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MPH vs Kts

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drone_pilot

Well Known Member
Hello All,

I've done some searching but I'm not finding the answers. I've got dual Dynon Skyviews. I can set the airspeed units to either knots or mph.

The Cessna I currently fly displays both on the gauge but uses mph in the POH. What is the thinking when it comes to setting up the airspeed units on an airplane? Is there any significant advantage to using one unit of measure over the other?

Thanks!

Ben
 
ATC will always refer to speeds in knots. That's a pretty good reason to use knots.

Use mph to impress your non-pilot friends about how fast your plane is!
 
It's aviation...use knots and nm :) ATC uses knots, weather reports and forecasts use knots, TFRs use nautical miles, the list goes on and on.

Plus, 1 nm = 1 minute of arc in longitude (at the equator), so it's cooler than the arbitrary distance of a statute mile :).

BTW...one of the most cringe-worthy erroneous expressions, IMO, is "knots per hour". Grrr....
 
If you have an EFIS, it should be easy to swap between the two with a few button pushes.
 
It's a mystery to most of us living outside the USA and also to quite a few living inside the USA as to why mph is used in aviation at all. Even inside the USA it's normally only experimental or GA which uses mph instead of knots. I've flown US-built military aircraft from the 1970s and they were always in knots. Knots is the international standard for aviation and is the language ATC will always use, the units upon which performance categories are based, with nautical miles being the standard distance unit on charts, and so on.

Suffice to say it's a hangover from days long gone by. I did read a great story about the Mooney 201 which apparently was named because it could reach 201 mph in level flight, and "Mooney 175" (in knots) just didn't have the same marketing "zing"! Maybe they should've gone full metric and named it the "Mooney 323"?
 
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dutchroll said:
...

Suffice to say it's a hangover from days long gone by. I did read a great story about the Mooney 201 which apparently was named because it could reach 201 mph in level flight, and "Mooney 175" (in knots) just didn't have the same marketing "zing"! Maybe they should've gone full metric and named it the "Mooney 323"?
Click to expand...

Maybe it was because Mazda had a cart named the 323?
 
But (there's always a but) VFR visibility is listed in statute miles! Class D airspace is 5 statute miles IIRC.

-Marc


RV7A Flyer said:
It's aviation...use knots and nm :) ATC uses knots, weather reports and forecasts use knots, TFRs use nautical miles, the list goes on and on.

Plus, 1 nm = 1 minute of arc in longitude (at the equator), so it's cooler than the arbitrary distance of a statute mile :).

BTW...one of the most cringe-worthy erroneous expressions, IMO, is "knots per hour". Grrr....
Click to expand...
 
Plummit said:
But (there's always a but) VFR visibility is listed in statute miles! Class D airspace is 5 statute miles IIRC.

-Marc
Click to expand...
What does visibility have to do with speed? :confused:

rv9builder said:
ATC will always refer to speeds in knots. That's a pretty good reason to use knots.

Use mph to impress your non-pilot friends about how fast your plane is!
Click to expand...

IMHO this is the best reason/response. In the end, it will be whatever you want for your airplane. :cool:
 
If you are going to talk to ATC, whether it's VFR flight following or IFR, then use knots, no question. If you don't plan on ever speaking with ATC then use MPH to impress.:rolleyes:
 
Knots 'controversy'

As a retired sailor (Blackshoe navy - nuke subs), I have always used knots, so they're a bit more comfortable. I suspect that the use of mph stems from our early adoption of the English system of measurement. That said, I refuse to switch to metric - just the dinosaur in me, I guess.
 
Your call

Me too for all the above reasons. Kts. Only one little addition.
The more you fly the more math you will be doing in your head as you go.
It may help you to make a cheat sheet and start getting used to a few conversions. This is one you will know by hart after a while " 1.151" Sm. to Kts.
I find myself even doing it on a road map some times. Hope this helps. Yours as always, R.E.A. III #80888
 
dutchroll said:
Suffice to say it's a hangover from days long gone by. I did read a great story about the Mooney 201 which apparently was named because it could reach 201 mph in level flight, and "Mooney 175" (in knots) just didn't have the same marketing "zing"! Maybe they should've gone full metric and named it the "Mooney 323"?
Click to expand...

