goinv
Topic Author
Posts: 255
Joined: Wed Jan 05, 2005 11:16 pm

### Overspeed Danger?

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
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Jetlagged
Posts: 2564
Joined: Sun Jan 23, 2005 3:00 pm

### RE: Overspeed Danger?

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.
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Starlionblue
Posts: 17896
Joined: Fri Feb 27, 2004 9:54 pm

### RE: Overspeed Danger?

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.

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"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

Posts: 836
Joined: Mon Jun 15, 2009 5:35 pm

### RE: Overspeed Danger?

Isn't this what the Q-corner (coffin corner) is all about?
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Starlionblue
Posts: 17896
Joined: Fri Feb 27, 2004 9:54 pm

### RE: Overspeed Danger?

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." - John Ringo

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