Airplanes nose pinching problem. Help please!

[SoulReaper1702]

I am having trouble with the pitch of my plane. The plane takes off perfectly however 3-4 seconds after launch the airplane tilts down. I have tried moving the COL behind the COM but that doesnt seem to help.

[Vanamonde]

1 hour ago, SoulReaper1702 said:

I am having trouble with the pitch of my plane. The plane takes off perfectly however 3-4 seconds after launch the airplane tilts down. I have tried moving the COL behind the COM but that doesnt seem to help.

How about pics of the plane in question? 

[mikegarrison]

Moving the CoL back relative to the CoM should increase the pitch-down moment, so that would not be expected to help. I would assume that what you need is more restoring force from your tail. You probably don't have enough control authority in your elevators.

The plane is like a lever, with the CoM as the pivot. You need the forces to balance on both sides of the pivot. If your plane is pitching down, then you need to either move the CoM back (not forward) or you need to get more downforce (or less lift) from the tail.

If you are using canards, then it means you need to either get more lift from your canards or you need to move your main wing a little forward.

Edited May 16, 2018 by mikegarrison

[adsii1970]

1 hour ago, SoulReaper1702 said:

I am having trouble with the pitch of my plane. The plane takes off perfectly however 3-4 seconds after launch the airplane tilts down. I have tried moving the COL behind the COM but that doesnt seem to help. 

You can do a couple of things while in flight. Do a right click on your tail fins and play with the limiter settings. Sometimes manually adjusting them until the indicator on the nav ball indicates a +5 degree will solve the problem. What would help me help you better is for a few screen shots of your craft - one in the hangar with the COM indicator on and then one in flight as your plane pitches down. If you can do that, I'll give it my best shot at diagnosing it for you.

[SoulReaper1702]

3 hours ago, adsii1970 said:

You can do a couple of things while in flight. Do a right click on your tail fins and play with the limiter settings. Sometimes manually adjusting them until the indicator on the nav ball indicates a +5 degree will solve the problem. What would help me help you better is for a few screen shots of your craft - one in the hangar with the COM indicator on and then one in flight as your plane pitches down. If you can do that, I'll give it my best shot at diagnosing it for you.

Link to a picture from the side.

https://drive.google.com/open?id=1S_uOaos8VBGgmi2hHpjEr1Vu6gkJVePs

Link to a picture from the top

https://drive.google.com/open?id=1DGfh6jr89ocmcqzIX4PMI3cZYtqhrkOU

[Vanamonde]

Looks like your COL is so close to the COM that the plane is having trouble exerting torque to apply control authority. That's going to cause trouble on any axis and it just happens that in this case it's nosing down. Counter-intuitively, I believe shifting the wings back slightly would help. Although that would make the plane more nose-heavy, it will also help the control surfaces do their job. 

Also, now that it's clear your question is about a KSP craft, the thread has been moved to Gameplay Questions. 

[Val]

I think another issue could be the control surface that you've added to the swept wings. It appears that the axis of rotation for the control surfaces crosses the centerline in front of CoM, which used to cause them to work opposite direction of what you want.

[SoulReaper1702]

6 hours ago, Vanamonde said:

Looks like your COL is so close to the COM that the plane is having trouble exerting torque to apply control authority. That's going to cause trouble on any axis and it just happens that in this case it's nosing down. Counter-intuitively, I believe shifting the wings back slightly would help. Although that would make the plane more nose-heavy, it will also help the control surfaces do their job. 

Also, now that it's clear your question is about a KSP craft, the thread has been moved to Gameplay Questions. 

I will check that. So what I want is my COL behind the COM ?

[SoulReaper1702]

2 hours ago, Val said:

I think another issue could be the control surface that you've added to the swept wings. It appears that the axis of rotation for the control surfaces crosses the centerline in front of CoM, which used to cause them to work opposite direction of what you want.

I do not understand. Are you talking about the elevators on the wings ? 

[Lisias]

4 hours ago, Val said:

It appears that the axis of rotation for the control surfaces crosses the centerline in front of CoM, which used to cause them to work opposite direction of what you want.

He's talking about this. When this happened to me, it took so much time to detect that I got sympathized with the fellow Kerbal and made this image to make it clear. :-) Essentially, my elevons became airbrakes! :-D

On the bright side, when having a odd number of surfaces on my wings, I usually make two of them as airbrakes (attaching the deploy to the brake action, and reversing the deploy of one of them) and save two parts (symmetrical airbrakes) on my designs. I had to learn something from this stunt! ;-)

In time, @Val, since the CoM usually moves as the fuel is consumed, would be more appropriate to make the elevons shift functions based on the CoL? This would make things a bit more realistic - and logical, as the rotation pivot is the CoL...

