Indicated Stall Speed
02-24-2008, 07:01 PM
#1
Gets Weekends Off
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Joined APC: Aug 2007
Posts: 156
Indicated Stall Speed
Just wanted to see what you guys think.
As I understand it, Indicated stall speed changes only with load factor, weight, and, power (available).....................
Does indicated stall speed change with altitude?
I cant seem to find any clear answers.
Thanks guys......and gals
As I understand it, Indicated stall speed changes only with load factor, weight, and, power (available).....................
Does indicated stall speed change with altitude?
I cant seem to find any clear answers.
Thanks guys......and gals
02-25-2008, 06:25 PM
#4
Prime Minister/Moderator
Joined APC: Jan 2006
Position: Engines Turn Or People Swim
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Assuming fixed GW, CG, and AOA...
At higher density altitude:
1) The true airspeed necessary to generate sufficient lift increases. This means that you have to fly FASTER to maintain your stall margin.
2) The reduced air density causes the AS indicator to read lower than true.
As the pilot flys faster (to keep indicated airspeed at normal values), the TAS will then be higher than at sea level.
Convienently, the amount of reduction in IAS at higher density altitudes approximately matches the required increase in TAS to maintain normal stall margins...so we just fly the same numbers regardless.
Note: your groundspeed will be higher on landing, so the distance remaining markers will go by more quickly.
At higher density altitude:
1) The true airspeed necessary to generate sufficient lift increases. This means that you have to fly FASTER to maintain your stall margin.
2) The reduced air density causes the AS indicator to read lower than true.
As the pilot flys faster (to keep indicated airspeed at normal values), the TAS will then be higher than at sea level.
Convienently, the amount of reduction in IAS at higher density altitudes approximately matches the required increase in TAS to maintain normal stall margins...so we just fly the same numbers regardless.
Note: your groundspeed will be higher on landing, so the distance remaining markers will go by more quickly.
02-26-2008, 09:24 AM
#5
Gets Weekends Off
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Joined APC: Aug 2007
Posts: 156
Originally Posted by rickair7777
Assuming fixed GW, CG, and AOA...
At higher density altitude:
1) The true airspeed necessary to generate sufficient lift increases. This means that you have to fly FASTER to maintain your stall margin.
2) The reduced air density causes the AS indicator to read lower than true.
As the pilot flys faster (to keep indicated airspeed at normal values), the TAS will then be higher than at sea level.
Convienently, the amount of reduction in IAS at higher density altitudes approximately matches the required increase in TAS to maintain normal stall margins...so we just fly the same numbers regardless.
Note: your groundspeed will be higher on landing, so the distance remaining markers will go by more quickly.
At higher density altitude:
1) The true airspeed necessary to generate sufficient lift increases. This means that you have to fly FASTER to maintain your stall margin.
2) The reduced air density causes the AS indicator to read lower than true.
As the pilot flys faster (to keep indicated airspeed at normal values), the TAS will then be higher than at sea level.
Convienently, the amount of reduction in IAS at higher density altitudes approximately matches the required increase in TAS to maintain normal stall margins...so we just fly the same numbers regardless.
Note: your groundspeed will be higher on landing, so the distance remaining markers will go by more quickly.
Good info! Thanks Rick
06-27-2009, 06:54 PM
#7
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Posts: 248
Originally Posted by rickair7777
Assuming fixed GW, CG, and AOA...
At higher density altitude:
1) The true airspeed necessary to generate sufficient lift increases. This means that you have to fly FASTER to maintain your stall margin.
2) The reduced air density causes the AS indicator to read lower than true.
As the pilot flys faster (to keep indicated airspeed at normal values), the TAS will then be higher than at sea level.
Convienently, the amount of reduction in IAS at higher density altitudes approximately matches the required increase in TAS to maintain normal stall margins...so we just fly the same numbers regardless.
Note: your groundspeed will be higher on landing, so the distance remaining markers will go by more quickly.
At higher density altitude:
1) The true airspeed necessary to generate sufficient lift increases. This means that you have to fly FASTER to maintain your stall margin.
2) The reduced air density causes the AS indicator to read lower than true.
As the pilot flys faster (to keep indicated airspeed at normal values), the TAS will then be higher than at sea level.
Convienently, the amount of reduction in IAS at higher density altitudes approximately matches the required increase in TAS to maintain normal stall margins...so we just fly the same numbers regardless.
Note: your groundspeed will be higher on landing, so the distance remaining markers will go by more quickly.
