Written by Mark Christy Saturday, 06 February 2010 22:34
If you are running a flybarless system then you might as well stop reading now.
The majority of RC helicopters on sale run a Bell/Hiller system to control the main rotor head. What this means is that the swashplate connects directly to the main blades (Bell) AND directly to the flybar system (Hiller). Not only that but the swashplate to main blade connection goes via a mixer system on the flybar which allows the flybar to have an influence on blade pitch (hence Bell/Hiller).
Because of this connection going via the flybar to the main blades you have probably heard the term "flybar ratio" or "bell ratio" and that you should increase or decrease this ratio. But what is this ratio and what is the effect of raising it or lowering it.. and what's a good value anyway?
The flybar is essentially a stabilisation device that feeds into the main blades. Therefore the more input the flybar has into the main blades then the more powerful the effect of the flybar system. In my earlier article about paddles I talked about the effect of different sizes and weights of paddles.. well this translates also on to the bell ratio. If a gust of wind hits the model and it has a very low bell ratio then the model will react like it has very heavy blunt paddles and move with the wind. If you have a high bell ratio then the model will react to the gust like it has light weight aggressive paddles and lean into the gust.
When we talk about the ratio we mean the number of degrees of blade movement for each degree of flybar movement. Traditionally most machines are about 1 degree of flybar to 0.7 degrees of blade movement.
In theory therefore you would want to run as high a ratio as possible until the model starts over correcting for wind gusts. However as with all things helicopter orientated it isn't as simple as that depending on your flying style.
The flybar as mentioned earlier is a stabilisation device. Therefore the higher you go on the ratio the more stable the machine will feel but also the less "snappy" on the controls as the flybar fights back against your control inputs.
Therefore most 3D pilots like to run a low bell ratio to make the model as reactive as possible. The downside being that the machine is badly effected by gusts of wind and suffers large trim changes.
F3C pilots like to run near to 1:1 ratio so that the model will sit nearly hands off in the wind no matter which direction the wind hits the model from.
Generally therefore as a basic rule of thumb the heavier and more docile the paddles you run the higher the bell ratio you will want. The lighter and more aggressive the paddles the less ratio.
Yet.. there is another BUT..
As the mixing arm is moved by flybar input and swashplate input it means as you adjust the ratio your pitch range will also change. Generally the higher the ratio the less blade pitch range you will have.
To know what your bell mixer looks like there are 4 types of mixers that I can think of from the top of my head.
1) Blade Grip. The mixer is quite short and sits on the blade grip leading edge and is 90 degrees to the flybar. Vibe 50, Aurora, Xcell.
2) Trailing. The mixer is longer and connects to the trailing edge of the blade grip and runs parallel to the flybar from one side of the head to the other.. Align, JR Airskipper, Raptor.
3) Leading. The mixer connects to the leading edge of the blade grip and runs parallel to the flybar and does not go from one side of the head to the other. Knight 3D, Hirobo Freya Evo and Scedeau Evo.
4) Right Angle. The mixer connects to the leading edge of the blade grip and runs at 90 degrees to the flybar. Kyosho Caliber & My Aurora!
Leading on from the bell mixer we get to Delta 3 mixing.
What this refers to is how the blade reacts to unwanted external inputs.
Now for years we talked about Delta as being either positive or negative.. but could never decide which was which as people had different definitions. Therefore it is generally referred to as "correcting" or "uncorrecting" Delta.
How Delta mixing works (or features on your model) is dependent on where the blade grip control point (ball) is in relation to the pivot point of the spindle. On the JR Vigor the ball control point was inline with the center of the head and that model featured a center teeter point. Therefore there was no Delta introduced into the rotorhead. On most other models the ball control point is short of the center teeter point of the head (if the head has a center teeter point!) and depending on whether the grip is leading or trailing means you end up with correcting or uncorrecting delta. This is because as the spindle pivots the blade grip control ball is moving up or down as well as it is not inline with the pivot point. Therefore the pitch on the blade will change.
WIth correcting delta as the blade flaps up the delta effect will reduce the blade pitch to try to bring the blade back down.
With uncorrecting delta the opposite happens and it will try to force the blade up.
Brilliant.. lets all run maximum correcting delta. Not so fast! As mentioned in my swashplate timing article everything on a helicopter happens 90 degrees out of phase. Therefore with any correcting or uncorrecting adjustment it will be happening 90 degrees too late and therefore making the rotor disk unstable.
For general sport or 3D flying it is generally thought that 0 degree delta is the best choice.
However for F3C things get a little more interesting. For F3C hovering uncorrecting delta may be better for the model. The reason for this is that the delta actually has an effect on the body of the model more than the blades. Imagine the model is hovering in front of you with the nose of the machine pointing left and the wind in your face. As the wind hits the model the blade disk stays pretty much where it was but the body moves towards you a little causing the machine to roll right slightly into the wind. As the advancing blade over the canopy rises in the extra lift generated by the wind the uncorrecting delta will exaggerate this movement. As the effect will only happen 90 degrees later the force will be applied when the blade is inline with the wind and furthest from you.. therefore pulling up and rotating the body of the model back to level!
As you can see.. to get a model setup "perfectly" requires a balancing of all the aforementioned items.
The order I go about setting any machine up for F3C is the following:
1) Choose the heaviest set of paddles and shortest flybar I can that still gives me a good roll rate. (on a 90 that is 40g and 490mm)
2) Adjust the bell ratio until the model will not move forwards or backwards when pointing into wind in the hover. (1:1)
3) Adjust the delta mixing so that in a cross wind the model also stays level and resists movement as much as possible. (Small amount of uncorrecting).
4) Adjust swashplate timing so that model reacts cleanly to control inputs. e.g. forwards means forwards and not forwards and left!
With this I can hover my model in any direction in any wind and it will almost stay there on its own and not suffer any trim changes. However in aerobatics it still has enough control authority to do a reasonably fast flip at maximum cyclic deflection AND have around 22 degrees of pitch range. The machine will also roll and loop dead straight without any mixing in the transmitter (so long as the washout phasing can be adjusted mechanically!).
For my 3D machine I would do exactly the same except I would run lighter paddles and less bell ratio.
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