Speed/knots wrong way around?

[Ari]

Sorry - meant MPH/Knots wrong way around (can't edit title).

This can't be right surely?

Edited by Ari on Monday 2nd February 11:52

[droopsnoot]

According to a quick search, 1 Knot = 1.15078 mph, so that seems the correct way around to me. It doesn't fit with the relationship between the mph and knots figure above, would that be because it's above sea level (by quite some way)?

Edited by droopsnoot on Monday 2nd February 12:17

[Ari]

If (as you correctly say) 1 Knot = 1.15078 mph then 100 knots is 115mph. In other words the MPH figure should always be higher.

[Simpo Two]

droopsnoot said:

According to a quick search, 1 Knot = 1.15078 mph, so that seems the correct way around to me.

No, nautical miles are longer than miles so you get fewer knots than mph.

Perhaps one is air speed and one is ground speed, and a head/tail wind is causing the mismatch? I suspect the mph reading is groundspeed and there's a headwind.

Actually scratch that, I just saw there's a 62mph tailwind... someone else can do the maths and see if it works.

Edited by Simpo Two on Monday 2nd February 13:48

[Mr Pointy]

Google thinks 464mph is 403 knots

[Ari]

Google is correct - hence my confusion.

[IanCress]

Those figures must be groundspeed, you'd never do 521 knots airspeed at 36000 feet.

Still doesn't answer where the mph figure comes from, but 464 knots doesn't equal 521mph, although not far off.

[JP4]

521 Knots is certainly possible at 36,000. Depending on the absolute temperature a Mach no of about 0.88 will give that speed if not higher if temperatures are higher than normal.

[barpilot]

I'd go with the the units are the wrong way around and the calculation is slightly out. Just hope the pilot doesn't use that screen...

[Muddle238]

At high altitudes including the cruise phase, the aircraft will be flying on a Mach number due to aerodynamic properties at high altitudes, typically somewhere between Mach .76 and .82, depending on weight and air traffic restrictions. Therefore the flight crew won't be looking at an actual indicated airspeed, instead they'll have a spot wind readout, a true airspeed, ground speed (for reference), but the aircraft automatics will be maintaining the most economic Mach number in that above range (depending on type).

The actual relationship between airspeed and Mach number is that as you climb at a constant indicated airspeed, the critical Mach number (point at which airflow at some point on the airframe travels supersonic) will decrease, eventually meeting the IAS. Therefore you switch to fly at a constant Mach number, the IAS decreases but as you climb naturally the true airspeed still increases, which is why that image the OP posted shows 500 odd knots, equivalent of Mach .76-.82, but indicating on the speed tape around 240-260kts.

[dazco]

IanCress said:

Those figures must be groundspeed, you'd never do 521 knots airspeed at 36000 feet.

Still doesn't answer where the mph figure comes from, but 464 knots doesn't equal 521mph, although not far off.

Why do you say that? I always thought this was easily done but obviously I was wrong.

[SimonV8ster]

JP4 said:

521 Knots is certainly possible at 36,000. Depending on the absolute temperature a Mach no of about 0.88 will give that speed if not higher if temperatures are higher than normal.

What have you been doing for the last 3 years ?!

[hidetheelephants]

SimonV8ster said:

JP4 said:

521 Knots is certainly possible at 36,000. Depending on the absolute temperature a Mach no of about 0.88 will give that speed if not higher if temperatures are higher than normal.

What have you been doing for the last 3 years ?!

Clearly waiting to deliver a pithy factoid!

[IanCress]

dazco said:

IanCress said:

Those figures must be groundspeed, you'd never do 521 knots airspeed at 36000 feet.

Still doesn't answer where the mph figure comes from, but 464 knots doesn't equal 521mph, although not far off.

Why do you say that? I always thought this was easily done but obviously I was wrong.

A that height indicated airspeed would be much lower, around 250 knots. 520 knots IAS would put you well over Mach 1.

[dazco]

IanCress said:

dazco said:

IanCress said:

Those figures must be groundspeed, you'd never do 521 knots airspeed at 36000 feet.

Still doesn't answer where the mph figure comes from, but 464 knots doesn't equal 521mph, although not far off.

Why do you say that? I always thought this was easily done but obviously I was wrong.

A that height indicated airspeed would be much lower, around 250 knots. 520 knots IAS would put you well over Mach 1.

I'm sorry, I am still none the wiser.

[droopsnoot]

Simpo Two said:

droopsnoot said:

According to a quick search, 1 Knot = 1.15078 mph, so that seems the correct way around to me.

No, nautical miles are longer than miles so you get fewer knots than mph.

Ah, yes, not sure how I missed that even though it was the premise of the original post and I thought I'd checked it twice. I started reading up on how 1 knot = 1 nautical mile per hour, and that a nautical mile is basically the distance of one minute of arc and presumed that it was based on the surface, and that at 36000 feet of course, 1 minute of arc would be longer. However, all the later posters clearly know far more about this, and it wouldn't make sense that a nautical mile was adjusted based on altitude but a normal mile was not.