Stock propeller-driven craft! Can you improve it?

[Stewcumber]

Today I thought I'd experiment with what I call "indirect thrust movement"; as in jet engines powers something that in turn makes my craft move. I've dabbled before and made spinning axles,where the axle has engines attached:

Elegant, like a drunk hippo falling down stairs.

Today though I thought I'd give a different attempt. What if I have fixed engines pushing a cylinder around? I've seen this on this forum somewhere before so it certainly isn't my idea. It allows you to control the speed of the propeller as well as steer your vehicle. My first attempt looked like this and went 12m/s

I then experimented with different propeller pitches, number of blades, lift factor/size of blades...

This didn't help a huge amount. I eventually got my speed up to about 27m/s with some fiddling with the above propellers designs - smaller, faster propellers seemed better.

However I then started messing with my "engine" bit rather than the propeller which netted bigger speed gains - see my theories below. I used larger "wings" on the engine itself and adjusted the angle of the wings and engines. This netted me 40M/s. I then increase the the engine count to 3 per wing, this resulted in 50m/s yay!

Aaaand across the line! - Jeremy Clarkson

Having changed lots of parts I've got a couple of theories that I'd be grateful if a much cleverer person than I could comment on like an aero engineer!

• Propellers angled closer to horizontal (higher pitch?) will result in the most forward thrust at low RPM but won't be able to spin as fast, by definition almost, as there is greater resistance on the blades in order to generate that thrust. Vice versa is also true. As such, these propellers are suited to slower engines. In general, all of these engines appear to spin slowly (note the angle I used in the image above)
• If the propeller (for example) is not the weakest link in the system, changing the propeller won't influence things much. For instance I went from 8 double blades to four single blades and my speed improved despite reduction in "lift" generating surfaces. From this I assume that the engine spins so slowly that the increased lift of the larger propellers would benefit from a faster engine. The weight of the blades themselves could be causing the engine to spin slower to such an extent that the lift benefit of the propellers was outweighed by the faster engine speed of the smaller propellers (there is probably a formula for that somewhere, oh God help...)
• The engine would be most efficient if the thrust hit the blades of the engine at a 90 degree angle so all the thrust goes into a "forward" anti-clockwise / rotation motion, instead of an angle, so the blades have been angled as such (1 in image). Like hitting a nail with a hammer at an angle will cause the nail to bend and not go into the wood.
• The engine would be most efficient at converting the jet thrust into RPM (i.e. fastest) when the jet thrust made contact with the axle as far away from the centre of the axle as possible. Like pushing open a door from the outside edge, vs the inside edge (2).

 

The engine could potentially be improved a lot I think. I have a plan for tomorrow of doing something like this, with 8 blades / engines.

 

I am quite proud of the design and method of getting the axles to decouple and spin, using 8 girder things to form a octagonal mount.:

The axle/mount doesn't seem to offer much resistance at all; when I go to 0 thrust, the propeller keeps spinning for ages. It also doesn't jump around at all and it is also very robust as you can see from the crashes:

https://www.dropbox.com/s/1hnc28wk0szcli3/KSP_x64%202018-10-30%2020-31-32-06.avi?dl=0 (151mb crash video)

I might make a challenge if anyone would be interested - how quickly can you get to end of the runway using propellers!

Anyway I would be pleased to see other peoples' propeller vehicles (unless it is just me :D)

 

Edited October 30, 2018 by Stewcumber

[Rocket In My Pocket]

I don't fool with stock propellers much, but I enjoyed reading this, and taking your engineering journey with you.

Nice job!

[Chel]

I believe this thread should go in the Spacecraft Exchange, as it seems to be the place for it

[qzgy]

*ahem*

Not to take anything away from this very interesting post, you've actually done pretty well here, getting a good amount of the basics right. You did ask for comments and I am by no means an engineer, but I have messed with this stuff a bit... So comments on your comments!

6 hours ago, Stewcumber said:

Propellers angled closer to horizontal (higher pitch?) will result in the most forward thrust at low RPM but won't be able to spin as fast, by definition almost, as there is greater resistance on the blades in order to generate that thrust. Vice versa is also true. As such, these propellers are suited to slower engines. In general, all of these engines appear to spin slowly (note the angle I used in the image above)

Up to a point yes I guess. Having the edge of the blades more inline with the axle that you use will generate higher thrust up to a point because of the higher angle of attack. But also because of that, you get increased drag which I think you also have figured out. From what I've learned (of not really my own original research), generally for the most thrust you want a high speed, lowish pitch blade. Finding the correct pitch is a lot of fine tuning and often times, you want the inner and outer pitch to be a bit different to make the most of the rotational speed you have.

