StarJar
Well-Known Member
Here's my question:
I've been studying a little about helicopter rotor airfoils, and learned that fairly symetrical ones are desired, because twisting is not handled well, by long, thin blades. Symetrical airfoils are preffered because they have no twisting force (Cm).
However, in a joined wing or joined blade, wouldn't this 'almost symetrical' requirement go out the window? Since I'm researching for a HPA (Human powered aircraft), the blades have different proportions than a 'real' helicopter blade.
For example, the rotors on the Gamera II have an aspect ratio of about 10. On a 'real' helicopter the aspect ratio is many times this. Yet the Gamerra II uses a Selig 8037 airfoil on the rotors, which has a very curved bottom, and just a little camber.(close to symetrical.)
The spar, (for the 40 ft. long rotors, with 4 ft. cords), supposedly weighs only 4 1/2 lbs. (cantelevered in the center) using balsa wood and carbon fiber. The airfoil is 16% thick . So combined with the relativley short aspect ratio of the blades, I'm wondering why they didn't use more camber.
Say they made the spar twice as heavy, to handle the twisting, (adding 18 lbs.) that is only 10% more weight to the gross weight. There are other airfoils, with twisting force, that have much more lift, with essentially the same drag. Also the thing about the pedal-power helicopters is that, the wing essentially sees the same angle of attack all the time, adding some simplicity .
My intrest is in a joined-tip configuration, (See attachment below, far right) I would like to get twice as much lift (at least 2d polar-wise), with the same drag. Is this something I should try for? Is it just a matter of handling the twisting moments? That's my question.
And a sub-topic; I wonder why they used essentially long arms, to hold the rotors, rather than big triangles emanating from a tall center strut. (See bottom attachments, left and center) That doesn't make the best sense to me, but maybe I'm missing something. (A perimiter of lightweight Spectra line and then jury struts on big triangles coming off a tall center tube, would be lighter simpler and cheaper, or am I missing something?)
I've been studying a little about helicopter rotor airfoils, and learned that fairly symetrical ones are desired, because twisting is not handled well, by long, thin blades. Symetrical airfoils are preffered because they have no twisting force (Cm).
However, in a joined wing or joined blade, wouldn't this 'almost symetrical' requirement go out the window? Since I'm researching for a HPA (Human powered aircraft), the blades have different proportions than a 'real' helicopter blade.
For example, the rotors on the Gamera II have an aspect ratio of about 10. On a 'real' helicopter the aspect ratio is many times this. Yet the Gamerra II uses a Selig 8037 airfoil on the rotors, which has a very curved bottom, and just a little camber.(close to symetrical.)
The spar, (for the 40 ft. long rotors, with 4 ft. cords), supposedly weighs only 4 1/2 lbs. (cantelevered in the center) using balsa wood and carbon fiber. The airfoil is 16% thick . So combined with the relativley short aspect ratio of the blades, I'm wondering why they didn't use more camber.
Say they made the spar twice as heavy, to handle the twisting, (adding 18 lbs.) that is only 10% more weight to the gross weight. There are other airfoils, with twisting force, that have much more lift, with essentially the same drag. Also the thing about the pedal-power helicopters is that, the wing essentially sees the same angle of attack all the time, adding some simplicity .
My intrest is in a joined-tip configuration, (See attachment below, far right) I would like to get twice as much lift (at least 2d polar-wise), with the same drag. Is this something I should try for? Is it just a matter of handling the twisting moments? That's my question.
And a sub-topic; I wonder why they used essentially long arms, to hold the rotors, rather than big triangles emanating from a tall center strut. (See bottom attachments, left and center) That doesn't make the best sense to me, but maybe I'm missing something. (A perimiter of lightweight Spectra line and then jury struts on big triangles coming off a tall center tube, would be lighter simpler and cheaper, or am I missing something?)
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