Knots Equivalent Airspeed
Discussion
I've recently been reading Paul Crickmore's "Lockheed Blackbird - Beyond the secret missions" which despite the slightly silly title, is one of the better SR71 books I've read. There are several references to KEAS in it and some numbers that didn't make much sense to me in the context of the aircraft's true speed. Can someone explain in idiot terms what KEAS is and why it's significant to high speed/high altitude flying?
Aircraft airspeed indicators are calibrated in Knots (not MPH or KPH). Therefore pilots normally talk about Knots when talking about airspeed.
Airspeed is measured in an aircraft with reference to pressure sensors mounted outside the aircraft called a Pitot (often in the form of a pitot tube). Even though change in pressure can give you a speed indication, pressure is also altered by altitude - so the indicated airspeed showing on the airspeed indicator will also be affected by the altitude of the aircraft. And finally, at high Mach numbers, pressure changes occur around the aircarft due to shock wave development etc and that will also affect the indicated airspeed.
All these factors have to be taken into account when arriving at the actual airspeed of the aircarft.
https://en.wikipedia.org/wiki/Equivalent_airspeed
Airspeed is measured in an aircraft with reference to pressure sensors mounted outside the aircraft called a Pitot (often in the form of a pitot tube). Even though change in pressure can give you a speed indication, pressure is also altered by altitude - so the indicated airspeed showing on the airspeed indicator will also be affected by the altitude of the aircraft. And finally, at high Mach numbers, pressure changes occur around the aircarft due to shock wave development etc and that will also affect the indicated airspeed.
All these factors have to be taken into account when arriving at the actual airspeed of the aircarft.
https://en.wikipedia.org/wiki/Equivalent_airspeed
As Eric explained, what matters is the "equivalent" speed of the air passing the aircraft, when it comes to flight controls and aircraft dynamics.
As Lift and Drag are factors that result from the effective change of momentum of the air flowing over and around the aircraft, and lift and drag are what determines "how" an aircraft flies, that are what the pilot, from a controls (rather than a navigation) perspective cares about. Momentum is change is velocity multiplied by the objects mass, which of course depends on it's density for a fluid (ie air)
So, as air density changes, either due to static pressure (altitude) or dynamic pressure (velocity, esp. transonic and at high Mach) then the effect of that air hitting the airframe and its control surfaces change, even for a fixed absolute velocity.
So, flying fast and low, where air is dense, means a small change in velocity of the air makes a big change to momentum of that air, as opposed to flying slow and high, where the opposite is true.
To try to make head and tail of all this complex physics, and especially for aircraft flown 'manually' with a broad range of operating speeds and altitudes (SR-71 being case in point) it was simpler to introduce KEAS to boil down to a number on a dial that, if held constant, made the aircraft fly in a consistent manner.
For say the pilot of a little Cesna, which only flies slowly and at low altitude, these effects are small and (mostly) can be ignored, but for the pilot of a SR-71, which rotates off the runway at a true airspeed of 180 knots, but can also fly at a true1905 knots at altitude, using true airspeed would make for a large number of limiting, yet highly disparate speeds for the pilot to remember in order to operate the aircraft safely.
Flying against Equivalent speed actually means the SR-71 only flies in a narrow speed range. In fact, for that plane, iirc, 420 KEAS is the maximum dynamic pressure limit of the aircraft, and again, irrc, most normal manual flight (as opposed to high altitude cruise) was done between 260 and 330 KEAS.
As Lift and Drag are factors that result from the effective change of momentum of the air flowing over and around the aircraft, and lift and drag are what determines "how" an aircraft flies, that are what the pilot, from a controls (rather than a navigation) perspective cares about. Momentum is change is velocity multiplied by the objects mass, which of course depends on it's density for a fluid (ie air)
So, as air density changes, either due to static pressure (altitude) or dynamic pressure (velocity, esp. transonic and at high Mach) then the effect of that air hitting the airframe and its control surfaces change, even for a fixed absolute velocity.
So, flying fast and low, where air is dense, means a small change in velocity of the air makes a big change to momentum of that air, as opposed to flying slow and high, where the opposite is true.
To try to make head and tail of all this complex physics, and especially for aircraft flown 'manually' with a broad range of operating speeds and altitudes (SR-71 being case in point) it was simpler to introduce KEAS to boil down to a number on a dial that, if held constant, made the aircraft fly in a consistent manner.
