is modelling aircraft landing difficult? i''ve just been recalling collision detection algorithms causing "shaking" if trying to implement resting contacts, on the ground. i was wondering how they are handled in today''s flight sims. a2k

did you mean:
"is simulating an aircraft landing difficult?"

or?

I''m sure that if you wanted accurate Bournulli''s Principle simulations, as well as gravity and everything else, it could be pretty difficult, but if you do a greatly simplified version, it can''t be too difficult

quote:

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Original post by Gee
did you mean:
"is simulating an aircraft landing difficult?"

or?

---

the terms "flight model" and "physics model" are what i was referring to.

a2k

If you''ve modeled the physics of the airplane correctly, you''re more than half done. After all, an airplane about to land is just an airplane flying at a really low altitude, usually dirtied up (flaps extended) to lower approach speeds.

The other thing you''ll want to simulate is ground effect. Ground effect improves the airplane''s lift efficiency, and in my experience at least is manifest at less than 10 feet or so above the runway. The result usually feels like you''re floating down the runway.

The landing happens (or rather, ideally should happen) when the aircraft no longer has sufficient airspeed to keep it aloft, allowing it to settle down onto the ground. For heavier aircraft or aircraft with violent pitch-down stall tendencies (like most low wing planes), you''d want to keep it just above stall.

Other factors you may want to throw in are gusts and crosswinds, which can definitely add some excitement to your landing (speaking from experience here!). For crosswinds, your airplane will have a tendency to yaw, fighting to turn the nose of your plane into the wind, which of course must be corrected with the appropriate rudder pedal.

Lastly, if you REALLY want to get technical, many people employ a technique known as a slip to compensate for a too-steep straight-in approach. Slip involves cross-controlling the airplane; that is, ailerons one way, rudder the other. The result is a goofy, cock-eyed attitude and a markedly accelerated rate of decent.

Hope this helps,
John Nagle
http://www.Jobie.com

thanks for the input AP, i''ll definitely hold your ideas in high consideration. unfortunately, the problem that i''m dealing with is related to collisions and resting contacts. for instance, if i fly a plane in IL-2, and have it skim the ground, the craft bounces around, and once the gears break, the fuselage starts skidding until it comes to a stop. now, with collision algorithms, resting contact forces are needed to prevent "bouncing around" on the ground. not sure how to implement the resting contacts.

a2k

As my first flying instructor once said, "Landing is just crashing with style"!

You can either implement the collision response as an impulse based method or a constraint based method. One novel approach is to combine the two. If the resultant restituation energy is less than a chosen threshold, move from an impulse to a constraint model.

The point of my comment above is to remind you that the lift force being generated at the point of touchdown of the main undercarriage is less than the weight of the aircraft, since the wings have (at least partially) stalled. If you''ve got a decent flight model going then the lift force will be proportional to the airspeed for a given angle of attack (and subsequently coefficient of lift). So, as the aircraft rotates nose-up near the end of its decent and reduces power, it starts to wash off airspeed and thus lift. At the point of touchdown the net weight of the aircraft is still quite low, so the landing struts dont take much of a load... that''s why they don''t break unless you try and achieve touch down from 10ft up, or higher! My worst was about 5ft... and I had a sore ass after that one! As the aircraft decreases airspeed the net weight increases toward the resting mass of the aircraft.

is this the sort of information you were looking for? If not, throw this post in the trash bin and please give us some more details on what you need help with.

Cheers,

Timkin

yes, that's the closest i've been looking for so far. so far, the points are being responded to as collisions. in addition, a contact force is applied just to overcome the force of gravity. now, i'm not sure how to implement the "constraint" model that you've suggested. would that be the same thing as walls in quake simply preventing characters from penetrating through?

a2k

[edited by - a2k on July 16, 2002 12:24:00 PM]

Yes, that''s the gist of it. There are many papers available on the web regarding impulse and constraint based collision methods. You should start with the Articles & Resources Link at the top of this page. Also check out Chris Hecker''s Annotated Bibliography.

As to more on flight mechanics, if you''ve got access to a decent library and don''t mind a technical book, I highly recommend Barnes W. MacCormick''s "Aeronautics, Aerodynamics & Flight Mechanics".

Cheers,

Timkin

actually, my physics model is already based on hecker''s demo, not to mention baraff''s papers as well. it''s just the collisions thing that are really getting to me, because papers i''ve read on collisions always discuss single point collisions, but not multiple collisions. baraff has been the closest to deal with this topic with resting contact, but his solution discusses a complex topic with only a few paragraphs.

i''ll dig deeper.

a2k