windshield fabrication

Discussion in 'Workshop Tips and Secrets / Tools' started by aeromike49, Dec 16, 2012.

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  1. Dec 18, 2012 #21

    StarJar

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    I just had a bright idea... Keep a spray bottle of water handy to knock away all the dust (small grit) before using your soft cloth and plastic cleaner/Pledge.

    edit; Hmm, but then you have a wet window/windshield. (Scratches head.)
     
    Last edited: Dec 18, 2012
  2. Dec 18, 2012 #22

    PTAirco

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    SVSUSteve - of course any material will break at some point. Duh. I assumed we're discussing Lexan vs Plexiglass. But Lexan will not shatter; it is almost rubbery in comparison with plexiglass. Go make up a test frame, one with Plexiglass and one with Lexan and take a ten pound hammer and see what happens. No "magical thinking" is required to prove this.

    For a typical thickness windshield on a light aircraft, Lexan will be far more impact and crack resistant and totally shatterproof unless the bird that hits is filled with acetone or MEK. And plexiglass doesn't like that stuff either.
     
  3. Dec 18, 2012 #23

    SVSUSteve

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    Been there, done that. I reiterate that a few swings with a sledgehammer is not an accurate representation of the force involved with a bird strike. You often wind up with a cracked piece of Lexan or a punched out hole like Dan described. You fire an one or two pound gelatin block at it (to simulate a bird strike involving a modestly sized bird) out of a small homemade air cannon and you wind up with shards of polycarbonate unless it is backed up by either a second sheet or there is something there to make it more resilient. One of my friends (a civil engineer by trade) owns a homemade one that can fire something up to the diameter of a Thanksgiving turkey or the length of a standard two by four depending upon which "barrel" it is fitted with. He got the OK to build it (to make sure he didn't violate any potential legalities) to test home designs for damage resistant to cyclonic or tornadic winds. We also used it as part of a test for the fuel tank wall designs for the Praetorian.

    I will agree with you that it is better than the variety of acrylic we commonly see sourced for aircraft windshields. However, it is not as tolerant or protective as you want to make it seem otherwise we would not have the issue with birdstrikes that we do. You're entitled to your opinions on the subject but Dan and I am also entitled to counterbalance that to a reasonable degree. This is a tricky problem but not one that is insurmountable

    Honestly, the best way to avoid a bird strike is to climb quickly to avoid the range at which most occur (below 3,000 feet). It's a lot like avoiding collisions with bumbling pilots due to the limitations of see and avoid. Chalk up the risk of birdstrikes as another reason why I prefer not to fly low and slow any longer than absolutely necessary.
     
  4. Dec 18, 2012 #24

    PTAirco

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    5lbs bird. 5lbs sledgehammer. Bird will punch through a certain thickness of Lexan, but sledgehammer will not. OK, I am totally convinced.

    In 25 years of working with polycarbonate I have never seen "shards" of it. It tears, eventually, sure, but you don't get shards.
     
  5. Dec 18, 2012 #25

    SVSUSteve

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    The difference is simply one of velocity. Nothing more, nothing less. A duck flies at about 40 mph (about 35 knots) so you are most likely looking at closure speeds of 100+ knots. I doubt anyone could swing a sledge that fast since professional baseball players apparently swing the bat at around 50-70 mph.

    Perhaps shards was the wrong choice of words. I just mean separated pieces. Sorry for the confusion.
     
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  6. Dec 18, 2012 #26

    Toobuilder

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    Not my area of expertise, nor do I have a strong opionon one way ot the other. But in addition to my engineering department AND the company that builds our Mil Spec canopies and windscreens telling me not to use PC, I found it compelling that you just dont see people offering aircraft windscreens/canopies as an option.

    Anyone have an opinion concerning the apparent UNpopularity of Lexan in the GA world?
     
  7. Dec 18, 2012 #27

    StarJar

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    Don't forget that the "duck" will be influenced by the direction of air moving upward, up and over the aircraft. This combined with backward angle of the windshield creates much less than a direct blow. Just trying to convince myself that flying can be fairly safe again.:gig:
     
  8. Dec 19, 2012 #28

    Dan Thomas

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    There will always be risks to flying. If we try to eliminate them all we'll never go flying.

