heliflyknow Posted January 1, 2016 Report Share Posted January 1, 2016 I recently watched a YouTube video of a man flying a contraption that can best be described as a lawn chair with 30 RC helicopter blades/motors attached with a trellis frame. After asking why, then, how fast/high can it go, and it's endurance, I wondered: does the limit of ground effect vary for multi rotored aircraft? Does this contraption's rotors act as a single larger rotor, or is it impossible for it to benefit from ground effect since it cannot get within one rotor diameter? Do tandem rotor aircraft or coaxial aircraft have a higher threshold for ground effect or is the difference so negligible it can't be felt? Quote Link to comment Share on other sites More sharing options...
iChris Posted January 6, 2016 Report Share Posted January 6, 2016 (edited) I recently watched a YouTube video of a man flying a contraption that can best be described as a lawn chair with 30 RC helicopter blades/motors attached with a trellis frame. After asking why, then, how fast/high can it go, and it's endurance, I wondered: does the limit of ground effect vary for multi rotored aircraft? Does this contraption's rotors act as a single larger rotor, or is it impossible for it to benefit from ground effect since it cannot get within one rotor diameter? Do tandem rotor aircraft or coaxial aircraft have a higher threshold for ground effect or is the difference so negligible it can't be felt? Rotor performance is affected by the presence of the ground or any other boundary that may alter or restrict the flow into the rotor or restrict the development of the rotor wake. An increase in performance is most notable at rotor heights less than 1 rotor diameter. Above 1 rotor diameter ground effect diminishes and by 2x times the rotor diameter ground effect is effectively nonexistent. Since the blades are not overlapping you can use the basic momentum analysis where we can assume that each rotor carries (supports) its equal share of the total weight (thrust). In this case 167kg (361 lbs.)/54 = 6.7lbs. Hence, we can calculate the disk-load and power required of each of the 54 engines to produce 6.7 pounds of thrust. Moreover, any other arrangement of engines or rotor size as needed. https://youtu.be/t5JgnMJzCtQ The Swarm man carrying multi-rotor airborne flight testing montage. 54 counter-rotation propellers, six grouped control channels with KK2.15 stabilization. Take off weight 148kg, max lift, approx. 164kg. Endurance10 minutes. Power approx. 22KW. Just a bit of fun for my self, never intended for making a significant journey or flying much above head height. Approx cost £6000. Props at 18 in were the largest I could find with contra rotating pitches and so defines much of the layout. Also the fine pitch allows them to turn at high speed which reduces motor weight for a given power. Motor KV chosen to work with 4 cell batteries with 20% or so control margin. This low margin maximises the ESC efficiency, higher margin results in higher circulating current losses in the ESC and motor, reducing endurance. 54 chosen as this fits the hexagonal close pack layout. 6 more could be added in the centre. 18 in props at 5000 rpm though hazardous, are still much less so than 6, 5 ft ones. Where could you get a low power 5ft prop from weighing less than 1Kg with opposite pitches? The 54 gives good redundancy as mechanical and power electronic systems are the main failure areas. I have had one ESC randomly fail already. Controls could be made fully redundant (9 x KK2.15s) so only the control sticks (and pilot) are single failure points. Redundancy increases the likely hood of failure but reduces the consequences. Props are standard RC aircraft types used at a relatively low fraction of their thrust/rpm capability. So should be reliable though needlessly heavy. Being inline with my head the polycarbonate dome is for protection. The polythene bubble balloons up catching some ground effect pressure rise contributing to lift. It also doubles as a rain shelter... Batteries, ESC and motor are close together to minimise wiring weight which is significant because of the high current 30A. Also the battery damps vibration. Individual batteries does mean some may run down a little quicker than others. The front and rear groups are used for pitching, a more common manoeuvre so far than roll. No one has spotted the biggest flaw! That is the large number of props running at high speed means the net torque reactions are relatively low, so the craft has little yaw authority. So it probably needs a tail rotor for spot turns. Edited January 7, 2016 by iChris Quote Link to comment Share on other sites More sharing options...
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