Edited May 17, 2018 by Lisias
beter phrasing

[Val]

54 minutes ago, Lisias said:

In time, @Val, since the CoM usually moves as the fuel is consumed, would be more appropriate to make the elevons shift functions based on the CoL?

It can't be based on CoL, because moving the control surfaces moves CoL. That is actually the whole purpose of control surfaces. Moving the CoL around to make the craft change attitude.

 

54 minutes ago, Lisias said:

This would make things a bit more realistic - and logical, as the rotation pivot is the CoL...

No. The pivot is CoM.

[Lisias]

2 minutes ago, Val said:

No. The pivot is CoM.

Thanks for the explanation! (in time, what did you use to generate the animated images?)

[Val]

1 hour ago, SoulReaper1702 said:

I do not understand. Are you talking about the elevators on the wings ? 

Yes. It is probably not the only problem you are having, but I think it's a contributing factor.

1 minute ago, Lisias said:

Thanks for the explanation! (in time, what did you use to generate the animated images?)

I first recorded it as a video clip, then made it to a .gif with http://gifmaker.me/

[adsii1970]

Here's my two cents, although it seems that the issue has already has been answered.

In this example I've given you, I try to keep the COL a little behind the COM. This way, the craft has a natural tendency to want to have upward drift rather than the alternative you've experienced. In fact, here's my latest craft - the DC-7 (the bottom image is the real aircraft):

[Val]

5 minutes ago, adsii1970 said:

I try to keep the COL a little behind the COM. This way, the craft has a natural tendency to want to have upward drift rather than the alternative you've experienced.

No. If you place CoL behind CoM, then the craft will have a natural tendency to dive towards the ground unless some up elevator is applied.

[adsii1970]

4 minutes ago, Val said:

No. If you place CoL behind CoM, then the craft will have a natural tendency to dive towards the ground unless some up elevator is applied.

For some reason, I'm not having that problem... You're probably right and I've found some sort of strange KSP physics flaw...

[Val]

1 minute ago, adsii1970 said:

For some reason, I'm not having that problem... You're probably right and I've found some sort of strange KSP physics flaw...

Very likely. As the CoL doesn't take into account lift and drag from parts with no Lift rating. Which also means my statement is only accurate for crafts with very few non-Lift rating parts.

Torque from off center engine thrust can also be a factor.

[SoulReaper1702]

12 minutes ago, Val said:

Very likely. As the CoL doesn't take into account lift and drag from parts with no Lift rating. Which also means my statement is only accurate for crafts with very few non-Lift rating parts.

Torque from off center engine thrust can also be a factor.

So what should I do val?

[Val]

7 minutes ago, SoulReaper1702 said:

So what should I do val?

I'd try with a simpler wing layout to reduce the possible amount of faults.

• A simple straight wing, with optional Ailerons.
• One set of horizontal fins at the back for elevator control
• Keep the vertical fins as is.

Keep the CoL close behind CoM as you have it now. If you can get that to work, you can add more and modify layout until you find out what made it go wrong.

[SoulReaper1702]

5 hours ago, Val said:

I'd try with a simpler wing layout to reduce the possible amount of faults.

• A simple straight wing, with optional Ailerons.
• One set of horizontal fins at the back for elevator control
• Keep the vertical fins as is.

Keep the CoL close behind CoM as you have it now. If you can get that to work, you can add more and modify layout until you find out what made it go wrong.

I idid so, however it did not help.

Heres pictures of the simpler wings

https://drive.google.com/open?id=1Yupvwbu1Yk5mBkjn94Rad9eMrMyQ9mxm

https://drive.google.com/open?id=1i0RcoeQ0-yhw5erRH821psg7tAOAOhCd

6 hours ago, adsii1970 said:

For some reason, I'm not having that problem... You're probably right and I've found some sort of strange KSP physics flaw...

Any tips that may resolve the problem i have mate?

[kerbonara]

On 5/16/2018 at 5:50 PM, SoulReaper1702 said:

I am having trouble with the pitch of my plane. The plane takes off perfectly however 3-4 seconds after launch the airplane tilts down. I have tried moving the COL behind the COM but that doesnt seem to help.

Can you post a pic of your craft?

I was not able to open it, can you post it somewhere else like imgur? I’d love to help.

Edited May 17, 2018 by kerbonara

[Foxster]

I've had these problems before with swept wings. They just seem to cause any attached elevons to work upside-down - no matter how attached to wings or where they are mounted. 

I tend to avoid the swept wings or else never put elevons on them (you can attach them elsewhere and move them to look as though attached to the wings with shift-offset). 

Edited May 19, 2018 by Foxster