@rick yonika ; for any motor being articulated by a servo ; i recommend triple blade props with some flex in them ; this gives a "much" smoother result and almost no feed back forces associated with 2 bladed props (from my Scarab tricopter tests) during the motor pivot phases, especially if they are fast servos.
I must admit I haven't actually thought much about the efficiency of the different control methods before but I'd imagine that thrust vectoring using the tilt servos is actually more efficient than modulating thrust from a static mounted motor/prop, as configured on a conventional quad. The reason I say that is I would expect the motors on the tilt rotor to spin at a more stable RPM as pitch and yaw attitude, apart from roll, is controlled by the direction of thrust, not just by the amount of thrust of each motor. By maintaining a more stable RPM and power consumption, peak accelerations or motor and props, including battery droop etc can be avoided. I'm not sure of the magnitude of improvement, but it will be better.
Extrapolating this to a slow moving inspection drone, I'd expect adding another larger prop just for hover, but a non-tilting static mounted one, would be capable of increasing hover times even further, as the two tilting motors would maintain attitude control, whilst the third main rotor only produces lift. Taking this a step further actually leads to another design "feature" of such a asymmetric motor platform, namely that in slow forward speeds the main rotor will actually autorotate just from the forward tilt motor thrust, turning the platform into a hybrid heli/gyrocopter. I have a pusher hex that operates exactly like that.The wing for cruise is optional. :-)
@JB - Thanks for the reply - interesting stuff. So my reason for asking the question is a thought experiment on the minimum system configuration required for a slow speed inspection drone. It looks like the servo controlled motors are able to provide a high level of maneuverability and the perhaps the wide separation of the motors is helping with stability. Maybe steering the motors rather than relying entirely on changes in prop speed is improving hovering efficiency.
No problems. We've been pondering these challenges for a while, and most of our experience is from optimizing quadplane performance, which are subject to the same dynamics.
LD
If I may intrude on the question. The wing allows for much greater range overall and more payload to be carried for a given amount of power (thrust) than can be done just by using propeller lift alone. In the case of stationary hover the wing becomes "dead weight", but depending on the wing and it's AoA the wing used in the video will probably start assisting lift as low as 4-5m/s, plus the wing in ground effects on takeoff etc. and even the prop wash will start to generate some lift as well over the wings. Even a small lateral movement of air, from wind or the aircraft translates into lift, plus the attitude control via the tilt motors makes tip stalls "impossible" meaning even higher AoA can be used than in a non-VTOL aircraft, magnifying the lift effect further. The point I made in my comment, which is in line with your thinking, is how far can one reduce the wing area (and weight/drag) overall and still be able to maintain enough lift for forward winged cruise, if one primarily relies on the prop lift for hover and takeoff/landing and ignores the wings resulting poor slow speed lift characteristics.
@Rick - Let me ask a really dumb question here: Why bother with the wing? Isn't this actually a two motor multi-copter, at least in the way that it's being flown in the video? I know for longer range operation you can make use of the lift generated by the wing but at slow speeds it looks more like a dead weight than a lift generating surface.
HI, JB I have been going in the other direction down to 2 motors 2 servos and 18650 batterys been getting good results, but I will think about your hypothasis( ok I can"t spell)
In my opinion the propulsion weight is not as important a factor as getting the right prop size and pitch for efficient forward thrust. We get 1.7kg of thrust from 35g motors. and around 1kg from 18g motors, so there's plenty of propulsion performance options available with low mass. Distributing this across the frame by using a third motor is fairly trivial, but allows the hover motor/prop to be larger and less pitch for hover which is more efficient, and the forward tilt motors smaller and with a higher pitch as they only do about 30% of lift in hover(and are responsible for attitude control). For efficient forward flight you need thrust to equal drag at desired cruise, conversely efficient hover is thrust/lift at the minimum of air velocity (high volume low speed). So decoupling forward propulsion from hover has the effect of better performance from both, in this case the performance gain makes adding the weight of a single fixed motor and prop negligible. Besides, adding more weight just means you have to fly faster to achieve similar lift to drag ratio etc etc leading to a point where the low speed wing aerodynamics and wing flight envelope can be largely neglected all together, and you end up with cruise missile style wings. ;-)
Comments
Hi, makes a lot of sense ,I will try some experiments, all the servo problems have been on impact, the plastic gear,
but something has to give.
thanks
@rick yonika ; for any motor being articulated by a servo ; i recommend triple blade props with some flex in them ; this gives a "much" smoother result and almost no feed back forces associated with 2 bladed props (from my Scarab tricopter tests) during the motor pivot phases, especially if they are fast servos.
