FlyBeetle

My Feedback: (9)

This is not really airplane related but I wasn’t sure what forum to post this in and I thought that this may be the best place to start. I am considering building an R/C vehicle that would be driven on slippery surface similar to ice and would weigh up to 130 lbs (it is for a robotics competition). Due to restrictions in the rules for this competition all vehicles have to use unaltered wheels provided in kit of parts (low friction to the ground) so instead of driving the vehicle through the wheels we were considering building something driven by one or more props. I have seen few videos of vehicles called air sled or snow plane, but I am not sure what would be the proper name, (think of something ala swamp boat with wheels). Due to size limitations, overall width and length are limited to 28 and 38 inches, but overall weight could get up to 130 lbs. We also think this type of set up may be better in changing direction as compared to those low friction wheels. The playing surface will be about 30 x 60 ft rectangle. Here are some questions related to props:

-difference between pusher and pull props. In our case the prop(s) would most likely be driven by belt or chain so could be mounted in either direction. Also, we would like to use the prop(s) to slow down or reverse direction (electric motors and speed controllers allow reversing motor direction).
-recommendations on 2 vs. 3 blade props. Again, the vehicle will be heavy. We are not looking for top speed but torque to get the vehicle moving from stop quickly.

-recommendations on using one large prop vs. using multiple smaller props. For example using one 26” prop versus 2 or even 4 motors with 13” props

-any comments if building a shroud or tube around the prop would improve the efficiency?

-use of props intended for electric or glow motors. Again, the props will likely not be driven by motor directly so we can play with optimizing gearing ratio.

-calculating thrust produced by any of the configurations listed above.

-calculating max thrust given battery voltage and max amperage draw. The battery is motorcycle style 12 volt battery and max amperage will be limited by power distribution system. The batteries charge has to last only about 3 minutes and batteries can be swapped between matches.

Thanks for any info or links to sites that may be useful.

wellss

I would think for directional stability, the source of thrust should be in front of the center of gravity.

The prop(s) should be the largest possible.

Simply reversing direction probably won't give you efficient reverse thrust. Maybe consider a separate system (behind the CG) for braking, like maybe a drag chute or anchor?

BMatthews

130 lbs is a heck of a lot of weight to move around with air thrust that you can get from motors that'll run off a 12 volt battery. I would suggest you plan on making the vehicle far lighter in order to achieve a better power to weight ratio.

Motorcycle batteries are rated at around 50 to 70 amps for starting purposes but they are not designed to operate at that sort of output for even up to a minute, let alone more. At those currents you're in serious danger of boiling the electrolyte if you hold the current to that amount for more than around 30 seconds. Up to around 20 to 30 amps would be as much as I'd suggest you're safe with.

That equates to around 360 watts maximum for power. That is not a whole lot of watts to move any sort of serious weight around using a prop. I'd suggest that something around 30 to 40 lbs would be a better target weight. It would ensure a more timely response of the vehicle to commands of any sort.

For the best static and slow speed thrust you want a lot of diameter and not much pitch on the prop. But temper this with the need to spin the blades at a speed where the Reynolds numbers are high enough that the air seems more "solid" or "water like" to the blades vs how an air circulator fan sees the air at much slower RPMs. So you'll want to keep the prop spinning at up around 4000 to 6000 rpm for best results. The size and pitch will depend on the motor and current and what the Kv value of the motor is. The diameter/pitch ratio should be higher than 2. So something like a 14x7 would be fine. Same with a 16x6. But I wouldn't suggest stuff like a 14x10 which would run the blades in a stalled condition at lower speeds. This just uses up power without creating thrust.

Some static testing of props using an ammeter and thrust gauge will soon zero you in on the optimum choice. Or see if you can google around for static thrust propellor test data. I know that from doing my own in connection with a good buddy years ago any prop with more than around 6 inch pitch showed signs of running in a stalled mode when static. So for best initial pull away you'll want no more than that much pitch. Also consider that when reversing the prop for stopping that now the airfoil will be running "upside down" and also pushing into the oncoming air. All of which makes the blade shape far less ideal for slowing and stopping. So unless you plan on rotating the post with the prop so the backside of the prop faces forward you can expect the slowing down to be less efficient than the speeding up.

All in all shrouds do help the amount of thrust but they rely on the prop tips being very, very close to the inner surface of the shroud. Also the shroud needs to be in the shape of a proper venturi with a decently long intake path or else the disturbances from the blade causes unsmooth airflow into the blade and the shroud hurts more than it helps. An article for intake shrouds on a ducted fan from years back indicated that you want the intake to be roughly 1/4 the diameter in depth. Then you want a little of a guide behind the prop to direct the flow. So for a 14 inch prop you'd want to provide around 3.5 inches of intake venturi along with perhaps another 2 inches behind the prop for a total of a 5 inch long shroud. Just putting a ring around the prop won't do diddly for thrust although it's not a bad safety idea.

The rest of the stuff you're asking is either not something you can easily determine without knowing a bunch of other stuff first or not something that can easily be answered due to not fullying knowing your resources and building skills. However I will offer the idea that gear or pulley drives to the props is going to introduce a whole level of complexity that you likely don't really need or want. Also you haven't really fully described the "mission" and operating conditions so it's hard to offer up much else.