The "days long gone by" date back about four decades...:)

In the mid 1970's GAMA (General Aviation Manufacturing Association) recommended all manufacturers standardize on knots and so most but not all switched (Maule for example still to this day uses MPH calibrated airspeeed indicators).

My 1977 Tiger was made in the first year the factory switched from mph to knots on the ASI and placards.
 
Robert Anglin said:
It may help you to make a cheat sheet and start getting used to a few conversions. This is one you will know by hart after a while " 1.151" Sm. to Kts.
Click to expand...
Another handy quick conversion. For centigrade to fahrenheit, double the degrees centigrade then add 30.

Example: How many degrees fahrenheit is 15 degrees centigrade? Answer; 15*2 = 30, then add 30 which gives you 60 degrees F. While not exact (in this case it is actually 59F) it will be close enough for most of our applications and especially for getting an idea of what the temperature is.
:cool:
 
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Knots vs mph

I'm not throwing any sand in the gears here as I use both knots and mph depending on the airplane I'm flying, however if you look at the performance specification of every RV model, Vans aircraft (ya the mothership) uses MPH. Go figure
 
vetterman said:
I'm not throwing any sand in the gears here as I use both knots and mph depending on the airplane I'm flying, however if you look at the performance specification of every RV model, Vans aircraft (ya the mothership) uses MPH. Go figure
Click to expand...

Of course. MPH "briefs" better to buyers :D
 
1:60 rule works with knots

1 NM = 6000' is the basis of the 1:60 rule. You're 3 degrees off the localizer at 10 NM from the field. How far off are you (so you can plan your lead point turn)? 3/6 or 1/2 mile, or 3000'. The math also works well for desecent planning. Most of this is unnecessary if we rely on the electronics, but Inkeep the mental math sharp for when the electronics fail.


David
 
GalinHdz said:
What does visibility have to do with speed?
Click to expand...

I was referring to the statement that if its aviation, use knots. The FARs make reference to the VFR cloud clearance and visibility requirements for different airspace in statute miles.

-Marc
 
rzbill said:
So whats arbitrary about 1000 paces of an ancient Italian dude in fighting gear?? :):):rolleyes:

Seriously though, I agree with the crowd. Knots.
Click to expand...

The knot (actually nautical mile) was only a good guess when it was invented...:)

1852*m is the length of the internationally agreed nautical mile. The US adopted the international definition in 1954, having previously used the US nautical mile (1853.248*m).[4] The UK adopted the international nautical mile definition in 1970, having previously used the UK Admiralty nautical mile (6080*ft [1853.184*m]).

How the mile changed over time and by country away from the Roman soldier definition is also interesting -

https://en.wikipedia.org/wiki/Mile
 
i use both from my complicated past. in the plane i use both also. it is easy to go back and forth. 7 mph = 8 knots.

how did they come up with knots? well before we were in the air. the sailors of long ago had a rope with knots in it it. throw it out in the water and count the knots that go out in a specified time. :rolleyes:
 
i use both from my complicated past. in the plane i use both also. it is easy to go back and forth. 8 mph = 7 knots. 80 mph = 70 knots.

how did they come up with knots? well before we were in the air. the sailors of long ago had a rope with knots in it it. throw it out in the water and count the knots that go out in a specified time. :rolleyes:
 
A long time ago, in a University far away, I had a professor who thundered that "if you're going to be aeronautical engineers and work in professional aviation, you will use knots and nautical miles!! You build flying machines - Statute miles are for civil engineers, and they build targets!" Any work or papers turned in with mph or feet per second were marked "fail".

I suspect that the use of statute mph in small airplane's goes back to the earliest of days when farm boys and tinkerers in early twentieth century America started building their own airplane's in barns and workshops, and conversed in speeds and distances that were familiar to them. When aviation went big time professional, and international, the standard became knots and nautical - and yup,it's all arbitrary. But you're not going to change ATC and the professsional world, so use the standard.

And yes, I frequently fly airplane's with ASI's marked only in mph, and I learn the speeds in mph for those machines - I don't convert. But it is rare when I don't have a GPS along that is talking in knots, so that's what the outside world hears (if I need to talk to them).

Oh, and if you want to be cool at the Reno Races, you talk in mph because the numbers are bigger. Just humor them.....
 