I think your on to something.alt hold is tricky ,any breeze requires motor speed reduction as does any forward flight
Rick
@JB - Wow!
@LD
I must admit I haven't actually thought much about the efficiency of the different control methods before but I'd imagine that thrust vectoring using the tilt servos is actually more efficient than modulating thrust from a static mounted motor/prop, as configured on a conventional quad. The reason I say that is I would expect the motors on the tilt rotor to spin at a more stable RPM as pitch and yaw attitude, apart from roll, is controlled by the direction of thrust, not just by the amount of thrust of each motor. By maintaining a more stable RPM and power consumption, peak accelerations or motor and props, including battery droop etc can be avoided. I'm not sure of the magnitude of improvement, but it will be better.
Extrapolating this to a slow moving inspection drone, I'd expect adding another larger prop just for hover, but a non-tilting static mounted one, would be capable of increasing hover times even further, as the two tilting motors would maintain attitude control, whilst the third main rotor only produces lift. Taking this a step further actually leads to another design "feature" of such a asymmetric motor platform, namely that in slow forward speeds the main rotor will actually autorotate just from the forward tilt motor thrust, turning the platform into a hybrid heli/gyrocopter. I have a pusher hex that operates exactly like that.The wing for cruise is optional. :-)
@JB - Thanks for the reply - interesting stuff. So my reason for asking the question is a thought experiment on the minimum system configuration required for a slow speed inspection drone. It looks like the servo controlled motors are able to provide a high level of maneuverability and the perhaps the wide separation of the motors is helping with stability. Maybe steering the motors rather than relying entirely on changes in prop speed is improving hovering efficiency.
Rick
No problems. We've been pondering these challenges for a while, and most of our experience is from optimizing quadplane performance, which are subject to the same dynamics.
LD
If I may intrude on the question. The wing allows for much greater range overall and more payload to be carried for a given amount of power (thrust) than can be done just by using propeller lift alone. In the case of stationary hover the wing becomes "dead weight", but depending on the wing and it's AoA the wing used in the video will probably start assisting lift as low as 4-5m/s, plus the wing in ground effects on takeoff etc. and even the prop wash will start to generate some lift as well over the wings. Even a small lateral movement of air, from wind or the aircraft translates into lift, plus the attitude control via the tilt motors makes tip stalls "impossible" meaning even higher AoA can be used than in a non-VTOL aircraft, magnifying the lift effect further. The point I made in my comment, which is in line with your thinking, is how far can one reduce the wing area (and weight/drag) overall and still be able to maintain enough lift for forward winged cruise, if one primarily relies on the prop lift for hover and takeoff/landing and ignores the wings resulting poor slow speed lift characteristics.
@Rick - Let me ask a really dumb question here: Why bother with the wing? Isn't this actually a two motor multi-copter, at least in the way that it's being flown in the video? I know for longer range operation you can make use of the lift generated by the wing but at slow speeds it looks more like a dead weight than a lift generating surface.
HI, JB I have been going in the other direction down to 2 motors 2 servos and 18650 batterys been getting good results, but I will think about your hypothasis( ok I can"t spell)
Rick
Hi Rick
In my opinion the propulsion weight is not as important a factor as getting the right prop size and pitch for efficient forward thrust. We get 1.7kg of thrust from 35g motors. and around 1kg from 18g motors, so there's plenty of propulsion performance options available with low mass. Distributing this across the frame by using a third motor is fairly trivial, but allows the hover motor/prop to be larger and less pitch for hover which is more efficient, and the forward tilt motors smaller and with a higher pitch as they only do about 30% of lift in hover(and are responsible for attitude control). For efficient forward flight you need thrust to equal drag at desired cruise, conversely efficient hover is thrust/lift at the minimum of air velocity (high volume low speed). So decoupling forward propulsion from hover has the effect of better performance from both, in this case the performance gain makes adding the weight of a single fixed motor and prop negligible. Besides, adding more weight just means you have to fly faster to achieve similar lift to drag ratio etc etc leading to a point where the low speed wing aerodynamics and wing flight envelope can be largely neglected all together, and you end up with cruise missile style wings. ;-)
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