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The Bernoulli Effect

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The bernoulli priciple tells us that: ‘when a gas increases in velocity , its pressure decreaces. ……..

This is one of the classic ‘miss quotes’ of Bernoulli Theorem….

What Bernoulli in fact states is:

Total Pressure = Static Pressure + Dynamic Prressure, there is a specific requirment under Bernoulli that the value of Total Pressure MUST be a constant (for any given altitude)

What this means is that as the velocity of the flow past a surface increases (i.e. greater airspeed) the result is the Static Pressure acting on that surface will decrease.

It is important for PG pilots to understand that in the case of a PG wing it is the diffrence in Static pressure acting on inside/outside the wing membrane that gives our wings their ‘rigidity’, hence the concept of ADDING some brake in rough air to ‘pressurise’ the wing will in fact result in a REDUCTION in the diffrence in out/in Static pressure and so INCREASE the risks to the pilot as the wing in fact becomes less ‘rigid’ and of course any gust is now a higher % of the airspeed!

NB. Fly at near 20kts and a 5kt gust is 25% of your airspeed and the glider (tail gust situation) remains above 15kt ‘instant’ airspeed… while slow to 15kts and the gust is not 33% of your airspeed and a ‘tail gust’ drops the gliders ‘instant’ airspeed to below flying speed i.e. 10kts!

Note: there are significant safety implications inherent in the core Bernoulli theory errors that were introduced into PG ‘normal’ flight theory in the early days of the sport (due to the fact we fly ’soft’ wings!)

The safety implications can be seen in the fact that 95% of the PG wing theory I teach DIRECTLY (yes I did use Capts!) contridicts what is normaly taught…..

As a result over the last 10+ years the accident rate my students have seen (75% of my students are RE-training BHPA ‘pilots’ is less than 5% of the accidents rates of the ‘normal’ trained UK PG pilots…

So... what do you think? Please leave me a comment.

One Comment to The Bernoulli Effect

  • MurrayHay responded:
    A good (and intresting) example of Physics (Bernoulli’s Theorem) at work on the BOTTOM (AND of course the Top) surface of the wing can be seen on my ‘Demo Tip Touch Harness Control’ YouTube video http://www.youtube.com/watch?v=xaRhIwceWd4
    Here in 15+Kt conditions it’s fairly easy to see the ‘Lift’ acting OUTwards from the UNDER wing surface (due to Bernoulli) is almost ballanced by the ‘Lift’, again OUTwards from the TOP wing surface (Bernoulli) generated by the air flowing PAST the surfaces of the wing.
    It should be very clear that IF the Physics was not the correct, and instead the dangerous ‘normal’ PG theory was right (remembering just how many pilot who were taught/belived in ‘normal’ training have been injured/died…) then clearly when the wing is rolled to ‘Tip Touch’ the ‘normal training’ concept of "2/3rds lift due to low pressure at the top surface and 1/3rd lift due to the underside" taught/promoted by associations (with a VERY poor accident history, 80 deaths, thousands of injured pilots…) like the BHPA in the UK, then the ‘Lift’ from the 15+ knot wind would, when the wing tips are close to the ground, be ‘dragging’ me sideways across the ground… instead as can be easily observed, the top & bottom surfaces have a normal flying appearance and it takes almost no effort for me to stay in the camera viewpoint.
    NOTE: This sort of demonstration, with students standing in front of the wing, and looking at it the same way as the airflow meets the wing, lets them clearly observe that the further dangerous concept of ‘Ram Air’ and a ‘Pressurised’ wing are also false, as it’s simple to spot that with the cell opening on the UNDERsurface, and set BACK from the leading edge, on modern wings the ’stagnation point’ where the air first meets the Leading Edge of the wing on most modern wings does NOT in fact have any (cell) openings…
    Murray Hay, with over 7,000 hours safe PG aviation experiance!

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