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Spemble

Stable Jet design

Question

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

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55 minutes ago, Spemble said:

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.

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

One place that passive stability comes from is from having low drag ahead of the CoM, and high drag behind the CoM, as you seem to understand. Try reducing the friction on your front wheel down to .3 or below, and the friction on your rear wheels to 1.5 or above. Also, those medium landing gears are one size bigger than you could possibly need -- and I've seen it happen that using extra heavy landing gear can cause this sort of veering. So try knocking it down to the "Small retractable"s, and do the friction trick. Additionally, did you use "Absolute Rotation Mode" to make sure your wheels are perfectly straight? Sometimes that helps too. There are many other tricks to prevent veering, and you are about to get bombarded with a lot of suggestions.

 

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You should never ever need to adjust the friction settings of the wheels, @bewing; that design, for example, has plenty of ground drag at the back at low speeds or at rest. It's problems are entirely aerodynamic.

 

The issue is very straightforward. It's CoL is so far back (and more or less aligned with the rear gear) that what happens is that the rear gears are being lifted off the ground, reducing their traction and drag, and causing the wheelbarrow effect. The rear gears and CoL being so far back also make the pitch-up rotation on takeoff difficult to achieve.

It looks to me like that design should have fairly minimal CoM-shift as fuel burns off (in fact I think there's more dead mass on the nose than the tail, which suggests the CoM will actually pull forward with burnoff). My solution, then, would be to, in order:

 

1. Remove all the gear;

2. Add a set of canards (the advanced ones, probably, but it's also small enough that you might be able to get away with a naked elevon) to the nose;

3. Slide the main wing forward until the CoL is dead center on the CoM;

4. Add the gear back (probably one size smaller than current; they're a bit over-sized), placed in the trike arrangement it currently has but with the main gear just barely behind both the CoM and CoL. The nose gear's larger lever arm will act to pull the CoM ahead of the CoL again, making the plane stable.

 

What this will do is produce a much more manouverable, but still stable, plane that will readily rotate up for takeoff and whose aerodynamics will make the nose tend to lift off first when on your takeoff run, keeping you stable on the ground.

 

 

12 hours ago, bewing said:

Well, you also have to multiply by the lever arm. So the one front wheel is probably a lot further ahead of the CoM than the rear wheels are behind it.

 

Are we looking at the same design here? That plane has a midbody CoM and wheels at both the front and tail with approximately the same lever arms :P

Edited by foamyesque

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Yes,  you haven't even left the ground yet.  It's not an aerodynamics problem it's an automotive one. Like a shopping trolley that won't go straight.

Quote

Additionally, did you use "Absolute Rotation Mode" to make sure your wheels are perfectly straight?

to expand on this - go to the Hangar,  make sure angle snap is set to ON, and that you are in rotation adjust mode (3).  Select your gear.  Press F on the keyboard and you'll see a message showing you are toggling between "absolute" and "local" rotation.   Make sure it's in absolute mode, then rotate the gear one notch of yaw , then snap it back to straight ahead again.   This will take out any stray rotation that crept in.  To be safe, do that for all three axes of rotations

 

You might know this, but in pushing the centre of lift so far back you've created an aerodynamic problem.      That plane will suffer excessive "trim drag" in flight , ie it will need a lot of force to hold the nose up, and creating that force will create a lot of drag.  

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!

Centre of lift - Ideal placement and payload/consumables

 The ideal is to have the centre of lift behind the centre of mass, but not MILES behind it.  If you arrange things so that centre of mass doesn't move when fuel burns off, you won't need to leave such a huge margin of safety.

The mass of your engines (at the back) needs to be balanced by the mass of passenger cabins / cockpits (at the front) .

Your fuel needs to be distributed around the CoM so that the mass of the fuel ahead of CoM balances the stuff at the rear.  Be aware that distance from CoM counts.    Eg. 10 tons of fuel close behind the CoM can be balanced by one ton in front if the front tank is well forward.   Real airplanes call these "trim tanks".

Examples of cg - neutral layouts

>Engine][Aft Fuel Tank][Cargo Bay][Fwd Fuel Tank][Cockpit>

                             ^
                            CoM


or maybe put the fuel in sponsons either side of the cargo bay

        <         Fuel        > 
>Engine][      Cargo bay     ][Cockpit>
        <         Fuel        >
          
                   ^
                  CoM

          If your engines are too heavy to be balanced by the cockpit, you need them either side of fuselage and not so far aft.  The epitome of this is the Skylon design.
        
                >Engine>
<Aft Fuel Tank][Cargo Bay][Fwd Fuel Tank][Cockpit>
                >Engine>
                
                    ^
                   CoM

 

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2 hours ago, Spemble said:

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? 

Your OP question tends to get asked fairly often. So it's actually in the FAQ. Scrolling back a couple pages through this forum would also locate a few threads where folks asked basically the same thing. Or, if your search-fu is strong, you can try to get the forum searcher to cough up a relevant thread.

But it seems pretty clear that the next time you get stumped, the problem is going to be much more complex. :) So it's unlikely that you will be able to avoid asking more questions on here. Diagnose as best you can, and then don't be afraid to ask. There are lots of questions dumber than yours that get asked. :wink:

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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

 

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Control surfaces!

 

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.

Always think of an aircraft as a balancing act. The farther the push, the more effect it has. fulcrum style. Closer to CoM, less effect it has.

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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.

Edited by Spemble

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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...

Edited by Spemble

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1 hour ago, Spemble said:

I probably just assumed that there would be more friction in the back because there are two wheels.

Well, you also have to multiply by the lever arm. So the one front wheel is probably a lot further ahead of the CoM than the rear wheels are behind it.

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