[Jet Aerodynamics]

I just made this as a joke, to test the wings

It flopped around like a dead fish around the Space Center (and broke the front landing gear), but then left the computer to come back to it half way around kerbin, skipping across the top of the atmosphere. ._. "Stable rocket" like a slap in the face. Thanks KSP. <3

4 minutes ago, Geschosskopf said:

1.  Give it enough thrust and wing area to fly at all.

2.  Build it so the CoM doesn't move much or at all as the fuel burns.

3.  Build it so the CoL is slightly behind the CoM.

4.  Turn on SAS.

I have done this countless times, but sometimes the plane flips around anyways, even with SAS. I'd really like to understand how KSP models it so I can build crafts that will reliable, and thus know how to fix things in the future, if another problem presents itself.

[Jet Aerodynamics]

Well, after hours of testing, reading, and testing again, I am still having a really hard time understanding the aerodynamics.

Previous post:

Even though my previous post was ironically the only ground drag issue that came into the equation with trying to make a stable jet, I'm still trying to build a stable jet. Rather, understanding how to make one that is.

I know CoM, CoT, and CoL, but I don't understand their results. There are numerous claims that if the CoL is behind the CoM, then the rocket will be stable.

KSP wiki:

http://wiki.kerbalspaceprogram.com/wiki/Center_of_lift#Flight_characteristics

Quote

Flight characteristics

Different flight characteristics are desired in different craft. If the COL or the lift vector is positioned or pointing off-center left-to-right, the craft will turn. If the COL is positioned too far toward the bottom of the craft, it will want to roll over. Assuming the COL is centered side-to-side, and possibly top-to-bottom, performance is basically as follows:

• COL ahead of COM → Flips or turns uncontrollably
• COL inside COM → Excessively maneuverable; needs constant course correction
• COL closely behind of COM → More maneuverable
• COL further behind of COM → More stable
• COL very far behind of COM → Very hard to correct course; likely to flip up or nosedive

foamyesque:

Quote

"pull the pull the CoM ahead of the CoL again, making the plane stable.

But my mind is vehemently refusing to accept this. Don't you guys mean the "center of drag"? I feel like this is very similar to the CoL, but is still different. You could have a ship with two wings in the front and then a bunch of random parts on the back, and would fly with the drag-inducing parts in the back like an arrow with feathers. Yet, the CoL would be right between the two wings, because they are the only things providing lift. I've had planes where the CoL is significantly behind the CoM, but it still wants to flip at high speeds, or at least doesn't want to just go straight.

And this flows into my second question.

You know, let's make a list. Welcome to the buffet of questions for all your answering needs:

1. Is the CoL the same thing as the "Center of Drag"?
2. If not, is there a good way to find how far the CoD (center of drag) is behind the CoM?
   1. I've been shooting my crafts way up in the atmosphere and then letting them fall to see if the CoD is behind the CoM. Kinda time-consuming.
3. If all parts create lift (KSP wiki quote below), why don't they change the CoL in-game?

   Quote

   As of version 1.0, all parts generate some lift as well as drag; however the lift-to-drag ratio is much higher from aerodynamic parts such as wings, control surfaces, etc. KSP also models drag created as a consequence of lifting force.

   1. KSP wiki: http://wiki.kerbalspaceprogram.com/wiki/Lift#Dynamics
4. The wings are symmetrical. Do they still produce vertical lift in ksp if they are not given any angle of attack?
5. Are there specific wings that do/don't produce vertical lift at 0° angle of attack?

I'm really sorry I'm asking the same question that everyone always asks, but this is a little different.

Thank you so much for taking the time 

- Spemble

[Stable Jet design]

20 minutes ago, AeroGav said:

Also, you are controlling pitch by pushing DOWN with the elevons,  but the airplane needs to pivot around it's main landing gear to get the nose up, and the pivot is behind the elevons so that's actually impossible.  It should have lifted off before you took your second pic.     Fix - either move the main gear forward to just behind centre of mass,  or set the elevons to control roll only and add some canards to "lift" the nose up, which works no matter how bad your landing gear is!

 

Ah! I didn't think about this, I just thought about what the control surfaces would do in flight! Those control surfaces in the back aren't going to push the nose up unless they are behind the back wheels.

Just as a note, I didn't press any keys in the pictures except z and space to put the throttle to full and go. That was all on its own.

Just as a quick question for the future, what would be the best way to figure these things out myself so I don't have to bug you guys? I guess just try to diagnose the problem and go through the different things that could cause that? For example, how would I diagnose the problem of the plane spinning in a cone shape (not just rolling, but firing the engine backwards in a cone)? It's hard to distinguish what could be causing it...

[Stable Jet design]

58 minutes ago, Raideur Ng said:

You need control surfaces! Well.. you need to move your center of lift closer to the center of mass, but mostly, you need a pair of control surfaces ahead of the center of mass to act as a fulcrum to allow you to maneuver. A pair of fins around the cockpit will help immensely.

Yes, control surfaces can help the problem, but they are just a cover up. I want it to be inherently stable, without SAS.

But you have a good point that the center of lift needs to be closer. I didn't take the time to think about the center of lift, but now it makes sense with the nose pointing down. The center of lift in the back is just ... well... lifting the back, thus rotating the the nose down.

[Stable Jet design]

19 minutes ago, bewing said:

You have a lot more aerodynamic drag at the back, yes. But not ground drag.

Ah, that's the trick. I don't know why I didn't think about the ground drag, I probably just assumed that there would be more friction in the back because there are two wheels. While this should be true usually, I also found that the jet was naturally pointing downwards, so it was probably digging the front into the pavement, making more drag/friction.

I purposely replaced the small with the medium landing gear because they are more forgiving for landing, and I wanted it to be really durable. And yes, I did use absolute mode, but that's also a good tip - ran into it before.

Now that part is pretty good, on to other things that need "debugging" with it. :/

Thanks bewing, - a thanks from Portland!

- Spemble

 

[Stable Jet design]

Hello

Why does this plane turn?

 

I know a fair amount about the center of mass and center of thrust, (not as much about the center of lift), but with this design,

there is clearly a lot more drag at the back, and like a badminton birdie, it should want to go straight as a toothpick. Nope.

 

Plz help.

I love ksp so much and want to go explore and do amazing things, but it is things like this that drive me batexcrements crazy and irritate me to the point of not playing.

I don't know how to solve these things myself through deduction, I've tried so many times.

 

Thanks in advance.

 

http://imgur.com/gallery/r3juG