I suspect the use of SM in airplanes was an early marketing idea. Trying to make it easier for the person to use something they were used to. Kinda like some early airplanes had a steering wheel instead of a yoke. Besides, it sounds faster.
:cool:
 
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If you're dealing with ATC a lot, use knots. If you're not, it doesn't matter. I'm strictly a day VFR pilot and rarely talk to ATC, and I'll be setting up my RV-3B in mph.

Dave
 
turbo said:
i use both from my complicated past. in the plane i use both also. it is easy to go back and forth. 7 mph = 8 knots.

how did they come up with knots? well before we were in the air. the sailors of long ago had a rope with knots in it it. throw it out in the water and count the knots that go out in a specified time. :rolleyes:
Click to expand...

Not to be confused with the sailors of long ago who threw a weighted rope with marks on it to measures depth - which led to a famous American name, Mark Twain.

That rope was marked in fathoms which were two English yards, based on the tip to nose measurement of some old royal dude (Henry I)...:)
 
Okay okay! Knots it is! I was just wondering as I have seen a lot of RV POHs in MPH. I was wondering if I was missing out on something.
 
I know I've mentioned this before but I fly from the left seat and Ann flies from the right. I have everything set up in kts. and nautical miles on my side of the plane and she has everything set up in statute miles on her side. She goes a little faster than me but she has farther to go so we usually get there about the same time.
 
Temps

And then there's the Temps in Centigrade, but I digress. I soloed in a 1946 J3 Cub and the ASI was in MPH. It wasn't till much later that I started flying planes with the ASI in knots.

Kinda reminds me of when the powers that be decided that gas (at least in California) should be sold in Liters... What a fiasco that was!

~Marc
 
az_gila said:
The knot (actually nautical mile) was only a good guess when it was invented...:)

1852*m is the length of the internationally agreed nautical mile. The US adopted the international definition in 1954, having previously used the US nautical mile (1853.248*m).[4] The UK adopted the international nautical mile definition in 1970, having previously used the UK Admiralty nautical mile (6080*ft [1853.184*m]).

How the mile changed over time and by country away from the Roman soldier definition is also interesting -

https://en.wikipedia.org/wiki/Mile
Click to expand...

Ah, but the *meter* was defined as being 1/10,000,000 of the distance from the equator to the pole :)

Someday I'm going to figure out how to set up an aircraft to have things like RPM replaced by rad/s, angle of attack in radians, etc. LOL!
 
Not that's hilarious! Mel, you made my day!

Mel said:
I know I've mentioned this before but I fly from the left seat and Ann flies from the right. I have everything set up in kts. and nautical miles on my side of the plane and she has everything set up in statute miles on her side. She goes a little faster than me but she has farther to go so we usually get there about the same time.
Click to expand...
 
Mel said:
I know I've mentioned this before but I fly from the left seat and Ann flies from the right. I have everything set up in kts. and nautical miles on my side of the plane and she has everything set up in statute miles on her side. She goes a little faster than me but she has farther to go so we usually get there about the same time.
Click to expand...

That just means she is a lot younger than you. As we all used to fly airplanes that were in Sm./ Hr. when we were younger. Now that it has been a while and we are getting much older we fly in airplanes that are in Kts. / Hr. Shame on you for slowing down on a young lady that is still up to a younger speed. You should treat her better than that. TEXAS always, no mater where you are, it is still an act of love. Yours Robert
 
I know I should use Knots, but I still use Miles on my D10A because I like that occasionally I break 200. If i'm indicating in Knots and that happens, there's something wrong...

Of course, I leave my GPS reading in km/h, because the passenger can see it easier than the D10A in front of me... And flying along at 300+ is way more impressive... :)
 
RV7A Flyer said:
Ah, but the *meter* was defined as being 1/10,000,000 of the distance from the equator to the pole :)

Someday I'm going to figure out how to set up an aircraft to have things like RPM replaced by rad/s, angle of attack in radians, etc. LOL!
Click to expand...

Yes, the French dudes in 1791 who set the N pole to Equator distance at 10,000 km (or 10,000,000 metres) were quite a bit more accurate.

They were only out by 2 Km. :)
 
az_gila said:
Yes, the French dudes in 1791 who set the N pole to Equator distance at 10,000 km (or 10,000,000 metres) were quite a bit more accurate.

They were only out by 2 Km. :)
Click to expand...

