Ducted fans? Just for looks?

[KerikBalm]

So with 1.8, we have new fan blades, and shrouds.

As far as I can tell, these are largely cosmetic, but the shrouds may have some niche uses for protecting fan blades from collisions (for designs where prop blades are close to the ground when landed.

I notice that the normal prop blades can work with the shrouds too.

Is there any advantage to these fan blades over the standard prop blades?

I was hoping that they would have a higher lift point offset to make them act as if they were operating at a higher RPM.

[RealKerbal3x]

@KerikBalm I think they give higher thrust, just as real ducted fans do. I believe they gain the higher thrust by channeling more air through a certain area than standard propellers.

Edited October 27, 2019 by RealKerbal3x

[Xyphos]

Higher thrust and operate at higher altitudes; normal fans usually don't go above 2km but I've seen a ducted fan vessel get up to 6km.
still not as impressive as 18km or 23km like the turbojet and rapier engines, but still great performance.

Edited November 2, 2019 by Xyphos

[Phreakish]

On 10/27/2019 at 1:22 PM, RealKerbal3x said:

@KerikBalm I think they give higher thrust, just as real ducted fans do. I believe they gain the higher thrust by channeling more air through a certain area than standard propellers.

In reality they increase thrust by reducing tip losses. Similar to winglets on wings, the duct helps improve the lift distribution profile over the span of the wing/blade (similar to high aspect ratios). It does this by making it harder for the higher pressure flow on the 'bottom' of the blade to reach the lower pressure flow on 'top' of the blade, maximizing the difference in the pressure of each flow path.

[KerikBalm]

On 11/2/2019 at 12:34 PM, Xyphos said:

Higher thrust and operate at higher altitudes; normal fans usually don't go above 2km but I've seen a ducted fan vessel get up to 6km.

 

On 10/27/2019 at 9:22 PM, RealKerbal3x said:

@KerikBalm I think they give higher thrust, just as real ducted fans do. I believe they gain the higher thrust by channeling more air through a certain area than standard propellers.

I really doubt this. Its the same RPM limit, its the same "thrust" mechanic with apparently the same offset. So their thrust is just a matter of RPM, theire "area" stat, and their pitch relative to the speed.

They have smaller area's than the corresponding size prop blades (although a large duct blade is physically smaller than a large prop blade).

So I'd think they give inferior performance for the same part count, but can be made a bit more compact.

[Guest]

Ducted fans are practical on Duna at low altitudes, unlike regular props.

Edit: I made a ducted fan electric plane that reached 8400 m altitude on Duna. Hover ceiling on Kerbin is over 14 km. This is a lot more than I've been able to achieve with regular rotors (or props). (Not as high as with rotors built from elevons however.)

Edited November 12, 2019 by Guest

[herbal space program]

On 11/10/2019 at 5:13 AM, Brikoleur said:

Edit: I made a ducted fan electric plane that reached 8400 m altitude on Duna. Hover ceiling on Kerbin is over 14 km. This is a lot more than I've been able to achieve with regular rotors (or props). (Not as high as with rotors built from elevons however.)

Yeah, I slapped together a solar-powered Mk1-based quad-rotor helicopter with regular blades and got it above 15km several times last night. I was also able to pitch it over, using 4 rear-mounted advanced canards as wings,  getting up to ~80m/s ground speed as an awkward-looking pseudo-airplane. Plastered with panels, I found I could fly it essentially forever in daylight. What I couldn't figure out how to do is land it  safely, as the amount of hysteresis involved in trying to use the torque limiter to go up and down was well nigh impossible to deal with. I suppose stopping the rotors and deploying a parachute would work!

...So has anybody tried to use something like this yet to get off Eve in a single stage? I'm nearly certain it should be quite possible based on my experience last night....

Edited November 13, 2019 by herbal space program

[Guest]

On 11/14/2019 at 12:13 AM, herbal space program said:

What I couldn't figure out how to do is land it  safely, as the amount of hysteresis involved in trying to use the torque limiter to go up and down was well nigh impossible to deal with. I suppose stopping the rotors and deploying a parachute would work!

Don't use the torque limiter to control hover, use blade pitch. You may need to use the RPM limiter to prevent Bad Things from happening in Eve's lower atmosphere.

I haven't attempted an Eve SSTO but did build a theoretically reusable Eve launcher.

[herbal space program]

8 hours ago, Brikoleur said:

Don't use the torque limiter to control hover, use blade pitch. You may need to use the RPM limiter to prevent Bad Things from happening in Eve's lower atmosphere.

I haven't attempted an Eve SSTO but did build a theoretically reusable Eve launcher.

After burning the better part of my weekend, I'm getting the hang of it so to speak.   Flying propeller aircraft is tricky!  When I went to bed last night, I had a plane that fell just shy of making LKO in a single stage using only electric props and aerospikes. The rocket stage had a vacuum dV of ~2.5km/sec with all the propellers attached, which is less than I would need to get so close to making orbit from the ground, but without the props I think I would have had enough dV to do it. The best I've been able to do so far hauling that rocket stage with the ducted fan blades is around 180 m/s at 13km with  the prograde marker like 20 degrees above the horizon. I can get higher than that, but only at the cost of a lot of speed, and from that altitude the drag of the propellers during the rocket burn is pretty  cumbersome, even if I do my best to manage it with the blade pitch control.

