Homemade Airspeed indicator

[Cadet_BNSF]

Hi all.

I am working on an airspeed indicator, and I am having some problems.

I can get dynamic pressure easily, and it appears to be correct, fluctuating only slightly(0.00 - 0.07 kPa) when at a dead stop. Those values appear to be fine, but they cause huge fluctuations in the airspeed I calculate from it. I get from a dynamic pressure of 0.04 kPa to an airspeed of 32ish mph. I am using this equation to calculate the airspeed from the dynamic pressure.

 

 

where A0 = 661.4788 knots

and P0 = 29.92126 inches Hg

and q_c is the dynamic pressure.

The overall output should be in knots. I'm at a loss as to why this doesn't work. If anyone has any clue, please let me know.

[ShadoxFilms]

There are a few things that stand out to me.

1. units: you mention kPa, knots, and in-Hg as units you used. Make sure your units are consistent, use ft/s, slugs/ft^3, and lbs/ft^2 for velocity, density, and pressure respectively (or m/s, kg/m^3, and Pa for metric)

2. there is a much easier way to calculate dynamic pressure, where qbar = .5*density*velocity^2, from here you can use Bernoulli's principle (I'm assuming you're in the incompressible regime, below mach 0.3ish) to calculate velocity. where P₀ = P + q. P₀ will be the total pressure readout from your transducer, P is the static pressure (based on your elevation/altitude). Since we assume incompressible flow, you can get density from your altitude as well from the 1976 Standard Atmosphere

from there, just rearrange the equation to give you  V² = 2(P₀-P)/density

[Cadet_BNSF]

1 hour ago, ShadoxFilms said:

There are a few things that stand out to me.

1. units: you mention kPa, knots, and in-Hg as units you used. Make sure your units are consistent, use ft/s, slugs/ft^3, and lbs/ft^2 for velocity, density, and pressure respectively (or m/s, kg/m^3, and Pa for metric)

2. there is a much easier way to calculate dynamic pressure, where qbar = .5*density*velocity^2, from here you can use Bernoulli's principle (I'm assuming you're in the incompressible regime, below mach 0.3ish) to calculate velocity. where P₀ = P + q. P₀ will be the total pressure readout from your transducer, P is the static pressure (based on your elevation/altitude). Since we assume incompressible flow, you can get density from your altitude as well from the 1976 Standard Atmosphere

from there, just rearrange the equation to give you  V² = 2(P₀-P)/density

I am reading both static and ram air pressure, so would I just put my static air pressure in for P then?

 

Also definitely in incompressible regime

Edited May 6, 2019 by Cadet_BNSF

[ShadoxFilms]

2 hours ago, Cadet_BNSF said:

I am reading both static and ram air pressure, so would I just put my static air pressure in for P then?

 

Also definitely in incompressible regime

If you have the ability to read static pressure then use that for P, and use your stagnation pressure for P₀

[wumpus]

I remember a rant (I think it was on the Van's Aircraft website) that insisted that the airspeed indicator was "the gauge that lies".  Perhaps you simply have too realistic a device.

- The rant was why you shouldn't put too much engine in your homebuilt aircraft.  When they say "don't exceed this velocity", they mean it and you shouldn't trust the airspeed indicator to indicate safety.

[mikegarrison]

16 hours ago, Cadet_BNSF said:

I can get dynamic pressure easily, and it appears to be correct, fluctuating only slightly(0.00 - 0.07 kPa) when at a dead stop. Those values appear to be fine, but they cause huge fluctuations in the airspeed I calculate from it. I get from a dynamic pressure of 0.04 kPa to an airspeed of 32ish mph. I am using this equation to calculate the airspeed from the dynamic pressure.

0.04 kPa at sea level should be about 8 mph. I suspect you multiplied by two where you should have divided (or something like that). Check your math.

15 hours ago, Cadet_BNSF said:

I am reading both static and ram air pressure

Dynamic pressure is simple if you are reading static and ram (total) pressure.

total pressure - static pressure = dynamic pressure

But any instruments are going to have correction factors. It depends on where you measure it. Airspeed indicators have traditionally used pitot tubes that measure both static and total pressure at nearly the same spot.

Edited May 6, 2019 by mikegarrison