Center of Drag?

[rehler]

It is supposed to be best to have the center of drag, the CG (which includes the rotor blades) and the prop thrust all in line with each other.

To determine the center of drag some people use the "two dimensional cut out" method, which is taking a photo (or drawing) of the gyro from the front view, then cutting out the profile of the gyro and pilot to create a flat area, then balancing the cut out on a horizontal rod to find the center of drag (the balance point of the cutout).

When this is done, do you count the entire rotor disk area tilted as seen from the front (elliptical shape)? If so, then the center of drag would be about a foot below the rotor head!

If you don't count the rotor blade area, then why don't you count it?

Or, do you count the area of the actual rotor blade (front view) rather than the "rotor disk" area. Perhaps in the left-right position or the fore-aft position or and average of both?

[Doug Riley]

Ken: A quickie answer: The center of drag (really center of pressure) determination is best done EXCLUDING the rotor. The idea is to have an airframe that's pitch-stable even when the rotor is edgewise to the air (i.e. when the rotor is applying very little aerodynamic force to the frame). Rotor "drag" is an element of rotor thrust and is best accounted for in a different way. Thinking of rotor "drag" as something pulling straight back at the teeter bolt will lead you into huge errors if you're not very careful.

The "paper cutout" method is quite a rough approximation -- since it's only a 2-dimensional silhouette, it doesn't account for the shapes of the elements of the frame. A streamlined wheel pant will have the same "drag" in this model as an oval flat plate of the same projected area, which obviously isn't correct. It's a start, though.

[Victor Duarte]

Hi Ken,
i wouldn't trust the 2d drag very much.
instead, i would decompose the elements in drag elements.
each one doesn't have the same drag.
you can have a big streamlined frontal area that drags less than a smaller but less aerodynamic area.
i would apply a coefficient of drag from 0.01 for streamlined to 1.0 to parachute like surfaces (pilot body for ex.) and try to evaluate roughly what is the drag of the other components (windshield, tank, wheels etc).
just my opinion, i'm not a specialist.

[quadrirotor]

With this rule of thumb, you could have it where the back lose its name!

[Al_Hammer]

Ken, good question.

The rotor drag changes with disk angle and that's why you don't know from a still photo, what it will be in flight. The rotor thrust will fall wherever it needs to, to balance the other forces and moments from prop thrust and airframe drag.
If all the other sources of drag combine to give a center of drag below the CG, then the Rotor thrust vector may need to move forward, depending on where the prop line of thrust is, so that the rotor is providing nose up moment to balance the nose down moment from drag.
It is convenient, from a vector standpoint, to think of rotor drag as being horizontal and ALWAYS passing through the CG. When you change disk angle, the amount of drag changes and the location of the LIFT vector changes,shifting forward or back of the CG and providing nose up or nose down moment.