• 1.7.3 Design Notes - Propellers, Rotors and EC usage


    Maxsimal

    We’ve raced to fix bugs and add a whole bunch of new parts and functionality in this update – it’s almost a new version unto itself.

    I just want to take this time to highlight some of the additions and changes. 

    Propellers:

    Our new propellers take a different route than jet engines in the game.  Jet engines, while historically more difficult to construct, are simpler for a pilot to manage.  Propellers are not meant to compete with them.  However, propellers do offer one unique benefit – you can build rotor craft to fly on planets without oxygen in its atmosphere with the electric rotors.

    I'll cover the basics here - but if you want a more in depth look behind the physics of propellers, this video goes into a lot of the actual real world mechanics - many of which are simulated by KSP. 


    Managing Propellers:
    So, what makes propellers so difficult to manage for a pilot?  It’s because you have to manage the angle of attack of the propeller – which is basically just a spinning wing.

    First, consider the case with a normal fixed wing aircraft, illustrated below:

    0OL2ZfL.png

    The above is easy to manage.  When you want to increase angle of attack, and get lift - at the cost of more drag – you point the nose – and your wings - above prograde.  In level flight, that just means pointing them above the horizon.

    Now, consider a spinning propeller on plane that is not flying – just sitting still with its brakes on and the propeller spinning.  In this case, all the airspeed is coming from the fact that the propeller is moving through otherwise still air – pictured below for what one section of the propeller would see.

    U5jx1iE.png

    The angle of attack is still large and the propeller can generate a lot of lift, though it also causes a lot of both drag and the lift in a direction not fully parallel with forward – both of which the torque of the rotor has to counteract.

    Now, picture what happens when the plane starts moving.  Now the airspeed across the propeller comes from both the rotation of the propeller and the movement of the aircraft.

    F7Wl6BZ.png

    Consequently, the angle of attack has gone down dramatically, and you get less lift.  This is why propeller planes either have to change the angle of propeller blades – called their pitch – or remain limited to a low airspeed.  You could increase RPM, but RPM is limited in both KSP and the real world because propellers lose effectiveness if the propeller tips go faster than mach 1.

    In KSP, you can adjust the angle of our propellers by setting their ‘Deploy’ field to ‘Extended’ in the PAW, and adjusting the authority limiter, as pictured below.  KAL-1000 can assist you with coordinating the settings on multiple sets of propellers if you build a multi-engine plane. 

    tsS4mfp.png


    Using aero-debugging visualization – F12 by default – can assist you by letting you visualize the lift off of the propeller – your aim should be to adjust the pitch so that the yellow arrows are as long and as far forward as possible.

    NJNhGQ9.png
     

    Rotor Changes

    Rotors have seen a significant set of changes and improvements, as well as the addition of the liquid-fuel consuming rotors, which model a turboshaft engine for a propeller plane and for a helicopter.

    Now all rotors are set to max out at 460 RPM – near the limit that our physics engine allows.  Further, now you can reach that RPM limit regardless of how little torque is applied.  Before the rotor RPM and torque were unrealistically interrelated.  However, just as a rocket can reach any velocity in space regardless of how much thrust you use - lower thrust just means it takes more time - now a rotor can do the same, barring things like atmospheric drag that would oppose it.

    Finally, rotor RPM is more stable, you won’t see the RPM numbers vibrating as much.

    This all required retuning.  Further, our initial pass for rotor power gave too much torque.  Rather than reducing the available torque, we’ve increased the resource usage and weight/cost requirements for a given motor power.   Be aware that you absolutely don’t need to always keep the motor size at 100% - for many applications, a lot less torque will be more than enough.

    Robotics Resource Usage Changes

    While we endeavor to simulate physical systems in as realistic way as possible, that has to be balanced against fun and development feasibility.   With 1.7.3, we’ve now improved our resource usage simulation for robotics parts, though it is still by not 100% accurate to the real world.

    What this means is that in 1.7.3  the traversal or rotation rate you request from a part will now impact how much EC/LF is used by the part, where before it was not a factor in the resource usage calculations.



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    XLjedi

    Posted (edited)

    Have to say thanks again, for adding rotors and props.  I had stepped away there for awhile and with the 1.7.3 update you've brought me back into the kerbal design fold!  Very happy with these and the way you went about implementing them is brilliant.

    Edited by XLjedi

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    However, propellers do offer one unique benefit – you can build rotor craft to fly on planets without an atmosphere with the electric rotors.

    I assume you mean without an Oxygen atmosphere, propellers shouldn't work in a vacuum.

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    9 minutes ago, Paul Kingtiger said:

    I assume you mean without an Oxygen atmosphere, propellers shouldn't work in a vacuum.

    No you just need to bring other rotors to spin up and act as fans.