For say the pilot of a little Cesna, which only flies slowly and at low altitude, these effects are small and (mostly) can be ignored, but for the pilot of a SR-71, which rotates off the runway at a true airspeed of 180 knots, but can also fly at a true1905 knots at altitude, using true airspeed would make for a large number of limiting, yet highly disparate speeds for the pilot to remember in order to operate the aircraft safely.
Flying against Equivalent speed actually means the SR-71 only flies in a narrow speed range. In fact, for that plane, iirc, 420 KEAS is the maximum dynamic pressure limit of the aircraft, and again, irrc, most normal manual flight (as opposed to high altitude cruise) was done between 260 and 330 KEAS.
ETA, of course true airspeed and in particular Equivalent air speed cannot be used for navigational purposes, as neither reflect the absolute speed over the ground (True airspeed does not compensate for wind, and Equivalent does not compensate for altitude)
For the SR-71, with its complex Inertial Navigation System and Star Tracking Scope, that wasn't a problem!
(most sources suggest those systems when properly aligned before flight could deliver the plane to within approx 100m or better after a 3000km flight!)
For the SR-71, with its complex Inertial Navigation System and Star Tracking Scope, that wasn't a problem!
(most sources suggest those systems when properly aligned before flight could deliver the plane to within approx 100m or better after a 3000km flight!)
Thanks, that's exactly the level of explanation I was after. The context KEAS was used in makes sense now, if I've understood your explanation properly, the SR71 had a narrow operating window of KEAS and this would have caused some difficulty during air to air refueling as the speed and altitude has to be exactly right.
I've learnt something today!
I've learnt something today!
Max_Torque said:
As Eric explained, what matters is the "equivalent" speed of the air passing the aircraft, when it comes to flight controls and aircraft dynamics.
I'd have thought what actually matters is the indication of speed given by a standard pitot-static tube (IAS), which is affected by density/altitude/temperature, and therefore is what affects dynamics? So the higher you go, the faster you have to fly to maintain a constant indicated airspeed?Put very simply, isn't equivalent airspeed is just indicated airspeed corrected for supersonic flight due to compressibility?
Eric Mc said:
Aircraft airspeed indicators are calibrated in Knots (not MPH or KPH).
Point of order- my old Pitts had an MPH ASI, and the Yak is calibrated in KPH. Most other types I have flown are in Knots though. You just have to make sure the right number for whatever type you're flying is indicated at the appropriate time.Some of the other engine indicators on the Yak are calibrated in particularly esoteric units - it could be turnip force per square farm-collective-micro-hectare for the clue that the Cyrillic annotations give you - but in general the indication of a white needle in a green dial arc is what you're checking for.
Older aircraft often had other forms of measurement. During WW2 most continental aircraft would have shown KPH and British aircraft showed MPH. It is the Yanks who liked their Knots and since they came to dominate post war aviation, Knots kind of took over - with a few rear guard actions from people like the Russkies.
It's the same reason that the generally accepted measure of altitude is in thousands of feet.
It's the same reason that the generally accepted measure of altitude is in thousands of feet.
Eric Mc said:
Flying at extreme altitudes where the air is so thin is not easy and a bit of a knife edge. The Space Shuttle was even trickier.
although it of course had a reaction control system that augments and then entirely replaces the conventional moving surface control system to maintain dynamic stability in all flight regimes....Vlad the Imp said:
the SR71 had a narrow operating window of KEAS
in fact, it was really very narrow. Due to the airframe being optimised for the high mach cruise (where the plane spends most of it's time) and carrying a vast fuel load, the airframe had extremely narrow dynamic loading limits and the engines themselves were similarly fussy about the vector and stability of the impinging airstream they needed to consume. Add in the aerothermodynamic constraints (airframe frictional heating at high mach) and it wasn't a plane you threw around the sky with abandon!The flip side of a plane that was "difficult" to fly was of course the fact that it actually got more fuel efficient the faster it flew, meaning it had a simply enourmous range capability for what was a fairly small aircraft.
Eric Mc said:
Older aircraft often had other forms of measurement. During WW2 most continental aircraft would have shown KPH and British aircraft showed MPH. It is the Yanks who liked their Knots and since they came to dominate post war aviation, Knots kind of took over - with a few rear guard actions from people like the Russkies.
I think a majority of US General Aviation types were delivered with MPH ASIs until the early 1970s, when the shift to knots came about. Some manufacturers stayed with MPH - I think Maule is an example. Depending on the level of certification, you can of course fit any calibration you like, provided the appropriate paperwork and approvals are available.Looking at the aftermarket / replacement ASIs available from a US parts vendor...
http://www.aircraftspruce.com/search/search.php?ca...
...I'd say it was an unscientific 65/35 split for Knots/MPH - MPH being more predominant in lower performance categories.
