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Guest Adverse Yawn

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Hi Guys Great explanation of KIAS etc, but what about mach speed (CS CX etc). That is to say the "transition altitude" ie the altitude at which the usual airspeed indicator becomes unreliable and can no longer give you an accurate indication of the airplane's speed. I believe at this altitude pilots begin measuring speed in relation to the speed of sound, eg 0.7 mach and so on. What I would like to know is, how is this measured? What instrument is used to measure the speed of the palne in relation to the speed of sound? Is this an accurate measure of speed at all altitudes? Can you predict the "Coffin Corner" ie when the speed of sound and the stall speed intersect?ThanksPeter Hayes

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Guest Gordius

I haven't got the answer here at work but I can tell you that it is based on TAS and simply another way of showing it on the gauge. Calculation of TAS is calculated using the formula which includes IAS. The formula used to calculate it is the bit I have at home but I am sure it will include a reference to altitude where the resisitance from the air density is much less.Stall speed is based upon the IAS needed to generate enough lift to remain airborne which in turn relies on air density. As far as predicting that altitude I'll leave that to the manufacturerd and their computer simulations etc but roughly speaking its the max altitude you will see for any aircraft.Stall Speed = Mach 1. No time left so a proper answer will follow in about 8 to 10 hours.What a rubbish answer. I wish I hadn't started. Oh well now that Ive typed that much out I might as well click on "Post Message". I'll have a better try at home :-)Andrew BrownRoaring Thirtieshttp://www.gordiusfs.pwp.blueyonder.co.uk/192029.png

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What? Stall speed has absolutely nothing to do with Mach number.Mach number is the ratio of the aircraft's speed to that of the speed of sound. Mach 1 means a 1:1 ratio - you're flying the same speed that sound travels. Mach .50 would be half the speed of sound, and Mach 2 is twice the speed of sound. The speed of sound depends on the temperature of the air and this is why it changes as you go higher into colder air. The temperature changes more quickly than the pressure with altitude, so this is why you see the inverse relationship between Mach number and IAS - you go higher and your IAS drops but your Mach rises. The opposite occurs as you descend.As for the relationship between IAS and Mach, at some point in your climb or level off, the Mach number you're shooting for like .74, .80, .86, etc will rise up to equal your current IAS. This is really the point where you would want to start paying attention to Mach number as if you continued to climb, you'd shoot past your target Mach even if your IAS stayed constant. It isn't so much that IAS becomes an unreliable indicator of your speed as it is that Mach is a much easier way for controllers and pilots to relate their speeds to each other for spacing purposes. IAS is still indicating airflow over the wings just like at sea-level, and it is still important, you could concievably stall at a very high Mach number if the pressure was such that your IAS was low enough. I know I've heard stories about the SR-71 being near the "coffin corner" as you put it for most of its flights at Mach 3, just because it was so high up in the atmosphere that it's IAS was extremely low for the wing's shape.Mach number is actually calculated like this:Speed of sound = SquareRoot(specific heat ratio * specific gas constant * temperature)You take that value, divide your TAS by it, and you have your mach number.As for the exact mechanism that an airspeed indicator in an airliner uses to perform these calculations, I'm not exactly sure.Here's some links that should give you a better understanding of everything:http://en.wikipedia.org/wiki/Mach_numberhttp://en.wikipedia.org/wiki/Airspeed_indicatorhttp://www.aerospaceweb.org/question/atmosphere/q0126.shtmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/mach.html


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Guest Adverse Yawn

>>What I would like to know is, how is this measured? What instrument is >>used to measure the speed of the palne in relation to the speed of >>sound? Is this an accurate measure of speed at all altitudes?>>Can you predict the "Coffin Corner" ie when the speed of sound and the >>stall speed intersect?Basically, Mach comes into play at speed exceeding 300kts TAS, this is because of air compressability which provides an over read situation for the IAS/CAS. Mach can be determined using two methods:1) From temprature: The Local Speed of Sound (LSS) = 38.94 * SQRT(air temp in Kelvin). Mach = TAS/LSS. This is not instrumented in a/c, but can be used by the pilot/engineer should they whish to! The 38.94 is a generlisation that produced good enough figures.2) A Machmeter determines Mach with the following formula: (pitot pressure-static pressure)/pitot pressure. Pitot pressure is the pressure within the pitot line and static within the static line. The Machmetere works of the pitot tube and static pressure systems, this removes the need to rely on accurate temprature readings.Just to concoct a situation. Guessing the figures here, but a jet at 50,000'. Such an aircraft would stall at say 200kts. At 50,000' 200kts EAS is 488kts TAS! At 50,000' the ISA temp is say -60.0 so the LSS is 38.94*SQRT(273-60) = 568kts. So 488/586 = Mach 0.86. Just assume the the Mmo (max Mach speed) is 0.85 then the poor sod has both stalled and exceeded his Mach limit! However, the aircraft will only stall at 200kts EAS, not a Mach number. The EAS the speed the aircraft thinks it is flying at, hence the danger of Coffin Corner for some jets.============================================================Important thing to realise!Mach is temprature dependant. Therfore, if you climb at a constant IAS, you TAS will increase. If you descend at a constant TAS, your IAS will decrease. If you climb at a constant TAS your mach will increase, if you descend at a constant mach you TAS will decrease. See the relationship?

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Guest Adverse Yawn

>>Mach is temprature dependant. Therfore, if you climb at a constant IAS, >>you TAS will increase. If you descend at a constant TAS, your IAS will >>decrease. If you climb at a constant TAS your mach will increase, if >>you descend at a constant mach you TAS will decrease. See the >>relationship?Achem! Apologies!! That should read:Mach is temprature dependant. Therfore, if you climb at a constant IAS, your TAS will increase. If you descend at a constant TAS, your IAS will increase. If you climb at a constant TAS your mach will increase, if you descend at a constant mach you TAS will increase. See the relationship?