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    Thank you very much for the article! The new parts really changed a lot in KSP. So much new things to experiment and play with.

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    This all required retuning.  Further, our initial pass for rotor power gave too much torque.  Rather than reducing the available torque, we’ve increased the resource usage and weight/cost requirements for a given motor power.   Be aware that you absolutely don’t need to always keep the motor size at 100% - for many applications, a lot less torque will be more than enough.

    Overall, I like the new changes, with the massive exception that resource consumption should be based on actual RPM (not maximum RPM setting), and on actual applied torque, not the maximum setting.

    Flying around with both set to max drains something like 0.4 LF per second from the 1.3m turboshaft engine, making it the most fuel hungry airbreathing engine. Most of the time that figure is 10x or higher than it needs to be if a player doesn't adjust the max torque and rpm settings manually.

    On the other hand, if they do... it can lead to very low power consumption, and planes that easily fly on solar power alone

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    On 7/18/2019 at 4:27 AM, 5thHorseman said:

    No you just need to bring other rotors to spin up and act as fans.

    WIle E. Coyote likes this.

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    On 7/18/2019 at 12:27 PM, 5thHorseman said:

    No you just need to bring other rotors to spin up and act as fans.

    Hahah sorry all, fixed the issue.  You knew what I meant

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    Love the new parts. Some additional feedback.

    Fuel and EC consumption should be governed by the "current" torque and "current" RPM. It's bizarre that setting the max RPM and max torque above the current values increases consumption.

    Just my 2 cents, thanks!

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    Nuke

    Posted (edited)

    ive been doing this thing where i deploy the blades and use the authority limiter to vary the pitch of the blades. its better than controlling torque or rpm limiters. 

    i still haven't been able to get cyclic control to work. thats kind of stopping my single rotor designs from working. 

    Edited by Nuke

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    5 hours ago, Nuke said:

    i still haven't been able to get cyclic control to work. thats kind of stopping my single rotor designs from working. 

    Unfortunately, it appears that the rotor blades do not have the capability to control attitude of rotor craft via cyclically-feathering blade pitch as helicopters do; despite the rotor blades' PAW having Pitch/Roll/Yaw toggles.  For the moment, conventional helicopter designs will require other means to control attitude, such as reaction wheels.

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    18 minutes ago, Raptor9 said:

    Unfortunately, it appears that the rotor blades do not have the capability to control attitude of rotor craft via cyclically-feathering blade pitch as helicopters do; despite the rotor blades' PAW having Pitch/Roll/Yaw toggles.  For the moment, conventional helicopter designs will require other means to control attitude, such as reaction wheels.

    probably something that can be modded in. some kind of controller to set the pitch of each blade based on the phase of the rotor. 

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    XLjedi

    Posted (edited)

    On 7/28/2019 at 4:14 AM, Nuke said:

    probably something that can be modded in. some kind of controller to set the pitch of each blade based on the phase of the rotor. 

    The more I play with it... the more I don't see any need for it.  I just call em "Kerbo-Kopters" and use a reaction wheel as my pseudo-cyclic controller.  I could also probably assist with your single rotor designs.

    Edited by XLjedi

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    1 hour ago, XLjedi said:

    The more I play with it... the more I don't see any need for it.  I just call em "Kerbo-Kopters" and use a reaction wheel as my pseudo-cyclic controller.  I could also probably assist with your single rotor designs.

    built a quad that came out pretty good without the need for reaction wheels. a tandem bicopter might work, giving good control on the pitch axis where it matters. maybe some degree of yaw control by using torque limits in concert with variable pitch. roll control would be handled fine with reaction wheels in hover and with some forward airspeed with ailerons on stub wings. the harrier jet has a compressed air system that takes bleed air off the engine and directs it to nozzles on the wing tips, nose and tail for better control in hover. easy to mod some rcs thrusters to run on intake air and electricity to simulate a compressor and jet nozzles. i always feel that reaction wheels are way op in this game and i try to avoid using them whenever possible (though it is still standard equipment on most spacecraft). 

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    Nuke

    Posted (edited)

    ok my khinook configuration was somewhat flyable but i had to incorporate roll thrusters. im also not getting the yaw rate i need. i think its a tuning problem. i think i need to find the neutral point, the minimum torque needed to maintain hover and make that the center point of the opposing torque curves for the 2 engines. of course it it works it should induce a pitch tendency, and so you would have to compensate by tweaking the blade pitch on the front and back rotors.

     

    i really like being able to micro manage my control systems.

    Edited by Nuke

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    We could use some sort of spring system that automatically pulls/pushes towards a preset position (that can only be changed in the VAB) without consuming resources.

    This would be incredibly good for landing gear.

    Making the variable pistons be able to change to this mode where they would maintain their current position as a spring without consuming resources, would also be good.

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