MakerPlane Open Source LSA Looking for Guidance on Windscreen Design

[JNicol]

Hi Everyone,

Just a quick note to ask for a little bit of help with attachment of wind shield to frame on the LSA design we are in the middle of.....

Here is a render of the design. The doors are still being designed, so you can ignore the side windows that you can see directly below the wing. The wind shield includes the front clear panel, the two side angled panels and the top curved piece that conforms to the airfoil shape above the main spar in the rendering.



I spoke to a couple of folks at Oshkosh and there were a couple of opinions about how the wind shield could be formed and attached. The reason we went with flat panels is ease of construction. The curved panel should be easy enough to bend on a form with a heat gun I think (I will try this soon). I have the EAA Sportplane Construction Techniques handbook as well as Ron Wanttaja's book Kitplane Construction and have delved deep into the windshield and canopy chapters.

A couple of questions for the experienced builders out there!

• Now one person has indicated that it would be quite easy to form the front and two side panels as one piece and not to worry about building with the three flat panels. Might even be able to do the top curved panel incorporated and do the whole thing as one. Anyone have thoughts on that? I have no experience with forming wind shields.... Could we put filler material between the structural beams and form it on the cockpit itself? Anyone have any references to point to? Another idea we are looking at is having a one-piece front panel that is flat and then curves over the top surface. You can see the structure in the 3D model below without the cross bar that you see in the above rendering. (Note that the spars have been hidden in the image below.)

• If we go with flat panels butting up against each other, what should I use to cover the seams/joints? My initial thoughts include an angled aluminium strip with silicon sealant. BTW, the structure will be composite. Perhaps we form a composite strip to cover the seams?

We have some update info on where we are at on the design in our forums here: http://makerplane.org/forum/viewtopic.php?f=22&t=84&p=428#p428

Thanks in advance for your help on this.

John

[Eric Witherspoon]

A couple of thoughts from my experience making the windshield for my Sonex:

1. If it's a sheet of polycarbonate, you can cold-form it (bend it in place). Down side to this is polycarbonate is pretty poor for fluid/fuel resistance. Spill any fuel on it, and it's quite likely to craze. Plus sides - you can beat on it with a hammer, don't have to worry drilling / countersinking fastener holes, and it is likely to have some impact resistance advantage. One disadvantage in the Sonex application is bending it puts stress on it, and the optics vary with stress. Then the Sonex design lays it down at a pretty low angle, so it's visibly wavy. For the more upright style of windhshield you are looking at, also with a rather large flat area, optics with polycarbonate should be ok - maybe a little wavy in the curves if you go with wrap-around, but still ok.

2. If it's a sheet of acrylic (typical aircraft windshield / canopy material), it's thermally formed. Two general ways are free-blown and molded. With molded, there's a big worry about inadvertently molding imperfections into the shape. Free-blown generally eliminates that risk, but the windshield shape you are considering here won't be made via that method.

Acrylic has very good / excellent fluid/fuel resistance. Spill some fuel on it, wipe it off, no big deal. Down side - drill it in any sort of stressed condition, it is likely to crack. And not just radiate little cracks out from around the fastener location, but crack all the way across the part. So it needs to be molded / formed to fit the supporting structure as closely as you can.

Though for what you are doing, I have seen single-piece windshields that wrap around a nice curve out front as well as taking a bend in the other direction to make a window up over the top of the cabin. So it's definitely possible.

How to attach? There's a bunch of ways. I've seen rivets. I've seen screws/nuts. I've seen screws into tapped holes. I've seen structural adhesives. Whatever you want. I used a structural adhesive on my Sonex's bubble in order to eliminate any stress risers at holes (the bubble on my airplane has exactly zero holes drilled through it).

Check out the Savor. This is a high-wing with the windshield shape I'm talking about.

From this article, he used polycarbonate, just accepting the fuel exposure risk. Pretty slick shape for a flat sheet.
http://www.eaa.org/experimenter/arti...9-12_savor.asp

Another consideration is that by bending up a flat piece to create some curved corners, a gentle curve along the cowl line, as well as a curve that wraps up over the wing is that all these curves will be MUCH stiffer than a flat sheet. A big, flat sheet like what you are proposing might need stiffener beams behind it to minimize the bowing-in. The same thickness of material with a big, gentle curve to it is going to have much more resistance to "oil canning" (flopping to an inward-displaced direction) than if it was curved in the at-rest configuration. Look at more aircraft windshields - there aren't a lot of flat ones out there.

[JNicol]

Hi Eric,

Thanks for the info and the link to the Savor! It is definitely a cool design and I can see how the polycarbonate sheet was cleverly bent in two directions from a flat sheet to create the wind shield. If we decide to stick with flat panels, any hints on covering the butt joints? I was thinking of aluminium or composite strips glued down over the joints?

Regards,
John

[FlyingRon]

For a nice straight bend like that, you can just just make a strip heater and bend the acrylic.
If you insist on butt joints, the best way would be to solvent weld them together. Again pretty standard acrylic technique.

If you're going to make a seperate frame as depcited, why not flush mount the panels to the interior?

Be sure whatever you're using as a sealent is compatible with the aluminum as well as the plastic.

[Eric Witherspoon]

Ron has a good point - if you are going with straight-line joints, and if this part is intended to be made available from a central source (rather than each person developing their own heating/bending tooling), just get the acrylic part bent - that way there is no joint to seal or cover.

I'm thinking about "coverings" - on the Sonex there are metal strips down each side at the bottom edge of the canopy bubble. These are simply aluminum riveted on. For sealant, on my airplane, I actually ran the structural adhesive on the inside of these metal trim strips and bonded the acrylic bubble material to the inside of the metal trim, then riveted the metal trim to the canopy frame. There is no external trim at all on the forward and aft sides of the bubble. The structural adhesive can be seen through the canopy material from the outside, and I masked that to form a neat, constant-width strip. Kind of like what you see on car windshields and back windows - they are just bonded on from the outside.

There's so many ways to get things done I think you need to know more about the specific geometry you are trying to work with - will it be a flat piece "wrapped" into a couple of curves, or will it be flat panes stuck onto a frame. Either way, there's lots of different shapes that could be needed to trim it out / finish it off.

For another idea, find the BK-one website and how they hold their windshield to the frame. It's an aluminum strip (to start with), but part of it is hammer-formed into a 3-d curve, and another part is aluminum that is flat-wrapped in 2 directions. I think a good bet would be to get your windshield supporting structure in place and your clear material at least cut to fit, then mock up covering/trim with paper patterns before transferring to another material.

[JNicol]

Hi Eric, Ron,

Thanks for the info. I am actually going to be starting a full scale mockup of the cockpit within the next week or so and will start playing around with different techniques to figure it out as suggested. I will put up progress in my forum as I do it. The information provided here has given me some good starting points, which is what I need!

John