Strange Vmo limits.
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Joined: Jan 2005
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From: Around
Strange Vmo limits.
This was discussed a while ago, but there was no conclusive answer. So...
On the BAe ATP (and on other turboprops) the Vmo curve has strange "breaks" in the curve at certain altitudes. The following is from the ATP:
SL ---> 15.000' - @SL, 227kts then increases linearly up to 230 at 15.000.
15.000'--->22.000' - at/above 15.000 it has suddenly reduced to 220 then it increases to 221 @ 22.000'
22.000'--->25.000 (max) - decreases linearly from 221 at 22.000' to 208 at 25.000'
Anyone able to explain why? If it has to do with Mcrit due to the thick wings - why then the (sudden) decrease followed by an increase. And why the sudden "break" at 15.000'?
On the BAe ATP (and on other turboprops) the Vmo curve has strange "breaks" in the curve at certain altitudes. The following is from the ATP:
SL ---> 15.000' - @SL, 227kts then increases linearly up to 230 at 15.000.
15.000'--->22.000' - at/above 15.000 it has suddenly reduced to 220 then it increases to 221 @ 22.000'
22.000'--->25.000 (max) - decreases linearly from 221 at 22.000' to 208 at 25.000'
Anyone able to explain why? If it has to do with Mcrit due to the thick wings - why then the (sudden) decrease followed by an increase. And why the sudden "break" at 15.000'?
Joined: May 2003
Posts: 198
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From: france
hello bacalau,
i do not know the baeAtp, but vmo, basically, is a structural limitation: pitot pressure & expressed in eas, equivalent airspeed. rember the acronym: ICETea for IAS, indicated airspeed + position error( assumed mostly positive sign)= CAS, calibrated airspeed + compressibility error( always negative sign)=EAS, equivalent airspeed + density correction=TAS, true airspeed.
for info, these different speeds & correction must be added/substracted algebraicly.
so assume position error as constant & positive, then
ias + pos.error=cas & thus ias vs cas change in a similar, symmetrical way when altitude increases.
cas + compressibility error(-)=eas & compress.error becomes more & more negative when altitude increases.
now back to your aircraft & using the formula: ias/cas + compr.err(-) =eas
from sl to 15000ft : if you keep eas(the limiting structural speed), constant, then because the compressibility error increases in size & is negative, the ias/cas increases. hope this makes sense.
now, at a certain altitude,called the crossover altitude,here 15000ft, indeed a low figure,as you mentioned Mcrit & a thick wing, due to the compressibility error becoming to important, ias/cas are not representative enough anymore & we change to Mmo. keeping limiting M constant, the vmo in ias/cas/eas now decreases with altitude cfr the formula which i will not elaborate: eas= 661xMxsquare root of p/p°.
661kts=speed of sound at sl.
M=mach number=tas/a ( a= local speed of sound, only function of temperature).
p=local air pressure, p°=std air pressure, 1013Hpa.
keeping mach constant with increasing altitude & decreasing air pressure, eas & ias/cas now decreases. this is evidenced on the airspeed indicator by the "barberpole"(vmo/mmo pointer) decreasing.
as, why at fl220 vmo increases again by only 1 kt, i don't know for sure, maybe a very small change in position error after all?
i do not know the baeAtp, but vmo, basically, is a structural limitation: pitot pressure & expressed in eas, equivalent airspeed. rember the acronym: ICETea for IAS, indicated airspeed + position error( assumed mostly positive sign)= CAS, calibrated airspeed + compressibility error( always negative sign)=EAS, equivalent airspeed + density correction=TAS, true airspeed.
for info, these different speeds & correction must be added/substracted algebraicly.
so assume position error as constant & positive, then
ias + pos.error=cas & thus ias vs cas change in a similar, symmetrical way when altitude increases.
cas + compressibility error(-)=eas & compress.error becomes more & more negative when altitude increases.
now back to your aircraft & using the formula: ias/cas + compr.err(-) =eas
from sl to 15000ft : if you keep eas(the limiting structural speed), constant, then because the compressibility error increases in size & is negative, the ias/cas increases. hope this makes sense.
now, at a certain altitude,called the crossover altitude,here 15000ft, indeed a low figure,as you mentioned Mcrit & a thick wing, due to the compressibility error becoming to important, ias/cas are not representative enough anymore & we change to Mmo. keeping limiting M constant, the vmo in ias/cas/eas now decreases with altitude cfr the formula which i will not elaborate: eas= 661xMxsquare root of p/p°.
661kts=speed of sound at sl.
