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Messages - MarvinMan

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Robotics / Re: Things you've built/invented/programmed (or want to)
« on: February 07, 2019, 07:05:21 pm »
Cool. I do electronic design professionally, and everyone I know considers anything RF to be a dark art.

Assembly language programming is getting to be a bit of an arkane ability these days, but it's certainly a good way to learn the intricacies of a particular processor, and useful for generating precise timings.

Challenges / Re: Autonomous Combat League
« on: January 27, 2019, 05:43:35 pm »
Things are even quieter than normal around here these days, but there are still a couple of us around.

Shame to hear the forums might be disappearing again, but I suppose it's hard to justify paying for the hosting when there are usually months between posts.

Screenshots & Videos / Re: Marvins Creations
« on: September 28, 2018, 07:51:46 am »
Interesting idea. I've not come across any obvious mechanical integrity problems with the props I've been testing, but I assume offsetting the fins is beneficial because it counters some of the stretch in the arcs, letting them effectively move to their optimal position under load/at speed.

Can hypercubes be used for de-stressing any structure? For example a string of continuums bent into a loop being turned into a stable structure that won't spring back when broken.

Screenshots & Videos / Re: Marvins Creations
« on: September 27, 2018, 09:24:16 pm »
I had noticed the graphs were broken when I uploaded them. I had hoped that google docs would make the spreadsheets more accessible than uploading them to dropbox.

When I started I wasn't particularly planning on publishing the data, so the labelling reflects my aversion to unnecessary typing. On the static results W is rotational speed of the prop, T is torque, and F is forward thrust (because T was already taken by torque). F/T gives a sort of measure of efficiency. On the second sheet, V is airspeed.

As torque and rotational speed are pretty constant at all airspeeds using variable pitch to control speed is probably better, but would need more work to characterise the pitch needed for peak thrust and zero thrust across the full speed range, especially if you did want to increase prop speed once in the air to take advantage of the improved airflow over the wings.

It appears that most of the energy input goes into just rotating the fins for no gain, so larger props allow more thrust/speed from a given rotational speed. The large props I tested using a cross piece as the hub were the best of the non-offset designs tested. Assuming it had enough thrust to get off the ground when retracted, a prop with pistons to increase its diameter might work well for efficient cruising or maximising top speed. I imagine such a design would make the handling of the plane awful, but it would be very fast.

Airspeed test rig.

Static thrust rig.

Screenshots & Videos / Re: Marvins Creations
« on: September 26, 2018, 10:55:05 pm »
Propeller infodump

Propellers in Rawbots are weird. The high torque they require and their dependence on airspeed make them ideal for quadrotors, but very tricky for planes. I was looking for a better design to power my planes, so I've characterised several different configurations to try to find out what works best. I've put the results in a couple of spreadsheets.

Static results

The first set of tests were for static thrust against torque, measured by balancing the thrust against a jet. The offset elbow type is the one seen fitted to my propeller plane above, and turns out to be the best fixed-pitch design over much of the range tested. Logically, a faster spinning, lower pitch prop should require less torque to provide the same thrust, however this only appears to be accurate at the lowest power levels. In general, a 45 degree pitch works pretty well. For best performance, use the largest diameter prop you can, and offset the fins slightly, like the offset elbow design.


Optimising the pitch over a range of airspeeds is where things get really weird. With a fixed 45 degree pitch and constant torque, the rotational speed only changes by 12% from stationary to a forward speed of 70 while the trust decreases linearly. This suggests that most of the torque requirement goes into just spinning the prop. When the pitch is optimised at each speed, the thrust shows more of a 1/x decay with airspeed, enabling much higher aircraft speeds to be attained (to the point where the wheeled test platform used for these tests reached escape velocity). Interestingly, the rotational speed of the prop stays largely constant throughout the test. For most sensible speeds, the pitch can be mapped linearly to forward speed.

Overall, it's probably better to just use a lightweight fixed-pitch prop optimised for lower speeds, but higher pitches will be needed to go fast.

General Discussion / Re: Is the development still being rebooted?
« on: July 02, 2018, 08:46:36 am »

The only thing I've seen is this:, although it doesn't exactly seem official. I think there was meant to be some sort of announcement months ago, but obviously nothing appeared.

Introductions / Re: Im back too!
« on: June 10, 2018, 09:35:46 am »
Welcome back.

I don't know what the deal is with that new site, but it does at least offer a more legitimate source to download the game, and presumably gets a little more publicity too.

General Discussion / Re: Rawbots development restart discussion
« on: June 04, 2018, 08:05:25 am »
There is of course this thing that appeared recently:

Still only has the same version of the game we've had for years and a load of old videos, but someones bothered to put it up.

General Discussion / Re: Rawbots development restart discussion
« on: March 02, 2018, 04:51:48 pm »
Apparently gmail thinks Rawbots forum updates are spam.

Anyway, there are a few of us still here, and the domain name got renewed recently.

General Discussion / Re: Rawbots development restart discussion
« on: September 24, 2017, 11:17:37 am »

I suppose it has been rather quiet recently, especially with the server going down for a while. But there are still a few of us here, and the server came back, so we're not going to just disappear again any time soon.

Introductions / Re: Petit Scarabee
« on: May 10, 2017, 07:37:44 pm »
We've been here 4 years already, it's not like we're going away any time soon.

It is a bit odd how we got that those couple of posts about new rawbots, and then it went silent again.

General Discussion / Re: A question
« on: April 14, 2017, 01:25:59 am »
Welcome back!

+1 on XFM, it is pretty much a standard part of the install these days.

We are hopeful for there to be a new version of the game, although things do seem to have gone quiet again since the last couple of demo videos.

Challenges / Re: Mini challenge: Extreme hill climb
« on: April 07, 2017, 04:02:54 pm »
I wish I knew how to stop the terrain getting screwed up like that. It only seems to happen on some of the stardust style maps.

Hopefully I've still got the half built version somewhere, as that must have loaded properly.

Challenges / Re: Mini challenge: Extreme hill climb
« on: April 06, 2017, 07:55:32 am »
I'd have said everything like that is fair game for this challenege.

Using hooks implies a walker-like design, which won't be trivial, and I expect mass drives would be tricky to control in this application.

BTW, did the map load correctly? I've just tried to open it again and rawbots has decided that the ramp is now off in space in two pieces somewhere between the planets.

Challenges / Mini challenge: Extreme hill climb
« on: April 05, 2017, 11:12:57 pm »
Extreme hill climb mini challenge!

Here's a quick challenge while we wait for the new version of Rawbots to surface. The goal is simple: make it to the top of the ramp, complete the zero-G hairpin and safely get down again.

Easy mode: anything goes. Use jets for as much free propulsion and downforce as you want.
Hard mode: no active downforce. Only use fins for downforce, so you have to keep going or you'll drift off into space.
Extreme mode: none of the above. Show off some more exotic solutions to the problem.


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