6 ms·

Total noob here, what's the difference between wrapping a quadcopter with a duct and this? Is it because a single rotor is much harder?

by est 8y ago

hoorayimhelping 8y ago
I believe a quadcopter has variable-pitch wings that spin to create lift. This is a fan that pushes a column of air down to create thrust. Helicopter blades pull the vehicle up from the top, rocket thrust pushes the vehicle up from the bottom.
- FredFS456 8y ago
  Quadcopters never have variable pitch props, they just vary the speed of them to vary thrust. Moreover, thrust from the bottom or top doesn't really matter (look up the Pendulum Rocket Fallacy)
  - dirktheman 8y ago
    Well, they usually don't have variable pitch. But there are a few rare examples of variable pitch quadcopters, for instance this monster from Curtis Youngblood: http://www.curtisyoungblood.com/legacy-product-support-curtis-youngblood/attachment/stingray-500/ http://www.curtisyoungblood.com/legacy-product-support-curti...
  - modzu 8y ago
    neat! but (speaking of the pendulum fallacy) if the fuel tank was attached with cables instead of a fixed structure, wouldn't that actually work?
  - ryanmarsh 8y ago
    For most applications you don’t need variable pitch blades on a quadcopter, they’re also very delicate and break easily on drones. I used to fly r/c helicopters before drones became popular. The pitch mechanism was routinely a source of problems for me. It’s pretty amazing to watch it work though.
- kjeetgill 8y ago
  Surely the only difference between push and pull is near the ground right?
  - Ao7bei3s 8y ago
    Another example would be rapid descents: the aircraft can become quite unstable when it gets into its own downwash. (A high control frequency makes all the difference here for quadcopters. Compare a toy and a hobby quadcopter, it's easy to spot the difference.) Also, lift generated due to the top and bottom of airfoils can be quite different.
- Ao7bei3s 8y ago
  No, not at all. The technical difference is how the overall thrust is steered: * Quadcopter: the speed of four (fixed pitch[1]) propellers is managed independently. Spinning a single propeller a bit more tilts the copter away from that propeller[2]. This is very unstable, so speed needs to be adjusted at kHz rates[3][4]. * "Electric rocket": the thrust of the single propeller is steered using the control surfaces at the bottom to deflect the thrust. This is called thrust vectoring. Since the propeller is not directly used to steer, the speed doesn't have to be adjusted that often. However, the position of the control surfaces does. * Helicopter: tilts the rotor plane[5] to tilt the aircraft, and inclines the individual rotors to change the magnitude of thrust. The rotor speed typically remains constant.[6] Helicopters are aerodynamically stable and don't require an automatic control loop [edit: no, see follow-up post!]. In theory the "electric rocket design" with a single propeller might allow higher efficiency: larger propellers are generally more efficient (thrust per watt). (However, the model in the video won't be very efficient, the props are quite small. And there'll be other tradeoffs too.) Also, this is a refreshing take on RC aircraft that hasn't been done as much before. Lift is generated on both sides of the propellers in both cases.[7] Optional reading: [1] Generally - variable pitch quadcopters exist. However that gives up the mechanical simplicity that's one of the main advantages of quadcopters over helicopters. Their advantage is that they can actively decelerate a lot more efficiently and also fly upside down easily. [2] And turns the copter (yaw). Quadcopters use two pairs of counter-rotating propellers to be able to cancel the yaw. [3] Quadcopters are unstable and need to measure linear and angular acceleration and correct the propeller speeds very quickly. Control frequency of 100Hz flies, 1kHz is somewhat standard, 32kHz is state of the art. (Note: Propellers have significant inertia. To give an idea, reversing prop turn direction in-flight takes ~1sec (and requires special symmetric props if it's supposed to work efficiently). [4] This is feasible since the propeller is smaller (less mass, less inertia) than for helicopters. [5] Due to gyroscopic effect, the tilt of the aircraft is 90 degrees out of phase. [6] Assuming the typical variable-pitch helicopter. There are other options both for real life helis as well as for models (fixed pitch, coaxial, ...) but they're rare (or very cheap toys). [7] Side remark: lift is generated due to flow turning, not due to different pressure differences caused by different airspeeds over top/bottom of wings as is taught in school. Search for "NASA incorrect lift theory" to learn more. https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html
theon144 8y ago
Well, in simple terms with four rotors you vary their speeds to tilt the copter whichever way and thus steer it. Here you have a single rotor, and just try to imagine what happens when you vary its speed. It might bob around, fly up and then crash into something, but you won't be able to point it anywhere. Imagine trying to fly a ceiling fan. This guy uses something called "thrust vectoring", i.e. "pointing the thrust of the single rotor in some specific direction" to steer it around, but for that you need something to redirect the flow of air, which he designed and custom 3D printed. Which is pretty damn cool.
- close04 8y ago
  But what's the benefit of taking this approach instead of the classic open propeller design or to ducted multi propeller designs?
  - blattimwind 8y ago
    Fun building it
  - A2017U1 8y ago
    The advantages of a single blade over the equivalent 4 smaller blades is efficiency. A tiny part of that is losses from extra motors but mainly its from blade length which dramatically affects power efficiency, up until the point where the tips go over the speed of sound, helicopters heavily optimise for this. The duct itself offers some advantage but it's used here to vector thrust for control rather than just go up and down. Battery power is quite constraining already for aircraft, ducted fan offers some benefit. The US military use a petrol powered ducted fan drone operationally, though i believe that's dual rotor, this is single rotor that offsets counter rotation with vectoring.
    - madeuptempacct 8y ago
      So, is this thing guaranteed to go faster in a straight line even without wings to provide lift? (Slight downward vector on thrust?) What would happen if rotors were added, ala Kamov helicopters.
toss1 8y ago
Mainly a completely different control mechanism and single vs multiple power-sources. On a multirotor, there are obviously 4/6/8 motor/rotor sets, typically positioned far from the mass centroid, so control is effected by 1) varying the thrust from each rotor (primarily pitch & roll), and 2) varying the speed of each rotor for inertial effects (primarily yaw). This uses a single rotor and thrust vectoring fins to influence the direction and reaction to the thrust. I'm more interested in how the efficiency numbers work out. Typically, a larger rotor produces more thrust per unit of energy. However, a ducted fan is also typically significantly less efficient than an un-enclosed rotor. Since I don't have numbers, my question is which effect dominates -- is it overall more or less thrust efficient than a multirotor? Either way, it is a very cool design & build -- nice work Ikarus!

