Speed Sensor Anyone???

[SteveB]

Some clarifications regarding speed and airplanes (simplified). Aircraft measure speed in many different ways (mostly in knots, which is annotated by the K prefix).

KTAS = True Airspeed - This is the aircraft relative to the air mass.

KGS = Groundspeed - This is TAS corrected for the movement of the air mass (headwind/tailwind).

KIAS = Indicated Airspeed - Speed measured by the Pitot-Static System.

KCAS = Calibrated Airspeed - IAS corrected for installation/instrument/position errors.

If the air mass is stationary, GS = TAS = how fast you see the airplane fly by as you stand on the ground.
CAS ~ IAS. These numbers are normally pretty close (depending on aircraft), and often times used interchangeably in non-critical situations.
Typically, CAS, TAS, and GS need some form of 'advanced' instrumentation to read the cockpit, such as GPS for GS, or an Air Data Computer + GPS + Flight Management System for TAS and CAS.

CAS/IAS differ from TAS, often by significant amounts, due to variations in the air mass, such as decrease in air density with increase in altitude. However, it is because IAS is affected by density that it gives a better picture of the aerodynamics of the aircraft and speed/mass/volume of the air flowing over the wing. Whether the aircraft is at 1000' or 30,000' in altitude, at 175 KIAS, air flowing over the wing is aerodynamically equivalent, however the TAS at 30,000' will be much greater.

So, why not measure the rotation speed of the prop to get the speed of the aircraft? Because, the aerodynamic forces on the aircraft and propeller change with the speed, altitude, weight, etc. So a fixed pitch propeller spinning at 2000 rpm at sea level on a standard day, no wind, with the aircraft total gross weight (TGW) of 3000lbs and a new paint job may fly at 150 KIAS (and ~150 TAS = 150 GS). But, at 5000' on a hot summer day with TGW of 4000lbs and having not been washed in months may only fly at 130 KIAS. But, TAS would be more like 160 knots, and there is a 25 knot tailwind, so the GS would be 185 KTAS!


And that is the simple answer


I should add that with a ship/submarine, you will encounter the same issue with counting prop revs, although to a lesser degree due to smaller variations in density and speed, but weight would make a much bigger difference because of the much larger range in weight capacities of a ship.