aircraft window material selection
aircraft window material selection
(OP)
Are many aircraft windows made of stretched acrylic? I've read a thread here that described the 747 window as a laminate with a thin layer of glass on the outsides. My question regards the choice between stretched acrylic and polycarbonate. The stretched acrylic is stronger and stiffer than polycarbonate, but is much more notch sensitive as shown by the Izod impact test. How do the manufacturers using streched acrylic overcome this problem, both in manufacturing and later maintenance. Is this why they use the laminated windows?
Tom Stanley
Tom Stanley





RE: aircraft window material selection
RE: aircraft window material selection
The question actually arises from powerboat racing. They look to the aircraft industry for their technology but they don't have the rigorus standards and quality control before and after manufacture that the aircraft industry does. There has been a long term controversy about the best material for their windshields and the bird strike test is often referred to, although their incidents are somewhat different.
I was wondering if the aircraft industry also had the controversy but it looks as though they can minimize the probability of developing small stress raisers leading to the notch sensitivity problem.
RE: aircraft window material selection
bird-strike ? maybe a heavier bird, but impact velocity is much less.
stress raisers ? ... fatigue ?
RE: aircraft window material selection
RE: aircraft window material selection
The key question on the material choice is whether to go for the better stiffness and strenth of the stretched acrylic, or a better notch sensitivity with the polycarbonate. I'm assuming with aircraft there are strict standards regarding hole preperation for bolting of the windshield into the aircraft frame. Correct?
Thanks for your help.
Tom Stanley
RE: aircraft window material selection
It's not just notch sensitivity-- it is the entire spectrum of fracture toughness, meaning crack initiation, crack propagation, etc. Polycarbonate is substantially tougher than acrylic, and will outperform acrylic in the type of dynamic (i.e. high strain rate) event you just described.
RE: aircraft window material selection
RE: aircraft window material selection
The windshield pane(s) is(are) only 1/2 the picture: the windshield frame and substructure is the other 1/2 of the picture. The frame/substructure is essential for transmitting shock windshield loading to the forward hull... without the pane shooting out of the frame. Crushing loads due to inverted contact with water will be a bear to handle!
Comments.
Acrylic-polycarbonate-polycarbonate-acrylic laminates are a good match for what You are doing.
The acrylic inner and outer plys provide high quality abrasion/erosion protection for the structural (soft/tough)polycarbonate structural plies. In addition, acrylic [or an acrylic replacement], has the ability to accept long-lasting abrasion/scratch resistant coatings. DO not accept a polycarbonate windshield with JUST hard coatings: Repair can be impossible. Also, do not accept a wndshield with only one acrylic ply: the opposite side of the pane is will be abrasion prone (repair is difficult-to-impossble to accomplish).
NOTE.
May want to conside an antiglare coating and a hydrophobic coating if sunlight-glare and water spray are significant factors.
Regards, Wil Taylor
Trust - But Verify!
We believe to be true what we prefer to be true.
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RE: aircraft window material selection
Regarding the frame, I have done a couple of fea models of the entire canopy, including the windshield. Using that tool the frame around the windshield can be strengthened. Right now though, the fea shows the windshield itself is the weak point.
In the slower classes, of course, cost becomes a factor. When cost will only allow a single ply we've been recommending the polycarbonate with the hard coatings. Some builders still insist on using a single layer of stretched acrylic though. That's when I start to worry about the notch sensitivity and now fracture toughness.
Tom Stanley
RE: aircraft window material selection
RE: aircraft window material selection
The greater the slant angle of the windshield the better... up to the point it becomes too wide and hard to manufacture and structurally support .. or becomes too heavy.
Also ensure that when roll-over occurs, the windshield can't inadvertently turn sideways and become a water-scoop. A semi-enclosed cockpit with side shields may be required.
Regards, Wil Taylor
Trust - But Verify!
We believe to be true what we prefer to be true.
For those who believe, no proof is required; for those who cannot believe, no proof is possible.
RE: aircraft window material selection
Mr. Taylor is also right in that slanting the windshield will help up to a point. The equations are well known for when the boat is running right side up. So some approximations can be done for the reverse. I don't have the software to find out more accurate forces through simulation. Once again it is a compromise between reducing the forces by slanting the windshield, keeping the span small to reduce stress, and giving the driver enough vision. The cockpits are fully enclosed as much as possible, with hatches for entry and egress (a different problem)
It is sure great to discuss this with knowledgable people willing to help. I appreciate it.
Tom Stanley
RE: aircraft window material selection
RE: aircraft window material selection
Tom Stanley
RE: aircraft window material selection
RE: aircraft window material selection
-Kirby
Kirby Wilkerson
Remember, first define the problem, then solve it.
RE: aircraft window material selection
RE: aircraft window material selection