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Guest Shalomar

Donny AKA ShalomarFly 2 ROCKS!!!Maybe you're ready for this maybe not...Your aircraft stall speed is based on indicated Airspeed. As you gain altitude, your Indicated airspeed drops. Meanwhile, the speed of sound is decreasing with altitude. But aircraft- even supersonic ones are designed to safely fly up to a specific fraction of the speed of sound (in the case of a supersonic aircraft, it would exceed one of course- like 1.5 or whatever).A lot of piston powered aircraft, especially non turbocharged ones, simply can't fly high enough to get in much trouble due to excess mach. They stall out first.But in many turboprops and all jets, the margin of error for mantaining speed gets narrower and narrower.The default Lear is a fairly new design, and has a speed range of 150 knots at 51,000 feet.It hasn't always been like that, and plenty of aircraft are flying, both in real life and FS, with narrower margins at the maximum altitude they are certified to fly at.Best Regards, Donny:-wave

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Guest Adverse Yawn

A classic example is the U2 spyplane. If flew with 2-3kts either side of the stall and the Mach buffet. It is rumouerd that the Russians didn't shoot one down at all, but a system malfunction required the pilot to handfly and so he lost control as it would be near impossible to manually fly withing those margins. True or false? Either way it does bring come the problems of Coffin Corner.

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Guest Gordius

Thanks for stepping in guys. Facinating reading by the way. Not sure if my comment>> Stall Speed = Mach 1. No time left so a proper answer will follow in about 8 to 10 hours.was read as stall speed IS mach l lol.I was wrong about having the forumula what I remembered seeing was at http://www.ivao.org/training/tutorials/Ipa...s/L5-Speeds.htm which is interesting reading if you want it.Andrew BrownRoaring Thirtieshttp://www.gordiusfs.pwp.blueyonder.co.uk/192029.png

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Guest Adverse Yawn

Hi Andrew,May I be sol bold as to refer you to the rule of squares method that I outlined in the earlier "Speeds!" thread. I think you'll agree that it is much more agile and accurate method for determining TAS than the method outlined in the Ivao document link :)

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>That is to say the "transition altitude" ie the>altitude at which the usual airspeed indicator becomes>unreliable and can no longer give you an accurate indication>of the airplane's speed. I believe at this altitude pilots>begin measuring speed in relation to the speed of sound, eg>0.7 mach and so on.There isn't a "mach transition" like there is when you switch from local to 29.92 at the transition altitude.Instead there is a climb profile, e.g. for the aircraft I fly, we climb at 290kts/M0.70. So once we get above 10,000ft, we fly 290kts in the climb. The Machmeter starts indicating our "Mach speed" once it gets above M0.40. When the Mach meter gets to M.70, we switch from knots to Mach and complete the rest of the climb at M.70. The altitude at which this happens is around 25,000ft sometimes higher, sometimes lower.Samething going down, we descend at 300/.74 So descend at M.74, when the knots = 300, then we switch to knots and fly 300kts in the descent until we get to 10000ft when we have to slow to 250kts. The transition back to knots happens around 26000ft, sometimes higher and sometimes lower, just like when you're going up.

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Guest msett1

Ok,So realistically I want indicated airspeed? What is closest to what pilots would really see? So if you higher would your IAS stay low but TAS actually be higer or mach speed?Matt

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Guest Ron Freimuth

>Hi To all that responded to my question>Many thanks, the explanations were excellent.>Regards>PeterH Some things I don't think were noted: Climb in jets is generally near 'best range' speed. Both in Jet transports, and even in F-18's if a performance climb isn't required. IAS is a reasonable measure of 'best range' at lower FL's, the heavier the AC, the higher 'best range' IAS is. 'Best Range' when Mach is less than about 0.60 occurs near 1.25 the IAS for Best Endurance (Cdo = Cdi). However, once the Mach number approaches Cruise Mach, Mach effects become significant. Best Range (or LRC) occurs at at nearly the same Mach, regardless of FL, Temp, etc. Actually, Long Range Cruise is used, this gives near maximum range. LRC is defined as the highest speed where range is 1% lower than the best possible. Thus, a B727 might climb at 280 to 310 kts IAS, but once at Mach 0.77 or so, the flight slope is increased so Mach holds at 0.77. The AC is now slowly decelerating (higher FL means lower Temp, thus lower TAS for a given Mach), one can climb faster at the same throttle setting since none of the thrust is required for acceleration. Once at desired/optimum FL, the AC is leveled. The throttle may or may not be reduced. Mach number slowly increases (five minutes is allowed in a 727 to get from Mach 0.78 to 0.80. EPR or N1 is set for the current weight and FL, only small changes should be necessary to hold cruise Mach. LRC Mach in a 727 is about 0.78, typical cruise is at 0.80. At higher Mach numbers fuel flow increases rapidly relative to TAS. More so at higher FL's than lower altitudes. Speeds for the B707-320C are about the same as for the B727 above. Lift related drag is what increases rapidly past some Mach number. Zero Lift drag (Cdo) doesn't increase near as much until over Mach 0.86 or higher. Lower weight means a higher FL can and should be used, since induced drag varies with W^2. Typical optimum FL's range from FL 290 when at near gross weight, to FL 420 or even 450 (B747) at low weight. Very few FS AIR files are set correctly to achieve the real performance variations noted above. Ron

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