Can't Pitch Up!

[BenKerman]

Hi,

So, I have used the Kerbal Space Program Player's Guide to make a "NiftyPlane 9000."  It is an SSTO spaceplane.  It has two Rapier engines, Big-S delta wings, and a Mk2 fuselage, among other things.

Happily, I can get it to orbit and back down from orbit (sort of -- I had to make a crash landing in the ocean.  The Kerbals survived).  However, after I re-enter atmosphere (Kerbin's atmosphere I mean), it can no longer pitch up.  Actually, now that I think of it, I don't think it was able to pitch up after warping forward several hours (I was trying to land at KSC during the day).  Any ideas why my NiftyPlane 9000 refuses to pitch up after time-warping?  When I try to "pull up" it just pitches down.  When I "pull down" it pitches down.  It looks like the ailerons are just always tilting downwards if I pull up or pull down.

Thanks,

Ben

 

 

Edited December 15, 2019 by BenKerman

[HvP]

It's difficult to say without knowing more about your craft. Posting a picture of the craft would greatly help diagnosis of the problem. I can give you some basic info to start with though.

It sounds to me like you may have multiple pitch, yaw, and roll inputs enabled for all your control surfaces and they will interfere with each other. You should always place your control surfaces in discrete pairs, each relating to the role that they perform, and then make sure to right click them in the space plane hangar and toggle off any function that they would interfere with (yaw, pitch or roll.)

It can be tricky to correctly place control surfaces on a delta wing craft, especially if you have designed it as a "flying wing" with no separate tail-plane or elevator. Your control surfaces may be trying compensate for more axis than they are able to control.

In a conventional airplane design, only the horizontal elevators in the tail will control pitch, so toggle off the roll and yaw in their right click menu. And the vertical rudder should control yaw, so switch off roll and pitch for that. And of course, the ailerons placed on the rear edge of the wing tips should only have roll enabled.

If your design is more like a flying wing with no rear stabilizer then you may simply not have enough leverage in the rear for pitch control authority. Maybe your elevator surfaces are simply too close to the middle of your plane. They need leverage to work, so try and get them to far rear if you can - you can try placing canards near the nose to help with pitch authority.

Another possibility is that your design has the center of lift too far to the rear. This causes the plane to be exceptionally stable, but it flies like a dart with very little deviation possible. This is very common for delta wing planes because most of the lifting surface is far to the back. The center of lift should normally be just behind center of mass. And don't forget to check CoM and CoL with the tanks both full and empty. The game determines which direction to move the control surfaces based on their location relative to the center of mass. If the surfaces controlling pitch right on the CoM then the game can become confused as to which direction they should deploy.

Edited December 16, 2019 by HvP

[Kerbal4]

Hmmm... just today i made a plane that can’t get off the runway. you must be pretty lucky for orbit! But it seems that yours may be a little too heavy at the front. if you bring some weight to the back you may get good. As Hvp said, you may have too many control surfaces, but maybe not enough, try either adding or taking them. . I would recommend making the middle section larger, or smaller wings or engines. have a look at the stock craft for a look at how they work. maybe use spaceplane wings, not delta wings, as they offer more controllability regarding center of mass. hopefully this helps. 

 

btw, this is my first post, please tell me if i am doing  something wrong, I’m kinda new.

[AHHans]

2 hours ago, Kerbal4 said:

Hmmm... just today i made a plane that can’t get off the runway.

That probably means that you don't have enough control authority to pitch up on the runway. Check where you have your elevators - the control surfaces that control pitch - in relation to the COM and the landing gear. In a traditional setup - with the elevators at the tail - you need to be able to push the tail of the plane down in order to pitch up. If the landing gear is too close to the tail, then the leverage arm can be too short to push the tail down and nose up.

[pmborg]

3 hours ago, Kerbal4 said:

Hmmm... just today i made a plane that can’t get off the runway. you must be pretty lucky for orbit! But it seems that yours may be a little too heavy at the front. if you bring some weight to the back you may get good. As Hvp said, you may have too many control surfaces, but maybe not enough, try either adding or taking them. . I would recommend making the middle section larger, or smaller wings or engines. have a look at the stock craft for a look at how they work. maybe use spaceplane wings, not delta wings, as they offer more controllability regarding center of mass. hopefully this helps. 

 

btw, this is my first post, please tell me if i am doing  something wrong, I’m kinda new.

