Pressure bubble
Nick
What is the most common FORMAL definition of induced drag ?
Why am I asking this apparent simpel question?
When calculating the detailed dynamics for my simulator, I needed a formal definition to aggregate the precise data so as to obtain a precise global figure (=integrated over one revolution) for induced drag.
In more detail:
First consider Plank, linear aerodynamics:
Given a certain flow we have ONE force on the airfoil, this force is "arbitrarily" decomposed to drag and lift with respect to some "conventional" reference frame. This is the frame that is conventionally taken with reference to some points of the airfoil profile. Rotating this reference frame gives different meanings to drag and lift (without changing the resulting total force). This of course will also change the definition of induced drag which is the drag caused by the lift because it is projected in a different (rotated) coordinate system.
Putting a wing at a rotated reference with respect to the plane may alter this reference and thus the precise NUMBER of induce drag.
Having the plane fly at a different pitch angle may again change the reference system and thus the notations of lift, drag and induced drag.
Puntosaurus used for instance "horizontal", can he give a precise definition ?
Now go to the heli
We can take the hub plane (that is perpendicular to the physical rotor shaft (this is what I took, because its the most natural one to calculate moments and torques around the rotor axis)
or the effective rotor plane or disk plane (once it starts flapping), which looks more natural when analysing all flow components around a particular blade element, or the direction the body goes (plane of motion)
So even the "induced drag" line of reasoning may not be as simple, question of references.
Back to the pressure bubble.
Again Nick I fully endourse your arguments (conditional to theabove caveat), but reiterate that this is not the only possible "angle". We can talk pressure, angles, molecules... all lead to the same: less power needed. I guess it is up to your personal preference.
May be the analogy to electricity does not appeal the the rotary world, but take for instance a car: why does it accelerate
- because the driver hit the pedel
- because the engine gives more power
- because carb pressure increase
- because sheer force between ground and tyres increases
I think all are correct...
d3