Need help with planebuilding (optimizing a turboprop)

[DDE]

First craft in years, so naturally I had to try to master the "simple" art of single-engine turboprops before moving on to rockets.

This thing can fly. I've got Pitchperfect autoadjusting blade angle. Front tanks are B9-switched to LF only, rear tank is half-full to make it at least bounce pack off its rear end when it loads.

Problems:

1. I've still got no idea about optimizing torque and RPM;
2. While I was fully prepared for some degree of induced roll, there's quite a bit of it. Overall, SAS can't handle it at all, and vigorous manual corrections are required to keep it steady (MechJeb 2, though, can).

NOTICE ME SENPAIS!!1!

Edited March 31, 2022 by DDE

[swjr-swis]

With the one you linked last and less changes:

Mostly I notice those prop blades generate much less thrust from the same available torque than the duct fan ones I used. You'd need to double up on them to get even close. Also, their optimal angle of attack seems to be more in the range of 7-9 degrees, although it's hard to pinpoint because the display values are very erratic, even when flying stable and level.

DDE Mustang demodded 2

 

[swjr-swis]

1 hour ago, DDE said:

it just doesn't fly. Even at 70 m/s I can't get the two main wheels off the runway! I've already drained fuel until it's back at 6 t, no effect.

At first sight: this general design, with the parts I can see, should definitely be able to fly, even fully fueled. So there's got to be something about the details of its design.

With only a screenshot and at that angle, it's hard to put the finger on it, but there are a few things that are obvious:

• There seems to be no angle of incidence on the wings. KSP does not simulate aerofoils - the simulated physics require wings to be placed at a (small) angle to the airflow to transform some of the drag into lift. There is much debate about optimal angles, but I find that 5 degrees tends to be a very nice approximation, that can be done easy and repeatably without mods: with the Rotate tool on snap, rotate the leading edge one fine-snap (hold shift) up. Just this one 'trick' will make a world of difference for any plane's performance in KSP.
• Next is the CoM (center of mass, yellow ball) and CoL (center of lift, blue ball). Generally it's good to start by trying to bring those two close together, the CoL overlapping and just slightly behind the CoM. Closer adds lift neutrality and maneuverability, but makes a plane more unstable, while a bit more distance adds passive stability at the cost of a tendency to lawn dart and resistance to pitch up. If I'm not mistaken, your plane has the CoL quite a bit behind the CoM, making it very hard to lift the nose and take off.
• Placement of the main gear can also make it hard to pitch up for take off, but it looks like you have it reasonably close behind the CoM, so that is likely not the issue.
• The elevons on the wings can be configured to 'deploy' on an action group, so they act as flaps. This adds lift at lower air speeds, also helping with take off. (also adds some drag, but the lift effect is worth it).

I rebuilt your plane from the screenshot you posted to see if could make it fly with the above-mentioned changes. You can download it from this link to compare with your own, or use the tips I mentioned directly in your own craft if you prefer.

Disclaimer: I am... not a fan of stock props, to put it mildly. I will refrain from expanding on my reasons, not the place or time. So I clipped a pair of Junos inside the Mk1 LF tanks to avoid dealing with the props. Basically this means extra weight, so if you have a good handle on your props, yours should perform even better without the Junos.

A rebuild of your plane from your screenshot. I think I have most of it, sans the prop blades, but you seem to have used a short triangular wing section on the nose that I can't find in my 1.12.3 install. I used the stock 'small delta wing' instead and clipped most of it into the center fuselage.

This here shows the angle of incidence I added on the wings, and the resulting stock CoM and CoL indicators. They are quite close, which I think is mainly due to the bigger wing section I used in place of the one you have. But this works very well.

I added the wing elevon deployment to the RCS action group to aid in take off. Tip: wait to deploy flaps until making about 50-60 m/s... the plane will accelerate faster and then pretty much hop into the air by itself.

Fully fueled, and weighted with two extra Junos, but this really should perform as well or better with the actual props.

Edited March 31, 2022 by swjr-swis

[Vanamonde]

Cute little plane.

Is this a craft sharing thread or a gameplay issues discussion? 

[DDE]

8 hours ago, Vanamonde said:

Cute little plane.

