FAR Aerodyamics - Oscillating pitch and stalling?

[SpaceToad]

Hey everyone,

I am currently working on an aircraft for FAR, but my elevators keep stalling and my pitch oscillates a lot.

Any clues?

Thanks.

 

[ferram4]

If your elevators are stalling under full control authority, then they deflect too much and you need to tweak them downwards to something reasonable.

Oscillating pitch under no control inputs indicates a plane that is statically unstable; your CoL is in front of the CoM.  The oscillating is caused by the plane pitching up uncontrollably, then stalling, becoming stable under stall, pitching down, stall ending, and then becoming unstable again.

Oscillating pitch under heavy pitch input means that the same stall, pitch-down, un-stall, pitch-up cycle is occurring, but is instead driven by the pitch-up created by control surfaces.  The solution is to reduce the amount that your elevators can deflect.

[SpaceToad]

1 hour ago, ferram4 said:

If your elevators are stalling under full control authority, then they deflect too much and you need to tweak them downwards to something reasonable.

Oscillating pitch under no control inputs indicates a plane that is statically unstable; your CoL is in front of the CoM.  The oscillating is caused by the plane pitching up uncontrollably, then stalling, becoming stable under stall, pitching down, stall ending, and then becoming unstable again.

Oscillating pitch under heavy pitch input means that the same stall, pitch-down, un-stall, pitch-up cycle is occurring, but is instead driven by the pitch-up created by control surfaces.  The solution is to reduce the amount that your elevators can deflect.

Okay, with that said, is there a way to preserve the control input but negate the oscillation? I am working to make a maneuverable fighter.

[ferram4]

If you want to be able to full-pitch but not stall the plane with the same control inputs, you don't have any options besides attempting to add leading-edge slats/flaps to the design, but even that may not work.  If you don't want that, then you're stuck.  Understand that you will achieve greater maneuverability if you reduce your pitch inputs and don't stall the plane out.

Alternatively, don't use full pitch-input when maneuvering the plane.  Which results in the same thing.

[kcs123]

Perhaps this picture helps to understand how to create less oscilating pitching ?

 

[Wanderfound]

What Ferram said above about pitch wobble with no control inputs includes SAS as a control input. But you may get pitch wobble under SAS even with your CoL in the right place. If it's pitch-wobbling with no input apart from SAS, it's likely that the oversensitive SAS is driving it. High control authority aggravates the problem.

The way to deal with that is by using a PID tuner to reduce SAS sensitivity. The Kerbal Pilot Assistant mod includes one.

As for hyper-mobile Kerbal fighters...see some of the more successful builds in http://forum.kerbalspaceprogram.com/index.php?/topic/119111-far-fighter-challenge-bd-armory-ai/

 

Careful tuning of wing placement and control surface settings are required, but get it right and you can peg the g-meter all day long. AoA-sensitive leading edge slats are helpful but not essential; AoA settings on elevators are likewise helpful but not always essential. Low wing loading allows for tighter turns; finned missile designs are interceptors, not dogfighters.

One non-obvious trick: use the Dynamic Deflection mod with the default settings reversed (i.e. reduce deflection at low pressure, increase it at high). This allows you to set control surfaces to deflect little at low dynamic pressure (e.g. at low speed, just after takeoff) while still retaining high-deflection ability for supersonic aerobatics. Again, it takes a lot of fine-tuning of the DD settings to get this right.

Edited January 4, 2016 by Wanderfound