Speed Sensor Anyone???

[Ramius]

Hi Bo!
Thank you and there is no such thing as "ignorant" as we all see situations differently which makes for the stimulation of great ideas! Counting the revs is a great idea and this sub has two motors. I guess you could add the two motor RPM's together and then divide them by 2 for average RPM. In a turn you speed up one motor and slow down the other. While I do not have an answer as to why this is not used in aircraft (for example) it seems there must be a reason. Aircraft seem to use a pitot tube while boats seem to use either pressure or a "paddle wheel" system. Maybe it has more to do with "flow" such as a "head wind" in aircraft? Best, Ed

[ardhuru]

While I do not have an answer as to why this is not used in aircraft (for example) it seems there must be a reason.

Wouldnt you get a wrong reading after factoring in the tail/head wind? The RPM measuring technique would work only in the absence of the above. For the same RPM, your actual speed would be different depending on whether the wind is aiding or resisting you.

[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.

[Ramius]

Thanks Steve!
I have been flying airplanes since 1973 and helicopters since 1994 (the real ones not the models) so I agree with you completely. Typically in most lakes or even a swimming pool the water is pretty much stationary. I will probably take an extra depth sensor and spin it in a plastic trash can filled with water at different speeds and see what "head-on" pressure readings I get. Hopefully there will be major changes in the readings? My fear is that the pressure at slow speeds will be to small to see. There are just so many details to work out on this sub project. For example, no one seems to know if First Person View goggles display video in underscan or overscan. This makes a different with trying to use an On-Screen Display character generator as with overscan as with a CRT TV you do not have the first column available and thus you are limited as to how many columns are useable! So many questions, so little time! Lol Best, Ed

[SteveB]

From your posts, it sounds like an impressive project and quite an undertaking.

My fear is that the pressure at slow speeds will be to small to see.

I can understand the concern. I would expect it would take some rather sensitive pressure sensors to pick up the differences in pressure at slow speeds.

Keep us updated on how things progress!