Is this a craft sharing thread or a gameplay issues discussion? 

The latter. Otherwise I would've probably shared the craft )

[Gargamel]

2 hours ago, DDE said:

The latter. Otherwise I would've probably shared the craft )

Let’s bump you over to Game Play Qs then, shall we?  
 

And psssst, you’re not alone not having built a new craft on a long time. 

[KerikBalm]

11 hours ago, DDE said:

First craft in years, so naturally I had to try to master the "simple" art of single-engine turboprops before moving on to rockets.

11 hours ago, DDE said:

1.  While I was fully prepared for some degree of induced roll, there's quite a bit of it. Overall, SAS can't handle it at all, and vigorous manual corrections are required to keep it steady (MechJeb 2, though, can).

I would suggest that twin engine contra rotators are "simpler". IRL, yesm they are mechanically more complex (2 engines instead of 1, or complex gears to make one engine drive 2 contrarotating props), but in KSP, they are just so much easier.

FWIW, I have barely used the turboprops, and instead mostly use the electric rotors. Turboprops just don't seem competitive with turbofans in KSP (haven't compared fuel efficiency, but its so much simpler to use a wheesley/panther/Goliath, all of which already have excellent fuel efficiency).

If I'm using props, its for extra-kerestrial worlds without O2, but I also test them on Kerbin, they can go quite fast. Note, I have found that the "blades" give much better performance (at least at high speed) than the "props"

 

I haven't compared efficiency of using fuel cells + electric rotors vs the air breathing turines. One would hope that LF consumption is similar, and its just the Oxidizer consumption that differs significantly, but I have my doubts.

Spoiler

Re-usable 2 stage eve cargo craft:

It doesn't balance with full tanks and no 2nd stage cargo craft

but with empty tanks and no payload, it flies fine:

 

11 hours ago, DDE said:

I've got Pitchperfect autoadjusting blade angle.

Never used it, seems like such a think would be convenient, much easier than manually adjusting blade angle.

11 hours ago, DDE said:

1. I've still got no idea about optimizing torque and RPM;

I suppose it depends what you are trying to do. Maximize fuel efficiency for a moderate speed cruise? or maximize speed - as seen above, you can get to nearly mach 1 with these props, but according to the part window, the blades are going super-sonic, and we've seen thats really load IRL (Tu-95 had prop tips going supersonic, and the XF-84H Thunderscreech was ridiculously loud) 

Generally speaking, I try to have RPM as high as possible, you can see in my screenshots that i have a rotor window open to monitor RPM. When RPM starts to fall, I increase torque (at least now one doesn't have to match the RPM setting to the actual RPM, before if you set max RPM at 460, but only achieved 230 rpm, it would consume power as if it was at 460 RPM, so reducing max RPM setting would reduce power/fuel consumption without affecting power output).

I figure I want the lift vector from my blades to be pointing forward as much as possible for maximum efficiency, and that means higher RPMs (at least at significant forward speed, doesn't neccessarily apply to slow speed planes (helicopters).

Also, I noticed that different types of blades have different optimum AoAs. Generally speaking, you want to keep the blades inedned for use in ducted fans to between 4-5 degrees AoA, but the standard propellor blades do better at higher AoA (I forget what AoA exactly, I think its more around 10).

Similarly, the Helo blades have a different optimum AoA, I hope the mod takes that into account. You can use helo blades, I tried to use them in tilt-rotor craft:

but so far I've had the best results with the ducted fan blades. The downside is, of course, higher part count relative to the massive helo blades.

 

[DDE]

@KerikBalm, yeah the ducted fan blades should basically result in an unducted propfan a la Progress D-70.

[DDE]

Also, separately, I believe I started off on the wrong foot. The Pitchperfect setting I used ties throttle to blade angle. What if I switched it to the setting that maximizes thrust at all times, and went back to managing RPM and perhaps torque?

[DDE]

So, take two. It still veers to the left, but at this point I'm convinced it's a feature of the Dessert Runway.

I do have a new problem, though: it just doesn't fly. Even at 70 m/s I can't get the two main wheels off the runway! I've already drained fuel until it's back at 6 t, no effect.