    Automobile windshields are designed to resist and deflect stones and birds and so on, but occasionally we hear of someone killed by unusual debris. Last year someone in Calgary was killed by a small brake drum that fell off a scrap-car truck. This fall a woman was killed in Southeastern BC by a rock flung from between the dual tires of a dump truck. Both of these came through the windshields of the cars they were in. What do we do next? Demand windshield able to withstand brake drums and four-inch rocks, just to eliminate the one-in-50-million chance?

    I have encountered big birds at altitude. I once had to dodge a Bald Eagle coming at me; he would have taken the tail clean off the airplane, or come though the windshield, if I hadn't. I know an instructor who had a student come back from a solo flight, barley able to control his 172, because a turkey vulture struck the leading edge of the wing about midspan and flattened the metal back against the spar. The drag and turbulence from that big squashed flat spot nearly ended the flight. Another friend, flying in Africa, ran into another big buzzard and that one came into the cockpit, cutting everyone up with sharp shards of plexiglass. The drag and turbulence and blood in his eyes made control difficult and he ended up ditching it in a swamp. Everyone got out OK, amazing when you consider the ferocious occupants of swamps in the Okavanga Delta. And a former student of mine spun a 172 and had the plexiglass windshield shatter in the dive recovery near Vne and cut himself and his girlfriend up with more sharp bits of plexiglass. That one was probably due to an old, crazed windshield. Plexiglass doesn't age well, either.

    Yes, birds are a real hazard. I know of some pilots that are afraid of getting too high, either because of acrophobia or because they're afraid of a midair collision, but low flying kills plenty of guys, too. Birds, powerlines, cell towers and their guy wires are bad enough, but the failure of the engine at such low altitude leaves very few options or time to plan a survivable encounter with the ground.

    Geese seem to like to use airports as navigation checkpoints.

    Dan
     
  9. Dec 20, 2012 #29

    Battson

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    At 100kts it makes no difference whatsoever to the momentum of a 5lb bird.

    Unless it's specially designed for it, any GA windscreen will most likely not survive bird-strike from something as large as a duck or big seagull. Material independent. As a rule they are sized to resist airflow at their design airspeed, and the incremental force involved with birdstrike at that airspeed is a bridge too far for any GA windshield. Songbirds maybe, waterfowl no. As with most things general aviation, the incremental weight of a bird-proof screen is what makes it prohibitive when you consider a significant proportion of strikes are larger bird species commonly found around airports.

    Aeromike -
    Any material which is an amorphous solid will reflect and refract the light that passes through it, to some extent. So no matter if you used glass / acrylic (plexi) / lexan - it will bend the light in some way as it strikes the surface at an angle. If you have a curved surface you may see a change in the way it bends it much more obviously, as the angle to your eye changes across the curved surface. Is that what you're talking about, or do you mean it's muddled the image?

    We don't usually notice the way amorphous solids bend light, such as with sunglasses or a windshields, because they cover most of our field of view. It sounds like maybe you have a smaller free-standing windshield which makes the effect more obvious?

    If it's a muddled image, then that is more of a manufacturing thing and something which you already have advice on.
     
    Last edited: Dec 20, 2012
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  10. Dec 20, 2012 #30

    StarJar

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    Any bird in flight will be effected by the airstream. Maybe only a slight amount, but just a little is significant, combined with the windshield slope angle. And besides after passing through the propeller, it won't be a 5 lb. lump, any longer. Contact with the blades will slow it's velocity as well, like getting hit with a 100 mph bat. At this point, I'd like to hear more about actual cases in single engine planes, rather than speculation.

    I'
     
  11. Dec 20, 2012 #31

    Battson

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    I can see where you're coming from, but it doesn't really work out. I can do a calculation for you if that is necessary, but suffice to say the energy associated with the relative velocity of the bird and plane will dwarf any minor effects that changes in the bird's velocity would have. ASSUMING there would be enough time for the small force the air pressure preceding the windshield to overcome the momentum of the bird and meaningfully change it's velocity, which there isn't, as the forces are small and the time they act on the bird for is even smaller - because the plane is moving so quickly. If it were a smaller, lighter bird or large insect then it might happen as you imagine.

    You wont hear too many first hand accounts, as bird strike on the windscreen is quite often fatal.

    I can't imagine how contact with the blades would slow the bird. They are designed to accelerate airflow (and objects in the airflow) in the direction of the windshield. What is the through process there?

    You are correct about the angle of the windshield being a factor, because the shallower the slope, the smaller the change in the velocity of the bird and the less momentum change overall - which directly relates to the force the windshield must be strong enough to withstand. Smaller momentum change means smaller force on the windshield.
     
