Originally Posted by
Pace
Surely when a large aircraft manufacturer works out premeditated and forecast performance they use more than just mathematical equations?
I am far from an expert on this so on my part its guesswork but they have access to wind tunnels where they can test and modify structures which make up the aircraft?
Generally it would be done "in silicon" first of all - computer design tools which combine aerodynamic and structural estimation.
The results would then be checked by hand calculation by reference to historical data sheets - such as ESDU.
Then sample critical aircraft structure would be built and tested to destruction, and representative models would be put in the wind tunnel, matched as far as possible to Reynolds number, then scaled to the complete aircraft.
Then it's flight tested, to validate the aerodynamic estimates.
Take winglets which may be used on a progressive design?
they will be designed, tested and modified to create an effect?
They will be tested alone in the wind tunnel with sensitive instruments measuring everything from pressure to airflow and then tested with the wing?
Yes.
The main instrumentation will be forces and moments - but it's common to pressure tap sections of lifting surface to generate pressure distributions. When I worked in the tunnel on what is now Typhoon, we also had moveable control surfaces, and re-tested at lots of control settings, AoA values, and even some Beta variations.
Flow visualisation isn't used that much, but has value in identifying interference drag issues, and boundary layer thicknesses for instrument positioning particularly.
That data will then be used to workout performance gains.
I am sure its not a case of sitting down with a blank sheet and saying x+y-z=127.7 KTS
The equations are vitally important, and any performance engineer lives and breathes them. But they require historical and wind tunnel data as part of the inputs to make them really trustworthy. You can't work from theory alone - but also you can't go from wind tunnel results to a full scale aeroplane without a pretty solid set of flight mechanics formulae to work with.
And even then, they're not as trustworthy as flight test data - and there will be differences.
G