Edited March 31, 2022 by DDE

[KerikBalm]

22 hours ago, DDE said:

So, take two. It still veers to the left, but at this point I'm convinced it's a feature of the Dessert Runway.

I do have a new problem, though: it just doesn't fly. Even at 70 m/s I can't get the two main wheels off the runway! I've already drained fuel until it's back at 6 t, no effect.

Do you have some wing incidence?

At least have the nose point up a little when just resting on it's gear

On 3/31/2022 at 6:29 PM, DDE said:

Also, separately, I believe I started off on the wrong foot. The Pitchperfect setting I used ties throttle to blade angle. What if I switched it to the setting that maximizes thrust at all times, and went back to managing RPM and perhaps torque?

I don't know about that mod. Generally, I set RPM to max, and use an axis group for blade pitch.

I apply a little throttle to spin up the blades at 0 AoA, then I increase blade pitch, and keep increasing it as speed increases, adjusting throttle (having torque linked to throttle) to keep RPM up.

It is much more complicated than just using a wheesley.

[TheFlyingKerman]

Would it be simpler just linking the pitch angle with the pitch using KAL-1000? I am still new to propellers and this is what I've got so far

https://kerbalx.com/TheFlyingKerman/Electra-2A

if anyone would like to further optimize it.

It seems aiming for 400-440 rpm gives the best speed for the least power consumption.

[DDE]

On 4/1/2022 at 12:11 AM, swjr-swis said:

Next is the CoM (center of mass, yellow ball) and CoL (center of lift, blue ball). Generally it's good to start by trying to bring those two close together, the CoL overlapping and just slightly behind the CoM. Closer adds lift neutrality and maneuverability, but makes a plane more unstable, while a bit more distance adds passive stability at the cost of a tendency to lawn dart and resistance to pitch up. If I'm not mistaken, your plane has the CoL quite a bit behind the CoM, making it very hard to lift the nose and take off.

Correct. Rather excessively stable.

On 4/1/2022 at 12:11 AM, swjr-swis said:

The elevons on the wings can be configured to 'deploy' on an action group, so they act as flaps. This adds lift at lower air speeds, also helping with take off. (also adds some drag, but the lift effect is worth it).

I was trying to use the inner set of elevons as flaps, but at 20 degrees they created more drag than lift.

On 4/1/2022 at 12:11 AM, swjr-swis said:

I think I have most of it, sans the prop blades, but you seem to have used a short triangular wing section on the nose that I can't find in my 1.12.3 install. I used the stock 'small delta wing' instead

That is what what they were. I was desperately trying to push CoL back forward. It was right at the aft tip of the central fuselage - the tail stabilizer creates a heck of a lot of lift, it seems.

On 4/1/2022 at 12:11 AM, swjr-swis said:

the bigger wing sectio

No. I believe this is a very exact rebuild.

On 4/1/2022 at 12:11 AM, swjr-swis said:

There seems to be no angle of incidence on the wings. KSP does not simulate aerofoils - the simulated physics require wings to be placed at a (small) angle to the airflow to transform some of the drag into lift. There is much debate about optimal angles, but I find that 5 degrees tends to be a very nice approximation, that can be done easy and repeatably without mods: with the Rotate tool on snap, rotate the leading edge one fine-snap (hold shift) up. Just this one 'trick' will make a world of difference for any plane's performance in KSP.

On 4/1/2022 at 9:13 PM, KerikBalm said:

Do you have some wing incidence?

Hm. Interesting how I'm not used to it. Wonder if having started playing back in the souposhere era has something to do with such recalcitrance.

The angle of incidence got it to fly, but it retained the bad habit of augering in. Takes all the pitch authority to keep it airborne. Yours, as I tested, is about half as bad; could be the effects of a tractor vs pusher.

Went back and carefully redid the wing angle, including the dihedral you didn't mention. After some troubleshooting of the engines that had to be taken after the rebuild, experiments showed that the anhedral destroys lift - the plane can barely tear itself off the runway again (despite weighing 6 t vs your 10 t). Unfortunately, I can see why you're using the anhedral - it counters the augering-in effect induced by the mutual position of CoM and CoL.

