Why does my plane keep pitching down?

[aluc24]

Hi guys, I'm trying to make a low-tech plane (no command modules, just command seats), for these short science missions. I'm playing with FAR. The plane looks like this:

The problem is, as soon as it hits 60-70 m/s, the nose starts pitching down violently, and I can't keep it up.

Thing is, I know quite a lot about aerodynamics, and I've built quite a lot of planes, and none of them ever did this to me. Center of Thrust is in line with COM.

1. If I move COE forward, closer to COM, the nose either drops down or shoots into the sky, which is expected.

2. However, if I move COE further back, the nose-drop tendency increases, and I can't pitch up anymore.

I tried every possible step of COE and COM distances, everything in between these two extremes. No matter what I do, the nose drops. I have plenty of control authority. I even added a Reaction Wheel at maximum setting (with SAS in flight, of course), and even it can't handle the nose drop. I tried adding a tail piece with elevons to gain more leverage, but it doesn't help either.

 

I'm stuck. I've been at this for 3 hours now, micro-managing wing position, trying to find a sweet spot of COM-COE distance, but it just doesn't exist here. Here is my craft file, if you want to take a look... It needs FAR, TakeCommand and DMagic orbital science addons only, I guess most of you have it anyway.

https://ufile.io/z3cwm

I would really appreciate the help

Edited July 20, 2017 by aluc24

[Rodhern]

Hi @aluc24

Thank you for sharing your fun little craft. It is interesting and it doesn't actually fly quite as bad as you seem to indicate, but it is fairly easy to loose control though.

There is a few little quirks to this answer. First, the root cause of the problem most likely is that your pitch control surfaces do not have much of a lever arm. You may not notice it when you fly but the pitch control is mostly provided by the small reaction wheel. Eventually the aerodynamic forces will be too great for the reaction wheel to keep the craft entirely stable. Another reason you may not notice this, is that, at least in my KSP, the FAR Editor gets somewhat confused by the elevon pitch control on your craft. Once out on the runway though it all makes sense.

I know I don't win any fashion competitions for this design; but I do believe it tells you in just one image what the problem/fix is:

 

Edited July 20, 2017 by Rodhern
changed png to jpg

[aluc24]

1 minute ago, Rodhern said:

Hi @aluc24

Thank you for sharing your fun little craft. It is interesting and it doesn't actually fly quite as bad as you seem to indicate, but it is fairly easy to loose control though.

There is a few little quirks to this answer. First, the root cause of the problem most likely is that your pitch control surfaces do not have much of a lever arm. You may not notice it when you fly but the pitch control is mostly provided by the small reaction wheel. Eventually the aerodynamic forces will be too great for the reaction wheel to keep the craft entirely stable. Another reason you may not notice this, is that, at least in my KSP, the FAR Editor gets somewhat confused by the elevon pitch control on your craft. Once out on the runway though it all makes sense.

I know I don't win any fashion competitions for this design; but I do believe it tells you in just one image what the problem/fix is:

Thank you for your answer, @Rodhern . If you mean to imply that I need to move elevons further away to give them a longer lever arm, I actually tried that, putting a long tail, moving elevons there, and balance-testing again. Thing is, it didn't solve the problem. Even with huge control authority, the plane either loses control (COE too close to COM) or pitches down violently (COE too far away from COM) as soon as the speed builds up.

This continues to baffle me. I've built dozens of planes, and never did any one of them do this, even with far less control authority. I could solve this the brute force approach - make a VERY long tail with huge elevons back there, but that would defeat the purpose. I still don't get why the plane suddenly locks itself in a nose-down attitude. It's almost as if it stalls.

[Rodhern]

2 minutes ago, aluc24 said:

This continues to baffle me.

That is indeed strange. The craft on the above image flies just fine for me. I have no explanation what so ever for why it would lock itself in a nose-down attitude.

[aluc24]

Just now, Rodhern said:

That is indeed strange. The craft on the above image flies just fine for me. I have no explanation what so ever for why it would lock itself in a nose-down attitude.

