Strange space-plane behaviour at high speed

[MartGonzo]

Hi there, I'm hoping someone can give me some insight into why this particular space-plane develops odd characteristics at high speed, such as pitch or even very minor yaw causes uncontrollable roll, or why the roll control inverts at around 800 m/s.

It's fine at low altitude, low speed, Q rolls left, E rolls right, but that switches over at high speed.

The rudders are set to yaw only, the front elevons (ailerons? I always get confused on which) are set to pitch only and the rear ones are set to pitch and roll.

Edited July 4, 2015 by MartGonzo

[rofltehcat]

I yesterday had a problem as well where my front control surfaces would suddenly invert. I think it was linked to a certain altitude or speed because after a few tries it cruised right up to 10k. But then when I tried landing the same ting happened when it was around 500 m, 100-250 m/s and it crashed (was unmanned though).

So I removed the front control surfaces and it flies fine now, although it needs a bit longer to turn.

Edited July 4, 2015 by rofltehcat

[MartGonzo]

I yesterday had a problem as well where my front control surfaces would suddenly invert. I think it was linked to a certain altitude or speed because after a few tries it cruised right up to 10k. But then when I tried landing the same ting happened when it was around 500 m, 100-250 m/s and it crashed (was unmanned though).

So I removed the front control surfaces and it flies fine now, although it needs a bit longer to turn.

Interesting, I'm not sure it's the same issue though as it happens only at high speed and I haven't had any issues at low speed or altitude but I'll try removing those front control surfaces anyway.

Thanks rofltecat.

[selfish_meme]

I have heard of an issue with CoM and inverted controls, is your CoM in front of your control surfaces at the start and as you lose fuel it moves backwards? Do you use Stock Bug Fix Modules mod, and is it updated?

[Daveroski]

As the objective of a spaceplane in atmosphere is to go in as straight a line as possible, using the atmosphere to aid in lift where possible, we have to consider that controls for roll and yaw are for stabilisation at most. Even the controls for pitch need only be rudimentary. This has advantages and disadvantages in that at supersonic velocities it is better to have as few control surfaces as possible as they all produce extra drag even in stable flight. The disadvantage being that at subsonic speeds, the craft may feel ungainly. Even the amount of actual lifting surfaces can be better at subsonic but detrimental at supersonic.

Of course in space all these lifting and control surfaces are only a hinderance as they only provide extra mass to the craft.

When constructing a spaceplane we need to consider five things

It must be as streamlined as possible to prevent drag in the atmosphere.

It should have enough atmospheric engine power to achieve as high a speed as possible by the time the edge of the atmosphere is reached.

It should have sufficient vacuum engine power to be able to push the craft into a stable orbit from the edge of the atmosphere or from apoapsis, which ever is most applicable.

It should have as small a mass as possible. Reducing initial structural mass to as little as possible.

It should have as few control surface and lift surfaces as possible yet still be able to maintain a glide-path for re-entry into an atmosphere.

I suspect the problem with your design may fall into the category of too much in the way of control surfaces.

To simplify the matter greatly, Think of a supersonic vehicle as a large cube with a pointy end. While travelling at supersonic speed in atmosphere in a straight line uniform airflow is maintained.

If this supersonic craft were to change its attitude even slightly, it would present a much larger or at the least a very different profile into the airflow.

To roll a craft for example, will cause two control surfaces to travel in opposite directions from each-other and the craft that they are attached to. At the instant of presenting more surface to the atmosphere, more drag is instantly applied. In the case of a roll it is rather asymmetrical and unless the craft was travelling precisely into the airflow without deviation, more surface area will be presented on one side of the craft than the other. This will of course cause the craft to yaw in the direction of the side with the greatest drag.

To consider how your craft will behave, first imagine it flying true into the airflow. then imagine it flying slightly out of true. As you can imagine as supersonic speeds even slightly out of true will present much more of the craft on one side into the airflow than the other. At such speeds even minor deviations can very quickly lead to loss of control.

At best massive amounts of DV can be lost merely to course corrections.

Yawing of the craft at supersonic speeds is to be avoided wherever possible as even though one may turn the nose slightly to one side, presenting the other side of the craft into the airflow causes a massive drag on that side and so can easily counter any lateral movements of the craft.

To summarise, at supersonic speeds, while one may attempt turn a craft to port, drag may then turn the craft to starboard.