Spoiler

So I went back and gave just a tiny bit of incidence to my tail stabilizer. Didn't help. Tried the opposite. Didn't help. Raised the wing a bit. Didn't help, and I once again lost the ability to fly.

Now this should load even with any traces of PicthPerfect and Kerbalism; I deleted the most important part of a plane - the ReStock+ navlights.

https://pastebin.com/R5Zic5RB

I think this deserves no more than one round of troubleshooting before I dump propellers for good. Guess the ground broke me.

[FleshJeb]

Your COL is still way too far back. Put it right on top of the COM and let SAS handle the stability.

I think those engines decrease in mass if you set the max torque lower on the part (the electric ones do). Drop them to about 10-15%, they have PLENTY of torque. That should push your COM back to a reasonable spot.

Don't bother with flaps in KSP, they just push your nose into the ground. If you have to, put some control surfaces on the leading edge that deploy upward.

I think one of those icons is RCS Build Aid--Check your center of thrust by putting a tiny rocket engine on the axis of the motor. You can also get a better handle on the aerodynamics by temporarily taking the blades off and looking at the plane with CorrectCOL.

Incidence + dihedral on wings is generally counterproductive in KSP. Anhedral vertical stabilizers is good to help the counter-roll issue from rudders.

With props, max RPM always. Vary the torque to just barely keep the RPS maxxed. For blade angle, I build them as a flat disk, and deploy from 0 to about 65 deg. I keep a KAL linked to my throttle and do a linear torque from 0-100, and blade-pitch is 0-10% @ 0 deg, 10%-67% is 25-55deg with a slight curve. 67%-100% is 55deg-65deg, although most low-power builds can't get to 65 and it's actually counterproductive to go that high. So throttle ends up being about picking what's best for the speed regime, rather than just MORE POWER.

[swjr-swis]

9 hours ago, DDE said:

The angle of incidence got it to fly, but it retained the bad habit of augering in. Takes all the pitch authority to keep it airborne. Yours, as I tested, is about half as bad; could be the effects of a tractor vs pusher.

(...)

Unfortunately, I can see why you're using the anhedral - it counters the augering-in effect induced by the mutual position of CoM and CoL.

5 hours ago, FleshJeb said:

Don't bother with flaps in KSP, they just push your nose into the ground. If you have to, put some control surfaces on the leading edge that deploy upward.

(...)

Incidence + dihedral on wings is generally counterproductive in KSP.

I feel like I've been issued a challenge here... twice.

Fine, I'll do the thing with actual props (I really don't like props) just to prove the point.

Took the version I had already made and redid it without Junos, with props, and then tweaked some more things to get the CoM/CoL and fuel balance adjusted the way I couldn't before because I was trying to stay as close to the original as possible. So DDE-proptest2, with props now:

Spoiler

Starting from the first I posted, but removed the Junos and used the actual props. Used the ducted fan props though... the others looked too small.

Still very close to the original, but with some small changes to get the balance how I like.

Taking off without touching the stick. And flaps down, just cause I was just told not to bother with them.

Cruising at mach 0.88 or 278 m/s @ 6.5 km.  Throttled back to 23% to conserve fuel while maintaining max RPM, with blade AoA at 4.5 degrees for max thrust (forward lift).

It definitely takes off and flies, and seems to do so pretty well (for a prop).

 

All the previous suggestions I posted are still part of this as well, sine I started off with that version.

I removed the Junos, set the prop engines to full RPM and output (because why not), replaced the prop blades by ducted fan blades cause the others looked too small, added prop blades deployment angle to the 'translate F/B' action group, redid the center aft section, redid the tail stabilizer, shifted the main wing section one fine tick back for CoM/CoL balance, and added some more fuel to the nose.

Throttle (engine torque) is now mainly used to spin the rotors and decide how much fuel you spend doing so, while translate forwards/backwards (H/N by default) is used to adjust blade deployment angle, to keep blade angle of attack between 4-5 degrees at all circumstances, and through that, keep thrust ('forward lift') maximized.

