Boeing 777-400
Topic Author
Posts: 115
Joined: Fri May 28, 1999 9:39 am

Mph? Knots?

Thu Aug 19, 1999 12:28 am

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?
 
Guest

I Have Been Wondering That Myself...

Thu Aug 19, 1999 12:30 am

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
 
Panman
Posts: 603
Joined: Mon Aug 09, 1999 8:25 pm

RE: Mph? Knots?

Thu Aug 19, 1999 12:51 am

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.
 
Boeing727
Posts: 814
Joined: Sat May 22, 1999 1:32 am

RE: Mph? Knots?

Thu Aug 19, 1999 1:11 am

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
 
flyf15
Posts: 6633
Joined: Tue May 18, 1999 11:10 am

RE: Mph? Knots?

Thu Aug 19, 1999 1:57 am

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.
 
Guest

RE: So....

Thu Aug 19, 1999 2:08 am

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

DL 604
 
Guest

RE: Mph? Knots?

Thu Aug 19, 1999 2:24 am

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.
 
Buff
Posts: 1066
Joined: Fri Mar 16, 2007 1:29 pm

RE: Mph? Knots?

Thu Aug 19, 1999 2:58 am

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
 
Guest

RE: Mph? Knots?

Thu Aug 19, 1999 3:39 am

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.
 
A330
Posts: 688
Joined: Wed May 19, 1999 12:31 am

RE: Mph? Knots?

Thu Aug 19, 1999 7:33 am

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!
 
Buff
Posts: 1066
Joined: Fri Mar 16, 2007 1:29 pm

RE: Mph? Knots?

Thu Aug 19, 1999 10:16 am

" 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
 
Continental777
Posts: 197
Joined: Tue Oct 10, 2000 4:09 am

RE: Mph? Knots?

Thu Aug 19, 1999 10:22 am

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
 
Buff
Posts: 1066
Joined: Fri Mar 16, 2007 1:29 pm

RE: Mph? Knots? Dohhh!

Thu Aug 19, 1999 10:23 am

Can't read my own calculations!

6080 ft = 1854.4 metres

Yikes.

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

Buff