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Maximum speeds for jets

Pro Member Trainee
Robert (chuckabob) Trainee

Can anyone tell me why Flight Sim X jet speeds go into overspeed at about 350 kts while real jets travel over 500 kts?

Pro Member Trainee
mitch0508 Trainee

Hi

Your getting confused between knots and mph! Real jets fly at around or above 500MPH not Knots!!! In knots thats about 300 or a bit above! So your going overspeed because your doing over about 550 knots!

Thanks

Pro Member First Officer
Bob (Traches) First Officer

Actually, I believe you're getting airspeed and ground speed confused. Next time you're at cruise, open up your GPS and see what speed it gives you. Your DME should give you the same information, assuming you're heading directly towards or away from a VOR station.

Pro Member Chief Captain
Karlw Chief Captain

Actually you are getting IAS confused with true speed

IAS stands for Indicated Air Speed. The airspeed is taken from the Pitot tube which relies on ram air pressure to determine speed. As you go higher the pressure get's lower meaning that you are getting less ram air in the pitot while in fact you are going faster.

Basic concept hope I explained it well

Regards Cheers!

Pro Member Trainee
Robert (chuckabob) Trainee

So, when I track a real flight on the internet using a flight tracker, and it says the plane is traveling 503 kts @ 37,000', what is that really telling me? How does that relate to knots and MPH?

Pro Member Chief Captain
Karlw Chief Captain

Its giving you ground speed in knots you can go 400-500 knots at sea level or relatively close to the ground. however if you were to maintain speed during a climb to FL310 you would notice the IAS goes to 300 or around there.

Again more air pressure means a higher indicated airspeed less air pressure means a lower airspeed indication

regards

Pro Member Chief Captain
Jonathan (99jolegg) Chief Captain

In reality, nobody really knows what you're getting confused with.

Yes, IAS will never go above 350 KIAS in the cruise. Your TAS (True Airspeed) will be around 450-550kts, your groundspeed could be similar and your miles per hour (mph) could also be similar. They're all different for different reasons.

The chances are, you're getting confused with mph and kts. You know aircraft cruise at around 500mph and you're trying to get the speed tape to reflect that.



Last edited by Jonathan (99jolegg) on Fri Jul 10, 2009 8:09 am, edited 1 time in total
Pro Member First Officer
Pete (pdegraff) First Officer

GUYS!

I am SHOCKED that you don't seem to know the relationship between knots and miles per hour!

To get mph from knots multiply knots by 1.1515

(100 knots is 115 miles per hour. 200 knots is 230 mph. Got it?)

To get knots from mph divide mph by 1.1515

A little bit more: A nautical mile is 6,080 feet. (There's more to it but it's for the advanced class.) A knot is one nautical mile per hour. (It is NOT EVER stated as knots per hour.)

A statute mile (the basis of mph or "miles per hour") is 5,280 feet.

You guys in metric land can do the conversions into metric by yourselves, *I HOPE*, but world air travel uses nautical miles and knots.

Respectfully, Pete

Pro Member Chief Captain
Tailhook Chief Captain

Nice post Pete, thanks for breaking it down so concisely!

For those interested in why nautical speed became expressed in "knots", here's a bit of history:

Using the definition of a nautical mile for distance at sea, the challenge was to measure speed -- i.e. what is the ship's speed in nautical miles per hour?...

...The device that sailors used to make their speed measurement was called the "chip log." Chip as in chip of wood, and log as in to record in a log. The chip was a wedge of wood about 18" in size; it was tied to one end of a rope on a large spool. The rope had knots tied into it about every 47'3" (more about how that was calibrated below).

The wooden chip was thrown overboard at the ship's stern (back end). Because of its wedge shape, it would "grab" the water and start pulling out rope as the ship moved forward at some yet unknown speed. One man would hold the spool of rope as it played out; another man would start a sandglass filled with 30 seconds of sand; and a third man would count the knots as they passed over the stern board. When the 30 seconds of sand expired, the time keeper would call out and the counting of knots would stop.

