mc71

This is a little off topic but its fun to play around with and I'd like as many opinions as possible.

I am a second year mech engineering student, one of our assignments is a design and build competition, the basic requirements are a vehicle to move toward a fan, using an impellor to drive one or more wheels. The judging will be based on both starting distance from the fan and time over the final 2.5 meters.

I have a few theoretical questions.

Is it better to have a high or low pitch on an impellor?

Is it better to have the impellor turning the same way as the fan or against it?

Can anyone give me suggestions, either scientific or from experience?

Matt

djlyon

The blades on the impellor are little wings. When facing the fan you want them to produce max lift at min drag. This would be very high pitch. To win this race make sure you make or get a good estimate of the air speed of the fan and your expected impellor speed so you can calculate a good velocity vector. That will allow you to optimize the blade pitch for max lift and min drag. Sence you are using the impellor to drive wheels you want max torque out of it not max speed.

I would turn the impellor in the same direction as fan.

It would take quite an analysis to determine which is truely better or if there is a real difference.

Denis

Ben Lanterman

Fun project. Get a fan and do some model work with some balsa impellors.

Design the body shape to have a minimum drag, perhaps knowing the fan rotation you could take advantage of body shape and get a sail boat effect.

Does the impeller have to be a fan shape or could you use a vertically rotating cylinder?

You could get torque and possibly a favorable lift vector. There is a vertical configuration that looks like a old combine front rake assembly that is used in wind generators. With proper adjustment of individual wings as it rotates you could maximize the torque by having full flat (or optiimum angle of attack) on the going aft power segment of the rotation and zero incidence on the forward going segment.

Minimize weight, drag and friction and have just enough traction with ground to overcome the aero and friction drags.

Tall Paul

Looks like a job for a generator feeding the motor to drive the wheels.
Much less mechanical friction involved.
Ollie specified all the problems this has, over on E-Zone.

djlyon

Watch it guys. This is a class assignment for him with competition at the end. Giving him to much direct advice might be cheating.

I do like the rotating cylinder though.

Denis

djlyon

It really does sound like a fun challenge though. Maybe it'll catch on all over and it can called the Aussie Cup race.

Denis

Ben Lanterman

Speaking of class assignmients. I really wish I could do the college thing again with today's resources - Computers and internet. Instant access of data and the ability to manipulate it not having to use a slide rule. When I think of the amount of time that could be used for actual learning in the old days that was used up by having to hand chug with that blasted slide rule. Yeccch ......

djlyon

All I can say is when my slide rule said 5x5 was 24.8 then bygod 5x5 was 24.8.

Yea wish I could do it again (aero deg) with todays resources.

mc71

It's not cheating...

...........It's seeking advice from experts. (yeah that works)

We are most of the way through the design process we are just trying to pull the last of the energy out of the wind and there are some things that we just don't understand yet.

The thing I like about the project is that there are so many problems to design around. The basic specs are that the vehicle cannot change external shape or size, it is to have zero potential energy when it is put on the test track, the wheels must turn when the impellor turns and it has to fit inside a 30cm cube. A few guys from our local club had a go at buiding some the last time this comp was held. These were engineers and scientists from all backgrounds, and it was very plain to see that something that was very good theoretically was no match for something that was well worn and well tested with limited theoretical knowledge.

I'll post the specs when I can get a hold of an electronic copy, maybe someone else wants to have a go?

Matt

Tall Paul

.
Way back when, someone bragged about doing the computations for the AH-56 on his 5 inch Pickett... someone else commented..."Shoulda used a 10 inch."

mc71

Heres a picture of our design, I really hope our competitors can't find this! Its made from blue foam and balsa, the strcture weighs just 3 1/2 grams and is as strong as buggery ( aussie-ism)

Matt