Dragonfly Canopy Shape Survey

The inspiration for this section comes from my efforts while helping to refine the KR-2S in 1996. The canopy used on the Dragonfly tandem wing was becoming a popular alternative to the scarcer (at the time) and heavier stock Rand model. Given that no two builders seem to build the fuselage exactly the same, the extremely flexible 1/8" Dragonfly canopy is a natural, fitting in well with the "whatever you want it to be" spirit of the KR series planes.

Since the it was intended to be used on an entirely different design, the problem is that ready documentation for the exact dimensions of the Dragonfly canopy (as used on the KR-2S) was not available. This makes using this alternative somewhat problematic for builders.

In 1997, I purchased a Dragonfly canopy from Aircraft Windshield Company in Los Alamitos, CA, and kept it on hand until I could get my shop together.

The following documents my efforts to formally survey the dimensions and shape of the Dragonfly canopy and construct a standardized baseline CAD model that anyone interested in using this canopy on their own project could use as part of their design effort.

3D and 2D AutoCad DWG files (v.14) of the canopy model are available for download. For those who don't have a CAD system, 1-1/2" = 1'-0" scale drawings are also available as Adobe Acrobat (PDF) files for printing and use with traditional drafting methods.

Survey Process Overview

	This is the laser level I used to do the line projections and alignment checks. It's a 16" torpedo style with both plumb and level reference vials built in. It's also housed in a machined aluminum case. You can see the line projecting prism, also with plumb and level vials, which attaches magnetically to the front of the laser. You can attach it to a regular camera tripod, or a surveyor's tripod - if you have one. I shopped this tool around. The most similar one I could find cost over $400 and that's before adding the line and cross prisms at over $50 apiece. Most of the one's at the home center stores had short plastic bodies and were in the $150 range - also w/o the prisms. This unit here, I picked up through Northern Tool, Co. (http://northerntool.com) for $37. The cross and line prisms were $10 apiece. The level itself is a high quality tool, but you can see why the prisms only cost ten bucks each (I had to clean out the machine burrs with a cotton swab). They work great though. Playing around with it, I could see the laser spot at night at over 400 yards! It's got a nice tight spot up close too - something the home center lasers didn't have. Shipping was free from the catalog I ordered out of. I think this is one of the better deals floating around out there if you are considering a laser tool. The Northern Tool catalog number for the laser is #37781. Line Prism is #377802. Cross Prism is #377801.
This mock up jig represents a fuselage with an exterior width of 42-1/2" at the shoulders. For my plane, this will yield a clear interior width of 40" - about the same as a Cessna 152.
	The "frame rails" are straight & parallel. They are also rabbeted so the bottom of the canopy would be visible during the survey.
A centerline was marked on the supporting beams during layout. Once the jig was leveled (no easy task on my shop floor!) and the rails checked for parallel (again), I used the laser with a line drawing prism and a plumb bob to transfer the centerline to an adhesive measuring tape on the shop floor (for measuring overall length).
	Here is the canopy after being clamped in place and checked for alignment. The lower aft corner and the extent of the bottom flat section of the canopy are marked on the frame rail measuring tape.
Without moving the laser, I was able to transfer the centerline to the canopy and pinpoint the forward edge with the plumb bob. An identical setup (in reverse) was used to mark the aft edge of the canopy on the floor tape.
	Here I measured the height of the aft edge above the bottom of the canopy taking the height of the frame rails, and the level (used as a support for the steel ruler) into account. I used a similar technique to measure the forward height of the canopy as well.
Using the laser (moved perpendicular to the frame rail), the aft edge was projected onto the frame rail measuring tape. The distance from this point to the lower aft corner, combined with the aft height allows calculation of the aft angle cutoff of the canopy. The neat thing here is that the plumb line lights up like a neon sign (in dim light) when the laser is perfectly lined up. You can't ask for a better double check of alignment.
	This shot shows the laser being used to set the zero point for measuring the canopy height. I wish all the pictures could show just how useful this little gadget is!
Here's the setup to measure the canopy height. Once the base line had been established at the bottom, I raised the laser straight up, until the line just barely projected onto the lower edge of the level. The line is projected onto the vertical measure and marked. The difference between the upper mark and the lower is the canopy height!
	This is the primary shot I used to digitize the canopy profile into my CAD system.
This is the shot from the aft end of the canopy used to digitize the section view into the CAD system. Using the hard measurements I got from the laser survey, I corrected the digitized photo lines for size and aspect ratio. Since the pictures also show linear perspective, this had to be accounted for as well in the computer model. Since my modeling system can show either orthographic or perspective views, it was easy to overlay the model with the picture to see if I was in the ballpark. Once the basic profile and section views were complete, I made a surface model of the canopy...
	This is the "final" product of the survey - a parametric computer model. With a little FEA work, the canopy model can be "deformed" to predictably fit just about any fuselage shape. This model uses "automatic" fairing techniques (three view controlled tangency) that my company developed to assure a smooth predictable surface without much fuss.
	I was close, but it took a couple of tries to get the computer model to reflect the actual shape of the canopy. I plotted out section templates at various longitudinal positions and cut them out of cardboard to check the accuracy of the surface. I discovered here that the canopy had taken a "set" - It wasn't the same shape one side to the other despite my storage method to prevent just such an occurrence. I turned up the heat in the shop and left it overnight clamped in the jig. In the morning, I checked it with the template again and BINGO, most of the adverse shape had disappeared. This photo doesn't do justice to the 1/16" all around match from the canopy to the computer model.
This is the curvature map for the canopy. It doesn't serve much purpose here other than to show the extent of shape - and it looks cool. If this were a boat hull I'd use it to find high and low spots as well as other areas of unfairness in the surface model. Smooth transitions of one color to the next are the hallmark of a fair surface - looks like this one fills the bill.

DRAGONFLY CANOPY - (42.5" Outside Maximum Width)
LOA (Overall Length):	45.5"
Maximum Height:	15.25"
Aft Height:	13.25"
Forward Height:	6.375"
Length of Bottom Flat:	21.25"
Width (This configuration):	42.5"

Just What Shape Is A Dragonfly Canopy?

Survey Process Overview

Results Of The Survey

Files for Download