Rotor krash course?

[Isidro22]

Ok so I am one of the minority of people trying to make a large functional electric submarine and I am using the mk3 sized parts. I have the whole ballast idea down, but I'm trying to figure out the rotor. I know we have to use aero surfaces not related to breaking ground, but I am using the heavy rotor from the DLC.

Starting off everything seems OK, but when I put max throttle to the rotor the control surfaces shear off. I have been looking to see if there is a magic target angle of attack and torque limit for hydrodynamic use but have been unable to find any relevant information.

I can get it moving .3 with two rotors so I figured I just need to add the ballast so it sinks and overcomes that dreaded water super drag.

I am on the PS4 so I can't really supply pics that easily. Any help would be appreciated!

[FleshJeb]

The aero parts function with the same parameters in water as they do in air. Max lift at 25 degrees AoA, max lift-to-drag at 5-7 for supersonic, 3 degrees for subsonic.

It sounds like your props are sticking partially out of the water to start, and then getting destroyed on impact? KSP has some funny stuff going on with water impact. For a perfect zero-AoA entry they should be able to take 3x their impact rating (15m/s for control surfaces?). I believe this impact resistance gets reduced with the sin of the angle. (I could be wrong, so folks can correct me.)

What I would do is build it with the surfaces flat to the disk, and have them set on deploy at 0 degrees. Start with a fairly low RPM, and slowly increase the deploy angle and RPM until you're moving. (What's the "unstick" speed of KSP water? 0.4m/s?)

If you didn't know, optimal deploy angle depends on the speed the craft is moving. The prop blades are drawing a corkscrew through the air/water, and that corkscrew gets stretched out the faster the craft goes. You want the blades to have an AoA relative to that corkscrew (say 3deg).  Generally speaking, aim for the maximum RPM at all times, and adjust your deploy angle to preserve that.

I haven't done an electric sub yet (I just know entirely too much about KSP physics), so I'd love to see the eventual results.

[Isidro22]

Ok I see, would it be better to set the KAL (I think that's what the controller is called) to modulate the AoA with throttle and set the torq and rpm to constant after the craft submerges? I have had it set to max rpm always and been controlling torq with throttle while experimenting with the angles needed.

The props are definitely sticking partially out of water, maybe going ahead and ballasting it properly (pun there?) will eliminate the prop strike problem. I've no idea if it can even get underway yet as right at the edge of .4 the props turn to magic smoke and escape me 

[FleshJeb]

@Isidro22 That's a very workable solution. For testing, I'd put torque, RPM, and AoA on 3 separate KALs and control them with the onscreen sliders, so that I can see the interrelationships. Or you can use the throttle and the RCS keys to control the KAL positions. Once you've got some data points you like, you can put all three on the same KAL and control that with the throttle. There are a lot of good ways of doing it, so pick whatever scheme works for you.

Remember also that you can reverse the rotors independently with action groups, as well as toggle the deployment or reverse the deployment of the props. Good for rapid stops and tight turning.

I would definitely ballast it more (good pun).  Of note, water impacts don't get calculated until the attachment point of the part passes between air and water. For control surfaces, I think that's the midpoint of the fat edge of the part, so if you've got the topmost one just over half submerged, you won't have to worry about it any more.

[Isidro22]

One more thing, how many blades would be considered optimal? I assume more lift area is good but also makes drag right? 

[18Watt]

3 hours ago, Isidro22 said:

One more thing, how many blades would be considered optimal? I assume more lift area is good but also makes drag right? 

I have found this to be very much dependent on the vessel, at least with air propellers.  The optimal number for one vessel is probably not the optimal number for a different vessel.

For trial and error I suggest starting with a small number of blades (2, 3, or 4), because that will be easier to work with as you experiment with how to control them smoothly.  When you think you have the best performance you can get, try adding two more blades, see if that improves things or not.

I don't think you are going to find a 'one-size-fits-all' answer, you will have to experiment.

[FleshJeb]

5 hours ago, Isidro22 said:

One more thing, how many blades would be considered optimal? I assume more lift area is good but also makes drag right? 

I agree with @18Wattwith a couple of additions:

• I find anything less than 4 blades can be a bit jerky. I don't know if that's the physics engine or what. It's somewhat reflective of real life.
• The goal is to maintain maximum RPM possible because that results in a blade angle that's more perpendicular to the direction of the craft. The steeper the blade, the more thrust they waste pushing sideways, and the less they push backwards. Of course, you'll need a minimum amount of thrust to break the water "stickiness", so that may require more blades.

General submarine advice: A submarine is just an airplane in a medium that's 1000 times denser.

 

[Isidro22]

Thanks I appreciate the advice!

[Isidro22]

Is there any functional difference in the normal rotor model vs the S model or is it just visual? Didn't see any answers on that when I searched.

Also have you encountered a bug where half of the rotor blades freak out and flip the trust backwards?

[FleshJeb]

16 hours ago, Isidro22 said:

Is there any functional difference in the normal rotor model vs the S model or is it just visual? Didn't see any answers on that when I searched.

Also have you encountered a bug where half of the rotor blades freak out and flip the trust backwards?

Look at the difference in max torques. I believe they're identical, except for the shaft diameter. Of course this might make your prop radius smaller, and the S has less volume, so it will be less buoyant.

There's an issue with deploying control surfaces, where if you edit your craft at all, the deployment can invert when you load the craft to the flight scene. It's weird and it sucks. The solution is to just hit the "invert deploy" button on the relevant surfaces in the flight scene. I hope this is what you're running into, anyway.

[Isidro22]

2 minutes ago, FleshJeb said:

Look at the difference in max torques. I believe they're identical, except for the shaft diameter. Of course this might make your prop radius smaller, and the S has less volume, so it will be less buoyant.

There's an issue with deploying control surfaces, where if you edit your craft at all, the deployment can invert when you load the craft to the flight scene. It's weird and it sucks. The solution is to just hit the "invert deploy" button on the relevant surfaces in the flight scene. I hope this is what you're running into, anyway.

While watching them they flip back and forth from reversed to normal, does that sound like what you are thinking of?

[FleshJeb]

4 minutes ago, Isidro22 said:

While watching them they flip back and forth from reversed to normal, does that sound like what you are thinking of?

Nope. That's another problem. Control surfaces will flop direction depending on whether the axis of the surface is pointing ahead or behind the COM of the craft. I think I saw in your other post that you were using the angled elevons--That might be the problem. Try different ones.