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01-07-2017, 09:52 PM
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Join Date: Jan 2014
Location: Hobbs, NM
Posts: 239
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MPH vs Kts
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
__________________
Ben
RV-7A Tip Up Airworthy on 12/20/2016
RV-10 Emp 95% complete - sold
RV-14A Empennage Done waiting on front fuse kit to show up.
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01-07-2017, 10:13 PM
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Join Date: Jan 2005
Location: Irvine, CA
Posts: 873
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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!
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Mark
RV-12iS Fuselage
RV-9A Project: Sold
VAF donation made for 2020
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01-07-2017, 10:22 PM
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Join Date: Jul 2013
Location: US
Posts: 2,251
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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....
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01-07-2017, 10:38 PM
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Join Date: Jan 2005
Location: SC
Posts: 12,887
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If you have an EFIS, it should be easy to swap between the two with a few button pushes.
__________________
Bill R.
RV-9 (Yes, it's a dragon tail)
O-360 w/ dual P-mags
Build the plane you want, not the plane others want you to build!
SC86 - Easley, SC
www.repucci.com/bill/baf.html
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01-08-2017, 01:17 AM
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Join Date: Jun 2010
Location: Sydney, Australia
Posts: 123
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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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Mike
Down Under
Last edited by dutchroll : 01-08-2017 at 01:19 AM.
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01-08-2017, 04:58 AM
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Join Date: Jan 2005
Location: SC
Posts: 12,887
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Quote:
Originally Posted by dutchroll
...
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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Maybe it was because Mazda had a cart named the 323?
__________________
Bill R.
RV-9 (Yes, it's a dragon tail)
O-360 w/ dual P-mags
Build the plane you want, not the plane others want you to build!
SC86 - Easley, SC
www.repucci.com/bill/baf.html
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01-08-2017, 05:30 AM
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Join Date: Apr 2010
Location: Covid Country-SoCal
Posts: 1,081
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But (there's always a but) VFR visibility is listed in statute miles! Class D airspace is 5 statute miles IIRC.
-Marc
Quote:
Originally Posted by RV7A Flyer
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....
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RV-10
N814RV
2020 Donation Made
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01-08-2017, 06:39 AM
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Join Date: Mar 2010
Location: KSGJ / TJBQ
Posts: 2,039
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Quote:
Originally Posted by Plummit
But (there's always a but) VFR visibility is listed in statute miles! Class D airspace is 5 statute miles IIRC.
-Marc
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What does visibility have to do with speed?
Quote:
Originally Posted by rv9builder
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!
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IMHO this is the best reason/response. In the end, it will be whatever you want for your airplane. 
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Galin
CP-ASEL-AMEL-IR
FCC Radiotelephone (PG) with Radar Endorsement
2020 Donation made
www.PuertoRicoFlyer.com
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01-08-2017, 06:44 AM
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Join Date: Oct 2011
Location: Kennesaw, Ga
Posts: 825
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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. 
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Amir
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RV-7
RV-10 - Sold
Supporting VAF since the first visit
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01-08-2017, 06:51 AM
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Join Date: Jan 2010
Location: Green Cove Springs, FL
Posts: 234
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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.
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RV6 N585SS - building on!
O-360-A1G6D converted to A1A
Dual Powertek EFII
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Astronics PPS
Hartzell C/S
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