Plane pitching up after fuel consumed

[Laughing Man]

Ok I'm sure there's an answer to this somewhere but I can't seem to find it so I'll just go ahead and ask:- I've got a neat little plane 3 fuel tanks long. After a long-ish flight and the front tank is empty it starts to pull up like there's no tomorrow. I've got the CoL just a tiny tiny tiny tad in front of the CoM to help with take-off so I can't really push the wings any farther back. I've toyed with the idea of putting a fuel line between the front tank and the rear so the rear one goes first, but now that I know fuel consumption negatively affects the performance of the plane I'm reluctant to do this.

The only change I've made to the design so far is to swap out the normal sas for the more advanced one and I'm flying the thing atm to see if it's made a difference. Other than that I can't really think of a solution to this particular problem. Has anyone found something that can help?

[Vanamonde]

Pictures of your craft always help diagnosing problems. Can you post some?

[Laughing Man]

Sure, just need to finish this flight. I should mention I'm trying to fly from ksc to the north pole, the plane is fairly stable so I just leave the sas on and go into steam browser, coming out every so often to adjust pitch.

[Taki117]

What I might suggest is a Mod like TAC Fuel Balancer so that you can draw fuel from all your tanks at once. Also, if you are just using Liquid Fuel and jet engines, you might not actually need all that much to get there.

[Tank Buddy]

Well, you should have your CoL behind your CoM, so you don't have any pitching up because the CoL is too far forwards after fuel drains. You could also stop the fuel flow from the front tank by clicking the arrow in the right-click menu.

You can place the rear landing gear on the CoM to give you more torque when lifting off.

P.S. All SAS modules have the same torque, so changing the SAS won't help, only adding more.

[Laughing Man]

http://i1373.photobucket.com/albums/ag385/Callum_Booker/2014-03-30_00009_zpsc0bbc040.jpg

http://i1373.photobucket.com/albums/ag385/Callum_Booker/2014-03-30_00010_zps5499b6ec.jpg

http://i1373.photobucket.com/albums/ag385/Callum_Booker/2014-03-31_00002_zps24c0ddcc.jpg

I was able to make it to the North in the end, but the plane kept doing loops at random intervals after the first tank went then again after the middle went. The weird thing is, after I was able to regain control it went back to being ok again so long as the sas was on.

EDIT: CoM and CoL - http://i1373.photobucket.com/albums/ag385/Callum_Booker/2014-03-31_00003_zpsa59762e4.jpg

EDIT2: Isn't there a way to transfer fuel between tanks with CTRL and +? I tried this but it wouldn't work.

[MockKnizzle]

If your CoM is already behind the CoL will full tanks, then as the front tank drains it will shift further back and exacerbate the problem. My suggestion is adding canards or changing the design of the wing some other way so that the CoL is more properly positioned relative to the CoM. Make sure to check other fuel loads by using the tweakables to empty the front-most tanks so you can see if your setup works in the worst-case balance scenario.

[mellojoe]

Did you know:

You can right-click on fuel tanks in the VAB/SPH and empty them of fuel? This way, you can see what your CoM/CoT/CoL look like with partial or completely empty fuel tanks. Since learning this, I have drastically changed how I design my planes. I build OUT a lot more than I build LONG. It helps keep the CoM stable as the fuel tanks drain.

GOOD LUCK!

[Laughing Man]

Did you know:

You can right-click on fuel tanks in the VAB/SPH and empty them of fuel? This way, you can see what your CoM/CoT/CoL look like with partial or completely empty fuel tanks. Since learning this, I have drastically changed how I design my planes. I build OUT a lot more than I build LONG. It helps keep the CoM stable as the fuel tanks drain.

GOOD LUCK!

Wow, this helped soooo much! After some fiddling I realised that shifting the CoL back a fraction had a huge effect on how much runway my plane needed to take-off and so I needed more lift. So I added another couple of deltas at the back and shifted the wings forward. It takes off just fine now with the CoL slightly behind CoM with full tanks. It'll climb fine, but now it has a slight tendecy to pitch down which is a little unfortunate. However with the front tank drained the CoL goes forward, which is great cos I liked the plane having a tendecy to climb. So now the longer it flies the better it climbs!

Thanks for the top notch help gents!

[tutrakan4e]

My first SSTO plane crashed right before it landed because the CoM had shifted back, so what I did on my other SSTOs was to pump all fuel innto the front tanks while still in space.