Typical take off procedure (hands off the stick):

• rightclick one of the blades and pin the PAW to the screen somewhere - to keep an eye on angle of attack
• do the same with one of the engines (current RPM)
• engage brakes, SAS stabilize
• deploy flaps (RCS) - mainly to irk Fleshjeb, I confess
• stage (or AG1) to start the engines
• throttle up to 100% (max torque) to spin up the rotors
• disengage brakes
• press and hold H (translate forward) to increase blade AoA and start generating thrust (best blade AoA for max thrust is between 4-5 degrees)
• keep holding H until airborne
• retract gear, retract flaps
• while gaining airspeed, keep tapping H as needed to keep optimal blade AoA (which maximizes thrust)
• when she starts to arrive to cruising altitude, you will need to slowly lower a bit again - translate back (N)
• optional but recommended: as you gain altitude and when convenient, lower throttle (torque) to where it just maintains max RPM. It won't change thrust, but it helps save fuel. At cruise altitude throttle can be down to 22% and lower.

You'll notice while doing this that it's hard to manually do all of this optimally, because you'd need 5 independently focusing eyes, three hands, and two brains to process it all simultaneously. Hence my intense dislike for how they implemented prop engines in KSP. Luckily, it's really not required to do it optimally... the plane will still take off even if you lapse a bit at keeping blade AoA near 4.5 degrees during the whole take off, because there's plenty of thrust margin either way.

 

5 hours ago, FleshJeb said:

I keep a KAL linked to my throttle and do a linear torque from 0-100, and blade-pitch is 0-10% @ 0 deg, 10%-67% is 25-55deg with a slight curve. 67%-100% is 55deg-65deg,

That would help (not sure about the higher end pitch though... I had to reduce pitch past a certain speed to maintain thrust, from a max around 52-53 degrees down to 47-48).

Ideally, torque should follow some type of atmo curve, while blade deployment angle should follow a speed curve. Neither one are things that KAL can do, but if it could, prop thrust would be reduced to simply throttling up and down, like the jet and rocket engines, and we could devote our attention to flying the craft instead of micromanaging engine components.

 

Edited April 5, 2022 by swjr-swis
spurious emoticon spawning despawned

[KerikBalm]

1 hour ago, swjr-swis said:

Mostly I notice those prop blades generate much less thrust from the same available torque than the duct fan ones I used. You'd need to double up on them to get even close. Also, their optimal angle of attack seems to be more in the range of 7-9 degrees,

As I mentioned earlier:

On 3/31/2022 at 10:08 AM, KerikBalm said:

Also, I noticed that different types of blades have different optimum AoAs. Generally speaking, you want to keep the blades inedned for use in ducted fans to between 4-5 degrees AoA, but the standard propellor blades do better at higher AoA (I forget what AoA exactly, I think its more around 10).

Similarly, the Helo blades have a different optimum AoA, I hope the mod takes that into account. [...] but so far I've had the best results with the ducted fan blades. The downside is, of course, higher part count relative to the massive helo blades.

 

I will be giving the "challenge" a try as well, and posting my tweaked version... or if I can get it to fly with no tweaks, I will post pics of that... KSP starting now.

*edit*, so for some reason the craft file I used only had one set of props giving thrust (even though I couldn't figure out what the problem was). SO I just removed the engiens and props, and rebuilt it exactly as it appeared to have been, and then it worked just fine. Alll I did was adjust the control to have manual control over blade pitch, and increase max rpm to 460:

With manual control of pitch, I could even use the props as brakes/thrust revers to slow down fast.

Just set throttle so that it consumes lf at 0.12 per sec, and adjust pitch to adjust speed.

When manueviering and changing speed rapidly, its a bit hard, but I slow down before maneuvering and have blade AoA rather low so that it if increases during manuevering (as the plane slows down), the blade AoA increases in a way that makes it more efficient and generates more thrust. This keeps it managaeable to adjust pitch while maneuvinering.

 

Edited April 5, 2022 by KerikBalm

[FleshJeb]

16 hours ago, swjr-swis said:

mainly to irk Fleshjeb, I confess

Hey FLAP YOU, buddy!

Hey, I'm glad it worked. (I bet it would have taken off without the flaps ;P )

They've definitely changed some stuff since I last played (1.9ish)-The turboprops didn't have alternators, and the optimal AoA for all blades was the same. Did you have the aero debug options on? The PAWs will show lift AND drag from the blades.