Hello you made it congratulations, to do this task the following:  (is very useful to turn on cog (centrer of gravity) and coLift (center of lift) in ship editor)

rules make the task easier:

• You should have cog in front of coLift (allow a easier stall recovery)
• the main wheels should also be located below coLift (easier to rotate)
• Then is just a question of have a arm (distance from cog and elevons) bigger enouf and elevons on tail big enouf to liftoff at rotation speed.

Edited December 16, 2019 by pmborg

[Zhetaan]

4 hours ago, Kerbal4 said:

btw, this is my first post, please tell me if i am doing  something wrong, I’m kinda new.

Welcome to the forum!  It's nice to see new people here.  You're not doing anything wrong, though I will say that you misread HvP's post.

4 hours ago, Kerbal4 said:

Hmmm... just today i made a plane that can’t get off the runway. you must be pretty lucky for orbit! But it seems that yours may be a little too heavy at the front. if you bring some weight to the back you may get good. As Hvp said, you may have too many control surfaces, but maybe not enough, try either adding or taking them. . I would recommend making the middle section larger, or smaller wings or engines. have a look at the stock craft for a look at how they work. maybe use spaceplane wings, not delta wings, as they offer more controllability regarding center of mass. hopefully this helps.

Note that @HvP did not say that there could be too many control surfaces.  HvP said that there could be too many control inputs.  That is not the same thing.

Too many control surfaces is something that can happen, but it usually results in an unstable plane with too much control authority--in other words, tapping the ailerons to level the plane instead results in flying upside down, and trying to go back the other way results in flying sideways, because you have so much control that you cannot make fine adjustments.

Too many control inputs means that each control surface tries to operate in all three axes--pitch, yaw, and roll.  Normally, planes devote a control surface to controlling one axis at a time:  elevators control pitch, rudders control yaw, and ailerons control roll.  Occasionally, one surface will be used to manage two axes as in the case of elevons that manage pitch and roll (in fairness, though, I've never seen elevudders or rudderons).  KSP doesn't know that a given control surface is supposed to be an elevator and another is supposed to be a rudder--even though some parts are called elevons in the VAB, nothing prevents you from using them as rudders (or as landing legs, if they've the crash tolerance for it).  The game provides a way for you to instruct the parts to operate only for pitch control, but you need to select it when you place the part.  If, for example, you leave your rudder set to roll rather than allow it only to control yaw, then an attempt to roll will also turn the rudder, even though its total contribution to roll is, at best, two or three percent of its contribution to yaw.  The result is that the plane rolls (provided that you have ailerons as well), but rather than rolling while maintaining forward motion, it also yaws and invariably pitches down.  With a full complement of control surfaces, they all end up fighting one another.

The other part of that problem is what @AHHans mentioned.  When your wings are too close to the centre of mass, the control surfaces on the wings are also too close to the centre of mass.  This creates two problems.  One is that the surfaces lose control:  if you imagine that the control surfaces apply a force like a wrench turning the plane about its centre, then having the control surfaces father from that centre provides a longer wrench, more leverage, and thus more control.  Having the surfaces too close to the centre shortens the wrench to the point that it has no control.  Worse, if the centre of mass moves about because of a shifting fuel load, then the control surfaces that were slightly to the rear of the plane may find themselves slightly to the front once the centre of mass moves past them.  Since KSP moves those surfaces according to their relationship to and direction from the centre of mass, it means that elevators that work correctly on the runway may suddenly switch and work backwards at some point in flight.

I'd also like to see a picture of the plane in question.  It doesn't make sense for time warp to change control surface function, but I've seen stranger things.

[BenKerman]

Here are some screenshots.  Thanks for the suggestion that the issue has to do with multiple inputs enabled for the control surfaces.  Would you say that's an actual aerodynamic behavior or is it a glitch in the game?

I'm sure I've seen odd glitches in the game.  For example, I landed something at an angle once, and it stayed stuck at that angle.   The craft was balancing on a single leg while the others floated high in the air.  The other legs should have dropped down to the ground.  I can attach a screenshot if anyone wants to see it.  Besides that, before I landed that craft, if I turned off SAS, the craft would start to spin wildly.

I did remove a "floating battery" which was living inside the tail section (the Mk2 to 1.25m transition piece).  I mean, I was trying to attach the battery to the tail, but the battery wound up inside the tail.  I decided to ignore it.  After this pitch up problem happened, I went back and removed the battery.  Afterwards, the pitch up problem went away.

