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Cockpit 1 Mechanical Design

UPDATE Jan '08, I've made available a series of high-resolution CAD images of the main assemblies of the motion cockpit for any other DIY'ers who are interested in the construction principals - you can see them here.

DIY 3DOF Motion Cockpit - Drive Motor with FeedbackFor the FSX/FS2004 driven flight sim motion cockpit I've gone for a three degree of freedom system two rotational DOF's to simulate the forwards/reverse (fore/aft) forces and the sideways (lateral) forces acting on the pilot, and a linear vertical or heave DOF to try to deal with some of the vertical force effects. The fore/aft forces arise mainly from changes to the aircraft's forward speed (velocity) such as on take-off and landing, but also as a result of the pitch attitude of the aircraft. Testing has suggested that lateral forces during turns are generally light and mainly arise from slippage effects but the roll motion is important in developing a feeling of roll so there is a contribution from the roll angle of the aircraft, and the vertical forces arise from the heave movements of the aircraft in flight and also from discrete events such as touch-down bumps on landing or running off the end of runways (I know, I should practice more).

 

Testing of the system has brought up some surprises for me in which of these DOF's is the most active under different flight conditions mainly heave with not much in the way of sideways accelerations.

 

 

DIY 3 DOF Motion Cockpit - Wall Frame

Support Frame

 

The working principal is to try to simulate the force effects on the pilot - not to simulate the apparent aircraft movements. Shown in a lower image the flight sim motion cockpit is covered with a light-weight hood and when in flight the pilot has no visual signals of the external movements of the simulator all he/she sees is the view of FS2004 on the computer screen and at the same time feel forces that match what's happening in the virtual world he's flying in.  For a more lucid explanation of this check out ForceDynamics' "how it works page" here (and while you're there watch their videos - that's performance that will be hard to match).

Looking at the CAD images you'll see I've gone for a one-sided or cantilevered design for the flight sim motion cockpit. This is quite a bit different from most of the movement platforms I've seen. It does introduce some structural challenges but it leaves one side completely open for pilot access and reduces the overall footprint of the equipment. It is installed up against a solid wall which provides much of the load support for the structure. The cantilevered design also makes the heave DOF drive design relatively simple and allows important counter balance weights to be incorporated into the arrangement with out too much difficulty all of which are positioned away from the side where the users would congregate. The arrangement also allows fairly generous movement ranges to be designed-in which directly affect the range of flight conditions that can be simulated these are detailed below. Testing has shown that I don't need all of the pitch and roll movement available, although it does look like the more heave stroke there is the better.

The question of weight balance in the system is important because it directly affects the power required to drive the cockpit movements, and for a given set of drive motors it affects the response times they are able to induce in the simulator. The lower the drive power required the smaller the motors need to be and the cheaper the cockpit build will be - power capacity affects most of the elements in the mechanical and electrical power transmissions.

The rotational DOFs are brought into balance by trying to ensure as much as possible that the axes of rotation of the pitch and roll movements pass through the centers of mass of the moving cockpit and pilot. This geometry is built into the design and some minor adjustment should be possible by slightly altering the seating position of the pilot. However the heave DOF can only really be brought properly into balance by adding counter weights, my provisional plan here was to use a number of 15kgf batteries for this because they are heavy, compact and I've got several old ones sitting about. In the end I used a combination of batteries and bungee or shock cord springs which reduces the added mass in the system.

I have used four inexpensive 24V 200W DC PM motors to drive the movements. Two motors drive the heave DOF which requires the most power, and one each for the rotational DOF's. Each motor is fitted with a worm gearhead which limits to an extent the ability of the simulator to back drive through the motor and helps the simulator hold position better without drawing much power. It's important to say that the 200W rating of the motors is their continuous use rating. Like most DC PM motors they are capable of short term outputs much higher than this. It's modest commercial aircraft levels of performance that the design is aimed at and it looks as if it will be the heave DOF that will be most affected by any power limitations there are.

I did the inertia and acceleration sums and was interested to see what the performance actually turned out to be. Generally the power consumption is less than I anticipated. Only the heave movement motors and controllers show any signs of warming (indication of current draw) and the 20Amp speed controller driving both heave motors stays well within its normal temperature range. I suspect that less than 200W is needed for the two rotational DOF's.

Mechanical power transmission is by roller chain drives which are quiet, stiff, non-slip and relatively tolerant of misalignments between the moving parts, they are also relatively inexpensive to build. One unanticipated effect of the chain drive on the heave movement was the transmission of chain on sprocket vibration through to the cockpit at the higher heave speeds. In the end I made up and added a mechanical vibration isolation unit which removed this vibration transmission - I have found that anything you can do to smooth the movement improves the simulation effect.

Some technical details -

Heave displacement - +/- 200mm (more if the structure height is increased, an advantage of using counterbalance weights is that the heave DOF remains in balance over the full stroke length). If I was to build it again I would add more heave stroke length to give more scope for acceleration development.

DIY 3DOF Motion Cockpit - Part Assembled

Support Frame with Heave Cradle assembled

 

DIY 3 DOF Motion Cockpit

With Cockpit and Screen Added - scope is there for a more realistic cockpit fit-out.

 

Motionulated Forward Acceleration - it became clear during testing that the full angle wasn't needed to deliver strong force cues and the rig is now limited to about +/- 30 Deg Pitch.

 

DIY 3 DOF Movement Cockpit

 

 

DIY 3DOF Motion Cockpit - Drive Motor with FeedbackPitch - +/- 60 Degrees the pilot needs to be strapped in! Theoretically a 60 Deg pitch backwards of the simulator simulates a forward acceleration of nearly 0.9g . On testing it became apparent that to deliver impressive force cues as much as +/- 60 Deg isn't needed. The normal working range of the rig is now limited to +/- 30 Deg, which also reduces the operating stresses on the kit.

 

Roll - +/- 40 Deg enough to simulate lateral accelerations of about 0.6g, again a fairly respectable level. Again on testing it was clear that this capacity was likely to be unused. In reality there is little lateral forcing felt in a banked aircraft - especially if the turns are coordinated. What does seem to matter in the simulator is cueing roll rate - ie delivering a sensation of rolling that matches the on-screen visuals. This can be done by rolling in the right direction but at a lower rate and then slowly washing-out back to zero at a rate unnoticed by the pilot. If large roll angles are maintained in the cockpit the sustained sideways forcing felt by the pilot is not realistic. Where this might be different is during ground manoeuvres - turns on the ground do produce sustained lateral forcing.

 

However, it turned out that the un-needed angle capability was in fact needed - but to provide a runoff movement following triggering of the cockpit's limit switches. It takes a moment for the movement to slow to a stop even when the power is cut to the drive and the extra rotation allows this deceleration to happen before the actual physical end stops are hit. It was wise to design the capacity in - I just didn't use it the way I'd intended.

 

(For interested readers the human model shown in the CAD design is derived from body shells made available by M P Reed at the University of Michigan. They are based on anthropometric data for crash test dummies, data which also contains very useful mass and inertia data - see his downloads page.)

The CAD images on the page show the flight simulator motion cockpit in a range of operating positions the light-weight cockpit hood fitted on the built design but not shown in the CAD images is very important for developing  the full effect of the simulation.

 

 

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