Open-EZ scales

In previous posts, I had mentioned that the scales of the Open-EZ scanned docs (the 'full-scale' A1 through A14 drawings) might be a little off. Here, I will explain what I meant and how I came to this discovery.

First of all, let's discuss aircraft references. If you've done your homework and read the plans, you would know that on page 3-19 and 3-20, it explains how things are referenced on the airplane. I'll summarize it here:

Aircraft measurement reference system:

The three basic references are called fuselage stations, butt lines, and water lines. All three are given in inches from some arbitrarily chosen reference - so fuselage station 100 is 100 inches away from fuselage station 0. These are abbreviated as F.S., B.L. and W.L.

Fuselage stations (F.S.) are used to define the location fore and aft on an airplane. F.S. 0 is typically located at the nose of the aircraft and increases as you travel aft. Note that once the system is set, it is fixed and does not move for the life of the aircraft - typically, the origin of the system is defined by the engineer who designed the aircraft.

Waterlines (W.L.) are used to define vertical locations. Waterline 0 is generally found near the belly of the aircraft and increases as you travel up. Negative numbers are acceptable – so a W.L. of -10 means 10 inches below the reference waterline. Think of a boat floating in water - hence the name 'waterline'.

Butt lines (B.L.) define positions inboard and outboard. The aircraft’s centerline is typically B.L. 0 and is the case for the Long-EZ. So a B.L. 157R means 157 inches to the right of centerline.

Using these three references, any point in the airplane can be described with a fuselage station, butt line, and waterline - think of it as your X, Y and Z locations of a giant grid system set up around your aircraft.

Ok, so now that we have a clear understanding of our aircraft reference system, let's discuss how this relates to the scales of our Open-EZ drawings.

Whoever scanned the Open-EZ full-scale drawings (NOT ME), hand drew scale lines at the bottom and side edges of every page. The idea was to make it easier for us to print and verify the scale - simply by taking a ruler up to it and hopefully measuring the value that is indicated on the page. Here is an example of Drawing A7:

Notice the hand drawn scale reference at the very bottom...

So in a perfect world, you print this page out, measure the distance from line to line and cross your fingers that you measure the stated value. If not, you adjust your printer settings and try again until you nail it.

Most people would follow these rules and be happy with the results... but if you've been following my blog, I'm sure you've noticed that I'm not 'most people'. Of course, my OCD kicked in and said, "How can I cross-verify these scale references?"

Yup... you guessed it... aircraft measurement reference system! 

So if you look close, you'll notice that most every drawing has a combination of F.S., B.L., and/or W.L. I've circled a few on Drawing A7 to demonstrate.

 If you take a ruler and measure the distance between WL20 and WL10, you should measure 10 inches (if your drawings were printed to scale). Likewise, if you measure the distance between F.S.30 and F.S.20, you should measure 10 inches.

Using 3D-CAD (computer aided design) software, I imported every drawing and scaled them according to the F.S., W.L. and B.L. reference marks. I then measured the hand-drawn scale lines at the bottom and sides of the pages and noticed that some hand-drawn reference scales were off by over an inch.

I get asked, "Is it ok to use the Open-EZ drawings?" all the time. My answer is, "As long as you scale them accurately via the F.S., W.L. and B.L. reference marks, you'll be fine - just don't use the hand-drawn reference scales at the bottom of the pages since those can not be trusted."

I hope this helps clarify the Open-EZ scales issue...


Crockett Motor Works (CMW) said...


Tango Charlie said...

Thank you for this informative post! It takes all the mystery out of earlier comments regarding the inaccuracies in the drawings. Thank you, thank you, thank you!
Really enjoying watching your progress via this blog. Keep it up, man!

Jörg Krause said...

Thank you for your wonderful bolg.

Keep it up!

May I ask what cad software you have used?

Ary Glantz said...

Thanks for the positive comments!

I use Catia V5.

Jon Matcho said...

Ary, good save on that sheet you show above, but the airfoil templates do not have enough WL and BL reference marks.

Please read and share this post regarding the Open-EZ Rev 5 template inaccuracies: