Author Topic: Life limped along at subsonic speeds  (Read 2532 times)

Offline calura

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Life limped along at subsonic speeds
« on: July 16, 2013, 11:07:15 PM »
Hooray for new subforums!  OK so I should probably put this in the SSO section but this one needs filling. 

Listening to episode 174 was great, because sonic booms etc are something I wish I knew more about, and Patrick was asking all the questions that came up in my head too.  Except one.  Jacob said something about how once you get to mach 5, all the forces change.  Which forces, in what way?  And WHY?

Offline adrive7

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Re: Life limped along at subsonic speeds
« Reply #1 on: July 17, 2013, 11:01:57 PM »
I guess I have to start checking the forums now  :P

<Ben Voice>OK, So.</Ben Voice>

The problem with going over Mach 5 is that the air you are flying through is generating an enormous amount of heat. That heat causes all kinds of issues, which is a topic called "Aerothermoelasticity." Aerothermoelasticity looks at the heat generated by friction with the air, how that heat affects the aircraft structure, and subsequently how that affected structure affects the flight characteristics of the aircraft.

So, all aircraft deal with aeroelasticity, also known as flutter. You can't build a completely rigid wing or you'll end up breaking things. If you've flown on a commercial airliner you've probably noticed the wing tips bounce up and down. This is normal.

Pretty picture:


As your aircraft increases speed, you stiffen up the structure, but flutter still happens. You're just going faster when it does.

Once you hit hypersonic speeds, it's not just aerodynamic loads affecting the structure. The heat from friction comes into play. Imagine if the wings are made of wax. That didn't work out well for Icarus, and the same issues come up for hypersonic aircraft. The material making up your structure starts to get more flexible and weaker. This leads to elastic effects, namely your wings flapping around. At Mach 5. This flapping (flutter) can lead to aerodynamic control issues or worse, structural failure.

More pretty picture:


So even if we can figure out the propulsion issues (lighting a match in a hurricane), there are perhaps even larger issues to be solved in the world of Aerothermoelasticity.

I know a guy doing research in the area, I'll have to convince him to come on the show and talk about it.

Here's a paper of his: http://www.asdjournal.org/index.php/ASD/article/viewFile/11/Crowell_ASDJ2011.pdf

Joe
Speaker of Technical Things.

Offline bn

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Re: Life limped along at subsonic speeds
« Reply #2 on: July 18, 2013, 09:53:43 PM »
<ben voice> wow! </ben voice>
Titanium Physicists has a pro-bee-analogy agenda. That's certainly no secret.

Offline stephako

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Re: Life limped along at subsonic speeds
« Reply #3 on: July 22, 2013, 04:29:46 AM »
Wow cool pictures. Me like pictures :)

And by the way: Super cool show. It made me realize that my engineering vocab definitely needs some polishing
Null results, open questions and a bit of my writing: JUnQ

Offline adrive7

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Re: Life limped along at subsonic speeds
« Reply #4 on: July 22, 2013, 06:38:47 AM »
Thanks! Glad you like it.
Joe
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