Why is a C150 airspeed static port located in the propeller slip-stream?

Question

Wouldn't the prop's slip-stream raise the airspeed and giv false reading. I know it doesn't but then why is it?

Answer 1

Static ports wont detect the the ram air, it will only detect the ambient pressure.

It is the Pitot tube which detects ram air pressure and displays it in Airspeed indicator.

Be concerned only when the Pitot tube is in the Props slipstream, as this will surely display false reading in ASI.

Hope this helps.

Answer 2

Anywhere on a single engine airplane's fuselage is in the propeller's slipstream. Where else you gonna put it?

Answer 3

Because the propwash doesn't change the air pressure which is all a static port does, measures air pressure. And it's not an airspeed static port. Airspeed is sensed by the pitot tube. It does use static pressure for comparison but the static port itself does not measure airspeed.

Answer 4

As long as you fly with the ball in the middle, I don't think it really matters where the static port is located. Increased air speed doesn't really mean increased pressure, right?

But if you like side-slips like I do with my Kitfox (I don't have flaps) be aware of the change of pressure from one to the other side of the fuselage. To counter this, I have a static port on each side, just foreward of the elevators.

Answer 5

Static ports in the slipstream WILL affect airspeed readouts.  When velocity is increased, pressure is decreased.  Slipstreaming air has a higher velocity, and therefore a lower pressure.  That's Bernoulli's Principle.  Remember that airspeed is detected by ram air being ducted into expandable diaphragms inside a case that is filled with static air.  If the static air pressure decreases, those diaphragms expand.  The diaphragms expand because the pressure surrounding them is less.  This, through a series of linkages, shows an INCREASE in indicated airspeed.

I'm not very familiar with 150s, but I would suspect that the actual static port hole is not directly in the slipstream.  Sometimes they mount the hole on the backside of a slight protrusion (looks like a penny on the side of an airplane).  That way, the slipstream won't really affect it.

Also, remember CAS is corrected for instrument position error.  If there was a significant effect in airspeed caused by the location of the static port, this would be corrected for by a conversion from IAS to CAS.

Answer 6

I thought others would know better, but yes, it does effect it, but the VSI and airspeed indicator are calibrated to deal with it.

Answer 7

It does affect it,   I agree with the person who said that. the air going by it causes a suction due to the bournelli principal. There is nowhere on the aircraft within a reasonable distance where there is no prop wash or airflow traveling by the hole. The rest of the system is calibrated to offset this.

Answer 8

As someone who teaches a course on pitot-static system testing and certification I can say that the effect of the propeller is minimal. The size of the static port on a Cessna 150 is approximately 1 millimeter. Since it is so small, so is the effect of the propeller on the static system, so small an effect that it does not register on the instruments.

To rebut a previous answerer, no the instruments on a plane this small are not calibrated to to compensate for this effect. Between the low speeds and low altitudes these planes fly, there is no need for what is called "static defect correction" which compensates for errors induced by the airframe, and this is done for business jets and larger aircraft.

Answer 9

The key is the name of the port:STATIC. The static port is located in a dead spot on all aircraft. There is no suction or pressure at that point. How it was determined on the 150, I don't know, but there are devices that can be placed on the fuselage in a wind tunnel to measure the effects of passing air to find the dead spots.