As I said before, this plane is not my own design, it's from The Kerbal Player's Guide, by Manning, Nugent, Fenwick, Allan, and Buttfield-Addison.  Amazon link here:  https://news.google.com/?hl=en-US&gl=US&ceid=US:en

 

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Edited December 17, 2019 by BenKerman

[Kerbal4]

7 hours ago, BenKerman said:

Here are some screenshots.  Thanks for the suggestion that the issue has to do with multiple inputs enabled for the control surfaces.  Would you say that's an actual aerodynamic behavior or is it a glitch in the game?

I'm sure I've seen odd glitches in the game.  For example, I landed something at an angle once, and it stayed stuck at that angle.   The craft was balancing on a single leg while the others floated high in the air.  The other legs should have dropped down to the ground.  I can attach a screenshot if anyone wants to see it.  Besides that, before I landed that craft, if I turned off SAS, the craft would start to spin wildly.

I did remove a "floating battery" which was living inside the tail section (the Mk2 to 1.25m transition piece).  I mean, I was trying to attach the battery to the tail, but the battery wound up inside the tail.  I decided to ignore it.  After this pitch up problem happened, I went back and removed the battery.  Afterwards, the pitch up problem went away.

As I said before, this plane is not my own design, it's from The Kerbal Player's Guide, by Manning, Nugent, Fenwick, Allan, and Buttfield-Addison.  Amazon link here:  https://news.google.com/?hl=en-US&gl=US&ceid=US:en

 

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might want to bring the wings forward a bit. Also maybe some screenshots with the COM and COL overlay. just instinct tells me that this is a bit front heavy and would have trouble pitching up. 

[HvP]

On 12/16/2019 at 7:07 PM, BenKerman said:

Here are some screenshots.  Thanks for the suggestion that the issue has to do with multiple inputs enabled for the control surfaces.  Would you say that's an actual aerodynamic behavior or is it a glitch in the game?

Your screenshots seem to confirm that all control inputs are enabled for all your control surfaces. You should deactivate yaw and roll for the largest inner pair of Big-S elevons at the rear of your plane, leaving the pitch input active. Then, for the outer pair of elevons at the tips of the wings, deactivate pitch and yaw, leaving only roll active. For the vertical tail fin make sure that roll and pitch are off and leave only yaw activated.

The reason for this is not really a glitch in the game, it's just the nature of needing to isolate inputs for effective control so that you don't get crossed signals. There's just no reason that a vertical rudder should be trying to respond to pitch inputs - it won't be able to do anything but mess up the yaw of the craft. Additionally, having roll input also assigned to the control surfaces for pitch will often cause them to operate asymmetrically, lessening their effectiveness.

The problem is exacerbated when SAS is turned on. Because SAS is constantly trying to keep the aircraft flying straight under most circumstances it is always adding a few degrees of roll here and a few degrees of pitch there - but because roll and pitch are both assigned to the same control surfaces it can't add roll without ALSO adding pitch. Sometimes this results in overcompensation, sometimes the inputs cancel each other out, and sometimes they end up doing the opposite of what they should be doing. The answer is usually to keep the inputs and outputs separate and discrete.

As for the general design of the craft, as @Kerbal4 implied, the wings are quite far back in delta wing craft like this which often causes very stable, very flat flight characteristics. In other words, they are inherently hard to get upwards or downwards pitch under the best of circumstances. Or alternatively, they may not have enough lift in the front causing them to nose down. In this case however, those tendencies are probably offset by the wing segments near the forward section of the craft, the lifting body properties of the MK2 fuselage, and the very large elevons in the back.

It'd try it first with the control inputs isolated for only the pitch, yaw and roll inputs required by each pair of control surfaces. And then if you are still having difficulty with the controls report back and let us know what is happening.

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I landed something at an angle once, and it stayed stuck at that angle.   The craft was balancing on a single leg while the others floated high in the air.

This could have been due to very strong reaction wheel torque, or an odd buggy characteristic of the landing gear and wheel physics in the game. The Unity platform that the game is based on has had strange and unpredictable behavior with the wheels, landing gear and landing struts for quite a while that sometimes cause jumping around, weird bouncing oscillations, or gliding across the surface. I wouldn't be surprised if it also caused some craft to become stuck at odd angles.

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Besides that, before I landed that craft, if I turned off SAS, the craft would start to spin wildly.

There are any number of reason for a plane to start spinning without SAS. It could have just been an unstable design that was barely kept level by SAS. Or sometimes part clipping and "Kraken" forces can cause wild buggy behavior. The jury is out on that one.

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I went back and removed the battery.  Afterwards, the pitch up problem went away.

