Posts Tagged ‘UAV’

Karoliina has some thoughts on plane design, looking at High Altitude Long Endurance (HALE) UAV:s as inspiration for high L/D craft, to ultimately cruise at fast speed with little power. I disagree somewhat, and I’m sketching out why, below.
Probably the drones, like sailplanes, want low sinkrate and high L/D is secondary, because they need to just stay aloft and not go anywhere.
Power needed is P = v*D (we assume). Since D = CD*v*v, P = v^3*CD.
Lift is L = CL*v*v so v = (L/CL)^(1/2). (Note this CL is different from the L = 0.5*rho*A*cl*v^2, so CL = 0.5*rho*A*cl. It’s more practical here.)
Power thus is P = v^3 * CD = L^3/2 * CD * CL^-3/2.
The lift equals mass, so the power needed is
  1. proportional to mass^1.5,
  2. proportional to the drag coefficient and
  3. inversely proportional to the lift coefficient^1.5.
This means the lift coefficient CL needs to be large for the craft to be able to loiter for a long time. So long wings and somewhat cambered profiles. A little drag doesn’t hurt as much as low lift so struts are a possibility.
Instead, for a piston cruiser, the L/D needs to be maximized for a certain minimum trip fuel consumption, not per time. Basically, you want to minimize delta_E = P*delta_t = P * delta_x/v so the cost function J = v^2 * CD = L * CD / CL which minimizes at maximum L/D. The CL term is less important compared to the loiterer. As a first guess this should optimize to a less cambered airfoil and smaller or shorter wings. And no struts.

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Just a theory of the recently photographed “Beast of Kandahar”. I was browsing the NTRS (for another project), when I ran into this:

Lockheed Martin Two Hinge Morphing Wing wind tunnel test configuration at the Langley transonic tunnel, published in 2007

A hinge, actuators or a locking mechanism could explain the wing humps (if more conventional structures are still used). The high speed dash configuration could be used when a craft is needed for surveillance at some site where there is no UAV nearby. Or perhaps for combat missions, but I doubt that…

The pdf is here and is titled “Validation of the Lockheed Martin Morphing Concept with Wind Tunnel Testing”.

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