The faster the ship was sailing, more knots and a longer length of rope were played out. The number of knots in the rope that were counted in 30 seconds, then, was equal to the speed of the ship in nautical miles per hour. A "knot", therefore, is not a nautical mile, it is a nautical mile per hour. Thus 1 knot was equivalent to 1 nautical mile per hour; 5 knots were equivalent to 5 nautical miles per hour; etc.
http://www.onlineconversion.com/faq_07.htm

Ironically the confusion is caused by non-metric values. It should also be noted that children from "metric land" seem to have no problems learning percentages in school Shocked

...anyway, for the benefit of those who have not yet graduated from advanced class the map below depicts an "Us And Them" scenario... or: "non-metric lands" versus "metric lands".
So, what do we have Think ...Everything in green is metric, the rest, yes, indeed it is the rest of the world i.e. U.S.A., Liberia and Myanmar/Burma is non-metric. Three countries standing shoulder to shoulder as an axis of... "Non-metric Lands" Very Happy
Welcome to the 21st century.

Whatever happened to globalization?

Pro Member First Officer
Bob (Traches) First Officer

To be fair, england still uses miles for their road system.

Pro Member First Officer
lionlicker First Officer

Nautical Miles or Knots(nautical miles per hour) are a navigators measure of choice because they fit into the globe's system of lattitude and longtitude quite nicely. Verticaly, there is always 60 nautical miles between each line of lattitude. Horizontally, there is Cos (Lattitude) *60 nautical miles between each line of longtitude.

Also when flying jets your primary speed reference should be:-
Below 20000 feet refer to indicated (IAS)
Above 20000 feet refer to Mach
Look up your jet's Vmo (Maximum operating velocity)

For jets (the CRJ700 for example), here is a list of typical cruise speeds at various altitudes
TAS - true airspeed in knots - varies with OAT outside air temperature

Altitude_____IAS___Mach______TAS
_4000______220_____________233
_8000______250____.43_______280
12000______285____.52_______336
16000______305____.60_______379
20000______305____.65_______403
24000______305____.70_______428
28000______313____.77_______466
32000______286____.77_______459
36000______259____.77_______451
40000______236____.77_______449

Pro Member Captain
nottobe Captain

the higher you fly the thinner the air becomes. In thin air, the pitot tube detects much slower air speed than it does at sea level, simply because much less air pressure blown into the tube. That is why the higher you go the slower your air speed indicator shows, but the ground speed never slows down with thinner air!!!

Pro Member First Officer
D5Tweddle First Officer

Here in (sometimes) sunny England we do use MPH for speed and miles for distance, its a near fifty/fifty split who use metric and non metric. All children at schools are taught metric.

Pro Member First Officer
Pete (pdegraff) First Officer

Dear Tweedle (and all others...),

There in (sometimes) sunny England you teach the children all about miles and miles per hour and that is nice

BUT YOU FLY DISTANCES OF NAUTICAL MILES AND AT SPEEDS IN KNOTS! - because that is the WORLD-WIDE AERONAUTICAL SYSTEM.

Respectfully,

Pete

PS: I like Lionlicker's table of Indicated Air Speed and True Air Speed at various altitudes. Study that to see that you are going much faster than your air speed indicator says when you fly at high altitudes.

He also correctly stated that IAS is affected by temperature because warmer air is thinner (less dense) than cooler air. The temperature effect is usually not nearly as significant as is a change of altitude - to an airplane - but with the notable exception of how nicely a hot air balloon rises through the air as long as the air IN the balloon is kept much hotter than the air outside of the balloon.

No one here has yet made any solid reference to the effects of headwinds and tailwinds - but they subtract or add (respectively) to one's speed over the ground. They DO NOT affect one's Indicated Air Speed or one's "True Air Speed". So glance at that upper left corner (Shift Z) to see how fast the wind is blowing and in what direction. NOW you have a truly practical application for your high school trigonometry because the wind is almost always at some angle to your direction of travel. To figure out its actual affect you must resort to trig.

Again, with respect,

Pete

Pro Member First Officer
D5Tweddle First Officer

You must a New Yorker COS YOU HAVE TO SHOUT ALL OF THE TIME!

I am aware that in the world of Aeronautics that the system is Nautical miles and knots, but what about height, is that in nautical miles? climb speed? weight? is this metric or not (I do know the answer to these, as you can gather I am being sarcastic).

Respectfully

Tweddle

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