Need to design a prop - For clockwise, and anti-clockwise operation

[beep684]

Hi,

Our school is taking part in a science competition, in which there are various problems. Our groups problem is that we have to design a propeller to move a model train along a track. There are a few sections, such as reversing and stopping before accuracy markers, as well as speed runs. The task sheet is attached.

I was hoping that a few of you guys might be able to give a few tips on the design of the prop. Unfortunately we do not know the specs of anything.

Apparently the materials we are given are a few sheets of plastic, scissors, a hair-dryer, and something to mount the prop with.


Thanks for any tips you can give,
Ben

[Sparky Paul]

That's a toughie..
All the best... :)

[JetPlaneFlyer]

You may find this page over an SmallFlyingArts web site usefull: http://www.smallflyingarts.com/Archives/Feature_Articles/Comprop/comprop.htm

Generally for max efficiency the prop blade angle should be greater near the hub and decrease as you move out toward the tip. The objective being that all points on the prop try to move forward the same distance per revolution. The blades should also have a slight 'camber' (which could be achieved by forming them over a cylinder) this improves their maximum lift coefficient, just like the wing of an aeroplane.
I think of pitch like gearing on a car… more pitch will be better for high speed at the expense of ‘pulling power’… less pitch will generally pull better at low speed but will run out of steam as the forward speed rises. Some experimentation would be in order to find the best compromise.

Pretty much any prop will generate 'reverse thrust' if spun backward. If the blades have camber then they will be more efficient in one direction than the other but some thrust will still be produced in the reverse direction.

I'd guess that the way to go would be to design a prop with cambered blades for maximum efficiency in the forward direction then live with reduced reverse performance because the reversing part of the task is not against the clock so maximum reverse performance is not necessary.

If however you wanted the prop to have identical forward and reverse performance at the expense of optimum performance in either direction then you would need a flat ‘un-cambered’ blade profile.

Steve

[Sparky Paul]

What is going to drive the prop?
And is it connected to the wheels?

[harveyweewax]

What is going to drive the prop?
And is it connected to the wheels?

After perusing the Word file Ben attached to his post, it appears to me that the prop is driven by a motor mounted on top of a train carriage. The motor gets its juice from the track via pickups on the carriage. To me, the critical parameter would be the distance from the track to the motor shaft and the pitch of the prop. After reading the challenges I think you would need a prop that would quickly accelerate and decelerate the train. I assume you would need a relatively low pitch prop.

Regards,

Sven

[Sparky Paul]

Presuming the wheels of the carriage aren't powered, and the power to the motor reverses the direction of the motor in the different phases of the test.. forwards to backwards, a fixed pitch prop will do the job.
The Eflite reversing pitch prop for backwards 3d flight has flat blades.

[beep684]

Thanks everyone for all your fast replies and good info. It will help us a lot.

I think we may go with what steve said about having cambered blades to give better forward thrust:

I'd guess that the way to go would be to design a prop with cambered blades for maximum efficiency in the forward direction then live with reduced reverse performance because the reversing part of the task is not against the clock so maximum reverse performance is not necessary.
Steve

The only thing we arent sure about, is the scale of the track, and therefore the carriage, as some of the smaller carriages are quite small, so there wont be much room for a large diamater prop. Because of this we think that a multi-bladed prop with three or four blades may work best, and would be like a fan. Do you think this would work well?

Thanks,
Ben