There's always my physics prof's favorite units...furlongs per fortnight. :) He claimed that if you used furlongs/slugs/fortnights, all the physical constants in the universe ended up being close to 1.0 X 10^something. LOL! (We never checked his math on that, though ha ha!).
 
az_gila said:
The "days long gone by" date back about four decades...:)

In the mid 1970's GAMA (General Aviation Manufacturing Association) recommended all manufacturers standardize on knots and so most but not all switched (Maule for example still to this day uses MPH calibrated airspeeed indicators).

My 1977 Tiger was made in the first year the factory switched from mph to knots on the ASI and placards.
Click to expand...
I still remember when we switched to metric down under in the 70s (except for aviation where we use ICAO units - knots, ft for altitude, etc). There was the pre-requisite bitching and moaning but everyone pretty much got used to it eventually.

I'm tri-lingual. More than comfy with the metric system for most of my life but also speak in imperial measurements on occasion if required, and of course "nautical" language too. The only thing I've never quite got my head around is degrees F. Oh.......and the drill gauge numbering system. My head makes much more sense of an actual dimension (inches or mm doesn't really matter) than a kinda random number assigned to it! :D
 
RV7A Flyer said:
There's always my physics prof's favorite units...furlongs per fortnight. :) He claimed that if you used furlongs/slugs/fortnights, all the physical constants in the universe ended up being close to 1.0 X 10^something. LOL! (We never checked his math on that, though ha ha!).
Click to expand...

Perhaps Vans should switch to the furlongs/fortnight for sales literature?

The faster RVs would be 8,400 furlongs/fortnight :)
 
az_gila said:
Perhaps Vans should switch to the furlongs/fortnight for sales literature?

The faster RVs would be 8,400 furlongs/fortnight :)
Click to expand...

I hope they're faster than that :)

Quickie on-line conversion tool says 170 kts = 525859 furlongs/fortnight! Impress your friends: "This plane goes over half a million furlongs per fortnight! And it only weighs about 35 slugs!"
 
RV7A Flyer said:
I hope they're faster than that :)

Quickie on-line conversion tool says 170 kts = 525859 furlongs/fortnight! Impress your friends: "This plane goes over half a million furlongs per fortnight! And it only weighs about 35 slugs!"
Click to expand...

Correct... I have to give up late night calculating. I screwed up the 8 furlongs per mile bit.

I didn't even think of looking for an on-line calculator that spoke furlongs/fortnight...:)
 
Metric

RV7A Flyer said:
I hope they're faster than that :)

Quickie on-line conversion tool says 170 kts = 525859 furlongs/fortnight! Impress your friends: "This plane goes over half a million furlongs per fortnight! And it only weighs about 35 slugs!"
Click to expand...

You know we are supposed to use metric if we can according to Jimmy Carter.
mm. per second would be about as good as you could want to state.
 
MPH or Knots

Here in Canada where I trained in 1981 it was done in MPH for me and I have always had an ASI that indicated MPH and knots were in small letters.

I have noticed the switch in the newer model planes I rent especially the ASI and struggle sometime with the :"conversion". Similar to our late PM sending us into "metric" chaos this does make for some trouble sometimes in the pattern; "have i trimmed for 70 MPH or have i trimmed for 70 knots?"

That is where the fun of this issue ENDS! I hate that momentary confusion on final and twinge of non confidence. So I fly er by the feel and use the ASI as a guide the way we were meant to.

I always just add 10 to the new plane ASI(knots) and hope it is "close enough".

Great thread here! Us old guys always need training eh?
 
At my Engineering school the problems on assignments and exams mixed up the units. The questions defined the desired units for the answer and you had to convert. Distance could be feet, meters, inches etc. Temps could be C or F, speeds in mph, kph or knots, and the worst, mass, could be kg or slugs. Slugs were my arch nemesis. Try to explain to a layperson why you buy hamburger at the grocery store in units of force (lbs) in America and units of mass (kg) in Canada. Try to explain that a model airplane servo torque rating in g/cm (not even g-cm) is wrong.

Interestingly, after that space probe slammed into Mars a bunch of years back, NASA has gone all metric to avoid a similar screw up. But at the aircraft company where I work, which is in Canada, which is officially metric, we work in imperial units. We will kick out the afm in metric for European customers if they ask for it.

