I can explain that.
A prop in hover (or climb) leaves a spiral vortex wake starting at the prop tip and spiraling down from it (you can find images where that is visible). This vortex wake structure induces a downward speed on the prop plane and it is that speed that determines the power required by the prop - it is the product of the thrust generated and the speed through the prop disc. So, if we could somehow reduce the speed through the prop disc we would need less power to achieve the same thrust.
When the copter is moving forward the oncoming wind has a component parallel to the prop disc and a component perpendicular to it. The perpendicular part adds to the induced velocity and so increases the power requirement. The parallel part is washing the wake away, so that the spiral wake is canted. That reduces the velocity induced by the wake (it is governed by the Biot-Savart law) and so the power required for a given thrust. At a moderate pitch angle, the parallel component is much bigger, so overall the power requirement is reduced. That is the "unloading" effect - the prop presents a lesser power load.
At high pitch angles (and in vertical climb) the perpendicular wind component is much bigger so the power requirement is increased - causing an increase in current draw.
To sum up, what we have here is a phenomenon where at moderate forward speed (moderate pitch angle) the copter needs less power than at hover to maintain a thrust that keeps it level in the air. As a result efficiency is improved and flight time is extended compared to hover (static) conditions.
BTW for that reason fully loaded helicopters need to take off at a "run" like a fixed wing aircraft to "boost" the available lift.
So, just to make sure my non-aerodynamicist brain is grokking this... A prop that exhibits "high unloading" is one where it uses much less power once it gets angled into forward flight, than it does in a static hover, correct? That would be a good thing, especially in racing contexts where forward flight dominates the situation.
ZzyzxFPV:
Apr 12, 2018 at 01:06 PM
I know this is an older article, so not sure if you're following comments...
Could you offer up a short explanation of what exactly "unloading" of a prop means? And how that relates in practice in flight?
Thanks!
GV27:
Apr 12, 2018 at 02:30 PM
I can explain that.
A prop in hover (or climb) leaves a spiral vortex wake starting at the prop tip and spiraling down from it (you can find images where that is visible). This vortex wake structure induces a downward speed on the prop plane and it is that speed that determines the power required by the prop - it is the product of the thrust generated and the speed through the prop disc. So, if we could somehow reduce the speed through the prop disc we would need less power to achieve the same thrust.
When the copter is moving forward the oncoming wind has a component parallel to the prop disc and a component perpendicular to it. The perpendicular part adds to the induced velocity and so increases the power requirement. The parallel part is washing the wake away, so that the spiral wake is canted. That reduces the velocity induced by the wake (it is governed by the Biot-Savart law) and so the power required for a given thrust. At a moderate pitch angle, the parallel component is much bigger, so overall the power requirement is reduced. That is the "unloading" effect - the prop presents a lesser power load.
At high pitch angles (and in vertical climb) the perpendicular wind component is much bigger so the power requirement is increased - causing an increase in current draw.
To sum up, what we have here is a phenomenon where at moderate forward speed (moderate pitch angle) the copter needs less power than at hover to maintain a thrust that keeps it level in the air. As a result efficiency is improved and flight time is extended compared to hover (static) conditions.
BTW for that reason fully loaded helicopters need to take off at a "run" like a fixed wing aircraft to "boost" the available lift.
ZzyzxFPV:
Apr 12, 2018 at 08:18 PM
Ah, thanks!
So, just to make sure my non-aerodynamicist brain is grokking this... A prop that exhibits "high unloading" is one where it uses much less power once it gets angled into forward flight, than it does in a static hover, correct? That would be a good thing, especially in racing contexts where forward flight dominates the situation.
Thanks!