Friction and thrust

[SteveJanesch]

I've been keeping up with "Propulsion that does not involve driving wheels" (and even contributed to derailing it), and have had this going through my head the last day or so:

Part 1: Friction
Assume the robot, battery, bumpers, and trailer tongue weight add up to 150 lb. With a coefficient of friction of 0.08 (on the high end of experimental data), the lateral force available for acceleration can be as high as 12 pounds. That's just the normal force (due to the total weight on the robot wheels) times the coefficient of friction, independent of wheel configuration.

Part 2: Thrust
(My apologies in advance for using non-standard terms for this, it's not really my field. Feel free to provide the correct syntax.)
I looked up equations for thrust (on wiki) of a driven fluid, which is the product of the mass flow and the velocity of the fluid. Using a value for air density of 0.075 lb/ft^3 and a flow rate of 60 ft^3/min or 1ft^3/s gives a mass flow of 0.075 lb(mass)/s. Forcing this through a nozzle at 100 ft/s, you'll get a thrust of 7.5 lb. Based on what teams did last year, this seems like it might be possible with a scavenged shop vac or leaf blower impeller.

Is that thrust calculation right, or even in the ball park? Could a jet-assist directed laterally really give a 60% or greater boost to the drive train?

- Steve

[writchie]

Electric Ducted Fan Airplanes generally perform at about 2 - 3 g/w where the watts are input watts (not motor watts). This is your starting ballpark and it translates to about 29 ounces of thrust from 330 watts of input power.

The only reasonable candidate motor for this is the fisher price. You could get perhaps 1 - 2 lbs from each.

IIRC we have to protect the FP motors for 30 amps. Allowing for some loss in the speed controllers we should get a shaft output of 150 watts if can present a torque load of about 21 oz-in with a motor speed of about 10,000 RPM. The motor could probably be pushed to 40 -50 amps for short times. You will need to find a propeller or ducted fan unit that can fit the motor and provide the suitable load. Ducted fan units in the right size range are generally sized for 1 - 2 kw brushless motors.

There are obvious safety hazards. The ducted fans are a bit safer.

I am a little surprised that FIRST hasn't closed down this option by rule. Its one thing to have 10,000 RPM in a closed gearbox and quite another to have it on a 3 - 8 inch rotating cutting blade. A rule say limited rotating parts to 3-5000 rpm-inches of diameter would take this off the table.