M=mach number=tas/a ( a= local speed of sound, only function of temperature).
p=local air pressure, p°=std air pressure, 1013Hpa.
keeping mach constant with increasing altitude & decreasing air pressure, eas & ias/cas now decreases. this is evidenced on the airspeed indicator by the "barberpole"(vmo/mmo pointer) decreasing.
as, why at fl220 vmo increases again by only 1 kt, i don't know for sure, maybe a very small change in position error after all?
Last edited by blackmail; 22nd January 2005 at 20:23.
Thread Starter
Joined: Jan 2005
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From: Around
Your answer seems reasonable, but there are a few things I still wonder about.
-Wouldnt the EFIS show CAS/EAS - doesnt the ADC normally compensate for the difference?
- If 15.000 is the crossover alt - why isnt there a linear decrease from then on?
- Why the next sudden break at 22.000'?
-Wouldnt the EFIS show CAS/EAS - doesnt the ADC normally compensate for the difference?
- If 15.000 is the crossover alt - why isnt there a linear decrease from then on?
- Why the next sudden break at 22.000'?
Joined: Feb 2002
Posts: 178
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From: -
I think the new version of the Boeing 777 is the first aircraft showing EAS directly.
If structural limits are based on EAS and indications are IAS, then speed should increase with altitude?
Other restrictions could be TAS and windshield resistance to bird strikes - although bird strikes are infrequent at high altitude.
If structural limits are based on EAS and indications are IAS, then speed should increase with altitude?
Other restrictions could be TAS and windshield resistance to bird strikes - although bird strikes are infrequent at high altitude.
Joined: Jun 2004
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From: Australia
Bacalau,
Checked a few of your numbers -
SL ---> 15.000' - @SL, 227kts then increases linearly up to 230 at 15.000.
230 KIAS at 15000 feet = 227 Knots EAS, the same as the CAS at Sea Level, where EAS=CAS. Obviously the Aerodynamis structural limit which is dependant upon EAS.
22.000'--->25.000 (max) - decreases linearly from 221 at 22.000' to 208 at 25.000'
221 KIAS at 22000 ft = M 0.505 : 208 KIAS at 25000 ft = M 0.506.
For practical purposes both Mach Numbers are the same, only M 0.001 difference due to rounding Up/Down of the relevant CAS. Obviously a Mach No. limit, and PROBABLY limited by propeller tip speed.
15.000'--->22.000' - at/above 15.000 it has suddenly reduced to 220 then it increases to 221 @ 22.000'
A bit of a mystery that one, although the other 2 are very obvious. More than likely a simplified speed in the transition area between EAS limitation and Mach No. limitation. I suspect that the constant 220 KIAS is a 'rounded off' version of the 221 KIAS at 22000 feet.
Some numbers for your thought processes -
227 EAS = 231 CAS = M 0.506 at 19988 feet Pressure Height.
I hope that that solves 2 out of 3 mysterys.
LGB,
At last a dream come true, I hope that you're correct in stating that the newer B777 versions will have direct EAS readout. Great for piloting, but the problem remains for ATC in speed separation for the EAS aircraft with all of the other CAS aircraft.
Regards,
Old Smokey
Checked a few of your numbers -
SL ---> 15.000' - @SL, 227kts then increases linearly up to 230 at 15.000.
230 KIAS at 15000 feet = 227 Knots EAS, the same as the CAS at Sea Level, where EAS=CAS. Obviously the Aerodynamis structural limit which is dependant upon EAS.
22.000'--->25.000 (max) - decreases linearly from 221 at 22.000' to 208 at 25.000'
221 KIAS at 22000 ft = M 0.505 : 208 KIAS at 25000 ft = M 0.506.
For practical purposes both Mach Numbers are the same, only M 0.001 difference due to rounding Up/Down of the relevant CAS. Obviously a Mach No. limit, and PROBABLY limited by propeller tip speed.
15.000'--->22.000' - at/above 15.000 it has suddenly reduced to 220 then it increases to 221 @ 22.000'
A bit of a mystery that one, although the other 2 are very obvious. More than likely a simplified speed in the transition area between EAS limitation and Mach No. limitation. I suspect that the constant 220 KIAS is a 'rounded off' version of the 221 KIAS at 22000 feet.
Some numbers for your thought processes -
227 EAS = 231 CAS = M 0.506 at 19988 feet Pressure Height.
I hope that that solves 2 out of 3 mysterys.
LGB,
At last a dream come true, I hope that you're correct in stating that the newer B777 versions will have direct EAS readout. Great for piloting, but the problem remains for ATC in speed separation for the EAS aircraft with all of the other CAS aircraft.
Regards,
Old Smokey