6 hours ago, Stewcumber said:

If the propeller (for example) is not the weakest link in the system, changing the propeller won't influence things much. For instance I went from 8 double blades to four single blades and my speed improved despite reduction in "lift" generating surfaces. From this I assume that the engine spins so slowly that the increased lift of the larger propellers would benefit from a faster engine. The weight of the blades themselves could be causing the engine to spin slower to such an extent that the lift benefit of the propellers was outweighed by the faster engine speed of the smaller propellers (there is probably a formula for that somewhere, oh God help...)

Yeah, often what is the weakest link is the bearing or the blower (the jet engines you have pointing at your spinny part). But it also could be that by reducing the mass, you were able to spin faster and thus get a better lift output. Sometimes more blades is not better as you mentioned (higher rotational inertia -> slower speeds). No idea if there's a formula for that but ummm..... yeah I guess mostly experimentation? 3 or 4 blades is usually pretty reasonable. You might also want to try to make the blades as big as possible, since your blade tip will be going faster and thus can produce more lift/thrust.

But often times, making a smoother bearing or how you're powering will make a great difference.

6 hours ago, Stewcumber said:

The engine would be most efficient if the thrust hit the blades of the engine at a 90 degree angle so all the thrust goes into a "forward" anti-clockwise / rotation motion, instead of an angle, so the blades have been angled as such (1 in image). Like hitting a nail with a hammer at an angle will cause the nail to bend and not go into the wood.

Yeah you want your blowers to be as close to as tangent as possible to the surfaces you are blowing on. Your analogy here is actually pretty good - the not-straight force would just be wasted. The actual blades themselves dont have to be to be at a 90 degreee angle though. I have used fairing bases and heat shields as the thing in which the jets blew and those worked fine.

6 hours ago, Stewcumber said:

The engine would be most efficient at converting the jet thrust into RPM (i.e. fastest) when the jet thrust made contact with the axle as far away from the centre of the axle as possible. Like pushing open a door from the outside edge, vs the inside edge (2).

Yup, greater torque (applying the force further away) can generally mean higher speed and thus more thrust. Though on the other hand, you want to make sure your engine is also as continuously powered as possible. And if you want to put it in something, not too big either.

So you're on a pretty good track already! Some more tips I guess (although for better ones, there are significantly more qualified people....)

 - I would highly recommend the mod Collide-o-scope and VOID. The first mod allows you to see the various colliders of parts in KSP which is super useful for fine tuning bearings and seeing the path of your jets to try and make them as tangent as possible. The second give you the rotational speed of your craft, which can give you the rotational speed of your blades - helpful for comparing fast and slow.  I'd also recommend the mod Editor Extensions Redux since it adds a number of quite useful features (but dont autostrut blades. I think that slows things down....)

 - I'm not sure how your bearing works actually - I'm not going to complain too much since it works (again no idea how), but I guess there's no harm in letting you know about some other styles using the RCS place anywhere ports because of their perfectly smooth collider - nothing to catch and slow it down. Yours does seem to work quite nicely though.

 - I wouldn't bother using wing panels to actually push - they are kinda big and cause drag while rotating. Maybe try instead like the vernier RCS ports or even more RCS balls. All that matters is that the jet can push on it even if only for a short period.

Keep at it though, you're doing already pretty good. and its a fun challenge to get something to fly using those. (I've done helicopters, but not planes yet.... Others have done planes though.) Good luck with it!

[KerikBalm]

Well, I haven't experimented with the "turboprops" or "blower" designs.

Isn't there a hard limit to the rotation rate of parts in KSP (although I think this can be modified with text editing)? Or does this apply to SAS? I've only made reaction wheel powered designs (switch to the rotor, use trim settings to cause it to spin even when you switch back to the main craft. Also I use control surfaces so that I can have variable pitch props, and I just alter the deployment range to figure out which one gets me the best speed via trial and error.

(flying on 3x duna with seas and 25% more surface gravity: 0.376 G)

Note that here the blade pitch angle is quite high, whereas this next one is much lower:

Note that the above one uses a fairing to contain the rotor, whereas the one below has structural fuselages:

On a modified Eve, back to the fairing rotor housing:

I love how you can pin a part menu, and then switch to a craft in the same physics bubble, and the part menu is still there, allowing me to adjust the pitch on the rotor blades while flying the plane, even though they are considered separate craft.

I also made my own rotor design for a stock (with structural tubes... I forget if those are MH parts only, or part of the core game too) centrifugal rotors for long duration voyages for my kerbals (RPing)

Spoiler

(the green science lab is a part I modded myself, I duplicated the science lab and made it a greenhouse for use with TAC-LS)

 

[Pds314]

Nice little plane for hopping around the KSC.
Although this one is electric, not a jet-powered turbine.

Old Helicopter I made with a compact turboshaft engine.
https://kerbalx.com/pds314/Turbocopter-XR-3



Older CRAZY powerful turboshaft plane. This thing was absolutely nuts.


Another attempted record-setter plane. It turned out to be too easy to break at over 200 m/s.


