Apr 19, 2012, 04:36 AM Closed Account Joined Nov 2011 1,246 Posts count me in...
Apr 19, 2012, 05:33 AM
Closed Account
Joined Nov 2011
1,246 Posts
Quote:
 Originally Posted by Balr14 Just an FYI, a longer flybar has greater tip speed, so it will have better stability than a shorter flybar.
...really? I severely debate that. In reality, the speed would be a constant along the length of the flybar, at a given amount of revolutions. but because of the less weight associated with the flybar, the motor would not have to work as hard, because of drag or friction created by the centrifugal ("centrifugal" from the latin "centrum", meaning "center", and "fugere", meaning "to flee", so an object "flee's the centre" or is "expelled from the centre") force of the weights, so therefore it would have more torque available to it and therefore would be able to spin faster. The further away any given weight is from the central point, the greater centrifugal force, and air resistance or drag, so the greater self-stabilisation or gyroscopic effect, which in turn would make the heli inherently more stable and ultimately make the heli less responsive to sudden changes to the cyclic pitch angle.

A shorter flybar or lighter end weights would make the heli more responsive, but less stable, as the gyroscopic effect / centrifugal force generated by the flybar must match the weight of the heli itself or it's affects (Long CoG, as Balr14 suggested). Imagine a heavy weight on a string (plumbob), on the initial attempt to influence any movement, the weight will resist moving, but if enough force is applied, in direct proportion to it's movement (pendulum effect), the weight will actually assist to maintain the motion and resist changes to it's motion or direction being slowed or stopped. A pendulum is newtonian physics at it's best.

Newtonian physics at work...

I. A body, if left at rest, will stay at rest unless acted upon, to wit, if a body is in motion, unless acted upon by external forces, will stay in motion (and maintain it's direction). This mainly applies to objects floating or moving in a vacumn, i.e in space, away from any planetary or stellar gravitational forces, but on earth, the friction of particles in the air, gravity, the earth's magnetic poles or the centripedal ("centripetal" from the latin "centrum" meaning "center" and "petere" meaning "to seek", so an object "seek's the centre" or is "attracted to the centre") forces caused from the rotation of the earth, will slow or stop an object in motion (to some extent the "Coriolis Effect" will affect earth bound objects, to a marginal degree, i.e ocean currents and weather patterns).

II. The force applied to a body to gain momentum, is in direct proportion to the weight of the body.

III. For every action, there is an equal and opposite reaction.

So, not only does the flybar assist the heli's blades get back to the neutral horizontal plane position via the dogbones linked to the main blade grips, once the servo's / arms have gone back to their centre "neutral" positions (hence why trimming to obtain a near perfect hands free hover is important, so the flybar is working with the CoG, not fighting against it and ultimately unbalancing and de-stabilising the heli), but because of the long centre of gravity (imagine the main shaft is the string and the heli is the plumbob weight) the flybar also assists to stabilise the vertical plane of the heli. The reason why a coaxial is so stable is because it has an incredibly long CoG, and combined with the flybar, they both resist changes from the vertical plane, it is also the reason why a coaxial with a long CoG (like most of them are) will not move any direction very quickly.

So, in effect, any helicopter with a flybar, will naturally want to impede any alterations outside it's natural movement along it's horizontal and vertical planes, or inother words, a flybar (depending on weight and length) will always try to inhibit any directional movement and will always try to maintain the heli stays in a stable hover. A flybarless system relies on more mechanical means via the servos to self-correct or self stabilise.