Try disabling or removing the Reaction Control Wheel, and build up some speed.

[Rodhern]

14 minutes ago, aluc24 said:

Try disabling or removing the Reaction Control Wheel, and build up some speed.

Bill and Bob removed the Reaction Control Wheel and went for a test flight.

Except for the first part of the take-off run where the craft skids on its bum before getting airflow enough to raise the tail, it is a perfect little plane. (Wright 2E.craft).

Bill and Bob buzzed the tower at mach 0.5. They claim it is one of the best planes they ever got to fly. Very stable with no discernible ill tendencies.

[aluc24]

13 minutes ago, Rodhern said:

Bill and Bob removed the Reaction Control Wheel and went for a test flight.

Except for the first part of the take-off run where the craft skids on its bum before getting airflow enough to raise the tail, it is a perfect little plane. (Wright 2E.craft).

Bill and Bob buzzed the tower at mach 0.5. They claim it is one of the best planes they ever got to fly. Very stable with no discernible ill tendencies.

Fantastic. Your Bill and Bob are a pair of geniuses. I have no idea why it is so stable. How did you achieve that? I'm trying to dissect this design, but I still don't understand. Especially the roll stability, how did you make that work?

[Rodhern]

Just now, aluc24 said:

Fantastic. Your Bill and Bob are a pair of geniuses. I have no idea why it is so stable. How did you achieve that? I'm trying to dissect this design, but I still don't understand. Especially the roll stability, how did you make that work?

The genius of Bill and Bob is that they made a blatant copy of your design and put their names on the side - the little buggers. Seriously, it is your design; take your uploaded craft file, remove the reaction wheel and move back the two pitch elevons somehow (e.g. the empty liquid fuel parts as shown). Maybe you have encountered some kind of bug or formula edge case - your initial 'nose down lock' experience still doesn't make much sense.

[aluc24]

Just now, Rodhern said:

The genius of Bill and Bob is that they made a blatant copy of your design and put their names on the side - the little buggers. Seriously, it is your design; take your uploaded craft file, remove the reaction wheel and move back the two pitch elevons somehow (e.g. the empty liquid fuel parts as shown). Maybe you have encountered some kind of bug or formula edge case - your initial 'nose down lock' experience still doesn't make much sense.

Probably... Well, it must be some bug or something. I can't seem to recreate it with this craft you updated. A mystery solved, yet still a mystery. Anyway, thank you very much

[Rodhern]

Hmm, just an extra observation. If I add a cockpit and re-root then the pitch control surfaces do count in the editor. When you take the 'Static Analysis' panel, enter a 'pitch setting' (e.g. 1) and press 'Sweep AoA' you can see the control surfaces deflect. They do not deflect on the command seat only version. That is, they do not deflect in the editor, but they do deflect in flight. Maybe somehow this quirk some times carries over to the outside flight scene? (I mean if FAR somehow got confused and did not deflect the control surfaces then a 'nose down lock' all of a sudden might not be such a far fetched behaviour). In that case it is probably a 'Take Command' related issue.

 

Edited July 20, 2017 by Rodhern
spelling

[aluc24]

22 hours ago, Rodhern said:

Hmm, just an extra observation. If I add a cockpit and re-root then the pitch control surfaces do count in the editor. When you take the 'Static Analysis' panel, enter a 'pitch setting' (e.g. 1) and press 'Sweep AoA' you can see the control surfaces deflect. They do not deflect on the command seat only version. That is, they do not deflect in the editor, but they do deflect in flight. Maybe somehow this quirk some times carries over to the outside flight scene? (I mean if FAR somehow got confused and did not deflect the control surfaces then a 'nose down lock' all of a sudden might not be such a far fetched behaviour). In that case it is probably a 'Take Command' related issue.

 

Interesting stuff. I have no idea. I did notice, however, that if I add a probe core (of course, re-balancing the place to account for the weight shift), the plane flies very differently. Something is definitely out of place here. Maybe KSP or FAR doesn't like a craft with no control module.