This is a really neat project. I noted in the discussion that by rotating with end tip motive power you eliminate the need for stiffening the wing, however doesn't this increase the need for better tensile strength along the wing? (in order to prevent it from pulling itself apart, something "regular" wings don't get a lot of stress on).
Also, in typical rotor craft the lift is highest on the outer edge of the rotor and least in the center where the airfoil speed is slowest. Does that affect where you put the payload? Is it on the edges of the wing or still in the center? At the center, if the wing is supported by the centripetal force of the wingtip motor's angular momentum, there is a huge torque in the middle if you pull it down. (much like pulling down on a suspended cable). How much deflection before you have the same problem as the stiffened wings of current efforts?
Increasing tensile strength is super easy compared to improving bending and twist resistance, especially when the aerodynamics not only constrains the thickness of the wing but also rewards high aspect ratios (the tip-to-tip length of a wing compared to its front/back dimension).
Imagine a wing that's 20+ times longer than it is deep, and is only 5% as thick is it is deep (so, for example, a 20m wide wing, that's 1m deep front-to-back, and 5cm thick top-to-bottom) - that's really hard to get stiff in bending along it's long axis, and in twisting stiffness around that long axis. This is why - as the article mentions, a sailplanes long thin wings account for 40+% of the airframe weight, when a stubby-winged but less aerodynamically efficient wing (like, say a Cessna 172) the wing might only account for 10-20%$ of the total airframe weight.
This TURN design minimises bending/twisting forces by replacing the load bearing and alignment forces with mostly end-to-end tension ones - which are much easier to resist (just load the structure up with "axial" carbon fibre...).
Comments
This is a really neat project. I noted in the discussion that by rotating with end tip motive power you eliminate the need for stiffening the wing, however doesn't this increase the need for better tensile strength along the wing? (in order to prevent it from pulling itself apart, something "regular" wings don't get a lot of stress on).
Also, in typical rotor craft the lift is highest on the outer edge of the rotor and least in the center where the airfoil speed is slowest. Does that affect where you put the payload? Is it on the edges of the wing or still in the center? At the center, if the wing is supported by the centripetal force of the wingtip motor's angular momentum, there is a huge torque in the middle if you pull it down. (much like pulling down on a suspended cable). How much deflection before you have the same problem as the stiffened wings of current efforts?
Increasing tensile strength is super easy compared to improving bending and twist resistance, especially when the aerodynamics not only constrains the thickness of the wing but also rewards high aspect ratios (the tip-to-tip length of a wing compared to its front/back dimension).
Imagine a wing that's 20+ times longer than it is deep, and is only 5% as thick is it is deep (so, for example, a 20m wide wing, that's 1m deep front-to-back, and 5cm thick top-to-bottom) - that's really hard to get stiff in bending along it's long axis, and in twisting stiffness around that long axis. This is why - as the article mentions, a sailplanes long thin wings account for 40+% of the airframe weight, when a stubby-winged but less aerodynamically efficient wing (like, say a Cessna 172) the wing might only account for 10-20%$ of the total airframe weight.
This TURN design minimises bending/twisting forces by replacing the load bearing and alignment forces with mostly end-to-end tension ones - which are much easier to resist (just load the structure up with "axial" carbon fibre...).