In the case of the DJI S1000 ... each frame arm is designed with an 8° introversive and a 3° inclination, making the aircraft more stable when rolling and pitching and more flexible when rotating. But the Cinestar Multirotors are designed flat..... Just wondering what others think about the Cinestar stability when rolling and pitching?
Good question, Graydon. I've not noticed what I would call a stability problem with the Cinestar, though. The moment you angle a motor away from vertical, you lose some lifting power as the thrust line is no longer vertical so I guess it's a trade-off between some apparent increase in stability and some loss of lift capacity. I suppose the best folks to ask would be those that fly both a Cinestar and the the S1000. Not sure anyone on the forum flies both, but let's see if anyone posts. Andy.
Seems most already agree on adjusting the motor inclination angles, a concept easy to implement with tubular motor arms. As far as the introversive angle concept..... While several Chinese MR manufacturers seem to be following the DJI introversive design concept, I am not sure if they realize why they are doing it from a laws of physics perspective...... but others such as Tarot remain with the flat airframe concept.
I’m not sure I’d characterize it that way. Most of us here on the FF forum keep things vertical. Like Andy, I’ve not seen anything in the way of engineering that shows precisely what is gained by offsetting things, especially when one is using a good flight control system.
I’ve been thinking about this. I try to look at problems and solutions as integrally linked. As in, no solution should be considered until you’re sure you know what problem’s being solved. So I realized there is an advantage to the tilting of motors toward the center of a multirotor. The issue at hand (the “problem”) could be defined by the nature of how a multirotor copter moves horizontally. We all assume that our multirotor copters must physically bank in order to move forward, backwards, etc., because of the physics of how these systems work. But if the motors all tilt inwards towards the CoG of the copter, applying additional power to a motor should move the copter in the opposite direction as the tilt. So, consider a simple case of a ‘+’ configured copter (with 4 motors at the cardinal compass points; motor 1 at the front of the copter), let’s assume each of the motors is tilted at some inclination towards the center of the copter. If you apply additional power to the motor on boom 1 at the front of the copter, the thrust will force the copter to move backward without necessarily banking the copter to the rear. I’m sure there is a point at which the copter still has to bank to achieve certain horizontal velocities, but in the simplest case, this could result in less banking movements during flight, which in turn could result in a more stable camera platform. Mikrokopter sells a small wedge that you can use to do the exact same thing with another axis of the motors (side-to-side) to improve yaw performance, but in my experience, yaw performance is not a pain point in aerial photography applications, especially with our modern 3-axis gimbals using two operators. Nevertheless, as Andy pointed out above, there is a point of diminishing return where you lose some lifting power. This makes me wonder whether the ideal multirotor platform might have active, articulating motor mounts (at least a small amount) in order to improve maneuverability without sacrificing overall lift. All this makes me want to find somebody with a wind tunnel to play with. Oh, and a room full of Ph.D.s to figure out what I’m seeing. And now that I have exhausted my pedestrian knowledge of physics, it’s time for an actual college graduate to continue, or to tell me I really should have stayed in school.
MK sells this angle adaptors for a long time and their only benefit is the yaw authority. Flat platforms has no stability problems for rolling or pitching. The inclination of the motor arms may help to keep the gimbal away from prop wash since the brushless gimbals are sensitive to the wind. I have experienced this on an X8 setup where the air from props was hitting the gimbal and causing some vibrations. Inclination would solve this without extending the arms.
I'm not sure that's quite right, Steve. If you increase power to motor #1, it increases the thrust of the motor along the thrust axis -- which, in this case, is initially vertical. Specifically if you have a situation where initially you've got the copter hovering flat and level, let's imagine that you've got 1 Kg of lifting force from all the motors (technically this force is an acceleration, so it would be 1 Kg/meter/sec/sec thanks to Dr. Newton, f = m x a. So I really should write this as KgF. If you want to impress your friends, call it a "kilopond" and watch their reaction. And yeah, 1 KgF is also 1 Newton. Confused yet? ) The combined thrust is a vector of 4 KgF pointing vertically, counteracting gravity and the mass of the copter that is (in this simple example), 4 Kg. This is mass not an acceleration - -and it must be 4 Kg this example because the copter is hovering. If it was more then the copter would be descending. If it was less the copter would ascending. Anyhow, if you now increase motor 1 to give 1.5 KgF, what must the aircraft do? It's going to nick (pitch) up because motor 1 is now lifting boom 1 and motors 2, 3, and 4 are still only generating 1 KgF. This causes the combined thrust vector to tilt over towards boom #3 (the tail of the copter). When that combined force vector shifts towards motor #3, there is now a vertical and a horizontal component to the force because it's tilted over. It is that horizontal force component that will cause the copter to fly in the direction of motor #3. In other words, the only reason that a copter moves in the X/Y (horizontal plane) is if you tilt the copter over and create a horizontal component of the combined thrust vector. Actually, it always has to bank or nick/pitch to achieve ANY horizontal velocity relative to the airmass in which it is hovering. That diminution of lifting power is caused because part of the combined thrust vector is forming the horizontal thrust vector and thus is not part of the vertical thrust vector. You don't need a wind tunnel. It all works in dead calm air! Err.... Err.... Andy.