FlyBeetle

My Feedback: (9)

First of all, thank you for replies. Here is some more background. The game is organized by FIRST ([link=http://usfirst.org]US FIRST[/link]http://usfirst.org) and is intended to get high school students interested in technology and engineering. Every year the game changes and students have 6 weeks to design, build and test their robots. There are over 3000 teams that participate (or participated at some time) in this competition and many teams have gained and passed on a lot of knowledge and experience to their students. In order to make this competition more attractive and fair for rookie teams (again, the main goal of this organization is to get kids interested in engineering), people who come up with games for each year try to come up with new challenges and games where strategy plays big factor.

One of the things that I have noticed is that many veteran teams "reuse" or "improve upon" their previous year chassis and thus have more time to develop function / game specific components of the robot (arms, grapples, ejectors etc.) while teams with less experience often start from scratch. Many of the previous games have been played out on carpeted field and many veteran teams have developed some great powertrains for that type of field. So this year the game organizers came up with competition that would make veteran teams re-engineer their chassis/ powertrain designs by changing the playing field surface to very slippery material. (this is also because NASA is the main sponsor of this competition and the lower friction is supposed to mimic challenges of lower gravity on moon (think 40 year anniversary of lunar expedition)).

The rules are quite long, but if you are interested there is a 3 minute animation video showing this year game challenge on [link=http://www.youtube.com/watch?v=pW0eCFPDoxc]2009 FIRST Lunar competition[/link] youtube. Basically, there will be 2 alliances of 3 robots on the field at each match. Each robot will pull a trailer and robots on opposing team as well as human players will try to shoot special balls into the baskets located on the trailer. So the main strategy of almost every team would be to design robot that would drive and steer about the field to make it more difficult to score balls into its trailer. Every team is free to design and fabricate their own chassis but there are some components (such as motors, batteries, electronic modules, speed controllers) that are provided to each team in kit of parts and can not be substituted. Based on size and weight of those components as well as size and weight limitations, the robots usually weigh about 130 lbs (some are lighter). Because of low friction through the wheels, our team considered not using the wheels to drive or steer the robot and wanted to investigate other options. If we decide to go the air propelled route, we would try to build the robot as light as possible, but as I mentioned, due to standard battery, motors, esc (that are the size of adult's fist) etc. it may not be possible to build it super light. Also since there will be 5 other robots on the field at the same time (bumping into each other), we have to build a robust chassis. We can make some weight goals and assumptions and knowing the coefficient of friction calculate max friction force between wheels and the ground but wanted to compare this to propeller thrust route. I think we are going to do some testing to evaluate this route, and we will need to review the specs for motor and battery but we were trying to "think outside of the box".

Thanks again for all help.

wellss

Propellers are not as efficient for converting power into motion.

For example: If you were able to apply 500 watts to a 50 kg object with 100% efficiency, you'd have an initial acceleration of 4.5 m/s^2 ( the power is the amount of kinetic energy that the object has after 1 second )

In order to get a 225 N static thrust force from 500W with a propeller, the diameter would have to be about 6.75 m

A typical propeller of say 0.2 m would produce about 20 N of thrust, and so the system is 9% efficient.

If you were to use 4 propellers of 0.2 m with 125 W driving each one, you're efficiency only goes up to 15%. Still not good....

Ice or some other similar surface doesn't have to be slippery. As you mentioned, it's all in the coefficient of friction and thus the material and surface area that is in contact with the surface. It's only slippery if there is not enough surface area. I'm thinking a track or belt type system that can be as wide as you need it to be. The wider, the better for stability. Besides, one hit in the propeller from one of these special balls and you're probably done!

HighPlains

My Feedback: (1)

I suggest you Goggle Jim Hall and Chaparral 2J for a winning solution to your problem.

Craig-RCU

That 2J is quite the concept; it's a reverse hovercraft. That may or may not work for flybeetle depending on how the special low friction wheels and surface interact, but it is definitely worth looking into.

Flybeetle, since you have to tow a trailer, I think you'll get better steering response by keeping vectored thrust props up front on your vehicle. Although, you might get better control over "trailer sway" with the vectoring at the rear of the vehicle, closer to the trailer. The props could still be up front though, just the thrust vectoring vanes would be placed at the rear. You might even have the positioning of the vanes adjustable fore to aft to be able to tune handling qualities. You'll need to protect the props from collision and breakage, and prop shrouds increase static thrust a bit. So, prop shrouds could solve both those problems. Braking could be accomplished with variable pitch props that can reverse their pitch. There are some little foam electric planes that have pretty simple prop pitch control mechanisms that you could probably copy.

If you copy the Chaparral 2J and have at least some power to the wheels, a "traction control system" that compares the velocity of the vehicle to the rotational velocity of the wheels would help keep the wheels from spinning during acceleration and braking. I don't know how much other hardware is allowed, but maybe a GPS or an optical mouse could keep track of the vehicle velocity.

Love2FlyMN

how much do the initial components weigh? I really wish I could be there to just help you with this as I have a few ideas in mind already. How heavy are the balls you are launching into the baskets? What if you used a propeller to "suck" the machine down to the ice to get added normal force? Like those little RC cars that you can drive on a wall and cieling. I don't know if anyone else suggested that I didn't feel like reading the rest of the posts. Let me know more of what you think, maybe even talk about it over the phone because this interests me. I realize this post is a bit older so I hope it's not too late for my suggestions!