[mellojoe]

... shifting the CoL back a fraction had a huge effect on how much runway my plane needed to take-off ...

Looking at the image you posted here:

EDIT: CoM and CoL - http://i1373.photobucket.com/albums/ag385/Callum_Booker/2014-03-31_00003_zpsa59762e4.jpg

Another tip: The position of your rear wheels controls more than you think about your takeoff profile. Your rear wheels are far behind your center of mass. Think of a lever or a see-saw. Your CoM is your fulcrum, the tipping point. Your rear wheels are positioned where a child's feet would be on a see-saw blocking your rotation. Your craft can't tip upward because the wheels are acting like a FULL-STOP to your see-saw / lever. Moving them forward (if possible) reduces their impact, thus making your plane rotate easier, thus making it easier to lift off. Obviously, you can't stick your rear landing gear so far foward that your plane falls over backward and crashes its arse into the runway, but if you can cheat them forward a bit it could help.

Second tip: Put the rear wheels in a position higher than the front wheel so that the whole craft on the runway is angled up for lift-off. This means that you won't have to actually rotate the craft upward, it is already there from the beginning. This can be acomplished by putting your rear landing gear on the SIDE of the fuel tanks and then angle them back straight up and down. Or put them on the wings instead of the fuel tanks. This gives you an angle of attack while on the runway. You might even find the plane will take off without you even having to touch the controls.

[Laughing Man]

Thanks for the tips I'll try that out. I'm going to keep desining away to work my way up to getting an ssto since I've never built one before. Also while I'm at it; I understand the sas modules have the same torque, does the advanced module have any advantages over the normal sas whatsoever? I mean it weighs 0.2 mass units more so I'm not sure what the justification for having one is...

EDIT: It's a shame the forums got wiped cos I had a topic showing off a rendezvous before the advent of manuevre nodes

[Amaroq]

My first plane design had the same problem. As a quick-fix solution, you can right-click on a fuel-tank, and press the green arrow next to the fuel; it will turn to a red X and cease drawing from that tank.

Since I use the Mark II fuselage, the Mk1-Mk2 adapter was perfect for this, I just disabled it, leaving some weight well forward of my CoM, and can turn it on as a "reserve tank" once everything else is gone.

[Claw]

Ok I'm sure there's an answer to this somewhere but I can't seem to find it so I'll just go ahead and ask:- I've got a neat little plane 3 fuel tanks long. After a long-ish flight and the front tank is empty it starts to pull up like there's no tomorrow. I've got the CoL just a tiny tiny tiny tad in front of the CoM to help with take-off so I can't really push the wings any farther back. I've toyed with the idea of putting a fuel line between the front tank and the rear so the rear one goes first, but now that I know fuel consumption negatively affects the performance of the plane I'm reluctant to do this.

The only change I've made to the design so far is to swap out the normal sas for the more advanced one and I'm flying the thing atm to see if it's made a difference. Other than that I can't really think of a solution to this particular problem. Has anyone found something that can help?

In v0.23, the SAS/ASAS units are no longer different in functionality. So SAS vs. ASAS doesn't do much since most capsules, cockpits, and probe cores come with the ASAS functionality. What does help is more SAS (or ASAS) units, simply because it adds more torque.

Putting a fuel line from the rear tank forward is an okay way to deal with CoM movement in flight. As you discovered from someone else's comment, you can then change the fuel config in the SPH to see where the CoM moves. (So empty the rear tank, etc...) Don't be too reluctant to try things out, worse thing that will happen is you'll learn not what to do.

http://i1373.photobucket.com/albums/ag385/Callum_Booker/2014-03-30_00009_zpsc0bbc040.jpg

http://i1373.photobucket.com/albums/ag385/Callum_Booker/2014-03-30_00010_zps5499b6ec.jpg

http://i1373.photobucket.com/albums/ag385/Callum_Booker/2014-03-31_00002_zps24c0ddcc.jpg

I was able to make it to the North in the end, but the plane kept doing loops at random intervals after the first tank went then again after the middle went. The weird thing is, after I was able to regain control it went back to being ok again so long as the sas was on.

EDIT: CoM and CoL - http://i1373.photobucket.com/albums/ag385/Callum_Booker/2014-03-31_00003_zpsa59762e4.jpg

EDIT2: Isn't there a way to transfer fuel between tanks with CTRL and +? I tried this but it wouldn't work.