Total noob here, what's the difference between wrapping a quadcopter with a duct and this? Is it because a single rotor is much harder?

by est 8y ago

Total noob here, what's the difference between wrapping a quadcopter with a duct and this? Is it because a single rotor is much harder?

hoorayimhelping 8y ago

I believe a quadcopter has variable-pitch wings that spin to create lift. This is a fan that pushes a column of air down to create thrust. Helicopter blades pull the vehicle up from the top, rocket thrust pushes the vehicle up from the bottom.

FredFS456 8y ago
Quadcopters never have variable pitch props, they just vary the speed of them to vary thrust. Moreover, thrust from the bottom or top doesn't really matter (look up the Pendulum Rocket Fallacy)
- dirktheman 8y ago
  Well, they usually don't have variable pitch. But there are a few rare examples of variable pitch quadcopters, for instance this monster from Curtis Youngblood: http://www.curtisyoungblood.com/legacy-product-support-curtis-youngblood/attachment/stingray-500/ http://www.curtisyoungblood.com/legacy-product-support-curti...
- modzu 8y ago
  neat! but (speaking of the pendulum fallacy) if the fuel tank was attached with cables instead of a fixed structure, wouldn't that actually work?
- ryanmarsh 8y ago
  For most applications you don’t need variable pitch blades on a quadcopter, they’re also very delicate and break easily on drones. I used to fly r/c helicopters before drones became popular. The pitch mechanism was routinely a source of problems for me. It’s pretty amazing to watch it work though.
kjeetgill 8y ago
Surely the only difference between push and pull is near the ground right?
- Ao7bei3s 8y ago
  Another example would be rapid descents: the aircraft can become quite unstable when it gets into its own downwash. (A high control frequency makes all the difference here for quadcopters. Compare a toy and a hobby quadcopter, it's easy to spot the difference.) Also, lift generated due to the top and bottom of airfoils can be quite different.
Ao7bei3s 8y ago
No, not at all. The technical difference is how the overall thrust is steered: * Quadcopter: the speed of four (fixed pitch[1]) propellers is managed independently. Spinning a single propeller a bit more tilts the copter away from that propeller[2]. This is very unstable, so speed needs to be adjusted at kHz rates[3][4]. * "Electric rocket": the thrust of the single propeller is steered using the control surfaces at the bottom to deflect the thrust. This is called thrust vectoring. Since the propeller is not directly used to steer, the speed doesn't have to be adjusted that often. However, the position of the control surfaces does. * Helicopter: tilts the rotor plane[5] to tilt the aircraft, and inclines the individual rotors to change the magnitude of thrust. The rotor speed typically remains constant.[6] Helicopters are aerodynamically stable and don't require an automatic control loop [edit: no, see follow-up post!]. In theory the "electric rocket design" with a single propeller might allow higher efficiency: larger propellers are generally more efficient (thrust per watt). (However, the model in the video won't be very efficient, the props are quite small. And there'll be other tradeoffs too.) Also, this is a refreshing take on RC aircraft that hasn't been done as much before. Lift is generated on both sides of the propellers in both cases.