Eric Mc said:
It's the same reason that the generally accepted measure of altitude is in thousands of feet.
The ex-Soviet Bloc nations stayed with metric altitude specifications for many years - although I think the Russians at least now use feet - and the Chinese still use metric altitudes.Max_Torque said:
although it of course had a reaction control system that augments and then entirely replaces the conventional moving surface control system to maintain dynamic stability in all flight regimes....
Up to a point. Once in the atmosphere there was a regime where the RCS was ineffective bit the aerodynamic controls were not fully effective. As I said, it was really tricky and involved very clever computer control to keep the thing pointing the right way.eharding said:
The ex-Soviet Bloc nations stayed with metric altitude specifications for many years - although I think the Russians at least now use feet - and the Chinese still use metric altitudes.
It is a bit of a mess. The mixture of metric and imperial measures has caused major problems on occasions (such as the Gimli Glider incident).Knots are used in the West because a Nautical mile (unlike a Statute mile or a Kilometer) is a Terrestrial measurement. That is to say that 1 Nm subtends 1 Minute of Arc at the earth's surface on any Great Circle.
It thus makes long range navigation easier. It also allows the easy use of the 'one in sixty rule' when calculating heading changes.
Additionally, while the length of a Nautical Mile must vary across the surface of the earth (owing to the fact that the earth is an oblate spheroid), from 6110ft at the poles to 6050ft at the equator, the std definition (the mean) is 6077ft (although there is another standard measured at 25 degrees of Latitude which works out as 6076ft).
The upshot of this is that aircrew take the Nm to be 6000ft, in doing so (and because time is measured is portions of 60) it makes things like terminal descent profiles on, say, an ILS, easy to calculate (an ideal 3 degree slope is 300ft per mile) as well as the rate of descent (a 3 degree slope requires a RoD in feet per minute of 5x your groundspeed in knots).
Additionally it allows you to calculate things like Std Closing Angles (SCA, ie heading change to regain track) based upon 60 / groundspeed in Nm per min. eg 420kts = SCA 9 degrees.
Finally it makes calculating things like drift and groundspeed in any circuit or holding pattern easy to do as a mental calculation.
Having flown into Russia, Ukraine and Belorussia where they operated in Km and m it made flying rather 'interesting'.
WRT EAS it takes account of Compressibility:
IAS (Indicated Airspeed) is what shows on the ASI, and is a function of Dynamic Pressure (as measured by the Pitot Head) minus Static Pressure (as measured by the Static Port(s)). Given that there may be a positional error at the static port(s) owing to the way the air is moving over it (them), a correction will be applied giving something called CAS or RAS (Calibrated or Rectified Airspeed). This positional effect is most marked at low IAS and when you have things like the u/c or flaps extended. It is effectively a correction for 'Instrument Error'.
EAS is CAS corrected for Compressibility (so comes into play at high speeds and altitudes - above around Mach 0.4).. Correcting EAS for the Relative Density of the air will result in TAS (True Airspeed).
It's perfectly possible, say, for an a/c to have an IAS of 200kts, EAS of 300kts and TAS of 400kts dependent upon the height flown and the relative air density (temperature).
It thus makes long range navigation easier. It also allows the easy use of the 'one in sixty rule' when calculating heading changes.
Additionally, while the length of a Nautical Mile must vary across the surface of the earth (owing to the fact that the earth is an oblate spheroid), from 6110ft at the poles to 6050ft at the equator, the std definition (the mean) is 6077ft (although there is another standard measured at 25 degrees of Latitude which works out as 6076ft).
The upshot of this is that aircrew take the Nm to be 6000ft, in doing so (and because time is measured is portions of 60) it makes things like terminal descent profiles on, say, an ILS, easy to calculate (an ideal 3 degree slope is 300ft per mile) as well as the rate of descent (a 3 degree slope requires a RoD in feet per minute of 5x your groundspeed in knots).
Additionally it allows you to calculate things like Std Closing Angles (SCA, ie heading change to regain track) based upon 60 / groundspeed in Nm per min. eg 420kts = SCA 9 degrees.
Finally it makes calculating things like drift and groundspeed in any circuit or holding pattern easy to do as a mental calculation.
Having flown into Russia, Ukraine and Belorussia where they operated in Km and m it made flying rather 'interesting'.