So I feel like unlike jet engines, propeller stages really don't help you making orbit on Kerbin, but what is cool about them is that after re-entry, you can fly essentially forever if you've got enough batteries and panels. Of course Eve is another matter, since there are so many bad effects from the thick atmo, but from what I've already done I can see it will be challenging!

Edited November 18, 2019 by herbal space program

[XLjedi]

It appears the ducted fan blades are bugged when compared to the rotors and propeller blades.  It becomes evident in multi-engine counter-rotating engine designs.

[herbal space program]

1 hour ago, XLjedi said:

It appears the ducted fan blades are bugged when compared to the rotors and propeller blades.  It becomes evident in multi-engine counter-rotating engine designs.

What I noticed is that for regular propellers, if you set the first one up clockwise and then clone it and just toggle everything to be backwards, it works fine, but it doesn't for the turbofan propellers. IOW, when you set the blade variant to CCW for a regular propeller blade, it will snap to a mirror image of the pitch of the CW version, but when you do that to a turbofan blade, it ends up at the wrong pitch. I had to adjust them all manually to get it right, but then they worked fine.

Edited November 18, 2019 by herbal space program

[XLjedi]

7 minutes ago, herbal space program said:

What I noticed is that for regular propellers, if you set the first one up clockwise and then clone it and just toggle everything to be backwards, it works fine, but it doesn't for the turbofan propellers. IOW, when you set the blade variant to CCW for a regular propeller blade, it will snap to a mirror image of the pitch of the CW version, but when you do that to a turbofan blade, it ends up at the wrong pitch. I had to adjust them all manually to get it right, but them they worked fine.

You can never get a ducted fan to have the painted surface of the blades all facing in the same direction on counter-rotating engine designs.  If it were done correctly, the painted stripes on the blades for both engines would face in the direction of the lift-vector.  It works correctly for Rotors and Propellers, it's wrong on the ducted fan blades.

[Guest]

38 minutes ago, XLjedi said:

It appears the ducted fan blades are bugged when compared to the rotors and propeller blades.  It becomes evident in multi-engine counter-rotating engine designs.

Can confirm. Only solution I've discovered is to switch off symmetry when assembling the craft, and place each engine assembly individually. Works fine that way but it's fiddly to build.

[XLjedi]

Just now, Brikoleur said:

Can confirm. Only solution I've discovered is to switch off symmetry when assembling the craft, and place each engine assembly individually. Works fine that way but it's fiddly to build.

No...  I don't use symmetry.  If anyone has a picture of a functioning counter-rotating ducted fan engine where both engines have the lift-vector stripes pointing in the same direction.  Then I may have to concede I'm doing something wrong and will have to ask how they managed it.  When I mount these new blades side-by-side with the standard propeller blades it becomes painfully obvious there is something not-quite-right about them to say the least.

[herbal space program]

1 hour ago, XLjedi said:

You can never get a ducted fan to have the painted surface of the blades all facing in the same direction on counter-rotating engine designs.  If it were done correctly, the painted stripes on the blades for both engines would face in the direction of the lift-vector.  It works correctly for Rotors and Propellers, it's wrong on the ducted fan blades.

What I'm saying is you have to do do that manually for the ducted blades, using the rotation tool,  but not for the other ones, so there is something wrong about how the two variants are configured for that series of parts. You can actually get thrust from a ducted blade even if it's mounted backwards, but that's another matter.

1 hour ago, XLjedi said:

No...  I don't use symmetry.  If anyone has a picture of a functioning counter-rotating ducted fan engine where both engines have the lift-vector stripes pointing in the same direction.  Then I may have to concede I'm doing something wrong and will have to ask how they managed it.  When I mount these new blades side-by-side with the standard propeller blades it becomes painfully obvious there is something not-quite-right about them to say the least.

 

Anyway, here's a pic of my ducted fan plane with two pairs of counter-rotating propellers. As you can see the thrust vectors look more or less the same for all 4 of them.

 

Edited November 18, 2019 by herbal space program

[XLjedi]

@herbal space program  if you wouldn't mind posting a link to your plane, I'd like to see if it changes my opinion at all.

[herbal space program]

9 minutes ago, XLjedi said:

@herbal space program  if you wouldn't mind posting a link to your plane, I'd like to see if it changes my opinion at all.

I don't have a link to the craft file, but here's a pic from the SPH where you can see them all facing the same way:

 

BTW, I'll add here that I tried this with and without those ridiculously heavy shrouds and I noted no obvious difference in performance or how all the aero indicators looked.

Edited November 18, 2019 by herbal space program

[XLjedi]

@herbal space program  I must be doing something wrong then.  I dont use symmetry when placing the engines, so what do you mean by “manually”?

[herbal space program]

20 minutes ago, XLjedi said:

@herbal space program  I must be doing something wrong then.  I don't use symmetry when placing the engines, so what do you mean by “manually”?

