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Precise control of climb angle : Spaceplane How To


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Real pilots work in very fine increments.  A jet airliner flies at a speed and height precisely calculated to give the lowest fuel burn at the current weight.   Adjustments are made with the autopilot or by adding / removing one notch of trim.   In the back, passengers pouring drinks into plastic cups never even notice.

Harsh manoeuvres don't just spill beverages, they also create drag, so if you're a test pilot looking to set a speed or altitude record you also need to climb at the right angle.

Unfortunately when flying with keyboard, control inputs are "all or nothing".    At high speed, a brief tap of the S key results in a brief 15G pull-up.    KSP joystick support is still patchy, an uncalibrated controller does little better.

What we really want is something that lets us adjust angle of attack in one degree increments.   When trying to get through the sound barrier, we want to lower our AoA close to prograde for minimum drag, not be stuck at 5  or 10 degrees above or below it like a giant airbrake.      During the speedrun, when trying to milk as much speed out of the airbreathing engines as possible,   we need absolutely level flight.   At these speeds, a climb angle of 1 degree can stop us accelerating, or send us into air too thin for the engines to have power very quickly.   A dive of 1 degree quickly sends us down into the soupy lower atmosphere where drag, heating and fuel burn are enormous.

Other than autopilot mods, what can our intrepid pilot do ?

Technique 1  - SAS  Stability Assist lock


Simply pressing T activates SAS in it's default Stability Assist mode.  In this mode, SAS makes control corrections to try and hold the nose attitude same as it was last time you touched the controls.   At high speeds,  due to the curvature of the planet, this results in a slowly rising AoA.    When it gets too high, pressing any of the control keys, resets SAS's target attitude and drops the nose a couple of degrees.        For the same reason, this technique is less useful in situations where frequent corrections are being made , since every time you correct a roll or a yaw, the nose drops, which you might not want.

Technique 2 -  Pitch Trim

Pitch trim cannot be used while SAS is active.  However, if your plane is stable enough to fly without it, and you're not in a situation requiring rapid changes in pitch, it can give very precise control.     

Hold down the ALT key and press S to increase nose-up trim.   ALT + W to decrease it, and ALT X to remove all trim.

The effect of trim is like pulling back very gently on an analog stick and holding the controller there at the same angle throughout the flight, minus the wrist cramp.


Technique 3 -  Authority Limiter Adjustment


The elevons at the back of the main wing are set to control pitch and roll.   Note that as they are behind CoM, they get the nose UP by pushing the tail DOWN.  At the front is a canard, currently doing nothing.        And we have SAS enabled in Prograde Hold mode, so the rear elevons are controlling our pitch so that we have almost zero AoA.   This means our wings aren't producing any lift so the plane is in a dive.

After clicking the "Deploy" button on the front canards, they deflect and start creating a large amount of lift that picks the nose up.  If SAS wasn't running that would cause the plane to fly with an AoA of about 10 degrees, but because SAS is still trying to hold prograde, it fights the effect by generating lift with the elevons at the back of the ship.    This results in the plane flying at about 3 degrees AoA,  an efficient angle for making enough lift to stay airborne whilst keeping drag low.


Note you can adjust the pitch angle  by tweaking the "Authority" slider.    If you reduce the "authority" of the canard, it deflects at a lesser angle and our nose drops.   If you lower the authority of the elevons that are fighting it, our nose goes up.

Here we are doing the speedrun.    17km is a good altitude for a speedrun on the Whiplash - above this height, thrust starts to fall faster than drag.   By tweaking this slider, I can get us to fly level here quite easily.   This is easy to do because SAS is maintaining control of the plane  and I don't actually have anything else to do as a pilot other than play with this slider !


Picture - Elevons lifting the back end, Canard lifting the front.


Here's a video of me flying a different craft, where i accidentally discover this technique during the speedrun 


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