The International Cessna^® 170 Association

I have always done my flight planning in kts (and measured distances in nautical miles) until I bought my 170B. Given that the airspeed is in MPH and not kts, do you measure distance on the map in statute miles ?

Thanks All

Kevin

Kevin - I contiue to use knots. I flight plan at 85 knts/7.9 gph. My GPS reads out ground speed in knts. so the airspeed indication in MPH just has not been an issue. Without a GPS and a minimal panel I would convert airspeed to knots rather than vice versa.

Kevin,

One knot is is one nautical mile per hour.

One nautical mile is 1.15 statute miles

A stock C-170 will cruise at about 115 statute miles per hour.

That is 100 knots.

Measure on the chart (We aviators do not fly on maps) in either statute or nautical miles, your choice.

How much simpler can it get.

Robert,

If you're actually flyimg at 85 knots, thats about 98 miles per hour. at 7.9 gph, thats about 12.5 miles per gallon. Thats not at all good performance for a stock C-170. You must be running a very soft power setting and a very rich mixture.

Over the years on a stock engined C-170B, I have averaged about 7 gph on a short (one and one half hour) cross country and about 6.7 to 6.8 0n a longer trip with a TAS of 115 mph.

Kevin,

I know that this is off-topic from your question, but since the topic of gps came up in replies I thought that I would add... it is helpful to select the speed display on your gps to match your airspeed indicator units. On final I use the gps as a quick check to see if I misread the wind direction. If the gps displays 80mph (groundspeed) and the airspeed indicates 70 mph, it means I screwed up and have a 10 mph tail-wind and execute a go-around. If the units do not match, it takes quick math (knots x 1.15 = statute)which I don't have much of left in my brain on short final.

Fly safe,
Bob

Charlie170B wrote:If the gps displays 80mph (groundspeed) and the airspeed indicates 70 mph, it means I screwed up and have a 10 mph tail-wind....

Or you could be landing at a high density altitude airport on a calm day...

Miles

PS: At about 7500ft DA, your indicated airspeed in mph is very close to your true airspeed in knots.

My ASI read in mph too, but I think it's smart to always do all your flight planning calcs using nautical miles, as its length is actually derived from the length of an arc minute on the earth...which are used on sectional charts.

Well Zane I think I understand your message to be to use the same measuring system as the charts use to avoid conversion errors.

But the term knot was not derived from the length of an arc minute though that is know what the term describes.

This from Wikipedia (which I know is not gospel)

Origin
Until the mid-19th century vessel speed at sea was measured using a chip log. This consisted of a wooden panel, weighted on one edge to float upright and thus have substantial water resistance, attached by line to a reel. The chip log was "cast" over the stern of the moving vessel and the line allowed to pay out. Knots placed at a distance of 47 feet 3 inches (14.4018 m) passed through a sailor's fingers, while another sailor used a 30 second sandglass (28 second sandglass is the current accepted timing) to time the operation.[citation needed] The knot count would be reported and used in the sailing master's dead reckoning and navigation. This method gives a value for the knot of 20.25 in/s, or 1.85166 km·h−1. The difference from the modern definition is less than 0.02%.

I figure if I'm going to fly an old airplane, might as well go REALLY old-school. so I figure my flying in cubits. gives me a nice big number for a groundspeed too, very satisfying

We're all pretty good at math otherwise we wouldn't be here. But why mix it up and invite a mistake? Most everyone knows how to convert knots to statute and vise versa. When I fly the 170 everything I do is sm because all of my instrumentation is statute, therefore to minimize mistakes I calc in stat. miles and speeds. Last year I had over 100 hrs and 8000 km or 5000 sm in high performance sailplane and every thing in it is measured in kts in the glider (at least in the U.S. / metric in Europe and the rest of the world) so for the most part I calc in kts. but for FAI awards and records we also use the metric system such as 300 500 750 and 1000km flight awards. It just adds to the work load.
Russ

I use knots in a standardization effort. My airspeed is marked in both mph and kts (as they commonly were in the mid '70's.) With my O300C engine, and McCauley 1C172-EM7655 cruise-prop (same performance model as a MDM prop) turning 2450 rpm (determined to be accurate by stroboscopic test) N146YS consistently delivers 104 kts and burns 7.9 gph.

(But I'll never win a short-field takeoff contest. In the summer of 2002, taking off from 105-degree F, Winslow AZ at un-documented weights due to it being Jamie's first long cross country trip in the airplane,... the airplane flew only due to curvature-of-the-earth.)

(I guess you guys now know the explanation of the "mostly" in my signature.)

I mostly use mph in my plane because that's the way it is set up. If I'm in a plane that is set up in knots, I use knots. If I fly IFR, I switch to knots because that's the way IFR is.

It doesn't matter to me either way but I don't mix them any more than I have to because I'm lazy and don't like to do conversions in my head. My GPS is so easy to convert back and forth, I change it to suit the situation.

Well, with a scattering of poll results in, it looks like Knots and MPH will have nearly equal numbers of committed delegates at the convention. The superdelegates will have the final say.

Kevin, it looks like you can go either way for now, and probably indefinitely!

Bruce, don't be so hard on Zane. The length of a nautical mile was indeed chosen to agree with one minute of arc on a great circle course, because it greatly simplified the calculations of ocean distances before the advent of computers. Given two points of latitude and longitude, you can compute the distance between them very quickly using only trig tables.

The knots on a sailor's line were spaced to give a ship's speed in nautical miles per hour (not KM, as the Wikipedia article seems to suggest).

Best Regards,

John

Also a nautical mile is shorter at the Equator than at the poles, so your unit of measurement is not consistent. This could equate to about 60'.

From Wikipedia:
The historical definition differs from the length-based standard in that a minute of arc, and hence a nautical mile, is not a constant length at the surface of the Earth but gradually lengthens with increasing distance from the equator, as a corollary of the Earth's oblateness, whence the need for "mean" in the preceding sentence. This length equals about 1,861 metres at the poles and 1,843 metres at the Equator, a variation of one percent

I decided back in the seventies when I started flying that I wasn't flying a boat. I'm only VFR and have always changed my airseed indicator to have MPH on the out side of the arc. When the need arises I just convert off the gauge. I also use statute miles and my GPS reads MPH. GPH, MPH and time works for me. Some how I manage to get where I'm going. Lance

P.S. I've been forced to learn the metric system three times now for different reasons. Each time I don't need to use it anymore I promply forget it.