Here are my comments for your pictures. There's only two things I would say you "need" to fix, but the rest of the comments are up to you.

1) Typically you'll want to put the rear landing gear wheels slightly behind the CoM.

- This will allow your plane to actually rotate for takeoff, and will also prevent excessive stress on the rear gear (causing them to bend and crash). Watch out for tail strikes...

2) For most designs, the CoL should be a little behind the CoM (as you discovered) and slightly above.

- If it's ahead, your plane will be negatively stable. Which means if it gets off the prograde marker too far (pitch or yaw), it will go out of control. (As you discovered.)

- If the CoL is too far behind your CoM, your plane will be overly stable. Which means it can be difficult to control and will exhibit lawn-dart behavior (constantly wants to pitch down).

(If you want to know why it should be slightly above, let me know and I'll get into that also. Or you can check out this article by Keptin, and this hands on demo by me. They explain a lot of these concepts including landing gear.)

The SAS is capable of compensating for a lot of things, but if your craft gets too far off of neutral the SAS can be slow to respond. You can see how much the SAS is working by looking at the trim indicators on the bottom left of the screen.

Other things to think about:

- You have a lot of aileron area. I would guess that you don't know about tweakables yet, so they probably also respond in pitch. (I.e. when you pull up or push down, they deflect.) This is okay if you want your ailerons to act as elevons, but if not it hurts your lift. If you do want to use elevons (they control roll and pitch), then you'll want to move them back some. Otherwise you can disable the pitch response by right clicking on the aileron and turning off pitch/yaw.

Landing Gear!

- The SPH lies about CoM with the landing gear on. Landing gear are labeled with mass in the SPH, but in flight they are considered massless by the physics engine. So, as such, if you're making fine tune adjustments of your CoL/CoM, remove your landing gear first. Otherwise you will almost certainly end up with a CoM that is more aft in flight than what the SPH indicates (sometimes putting it behind the CoL).

- Avoid tilting landing gear like you did. As your plane designs get a bit heavier, tilted landing gear will cause you problems.

Yes, you can transfer fuel between tanks in flight. You do this by right clicking on one tank. Then hold ALT while right clicking on the second tank. You can then select Transfer "IN" or "OUT" as appropriate. This only works if there's empty space available in your destination tank.

Wow, this helped soooo much! After some fiddling I realised that shifting the CoL back a fraction had a huge effect on how much runway my plane needed to take-off and so I needed more lift. So I added another couple of deltas at the back and shifted the wings forward. It takes off just fine now with the CoL slightly behind CoM with full tanks. It'll climb fine, but now it has a slight tendecy to pitch down which is a little unfortunate. However with the front tank drained the CoL goes forward, which is great cos I liked the plane having a tendecy to climb. So now the longer it flies the better it climbs!

Thanks for the top notch help gents!

I'm not sure why the CoL would drift forward when the front tank is empty. Unless you mean the CoM moves aft (which has the same effect). If your plane wants to pitch down, then it's probably one of two things: 1) your CoM is probably starting too far forward or 2) you don't have enough pitch authority. Move your wings slightly forward or swap out your horizontal stab/canard for one of the canard style flight controls.

If you design your CoL to be just ever so slightly aft of the CoM with your most aft fuel setting (probably with the front and middle tanks empty), that will likely be the best starting spot for your wings. Hopefully it keeps the CoL close enough to the CoM when the tanks are full. If not, then you'll have to mess with fuel lines.

There's a ton of other rules of thumb, but if you're wondering about wings, you can usually get away with something around 0.5 to 1.0 lift rating per ton of aircraft. You can feel free to go well outside of that, but it's an okay place to start. So to me it looks like the plane in the pictures has enough wing/mass as it is. Just needs a few tweaks to help with takeoff/CoM control.

[Claw]

Thanks for the tips I'll try that out. I'm going to keep desining away to work my way up to getting an ssto since I've never built one before. Also while I'm at it; I understand the sas modules have the same torque, does the advanced module have any advantages over the normal sas whatsoever? I mean it weighs 0.2 mass units more so I'm not sure what the justification for having one is...

EDIT: It's a shame the forums got wiped cos I had a topic showing off a rendezvous before the advent of manuevre nodes

I got ninja'd a bit while writing my giant post.

The ASAS + SAS modules used to be different. The SAS only provided torque while the ASAS provided torque and the ability to use flight controls for stability. That's no longer needed as the flight control stability functionality is now included. So the difference is purely cosmetic as of now.

