# Tandem-wing LSA/microlight concept and poll

## Which tandem-wing configuration would interest you the most (pick one in each of four categories).

• ### D2 - Pusher engine (engine and propeller at rear);

• Total voters
49

#### cluttonfred

##### Well-Known Member
HBA Supporter
I was just thinking that if supporting a cantilever beam at two points then logically the 1/4 and 3/4 points would minimize the stress on the structure, like so:

>>>|<<<>>>|<<<

Other factors like the maximum folded width could force you to change the spacing to thirds, for example, but you'd gain some weight to keep the same strength:

>>>>|<<>>|<<<<

That's my reasoning in any case. And it was 1/4 and 3/4 (or 1/3 and 2/3) *span* not *chord*. The chordwise position would be determined by the airfoil.

#### rtfm

##### Well-Known Member
I meant span also. My bad. A hand-shaped piece of Hoop Pine? A 100mm chord of Hoop Pine shaped to a NACA0018 airfoil shape can hold way, way more than the full weight of plane at 6G (and of course there are two of them), and a 1350mm length would weigh about 3.3kg. A bit labour-intensive to shape, but do-able. And very easy to epoxy to whatever I come up with to anchor the mast.
Mmmm...

#### rtfm

I think a NACA0018 is probably overkill, so I'll go with the 0015. I also found Carveco which seems very powerful and at only $15/month it's a bargain. Just working through the application to find out how to shape the airfoil. #### Sockmonkey ##### Well-Known Member Anybody got any info on how the Spratt controlwing avoided tip stall when turning at high AOA? #### rotax618 ##### Well-Known Member I found the best CNC software to cut 3D, double sided objects is Estlcam, its about$60-$70US. Creates 2 gcode files, one for each side of an STL, you cut one side and flip it over like a book and cut the other. #### cluttonfred ##### Well-Known Member HBA Supporter Sorry, I don't, the details of the Spratt system have always been a bit murky to me. Anybody got any info on how the Spratt controlwing avoided tip stall when turning at high AOA? #### Sockmonkey ##### Well-Known Member Sorry, I don't, the details of the Spratt system have always been a bit murky to me. What little I can find implies that a stalled tip simply drops down until the plane is rolled back to level flight and produces normal lift again. I don't know if that would be the case for a wing using fore-wing ailerons though. I suspect it would. Anyhow, there's a reason I want to know this. #### erkki67 ##### Well-Known Member I always liked the Spratt103 concept, but the designer is questionabl, so I don’t want to deal with him. There is another Spratt like concept from south-America the UL-X. as a tractor engined version I could like it. #### Martin R. ##### Active Member Anybody got any info on how the Spratt controlwing avoided tip stall when turning at high AOA? A real Spratt wing (also called free wing) is absolutely independend from other forces than the laminar air-flow. Like a weather vane! This requires that the fuselage hangs (a little bit in front of the center of pressure) like a pendulum on the wing. That’s why you can’t “force” the airfoil into a stall provoking AoA. Example: If you want to increase the AoA of the wing, you must move his control tab into the negative sense. NASA Report CR-3135 / pdf-page 11, 12 .... * Changing upwards or downwards an elevator which is fixed on the fuselage, just changes the pitching motion of the fuselage and not the AoA of the wing. (i.e. for a better sight or a better position of the landing gear etc.) Seen this way, neither a flying-flea nor a tandem-wing aircraft that we discuss in this thread, are real Spratt aircrafts. Because in our examples we fix mechanically the angle of incidence (AoI) between the wing and the fuselage (here i.e. on a flying flea): That’s why mixing the Spratt-system logic with tandem and/or flying flea system logic is very problematic and leads to false interpretations. * https://ntrs.nasa.gov/api/citations/19790013869/downloads/19790013869.pdf Last edited: #### cluttonfred ##### Well-Known Member HBA Supporter Interesting post Martin R. and good points, I was not aware that the Spratt was a freewing. One point of clarification is that a Pou-de-Ciel can function much like a freewing if the pilot simply holds the stick gently and allows it to buck a little to alleviate gusts and turbulence. I have often thought that a true freewing Pou-du-Ciel with the stick connected only to a pitch control tab by a push-pull cable and the wing allowed to take its natural incidence could be an interesting experiment. #### Sockmonkey ##### Well-Known Member A real Spratt wing (also called free wing) is absolutely independend from other forces than the laminar air-flow. Like a weather vane! This requires that the fuselage hangs (a little bit in front of the center of pressure) like a pendulum on the wing. That’s why you can’t “force” the airfoil into a stall provoking AoA. Ahh, I always thought the Spratt was just a split Pou-type wing. It answers my question, but doesn't solve my problem. #### BJC ##### Well-Known Member HBA Supporter I have often thought that a true freewing Pou-du-Ciel with the stick connected only to a pitch control tab by a push-pull cable and the wing allowed to take its natural incidence could be an interesting experiment. One axis control, plus a suggestion in pitch? BJC #### cluttonfred ##### Well-Known Member HBA Supporter There might be a little lag but I think you’d still have pitch control. That might be a good option for a fore-and-aft-only stick and rudder pedal like a Sky Pup. The stick becomes the speed lever, the throttle climb/descend, and the pedals for turns, just like your instructor told you to think about it (or at least mine did). #### Sockmonkey ##### Well-Known Member I imagine that the control tab arrangement might let you get away with both fore and aft wings on a tandem being freewings. So, on another tack, if you used ailerons on the fore wing of a flea-type for roll control, how do you prevent tip stall in a high AOA turn? #### Martin R. ##### Active Member I have often thought that a true freewing Pou-du-Ciel with the stick connected only to a pitch control tab by a push-pull cable and the wing allowed to take its natural incidence could be an interesting experiment. Why do you use your push-pull cable just for the wing-incidence? If you mount the control tabs separately on each wing, you could pretty simple produce an elevon-effect and we would have an interesting full 3-axis aircraft. Last edited: #### cluttonfred ##### Well-Known Member HBA Supporter Well, I was thinking of a single front wing, not two separate halves, and besides, a three-axis aircraft is not a Pou-du-Ciel! Here we go again…. ;-) #### rtfm ##### Well-Known Member I found the best CNC software to cut 3D, double sided objects is Estlcam, its about$60-\$70US. Creates 2 gcode files, one for each side of an STL, you cut one side and flip it over like a book and cut the other.
Hi. I have the full paid version of Estlcam, but I can't generate a usable stl file. Sketchup seems to produce garbage, and I don't have any other way of producing a stl other than extruding the airfoil and exporting as a stl.

#### rotax618

##### Well-Known Member
If you only want to cut 2 1/2 D objects, Estlcam will produce a path from .DXF files. I would personally use the path workbench in Freecad. There are plenty of Youtube videos showing how to import .DAT airfoil files, convert them to a sketch, draw your spar cutouts, lightening holes etc. and generate Gcode for almost any post processor.
anything drawn in Freecad can also be exported as .STL .OBJ .IGES etc, it is worthwhile learning and its FREE.

#### rotax618

##### Well-Known Member
Sketchup is useful for sketches, it is not a CAD program.
I drew this Flapjack model in Freecad and cut it out from foam using Estlcam.

#### rtfm

##### Well-Known Member
If you only want to cut 2 1/2 D objects, Estlcam will produce a path from .DXF files... and its FREE.
I would personally use the path workbench in Freecad.... and its FREE.
I'll definitely take a look. Thanks