Page 3 of 3 FirstFirst 123
Results 21 to 30 of 30
  1. #21
    Wannabe Pilot Mick Broom's Avatar
    Join Date
    Jun 2015
    Posts
    23
    Thanks
    1
    Thanked 5 Times in 4 Posts
    I think looking at the tail of most microlights can be a bit of an eye opener in loaded conditions

    Just to clarify the lack of torsional stiffness is due to the tail mount being mounted on the back edge of the tube which allows the tube to deform from its shape as its got no support beyond the tube edge. If you hold the tube to its round shape it cannot do this and any torsional twisting is as the tube would be normally.


  2. #22
    Co-Pilot goldrush's Avatar
    Join Date
    Feb 2009
    Location
    Peterhead
    Posts
    877
    Thanks
    0
    Thanked 53 Times in 50 Posts
    . and those things which do not flex... break........... even trees "bend in the wind"
    Wally Hayward


  3. #23
    Airfield Ops Peter Twissell's Avatar
    Join Date
    Dec 2016
    Location
    Norfolk
    Posts
    87
    Blog Entries
    1
    Thanks
    8
    Thanked 16 Times in 13 Posts
    Quote Originally Posted by goldrush View Post
    . and those things which do not flex... break........... even trees "bend in the wind"
    Not universally true, especially with materials which fatigue. Fatigue is the result of repeated strain (i.e. displacement). A stiff structure, such as a pyramid frame, will survive many more load cycles than a more flexible structure, such as a cantilevered single tube where the ultimate failure strength of both structures is the same.
    Hence wooden aircraft design is different from aluminium aircraft design.
    G-BZNP Still not dead


  4. #24
    Co-Pilot Martin Watson's Avatar
    Join Date
    Apr 2012
    Posts
    312
    Thanks
    10
    Thanked 29 Times in 27 Posts
    Quote Originally Posted by Peter Twissell View Post
    Not universally true, especially with materials which fatigue. Fatigue is the result of repeated strain (i.e. displacement). A stiff structure, such as a pyramid frame, will survive many more load cycles than a more flexible structure, such as a cantilevered single tube where the ultimate failure strength of both structures is the same.
    Hence wooden aircraft design is different from aluminium aircraft design.
    I'm being pedantic I know, but fatigue is the result of cyclic stresses, not strain. Structures for use in cyclic loaded situations (aircraft, ships, oil rigs, cranes, pressure vessels) are designed to avoid stress concentrations. Different materials have different responses to cyclic stresses so it's entirely possible to have a stiff structure in one material that will fail in fatigue long before another less stiff structure.

    The early DH Comet airliners suffered fatigue failure of the pressure cabin from stress concentrations at the right angles at the corners of the windows. The cyclic pressurisation and depressurisation each flight supplied the stresses. Round corners are used now.
    Martin
    BMAA 5370


  5. #25
    Airfield Ops Peter Twissell's Avatar
    Join Date
    Dec 2016
    Location
    Norfolk
    Posts
    87
    Blog Entries
    1
    Thanks
    8
    Thanked 16 Times in 13 Posts
    With apologies for continuing pedantry:
    Stress and strain are directly related, however it is often easier to consider fatigue in terms of strain in order to understand the effects of design elements.
    The mechanism of fatigue is the propagation of cracks which may start as minute surface imperfections. If the structure surrounding a crack is stiff, the stress concentration at the crack does not increase significantly as the crack propagates, because deflection is limited by the stiffness of the structure. Where the structure is less stiff, the propagation of the crack leads to increasing deflection and increasing stress at the crack apex.

    Edited to add: In the Comet window, the frame was the stiff element surrounded by the less stiff skin of the fuselage. Thus when a crack began in the corner, the surrounding skin did almost nothing to support it and the load became focused into the remaining material in the frame.
    G-BZNP Still not dead


  6. #26
    Co-Pilot Martin Watson's Avatar
    Join Date
    Apr 2012
    Posts
    312
    Thanks
    10
    Thanked 29 Times in 27 Posts
    All true on the macro scale Peter. But fatigue cracks grow because locally the stress exceeds the elastic limit - and so there is a plastic zone at the tip of the crack. As you say, surface imperfections are the source of the crack usually. This is because there is a concentration of stress at the crack tip of the imperfection. As the crack extends over repeated stress cycles the stress concentration increases until it is big enough to initiate brittle fracture and catastrophic failure.
    Hence fatigue is not simply a design of structure thing. The processes involved in making the parts (eg extrusion) and, crucially, which alloy is used are very significant.
    Which is why I think using different alloys in replacement parts is a no no.

    Anyway we are in danger of boring every other forum member - or maybe they switched off ages ago. We can chat about linear elastic fracture mechanics some more next time we meet. In the meantime we'd better leave this thread to news of Doms tst modifications.
    Martin
    BMAA 5370


  7. #27
    Captain MadamBreakneck's Avatar
    Join Date
    Feb 2009
    Location
    Essex UK
    Posts
    1,591
    Thanks
    75
    Thanked 171 Times in 136 Posts
    Quote Originally Posted by Martin Watson View Post
    ...
    Anyway we are in danger of boring every other forum member - or maybe they switched off ages ago...
    That leads us to the discussion elsewhere on here about the demand for technical articles in MF


    Back to just bimbling


  8. #28
    Co-Pilot Arielarts's Avatar
    Join Date
    Feb 2011
    Posts
    452
    Thanks
    25
    Thanked 52 Times in 50 Posts
    Quote Originally Posted by Martin Watson View Post
    Anyway we are in danger of boring every other forum member - or maybe they switched off ages ago. We can chat about linear elastic fracture mechanics some more next time we meet. In the meantime we'd better leave this thread to news of Doms tst modifications.
    On the contrary. It is quite educational. Thanks guys.


  9. #29
    Co-Pilot dompech's Avatar
    Join Date
    Sep 2010
    Posts
    465
    Thanks
    3
    Thanked 31 Times in 26 Posts
    Indeed crack propagation is certainly a science that I know little about so thanks for that insight chaps.
    Had a result yesterday. A fellow collector of broken airplanes had been hiding a new set of TST tail parts from me right under my nose! So I brokered a deal and my project now has a complete new (used with very few hours) rear end. Something that I now don't have to worry about, I knew the bits were out there somewhere, just didn't know they were on my doorstep.


  10. #30
    Co-Pilot dompech's Avatar
    Join Date
    Sep 2010
    Posts
    465
    Thanks
    3
    Thanked 31 Times in 26 Posts
    Skins removed from tail surfaces. Damaged frames (rudder and lower strake, one elevator) renewed. All other frames inspected for cracks and alternative skins fitted where needed. An hour and a half well spent in some lovely early autumn weather.


Similar Threads

  1. Thruster TST/X-Air
    By markladson in forum Microlight Discussion
    Replies: 4
    Last Post: 22-03-16, 14:02 PM
  2. SSDR Thruster TST
    By Bertie Lissie in forum SSDR & Foot Launched
    Replies: 7
    Last Post: 24-02-16, 22:04 PM
  3. Rans S4 vs Thruster TST
    By fireblade900 in forum Off Topic Discussion
    Replies: 3
    Last Post: 20-04-15, 19:02 PM
  4. thruster
    By slarti in forum Microlight Discussion
    Replies: 10
    Last Post: 15-07-12, 10:08 AM
  5. Thruster TST Mk1
    By cobra1972 in forum Microlight Discussion
    Replies: 20
    Last Post: 08-03-12, 21:20 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •