Blackbird
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Old Speed-Gauges And Adiabatic Compressible Flow

Fri Jun 15, 2007 1:41 pm

Older speed-gauges used on the first generation of Jetliners did not take into account the effects of adiabatic compressible flow. As a result as you got higher, the airspeed gauges often showed speeds much higher than the plane was actually flying at. By 20,000 feet you often had a speed gauge reading 20 kts higher than you're actual knots-equivelent airspeed, and from that point on the speed readouts got more and more erroneous. Later jetliners featured more accurate speed gauges that factored in the effects of adiabatic compressible flow, as do modern aircraft.

What was the first airliner to have a speed-gauge that factored in the effects of adiabatic compressible flow?


Andrea K
 
411A
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RE: Old Speed-Gauges And Adiabatic Compressible Flow

Fri Jun 15, 2007 2:00 pm

B707, equipped with the KIFIS instrument system.

KIFIS

Kollsman
Intregrated
Flight
Instrument
System

Not quite as accurate as modern air data computers, but darn close.

Indicated/calibrated airspeed, very close, indicated mach number less accurate, but acceptable.

Also, your statement about the airspeed indicator reading higher at altitude, is incorrect.
It read lower than the actual aircraft speed.
 
Blackbird
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RE: Old Speed-Gauges And Adiabatic Compressible Flow

Sat Jun 16, 2007 1:08 am

411A,

Modern airspeed indicators do read lower at higher altitudes. But unless I'm wrong, from what I read the older gauges kept reading higher and higher speeds at altitudes due to compressibility effects. Of course above 20,000 feet you'd just be using Mach number.

The early 707's do seem to use the old type speed-gauge... The 727 seems to use the newer type though that doesn't seem to be affected by compressibility effects could be wrong though.


Andrea Kent
 
Pihero
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RE: Old Speed-Gauges And Adiabatic Compressible Flow

Sat Jun 16, 2007 3:09 am

Andrea, 411A is right and you seem to have got it backwards : I've flown five generations of flight instruments, represented by :

  • the DC-4
  • the Nord 262
  • the 747
  • the 737
  • the Tristar
  • the A-320 ,

and their ASIs present the same phenomenon : For a given IAS, the higher you go, the greater the TAS.
The compressibility effect is quite negligible at lower Mach numbers and goes up to around 15% of the speed you'd read on the gauge.
The passage from IAS to TAS is as follows :

IAS -->+ Ki[/] -->CAS--> x [b]Kc-->EAS-->x 1/d -->TAS,
where,
Ki is the instrument correction - we used to use correction charts for navigation purposes. On the DC-4 / C-54, we still used this correction graphs.
CAS is the corrected AS
Kc is the compressibility correction and Kc = 1/.2 x M square ( approximated St Venant's law )
d is the air density at a given altitude.
EAS is the Equivalent AS, only used for aerodynamics purposes
As all these factors add or increase the progression, IAS becomes smaller than TAS the higher one flies.

[Edited 2007-06-15 20:10:40]
Contrail designer
 
Blackbird
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RE: Old Speed-Gauges And Adiabatic Compressible Flow

Mon Jun 18, 2007 10:47 am

I may be missing something, but from what I remember, the CV-880 for example reads 373 kts max at 0 feet, at 21,500 feet the 373 kts airspeed (at zero feet) now reads 393 kts... and it just goes up and up and up...

Andrea K
 
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RE: Old Speed-Gauges And Adiabatic Compressible Flow

Mon Jun 18, 2007 1:35 pm

You are refering to the 'barber pole'...the maximum speed allowed indicator.

Yes, that does increase the higher you go, up to a point, and that point is the maximum allowed for the aircraft.
There is a maximum of course, and that speed is referenced in the TCDS.
 
Pihero
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RE: Old Speed-Gauges And Adiabatic Compressible Flow

Mon Jun 18, 2007 8:20 pm

Quoting 411A (Reply 5):
You are refering to the 'barber pole'...the maximum speed allowed indicator

 checkmark 
As usual, you hit the nail right on !

