Turtle or Wing?

[McRat]

This is a really simple shape.  Can't bend the material very much, and then in only 1 axis.  It's semi-flex solar panels.

I have a limit of 2 feet tall, 40" wide, 10 feet long. 

Which attached shape (looking from side) is more aerodynamic, the Turtle or Wing?  Both have identical frontal area, ie - they look the same from the top, they are basically 2D with depth.  The "Wing" will have to have 3 exposed tires due to it's shape, which will be skirted.

I'm thinking turtle is easier to construct, and more stable, and the wing would run into problems because of ground effect trying to lift the wing. 

This is what me and the kids are going to put together for fun.  Not a serious project.  Panels came today, budget is $2000 total.  Model helicopter electric motor and speed control will be used.  3 tires, 2 motors.  Top speed is under 60mph.  Probably a LOT under 60.

[Jon]

Most of the solar racers are a hybrid of turtle on top and wing underneath.

Cheers
jon

[Rex Schimmer]

A wing, when close to the ground, even when it is not inverted to generate down force, will create down force simply because the air that goes under the wing has to speed up as the ground surface and the bottom of the wing make a venturi which makes the air on the bottom going faster than the air on the top and as my old buddy Bernoulli predicts the pressure on the bottom will be lower than the pressure on the top and you have down force. I would use a wing shape, probably one of the NACA 66 series, and keep the ratio of the thickness to the length as small as possible, work very hard at getting everything as smooth as possible and at the low speed you are predicting you will have laminar flow which is the lowest drag condition you can have. If it is going to be a 2 dimensional wing then you might think about some sort of end plate to keep the air from the top of the wing from migrating to the lower part of the wing. Lots of small things you can do to get drag extremely low, buy or borrow a copy of Goro Tamai's " The Leading Edge" it deal with sun racer aero specifically, don't get lost in the calculations just look at the ideas for low drag.

Rex

[McRat]

Sounds like we start with the Wing.

We will do the body first, then tow it behind a truck at 60 mph and figure out how much HP we are going need, then try to reduce that need.

We already found out that it's too hard to make enough amps to run a 12v LS1 starter motor.  200 watts at 40v won't even make it flinch. We need a higher voltage motor, hence an RC helicopter motor.  They are cheap, light and powerful, but not very efficient.  Cheap and light is good.  Trying to keep all wiring to 10ga to cut weight.

Body material is expanded polycarbonate.  Seems to be pretty strong, and really light.  Honeycomb is too much money.

We will test winglets, that should be easy.

[SPARKY]

McRat whaaaats up?

[Trouble]

McRat whaaaats up?

Hey, Sparky, it's McRat.  I get too involved with the "politics" of LSR when I post on this board.  So, I delete my user to keep my big mouth shut, but I just end up starting another user.  

The material we are using is not flexible enough to make a decent shape.  We would have to solder hundreds of individual cells which are fragile like butterfly wings.  With the amount of "free" time me and the kids have, we can't do anything but a flat plane for now.  It has generated 1050 watts, or about 1.3 hp.

First effort was a 25' long wheelbase two wheeler.  At this point, that's not going to work.  The center would hit things and it was too "wobbly".  

Now we are going to a 4 wheeler or 5 wheeler, with the same overall length, but only a 16' wheelbase.

Layered polycarbonate won't work.  It gets weak too easy.  It's aluminum now.

Having sprocket issues right now, and some Chinese parts ordered months ago have not arrived.

Allison (14 years old) is going to enter it in her 2012 HS Science Fair.  We are going to attempt to test it at El Mirage next weekend, but won't clock it.

She is learning, volts, amps, ohm's law, trig, and hopefully still some aerodynamics.  And of course, how to use hand tools and power tools.

We are also trying to get a bike ready to do Time Only at Elmo, but it won't be electric.  The solar car takes precident(sp) over the electric motorcycle.

We will post pictures of the solar car it as soon as we git er dun.  Hopefully the 16th.

No more LSR cars for us.  Too expensive.  Fun projects or motorcycles now.  We can do all the safety equipment on a motorcycle, including tires for less than the cost of a HANS device alone.  We could buy a 2012 Busa retail with safety equip, for less than what our car safety parts cost.

I occassionally drive a "stupid fast" street truck to work, I like to think it's the fastest production diesel truck on the road with a 197mph official timeslip (it went faster though).  To continue LSR with it would turn it into a 100% trailer queen and still cost thousands a year to keep, and that's just not how I run.  Trucks haul and tow things.  If they don't, they are little but poorly designed coupes, they aren't trucks.  It's like marrying a mannequin.  You pay for the wedding, but the honeymoon is disappointing for me.

See?  It only takes me 5 minutes to go from tech to trash.  

I hope to run Bville again and Elmo, but I'm moving back to my first love, which is bikes.  



 

[SPARKY]

keep having FUN