Ground taxi and automated control systems

[nickel]

Hi,

I am working on a simulation model and have a problem with taxi. I am trying to model a helicopter which is steered with differential braking. The helicopter also has an AFCS system with heading hold capability.

Now the problem is, when taxiing on the ground, AFCS tries to keep the heading constant. So, tail rotor opposes the differential brakes, an the heli is not maneuvrable at all!

Obviously, heading hold must be off on the ground. On the other hand, we would like to turn it on during take-off. Now the question is how is the transition made in real helicopters? Are there sensors on the landing wheels that turn on/off heading hold?


Thanks in advance

[Art of flight]

Generally the autopilot functions are split into two groups, engage the autopilot stability before take off and then engage the 'upper modes' eg heading, alt, IAS etc once through 60 kts and 400 feet.

As you suggest, smaller, wheeled helis use tail rotor thrust to change direction via the yaw pedals during taxi so heading hold would not be engaged at that time and in most helis is not available until around plus of 60 kts.

Perhaps someone operating pre-programmed uavs could give a better example?

[TiPwEiGhT]

Weight on wheels switches will not allow you to engage any AFCS upper modes on the ground in most aircraft.

TiP

[Shawn Coyle]

And in most helicopters with an AFCS, the first thing that happens on the ground is that the AFCS gets turned off. If you're landing on a ship, it's definitely the first thing to get turned off, unless you want the helicopter to try to 'fly' the ship.

[nickel]

Thanks for all replies. I think weight switches on the pedals are use in my case.

Shawn: Does truning off AFCS happen automatically or do you mean the pilot does it? I am interested in knowing how AFCS knows that the heli is landed.

[TiPwEiGhT]

Weight on wheels switches as I mentioned above work when the landing gear struts compress and decompress. Or the pilot can manually press a standby/disconnect switch.

[Gomer Pylot]

I've never seen an automatic autopilot, which is on all the time. The pilot engages/disengages it manually. AFCS is not the same thing as an autopilot or flight director, it just tries to maintain an attitude, not a heading, altitude, etc. Generally the entire AFCS is turned off on the ground, and especially on a ship, but not always, depending on the model. If you want to be realistic, you need to have the pilot engage and disengage the system manually, not have it come on automatically. That would never happen on a real helicopter that I've flown. I certainly wouldn't want the heading hold or altitude hold to come on without my explicitly turning it on. The results could be less than satisfactory, depending on the situation.

[gwelo shamwari]

As TiPwEiGhT stated, it depends on the Aircraft and Model. On the newer systems the WOW (Weight on wheels) switch deactivates the ACFS. However we have an older aircraft and the pilot has to manually disengage the Helipilot. I have seen this older system try and "fly the disc" while on deck but never yaw.

If you wanted to make it realistic, post the aircraft you are trying to model along with the equipment and I am sure someone here will help you out with the details.