The Bernoulli Effect

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…