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Author Topic: Bluebird K7  (Read 1725 times)

Jonty

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Bluebird K7
« on: April 22, 2016, 12:13:04 AM »

  Has anyone managed to make a successful model of Bluebird K7 using an electric ducted fan? I don't need it to be blindingly fast, just capable of a respectable speed.
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ooyah/2

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Re: Bluebird K7
« Reply #1 on: April 23, 2016, 05:06:12 PM »

  Has anyone managed to make a successful model of Bluebird K7 using an electric ducted fan? I don't need it to be blindingly fast, just capable of a respectable speed.

Jonty ,
  The answer is No as far as I know nobody has been able to get K7 running on Ducted fan, the air intakes are too small, my K7 at 1/12th scale sits so low in the water that when full power is applyed it buries itself under the water befope coming up on the plane, they can be made to run very well on Brushless motors and Lipo batteries.

There is only one person and I can't remember who built a 1/24th scale K7 and fitted a  fan and he had to drill holes in the engine cover to get enough air into the fan, I don't think that he ever had it up on the plane but if you troll Y-Tube you may find his results.

Also on this same subject have a look at D.Cambells K7 about 7 threads down from your own and the link to mine at 1/12th scale on brushless motor and 3s Lipo's.
https://www.youtube.com/watch?v=HOjmAhHC6us
  Hope this helps
George.
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Jonty

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Re: Bluebird K7
« Reply #2 on: April 23, 2016, 07:16:50 PM »

  Thanks, George. I've followed your thread with great interest. I had read up a bit on ducted fans and thought that might be the snag.
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Tim_M

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Re: Bluebird K7
« Reply #3 on: April 23, 2016, 07:50:24 PM »

Hello Jonty,
We had a debate about this about 18 months ago in the forum. I thought we documented it in my Bluebird blog but I can't find it now. It has been done (https://www.youtube.com/watch?v=bR7FJNUCsdI) but I don't think it really worked. We also concluded that it probably wouldn't work - but prove us wrong! Here's what you are up against:


1) Thrust. I did some fag-packet calculations. There's a very simple relationship between power, thrust and speed: Power = speed x thrust. If you take my 1/8th model, assume it will run at 30 mph (13.5 m/s) using about 700W at the prop. The thrust comes out at 53N (5.5 kgf). That's a BIG ducted fan! OK, they are ball-park figures but not a million miles out and suggest that even on a smaller model, you will need a potent fan.
2) Water ingestion. I did rough out a design but declined to publish it on account of I didn't want to be responsible for somebody losing an eye when the fan burst.
3) Intake area. Just as George said, the only (findable) attempt had the cowling riddled with holes to try to get some air in. Looking on HobbyKing, you'd be looking at a 120mm fan for my model. The intakes would need to be at least double the size they are now.


The thing to remember is that ducted fans are nothing at all like turbojets. They work on a completely different principle. DFs are just propellers, shrouded to improve the efficiency. They produce thrust by aerodynamic lift on the blades, not by ejecting a high-speed jet. And they are pretty poor at low speeds. The pitch of the blades is too high and they stall (think 'wheel spin'). They are designed to work on aircraft moving forward fast.


For my money, I think a water screw is the answer. If I were starting again from scratch I think I would do away with the strut and go for a sturdy prop tube (like George). More discreet. But then again, the objective it to get the thing going so fast so can't see all that apparatus hanging off the back anyway.  :-))



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ooyah/2

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Re: Bluebird K7
« Reply #4 on: April 23, 2016, 11:47:55 PM »

Jonty,
Here is the link to Noel Starlings build of the 1/24 scale K7, the model is about 13" long (https://www.youtube.com/watch?v=pTravoRdzb0 )

I have read that he built this for somebody who has reported that it runs well.
You can see the holes in the engine cowelling whioch if they were on my 1/12 scale madel it would sink it as I put on full power to get it up on the plane.

I also wanted to build in an E.D.F. but Ernie Lazenby who has much experience with jet powered K7 strongly advised me that I was wasting time and money
I even built a Gas turbine to put into a K7 but discovered that I would need about 4 times the pond size that my club uses so it would need to be Loch Lomond with a chase boat and all the kit to get it running so the engine was sold and I went to the 1/12 K7 with brushless and Lipo's.

On my you-tube video K7 looks fast, on the water, when you have the controls it's frightening fast, so from one end of our pond to the other I only get an 8 sec run but have most fun with it on  Loch lomond which is only 30 mins drive from me, where I can let it run some 400 + yds, I have to cut back the throttle a touch or it will take of just like the full size K7 and you must not try a turn at full power.

George,
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