WRT EAS it takes account of Compressibility:
IAS (Indicated Airspeed) is what shows on the ASI, and is a function of Dynamic Pressure (as measured by the Pitot Head) minus Static Pressure (as measured by the Static Port(s)). Given that there may be a positional error at the static port(s) owing to the way the air is moving over it (them), a correction will be applied giving something called CAS or RAS (Calibrated or Rectified Airspeed). This positional effect is most marked at low IAS and when you have things like the u/c or flaps extended. It is effectively a correction for 'Instrument Error'.
EAS is CAS corrected for Compressibility (so comes into play at high speeds and altitudes - above around Mach 0.4).. Correcting EAS for the Relative Density of the air will result in TAS (True Airspeed).
It's perfectly possible, say, for an a/c to have an IAS of 200kts, EAS of 300kts and TAS of 400kts dependent upon the height flown and the relative air density (temperature).
Edited by Ginetta G15 Girl on Friday 10th August 16:14
Eric Mc said:
Max_Torque said:
although it of course had a reaction control system that augments and then entirely replaces the conventional moving surface control system to maintain dynamic stability in all flight regimes....
Up to a point. Once in the atmosphere there was a regime where the RCS was ineffective bit the aerodynamic controls were not fully effective. As I said, it was really tricky and involved very clever computer control to keep the thing pointing the right way.When Columbia broke up on re-entry, the first indication of an off nominal event was the abnormal RCS firing that occurred as the system tried to maintain yaw stability with an effectively asymmetric drag profile ( Lead edge of the wing, the carbon carbon panels, had been damaged by the foam strike on launch). Loss of control and vehicle breakup only finally occurred when the wing spar failed due to heat stress and the combined control system was unable to maintain dynamic authority.
Interestingly, on the SR-71, with it's earlier, simpler, dynamic augmentation system, worst case augmentation was that of a high speed single engine surge (or an 'unstart' in SR parlance) that resulted in massive yaw accelerations, banging the pilots head against the side of the canopy in the best case, and a full loss of control and vehicle disintegration in the worst case!
Bill_Weaver_and_article_952
For this reason, the yaw gain on the system was high, and if pilots forgot to turn the system off whilst taxing, the rudders would slam round and bounce off their end stops when the a/c was turned off or onto the runway!!
Bill_Weaver_and_article_952
For this reason, the yaw gain on the system was high, and if pilots forgot to turn the system off whilst taxing, the rudders would slam round and bounce off their end stops when the a/c was turned off or onto the runway!!
Max_Torque said:
hence my useage of the word 'augments'. Control blending was written into the flight control system that used dynamic pressure as the primary arbitrator, and maintained vehicle stability across all regimes, ensuring that the moveable control surfaces were not overstressed or overheated (primarily used for high altitude trimming once past peak heating), and that the RCS was not active (minimal fuel consumption)
When Columbia broke up on re-entry, the first indication of an off nominal event was the abnormal RCS firing that occurred as the system tried to maintain yaw stability with an effectively asymmetric drag profile ( Lead edge of the wing, the carbon carbon panels, had been damaged by the foam strike on launch). Loss of control and vehicle breakup only finally occurred when the wing spar failed due to heat stress and the combined control system was unable to maintain dynamic authority.
The Shuttle Orbiter was an aerodynamic mess. Trying to design an aircraft that can handle flight regimes from Mach 22 all the way down to a touchdown speed of around 200 knots is very difficult. I've heard the Shuttle carried a shed load of ballast into orbit on every mission - because it needed that additional weight to keep the centre of gravity within limits during the descent and landing.When Columbia broke up on re-entry, the first indication of an off nominal event was the abnormal RCS firing that occurred as the system tried to maintain yaw stability with an effectively asymmetric drag profile ( Lead edge of the wing, the carbon carbon panels, had been damaged by the foam strike on launch). Loss of control and vehicle breakup only finally occurred when the wing spar failed due to heat stress and the combined control system was unable to maintain dynamic authority.
Eric Mc said:
The Shuttle Orbiter was an aerodynamic mess. Trying to design an aircraft that can handle flight regimes from Mach 22 all the way down to a touchdown speed of around 200 knots is very difficult. I've heard the Shuttle carried a shed load of ballast into orbit on every mission - because it needed that additional weight to keep the centre of gravity within limits during the descent and landing.
The lifting bodies that NASA experimented with prior to the Shuttle had very tight centre of gravity limits too.The reason that Bruce Peterson survived the M2-F2 crash was actually down to an initial lack of weight at the front of the craft. The normal solution would've been to add ballast but someone (no one knows who) at Northrop decided that building the canopy and surrounding structure out of steel instead of aluminium would have the same effect.
The result was an 18G structure instead of a 6G one which did a much better job of protecting him when it rolled across the lakebed.
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