The need for speed....

[sir_crashalot]

Hi guys,

I have a question about which I have been looking for an answer since a long time. As I fly around the world now in my (t)rusty DC-3 I have three devices that tell me about my speed. First there is the IAS, it reads about 170 knots. Second is my GPS, which reads 211 knots. Third is DME for NAV 2, which reads 199 knots. So three devices, three different speeds. But what is my actual speed?

Crash

[Mobius]

IAS is your airspeed, your speed through the air, not corrected for a headwind or tailwind.  

GPS speed is your true groundspeed, or the speed at which you would be travelling over the ground.  

And for DME speed, in real life you need to be flying on a VOR radial directly at the VOR for that speed to be correct.  I don't know if that is modeled in FS but if it is, that would explain it
.

[Rocket_Bird]

Yep, everything what Mobius said.

GPS, im not sure how accurate, but it should be the most accurate airspeed of the three.

DME uses radio waves sent and sent back and the time difference between to measure your speed.  So if your not flying directly on the radial, your airspeed is going to be slow on the indicator.  

[MattNW]

Hi guys,

I have a question about which I have been looking for an answer since a long time. As I fly around the world now in my (t)rusty DC-3 I have three devices that tell me about my speed. First there is the IAS, it reads about 170 knots. Second is my GPS, which reads 211 knots. Third is DME for NAV 2, which reads 199 knots. So three devices, three different speeds. But what is my actual speed?

Crash

Your isntruments are showing two of your airspeeds. The one on the DME is explained above but the other two are your Indicated Airspeed, shown on your Airspeed Indicator and your Ground Speed, shown on the GPS. There's another airpseed you need to be concerned with but that one you will have to calculate. It's your True Airspeed.

I copied this from the Learning Center in the sim.

Indicated airspeed (IAS)
The speed shown on the airspeed indicator uncorrected for variations in atmospheric density, installation error, or instrument error. Except at sea level under standard atmospheric conditions, IAS does not correspond to the aircraft's actual speed through the surrounding air (that is, its true airspeed, or TAS).



True Airspeed (TAS)
An aircraft's actual speed through the surrounding air. As an aircraft climbs, the surrounding air becomes less dense. Therefore, indicated airspeed tends to decrease as altitude increases. To determine how fast the airplane is really moving through the air, the pilot calculates TAS based on the aircraft's current pressure altitude and the outside air temperature. A pilot must know TAS to solve navigation problems and file flight plans. As a rule of thumb, at a given indicated airspeed, true airspeed increases about 2 percent for each 1,000 feet (305 meters) of altitude. Therefore, an aircraft flying at an indicated airspeed of 100 knots at 10,000 feet (3,050 meters) is actually flying at about 120 knots through the air.


Ground Speed
An aircraft's speed relative to the ground; an aircraft's true airspeed corrected for the effects of a headwind or tailwind. For example, if an aircraft is flying level at 120 knots with a 15-knot headwind, its ground speed is 105 knots.

[gr31]

OK to everything so far as it goes, except the comment about why DME speed is slow if your going towards the DME. Remember, DME is measuring slant range to the DME, so the speed shown is a little off because of that. Since the slant range is a greater percentage of your total distance as you near the VOR, the speed error will increase. The three speeds are just measuring three different things.

[Mobius]

Yeah, that's right, but if you are flying perpendicular to a VOR, you slant range isn't changing so your groundspeed reading off of it would be incorrect.

[Ridge_Runner_5]

For example, flying around a navaid in a circle at the same altitude would read a speed of 0, since your distance to station is not changing.