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Sacramento, CA
Tags: Syma X9
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Someonespadre
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December 28, 2012 - 5:43 am

I was in the hardware store with my wife.  They had some of these S107G helicopters on display for $25.  I said, "I want one of those." She said, "OK but you have to wait for Christmas."

 

I'm mostly interested in how it works.  When I took the initial Flight Instructor oral exam (5 hours of one on one with an FAA Inspector peppering me with questions) I had to answer questions about a lot of things including aerodynamics and how all of the systems work (such as the constant speed propeller).

 

My flying skills are still pretty rough, nothing like the videos I have found in various places on the internet.  I haven't had too much luck making a flying turn.  I can turn in a hover OK but I need to fly it more :-) .  There is a lot going on aerodynamically. Every control movement affects some other control setting.  Sometimes I set the throttle in a hover then if I go forward it seems to climb.  I think it should descend due to directing some of the lift forward.  The forward motion may generate some lift too.  Of course my ham fisted flying skills could be a factor too.  I need to do more test piloting.

 

So I see it has two counter-rotating main rotors.  I guess it increases/decreases the speed of one of them to cause the helicopter to yaw right or left.  It also has the gyro stabilizer on top which has mass and rotates the same direction as the top rotor.  The gyro stabilizer doesn't affect yaw because it has no "bite" on the air like the rotors.  The induced drag of the rotors generating lift would cause the fuselage of the helicopter to yaw about the main shaft in the opposite direction, the two cancel each other out.

 

I see that if the helicopter rolls or pitches then the gyro stabilizer wants to stay in the same plane of rotation (horizontal) and that increases the pitch of one top rotor blade and decreases the other causing roll or pitch back to horizontal.  There should be no yaw due to the increased lift on one side because the other side with less lift cancels.

 

I noticed that if you pitch the nose down it wants to go back to level and I think this is due to the gyro stabilizer working at cross purposes to the pitch control which is the horizontal tail rotor.  It would seem to me there must be some loss of efficiency in forward motion due to the gyro and the tail rotor opposing each other.  My impression is that it goes faster forward than it does aftwards.

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zedorda
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December 28, 2012 - 7:29 am

Yes the tail rotor is fighting against the flybar for control as you try and move forward. And just to be clear the gyro is a chip on the PCB that can detect if the helicopter is level like a cell phone does. The flybar is for stability but doesn't interact with the gyro system which controls the two main rotor blades. Don't get me wrong the flybar is included in the calculations for the gyro chip to work accurately.

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Someonespadre
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December 28, 2012 - 8:13 am

I didn't know the gyro is a separate chip.

I was flying it just now.  I am not sure whether there is a climb/descend response to pushing forward on the stick.

It does turn left a lot better than it turns right probably because the flybar is going right.

That is like a Cessna 152 stall/spins beautifully to the left but not so good right (the prop turns clockwise viewed from the cockpit).  The C150 has a shorter nose and spins better.  The C152 with a more forward center of gravity required a swift backpull along with a swift rudder pedal to the left just before the stall to get it to drop into a good spin.

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zedorda
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December 28, 2012 - 10:03 am

Your probably right because I have noticed the bias turn rate also but didn't think much into it but yeah depending on which way the flybar spins would have inertia over the other.

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CPD
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December 28, 2012 - 3:15 pm

It's fairly basic in operation--turn? One blade set speeds up or slows down, (I still can't tell which it is) while the other stays the same. (or is it doing the opposit of the other... confused) Forward/reverse? The tail rotor spins to add lift to or pull the tail down to angle the helicopter. Since you can't angle the blades like the 800g, the whole heli needs to angle. The balance bar keeps the top rotors level in order to keep it stable when transitioning between turns, stops forward/reverse, etc. If it didn't pivot, then the would be uncontrolable and just crash.

 

The complex part is the physics of interactions between it, the air and objects around it. That can be quite screwey depending on what environment you're flying in.

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Someonespadre
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December 28, 2012 - 4:50 pm

I tried to fly it outside at dusk.  Even on a still evening (Tule Fog tomorrow morning) the air moves a surprising amount.  It couldn't overcome the general flow of the air in my backyard which is almost imperceptible. It almost drifted into my neighbors backyard.  It did respond to control inputs, though.

 

I can't fly it down the hallway, it seems to be attracted to the walls.  It seems to need a bigger room.

 

I did fly it back and forth right at eye level a few feet away.  I could see the fly bar staying stationary relative to level while the rotors pitched back and forth.  I need to get some video of that.  The fly bar has inertia so it wants to stay in the same plane but as the pitch is held constant it eventually tilts forward or backward (depending upon direction of travel).

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CPD
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December 29, 2012 - 2:02 am

Yeah, these things really get blown around, even when it doesn't look like there's any wind. Did a video a while back when I tried to fly mine outside, it didn't work so well. Didn't really have enough wind to blow the flag on our porch, but it flew the heli into plenty of trees!

 

Mine fly down halways fine, but then again I don't know what kind of drafts or air currents are in your house or the shape of the hallway. Might be suffering some bathtub effect if it's shaped wrong. (stilll want to know why so many of the terms for how these things perform come from the bathroom...) BTE is basically air from the heli goes down, hits the ground, spreads out, like it naturally does, but hits a wall/mountain/building/etc. and naturally has nowhere to go but upward. Then the heli is sucking down air, which creates a vacume, where the air going up the side(s) is sucked around and back into the blades. This severely reduces the amount of lift and is a MAJOR problem for real helicopters flying in mountains, where it will happen to one side but not the other and some other screwey stuff will happen.

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