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DIY Control Loader Flight Column - p2

DIY Force Feeback Flight Column

Aileron mechanism

DIY Force Feeback Flight Column

 2nd Stage Elevator Drive

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The single most important feature of a force feedback system is the quality of the force feedback - I haven't yet figured out how to communicate this by internet however. So you'll have to make do with an honest description and some video clips....


I think the force feedback is reasonably good. The force application is generally smooth and although there is no sense of motor cogging torque fluctuations in the feel there is discernable commutation related ripple. I think that the elevator forcing is slightly smoother than the aileron forcing, but am not too sure why.


The performance is sensitive to the set up of the mechanical transmission  also - for example drive belts that are too slack introduce lag, if they are too tight they introduce friction loading at the bearings which makes the movement slightly sticky.


With the aileron axis at higher loads (eg when the ailerons are deflected into a higher speed airflow) making very small one-handed position adjustments can bring through discernable but small force fluctuations in the feel - due to movement of the motor brushes over adjacent commutator segments in the motor. Most of the time, with two-handed operation, lower forces and faster movements the fluctuations are less noticeable. I think there is a good chance that I am more sensitive to this than others might be because I spent a lot of time trying to eliminate variations in the force feel - so I always notice them. For a sim pilot otherwise occupied by the tasks of flight and all the associated cockpit activity these small force fluctuations might not register. However I also think this might be quite a personal thing which might be more noticeable to some people than others.


The elevator feel is good I think, however as with the aileron axis, motor commutation based force fluctuations are still discernable, and especially at the higher load levels. These appear at a higher frequency than the aileron drive due to the higher transmission ratio used in the elevator drive. The best way I can describe this is as a slight rumble in the feel when the column is displaced towards its end of travel when there is a fast airflow over the elevator surfaces. Again, these force fluctuations with two-handed operation at normal force levels might not register, but I don't like them.



In terms of the overall feel the effect most noticeable in flight is the way in which the controls stiffen up as the airspeed increases. The feel is very different cruising at speed when compared to a much slower landing approach for example. The way in which the trim position changes with airspeed is also noticeable. Prop wash effects with the aircraft on the ground are interesting - the elevator stiffens as the engines are powered up if prop wash effects are included in the configuration. I like the difference the force feedback makes - you sort of get used to the information flow coming back which tells you almost subconsciously of the condition of the aircraft. For example the reducing control stiffness can alert you to dropping airspeed before you have noticed it on the instruments.


I haven't thought much about incorporating failure situations that would affect the control forces - any suggestions?




Perhaps a few video clips will give some sense of the flight column operation. Please note that the programmed behaviour of the flight column is still preliminary and the clips illustrate typical behaviours. One of the great benefits of taking a digital approach to the force feedback is that, within reason, any loading behaviour can be programmed for the system.


This first clip below shows the column being activated with an FSX flight in the air and paused. When activated the forces bring the controls to their trimmed balance position and the flight can be resumed. You can't see the forces off course but note how the controls return towards center when released. When the force feedback is disengaged the column returns to its inactive response.



In the second clip below the scenario is similar. Included in this one is illustration of the effect of adjusting the trim pots. The trim system is completely separate from FSX's internal system. Adjusting the trim pots on the column causes the trim surface force magnitude to be altered as it is deflected more or less into the airflow - this alters the force balance on the main control surface and shifts the control axis zero load position. The trim can be used to bring the column to balance at any position. However the trim force magnitude is airspeed dependent, so the balanced position does not remain in the same place if the airspeed changes.



For this third clip below the aircraft is on the ground and stationary. The force feedback software allows the elevator to be configured as load balanced or unbalanced. In this case the unbalanced settings are used and the elevator weight draws the flight column forward when there is no airflow. On some propeller driven aircraft prop wash generated on power up can induce aerodynamic loading on some control surfaces. In this clip powering up the engine causes the elevator to move to its trimmed balance point.... and to fall forward again when the power is removed.




And finally the clip below also shows prop wash effects, but this time using a load balanced elevator configuration - ie one in which the elevator remains in the position to which it is moved. Unless of course there is significant air flow - then the control surface is forced to the load balanced position.





DIY Force Feeback Software

DIY Force Feeback Software

The force feedback model calculates control force from control surface position, airspeed, trim forces, weight and aircraft acceleration information. Moderate damping and friction effects can be added as can some "manufactured" effects such as engine vibration and stall buffeting. How good these latter effects are I'm not sure.


The software is still in a preliminary state but it does allow a fair bit of configuration freedom with settings to be saved for different aircraft and set ups. All the effects are tuneable in terms of intensity and there are also a number of more fundamental parameters available governing the extent of internal filtering applied to various terms. The software includes output for the Rudder control loading but I haven't been able to do any refinement work on this as I haven't built a set of rudder pedals.




Overall I think I've made progress with the force feedback system. However it is indeed a tricky thing to get right, the drive motors are expensive and the mechanical design and build will be fairly unforgiving of poor design or workmanship. The performance is fairly good - but not perfect.


In the UK the MSS-4 motors cost roughly UKŁ400 each, the MSS-2 is slightly cheaper and might be adequate for the aileron. I wouldn't go smaller than the MSS-4 for the elevator drive. The remaining electrical and control hardware and mechanical components might cost around the UKŁ300 mark which would give an approximate build cost for the force feedback flight column of UKŁ1100.


Now I need to mull over where to take the project next - a more refined column design might be worth looking at. Using better quality ball bearings rather than cheap needle roller bearings might help further reduce friction levels when the drive is fully loaded. I would also like to investigate better location of the aileron drive motor which I have just stuck on the column at the moment. This works from a transmission point of view but it does affect the weight balance of the elevator axis (which is compensated for by the software) and it doesn't look good. A wider belt on the final elevator drive stage might be worth trying - the belt tensions in that stage are quite high. The question is whether or not a well set up mechanical drive which is stiff with low friction will help reduce force ripple to the extent that it can be accepted by the user. I also think it might be worth experimenting with a much stiffer geared transmission for the elevator drive - although backlash control might be a problem there if lower cost gears are used.




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