I've not seen this behavior before. Batteries don't usually have much effect on flight physics at all. They don't even really have any physical characteristics themselves except that they add a little weight and drag to the part they are attached to. Is it possible that you also accidentally attached an extra copy of the control surface inside the plane? I've done that before and the results were not pretty.

I've not investigated this design from The Kerbal Player's Guide (I think you may have pasted the wrong link) but I'm sure it's a tested and functional design. Was it this plane that started working correctly after you removed the battery? If so, I still think it could benefit from isolating the control inputs.

Edited December 18, 2019 by HvP

[Vanamonde]

Whatever the actual aerodynamics may be, I have always found it impossible to get a KSP aircraft off the ground if the only pitch control is at the back. I always make either wings-and-tail configurations or wings-and-canards. 

[Vince_K]

3 hours ago, Vanamonde said:

Whatever the actual aerodynamics may be, I have always found it impossible to get a KSP aircraft off the ground if the only pitch control is at the back. I always make either wings-and-tail configurations or wings-and-canards. 

try to put yours rear landing gear near the center of mass. it's more easy to lift the nose.

[Vanamonde]

2 hours ago, Vince_K said:

try to put yours rear landing gear near the center of mass. it's more easy to lift the nose.

Yes, even with that it still doesn't seem to work for me. 

[HvP]

21 hours ago, Vanamonde said:

Yes, even with that it still doesn't seem to work for me. 

From my understanding, what you're experiencing is an inherent property of tailless delta-wings at low speeds. The delta shape just plain (haha) requires high angle of attack to gain lift without a bigger pitch lever either from a rear tail-plane or canards. I also prefer not to use them unless they happen to solve some weird weight/lift distribution puzzle for a particular design.

The real advantage of delta-wings is to move the shock cone created by supersonic flow to the rear wing tip, allowing the leading edge and other control surfaces to behave more like they were operating at subsonic speeds. But KSP doesn't model this complex air flow which makes delta-wings more aesthetic than practical.

Edited December 20, 2019 by HvP

[Vanamonde]

Wings toward the back has an additional benefit in KSP, @HvP. Since the engine is the heaviest part, the lift has to be somewhere close to it for the sake of balance. So I still build that way. I just put canards up front as well. 

[HvP]

54 minutes ago, Vanamonde said:

Wings toward the back has an additional benefit in KSP, @HvP. Since the engine is the heaviest part, the lift has to be somewhere close to it for the sake of balance. So I still build that way. I just put canards up front as well. 

Good point.

I don't build a lot of space planes, and I tend to favor passenger jet styles instead of fighter jet designs. So my engines tend to be attached to nacelles on either side of the fuselage and under the wings which often makes weight distribution a little easier, for me at least.

[BenKerman]

Hey!  Thanks for responding to my posts, everyone.  I'm the OP here.  I'll disable the extra inputs to control surfaces if I do another plane.  Planes really are hard, rockets are easier.  

Yes, my link to the Kerbal Player's guide was incorrect.  Here is a good one:  https://www.amazon.com/Kerbal-Players-Guide-Easiest-Program/dp/1491913053/ref=sr_1_1?keywords=kerbal+player's+guide&qid=1576966447&sr=8-1

I think I said I would post more screenshots of the NiftyPlane 9000 with CoM, CoL, CoT displayed, here it is:  

 

Edited December 21, 2019 by BenKerman

[HvP]

@BenKerman The various markers in your screenshot look to all be in the right place for decent flight.

Are you still having difficulties with pitch control? If the elevons are still not moving in the right direction on reentry then it could be that your CoM is moving behind your CoL due to using fuel. If the plane is still flying straight without tumbling though then that isn't the problem. What do the markers look like after you have drained the fuel from the tanks?

The only other possibility I can think of is that the CoM moves too far forward after the tanks drain. This would make it difficult to get leverage from your elevons and causes your nose to dip more and more. But it wouldn't cause the elevons to move in the wrong direction; they just wouldn't have as much force.

Edit to add: one final possibility is that your control point is changing for some reason. This often happens after docking, but I don't see a docking port on the the craft.

Edited December 22, 2019 by HvP

[BenKerman]

Hi HvP, no, I haven't played with NiftyPlane 9000 for a few days now.  I think I've gotten what I need out of it.  The pitch problem went away after I removed the battery (I also stopped time-warping).  I haven't even disabled the extra control inputs.  After removing the battery and stopping time-warping, I was able to safely land on Kerbin after orbiting.  In the future I may disable extra inputs to control surfaces if I run into an issue.

I didn't check CoM and CoL with empty tanks.  I don't think that had anything to do with the pitching problem, because the pitching problem happened even in vacuum.