...or maybe I'd better go back to school and learn physics all over again.
Last edited by stormforce; Apr 19, 2012 at 08:01 AM.
 Apr 19, 2012, 05:59 AM Eugene Australia, VIC, Delacombe Joined Oct 2006 918 Posts You are totaly correct Mick and thanks for the info
 Apr 19, 2012, 07:13 AM Registered User Joined Jan 2012 521 Posts While I am not able to join you order I wonder if you have any info on the 9117? any specs at all?
Apr 19, 2012, 07:35 AM
still a lot to learn!!!!
Australia, NSW, Laurieton
Joined Jan 2012
306 Posts
Quote:
 Originally Posted by Whizgig Ok anyone that is in Aus and wants to put in an order for the 9117 at a cost of US\$51.00 + shipping send me a PM stating what colour I might have to order all the same colour I can get a box of 4 or 6 depending on how many people want it all payments will be notified shortly after conformation of how many and the cost of shipping.
Count me in Eugene, what choice of colours do we have???
Apr 19, 2012, 07:36 AM
Eugene
Australia, VIC, Delacombe
Joined Oct 2006
918 Posts
Quote:
 Originally Posted by freezywilly While I am not able to join you order I wonder if you have any info on the 9117? any specs at all?
well its basicly the 9104 upgraded to 4ch 2.4ghz same as the 9116 PCB and TX etc
Last edited by Whizgig; Apr 19, 2012 at 08:37 AM.
Apr 19, 2012, 07:38 AM
Eugene
Australia, VIC, Delacombe
Joined Oct 2006
918 Posts
Quote:
 Originally Posted by Auflyer Count me in Eugene, what choice of colours do we have???
Ok its only avalable in blue or red(orange) same as the 9116
Apr 19, 2012, 07:40 AM
still a lot to learn!!!!
Australia, NSW, Laurieton
Joined Jan 2012
306 Posts
Quote:
 Originally Posted by Whizgig Ok its only avalable in blue or red(orange) same as the 9116
Put me down for a blue one
Apr 19, 2012, 07:58 AM
Cranky old fart
Germantown, WI.
Joined Oct 2007
21,807 Posts
Quote:
 Originally Posted by stormforce ...really? I severely debate that. In reality, the speed would be a constant along the length of the flybar, at a given amount of revolutions.
Absolutely true, but the further from center the tip is, the further the tip has to travel to complete a rotation, at any given speed. The circumference of a 100m circle is 314m, the circumference of a 50m circle is 157m. If you have to run around the outside of the 100m circle in the same time as somebody running around the outside of the 50m circle, you will need to run twice as fast.
Apr 19, 2012, 06:18 PM
in Texas
Joined Mar 2012
29 Posts
Quote:
 Originally Posted by Balr14 Excellent observation and it makes perfect sense! That heli has a long vertical CoG. From the top of the rotor to the bottom of the landing gear it's quite long. It weighs a lot down low, with heavy landing gear and the battery is mounted down low. It's a perfect pendulum. The purpose of the 45° offset flybar is to damp movement, which it does very well. But reducing flybar weights or length will reduce the damping effect and facilitate more pendulum effect. Just an FYI, a longer flybar has greater tip speed, so it will have better stability than a shorter flybar.

By "long CoG" do you mean that the CoG (which is a point, after all and not a line with length) is located well below the rotor, perhaps too low, and this exacerbates the "pendulum" effect?

I have noticed that many competitive 3D designs will locate the battery up higher. I would guess this is on purpose and helps achieve faster roll rates, etc, and less pendulum efffect?

The "greater tip speed" giving more stability, I think, is simply because there is more angular momentum, therefore is a more stable gyro than one with less angular momentum. Angular momentum is also a function of the mass (of the weights effectively) as well as the length and angular velocity that gives the tip speed. I do have plans to increase the weights on the cutdown flybar. I do think I have detected a decrease in operating temp of the little tailmotor..... Actually I plan to acquire an infrared thermometer to measure this and try to quantify my anecdotal suspicion. If it really is lower I think it is attributable to much less air drag of the shorter flybar, ergo less torque for the tail motor to counteract.

I have very little experience with helicopters at all, so I appreciate insights from any direction.

Thanks.

rawdog
Last edited by rawdog; Apr 19, 2012 at 09:20 PM.
 Apr 19, 2012, 10:10 PM I need a bigger shed.. Australia, TAS, Launceston Joined Nov 2011 1,498 Posts I have started flying a HK450 now, so have no use for my Z101 (DH9116 imitation) now (which needs repairs, or good for parts). Anyone in Australia can have it for free, but pay for shipping of course (I still have original box, so it'll be safe). -Its had 27 flights, motors seem fine. -Inner shaft needs to be replaced or modified with new rod. -Landing gear is broken. -Stock tx is 47mHz, mode 3 (I think) and not programmable, best use would be the bin and replace PCB with DH9116. -A few minor damages that have been fixed with CA, like the canopy lugs on frame, and plastic where canopy screws on underneath. -Servos have been replaced with the turnigy ones I have posted a while ago. Still have one stock servo for a spare which is fine. Think thats about it, I can post pics if necessary.
Apr 19, 2012, 10:29 PM
Closed Account
Joined Nov 2011
1,246 Posts
Quote:
 Originally Posted by Balr14 Absolutely true, but the further from center the tip is, the further the tip has to travel to complete a rotation, at any given speed. The circumference of a 100m circle is 314m, the circumference of a 50m circle is 157m. If you have to run around the outside of the 100m circle in the same time as somebody running around the outside of the 50m circle, you will need to run twice as fast.
..yep, I get ya. So, in essence, the faster tip speed would further enhance the gyroscopic / self-stabilising effect, the further the weights were from the central point.

