Another Plane

[KerbMav]

This THING only wants to take off if I boost speed with the rocket engines on the runway - and it only takes off because the runway drops at the and ... and then it wants to pitch over in a backflip ...

I know I should move the rear landing gear closer to the CoM, but then it will sit on its tail ... the back is quite heavy.

An earlier design had half the fuel behind the cockpit, but that made it even more impossible to raise the nose for takeoff.

http://imgur.com/a/jBItN#0

[Amoun]

Hmmmm.......Try to make the CoL more forward?

Gonna be annoying though because it will always bring its nose up.

Perhaps more lift would help?

I would try lift+still move the gears closer to the CoM (fraction or not, it may be sufficient) with a set of gears on the back, angled so it doesn't make contact with the runway unless you pitch up.

[Jean Deaux]

I found this to be a very handy guide.

http://forum.kerbalspaceprogram.com/showthread.php/46067-Spaceplane-SSTO-Troubleshooting-Guide

[Johnno]

You need control surfaces forward of your CoL, and to move your rear landing gear closer to CoM. Keep moving them right onto the edge of what makes your plane tilt backwards. Also, more lift likely wouldn't hurt if you're stalling straight off.

[Dispatcher]

I don't know the mass of your mod parts, but it LOOKS massive. I'd agree about getting more lift somehow; probably more lift surface. Its good to see that you are using 909s as they are less massive, have good iSP and the thrust is OK.

[wasmic]

Definitely needs more wings. Also, remember that the CoM shifts when you retract the landing gears - to find out where your CoM will be in flight, remove all the gears. You can do like me and only add landing gears after the rest of the plane is finished. The reason for the shifting CoM is that gears, for some reason, don't have a variable drag. Instead, they simply set their mass to 0 when retracted, and to the normal reading when extended.

[ChevronTango]

I don't know the mass of your mod parts, but it LOOKS massive. I'd agree about getting more lift somehow; probably more lift surface. Its good to see that you are using 909s as they are less massive, have good iSP and the thrust is OK.

I cannot emphasise this enough, and my evangelism across many threads in this forum seems to be doing little to get this point as common knowledge. More lift is indeed what a plane sticking to the runway needs however the runway is a special case. With an angle of attack of zero the wings generate absolutely no lift at all. Whats worse is that if there is a fractional downward angle, say by the front landing gear being slightly higher (even a miniscule ammount) which causes the nose to point down, then you end up generating lift INTO the runway (counter intuitively, but think about when you point the nose down mid flight, or perhaps roll 180 degrees whilst pointing up, the effect is the same conceptually). The solution is either more control surfaces, which on command input will generate a large Torque in the right direction, or to angle the wings upwards slightly using alt+WASD, which assures the wings are in fact generating lift upwards. Adding more wings naievely without adjusting the angle of attack will not affect a solution and in some circumstances may make the problem worse.

When you fall off the end of the runway the torque allows the wings to point upwards and they suddenly generate lift, which is why the plane suddenly flies perfectly. With smaller aircraft the problem isn't noticeable because the control surfaces induce a very large torque relative to the mass so you can pitch up easily, then generate lift off the runway.

[BubbaWilkins]

Chevron covered most of it.

But if it flies at all, then the first and foremost issue is landing gear placement. Your rear landing gear need to be near/slightly behind the CoM. This allows them to serve as a pivot/leverage point for your control surfaces to push the nose into the air. With the gear configured in the images, all your control surfaces are doing are pushing the landing gear into the ground harder.

[KerbMav]

Seems like Id have to start over again mostly.

It flew once, with all engines full on the runway to gain enough speed and youre right, it only took off as the runway ended. It veered in all directions, up mostly and to the sides - and suddenly it flew straight like high class passenger liner ... until up to 6-7000m where it dropped its nose and started dancing again - chuted my Kerbals out.

Problem with the landing gear is, that if I move it closer to the CoM, the plane will tilt back and plant its bottom on the runway.

To do: move the CoM, add more wings, increase AoA, raise front gear, maybe add two more jet engines.

