Ground speed of a plane

[chrisb]

Lets make this simple flying at 5,000ft at 100kts with no wind

 

Is my ground speed really 185kmh?

 

Under the same conditions would a 350nm trip take 3.5 hours?

 

 

[Guest steve-nz]

In theory yes, but still have to account for climb, and any wind changes throughout your planned route.

 

 

[chrisb]

to make it simple, no wind changes and you start and finish the 350nm trip at 5,000ft

 

 

[metalman]

Basically yes,,there's the difference between indicated and true airspeed, as a rough calc true is 1.5 knots per thousand feet.

 

If your indicating 100kias @a050, then true airspeed would be about 107 true airspeed, air density makes a bit of difference but as a rough guide I find this works out okay. You'll notice the higher you go the lower your airspeed seems, but by working out your true speed you can get a better picture of your speed.

 

Matty

 

 

[rgmwa]

to make it simple, no wind changes and you start and finish the 350nm trip at 5,000ft

In that case, and assuming 185 kmh is your true airspeed, your actual ground speed would be less - about 184.9557 kmh.

 

rgmwa

 

 

[facthunter]

Keep your units the same. Nautical miles and knots or kms and kms./ hour. When the air thins out.(with altitude increase) your ASI will show a lower figure than your TAS. Your groundspeed is what you use to work out your time intervals, and there will normally be a wind component to make things a bit more complicated. Your climb segment will be separate normally, though some ignore it. It wouldn't hurt by doing the thing correctly from the outset, so I would use a climb, cruise and descent phase. If you plane is capable of doing it safely you can use a powered descent at a higher airspeed and use your potential energy to advantage to reduce time. Nev

 

 

[Captain]

Or also install a GPS to aid monitoring of trip progress, take the guesswork/calculation out of true speed, obtain an indication of winds aloft, know for certain your vmg and time to run.

 

 

[Ultralights]

a TAS at 5000ft ISA conditions, would be 108 kts.

 

 

[Zibi]

Keep your units the same. Nautical miles and knots or kms and kms./ hour.

Oh how I wish that someone would tell that to CASA and RAAus, instead of having rules like the VMC visibility (distance from cloud) where you have ft, m and nm all mixed into a a single rule...

 

 

[facthunter]

Those confusing mix of units probably come from the fact that elevations are in FEET, except in a few countries (Russia (Indonesia etc ) where they are in metres. Nev

 

 

[chrisb]

a TAS at 5000ft ISA conditions, would be 108 kts.

So if IAS is still reading 100kts and there is a 20kt head/tail wind does is affect ground speed by the same amount?

What if there was a 20kt crosswind, how will it affect ground speed?

 

 

[pylon500]

about 184.9557 kmh.

Why is anybody in aviation talking about kmh?

 

 

[metalman]

So if IAS is still reading 100kts and there is a 20kt head/tail wind does is affect ground speed by the same amount?What if there was a 20kt crosswind, how will it affect ground speed?

As you start on the cross country part of your certificate( or PPL) you'll learn about the difference between heading and track, but the basic idea is that you will have a certain amount of wind coming at the aircraft either from the sides, rear or front, working out what angle you have to fly the aircraft in relation to the wind will determine how much difference there is between what the compass is reading and the aircrafts track across the ground. Next time your on final with a bit of cross wind have a quick look at the compass, it will read different to the runway heading . Depending on the component of headwind to crosswind ( or tailwind ) will have an effect on the ground speed,

Matty

 

 

[kaz3g]

?.... Next time your on final with a bit of cross wind have a quick look at the compass, it will read different to the runway heading .Matty

Hi MM

 

Unless you do a wing down (into wind) approach instead of crabbing :-)

 

Kaz

 

 

[chrisb]

Yeah last time I had a flying lesson it was exceptionally windy the pilot took of and landed at about 30 degrees difference to the runway

 

 

[dazza 38]

I like the into the wing down into the wing method. Each to their own

 

 

[Ultralights]

yes, a 20 kt headwind, would take 20kts off your 108 kt ground speed, a tailwing would add 20kts, so your ground speeds would be 128kts or 88kts. even though the ASI will still read 100kts all the time..

 

also, if the wind is coming from you at a 90 deg angle, it will still have a headwind component, as the nose of the aircraft will point slightly into wind as you point it to maintain track across the ground.