[7] Optional reading: [1] Generally - variable pitch quadcopters exist. However that gives up the mechanical simplicity that's one of the main advantages of quadcopters over helicopters. Their advantage is that they can actively decelerate a lot more efficiently and also fly upside down easily. [2] And turns the copter (yaw). Quadcopters use two pairs of counter-rotating propellers to be able to cancel the yaw. [3] Quadcopters are unstable and need to measure linear and angular acceleration and correct the propeller speeds very quickly. Control frequency of 100Hz flies, 1kHz is somewhat standard, 32kHz is state of the art. (Note: Propellers have significant inertia. To give an idea, reversing prop turn direction in-flight takes ~1sec (and requires special symmetric props if it's supposed to work efficiently). [4] This is feasible since the propeller is smaller (less mass, less inertia) than for helicopters. [5] Due to gyroscopic effect, the tilt of the aircraft is 90 degrees out of phase. [6] Assuming the typical variable-pitch helicopter. There are other options both for real life helis as well as for models (fixed pitch, coaxial, ...) but they're rare (or very cheap toys). [7] Side remark: lift is generated due to flow turning, not due to different pressure differences caused by different airspeeds over top/bottom of wings as is taught in school. Search for "NASA incorrect lift theory" to learn more. https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html

theon144 8y ago

Well, in simple terms with four rotors you vary their speeds to tilt the copter whichever way and thus steer it. Here you have a single rotor, and just try to imagine what happens when you vary its speed. It might bob around, fly up and then crash into something, but you won't be able to point it anywhere. Imagine trying to fly a ceiling fan. This guy uses something called "thrust vectoring", i.e. "pointing the thrust of the single rotor in some specific direction" to steer it around, but for that you need something to redirect the flow of air, which he designed and custom 3D printed. Which is pretty damn cool.

close04 8y ago
But what's the benefit of taking this approach instead of the classic open propeller design or to ducted multi propeller designs?
- blattimwind 8y ago
  Fun building it
- A2017U1 8y ago
  The advantages of a single blade over the equivalent 4 smaller blades is efficiency. A tiny part of that is losses from extra motors but mainly its from blade length which dramatically affects power efficiency, up until the point where the tips go over the speed of sound, helicopters heavily optimise for this. The duct itself offers some advantage but it's used here to vector thrust for control rather than just go up and down. Battery power is quite constraining already for aircraft, ducted fan offers some benefit. The US military use a petrol powered ducted fan drone operationally, though i believe that's dual rotor, this is single rotor that offsets counter rotation with vectoring.
  - madeuptempacct 8y ago
    So, is this thing guaranteed to go faster in a straight line even without wings to provide lift? (Slight downward vector on thrust?) What would happen if rotors were added, ala Kamov helicopters.

toss1 8y ago

Mainly a completely different control mechanism and single vs multiple power-sources. On a multirotor, there are obviously 4/6/8 motor/rotor sets, typically positioned far from the mass centroid, so control is effected by 1) varying the thrust from each rotor (primarily pitch & roll), and 2) varying the speed of each rotor for inertial effects (primarily yaw). This uses a single rotor and thrust vectoring fins to influence the direction and reaction to the thrust. I'm more interested in how the efficiency numbers work out. Typically, a larger rotor produces more thrust per unit of energy. However, a ducted fan is also typically significantly less efficient than an un-enclosed rotor. Since I don't have numbers, my question is which effect dominates -- is it overall more or less thrust efficient than a multirotor? Either way, it is a very cool design & build -- nice work Ikarus!