Effect of cabin pressure on aircraft windshield

Discussion in 'Flight Following' started by Tantalum, Dec 6, 2018 at 3:52 PM.

  1. Tantalum

    Tantalum En-Route

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    Reading the most recent "I learned about flying from that" link here made me think about something..

    In the article they guy writes that his face was blasted with 310 knots windspeed at 25,000 when the windshield on his Lancair Evolution failed

    He also writes that the 6.5 PSI cabin differential ensured that the glass blew outwards, that's why there were no pieces of glass hitting his face or the cabin

    My question is 2 part:
    A.) for a given square inch.. what force is greater, the 6.5 inches pressing outwards, or the 310 knots of wind pushing inwards. I imagine this would be a very easy calculation assuming the mass of air at that altitude is known (I recall from physics it was around 30 g/mol at sea level).. anyone know? Curious what the *actual* force is on the windshield is at that speed..

    B.) If the cabin 6.5 PSI is in fact greater pushing out than the 310 knots of wind pushing in.. than surely those tiny pieces of glass can't carry that momentum so the minute they're presented with the 310 knots of wind and the pressure has been released they'd come back in the cabin.. right? 6.5 PSI is not insignificant, but spread across small pieces of glass? How much forward travel relative to the plane could a tiny piece of glass actually achieve? Maybe a quarter inch? Wouldn't it just get blown right back into the plane?

    Anyway.. made me think about the actual physics that are at play here
     
  2. Stewartb

    Stewartb Final Approach

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    Cessna windscreens are in negative pressure on the outside. The ones with straps have straps to keep the windshield from being pulled out, not pushed in.
     
  3. Tantalum

    Tantalum En-Route

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    Interesting, what makes the straps required vs not? Is it an AD, based on age? Which Cessna? Pressurised 210 or their jets? 310 knots is a lot of windspeed.. it must have *some* effect?

    I believe it, because the guy wasn't cut and had no glass in the plane, but would love to see the actual math behind it too
     
  4. midwestpa24

    midwestpa24 Line Up and Wait

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    One thing to keep in mind is the airflow over the windshield. Most are sloped in some fashion, and will have some aerodynamic effects occurring. The airflow on the outside may even produce some small amount of lift.
     
  5. Stewartb

    Stewartb Final Approach

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    When I converted my 180's windscreen to strapless I had to add a doubler to the top edge of the plex and capture that with an aluminum channel supplied with the STC. I assume the other Cessnas without straps have a similar arrangement.
     
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  6. Hank S

    Hank S En-Route

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    Think of it as 6.5 pounds on each square inch of windshield, which will bow it out; then when (if?) it cracks, the whole thing splinters and blows outwards. There are a lot of square inches in the windshield, and 6-1/2 t8mes as many pounds pressure acting on it.
     
  7. GRG55

    GRG55 Final Approach

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    :yeahthat:

    6.5 psi differential pressure acting on a one foot wide X two foot tall windshield area is 1872 lbs of force. Outward.
     
  8. AKiss20

    AKiss20 Pre-Flight

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    At standard temperature, the density of air at 25,000 ft is about 0.52 kg/m^3. 310 knots is 159 m/s so the dynamic pressure is 6606.2 Pa or 0.88 PSI.
     
  9. Zeldman

    Zeldman Final Approach

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    A pilot at a company I worked at lost a windshield in a C-414. When it went, it sucked the glare shield and every lose piece of paper. It also took his headset and glasses, but they stayed in the plane. It was the right side, the less expensive side.

    This was in winter right after dark. When he landed his first words was, "It's cold".


    They got out of the plane and he, along with the patient and med crew switched planes and took off again.
     
  10. Tantalum

    Tantalum En-Route

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    Damn! Thanks for the replies.. still crazy to think and a weird thought.. I wonder what PSI equalizes the pressure?