Airspeed vs Groundspeed.
11 November 2011, 09:33 PM
#1
Scooby Regular
Thread Starter
Join Date: Oct 2003
Location: Runway two seven right.
Posts: 6,652
Likes: 0
Received 0 Likes
on
0 Posts
11 November 2011, 09:52 PM
#3
Scooby Regular
Join Date: Nov 2006
Location: Enginetuner.co.uk Plymouth Dyno Dynamics RR Engine machining and building EcuTek SimTek mapping
Posts: 3,662
Likes: 0
Received 0 Likes
on
0 Posts
Wet air day almost worth it there!
I bet the man had one eye on the ASI the whole time...
I bet the man had one eye on the ASI the whole time...
12 November 2011, 11:39 AM
#4
Scooby Regular
Join Date: Aug 2002
Posts: 39,877
Likes: 0
Received 0 Likes
on
0 Posts
Its straightforward enough.
There are three considerations, indicated airspeed, true airspeed,and groundspeed.
Indicated is the pressure airspeed measured by the pressure created in the instrument by the movement of the aircraft in relation to the air.
True airspeed is the actual speed relative to the air as above except that as you go higher. the air density and pressure reduces so the true airspeed has to be higher to generate the same pressure in the instrument as at lower altitude. At 40K feet the TAS will be approximately twice the IAS. This IAS is the important speed for aircraft handling.
You could stall the aircraft with a TAS of well over 200 knots because the IAS would be too low for the aircraft to continue flying at the much higher altitude.
Groundspeed is the true airspeed but modified by the local wind speed. A TAS of say 250 knots will produce a groundspeed either plus or minus the wind speed depending whether it is a head or tailwind. 20 knots on the nose would convert the TAS as above to a groundspeed of 230 knots.
One of our training exercises on the Harvard was practising flying at the slowest possible speed in order to learn delicacy of touch. If there was a Chinook wind blowing as often happened just east of the Rockies you could get a strong enough headwind which would make you fly backwards relative to the ground at a groundspeed of as much as 25-30 knots. That looked really weird both from the cockpit and when watching someone from the ground!
At high altitudes, the Mach number which is the speed relative to the speed of sound is essentially a TAS indicator, but in different units.
If you are landing or taking off at a high level airfield such as Nairobi or Khatmandu, you fly the aircraft using the IAS as usual but the TAS will be higher thus giving a greater grounspeed. You really notice the difference with the extra braking which is required to stop and the extra runway you have to use to accelerate to the unstick speed for takeoff.
Les
There are three considerations, indicated airspeed, true airspeed,and groundspeed.
Indicated is the pressure airspeed measured by the pressure created in the instrument by the movement of the aircraft in relation to the air.
True airspeed is the actual speed relative to the air as above except that as you go higher. the air density and pressure reduces so the true airspeed has to be higher to generate the same pressure in the instrument as at lower altitude. At 40K feet the TAS will be approximately twice the IAS. This IAS is the important speed for aircraft handling.
You could stall the aircraft with a TAS of well over 200 knots because the IAS would be too low for the aircraft to continue flying at the much higher altitude.
Groundspeed is the true airspeed but modified by the local wind speed. A TAS of say 250 knots will produce a groundspeed either plus or minus the wind speed depending whether it is a head or tailwind. 20 knots on the nose would convert the TAS as above to a groundspeed of 230 knots.
One of our training exercises on the Harvard was practising flying at the slowest possible speed in order to learn delicacy of touch. If there was a Chinook wind blowing as often happened just east of the Rockies you could get a strong enough headwind which would make you fly backwards relative to the ground at a groundspeed of as much as 25-30 knots. That looked really weird both from the cockpit and when watching someone from the ground!
At high altitudes, the Mach number which is the speed relative to the speed of sound is essentially a TAS indicator, but in different units.
If you are landing or taking off at a high level airfield such as Nairobi or Khatmandu, you fly the aircraft using the IAS as usual but the TAS will be higher thus giving a greater grounspeed. You really notice the difference with the extra braking which is required to stop and the extra runway you have to use to accelerate to the unstick speed for takeoff.
Les
12 November 2011, 12:31 PM
#5
Scooby Regular
Join Date: Nov 2006
Location: Enginetuner.co.uk Plymouth Dyno Dynamics RR Engine machining and building EcuTek SimTek mapping
Posts: 3,662
Likes: 0
Received 0 Likes
on
0 Posts
Not having a calculator handy, I always found that flying too fast was better than flying too slow!
Trending Topics
Thread
Thread Starter
Forum
Replies
Last Post
GazJenno
Projects
36
11 June 2013 06:17 PM
Malcolm RC
General Technical
17
12 February 2013 05:05 AM
