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why MPH instead of knots?

everyone but us use metric.

Knots are not metric. It comes from using nautical miles vs statute miles, neither of which are part of the metric system. Nautical miles were created to be easily divisible from longitude and latitude measuring systems. The name comes from the measuring system used 100's of years ago, number of equally spaced knots on a rope hanging off a ship.
 
Well I for one am knot going to get involved in this debate. I'm in knots just thinking about it. And I don't have enough popcorn.
Art
I fly in knots also.
 
Fun With Units….

Way back in the last century, when I was in Aero Engineering school, we had a visiting professor (from GD in Forth Worth, home of the F-16….) for our senior design project. He announced that the use of ANY speed units (mph, fps, etc…) other than knots would be cause for an immediate “F” on anything submitted for grading because “that’s how the industry works, and you’d better get used to it!” (Of course, all your computations were in fps, so you had to convert in the end….)

Didn’t bother me - I was already a pilot before I got to college, and was used to knots because that’s what the “system” used - even though I learned to fly in the days where most GA airplanes were still calibrated in mph.

Of course, I went on to run missions with a flying machine that used knots, feet-per-second, and Mach - depending on the phase of flight. And once we started flying with our friends overseas, we added meters-per-second to the mix. All in a days work.

I chuckle every September when I get together with my “fast friends” at the Reno Races and they all do everything in mph (even the airline and military pilots who use knots in their day jobs) because…yup, the numbers are bigger that way!

My own airplanes? Everything is in knots because that’s what my profession uses (for atmospheric flight), and what ATC expects.

Paul
 
Maybe the RV-15 will have a cruise speed in MPH and a stall speed in Knots

Dick Rutan and Jeana Yeager were showing a VariEze, or maybe Long Eze, with cruise speed listed in MPH and stall speed in knots.

For years, I've heard that this is basic airplane marketing: Cruise speed stated in MPH, stall speed stated in knots. Obviously! Right?

Airspeed indicators should be scaled from 0-100.
0 = Vso
100 = Vne

You joke, but that's basically the kind of reasoning that was used to invent the Fahrenheit scale ;)
 
But Have We Considered..

I wonder how many times the knots/mph/kilometers/furlongs per fortnight decision was made by what was on hand when it was time to install the airspeed indicator?
 
Function vs bragging rights

I fly using Knots
I describe my flights using MPH to the plus 50 crowd
I describe it using KPH to those under 50 (oh, so easily impressed!)

My first 7A had a Flight Data Sys AFP-30 in it so I could also report cruise at Mach 0.256, but that seamed so un-Bragg able….
 
But didn’t get it quite right so they needed to invent the centigrade scale.

Fahrenheit:

100 degrees = Human body temperature +/- 5%
0 degrees = You're dead

To answer a question some of you may have, the zero point was chosen as the stable temperature of a saturated solution of ammonium chloride mixed with water ice, which achieves a very specific temperature when mixed. Conveniently, the freezing and boiling points of water happen to be nearly 180 degrees apart, as an interesting physical metaphor.
 
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Ancient history

For responders who are not really, really old here is the history of speed units that I experienced. I was in graduate school studying aero engineering when I was learning to fly, late 1960s. The FAA standard at that time was MPH. During that period the flight instruction world was in a bit of a tizzy because the FAA announced that they were going to "modernize" the use of speed units but did not say what the change would be. Most people thought it would be a change to metric, kilometers per hour or meters per second. I was astonished when the modernization was announced as a change from MPH to knots (early 1968 I think). Knots is a really old unit that originated with square-rigged sailing ships, and I still don't know how that seemed like modernization to the decision makers.

Anyway, all airplanes built before around 1970 used MPH, by FAA regulation. Van is roughly my age and started out in that scenario.
 
A nautical mile equals 1 minute of latitude. Using knots matches up with almost all charting. It simply makes sense.
 
I'm super old-fashioned. I like MPH. The origin of the statute mile is that it was the average distance a Roman Legion covered in 1000 paces (what we would think of as both a left and right step). That's a lot grander than a latitude minute.
 
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,
Knots is a really old unit that originated with square-rigged sailing ships, and I still don't know how that seemed like modernization to the decision makers.
And a statue mile is modern? Statute mile is the result of Roman influences and plowing oxen.

"It originated from the Roman mille passus, or “thousand paces,” which measured 5,000 Roman feet. About the year 1500 the “old London” mile was defined as eight furlongs. At that time the furlong, measured by a larger northern (German) foot, was 625 feet, and thus the mile equaled 5,000 feet."

If the mile originated with 5000 Roman feet, how did we end up with a mile that is 5280 feet? Blame the furlong. The furlong wasn’t always just an arcane unit of measure that horseracing fans gabbed about; it once had significance as the length of the furrow a team of oxen could plow in a day. In 1592, the English Parliament set about determining the length of the mile and decided that each one should be made up of eight furlongs. As a furlong was 660 feet, we ended up with a 5280-foot mile."


"Each nautical mile originally referred to one minute of arc along a meridian around the Earth. Think of a meridian around the Earth as being made up of 360 degrees, and each of those degrees consists of 60 minutes of arc. Each of these minutes of arc is then 1/21,600th of the distance around the earth. Thus, a nautical mile is 6076 feet."

But where did the unit if foot or feet come from? “foot” is 12 inches long and was actually the length of King Henry I's foot. The inch was the length of 3 grains of barley end-to-end or the width of a man's thumb. The length between someone's outstretched arms was called a fathom.... Imperial unit standards are now referenced to metric units, example one yard is 0.9144 meter or 12 inches 0.3048 meters.

But what about a meter or metre? The metre was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole along a great circle, so the Earth's circumference is approximately 40000 km. Clearly measuring the earth is not easy. In 1799, the metre was redefined in terms of a platinum bar in Paris. Then it got real complicated using cadmium isotope wave length, Krypton-86 wave length and methane-stabilized helium–neon laser, finally the distance traveled at speed of light in a vacuum divided by 299,792,458.

Moral of the story it is a standard that is fixed and known, based on some arbitrary choices.
 
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Ok .. now is it IAS, CAS or TAS??

I'm shopping for a LSA and can't seem to find the True Air Speed of the the RV12 with the ULS engine at .. say 8000ft msl? Anyone?
 
I'm shopping for a LSA and can't seem to find the True Air Speed of the the RV12 with the ULS engine at .. say 8000ft msl? Anyone?

Check out this RV-12 performance webpage from Van's:

The cruise speeds listed would be True Airspeed (TAS).

i-xZgdVzs-L.jpg



Also, check out the POH's for the RV12 and the RV12iS for more detailed performance specs.
 
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Are those range values right? The 912iS is a lot more fuel efficient than the ULS. The POH says 493/368 nm and 547/440 nm for the iS/ULS respectively.
 
Are those range values right? The 912iS is a lot more fuel efficient than the ULS. The POH says 493/368 nm and 547/440 nm for the iS/ULS respectively.

That question came up awhile back in the RV-12 forum. IIRC, 'rvbuilder2002' (Scott M.) addressed it and explained it. Maybe he or an RV-12 folk can point us to the post with the explanation.

EDIT: This is the thread that discusses the range numbers:

 
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Yes, I recall that thread. What I was querying was the figures in post #68 that show very little difference in range in between the iS and ULS. That doesn't look right. Maybe a typo?
 
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