High power high rpm universal joint?

[Gordon]

Hi guys

Sorry to encroach on your patch, as I'm a flyer, not a boater. I fly electric, and am trying to work out how to put around 2-2.5kW into an electric ducted fan (EDF) powered model plane. The plane will be a 65in long, 52in wingspan Folland Gnat weighing about 13-14lb. I have speed controllers and battery packs from other EDF projects, and the kit has been lying around for some time. So now I am working on the drive system for this project.

A commercial fan unit which will handle the power exists (called a Wemotec HW-730, and I have one), but no single electric motor or battery set exists which can provide that max power, and permit around 4-5 minute flights with throttle management. So the way to go is to divide the required power between 2 motors, 2 controllers, and 2 battery packs; 1-1.25kW is not an unreasonable target power for motors or cells at the duration required. For example, 2 x 25-cell packs, each delivering 45A, will provide a max power input of 2250W, which is in the right ballpark without overstressing cells (eg RC2400 nicads) or motors (big brushless ones, eg Plettenberg 300-30-A2S, Lehner 2250, Hacker B50-XL, maybe even Kontronik Tango) if sensible throttle management is used.

Linking the motors to drive the fan unit at over 28000 rpm is the next problem. What I want to do is link the motors in-line (I don't want to use gears or belts) and was wondering if you boat guys could recommend a suitable high-speed universal (Hooke or CV or any other) joint which would happily transmit about 3 horsepower at up to 30,000 rpm, fit a 5mm dia electric motor shaft, and be perfectly balanced.

Best regards

Gordon

[martin richards]

Can't see the problem. If I understand your idea, you're going to put one motor behind the other with the shafts in line. Why do you need a U joint? A solid coupler would seem the strongest and mechanically most efficient method as long as your motor mounts are rigid enough.
Btw some electric boaters use single motors approaching that level. See
http://www.rumrunnerracing.com/feforums/ and tap into their more expert knowledge although some of them also visit this site.

[Dick Crowe]

Wow Gordon,
That sounds like quite a project.
I've been running the Hacker XL's for a few years and have been very pleased with the performance. On 24 cells I run a 9XL in my oval hydro and a 8XL in my straightline hydro.

At the 2001 straightline event a guy named Chris Fine showed up with a boat running 2 hackers inline. He had Hacker build him motors with drive shafts out each end. I don't know if this is something Hacker commonly does, but you could sure contact Chris for more information. His business is www.finedesignrc.com . He's a little slow on e-mail as he's still grasping the wonders of the internet, but will talk your ear off if you call. He's a wealth of information. As I remember it he used a coupler that had a short piece of flex shaft in the middle.

I can only imagine what this project will do. I'm running in the 90's with my 24 cell boat and a Hacker. They are some brutal motors.

If you're interested, the striaghtline website with a ton of videos and all the NAMBA electric records is at www.drcwebservices.com/lasaw

Good luck,
Dick Crowe

[Gordon]

Thanks guys and I'll follow up your links.

Ref the Hacker with the shaft at both ends, I know that Mega do this for helicopters, so maybe most manufacturers will do that as well.

The reason I was hoping to use flexible couplers is so that the installation would be independent of the chosen motor. The fan unit would be built into the fuselage, and the motors be linked to the fan shaft, one in front of and one aft of the fan. I'd use the UJs to make motor alignment less critical. However, as this is a specialised application and I'm not going to market it, on re-thinking in the light of your comments, I guess that trying to make one installation fit all motors isn't as important as it seemed at first.

Best regards and thanks again.

Gordon

[Dick Crowe]

Gordon,
I really think that a flex shaft would accomplish just what you are looking for. They are way more forgiving the u joints and will allow for any motor misalignment. I used U joints years ago but didn't have much luck.

If you're interested in this call Chris Fine at www.finedesignrc.com . He's got every coupler and combination there is. Just tell him what you are doing and he'll get you all fixed up.

See ya,
Dick

[Pagemaster]

Dick wont a single 2280 lehner provide the 2250watts with 2 32cell packs in parallel for 4-5minutes?

[Dick Crowe]

You know what, you're right. I keep forgetting all the motor alternatives today.

That's what I would do.

See ya,
Dick

[Pagemaster]

Dam I feel good! Dick Crowe telling me I'm right.:) :) :)

Gordon hope to see video or pictures of that monster.

[Gordon]

Originally posted by Pagemaster
Dick wont a single 2280 lehner provide the 2250watts with 2 32cell packs in parallel for 4-5minutes?

Now that's an interesting suggestion. I've looked at the Lehners a lot, and the 2280 should handle the power, and be lighter than 2 motors. In fact just one motor would be a heck of a lot easier to install than 2, and probably cheaper too.

I believe that Lehners are better known in the boat world, and they are only now finding their way into electric ducted fan, so there's not much experience of them there. However, they do have a really wide range, both in power handling and rpm/volt, so it should be possible to find something from their catalogue.

Has anyone out there used parallel-connected nicad packs at high currents? I have no problems with charging 2 packs separately, (as I believe they should be as the peak detection could have trouble if I charged both packs in parallel) as I have enough chargers. I just wondered if there are any extra gizmos needed when the charged packs are connected in parallel, to ensure they discharge together OK.

Oh, and another question. It's been suggested that Lehners, being 2 pole motors, need to operate at over 27000 rpm when on max power, to avoid overheating. Presumably this has to to with eddy currents and other strange magnetically-induced effects. Can anyone comment, please?

Thanks again guys! Oh, and there'll be photos and a video, you can be sure.

Gordon