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If a small scale UAV (wingspan <2 metres) was to be fitted with solar panels on its wings to have more power available, would the addition of these panels cause an increase in the drag coefficient, and consequently aircraft drag?

Though wind tunnel testing would be required to ascertain the exact change in drag, can one theoretically anticipate a change in drag on account of these solar panels? Furthermore, will the minimal variation in surface thickness on account of the panels result in the same?

The solar panel thickness is of the order of micrometres.

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    $\begingroup$ What kind of solar panels are you thinking about? Of course if they are not as smooth as the wing they will add drag. Putting them under a clear material that follows the airfoil shape will prevent that issue. $\endgroup$ – fooot Jun 24 '15 at 16:59
  • $\begingroup$ I am referring to mono crystalline silicon based solar cells, which are quite smooth. They would indeed follow the airfoil shape. I wish to know to what extent would the drag increase compared to the drag values obtained from simulation softwares such as XFLR5? $\endgroup$ – Pranav Jun 24 '15 at 17:08
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    $\begingroup$ By adding drag would you also mean the induced drag that would come along with having heavier wings? Assuming, of course, that it would make the wings heavier... $\endgroup$ – Jay Carr Jun 24 '15 at 17:09
  • $\begingroup$ I was primarily referring to the parasite drag. I have not considered the possibility of induced drag, as the weight addition would be minimal. Moreover, wouldn't the coefficient of lift reduce which will in turn reduce the induced drag? $\endgroup$ – Pranav Jun 24 '15 at 17:12
  • $\begingroup$ It would depend on the airfoil design. But if the airfoil remains unchanged and weight is added, the AoA will have to increase to deal with it. And that will increase induced drag so... But if the weight difference is minimal, I really wouldn't worry about it. $\endgroup$ – Jay Carr Jun 24 '15 at 17:21
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No, assuming the wing geometry and the coefficient of friction of the material on the surface of the wing do not change. So, swapping a section of aluminum or carbon fiber skin on the wing for a section of Plexiglas or silicate glass under which you place the solar cells will, all other things being equal, have a negligible effect on drag.

However, doing something like this, all other things are not necessarily equal. First off, the border between the normal skin material and the solar cell cannot be a perfectly smooth transition; at some level of magnification, there will be a noticeable seam which will reflect as an increase in the surface "roughness" of the with with solar cells compared to the one without. While nonzero, it's likely negligible if the parts of the wing are well-fitted.

Second, there will likely be a change in mass when removing a thin layer of aluminum, carbon fiber or polymer sheathing, and replacing that with Plexiglas with solar cells underneath (plus the power wiring, converters and other additional components that otherwise wouldn't need to exist). Classically, that has no effect on drag, but it will change momentum calculations which affect how much the drag matters, and also as was stated, heavier masses require higher angles of attack to increase lift and counter weight, which increases induced drag as the effective shape of the airfoil slicing through the air changes to present a larger cross-section. Wing stress moments will also be affected as the weight distribution of the plane between fuselage and wings has changed, and this can change the amount of effective dihedral due to wing loading under various conditions.

Lastly, there will be a nonzero difference in surface roughness between the majority of the skin or sheathing and the solar cells. For instance, a matte finish as some aircraft have is rough by definition to provide different reflective planes to scatter light. You want a highly polished surface on the solar cells to minimize energy loss through the outer layer. Even if most of the aircraft is finely polished, there's a difference in how fine a polish various materials can take; the harder the surface, the easier it is to take a fine polish.

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