Originally Posted by patton33
No, indicated stall speed should remain the same.
If you have a multi engine rating refer to your Vmc vs Stall speed diagram, should answer the question in graphical terms.
If you have a multi engine rating refer to your Vmc vs Stall speed diagram, should answer the question in graphical terms.
Originally Posted by Lalo37
Good info! Thanks Rick
Stall speed INCREASES with altitude. Whether you look at IAS, CAS, EAS or TAS, they ALL increase with altitude.
Since PFDs display CAS (not IAS), and TAS has no importance whatsoever in the stall margin, we should only really talk about CAS and EAS.
EAS is really the airspeed that we need to be concerned about when relating airspeed to stall margin, and the stall speed in EAS increases with altitude.
A good rule of thumb is the 2kts/5000ft rule, but at cruise altitudes the increase is even greater than this approximation and varies depending on aircraft type.
The CAS stall speed will increase by an even greater degree due to compressibility effects... and since this is what we see on the PFD, we should keep and even greater "buffer".
I urge everyone to review some good aerodynamics or performance books because IMHO there should be no confusion whosoever over such a basic and fundamental aerodynamic principle. High altitude aerodynamics must be one of the most poorly understood areas of flight training and unfortunately it is not taught enough (FAA requires it for a commercial but applicants are seldom quizzed on it....and my experience is that regional airlines do a very poor job at teaching it).
Unfortunately, there are WAY to many books that contain incorrect or in some cases outdated information, so be careful about what you trust.
Long story short -> if you ever find yourself having to deviate from company profile at high altitudes, remember stall speed will be higher than what you would otherwise encounter at sea level.
06-28-2009, 08:59 AM
#9
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I was addressing the question in a general aviation context, where IAS is what you normally use and speeds are low enough so that compressibility effects are minimal. If you are a flying a high-altitude turbine airplane with an ADC, you will obviously reference your speed cards.
06-28-2009, 11:22 AM
#10
With The Resistance
Joined APC: Jan 2006
Position: Burning the Agitprop of the Apparat
Posts: 6,191
"AIRSPEED is a term that can be easily confused. The unqualified term airspeed can mean any of the following:
a. Indicated airspeed (IAS) - the airspeed shown by an airspeed indicator in an aircraft. Indicated airspeed is expressed in
knots and is abbreviated KIAS.
b. Calibrated airspeed (CAS) - indicated airspeed corrected for static source error due to location of pickup sensor on
aircraft. Calibrated airspeed is expressed in knots and is abbreviated KCAS. Normally it doesn't differ much from IAS.
c. True airspeed (TAS) - IAS corrected for instrument installation error, compressibility error, and errors due to variations
from standard air density. TAS is expressed in knots and is abbreviated KTAS. TAS is approximately equal to CAS at
sea level but increases relative to CAS as altitude increases. At 35,000 ft, 250 KIAS (or KCAS) is approximately 430
KTAS.
IAS (or CAS) is important in that aircraft dynamics (such as stall speed) responds largely to this quantity. TAS is important
for use in navigation (True airspeed ± windspeed = groundspeed)."
Bottom line IAS/CAS isn't going to change much, or at all for stall speed with altitude unless there is a large static position error or mach related influence.
Light GA piston, nada. Jet at altitude and high mach, some but not huge in most aircraft.
a. Indicated airspeed (IAS) - the airspeed shown by an airspeed indicator in an aircraft. Indicated airspeed is expressed in
knots and is abbreviated KIAS.
b. Calibrated airspeed (CAS) - indicated airspeed corrected for static source error due to location of pickup sensor on
aircraft. Calibrated airspeed is expressed in knots and is abbreviated KCAS. Normally it doesn't differ much from IAS.
c. True airspeed (TAS) - IAS corrected for instrument installation error, compressibility error, and errors due to variations
from standard air density. TAS is expressed in knots and is abbreviated KTAS. TAS is approximately equal to CAS at
sea level but increases relative to CAS as altitude increases. At 35,000 ft, 250 KIAS (or KCAS) is approximately 430
KTAS.
IAS (or CAS) is important in that aircraft dynamics (such as stall speed) responds largely to this quantity. TAS is important
for use in navigation (True airspeed ± windspeed = groundspeed)."
Bottom line IAS/CAS isn't going to change much, or at all for stall speed with altitude unless there is a large static position error or mach related influence.
Light GA piston, nada. Jet at altitude and high mach, some but not huge in most aircraft.
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