    Last edited: Dec 20, 2012
  12. Dec 20, 2012 #32

    StarJar

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    OK The first part makes sense to me now. I see how the forces would be small and the reaction time miniscule.

    And I suppose you are right about the prop too, since the duck would not hit the face of the prop. Otherwise air would be hitting the face of the prop, which it doesn't.

    I stand corrected in both areas. Thanks.
     
  13. Dec 20, 2012 #33

    Battson

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    Sorry I don't mean to call you out, your thinking is right on the money. It's a fast collision.
    I relate it back to big birds on the road (50% the speed). Dammed if I want to drive into a duck at 60mph, even with a thick hardened glass windscreen!

    The angle of the windshield would certainly help your chances like you say.
     
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  14. Dec 20, 2012 #34

    SVSUSteve

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    The funny thing is that the higher you fly, the lower the risk of a mid-air in cruise flight. This is mostly because most of the VFR traffic without ATC contact is within the first few thousand feet of ground. This is a predominate reason why I want to fly at or above 18,000 feet whenever possible for as much of the flight as I can. While a lot of folks are afraid of getting creamed by a business jet or an airliner, the thing they should really be worried about is the "don't tread on me" no radio (or refuses to use the radio) jerk-off in something like an ultralight or a Cessna. Those are the ones you worry about and that is why I always sweat more coming down through about 5,000 or 6,000 ft AGL than I ever do going up through it.

    Thank you Battson. You beat me to pointing out that fallacy.

    The effect of the slipstream is negligible based on actual cases. As for passing through the propeller, the odds of it hitting the propeller seem to be smaller than one would think. Keep in mind that on most GA aircraft, the impact angle is not often completely straight on and the propeller does not blanket the entire profile of the window. Even if it does hit the prop (based on the couple of cases where I know of this happening) you're trading a shattered windshield for an unbalanced prop and the engine either shutting down on its own or having to be shut down to avoid the vibration potentially tearing the engine from the mount.

    I can't speak for anyone else but what I have mentioned is not speculation as it is based either on things I have seen personally, have seen documentation of (usually photographs) or was told about by reputable sources (NTSB or FAA personnel or their foreign counterparts).

    I can recall a dozen or so that weren't but only maybe two or three that were. It probably has a definite variation based upon the mass of the bird as all of the fatal crashes I have heard of involved birds over 8-10 lbs. One fatal case involved a brown pelican off of Florida and another involved Canada geese over Minnesota. In the latter case, the NTSB (so far as I recall) did not find direct evidence of the bird strike to the windshield but I heard from a friend who helped recover the aircraft from the crash site that parts of at least one goose was found within the cabin.

    As a point of fact, I know a friend who took a pigeon through the windshield of his Mooney at cruising speed (three came through total) and to the face without serious injuries. He suffered a black eye, a couple of cuts, broken nose and a chipped tooth but landed safely without further problems.
     
  15. Dec 20, 2012 #35

    Toobuilder

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    Well, I'm another survivor... Certainly wasn't a goose, but hurt plenty!

    https://www.homebuiltairplanes.com/...ht-safety-better-pilots/11233-birdstrike.html

    Concerning the use of Lexan - considering its extreme deformation in an impact, one should consider attachment points for that added stress. As it was with my encounter, some bird parts managed to tuck under the fiberglass and roll bar hoop. I can imagine a Lexan windscreen deforming enough to "unhook" from the rear bow and shoot the critter right into the cabin like a ski jump, then have the windscreen pop back into shape.
     
  16. Dec 20, 2012 #36

    SVSUSteve

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    That's ironic because you're the person I was thinking of with that post when I mentioned a quail incident. I just got the material of the windshielf mixed up. LOL
     
  17. Mar 24, 2015 #37

    dougwanderson

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  18. Jul 30, 2015 #38

    fly_boy_bc

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    I have a pristine 30 year old lexan windscreen.........

    I am also in the optical industry and PTAirco is exactly right. FEW things are as tough as Lexan...
     
  19. May 10, 2016 #39

    Arthur Brown

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  20. May 10, 2016 #40

    BoKu

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    I know that there are a couple of shops there that make bubbles for old Slingsby gliders, so I'd suggest you check the glider magazines and ask around in the glider community.

    Thanks, Bob K.
     

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