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Mph? Knots?  
User currently offlineBoeing 777-400 From United States of America, joined May 1999, 128 posts, RR: 2
Posted (15 years 1 month 2 days 3 hours ago) and read 4378 times:

Hi I was wondering how many Miles per hour is 1 knot, and does the number change as the altitude increases? EX- would 550 knots at 5000 feet be the same amount of MPH as it would be at 35000 feet?

12 replies: All unread, jump to last
 
User currently offlineDL 604 From , joined Dec 1969, posts, RR:
Reply 1, posted (15 years 1 month 2 days 3 hours ago) and read 4305 times:

You took the words right out of my mouth! I was going to post that question, but it looks like ya beat me to it!  
I was wondering myself, so help us both!   

DL 604


User currently offlinePanman From Trinidad and Tobago, joined Aug 1999, 790 posts, RR: 0
Reply 2, posted (15 years 1 month 2 days 3 hours ago) and read 4313 times:

As far as I am aware the ratio between mph and knots (nautical miles) remains the same. I believe 1 mile is something like 1.18 nautical miles (can't remember - doing this off the top of my head). Unlike Mach No which changes with density and altitude.

User currently offlineBoeing727 From United States of America, joined May 1999, 954 posts, RR: 0
Reply 3, posted (15 years 1 month 2 days 2 hours ago) and read 4303 times:

I usually add about 15% to the value of knots when I tell my passengers how fast we are flying in mph; that is indicated air speed, of course. It is amlost the same ratio as meters to yards, which comes handy when I play golf in Europe.

Regards, Boeing727


User currently offlineFlyf15 From , joined Dec 1969, posts, RR:
Reply 4, posted (15 years 1 month 2 days 2 hours ago) and read 4301 times:

From the dictionary:
Knot: A unit of speed, one nautical mile per hour, approximately 1.85 kilometers (1.15 statute miles) per hour.

A knot is, as I recall, a knot is one minute of latitude at the earth's surface.


User currently offlineDL 604 From , joined Dec 1969, posts, RR:
Reply 5, posted (15 years 1 month 2 days 1 hour ago) and read 4298 times:

So, if you are flying at 10,000 feet and climbing at 250 knots, how many miles per hour are you going?

DL 604


User currently offlinePurdue Cadet From , joined Dec 1969, posts, RR:
Reply 6, posted (15 years 1 month 2 days 1 hour ago) and read 4301 times:

250 Knots would be about 288 mph.

(250 kts) X (1.15 statute miles/ kt) = 287.5 statute miles per hour

That math was done quickly in my head - I appologize if the multiplication is wrong.


User currently offlineBuff From Australia, joined Mar 2007, 0 posts, RR: 1
Reply 7, posted (15 years 1 month 2 days 1 hour ago) and read 4295 times:

Flyf15 is correct, except for the relationship of a knot to latitude.

"Knots" means nautical miles per hour, so is a unit of speed.

A nautical mile is coincidentally equal to one minute of latitude, measured along a true north/south line, or in exact terms, 6080 feet.

Good question - something that most flyers take for granted everybody else knows, but doesn't!

Best Regards,

Buff


User currently offline24291 From , joined Dec 1969, posts, RR:
Reply 8, posted (15 years 1 month 2 days ago) and read 4290 times:

I'll try and put this in other terms. One knot means 1 nautical mile per hour. A nautical mile is longer than a statute mile, hence 100 knots is faster than 100 mph. This ratio remains the same as others have said.

I think your confusion lies with indicated airspeed (IAS) vs. ground speed. If you climb at a constant IAS of 250 kts. in calm wind for example, your ground speed will increase as you go higher. This is because as you climb, you pitot tube, which measures ram air pressure and delivers that information to your airspeed indicator, begins to move through less and less air. Its easy to maintain a constant IAS, however, because your airplane will move faster through the thinner air and keep the same volume of air entering the pitot. If you takeoff on a runway that is at 5,000 ft elevation, your airplane will move across the runway at a higher speed than when taking off from a runway at sea level (assuming standard atmospheric conditions), yet you IAS for rotation and takeoff (generally speaking) will always remain the same.

So when you're on an airliner, your airplane may be cruising at 250 kts. IAS, but your pilot may announce a ground speed of something like 600 mph.

It's actually hard to explain but I hope I did a good job.


User currently offlineA330 From Belgium, joined May 1999, 649 posts, RR: 7
Reply 9, posted (15 years 1 month 1 day 20 hours ago) and read 4279 times:

Let me give you the CORRECT definition of a nautical mile:
A NAUTICAL MILE AT ANY PLACE IS THE LENGTH OF AN ARC OF THE MERIDIAN SUBTENDING TO AN ANGLE OF 1' AT THE CENTRE OF CURVATURE OF THE PLACE.
At the pole: 6107.8 FT
At the equator: 6046.4 FT
STANDARD: 6080 Ft or 10852 m

for the mathematic freaks:formulae:
( 6077.1 - 30.7 x cos2*) in feet
* is tèta
6077.1 is the mean value

Tino



Shiek!
User currently offlineBuff From Australia, joined Mar 2007, 0 posts, RR: 1
Reply 10, posted (15 years 1 month 1 day 17 hours ago) and read 4282 times:

" STANDARD: 6080 Ft or 10852 m "

Sure you don't mean 2393.7 m?!

(I don't know the metric equivalent standard, but 6080 feet is correct in British measure. The differences in the Brit measure of course are due to the fact that the earth is not perfectly round.)

Just to lighten this subject up even more, using the 1 minute of arc measure, wouldn't a nautical mile increase with altitude, ie distance from the centre of measurement?!!

(I know it doesn't because the 6080' is a Standard Measure.)

Best Regards,

Buff


User currently offlineContinental777 From United States of America, joined Oct 2000, 81 posts, RR: 0
Reply 11, posted (15 years 1 month 1 day 17 hours ago) and read 4275 times:

Boeing 777-400 did you copy the name from my discussion. no one is supposed to know about that. They just talked about I don't know if they will even go on with it. there is a 49/51 chance

User currently offlineBuff From Australia, joined Mar 2007, 0 posts, RR: 1
Reply 12, posted (15 years 1 month 1 day 17 hours ago) and read 4278 times:

Can't read my own calculations!

6080 ft = 1854.4 metres

Yikes.

Best Regards, and apologies for the "make-up" post,

Buff


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