Difference between wind tunnel and real flight

[randomy]

Please help:

We know that static pressure is the pressure felt when moving with the fluid (air). So in real flight the Pitot-static tube is moving with the aircraft and gives static pressure X (lets call it X). BUT in a wind tunnel the Pitot static tube is fixed (i.e. not moving) and the static pressure is Y(lets call it Y).

My question is X=Y? (assuming that air density, temperature etc. are all the same).

I don't know if there is a simple answer to this, if there is then please explain!

Thanks in advance.

 

[randomy]

What I am basically asking is that in real flight the freestream static pressure would NOT change with airspeed, it would only change with altitude, which is how the altimeter works. BUT in the wind tunnel would the freestream static pressure vary with airspeed coming in?

 

[grant9076]

Please help:

We know that static pressure is the pressure felt when moving with the fluid (air). So in real flight the Pitot-static tube is moving with the aircraft and gives static pressure X (lets call it X). BUT in a wind tunnel the Pitot static tube is fixed (i.e. not moving) and the static pressure is Y(lets call it Y).

My question is X=Y? (assuming that air density, temperature etc. are all the same).

I don't know if there is a simple answer to this, if there is then please explain!

Thanks in advance.

Actually, the pitot tube measures total pressure. It is the static port which measures static pressure.

One difference between the wind tunnel and real flight is that the model aircraft is usually smaller than the real aircraft and is usually subjected to a lower Reynold's Number.

 

[randomy]

Yes I understand. But you see in the aircraft the pitot-static tube is far ahead therefore the ''atmospheric pressure is measured'' via the static port, which theoritically would only change with altitude not aircraft speed.

In the wind tunnel, if we place a Pitot-static tube far ahead of the body (i.e. freestream) would the static pressure measured change with the airspeed (i.e. as we increase the fan speed)?

 

[Anomander]

Good question randomy ...

The answer is quite simple .

In the Pitot-static tube , the hole in the tube facing the jetstream gives Stagnation Pressure ( or in other words the Total Pressure/ram pressure/Impact pressure) reading while the the holes ( or ports ) on the fuselage of the tube will give the static pressure reading .

So , now we get the stagnation pressure and the static pressure from the Pitot-Static tube.

In the aircraft and in the wind tunnel , we obtain the airspeed by using the bernoulli equation to calculate the dynamic pressure ( 0.5 * density * velocity^2 ). This in turn gives us the airspeed( Indicated airspeed in case of aircraft).

For altitude measurement, in aircraft we use a aneroid barometer using the static pressure reading from the static port . And well in wind tunnel you woudn't need that lol .(:P)So you see , the static pressure doesn't change, the total pressure changes and to find that change we measure the static pressure .

 

[randomy]

Thanks Anomander.

Yes agree. Its just that I performed some wind tunnel tests and the static port measurement varied with fan speed. I concluded that there could be 2 reasons for this:

1). Misalignement in the static port (i.e. not exactly in parallel with the air flow)
2). (which is my question) is that because you increase the fan speed, you increase the work/energy done (i.e. more power) and so the wind tunnel (static) pressure would change as well?

 

[caslav.ilic]

BUT in the wind tunnel would the freestream static pressure vary with airspeed coming in?

Depends on the type of wind tunnel.

If it's an open test-section tunnel, then, well, the test section is at atmospheric conditions, and static pressure should always be that of room conditions.

If the tunnel is with closed test section, then the static pressure will in general vary with airspeed in the test section.

In open-exhaust fan-driven tunnel the static pressure in the test section must equal all losses from the test section to exhaust port (i.e. that's what's driving the flow out), and these losses increase with airspeed in the test section. Similarly for a closed-circuit fan-driven tunnel, there the losses to overcome are from test section back to fan. I wouldn't know about magnitude of expected pressure deltas in these cases though, and wouldn't be entirely surprised if they are negligible for low airspeeds...

If, on the other hand, the tunnel is reservoir-driven, there, even barring any losses, and in ideal case of huge reservoir, it is the total pressure which is constant, i.e. equal to static pressure of the reservoir. So in such tunnels, major variations to static pressure may be expected with airspeed in the test section.

--
Chusslove Illich (Часлав Илић)

 

[Anomander]

Well there you go randomy, Caslav nailed your question head on ... great answer ... ! :)