Cheers !

PS : Due to undue rush, my progression is incomprehensible.
This is the correct version :
IAS + Ki -->CAS x Kc -->EAS x square root of d -->TAS

Apologies
Contrail designer
 
Blackbird
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RE: Old Speed-Gauges And Adiabatic Compressible Flow

Tue Jun 19, 2007 9:24 am

Yeah, but on the 727 and if you're on the barber pole you'll be at 350 kts until the MMO comes up (0.88 on the -100 Dual Mode, and -200/-200A).

The indicated airspeed doesn't keep creeping up and up and up and up... know what I mean? I think the 727 might be one of the first to be like this,

Andrea K
 
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RE: Old Speed-Gauges And Adiabatic Compressible Fl

Tue Jun 19, 2007 9:54 am

Quoting Blackbird (Reply 7):
Yeah, but on the 727 and if you're on the barber pole you'll be at 350 kts until the MMO comes up (0.88 on the -100 Dual Mode, and -200/-200A).

The indicated airspeed doesn't keep creeping up and up and up and up... know what I mean? I think the 727 might be one of the first to be like this,

Andrea K

On some aircraft, the Vmo Barber Pole is not static and moves in relation to barometric pressure and SAT. Your CV-880 example shows this and is not an indication of compressability error on the indicator. Another famous aircraft that has a moving Barber Pole is Concorde. At S.L. the Vmo is only 350 KIAS. This increases to 400 KIAS by 6000' and increases to almost 550 KIAS during the Mach Cruise Climb.

The 727 and most modern subsonic aircraft have a static Vmo. This is mainly due to simplify the system as there really is no need to have a variable Vmo due to the fact that when you intercept the climb Mach number, your IAS will decrease as you climb with a constant Mach number.

Cheers,

John
Cheers, John YYC
 
411A
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RE: Old Speed-Gauges And Adiabatic Compressible Flow

Tue Jun 19, 2007 10:04 am

All present and past jet transport aircraft have a barber pole indicator on the airspeed dial, and they work about the same...a certain maximum allowed speed at sea level, and goes up to the maximum at altitude (which varies by type), as indicated in the TCDS.

It really does work like this...honest.

Nothing has really changed over the years, with respect to this barber pole indicator.

Now, with piston transports, there generally was no 'barber pole', but in the TCDS you will notice that the maximum allowed speed at altitude was decreased by a certain amount, per thousand feet, above a certain altitude.
This was not a problem with these types, as they simply did not have the power available to readily exceed this maximum allowed speed, quite unlike jet transports.
 
Blackbird
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RE: Old Speed-Gauges And Adiabatic Compressible Flow

Mon Jun 25, 2007 5:06 am

411A,

I'm totally confused here. The older plane's like the 707, 720, and such, if you are on the barber pole, the number keeps creeping up and up and up and up, and the mach number goes up too as you go higher...

Planes like the 727, 737, 747, DC-10, L-1011 etc, if you fly on the barber pole, let's say 350 kts, as I go higher and higher, it still stays at 350 kts, the number doesn't creep up at all... The mach number does go higher and higher for the same EAS at higher altitudes, and eventually you end up reaching the plane's MMO, at which point the EAS reading (airspeed) starts falling off while holding the MMO.

Andrea K
 
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RE: Old Speed-Gauges And Adiabatic Compressible Flow

Mon Jun 25, 2007 12:12 pm

Just slightly confused, I think, Andrea.

The 'barber pole' on the airspeed indicator on the 'ole 707, was at a fixed value at sea level, and increased slightly as you climbed, BUT once it reached its maximum limiting speed (Vmo) it did NOT go higher.

The L1011 is the same.

The L1011 for example, at sea level, the Vmo is 350 knots, and is thus incdicated by the 'barber pole'.

However, if you are sitting on the ground at an airport which is at an elevation of 5000 feet MSL, the barber pole will indicate (approximately) 375 knots.

Although the numbers are slightly different between the B707 and the L1011, the action of the barber pole is the SAME.

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