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Subsonic And Transonic  
User currently offlineArzouni From UK - England, joined Aug 2012, 7 posts, RR: 0
Posted (1 year 11 months 3 weeks 2 days 1 hour ago) and read 5470 times:

i am doing some presentation on aviation and need some deep information about :

a. how to Explain the terms Subsonic and Transonic ?

b. and Describe three design features that can be used to delay the problems of
Transonic Flight and how the features achieve the delay ?

8 replies: All unread, jump to last
 
User currently offlineStarlionblue From Greenland, joined Feb 2004, 16991 posts, RR: 67
Reply 1, posted (1 year 11 months 3 weeks 2 days 1 hour ago) and read 5469 times:

Quoting Arzouni (Thread starter):
a. how to Explain the terms Subsonic and Transonic ?

Subsonic can describe any flight slower than the speed of sound. It can also describe flight slower than the transonic region. The borders of the transonic region are defined at the lower end where any part of the airflow around the airfoil becomes supersonic and at the upper end where all airflow around the airfoil is supersonic.

Quoting Arzouni (Thread starter):
b. and Describe three design features that can be used to delay the problems of
Transonic Flight and how the features achieve the delay ?

1. Swept wings. Delay the onset of shockwaves. Shockwaves depend on the span component that is perpendicular to the trailing edge, and this is a smaller component than actual airspeed on a swept wing.
2. Supercritical airfoils. Delay the onset of shockwaves.
3. Mach trimmers. Automatically trim out mach tuch.

I recommend the old but excellent "Handling the big jets" for information, but warn that it was written before supercritical airfoils.

[Edited 2012-08-04 05:28:53]


"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 2, posted (1 year 11 months 3 weeks 1 day 21 hours ago) and read 5396 times:

Quoting Arzouni (Thread starter):
i am doing some presentation on aviation and need some deep information about :

Those aren't really deep questions...you're not asking a.net to answer a homework assignment, are you?

Quoting Starlionblue (Reply 1):
The borders of the transonic region are defined at the lower end where any part of the airflow around the airfoil becomes supersonic and at the upper end where all airflow around the airfoil is supersonic.

This is a very good working definition. The more technical one is related to the transformations used to analyze inviscid compressible flow. For subsonic flow you can get the real pressure coefficients by dividing the incompressible (easy to analyze) pressure coefficients by sqrt(1-M^2). For supersonic you flip it around, sqrt(M^2-1).

However, when M is around 1, since this term is in the denominator of the fraction, the pressure coefficient "blows up". The area where it blows up to the degree it's useless is the transonic regime. As a result, the borders of "transonic" move around depending on how accurate an answer you need.

Tom.


User currently offlineArzouni From UK - England, joined Aug 2012, 7 posts, RR: 0
Reply 3, posted (1 year 11 months 3 weeks 1 day 5 hours ago) and read 5188 times:

Dear Starlionblue thanks for the information   it was useful ...

Dear Tom ( tdscanuck )
i am doing only a research on Subsonic and Transonic nothing have to do with a course work, i had a course work before and i asked for help by telling that am doing an assessment  
but thanks for the information u post  .


User currently offlinewingscrubber From UK - England, joined Sep 2001, 845 posts, RR: 0
Reply 4, posted (1 year 11 months 2 weeks 3 days 20 hours ago) and read 4818 times:

Arzouni, be sure to include a section on Whitcomb Area rule - this becomes very important at high subsonic/transonic speeds.
Also, be sure to research variable-geometry air-intakes for the engines, all supersonic jets require this, either by a moveable cone or ramp that allows the engine to breathe.



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User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 5, posted (1 year 11 months 2 weeks 3 days 18 hours ago) and read 4801 times:

Quoting wingscrubber (Reply 4):
Also, be sure to research variable-geometry air-intakes for the engines, all supersonic jets require this, either by a moveable cone or ramp that allows the engine to breathe.

They don't actually require it, it just cuts losses way down because an intake tuned to one speed will be terrible at another speed unless you adjust it. If you don't care about intake losses (e.g. some ramjet-powered missiles) then you don't bother with the moveable intake.

Tom.


User currently offlineAKiss20 From United States of America, joined Sep 2007, 603 posts, RR: 5
Reply 6, posted (1 year 11 months 2 weeks 3 days 10 hours ago) and read 4738 times:

Quoting tdscanuck (Reply 5):
Quoting wingscrubber (Reply 4):
Also, be sure to research variable-geometry air-intakes for the engines, all supersonic jets require this, either by a moveable cone or ramp that allows the engine to breathe.

They don't actually require it, it just cuts losses way down because an intake tuned to one speed will be terrible at another speed unless you adjust it. If you don't care about intake losses (e.g. some ramjet-powered missiles) then you don't bother with the moveable intake.

Also depends on your intended Mach number. The F16 which only goes to Mach 1.6-ish has a straight inlet because the normal shock losses at that speed aren't too terrible. The F14, which flies faster (like Mach 2.1 iirc) has a variable geometry and ramped inlet to induce oblique shocks which are less loss-y.



Change will not come if we wait for some other person or some other time. We are the ones we've been waiting for. We are
User currently offlinewingscrubber From UK - England, joined Sep 2001, 845 posts, RR: 0
Reply 7, posted (1 year 11 months 2 weeks 7 hours ago) and read 4544 times:

Quoting tdscanuck (Reply 5):
They don't actually require it, it just cuts losses way down because an intake tuned to one speed will be terrible at another speed unless you adjust it.

It's true that there are several examples where variable geometry is not used in some supersonic inlets, I'm just trying to help Arzouni direct some research; why are variable geometry intakes used on the aircraft, how do they work etc, as a general rule in old-school supersonic turbojet design they are required for exactly the reasons you mention.



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User currently offlineFlySSC From United States of America, joined Aug 2003, 7402 posts, RR: 57
Reply 8, posted (1 year 11 months 1 week 20 hours ago) and read 4223 times:

Quoting Arzouni (Thread starter):
a. how to Explain the terms Subsonic and Transonic ?

When I was flying on Concorde, "Subsonic" was any speed under Mach 0.94
"Transonic" speed was between Mach 0.95 & M 0.99.
"Supersonic" was Mach 1.00 and above


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