Seeing how the centrifugal forces generated by the flybar weights needed to match the long CoG are so critical for self-stabilisation, in theory, a shorter flybar would indeed de-stabilise the heli, given the same amount of motor revolutions. So if one reduced the weight of the flybar end weights by half, or shortened the flybar length by half, one would need to proportionally increase the motor rpm by half again, just to keep the same level of stability, but considering this is a fixed pitch helicopter, the faster rotor head speed would generate too much lift and may make it uncontrollable. In essence, a shorter flybar would be better suited to a variable pitched helicopter, where one can adjust the amount of lift generated by the blades to compensate for the higher head speed. There must also be a relationship between the distance from the central point to the end of the flybar and the CoG, I assume it must be proportional (I'm guessing, as I'm not going to do the mathematical calculations to prove this hypothesis, as I'm not much good with maths or algebra), otherwise it wouldn't work as well as it does.
Last edited by stormforce; Apr 19, 2012 at 10:53 PM.
 Apr 20, 2012, 07:23 AM Registered User Joined Apr 2012 3 Posts Can someone please help me. I have just purchased a 9116 and unfortunately it didn't come with instructions. Can anyone tell me where I can find the instructions as I'm really struggling to set it up and have no idea where to start on the remote either!!
 Apr 20, 2012, 07:48 AM Closed Account Joined Nov 2011 1,246 Posts Hi DMcDermott and welcome, My advise is don't touch the tx program settings, and only mechanically adjust the heli, as per my blog (click on my name in the side margin next to this post). It is far better to adjust the heli itself, than try to manipulate the settings via the tx. I have the manual in zip form here, if you really want it, but you'll have to pm with your email address so I can send it to you.
Apr 20, 2012, 08:28 AM
in Texas
Joined Mar 2012
29 Posts
Quote:
 Originally Posted by stormforce .. In essence, a shorter flybar would be better suited to a variable pitched helicopter, where one can adjust the amount of lift generated by the blades to compensate for the higher head speed.
Stormforce,

This statement makes great sense to me. The only question was how much could the flybar be shortened and still maintain control. It is too short on my experimental 9100/9116 at the moment, and/or the weights are too light. The real issue for how well a gyroscope works is how much angular momentum it has. The mass of the weights, the length of the arms, and rpm all affect this proportionally the same if I understand correctly (http://en.wikipedia.org/wiki/Gyroscope ).

The part I do not quite understand well enough yet is how/why a longer distance between the rotor blades and the "point" known as the Center of Gravity should destabilize, rather than make it more stable (and less manoverable). I will try to give my present understanding of this..... and I invite any critical comment... I appreciate the opportunity to learn from you guys....

What I think is the issue is that with the lesser angular momentum of shorter/lighter flybar, the gyro effect is reduced and the gyro re-orients very quickly (I am speaking of the flybar as a gyroscope, not the electronic "gyro" which is really sensing rate-change movement of the tail and is not a "gyro" at all)..... It seems the apparent purpose of the flybar/gyro is to maintain a reference to whatever plane (geometric plane) the pilot wishes the rotors to be flying in, and to provide feedback to the rotor cyclic pitch control to maintain that plane of the rotors. So, if in a hover the flybar rotates in a horizontal plane and gyroscopically maintains that plane of rotation even when a gust of wind rolls the helicopter sideways.... by maintaining the horizontal plane even while the helicopter body is now in a roll, it operates the cyclic to cancel that roll and maintain the horizontal attitude of the helicopter back to the hover. Same behaviour for an unintended pitch/roll in any direction. (I know you already know this... I am saying this to try to build to a point below...)

Similarly if the pilot intentionally overcomes the flybar tendency to maintain that level hover by giving forward cyclic, after the helicopter pitches forward and maintains that new attitude for a few seconds, the flybar/gyro will reorient in this new geometric plane of rotation and would tend to maintain that new attitude until the pilot gives yet-again-new commands to the cyclic, perhaps giving rearward cyclic to pull the nose up and come back to a hover. In this case the flybar/gyro will initially fight the return to hover, but will in a few seconds reorient back to the horizontal plane and will then try to maintain that hover attitude of the aircraft.

So, the difference with a shorter/lighter flybar is that it reorients much faster, in fact too fast. When the helicopter is sailing foward and you give it a rearward cyclic, by the time the body of the helicopter comes back to a hover attitude, the flybar/gyro is rotating in a very rearward-tending plane of rotation and even though the pilot has returned to hover on the stick, the helicopter pitches to the rear due to the flybar/gyro. The pilot tries to compensate, but merely begins a series of oscillations..... unless you train yourself not to overcompensate and to just exactly damp the oscillation.... Unfortunately, that is too much to expect.

The issue is to reasonably match the time constant of the helicopter's pitch and roll rate to the rate at which the flybar/gyro reorients, and make sure the flybar/gyro reorientation is always at least a bit slower.

The way I can see the distance from rotors to CoG coming in to this is that a longer distance (ie, more pendulum) increases the time constant of pitch/roll rate, and this would aggravate the oscillation.

So, I think Balr14 was right.

Even though my radically shortened/lightened flybar has proven too unstable (OK, ok, everyone knew this but me... :-) ) I would stress that an earlier experiment with the full stock length, but half-weight worked just fine I think and I have flown that configuration a lot over a number of weeks. It returns to hover just fine... Even though it is still capable of flybar strikes, the reduced weight renders it much less damaging.

This suggests to me that there is a happy medium.... perhaps with about an inch taken off each end and retaining close to full stock weight.... Bottom line here is that I think the time constant of the flybar/gyro's changes in attitude must be slower than the time constant of the helicopter's pitch/roll rate. If the flybar gets too fast, instability results.

Tom4416 suggested this less aggressive short flybar and I plan to try it, after experimenting a little more with the radical short flybar and bringing it up to stock weight.

Sorry for long ramble......

rawdog
Last edited by rawdog; Apr 20, 2012 at 08:37 AM.