> -----Original Message----- > From: Dan Lyke [mailto:[EMAIL PROTECTED] > Sent: Tuesday, October 25, 2005 11:09 AM > To: [EMAIL PROTECTED] > Subject: RE: [Flightgear-users] Helicopters in Flight Gear > > > > One of the questions I've got is: I've tried to work through the > > > linkages that'd make up helicopter controls, and every way I come up > > > with there's some force feedback. Am I hallucinating, or is it > > > subtle enough or played out in such a way that it's just better to > > > build controls which don't have springs? > > > Questions about control sensitivity and centering appear to > be asked on every helicopter sim forum, but maybe it's better > done off-line and I'll put the answers on a web page. > > Suggestions, either directly or for reading materials?
Okay, let me see if I can shine a light on the subject. We'll keep it public, since others may be interested. A brief review: Helicopter controls (longitudinal cyclic, lateral cyclic, collective, and pedals) will all be trimmed at different locations for different conditions of airspeed, altitude, weight, center of gravity, and pilot weight. Joysticks and spring-and-pot control systems for simulators can't replicate this, thus making simulation of helicopters in a low-cost flight simulator difficult. A real life example (simplified): Let's take just one control, the lateral cyclic. In simple terms, it controls roll moment. Let's call full left 0% and full right 100%. For one condition, trim might occur at 48%, another at 52%. So, we need to move the trimmed location of the controls. Let's envision a control rod attached to the bottom of the control stick (below the pivot point) that would move left and right as the lateral cyclic moves right and left. Drive a nail horizontally into this rod, so that it is lined up with the longitudinal axis of the aircraft. Now, place a spring-mounted V bracket, so that when the control moves left or right, the nail starts moving up the V, which starts deflecting the spring-mounted V, applying force to move the rod back to center. For one condition, the bottom of the V needs to be at the 48% position, and at the other in our example, 52%. So, let's allow the V to move in the same axis and directions as the control rod. Thus, it can be moved to 52%, 48%, or wherever it needs to be located for trim. Now, in our aircraft (actually, this is based on an H-60 control system), there are two ways to relocate the bottom of the V, which is the trimmed location. One way is to use the trim control, the so-called 'hat' which controls a motor to relocate the V. The other way is to use the trim release button. When the button is pressed, the V is allowed to move to relieve pressure on the spring, and when the button is released, the V is reestablished at the present location. If a pilot wanted to establish the aircraft in a banked turn, it could be accomplished in one of three ways: 1. Use the trim 'hat' to move the control to the desired location (this is slow). 2. Move the control to the desired location, applying force against the spring, and momentarily press the trim release button and release it, thereby establishing a new trim location and releasing the force on the V, and thus on the stick. 3. Press and hold the trim release button, move the control to the desired location without associated spring force, and when the control is in the desire location, release the trim relase button. Now, with the trim release button depressed and held, the forces to move the control will be measured in ounces, and with the trim engaged, the force is measured in pounds. Getting both of these correct is difficult, especially with a purely mechanical system. If the trim 'hat' is used, the movement has to be the correct speed and must be smooth, without jitter that might be induced by a stepped motor. Combined, these problems all add up to a system that is difficult to model. One can envision a screw driven by a stepper motor, with a V attached to block on the screw, held on with an electromagnet. Not an easy mechanical system, but one that would work in theory. The stepper motor would have to be smooth and quick, and there would have to be friction in the system when the trim release button is depressed and the magnetic V wasn't attached to the trolley, and the force on the V would have to be correct too. All very difficult problems, especially if you are selling this joystick for $100. Just as an aside, one H-60 aircraft that I worked on had a discrepancy written against it, a Priority 1 problem. Some of the pilots were getting sick, and declared it was a roll stability problem with the flight model. After doing some research and some deep thinking, we realized that the pilots were trimming with the two different methods, #2 and #3 above. Many of the siumulator engineers, and some of the pilots, moved the stick against the spring, the pressed and released the trim release button. This mode was checked to be correct, with the right amount of force. Some of the pilots, the ones complaining that the flight model was wrong, were pressing and holding the trim release button, and flying the aircraft to where they wanted it, and releasing the trim release button. There was a minor problem that had gone undetected in initial testing that when the trim release button was pressed, there was not any friction modeled in that control, so it was too easy to move, and the pilots were overcontroling it. Restoring the proper amount of friction in the system solved the problem. I hope this has answered some puzzled looks, caused some people to things new and creative things, and will eventually lead to making me LOTS of money. Okay, I can dream, can't I? Bill Galbraith _______________________________________________ Flightgear-users mailing list Flightgear-users@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-users 2f585eeea02e2c79d7b1d8c4963bae2d
