Control surfaces and supersonic flight [FAR]

[gompasta]

While testing many of my planes, i have noticed control surfaces becoming a lot less responsive once i exceed supersonic speeds.

What i want to know is, are these effects simulating a realistic effect (If so, anywhere I can read more about it?), or is it just a glitch/unintentional effect.

[Van Disaster]

Your centre of lift moves rearwards when you go supersonic so you will usually lose a fair bit of pitch authority ( a generic term for planes wanting to pitch down going transonic is Mach Tuck, although iirc that is actually a transonic rather than purely supersonic effect ); I'm not actually sure why you'd lose much roll authority, although obviously any changes in wing behaviour are going to effect aircraft control, especially if you've managed to couple roll and pitch somehow.

Alternatively you've pushed the speed-sensitive control button and it's not tuned well for the plane.

[ferram4]

There are two effects being simulated here; one is the shift in aerodynamic center* (center of lift) that Van Disaster mentioned and the other is the change in the effect that an airfoil's camber has on its lift. Let me try to explain with these wonderful MS Paint diagrams I whipped up.

The first is pretty simple to visualize; take a cross-section of the wing and look at it from the side, it would look something like this:

Where the green dot is where the location of the wing's AC at subsonic speeds while the red dot is the location of the wing's AC at supersonic speeds, assuming the airflow comes from the left. This effect happens to some extent with all aerodynamic shapes, causing objects to become slightly more statically stable at supersonic speeds.

The second effect can be explained this way; for a wing, lift is determined as a contribution of camber effects and angle of attack effects; camber is just how "curved" the wing is compared to the airflow, while the angle of attack is the angle between the airfoil reference line and the incoming air; more camber and more angle of attack cause greater lift if stalling doesn't occur. The image below draws the reference lines and profiles for an airfoil with and without flaps deflected:

What the image shows is that an increase in control surface deflection increases the camber and the effective angle of attack of the wing, both which increase lift. However, camber only produces lift at subsonic speeds; it is entirely dependent on the airflow at the trailing edge of the wing being able to affect the airflow at the leading edge of the wing. Once in supersonic flow the only way changes in lift can be achieved is through increases in the angle of attack of the wing; camber will only increase drag. This would be the result of any roll controls feeling sluggish at high speeds, since they're less effective at Mach > 1 than at Mach < 1. This is also why supersonic planes fare better with all-moving tailplanes / canards than tails with separate elevators.

The thing to take away from this is that your plane is becoming more statically stable in both pitch and yaw and that your wings and control surfaces stop being effective at lifting when you switch from subsonic to supersonic flight. You will have to either make your plane less statically stable to handle this and/or you will have to use more control surfaces to give you greater control authority. If you want to learn more, just try searching for "supersonic flow" or "transonic flow" on Google and you should find a bunch of relevant stuff.

*I dislike the term "Center of Lift" because it seems to imply that the only aerodynamic force that needs to be accounted for in determining stability is lift; it also seems to imply that there is a separate "Center of Drag" that would be located somewhere else on the craft, confusing otherwise intelligent people by giving them something else to worry about / try to calculate when there is no need to do so. Aerodynamic center lacks those implications / problems.

[Galacticruler]

While that is true for real life, that is incorrect in KSP, the aerodynamic model does not work that way AFAIK.

[Van Disaster]

You entirely didn't read that he is using FAR ( how? it's in big letters in the thread title ), and the author of FAR made the post above yours.

Aerocentre is unfortunately not a KSP term yet; ferram, maybe FAR ought to rename the button? ( thanks for clarifying the thing I saw in my head and couldn't get the words to explain, btw ).

[rakutenshi]

I've dealt with this problem partially by using Infernal Robotics/Pwings to make full moving canards to function as pitch trim, it doesn't quite override the mach tuck entirely, but it does help a lot.

Ways to help deal with mach tuck also include making the wings more delta than swept (a thicker chord with a swept leading edge reduces the effects of mach tuck on the control surfaces of the trailing edge), building some trim into the horizontal stabilizer/canards (i.e. set trim before takeoff), or simply having more thrust to punch through the trans-sonic envelope before it's much of an issue. as for the loss of pitch authority, try using B9's stabilators instead of normal control surfaces.

In the VAB, I make extensive use of the Static Mach Sweep displays. I don't really fully understand all the numbers and everything that it can display but I do know that if you set the maximum value on the mach sweep chart to 6ish, just try to fiddle with things until the green line is above the red line and life will be pretty good.

[Van Disaster]

The other option is to use high aspect ratio wings; at the moment FAR doesn't seem to penalise lack of sweep, so short-chord relatively straight wings will not move the aerocentre that much. I don't think it's something to make a habit of given I suspect it won't be viable forever, but it's also a workaround for me for pWings not being able to scale along individual axes.

I can't really read the sweep mach graph either ( well I can see what's going on, I don't know how to make use of it ) but when actually flying this shape doesn't even notice going transonic; at least it doesn't do anything that needs any compensation. I suspect that wing shape should really be like deploying airbrakes...