[Dispatcher]

"Adding more wings naievely without adjusting the angle of attack will not affect a solution and in some circumstances may make the problem worse." -- ChevronTango.

CT, I don't know why you quoted me and then made your remark about naively adding wings. Take a look at KerbMav's second photo and you will see that his forward wings are indeed pitched upward. I looked at that before I commented. I think he needs to either add more lifting surfaces (pitched up) or maybe he could move the forward wings ... forward.

[TheHengeProphet]

Okay, so... one issue I see here is that your center of lift is ON your center of mass. This means your plane will be rather manoeverable (if it gets in the air), but not for long as it will lose aerodynamic stability as the CoM shifts stern.

Judging by the parts used, that thing is obscenely heavy, and the surface area of your wings will not provide enough lift at the speeds you would be able to achieve by the end of the runway in order to perform liftoff. To my knowledge, that pod provides torque, which means you should probably move the reaction wheel module to the other side of the craft, or at least more toward CoM.

Given the design of your craft, I would move the main wings back, sligtly, add more wing to them, and add a set of canards to the front. I would also move any exhaustible sources of mass (Liquid fuel, oxidiser, payload bay) closer to the rear of the craft, and non-exhaustibles (battery, cockpit, RCS fuel-exhaustible, but less likely to be used up) to the front of the craft. Doing so will help ensure continued flight stability. Also, that is a heck of a lot of RCS fuel you are lugging around, so I would suggest dumping that huge tank for some radial tanks instead, lest you intend on fueling something in orbit with it.

Good luck on your venture, sir!

[KerbMav]

Okay, so... one issue I see here is that your center of lift is ON your center of mass. This means your plane will be rather manoeverable (if it gets in the air), but not for long as it will lose aerodynamic stability as the CoM shifts stern.

As I observed during my first real test flight - the first time it didnt salto mortale into the bay.

Judging by the parts used, that thing is obscenely heavy,

Nah ... about 30-50t ... depending on the iteration of the design ...

and the surface area of your wings will not provide enough lift at the speeds you would be able to achieve by the end of the runway in order to perform liftoff. To my knowledge, that pod provides torque, which means you should probably move the reaction wheel module to the other side of the craft, or at least more toward CoM.

Position of the reaction wheels matters now??

Given the design of your craft, I would move the main wings back, sligtly, add more wing to them, and add a set of canards to the front. I would also move any exhaustible sources of mass (Liquid fuel, oxidiser, payload bay) closer to the rear of the craft, and non-exhaustibles (battery, cockpit, RCS fuel-exhaustible, but less likely to be used up) to the front of the craft. Doing so will help ensure continued flight stability. Also, that is a heck of a lot of RCS fuel you are lugging around, so I would suggest dumping that huge tank for some radial tanks instead, lest you intend on fueling something in orbit with it.

I can try that, but I started out with much more fuel at the aft of the craft and only now managed for the plane not to sit down on its bottom.

RCS - I thought, if I am going up there, I could as well resupply my space stations reserves for other crafts to refuel.

[KerbMav]

Now what ... there is absolutely no reason for the CoL to be where it is ... updated CoL in FAR GUI, ran every simulation, saved, loaded ...

http://imgur.com/a/1HFcr#0

And it is not only a display bug, as soon as the plane takes off, it starts to roll uncontrollably to the right.

Edited September 9, 2013 by KerbMav

[tizalee]

You can adjust the weight to this problem. For this you can have engine slightly forward. Remove any unwanted weight from its tail. Try to take off at full speed. Use longer runway to get time for speeding up the plane.Try to make it much lighter than before.

[TheHengeProphet]

It is possible that some part attatched to your extremities has duplicated. I have found this to happen when moving some wings with attatched bits, which can mess up your CoL. Also, keep in mind that your air intakes cause drag, so if you have them ahead of the center of mass, you might expect the effect of a CoL ahead of your CoM, even though the displayed indicators to not state such.

[ChevronTango]

"Adding more wings naievely without adjusting the angle of attack will not affect a solution and in some circumstances may make the problem worse." -- ChevronTango.

