airplane load requirements (FARs) will give you a starting point.

If you don't have an engineering degree, or the equivalent in self study, I can suggest two books that may help:
1. Simplified Aircraft Design for Homebuilders by Dan Raymer
2. Design of Light Aircraft by Richard Hiscocks

The first is primarily a preliminary design guide. It does not go far enough into the structural/detail design to do justice to your stretch fuselage project. The second is written by a co-designer of the deHavilland Beaver among many other aircraft and is a very good guide to load analysis and structural sizing. After studying both, I think you will be in a position to decide if you need professional help or not.

Bob, I agree with you that "ordinary folk" can design a safe airplane with good handling qualities, but in order to do so they will have to have the equivalent of an engineering degree by self study. And the airplane will have to have been designed with adequate stress and deflection margins, passed both ground (vibration/flutter survey) and flight test appropriate to the aircraft category.
 
You can download the materials reference here:
http://www.everyspec.com/MIL-HDBK/MIL-HDBK+(0001+-+0099)/MIL_HDBK_5H_1804/

You can download an excellent if somewhat abstruse collection of structural analysis methods here - this is not at all a tutorial, and you definitely won't need all of it:
http://euler9.tripod.com/analysis/asm.html

You can purchase an excellent textbook and general reference for structural analysis here:
http://www.amazon.com/Analysis-Design-Flight-Vehicle-Structures/dp/0961523409

These are all standard resources in the industry, although the first one is now superseded, this covers the materials you're actually likely to use: aluminum, steel and titanium, as well as some of the more exotic materials available.

The important thing is that the RVs and probably the Rockets were designed using the technology described in these.

Dave
 
design

The Formula One racers have evolved from speeds in the 165 range to current speeds in the 265 range. Many of the racers at Reno are highly modified Cassutts. On some all that remains of the Cassutt is the fuselage frame. Some use some very exotic composite wings that were probably designed by an aero engineer. Others are likely eyeball engineered.
 
Unfortunately, one-off projects are expensive in general. How much is a spoon worth? Perhaps a dollar or two. Yet, a local machine shop would be hard pressed to make you one for less than a few hundred bucks. One-off engineering projects are kind of the same way. It takes a lot of costly work and the end result might not be worth it to you.

Even a minor change affects everything around it and a complete analysis takes a very long time... often many times longer than the customer imagines. Sadly, I have to turn down most homebuilt engineering projects because there's no way to make it economically feasible for either party.

Luckily, a sharp builder can often use simplified design methods for small projects. With conservative practices and careful testing, almost anything is possible. Indeed, many successful kit/plans aicraft have been developed with very little engineering.

I personally like to see creative modifications and custom aircraft. However, it's important to be careful, stay within your capabilities and ultimately accept the risks.
 
Thanks for the material suggestions guys. I went to Dan Raymer's talks at OSH this year. Interesting guy and useful reading material (I bought is distilled for homebuilders book). I am working my way through 'Stress Without Tears' right now. Many have suggested Bruhn's book as well, and it is referenced often in SWT.

The RV tie to my musings is that Van, Mac, etc. say 'get engineering analysis of your changes' and the engineering folks say 'you can't afford us'. Many will be thinking 'if you can't afford the engineering you shouldn't make the change' but I think that people's penchant for lawsuits today create that atmosphere. How about survivors suing the vacuum pump manufacturer when their family members die because the PIC didn't recognize a failure in IMC? I will haunt my family if they sue someone like that :p

I will definitely stay within my capabilities and accept the risks!
 
I'm one of those self-taught guys. A few comments....

The most important thing is to know what you don't know. The more you know the more you know what you don't know.

It's not really about math, which is often high school level or less. The appropriate equation can be selected from a reference page. The trick is application of the math. Most homebuilders lack a basic understanding of fundamentals (stress and strain for example), and quite a few don't grasp structural basics like tension, compression, shear and torsion. Start there.

Find an expert. I've found if I do my homework and ask organized, specific questions, experts are often happy to help. Some of those experts became great friends, and it all started with a question.

Don't be afraid to trade. Maybe the analysis does require a professional. Maybe the pro is designing or building something. What can you do for him? Don't laugh. A friend and I once mirthfully addressed each other as "The Professor" and "Igor", which tells you all you need to know about the working relationship.

Buy books. Download stuff. Build a reference library of your own. In addition to the usual bookstore sources, I've obtained old ANC publications from the EAA Library, SAE papers from the basement at Auburn, and Excel spreadsheets from everywhere. And obviously, don't forget a plain 'ole Google search. Here's a starter:
www.air.flyingway.com/books/Airframe-Stuctural-Design.pdf

The OP asked what a particular builder could have done to get structural review of his mods. To be blunt, there wasn't any need for professional review of the rudder and seatbelt mods; they were obvious bad ideas. Simply posting them here with a "Hey, wadda ya think?" would have been all the review he needed. The fuel tank mod is a whole different deal. Flutter margin is a subject for the pros. Not sure what is simple and what requires a pro? Ask.

We're allowed to do what we do because of "education and recreation". You have a responsibility to learn what you need to know in order to make a modification. Anything else is a dice roll, a Darwin test, another chance for a spectacular YouTube fail.....and has consequences for your peers.

I'd like to recommend two books for beginners because they are fun to read and excellent for the basics. Think of them as the engineering equivalent of an RV intro ride....you may get hooked:

Structures, or Why things Don't Fall Down, J.E. Gordon

Engineer To Win, Carroll Smith
 
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Who was the Igor (or was it "eye-gor"):D

Jeremy

P.S. I'm STILL trying to get the potato image out of my head:eek::D
 
jrs14855 said:
The Formula One racers have evolved from speeds in the 165 range to current speeds in the 265 range. Many of the racers at Reno are highly modified Cassutts. On some all that remains of the Cassutt is the fuselage frame. Some use some very exotic composite wings that were probably designed by an aero engineer. Others are likely eyeball engineered.
Click to expand...

IIRC from the Cassutt plans I saw in the 70's (a hangar mate was building one) they came on less than 12 sheet of drawings. No details, just enough to define the structural items.

All of the installation of systems - including the control system, I think - was was entirely up to the builder. The fuselage plans were basically how the steel frame was made up.

Sounds like they used most of the original design to me....:)
 
2nd the recommendation re "Engineer to Win"

That book should be sub-titled "engineering for non-engineers." Thanks for posting that recommendation, Dan.

LarryT

Larry Tompkins, P.E.
RV-6A W544WB
W52
Battle Ground, WA
 
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