I still have never gotten dihedral main wings with incidence to contribute to roll stability in any configuration. I HAVE gotten a zero incidence T-tail to work with 5 deg ANhedral, because Surface Prograde tends to hold my planes a fraction of a degree nose-down.

14 hours ago, KerikBalm said:

With manual control of pitch, I could even use the props as brakes/thrust revers to slow down fast.

I set my abort button to undeploy the props. Magic airbrake.

[darthgently]

I'm a complete hack at aeroplanes, but I've found putting the CoL slight above (as well as slightly behind) the CoM helps keep the dirty side down, aka roll control.  Kind of like hanging a model plane from the ceiling with fishing line attached to the top of the plane, the CoL pulls the top of the plane up so it wants to return to level flight.  I have no idea if my explanation makes sense, that is just how I process why it seems to work for me

Edited April 5, 2022 by darthgently
spelign erorrs

[swjr-swis]

On 4/5/2022 at 10:05 AM, KerikBalm said:

As I mentioned earlier:

I missed that, my apologies. Good though, because it's independent confirmation of a rather crucial detail that can be found nowhere in the game's information.

 

On 4/5/2022 at 10:05 AM, KerikBalm said:

so for some reason the craft file I used only had one set of props giving thrust (even though I couldn't figure out what the problem was).

The blades of one of the engines were rotated 15 degrees compared to the other engine. That's why the first change listed in my craft description was "- right prop blades rotated to make absolute/local rotation coincide (same as left blades)".

 

On 4/5/2022 at 10:05 AM, KerikBalm said:

Then manueviering and changing speed rapidly, its a bit hard, but I slow down before maneuvering and have blade AoA rather low so that it if increases during manuevering (as the plane slows down), the blade AoA increases in a way that makes it more efficient and generates more thrust. This keeps it managaeable to adjust pitch while maneuvinering.

Quoting this because it's a good tip when flying stock props.

 

10 hours ago, FleshJeb said:

Did you have the aero debug options on? The PAWs will show lift AND drag from the blades.

I did. Those are the numbers I referred to being very erratic when using the regular prop blades. The duct fan blades' numbers were much more stable and allowed getting a good reading.

[DDE]

On 4/5/2022 at 4:01 AM, FleshJeb said:

I think those engines decrease in mass if you set the max torque lower on the part (the electric ones do). Drop them to about 10-15%, they have PLENTY of torque.

My procedure for determining torque consisted of looking up data for a, of all things, Hawker Typhoon. And then probably fumbling up the conversion.

On 4/5/2022 at 11:05 AM, KerikBalm said:

Alll I did was adjust the control to have manual control over blade pitch, and increase max rpm to 460:

On 4/5/2022 at 11:01 AM, swjr-swis said:

Mostly I notice those prop blades generate much less thrust from the same available torque than the duct fan ones I used. You'd need to double up on them to get even close. Also, their optimal angle of attack seems to be more in the range of 7-9 degrees, although it's hard to pinpoint because the display values are very erratic, even when flying stable and level.

DDE Mustang demodded 2

Alright, so fearing from the above post that I'm looking at a case of PEBKAC, I took demodded 2, and then I put the arrangements for PitchPerfect's autocontrol back in place. It flew just fine.

Then I took down engine torque, and thus weight, down from 100% to 20%. And it still flew, even if it was a bit more prone towards nosediving at around below 50 m/s.

So, what foul witchcraft is this?

Quote

edits:
- right prop blades rotated to make absolute/local rotation coincide (same as left blades)
- added prop blades' deploy angle to the F/B translation action group, to be able to change blade pitch in flight
- readded leading wing sections to mid wing
- added oxidizer to aft tank for ballast
- slight wing offset adjustments to balance
- nullified control surface AoI to move CoL bit more forward
- added more fuel, including nose

Could do with bigger (or more) prop blades for thrust, but flies.

CoM back, CoL forward, and not one but two people reporting my prop settings were borked... I get the first part, now what was that about "make absolute/local rotation coincide"...

10 hours ago, swjr-swis said:

The blades of one of the engines were rotated 15 degrees compared to the other engine.

I guess that contributed to my problems.

Thanks all. I think I need a pause at this point.