The problem with establishing standards is that it requires humans to agree on something. Humans don't like agreeing on stuff in my experience and I think this thread illustrates that very well :D

Now about that primer....
 
sblack said:
At my Engineering school the problems on assignments and exams mixed up the units. The questions defined the desired units for the answer and you had to convert. Distance could be feet, meters, inches etc. Temps could be C or F, speeds in mph, kph or knots, and the worst, mass, could be kg or slugs. Slugs were my arch nemesis. Try to explain to a layperson why you buy hamburger at the grocery store in units of force (lbs) in America and units of mass (kg) in Canada. Try to explain that a model airplane servo torque rating in g/cm (not even g-cm) is wrong.

Interestingly, after that space probe slammed into Mars a bunch of years back, NASA has gone all metric to avoid a similar screw up. But at the aircraft company where I work, which is in Canada, which is officially metric, we work in imperial units. We will kick out the afm in metric for European customers if they ask for it.

The problem with establishing standards is that it requires humans to agree on something. Humans don't like agreeing on stuff in my experience and I think this thread illustrates that very well :D

Now about that primer....
Click to expand...

The colonies may have trouble, but I'm always amused at the UK's conversions.

All there are now used (EU based) to buying stuff in kilos and feeling C temperatures, but they still drive in miles and mph, buy petrol in litres but still use MPG for fuel economy...:)
 
sblack said:
Interestingly, after that space probe slammed into Mars a bunch of years back, NASA has gone all metric to avoid a similar screw up.
Click to expand...

I think you mean the one that *missed* Mars, Mars Climate Orbiter.

NASA had been specifying SI units for many years prior to that, but the lower-level software was incorrectly coded (did not adhere to interface spec for SI units, and not caught during V&V).
 
RV7A Flyer said:
I think you mean the one that *missed* Mars, Mars Climate Orbiter.

NASA had been specifying SI units for many years prior to that, but the lower-level software was incorrectly coded (did not adhere to interface spec for SI units, and not caught during V&V).
Click to expand...

Which units you used at NASA depended on which program you were working. The ISS actually has used both. Shuttle was built all imperial, but we used a lot of metric once we started working with the Russians. The truth was - we had to be able to work in both, and regularly convert.

When doing maneuvers, I mostly wanted to make sure I wasn't off by a factor of three (meters per second versus feet per second). And we changed from fps on orbit to knots on the way downhill.....:rolleyes:
 
Ironflight said:
Which units you used at NASA depended on which program you were working. The ISS actually has used both. Shuttle was built all imperial, but we used a lot of metric once we started working with the Russians. The truth was - we had to be able to work in both, and regularly convert.

When doing maneuvers, I mostly wanted to make sure I wasn't off by a factor of three (meters per second versus feet per second). And we changed from fps on orbit to knots on the way downhill.....:rolleyes:
Click to expand...

Yeah, Paul, in truth...even though the policy has been metric for a while, a lot of mechanical stuff on our flight vehicles has been and still is imperial. Old habits die hard. But for MCO, the specs on the s/w were clear...SI units. It just was coded wrong and never caught. :(
 
sblack said:
The problem with establishing standards is that it requires humans to agree on something. Humans don't like agreeing on stuff in my experience and I think this thread illustrates that very well :D
Click to expand...
I think it's more that people don't like change. Especially major change. It's big and scary and too hard. Even if it makes more sense in the longer term.

I like knowing everything is a factor of 10 and that prefixing it with "milli_ centi_ kilo_ etc" all means something consistent. It's consistent with the entire decimal numbering system. Dividing something up into quarters, eighths, sixty-fourths to get smaller but multiplying by 12, then 3, then 1760 to get bigger.......who the heck thought that system up? :)

But, as I said earlier, ya gotta be at least bilingual to get by these days! :D
 
N941WR said:
If you have an EFIS, it should be easy to swap between the two with a few button pushes.
Click to expand...

I flew off my test hours in MPH because I was comfortable with them after 20 years of renting and owning small Cessnas and Pipers, and because my transition trainer (Old Blue) used MPH. I switched the Dynon to KTS after I started flying around our countries and talking to ATC a lot. It took a few circuits to get used to the "new" approach speed, but I never considered changing back.

When I want to impress my non-pilot Canadian friends, I tell them cruise speed is 280 klicks, with top speed of 340 ;)
Jay
 
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