 

 

[Guest Andys@coffs]

Its basic trig, draw a line that's length represents your speed across the ground (108kts) and the angle of the line represents your direction through the air. One end of the line represents the time at the beginning of the interval (time 0 of 108kts)l, the other the time at the end (1 hr of 108kts) . at the time at the end of interval end of the line draw a line representing wind. direction represents where the wind is going. the end of that line that intersects the first is wind at the beginning of the same time interval and the other end represents time at the end of the interval.

 

The distance from the beginning of line 1 to the end of line 2 represents the resultant speed across the ground (all lines must be to a scale and the resultant line read at the same scale). the difference in angle between Line 1 and Line 2 is the affect the wind will have on your flight and the angle of the final line represents the uncorrected resultant ground track.

 

We don't draw triangles in the air, rather use computers or circular slide rules to do the same thing, however drawing it as a triangle on paper as you are learning makes it easy to visualize what is happening. Until you have the ability to visualize what is occurring the computer or circular slide rule methods will deliver a result that you will be unable to know if they are correct or not. If you can visualize the triangle in your head you can quickly guestimate the result and therefore know if the result delivered bythe alternates is correct or you've made a mistake.

 

Andy

 

 

[Old Koreelah]

Why is anybody in aviation talking about kmh?

Much as I love the simplicity of the metric system, and loathe how mediaeval measurements are creeping back into our lives, I must admit that navigating fluid like air or ocean is easier with nautical miles.

One degree of a great circle is 60 nm, so one minute = one nm. Simple.

 

A fascinating bit of trivia: degrees for measuring angles came down to us from the ancients. Kilometres for measuring distance are based on the distance, through France, from the north pole to the equator. So the earth has a circumference of 40,000 km.

 

Divide that by the ancients' 360 degrees and you get 111.11111111111km per degree.

 

Interesting, but not easy to navigate by; nautical miles are better.

 

 

[rgmwa]

Aviation must win the prize for the world's most bizarre collection of inconsistent units for measuring stuff. It's probably here to stay though.

 

rgmwa

 

 

[facthunter]

When you get into the air , you are in an airmass which is generally moving. ( Like a river) The higher you go the more it can get up to great speed. Jetstreams at FL350 plus go at 200 knots.( you won't get to fly there for a while). It can help you or slow you or push you off track. When you learn navigation this will all be revealed. Aircraft always have to take account of WIND speed and direction. Look at it as though when you move a certain distance THROUGH the air in a certain time. the airmass will have moved a certain distance as well. Where you end up is a product of both. Nev

 

 

[Guest Andys@coffs]

Much as I love the simplicity of the metric system, and loathe how mediaeval measurements are creeping back into our lives, I must admit that navigating fluid like air or ocean is easier with nautical miles.One degree of a great circle is 60 nm, so one minute = one nm. Simple......

But only at the equator. There are infinite great circles( which must pass through the equator twice) but only the equator is 1nm=1second of arc, and in any event for us RAAus types we will never fly at the equator (at least in anything which we have to worry about navigation ourselves)

 

Andy

 

 

[facthunter]

The earth is an oblate spheroid. The equator is the biggest great circle. This is due to it's rotation and this is the place where a degree of longitude will be at it's maximum size. ( It is zero at the actual pole points(Geographic not magnetic)) Degrees of latitude stay constant in size. Flat maps can never truly give an accurate indication of distance or area all over or even of shape, particularly projections like Mercator's which are hopeless near the poles.

 

Lambert's Conformal Orthomorphic based on two parallels of latitude is used for most navigational maps, where a great circle ( the shortest distance between any two points) is almost a straight line on the map. It's the type you will encounter most in aviation. Nev

 

 

[Guernsey]

Why is anybody in aviation talking about kmh?

When I had my Tyro I used mh not knots or kmh....it gave me a hell of a cruise speed. .

 

Alan.

 

 

[Old Koreelah]

The earth is an oblate spheroid. The equator is the biggest great circle. This is due to it's rotation and this is the place where a degree of longitude will be at it's maximum size. ( It is zero at the actual pole points(Geographic not magnetic)) Degrees of latitude stay constant in size. Flat maps can never truly give an accurate indication of distance or area all over or even of shape, particularly projections like Mercator's which are hopeless near the poles.Lambert's Conformal Orthomorphic based on two parallels of latitude is used for most navigational maps, where a great circle ( the shortest distance between any two points) is almost a straight line on the map. It's the type you will encounter most in aviation. Nev

You blokes had to complicate things with facts!