[rakutenshi]

I'm just guessing here but your coefficient of lift is probably due to the sheer quantity of wing you have.. looks like a sail plane to me, in a lot of ways. that likely helps with the transsonic bump since your c/l doesnt' drop below drag at .8-1.3 mach.

Also just noticed that you were doing the calculations with the AoA at 5 degrees, that does give some radically different numbers than with the nose closer to zero.

I did some fiddling about and I think the reason the sweep isn't as important as the thickness of the wing is because the PWing doesn't allow for the MAC to be adjusted independently of the root/tip scale factor, so if you have a long wing, you have a fat, draggy, inefficient wing, so P-Wing + FAR = go sailplanes... hrmf.

Edited September 17, 2013 by rakutenshi

[Van Disaster]

If I took the same wing and cranked the sweep up to something that it realistically ought to be - rear or forward sweep, doesn't matter - then transonic issues with aerocentre movement become way worse; this is without changing the wing area ( in as much as you can avoid that when you're moving pWings about at least ). I think partly it'll be because I'm flying pitched up and a little pitch on the controls when I go transonic, so cM doesn't go negative; the sweep mach graph is a bit exaggerated for readability.

That's more likely.

238m^2 wing area... I actually don't know if that's a lot; it flies beautifully though. 10.3m span vs 3m scaled chord, but I don't think I want to work out the aspect ratio given the way the chord length changes...

Edited September 17, 2013 by Van Disaster

[rakutenshi]

You do get good results with that design, there's no arguing that. Wish P-wings would allow you to thin the wings out. as much as I like the B9 Aerospace mod, I just can't bring myself to fiddle with patchwork wings after using P-wings.

I'm considering trying to do a variable geometry SSTO , sorta B-1 style, use a thinner wing sorta like yours and just crank it back when I hit past mach 2 to minimize the drag as much as possible.

[Van Disaster]

I tried it with wingtips; certainly worked for moving the aerocentre forwards, but the hinges we had back then weren't the strongest things so they bent up rather badly. Maybe when we need them there'll be more motivation to do a moving wing glove.

[amo28]

@Van Disaster: Where did you get those control surfaces from? I've never seen them before. Is that from Taverio's? Did PWing make control surfaces? TOO MANY QUESTIONS.

[Van Disaster]

They're from Taverius' pack.

[rakutenshi]

I did extensive tests last night, culminating in an unrecoverable inverted flatspin, and the sad fact that it doesn't seem that even the infernal robotics pieces with the fairly good strength of attachment simply aren't strong enough to handle the forces placed on a wing spar.

[Van Disaster]

Someone suggested using quantum struts to lock the wings in place, however there's a couple of issues with that; first when you turn them off the wings are going to bend like crazy ( I have some experimental craft with quantum struts, if I run them out of electric charge they make some hilarious shapes ) and secondly... IR and quantum struts have some hilarious interactions at times, you might find parts of your plane falling off when you turn them on again. We do actually need a rotating wing root piece.

[rakutenshi]

I'd settle for sweep actually improving mach flight characteristics

[Mighty1]

Sorry for reactivating an old thread but I have a problem with FAR.

I am trying to recreate the Skylon in carrier mode and since FAR is "what work in RL works with FAR" I came to a wall in my design process. Every time I reach speeds greater than Mach 1 the plane AoA keeps getting higher up to the point when it just flips on its tail. I know that my CoL goes back when going supersonic but no matter how much i change the wing positions it keeps flipping. The fuel drains from the front and back tanks at the same rate so it can not be a fuel imbalance problem.

I presume that it has something to do with the red numbers on the FAR stats but i have no idea what do these numbers mean. Is there a tutorial on the graphs & stats, preferably that keeps it simple like: if X is red then add moar control surfaces to this and that part of the plane etc. Please help me. Here are the pics.

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[Hodo]

Sorry for reactivating an old thread but I have a problem with FAR.

I am trying to recreate the Skylon in carrier mode and since FAR is "what work in RL works with FAR" I came to a wall in my design process. Every time I reach speeds greater than Mach 1 the plane AoA keeps getting higher up to the point when it just flips on its tail. I know that my CoL goes back when going supersonic but no matter how much i change the wing positions it keeps flipping. The fuel drains from the front and back tanks at the same rate so it can not be a fuel imbalance problem.

I presume that it has something to do with the red numbers on the FAR stats but i have no idea what do these numbers mean. Is there a tutorial on the graphs & stats, preferably that keeps it simple like: if X is red then add moar control surfaces to this and that part of the plane etc. Please help me. Here are the pics.

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Fixed your album link.

And your problem is simple.

Your CoM is behind your CoL.

It should be the other way around,

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Like in this SSTO spaceplane.

[Mighty1]

Thanks. I will try it out tomorrow. How large is the col change when going supersonic anyway?

[phoenix_ca]

Thanks. I will try it out tomorrow. How large is the col change when going supersonic anyway?

Er...none. As far as I know.