Freeflight Design Shop Forum - Tips

Re: aa motion ranges - minitutorial

Posted by Tom Gibson/FFDS on Wednesday, 7 November 2001, at 12:04 p.m., in response to aa motion ranges, posted by lee roden on Monday, 5 November 2001, at 9:27 a.m.

Landing Gear Mini-Tutorial

The landing gear can be difficult, since AA describes it in terms of extension (i.e. 0 is retracted and 100 is extended), while we think of it in terms of retraction (since we build them extended). So start thinking extension, and it will be much easier.

So, let's look at some example motion sequences:

Gear: 0-50:0H;50-100:2

This means that for the first half of the extension cycle (0-50), the gear will remain motionless and hidden (:0H), and for the second half (50-100) the gear will extend at twice the normal rate (:2). Why do we need twice the speed?

The total movement (default is 90 degrees) is determined by two things, the speed of the movement, and the percentage of the total cycle that the gear moves. Again, with a default of 90 degrees, 0-100:1 will move 90 degrees (1x speed over the entire cycle). But 0-50:2 will also move 90 degrees - the part will move twice as fast, but for only half as long. So you still end up with a 90 degree movement, but it will happen twice as fast (and that will be during the first half of the total gear extension cycle).

So, what do we want to happen? Well, we want the gear doors to open, and then the gear to drop down (but only after the gear doors are open). I'll assume that we want both the gear and the doors to move 90 degrees, to keep things simple. After you understand the basics, head over to Felix's table on how to calulate different angles.

OK, we've already talked about the gear - it will move during the second half of the sequence, waiting for the gear doors to open.

So, the gear doors. They will open during the first half of the cycle, allowing time for the gear to move later. The typical sequence suggested is:

1-50:2;50-100:0

What is this saying? While the gear is fully retracted (the cycle is at 0), don't show the gear doors (that's the "1"). Another lesson: if part of the cycle is not listed, the part will be invisible. The above sequence is equivalent to 0-1:0H;1-50:2;50-100:0. I usually leave it out if for a very short portion (like 0-1), and include the :0H entry if for longer periods (seems to work better).

If you want the gear doors to be visible with the gear fully retracted (i.e. if you have modeled the actual gear bays, for example, and these need to be covered), then you should start the sequence with 0-50 instead.

OK, then we say "from 1-50 move the doors at 2x speed" (:2), equalling 90 degrees. Finally, we tell AA to leave them motionless (:0) for the second half of the cycle (50-100).

Now most people are pretty happy with this, but I find the doors move too slowly for most aircraft - they usually "snap" open and closed in real life. Thus, I use:

0-37:0H;37-50:8;50-100:0

This causes the doors to snap open just before the gear starts coming down - much more realistic.

Anyway, I hope that this has helped to make things a little clearer!