Why does static press < when dyanmic press >?

[Halfbaked_Boy]

Ok, for me, this one's a bit like gravity. I know it happens, but I don't understand why.

The question is, why does a reduction in dynamic pressure result in an increase in static pressure, and vice versa?

We've all seen the tautliners on the motorway with their curtains sucked inwards (ooooh err!), this being due a partial vacuum forming inside the trailer because the air is being drawn to the outside, because that's where there's high dynamic pressure, i.e. relative wind, i.e. lower static pressure.

Same principle explains why, in general, the 'cabin altitude' of an unpressurised aircraft is usually slightly higher than its true altitude. Due to relative motion, the air is moving much faster (relatively) outside, giving a lower static pressure, therefore some air from the cockpit/cabin will be drawn outside.

I'm understanding the 'why', just not the 'how'!

Any helpers?

[The500man]

Static pressure - Air molecules collide freely with a surface and each other.

Dynamic pressure - Air molecules are moved across the surface at speed and are therefore unable to strike the surface as much as they used to.

Total pressure - The same in both cases because we regard the air as an incompressible fluid (which is not true but is unimportant at low speeds).

Hope that helps.

[Halfbaked_Boy]

Thanks for the reply :)

So, very simply put, it is because there is a very thin area outside where there are relatively few air molecules striking the surface perpendicular to them (i.e. the aircraft) because they are too busy whizzing by.

This brings me to my next question... If the pressure inside is slightly less than the outside of the aircraft (this is something, myth or otherwise, that has been 'confirmed' to me by numerous people), then why is it much harder to pull the storm window (i.e. on a PA28) to the open position, INTO the lower pressure interior when in flight/at higher speeds?

I'm probably getting many wires crossed here, but it's one of those little mindbenders in aviation that I've decided to sort out once and for all!

[BackPacker]

I don't think I understand your question completely, so I don't think I can answer it fully. However:

We've all seen the tautliners on the motorway with their curtains sucked inwards (ooooh err!), this being due a partial vacuum forming inside the trailer because the air is being drawn to the outside, because that's where there's high dynamic pressure, i.e. relative wind, i.e. lower static pressure.

I don't think this is a good example. Due to the boxy nature of these lorries, the airflow around them is far from laminar. Particularly close to any sharp corners. This turbulent air creates localized areas of high and low pressure, leading to the bulging outwards and inwards of the fabric.

Same principle explains why, in general, the 'cabin altitude' of an unpressurised aircraft is usually slightly higher than its true altitude. Due to relative motion, the air is moving much faster (relatively) outside, giving a lower static pressure, therefore some air from the cockpit/cabin will be drawn outside.

I'm not sure whether this is generally true. However, I'm equally not sure whether there are any certification requirements with regards to cabin altitude vs. formal "static" pressure altitude in unpressurized aircraft. I guess the pressure inside would depend most on where the gaps in the hull happen to be, in combination with the exact location and setting of the fresh/warm air inlets. If the inlets are mostly at the front, and there are very few outlets at the rear, pressure inside will be higher than ambient, and vice versa.

The only thing I have seen in POHs, in this respect, is a correction factor if you need to use "alternate static" for some reason, and alternate static happens to vent to the cabin. In that case there's a specific configuration you need to use (DV window closed, all air vents open, for instance) and a conversion factor to get to the real (pressure) altitude.

On the other hand, I know that the location of static ports is very critical. Their location needs to be determined through computer modeling, or through a lot of trial and error, to make sure that the static pressure as measured in the pitot/static system, is as close to reality as possible.

In most cases this actually requires multiple static ports, to equalize out hull effects in all flight regimes. And even then things like sideslipping will upset the static pressure inside the pitot/static system to some extent. (Though still less than dynamic pressure.)

Note that all this applies to typical spamcan speeds. Once you get close to Mach 1, there are all sorts of transonic effects. I'm happy to say I know nothing about those.

[The500man]

Well let's pretend that the pressure inside and outside your PA28 is the same total pressure to keep things simple. The outside of the window is experiencing low static pressure due to the airflow across it (higher dynamic pressure), whereas inside there is only static pressure which is pushing the window outwards. So it is therefore harder to pull it inwards.

[24Carrot]

I would agree with Backpacker on this one - it is complicated.

Your non-airtight cabin will have airways to the outside.

If they are predominantly forward-facing, (like the front of a pitot tube), you would expect the cabin pressure to be above atmospheric.

If they are predominantly sideways-facing, (like a static port), you would expect atmospheric pressure.

If they are predominantly backwards-facing, you would expect sub-atmospheric pressure.

In reality the flows will be rather complicated, so forwards and backwards will be hard to define, but already we can see there won't be any simple rule.

[foxmoth]

It is actually incorrect that the pressure inside the cabin is lower than that outside, it is lower than the free stream static pressure, but not that immediately outside the cabin.Another way to think about it is that the air moving round the airframe will have different pressures in different places - the reason you have a lower pressure inside the aircraft is that the air moving round the cabin itself accelerates causing a lower pressure just outside the cabin, basically sucking the air out and so reducing the pressure below the free stream static pressure, so trying to open the DV window is doing it against that "suck". This is also why, as mentioned before, the designer has to be careful of the sitting of the static vents, otherwise it will be measuring the pressure of the air affected by the fuselage and not the free stream static.

[Heston]

Bernouilli. Conservation of energy.

H

[Big_Buddha36]

I think you need to put the Venturi effect into play to fully understand what's going on.

As for your pa storm window thing. It's the internal static that's less - it's being drawn out by Venturi

The air pressure outside the window is lower (Venturi) but the static internally is "artificially" dropped because of this.

I think that's right

Bb

[Torque Tonight]

In an incompressible laminar flow field, each streamline has a constant total pressure. As air flows along a streamline, if the dynamic pressure changes, the static pressure will change in the opposite sense to maintain a constant total. This is related to conservation of total energy. Therefore flow velocity and the pressure exerted on the airframe are interdependent, hence how the aerofoil shape affects flow velocity on the upper and lower surfaces generating a lift force.

[peterh337]

Which way the cockpit pressure goes is not obvious and is type dependent. I think even details like the vent arrangement will affect it.

For example in mine (TB20) the airflow is back to front, and the cockpit pressure is lower than outside, by (e.g.) 100ft at 9000ft/140kt IAS.