Relative effectiveness of control surfaces

[Aegolius13]

So, the different aerodynamic control surfaces have different maximum deflection angles, and also different percentages of the wing area that can move.  This got me wondering whether there's a straightforward way to compare the effectiveness of the different surfaces in providing control authority.  (Of course there are also other factors as well when choosing a control surface -  such as ordinary lift/drag, aesthetics, heat tolerance (which seems uniform other than the FAT parts), and cost (not much of an issue for reusable parts).  

My uneducated guess is that you can measure the relative control authority of a given control surface has by taking the relative wing area, multiplying by the moveable percentage, and then multiplying again by some function (sine?) of the maximum deflection angle.  And then, if you want to figure out what surface gives you the most bang for its mass, you can divide that number by the mass of the part.  Does that all sound right?  Assuming that's all true, the Big-S elevons and tailfin look like the best choices, and the Delta Deluxe Winglet looks pretty bad.  

Also, there is a control authority tweakable slider.  Assuming your're trying to squeeze all possible control out of low-mass parts, is there any reason not to maximize this?  

[Raideur Ng]

All of that is generally valid, but one of the largest factors in control surfaces, specifically fully moveable ones, is location relative to the center of mass and lift. Control surfaces  near the center of mass will impart very little torque on the vehicle for their area. Surfaces farther will impart far more. Thus, having control surfaces far from the center of mass can impact significant control for even a relatively small wing area.

 

Test it. I assure you that having smaller control surfaces on the tips of the vehicle will result in superior control than large surfaces near the CoM/CoL. If this ISNT correct, inform us! That would be groundbreaking and something that should be known.

[bewing]

The Maximum Deflection Angle is only important in knowing whether that surface will have enough oomph to control your airplane, or whether it will "lock up" under extreme conditions -- sending your plane out of control. Under most circumstances you don't want any of the control surfaces coming anywhere near their limits, because they incur tremendous drag when they get more than a few degrees off prograde.

The Delta Deluxe Winglet is mostly a lifting surface. Its specs are a combination of the lifting surface typical specs with a control surface typical specs. When you take the lift into account, its cost and mass are right in line, AFAIK -- if you were to have both a wing and a control surface of those sizes, they would cost and weigh exactly that amount.

The control authority tweakable mostly has to do with SAS stability. If you set it too high, you can have inefficient oscillations. So you want it high, but not too high. So that one is a matter of testing on a particular design.

 

[fourfa]

Interesting analysis so far. My usual objective is to place the control surfaces as far from CoM as possible, and use as little as I can get away with - to minimize mass and more importantly, drag. That usually just requires a couple handling passes around the KSC. If I find that I'm dialing down the authority limit below 50, I look for a smaller one. 

One factor to consider is if you're using mods. Most I've tried will fly or hold angles using proportional control. Keyboard control is full max-authority deflection (bad as mentioned above). Joystick goes proportional. I have found profound differences in dV remaining in orbit between having a pilot aid hold a pitch for me, and flying the same profile by hand. Could be the extra drag from keyboard commands, or just pilot error. Many of the efficiency-challenge craft I've downloaded, I can't get to reach orbit by hand but breeze to orbit with pilot aids.

[Signo]

KSP loves "leverage" related to CoM. You must take this into account too.

It is easy to spot if you experiment with "T" tails. 

[GoSlash27]

Aegolius13,

 I almost always turn down the sliders on my control surfaces so that I won't have too much control authority. When the sliders are turned up and SAS is engaged, it can set up an oscillation that increases drag. I give it just enough control authority to feel pleasantly "heavy" at landing speed.

Best,
-Slashy

[Signo]

What Slashy said is especially true about "roll", the slider is limited to 45 for the craft in the pic I posted.

[Aegolius13]

Thanks all.  I did not mention distance from center of mass since I was talking about comparing one part to another for a given application, but it's definitely a bigger deal.

So it sounds like, to get a given amount of control authority but minimize drag, you may be better off with more surface area, but less angle of deflection.  That stands to reason.  And the non-moveable area of a surface is not really wasted, since it's as good as wing area   for providing lift (though it does not store fuel like the most efficient wings).  

 

[AeroGav]

On 14/12/2016 at 7:13 AM, Aegolius13 said:

My uneducated guess is that you can measure the relative control authority of a given control surface has by taking the relative wing area, multiplying by the moveable percentage

Correct.    I did some aerodynamics testing.  

All control surfaces and wing parts have the same lift/drag ratio at the same angle of attack and mach number.   However, 100% control surfaces have twice as much mass as a wing part (0% control surface) for the same relative wing area.

As has already been said,  the greater the distance from CG, the more leverage your control surface has and the more it can do with the same amount of force.

This means if you are putting a canard on something,  you want it right at the front to maximise the lever arm.  However, too much lift rating at the front of the ship brings your CoL forward, potentially making the airplane unstable.  So for canards i use 100% control surface parts only.

 

Quote

maximum deflection angle.  

Quote

Also, there is a control authority tweakable slider.  Assuming your're trying to squeeze all possible control out of low-mass parts, is there any reason not to maximize this?  

All wings and control surfaces use the same curve of lift and drag vs AoA.    If you are at a low deflection angle and increase it some, you get a lot of extra lift for not much extra drag.   If you are starting with a bigger angle, then a further increase in deflection results in a lot of additional drag but lift increases will be modest and subject to diminishing returns.   Beyond a certain point, the surface stalls and increasing deflection actually reduces lift, though drag continues to increase.

The tweakable appears to limit max deflection.    So if you take the tail fin,  and limit authority to 50%, it will deflect to a max of 15 degrees instead of 30.   The Advanced Canard, with a max def of 10,  deflects to 15 degrees when set to 150%.

Control surfaces are light compared to engines and fuel, in my opinion it's a false economy to go cutting them back to the minimum possible.

You want enough elevator power to be able to hold the nose up in cruising flight using small deflection angles, for minimum drag.  

If you want your plane to be idiot proof and impossible to stall or overstress, limiting authority will do this.  However it won't change the size of the nose up torque required to sustain level flight, so to keep trim drag low, you just need to make sure your CoM and CoL aren't too far apart and that the control surface is a decent size.    Airplane layout is key, if you can distribute fuel tanks fore and aft correctly, the airplane won't suffer CG shifts as fuel burns off, which lets you keep the CoM and CoL fairly close while retaining stability throughout the mission.

For ailerons, there's an even bigger issue - adverse yaw.

When you try to lift the left wing, the left aileron deflects to a greater angle of attack than the rest of the wing, in order to make more lift.    In the process, it creates more drag on this side of the airplane, which causes "adverse yaw", which is the nose pointing away from the direction you're trying to turn in.

Since i don't have rudder pedals, i like to limit the authority to 30% or less on my ailerons, which keeps adverse yaw to a minimum.    

 

[foamyesque]

I'm a bit weird in that I almost always max out the deflection authority for my pitch control surfaces. In ordinary flight, it's irrelevant, and on a well-designed craft SAS is perfectly able to handle it -- and when you execute a re-entry, that extra deflection range makes a real difference in being able to do controllable high-angle flight.