[Laughing Man]

-snip-

Sound advice. I removed the canards, pushed the wings up, moved the landing gear forward, turned off pitch control for the aerilons and did a little tweaking with the CoL (without landing gear). The pitching problem is gone and it's much more stable now, however the CoL is still slightly in front of the CoM once the front tank is empty, but I can live with that. Next time I'll design something wider.

[Claw]

Sound advice. I removed the canards, pushed the wings up, moved the landing gear forward, turned off pitch control for the aerilons and did a little tweaking with the CoL (without landing gear). The pitching problem is gone and it's much more stable now, however the CoL is still slightly in front of the CoM once the front tank is empty, but I can live with that. Next time I'll design something wider.

Honestly, slightly in front is not a big deal as long as you know it's there and are prepared to deal with it. If you're doing space plane design and you have that at reentry, it'll be a problem. But if you have a regular plane with enough pitch control, and you aren't being crazy with it, it's usually fine. You can still run a fuel line forward or lock out the front tank to keep the fuel forward. But if you're happy with it then good enough.

[Voculus]

Laughing Man, if the pitch down tendency is too much of a pain, you can always trim the aircraft for much more stable flight. ALT + W,A,S,D, etc. ALT + X will reset all of the trim. It's an extremely useful tool, no matter if you're using a keyboard or joystick.

[vencaslac]

your plane is in a very slow orbit (it's powered by engines)... it is not changing pitch... the inclination of kerbin's surface varies beneath it just like it does on orbital craft due to the fact that your sas is keeping the plane in the same orientation in refference to the skybox and there's nothing you can do about it except mod a dynamic orientation sas

edit: to put it another way: you're flying a straight line over a curved surface, the artificial horizon indicator assumes kerbin is a flat surface so what follows is that you're flying a curved trajectory. depending on how fast you're going the effect will become more and more pronounced as you move away from the surface faster, it's not lift - it's geometry. your constant downward readjustments just put your plane back on a trajectory that is parallel to kerbin's local surface

Edited April 1, 2014 by vencaslac

[Claw]

your plane is in a very slow orbit (it's powered by engines)... it is not changing pitch... the inclination of kerbin's surface varies beneath it just like it does on orbital craft due to the fact that your sas is keeping the plane in the same orientation in refference to the skybox and there's nothing you can do about it except mod a dynamic orientation sas

edit: to put it another way: you're flying a straight line over a curved surface, the artificial horizon indicator assumes kerbin is a flat surface so what follows is that you're flying a curved trajectory. depending on how fast you're going the effect will become more and more pronounced as you move away from the surface faster, it's not lift - it's geometry. your constant downward readjustments just put your plane back on a trajectory that is parallel to kerbin's local surface

That is true, but it causes a craft to look like it is slowly pitching up. The original craft was rapidly pitching up because the CoL was behind the CoM. The OP subsequently fixed it and was then having some pitch down tendency, which is usually a CoL design or flight control problem.

[Depth]

That also happens in reality at a rate of about 0.1 degrees per 10 km. Because of changes in pressure and airflow during flight, the pilot\autopilot automatically makes that small adjustment to fly level. In KSP the scale is much smaller but atmospheric variations aren't modelled, so unless the plane is trimmed for level flight it will simply fly up to a stall.

The SAS doesn't know you're in the atmosphere and does what it does in space; which is to LOCK your attitude to the star on the horizon your nose is pointing at. If you have enough thrust you'll eventually reach that star, but you'll leave Kerbin first, going up

[Claw]

That also happens in reality at a rate of about 0.1 degrees per 10 km. Because of changes in pressure and airflow during flight, the pilot\autopilot automatically makes that small adjustment to fly level. In KSP the scale is much smaller but atmospheric variations aren't modelled, so unless the plane is trimmed for level flight it will simply fly up to a stall.

The SAS doesn't know you're in the atmosphere and does what it does in space; which is to LOCK your attitude to the star on the horizon your nose is pointing at. If you have enough thrust you'll eventually reach that star, but you'll leave Kerbin first, going up

Also, while real world autopilots are capable of attitude-hold modes, nearly all autopilot usage is in an altitude-hold mode. So curvature of the Earth is automatically accounted for when flying barometric or gps altitudes. Attitude-hold modes that are based off an inertial navigation system also compensate for curvature.