CT, I don't know why you quoted me and then made your remark about naively adding wings. Take a look at KerbMav's second photo and you will see that his forward wings are indeed pitched upward. I looked at that before I commented. I think he needs to either add more lifting surfaces (pitched up) or maybe he could move the forward wings ... forward.

The critism wasn't of your intentions, merely of the wording, which I wanted to clarify less for you, and more for others reading this thread further down the line. Understanding how lift works in Kerbal is not well documented or well understood universally so I just wanted to emphasise the common misconception that most people infer from simply reading "it needs more lift".

Moving the wings about won't really help that much in my mind, but worth a shot. My recomendation is to increase the torque at rotate speed by increasing the number of control surfaces, and placing them strategically around the COM and the contact points on the runway to increase the ability to pitch up. Pitching the nose up by repositioning the landing gear would also do a massive amount to help.

Edited September 9, 2013 by ChevronTango

[KerbMav]

The plane by now stands stable on the runway and rolls without much veiring, it even takes off well before the end of the runway.

My problem now is that the CoL suddenly and for no visible reason is not centered anymore without me changing anything drastic.

It is possible that some part attatched to your extremities has duplicated. I have found this to happen when moving some wings with attatched bits, which can mess up your CoL. Also, keep in mind that your air intakes cause drag, so if you have them ahead of the center of mass, you might expect the effect of a CoL ahead of your CoM, even though the displayed indicators to not state such.

I used PartLister to check for this and anything I missed out on not being symmetrical.

The intakes are symmetrically placed too.

This has seized to be a project to get freight to my space station - it has become personal now ...

Edited September 9, 2013 by KerbMav

[ChevronTango]

My problem now is that the CoL suddenly and for no visible reason is not centered anymore without me changing anything drastic.

Several things could be causing this. Unbalanced fuel flow, part clipping or perhaps something to do with Landing Gear having no mass and drag. My advice would be to check the symetry on all the parts by removing them and then replacing them carefully in the Hangar, if the problem is a displacement perpedicular to the direction of travel. If the displacement is parallel to the direction of travel then I'd speculate its a fuel flow issue. Its a case of debugging it now to find the solution but if your stubborn enough you'll find the cause. Maybe post the craft file for it on here so we can take a look aswell.

[KerbMav]

Took off almost everything wingy - nothing. But: When I took off and reattached the small wings with the jet engines the COL was as centered as never before ... Butbut: The plane still rolls to the right.

Navball looks OK. There is the cockpit, a small "autopilot" probe core somewhere attached in the original orientation the VAB offered me and one big "SAS" wheel - all three being there since I started building this monster. But since the other fiasco ... I will rip them off and built in new ones.

What is getting to me is the wild way these problems turn up - first the thing wouldnt take off at all. Then it was unstable until a certain speed/height and flew straight up to 10k to then get out of control again. While trying to fix this the CoL drama started bringing with it the rolling after take off - which seemingly had nothing to do with the shifted CoL at all as it persists even after the CoL is straight!

And if that wasnt enough, before the last trial yesterday it split in two between the bay and the batterie! Which brings me to the conclusion, that there is a big problem with the SpaceJunk Cargo Bay and I should replace it with something else for testing.

Will post new pictures and try to find an easily accessible upload site.

[ChevronTango]

Took off almost everything wingy - nothing. But: When I took off and reattached the small wings with the jet engines the COL was as centered as never before ... Butbut: The plane still rolls to the right.

Well I'm glad progress is being made. Most plane's have some small exentricities that require trimming, otherwise it would be too easy and the game wouldn't need (A)SAS at all. Most RL plane's have some anomalous behaviour unique to them, which pilots often become accustomed to, so airline's tend to keep pilots on the same few aircraft to keep things comfortable.

turn on your ASAS and see if the problem goes away, I imagine it would. Best of luck with the future endavours though.

[KerbMav]

Here, take it, I cant take it anymore!

https://www.dropbox.com/sh/pld0n9smpl2w5ly/YjA-qjznv4

You will need:

SpaceJunk Bay (old version, but problem occured with the renewed mod too)

B9 M27 Overview Cockpit

AIES Superior-V3 Fuel Tank

the resized stock landing gear (whose part2.cfg is also in the box)