Sponsor Message:
Aviation Technical / Operations Forum
My Starred Topics | Profile | New Topic | Forum Index | Help | Search 
Overspeed Danger?  
User currently offlinegoinv From United Kingdom, joined Jan 2005, 264 posts, RR: 2
Posted (1 year 2 weeks 15 hours ago) and read 3658 times:

I ask my question on this basic premise :-

A flow of air molecules around the wings keeps the airplane aloft. Too few air molecules and the plane stalls.

Today, using flight simulator my speed envelope (is that the right phrase?) was something like 250Knts - 300Knts. This was at 37000 feet.

The way that I understand it is that because the air is thicker at lower levels the plane can keep aloft at lower speeds. As the air is thinner at higher altitudes the minimum speed is higher. So at 1000ft AGL I can fly at 200Knots. At 37000 feet AGL the air is thinner so I need to fly faster to keep the same amount of air molecules going by the wings.

But what about the upper speed limit?

As I decsend my maximum speed increases from 300knots to 340 knots. Surely as I descend the air gets thicker so the air flowing around the fuselage increases and along with the resultant stress that this creates ?

So basically what determines the maximum speed ? And why is it lower at higher altitudes ?

Sorry if this took some explaining


Be who you are, The world was made to measure for your smile. So Smile.
4 replies: All unread, jump to last
 
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2543 posts, RR: 24
Reply 1, posted (1 year 2 weeks 10 hours ago) and read 3539 times:

The maximum speed limit of the aircraft will be the lower of the maximum operating airspeed (Vmo) and the maximum operating Mach Number (Mmo). Vmo is effectively your thickness of the air concept, represented by the dynamic pressure of the air at that speed. On some aircraft Vmo may well increase with altitude somewhat as the air gets thinner. However there is also Mmo which limits Mach No due to shock wave formation which causes buffet. The speed of sound reduces with altitude (actually with static air temperature). Therefore the airspeed corresponding to Mmo will decrease accordingly. At a particular altitude the Vmo will coincide with Mmo. Above that altitude Mmo will be less than Vmo, so your maximum airspeed indication reduces. But it's not actually airspeed which is limiting maximum speed of the aircraft at higher altitudes, it's the Mach number.


The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineStarlionblue From Greenland, joined Feb 2004, 16994 posts, RR: 67
Reply 2, posted (1 year 2 weeks 8 hours ago) and read 3508 times:

To expand on that:
- Vmo is a structural limit. Too fast and you would get structural damage if you moved the control surfaces or experienced a gust.
- Mmo is an aerodynamic limit as shockwave formation affects lift and drag.

Regarding the relationships between the speeds, this is how I was taught.


E is EAS (equivalent airspeed), which is CAS (calibrated airspeed) corrected for compressibility.
C is CAS (calibrated airspeed), which is IAS (indicated airspeed) corrected for instrument and position error. Basically IAS.
T is TAS (true airspeed), which is EAS (equivalent airspeed) corrected for temperature deviation from ISA. This is the true speed of the aircraft through the air mass.
M is mach number, which is TAS divided by the LSS (Local Speed of Sound). The LSS is only dependent on temperature. For air, the LSS is 38.94 times the square root of the temperature in Kelvin.

The four graphs are for flight in standard conditions below the tropopause. The vertical axis is altitude, the horizontal axis is the value of the speeds. Each graph represents what happens if one speed (the black one) is kept constant while climbing of descending.

Looking at the leftmost graph, if you are climbing at constant EAS, then CAS, TAS and Mach number will increase. Say you are climbing at Vmo, at a certain altitude Mach number will reach Mmo, and you can no longer keep EAS constant because you would bust through Mmo. So from that point you climb at constant Mach number. Move over to the rightmost graph and you can see that if you climb at constant Mach number, EAS, CAS and TAS will decrease.

You can take the rightmost graph and put it on top of the leftmost to get the whole story of climb in a jet. There are also variations for temperature inversions and flying above the tropopause but I'll leave those as an exercise for the alert reader.

[Edited 2013-07-11 20:07:07]

[Edited 2013-07-11 20:07:25]

[Edited 2013-07-11 20:07:47]

[Edited 2013-07-11 20:08:05]

[Edited 2013-07-11 20:12:27]

[Edited 2013-07-11 20:13:56]

[Edited 2013-07-11 20:14:36]

[Edited 2013-07-11 20:15:29]


"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlinegolfradio From Canada, joined Jun 2009, 748 posts, RR: 2
Reply 3, posted (1 year 1 week 6 days 19 hours ago) and read 3270 times:

Isn't this what the Q-corner (coffin corner) is all about?

User currently offlineStarlionblue From Greenland, joined Feb 2004, 16994 posts, RR: 67
Reply 4, posted (1 year 1 week 6 days 11 hours ago) and read 3153 times:

Quoting golfradio (Reply 3):
Isn't this what the Q-corner (coffin corner) is all about?

Coffin corner is a related issue. Coffin corner is the altitude where the stall speed margin is decreasing (since EAS decreases at constant Mach Number) until there is no more margin to Mmo. Thus if you slow down you stall and if you speed up you get mach buffet.



"There are no stupid questions, but there are a lot of inquisitive idiots."
Top Of Page
Forum Index

Reply To This Topic Overspeed Danger?
Username:
No username? Sign up now!
Password: 


Forgot Password? Be reminded.
Remember me on this computer (uses cookies)
  • Tech/Ops related posts only!
  • Not Tech/Ops related? Use the other forums
  • No adverts of any kind. This includes web pages.
  • No hostile language or criticizing of others.
  • Do not post copyright protected material.
  • Use relevant and describing topics.
  • Check if your post already been discussed.
  • Check your spelling!
  • DETAILED RULES
Add Images Add SmiliesPosting Help

Please check your spelling (press "Check Spelling" above)


Similar topics:More similar topics...
"Danger" Panel On A380 Pedestal? posted Thu Dec 2 2010 17:21:37 by DreamsUnited
V1 - A Danger? posted Wed Oct 27 2010 11:52:04 by bueb0g
Etops Danger posted Tue Feb 2 2010 15:15:47 by Kimon
Take Off: Where Is The Danger With Tailwinds? posted Fri Dec 11 2009 11:27:11 by Dairy
Any Danger At Running Props Full Power? posted Tue Sep 2 2008 19:47:36 by Triebwerk
A380 Gear Retraction - Potential Danger? posted Sat Jun 28 2008 01:50:52 by Flexo
Passenger Danger From Uncontained Engine Failure posted Wed Dec 19 2007 21:37:34 by HangarRat
Planes And Fireworks: Any Danger? posted Sun Nov 4 2007 10:42:30 by BA787
Aircraft Overspeed Velocity posted Fri Jun 15 2007 06:27:27 by Blackbird
757-200 And BAe 146: Danger? posted Sun Feb 4 2007 14:55:50 by BA787

Sponsor Message:
Printer friendly format