Perhaps if you removed some of the control surface at the trailing edge and went for a more minimalistic approach, you may achieve a more stable craft.

[Xannari Ferrows]

Control surfaces are calibrated to the contact point of the wing it is attached to. If the CoM moves behind the contact point, the control surfaces will invert, even if the overall CoL is still behind.

It doesn't really seem like it'd be an issue with this plane, though. I'd still look into it regardless.

It's also possible that your CoM and CoL may be so close together that the craft becomes incredibly sensitive to rotation on applied lift forces.

Edited July 4, 2015 by Xannari Ferrows

[MartGonzo]

I have heard of an issue with CoM and inverted controls, is your CoM in front of your control surfaces at the start and as you lose fuel it moves backwards? Do you use Stock Bug Fix Modules mod, and is it updated?

Didn't Squad change the fuel flow so that it feeds from all available tanks equally to prevent the COM radically shifting in flight?

I'm not sure about that but I think I read that in one of the change logs.

I do use stock bug fix modules, I haeven't checked in the last couple of weeks but the version I'm using was only downloaded recently

regardless you may be onto something, I'll update that mod and look into playing around with the center of mass, there is space in each of the tanks so I could balance fuel and oxidiser and the wings are slightly angled so it shouldn't be hard to change.

As the objective of a spaceplane in atmosphere is to go in as straight a line as possible, using the atmosphere to aid in lift where possible, we have to consider that controls for roll and yaw are for stabilisation at most. Even the controls for pitch need only be rudimentary. This has advantages and disadvantages in that at supersonic velocities it is better to have as few control surfaces as possible as they all produce extra drag even in stable flight. The disadvantage being that at subsonic speeds, the craft may feel ungainly. Even the amount of actual lifting surfaces can be better at subsonic but detrimental at supersonic.

Of course in space all these lifting and control surfaces are only a hinderance as they only provide extra mass to the craft.

When constructing a spaceplane we need to consider five things

It must be as streamlined as possible to prevent drag in the atmosphere.

It should have enough atmospheric engine power to achieve as high a speed as possible by the time the edge of the atmosphere is reached.

It should have sufficient vacuum engine power to be able to push the craft into a stable orbit from the edge of the atmosphere or from apoapsis, which ever is most applicable.

It should have as small a mass as possible. Reducing initial structural mass to as little as possible.

It should have as few control surface and lift surfaces as possible yet still be able to maintain a glide-path for re-entry into an atmosphere.

I suspect the problem with your design may fall into the category of too much in the way of control surfaces.

To simplify the matter greatly, Think of a supersonic vehicle as a large cube with a pointy end. While travelling at supersonic speed in atmosphere in a straight line uniform airflow is maintained.

If this supersonic craft were to change its attitude even slightly, it would present a much larger or at the least a very different profile into the airflow.

To roll a craft for example, will cause two control surfaces to travel in opposite directions from each-other and the craft that they are attached to. At the instant of presenting more surface to the atmosphere, more drag is instantly applied. In the case of a roll it is rather asymmetrical and unless the craft was travelling precisely into the airflow without deviation, more surface area will be presented on one side of the craft than the other. This will of course cause the craft to yaw in the direction of the side with the greatest drag.

To consider how your craft will behave, first imagine it flying true into the airflow. then imagine it flying slightly out of true. As you can imagine as supersonic speeds even slightly out of true will present much more of the craft on one side into the airflow than the other. At such speeds even minor deviations can very quickly lead to loss of control.

At best massive amounts of DV can be lost merely to course corrections.

Yawing of the craft at supersonic speeds is to be avoided wherever possible as even though one may turn the nose slightly to one side, presenting the other side of the craft into the airflow causes a massive drag on that side and so can easily counter any lateral movements of the craft.

To summarise, at supersonic speeds, while one may attempt turn a craft to port, drag may then turn the craft to starboard.

Perhaps if you removed some of the control surface at the trailing edge and went for a more minimalistic approach, you may achieve a more stable craft.

I think you may be right, I knew that it was unnecessary to have all those control surfaces, really they were just added for aesthetic reasons, I knew they would have a slight drag effect but didn't consider they might actually be the cause of stability issues.

Thanks very much, you make everything much clearer, the asymmetric drag from control surfaces with even small corrections at supersonic speeds is undoubtedly causing the unexpected yaw and roll.

