Coaxial, contra-rotation

[kenneth_chiu]

I've been trying to figure out the best way to accomplish. My specific purpose is rotors, but I also just want to understand it in the general case. So far, it seems that there are 3 ways, none of them seem ideal:

1. Use three, stacked powered wheels. The first one is fixed to the chassis and rotates. The second one is fixed to the first one and rotates in the opposite direction, creating a net rotation of 0. The 3rd one is fixed to the second one, and also rotates in the contra direction, create a net rotation. This seems kind of kludgy.
2. Use a perpendicular powered cog which engages two unpowered cogs. This creates contra-rotation more elegantly, but has the theoretical disadvantage that the rotation must always be at the same speed. I have not tried this, but recall seeing a video of it, so I think it's possible to make work.
3. Create two, stacked platforms fixed to the chassis, making sure that there's clearance for the rotor blades. Braces could be used, since they are non-interfering.

Are there other options that are more elegant or flexible?

 

[Redstoneman]

I've built a double rotor VTOL prototype that has coaxial rotors built using the gear method. They actually work as intended, and never slip thanks to proper bracing, but the machine was only an experiment.

Download below.

Also, I don't know if that's what you meant, but I think that the A-23 in my signature uses the third method. Just a quick glance at the second gif below will tell you.
Gifs in the spoiler below:

Spoiler

What I assume you mean in the second method:

What I assume you mean in the third method, although I'm not sure:

Both of the above work really well for me, although there are two major differences between them:
1. Gear method will transfer torque to the hull if I'm not mistaken, as opposed to the one on the second gif
2. Gear method ensures that the top and bottom rotor spin at the same speed, while the other method doesn't

Hope I helped >u>

Attached Files
umbra.bsg

 

[Mumon]

here check this plane it has coaxial turbine fans in its engine:

http://forum.spiderlinggames.co.uk/.../war-machines/flying-machines/40050-basic-jet

Redstoneman the gear version only works at low speeds otherwise (even with ultra bracing) it will glitch

 

[Redstoneman]

Wrong, the machine you see in the first gif has the gears rotate at the speed of 2.85 when holding Up
It can go as far as 3.50 speed without the gears skipping or breaking the bracing, but that particular machine was not designed to withstand it.

 

[Mumon]

Redstoneman um..that is slow.. try stacking 3 gears with a speed of 2 each and then link the top one to the output no bracing or invincibility will help you it WILL glitch

 

[Redstoneman]

If you need more speed than that, add more propellers instead of more wheels/cogs. Same result, less space wasted.

 

[Mumon]

Redstoneman or dont use that method, anyways my argument still stands that this only works at low speeds. its a pity tho

 

[AxeSlash]

Use Easy Scale mod to scale the gears up so that they mesh better, then brace the f**k out of it. In real coax systems, the the rotors always DO spin at the same speed - yaw is created by varying the collective pitch of the top rotor vs the bottom rotor (collective up on top rotor + collective down on bottom rotor = net lift remains the same, but torque is induced on the airframe).

I'm toying with the idea of a 'floating' gear-driven system - the whole gear assembly is able to spin relative to the chassis, reducing any torque transmitted (yaw has to be accomplished by other methods). It is, however, big and ugly at the moment...

 

[[UTT] Narzu]

If you need more speed than that, add more propellers instead of more wheels/cogs. Same result, less space wasted.

This is not exact (at least not all the time). Propellers have a pitch, and the maximum linear speed that they can deliver depends on this pitch and the rotational speed (it's like a screw but in the air). More propellers will give you more thrust so you can lift more weight, but the more you approach the critical linear speed, the less you can get more by adding new blades, this is asymptotic.

If you correctly designed you machine and have a sufficient number of propellers, only more rotational speed will give you efficiently more linear speed or you can increase the pitch of the blades, which gives you more potential linear speed but it's at the expense of thrust when it's over 45°.

At the end you have to play with those 3 parameters : rotational speed, number of propellers and their pitch. 45° pitch is a safe bet to start with, then you should decide the number of propellers function to the machine's weight and finally adapt the rotational speed. In addition, this way let you easily set hoovering mode and boost mode if you want to.

Actually the 4th parameter would be the effective diameter of you propeller (larger = more power, but the system might be more easily broken). I say effective because if you want to make calculs in besiege the point of interest seems to be at the middle of the blade, not at the very end. You can see it if the blades start far from the rotor because you put braces.

Personally I set the rotational speed and the effective diameter the higher I can depending on what the system can bear without wrecking (on few custom props I can go up to x12 rotational speed for 4 bladed rotor), and then I lower the pitch, this brings more stability (angular momentum) and transverse flow effect is lower, but I have to deal with gyroscopic stuff...

collective up on top rotor + collective down on bottom rotor = net lift remains the same, but torque is induced on the airframe

Never thought about that, I need to try it! this is brilliant.


PS : kenneth_chiu, there is a 4th possibility : you can do the 3rd but with a 1 block chassis between your 2 propellers, which is then connected to the real chassis but keeping free rotation in the plan of the propellers (hope I'm clear enough)

 

[Redstoneman]

Jesus dude, what a thread necro.
Some time ago I made a quad with coaxial counter-rotating engines that don't transfer any torque to the hull. They work really well and ensure stability working alongside the nacelle tilting system, so the quad received a sniper turret.
The only way these rotors connect to their nacelles is with ball joints, which transfer absolutely no torque. Again, works really well.
http://steamcommunity.com/sharedfiles/filedetails/?id=723291639

 

[[UTT] Narzu]

Jesus dude, what a thread necro

Omg I see that now. Sorry I didn't pay attention to the date, this was the 1st post in the list, thought it was fresh.
This forum seems to die actually...

 

[Redstoneman]

This forum seems to die actually...

Come join the discord server then. There's plenty more talk in there.

 

[[UTT] Narzu]

I did that man! but well, it does look cool but it's only text chats if I understand well. How can we organize talks ? how can we capitalize on ideas ? everything looks so volatile.
I think I just need to look at it more than 5 mins to understand the thing. Also atm we are like 20 guys connected.. that's better than here I guess but the besiege community looks so tiny, I really thought it was much bigger.