Skyway 8-Inch Wheels: What A Drag

[Rick TYler]

One of the four launchers we've prototyped includes the 8-inch Skyway wheels from the KOP. They worked pretty well, but the notable thing is that at high RPMs the wide plastic spokes move a LOT of air. The drag from the spokes is dramatic. We are now looking into covering the spokes with a lightweight layer of something (I suggested either mylar or duct tape) to keep them from acting like cooling fans. One mentor's primitive guesstimation is that the aerodynamic drag might be costing us a 10% loss in RPM.

Just something else to think about...

[MikeJ675]

sounds like a good idea. If I remember correctly, duct tape is still not allowed on bots.

thin lexan or another plastic may work well. You may need to heat it to get it into the proper shape, though. If you have the ability to do so, making the cover out of carbon fiber/fiberglass would be pretty nice looking(you can buy this in non-hardened sheets and use a resin or epoxy to harden it over a mold I believe)

The main thing you need to make sure happens is that there is a good seal and that the cover piece is smooth. Assuming it's legal, shoe-goo is a good gap filler and adhesive(though I'd suggest bolts for anything spinning at high rpms)

[Rick TYler]

Using epoxy and fiberglass is a good idea. It turns out that another hobby of mine is building boats so I have a bunch of 6-ounce woven cloth and the appropriate epoxy, too. The hubs look and feel like a plastic that won't be epoxy friendly so we will probably have to attach the new "hub caps" with mechanical fasteners of some kind. Once they are drilled and slid over the shaft they won't go anywhere dangerous even if they come loose.

I see them painted in spiral patterns...

[John Neun]

We have made the same observation and know we have to reduce the drag, but haven't given it much thought yet. Thanks for the thread! I had envisioned something light like the mylar previously mentioned, bolted on. We would rather keep the mass as low as possible.

[Dick Linn]

Why not get a sheet of hard foam from Home depot and glue on there? Unless you need the mass.

[Tom Bottiglieri]

You could cover the entire hub of the wheel.

In effect turning this...



Into this...

[SkywayWheels]

Dude, you write:

"at high RPMs the wide plastic spokes move a LOT of air. The drag from the spokes is dramatic. "

If you carefully use a dremel or router or other abrasive machine you can shape the spoke without compromising the material.

As with any composite, an error in re-machining can be catasprophic. Be very careful and watch for stress-witnesses.

BMX racers have known for years that Skyway wheels can take abuse. Just sayin'.

[greencactus3]

Originally Posted by Rick TYler

I see them painted in spiral patterns...

depending on how fast they are spinning im seeing REALLY REALLY thin spiral lines
and if you're doing a DD then a bit of directed airflow to the motor to keep it cool will help in efficiency too.

[SkywayWheels]

bump.

[Madison]

We recently hooked up a rough and tumble prototype using these wheels coupled directly to a small CIM motor and also noticed what appears to be a significant current draw by the motor as a result of air drag.

We measured a draw of 27A with the wheels spinning under no load other than air resistance and friction and I figured that to mean there's about 1 lbf applied against the shaft or so. When passing a ball through the shooter, the current jumped a bit to 30A, suggesting an additional load of about .25 lbf. We intend to shield the spokes, of course, and also understand that the prototype rattles a lot and is not as efficient as it could be -- in fact, a quick test of the system before it was statically mounted produced a current draw of 66A!

All of that said, are these data in the same ballpark as anyone else? I've been using these numbers as a benchmark to determine my final gearing using a different motor and am getting expected results far different from what I see most teams implementing. I'm ignoring losses to ball spin at the moment, but it seems like that may account for as much as 45% of the energy put into the balls and that just doesn't seem right to me.

Any guidance?