I suppose aesthetics will have to suffer for the sake of functionality.

Control surfaces are calibrated to the contact point of the wing it is attached to. If the CoM moves behind the contact point, the control surfaces will invert, even if the overall CoL is still behind.

It doesn't really seem like it'd be an issue with this plane, though. I'd still look into it regardless.

It's also possible that your CoM and CoL may be so close together that the craft becomes incredibly sensitive to rotation on applied lift forces.

Indeed the CoM and CoL are very close together, I will try a bit more separation, along with the above suggestions on less control surfaces I think that will help greatly.

Many thanks everyone, you've been a great help

[Sharpy]

Control surfaces are calibrated to the contact point of the wing it is attached to. If the CoM moves behind the contact point, the control surfaces will invert, even if the overall CoL is still behind.

This is true for pitch, but roll? The centre of mass would need to move sideways past given control surface to make its roll behavior to invert!

[selfish_meme]

Sharpy is right, it may be drag related as the other suggestion, the inside control surface on the back of the wing should be pitch only, the outside ones pitch and roll.

[Xannari Ferrows]

This is true for pitch, but roll? The centre of mass would need to move sideways past given control surface to make its roll behavior to invert!

Roll is a fancy way of saying "This side of the craft pitches up, and this one pitches down."

[Red Iron Crown]

Roll is a fancy way of saying "This side of the craft pitches up, and this one pitches down."

I think what Sharpy means is that control surface deflection direction for roll doesn't change if the CoM moves forward or backwards.

[GoSlash27]

I'll second Daveroski's text- wall. I use absolutely minimal control surfaces on spaceplanes. I usually have only 2 active control surfaces and they have poor leverage for roll. I use vertical fins for yaw stability, but they are not active.

The more authority your SAS has, the more it will interfere.

Also, check those structural intakes in the nose. They may be generating enough drag to ruin your stability at high speed.

Best,

-Slashy

[Sharpy]

Roll is a fancy way of saying "This side of the craft pitches up, and this one pitches down."

Specifically, the left pitches up the right down, or vice versa.

Pitch, Yaw, Roll, the three most essential terms in aviation. Happily using "roll" to describe changing pitch is really confusing.

[Xannari Ferrows]

Pitch, Yaw, Roll, the three most essential terms in aviation. Happily using "roll" to describe changing pitch is really confusing.

I guess if you really wanted to, there's not really a difference between them. Yaw is pitch perpendicular to the cockpit [at least it should be], roll is two different directions of pitch, but pitch equates to symmetrical and parallel yaw. I guess they're just spelled differently.

[GoSlash27]

I suppose aesthetics will have to suffer for the sake of functionality.

Not necessarily. You can still have the control surfaces, just don't make them active. The canards in the nose should be adequate for pitch and roll and you don't need active yaw.

Best,

-Slashy

[Claw]

If you are using my stock bug fix add-on, I personally would appreciate if you try removing the Control Surface Fix and see if that solves the problem. One of the previous versions did have an inverting control surface issue which was resolved. But with more changes recently, it's possible it returned or is showing up in a different way.

I will try to replicate your plane when I have a chance, or if you can upload a .craft file that would be handy.

Cheers,

~Claw

[selfish_meme]

If you are using my stock bug fix add-on, I personally would appreciate if you try removing the Control Surface Fix and see if that solves the problem. One of the previous versions did have an inverting control surface issue which was resolved. But with more changes recently, it's possible it returned or is showing up in a different way.

I will try to replicate your plane when I have a chance, or if you can upload a .craft file that would be handy.

Cheers,

~Claw

You rock, nothing weird, just saying.

[Wolf Baginski]

I've been testing a plane, and things can depend on how they're being controlled. Default flight is manual+SAS and that is stable. Though doing a balanced turn is liable to lead to finger trouble. Use the MechJeb Spaceplane module and it hold altitude and heading, but can get into ocillations for pitch, roll, and yaw. Switching off some of the control surfaces helps.

One thing I have tried, with some good results, is pumping fuel between forward and after tanks to modify forward and after trim. That's what Concorde did to account for the shift in centre of lift in supersonic flight.

The whole thing went glitchy last night and I lost the option to switch on and off control services, but an action group to toggle a set of control surfaces might work.