Askew Pitot tube, airplane velocity measurement

In summary, the conversation discusses the effect of an incorrectly installed horizontal Pitot tube on the measurement of an airplane's velocity. The tube's incorrect placement can result in a wrong velocity measurement due to the inability to measure the stagnation pressure at the closed end of the tube. This leads to a reduced Bernoulli equation that only uses static pressure, resulting in a wrong velocity measurement. The conversation also brings up concerns about the laminar flow becoming turbulent after hitting the inner surface of the tube and the need to consider the height difference in the full equation.
  • #1
distalphalanx
3
0

Homework Statement


A horizontal Pitot tube (with one end open for airflow and another closed) is accidentally installed into an airplane askew, so that the tube is not horizontal. Instead the closed tube end is higher (in the y-direction of a xy-plane) than the inlet end. How does this affect the measurement of the velocity of the airplane?

Homework Equations


Bernoulli's equation. Solving for velocity it becomes v= √((2*(p_t - p_s)) / ρ_air)
, where p_t is the stagnation pressure (total pressure), p_s is the static pressure (pressure of the environment, atmospheric pressure at the height), and ρ_air is the density of air.

The Attempt at a Solution


I assume that in order to work properly, the Pitot tube needs to be horizontal. In this case, I suppose that the streamlines of the flow hits the wrong surface area (at the opening of a tube), and the flow isn't able to move to the closed end of the tube (which it should do in order to determine the stagnation pressure), because of the inclination.
As a consequence, the stagnation pressure at the end of the closed tube cannot be measured, since the stagnation pressure occurs at the point when the streamlines hit the inner surface of the tube (here v=0, p=max, which should ideally be measured at the closed end of the tube).

I assume that when there's no stagnation pressure to be measured (stagnation pressure at the measurement site in the closed end of the tube is 0, p_t = 0), the reduced Bernoulli equation for determining the velocity only uses the static pressure p_s, not the difference between the stagnation pressure and the static pressure (p_t - p_s).
So, the equation for the velocity reduces to v = √((2*(0-p_s)) / ρ_air) or v = √((2*(p_s)) / ρ_air) ?
This leads to a wrong velocity measurement, since the stagnation pressure can't be measured.
Am I on the right track with this issue, or am I in deep trouble?

Another concern I worked out related to this regards the laminar flow that comes into the tube. Does the laminar flow become turbulent after the point where the streamlines of the flow hits the inner surface of the pitot tube, and does this in turn affect the velocity measurement, since even here stagnation pressure can't be measured?

Thanks in advance for all your advice!
 
  • #3
Well looking at the the full equation and considering two points, p1+row(gh1)+(row*v1^2)/2= p2+row(gh2)+(row*v2^2)/2. You where using the equation assuming no height difference, but in the equation it says the closed end is higher than the open end. So I think you need to take into account the height difference. Also the stagnation pressure won't be zero, the stagnation velocity will be zero.
Note: I am not completely sure about this, but I think it makes more sense to consider height difference, and also the stag pressure won't be zero.
 

1. What is an Askew Pitot tube and how does it measure airplane velocity?

An Askew Pitot tube is a type of airspeed sensor used on airplanes to measure the velocity of the aircraft relative to the surrounding air. It consists of two tubes, one facing directly into the airflow and the other at a slight angle. The difference in pressure between the two tubes is used to calculate the airspeed of the aircraft.

2. How accurate is the Askew Pitot tube in measuring airplane velocity?

The accuracy of the Askew Pitot tube depends on several factors, including the design of the tube, the positioning on the aircraft, and the airspeed of the aircraft. Generally, it can provide accurate measurements within a few knots of the true airspeed.

3. What are the advantages of using an Askew Pitot tube over other methods of measuring airplane velocity?

One advantage of using an Askew Pitot tube is its simplicity and reliability. It does not require any electrical components and is not affected by electromagnetic interference. Additionally, it can provide accurate measurements at a wide range of airspeeds.

4. How does the Askew Pitot tube compensate for the effects of air temperature and density?

The Askew Pitot tube is designed to measure the difference in pressure between the two tubes, which is then converted to airspeed. This measurement takes into account the effects of air temperature and density on the airspeed of the aircraft.

5. Can the Askew Pitot tube be used in all types of aircraft?

While the Askew Pitot tube is commonly used in smaller aircraft, it may not be suitable for all types of aircraft. Factors such as airspeed range, positioning on the aircraft, and the need for high precision may make other methods of measuring airspeed more appropriate for certain aircraft.

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