Here's what I did: I started by placing two pairs of mounting points on the wings using mirror symmetry, then adjusted their heights/angles using the rotate/displace tools in absolute mode with angle snap. I then placed one engine, not using symmetry, on one of the left-side mounting points, set it to "octo" for blade placement nodes, then put the blades on the hub using 8-fold radial symmetry and angle snap. I set the rotor and blades both to "clockwise" and set the Deploy settings to "normal" and "extended", finally setting the Deploy Angle to 0 rather than 60. I then used the rotate tool, in Local mode with angle snap, to set the blade pitch to one click clockwise of flat. After doing that, I pulled that motor off, set it aside, and hit the undo key. I then put the removed motor on the opposite node, set it and the blade variant to "counterclockwise", and changed the Deploy setting to "inverted". With a regular propeller, that would be all you'd need to do, but somehow with the turbofan blades they snap to the wrong pitch when you select the CCW variant. So I used my rotate tool again on those blades, using local mode with angle snap, to rotate them to the mirror-image starting pitch of the ones on the other side. Once I had actually done that correctly for both of my CCW engines, everything worked right. Make sense? And yes, it was a royal pain in the exhaust nozzle figuring all that out!

Edited November 18, 2019 by herbal space program

[XLjedi]

@herbal space program  Thanks, I think I got it figured out now.  Your description was exactly what I had been doing with props and rotors, so it seemed very odd to me that I was not getting the same results.

I think the problem I was having related to the way I use the KAL-1000 to map power curves for my engines.  I had used radial attachment with my blades on the wrong (smaller side) of the rotary engines I was using.  So I first redid the blade attachment with the nodes set to Octo and I forced-snapped them to the nodes this time.   This mounted the blades in the correct location.

After I did this, the blade deploy angles were still not functioning correctly.  After tinkering with several different things, I finally deleted every reference from the KAL-1000 and then reassigned everything and finally it is behaving as I expected it would!  So for me, the blades now function exactly the same as the rotor and propeller blades... so I'm happy and have to retract on my bug statement. 

However, I did not notice a need to set the blade pitch one notch off of flat.  I'm using my throttle at 50% as the neutral or zero position and then forward and back is forward and reverse thrust respectively (based entirely on blade pitch).  So I may not notice them being off slightly? 

[herbal space program]

32 minutes ago, XLjedi said:

 However, I did not notice a need to set the blade pitch one notch off of flat.  I'm using my throttle at 50% as the neutral or zero position and then forward and back is forward and reverse thrust respectively (based entirely on blade pitch).  So I may not notice them being off slightly? 

No, that part was just a matter of personal preference for me, but it was part of my procedure so I included it. I'm using the torque limiter as my main throttle axis and have the blade pitch mapped to translate up/down, both with incremental rather than live axis control (I'm using a Nintendo-style game controller). I set it that way so I'd have  some forward thrust to start with and also so that I'd have enough control authority to get my blades nearly edge-on to the airflow to reduce drag while boosting out of the atmosphere. I found that when flying as fast and high as possible, you have to keep that pitch angle on just the right value or you suddenly aren't flying anymore. The only way I was able to get the best performance was to have the Alt-f12 aero markers on and use those to guide me. Also, as you get higher up you sometimes need to reduce blade pitch (i.e. lower "throttle") to go faster, because there's a complex interaction of the motor RPM, air density, and airspeed. It feels more like a trim adjustment than a throttle axis to me, that gets more and more critical the closer to the edge of the envelope you get. Since what I'm ultimately trying to do is make an Eve SSTO with a prop-driven initial ascent stage, I need to figure out how to get every last bit of height and speed out of those props while hauling a heavy load, so that trumps all other considerations. For that reason, I have props that only go like 120 RPM at max throttle on the ground, and only reach their 460 limit at ~13 km. I  think that makes stuff more complicated for me.

Edited November 19, 2019 by herbal space program

[Guest]

9 hours ago, XLjedi said:

If anyone has a picture of a functioning counter-rotating ducted fan engine where both engines have the lift-vector stripes pointing in the same direction.  Then I may have to concede I'm doing something wrong and will have to ask how they managed it.

Ask and ye shall receive.

Stationary, so you can see that all the rotors are contra-rotating:

In flight:

I built the plane normally, then removed one of the engine assemblies, switched off symmetry, and placed one at each wing separately. Then I switched the blade variants to the mirror image on one of the pods, changed the motor rotation direction, and inverted the deploy direction.

 

[XLjedi]

@Brikoleur  Yeah, I figured it out...  it was actually a problem with the KAL-1000.  For some reason, the Pitch/RPM/Torque settings I had in there became corrupted.  The only way to fix it was to delete every one of the settings in the KAL and start from scratch.  Just took me a long time to figure out that it wasn't the part that was the problem.  But thank you all, the pics and descriptions confirmed what I had thought should be the correct mode of operation.  So I at least knew there was something else going on here.

[Guest]

1.8 borked my KALs too. I have craft that use them to set prop blades and at least on one of them it now sets authority limiter rather than deploy limit, i.e. it now does nothing.