Cyclic pitch system. Sadly only for demonstration purposes. 13 separate craft cannot hold each other together under load. The actual cyclic pitch part works fine though.

Edited October 31, 2018 by Pds314

[Pds314]

When I tried to make an engine too compact for the number of blowers.


Old over-revving design which could briefly exceed the wobble RPM of physics-enabled parts, even with a prop, however what is not shown is the violent oscillations of each blade in and out:


Example of over-revving the prop, the blades are still attached and have proceeded to wobble back down to under 2850 degree/second.

Edited October 31, 2018 by Pds314

[qzgy]

4 hours ago, KerikBalm said:

Isn't there a hard limit to the rotation rate of parts in KSP (although I think this can be modified with text editing)?

Yes there is. 50 rad/s IIRC. I thought it was a limit of the physics engine. Could be wrong though (about the reason for the limitation).

[KerikBalm]

So Just FYI, if its 50 Rad/s, that is roughly 8 rps or more precisely, 477.5 rpm

If you have a prop blade 2 meters out (I think lift is calculated from the part center), then that means it can reach 2*pi*2*7.95775 m/s = 100 m/s at the maximum rpm that the engine allows.

Actually, just looked at the link:

https://forum.kerbalspaceprogram.com/index.php?/topic/98700-stock-helicopters-turboprops-14-problems/

Quote

If you want the ability to overrev turboshaft engines, go to your Physics.cfg in the KSP root folder, change line 89 which says "maxAngularVelocity = 31.416". Set the value to 50 or higher.

So lets take the stock setting, which seems to be 10 pi, rounded (). Then the max velocity of your blade with a 2m radius is 62.8 m/s. For the blade to not cause (drag against the direction of travel) when traveling forward at 100 m/s, (so 100 m/s in the direction of craft travel, and 62.8 m/s along an axis perpendicular to the direction of travel) its angle needs to be at least 57.9 degrees (Take tan^-1 (100/62.8)). At that rotation speed, blade angle, blade radius, and forward speed, the rotor blades should essentially nor create any lift nor induced drag.

Of course, there is a lot of G forces on those blades, so they will stretch outward and increase their radius. If they started at 2m from the center, and get to 10pi rad/sec...

a=v^2/r; r= 2, v= 62.8; v^2= 3944, a = 1972 m/s^2: roughly 200 G's... that will make the part move outward.

So... keep in mind the maximum RPM that can be achieved, the blade diameter, and how fast you want your design to go, and make sure the pitch angle is/can be appropriate to your intended speed (this is one reason you'll probably want a variable pitch propeller, your RPM limit in stock is roughly 300 rpm, real world props get much higher RPM... while a Cessna 172 will often have its prop RPM over 2500.... and that plane won't even fly 100 m/s (it does use a fixed pitch prop).

 

[Stewcumber]

Thanks for the help everyone. Look like I've got some more testing to do! 

Last night I improved the design by putting a larger cylinder in - the thrust from the jets was therefore hitting the engine further away from the centre of rotation. I did try using RCS engines instead of winglets to "catch" the thrust with less rotational drag however this didn't work at all... 

I also fixed some irregularities like the engines are clipped into the frame, now they're snapped to the edge of the frame like normal so it is easy to add and remove engines. This also pushed the thrust slightly away from the centre of rotation. 

I tried it out vertically to see if it would lift off that way but not enough thrust or too much weight to take off!

I'll have a look at those mods and kerikbalm's detailed advice when I finish work! 

[Pds314]

On 10/31/2018 at 6:27 AM, qzgy said:

Yes there is. 50 rad/s IIRC. I thought it was a limit of the physics engine. Could be wrong though (about the reason for the limitation).

The limit is only for physics-enabled parts. Physicsless parts can be spun up to 200+ rads/s without issue. The issue for physics-enabled parts comes from the movement in one timestep being more than a certain number of degrees. I think 120. This messes with the ability to maintain a constant radius from the attachment point and therefore a constant rotation speed.

In principle, you can exceed this limit. See above where I posted a prop turning at almost 60 rads/second. However, oscillations quickly go out of control in a matter of seconds, and having massive frame-to-frame differences in the angular inertia of a rotating object will lead to violent oscillations that consume arbitrary amounts of energy and reduce the rotation speed back to under 120 degrees/second.

The way I exceeded this limit in the posted image was that oscillations would be symmetrical. This isn't a stable configuration however and you can see what happens.

Sadly, there are no physicsless wings or control surfaces.

Edited November 2, 2018 by Pds314

[Pds314]

AFAIK KSP uses verlet integration.

I.e. it does this for every physics-enabled part every frame:

pos = pos + vel*timestep/2;

vel = vel + accel*timestep;

pos = pos + vel*timestep/2;

 

This is good compared to Euler integration because it does not produce systematic errors in energy under conditions of constant acceleration. I.E. it is simplectic.

 

The issue comes when those jumps are very large relative to the source of acceleration. I.E. the central hub.