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Author Topic: Electric Power for Keil Kraft Ladybird  (Read 304 times)
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frenchbaz
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« on: July 29, 2020, 05:54:30 AM »

I have been gifted a 1960's Keil Kraft Ladybird kit which I am building to the original plans. The kit was designed for free flight with a Mills .75 ic engine.  I would like to use electric power if possible to attempt to fly the model as free flight. It is a heavy (some use of plywood , with 3/16sq stringers) model and is renowned for being hard to trim (short body to wing ratio). I am not looking for remarkable flight times just to be able to show the person who donated the kit that it flies!! I am no electric flight specialist , using KP01 motors on 24" wing span models is as far as I have gone so far. Any advice on suitable motors and ESC ( obviously manually set as no RC) would be gratefully received.
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Frenchbaz (based in UK)
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lincoln
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« Reply #1 on: July 29, 2020, 08:59:56 PM »

If you wanted to be a Luddite, you could use a geared, brushed Speed 400. Maybe with a small 2S lipo.  Keep in mind that brushed motors need a different kind of speed control, or some kind of switch. A heavy motor is probably better than nose weight. The gears may be a little noisy. This is based on what I've seen in RC. I have an old Cleveland Viking that I plan to fly with RC and a geared Speed 400, if I ever get to it. It may take the entire second millennium to catch up on all my neglected projects.

I've seen listings for timed free flight ESC's. Some have DT's, too. I just don't remember where. I don't know if capacitors are good enough yet, but if so you probably wouldn't need a timer. A battery that can handle enough current will probably have more capacity than a free flight model needs. I understand that some lithium iron phospate batteries can be charged much faster than the usual lipos, but need a different charger or charger setting. They are supposed to be safer than lipos.
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Yak 52
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« Reply #2 on: July 30, 2020, 04:17:39 AM »

What does it weigh?

For a 42"model I would suggest a cheap brushless motor, 2 or 3 cell lipo and one of Derek Knights KP timers would be a simple and light solution. This timer gives you the ability to adjust thrust and the motor ramp down which are very helpful in trimming. It works as a profiler for the ESC.

You could use an brushed motor (perhaps larger GWS geared one would be suitable) and a simple cut off timer like the Peterborough FET timer which doesn't need an ESC but they have no thrust adjustment.


Jon
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Robmoff
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« Reply #3 on: July 30, 2020, 07:29:43 AM »

I did some comparisons a while back as to the power of the older small IC engines, and the Mills 0.75 managed to shove out less than 45 watts. Any modern 20 gram brushless would easily outdo that and may even prove to be too powerful, but if you have a flight controller with a throttle you should be OK. It would also be around 30 grams lighter, so with the Lipo and controller right up front you may not need much nose weight.
  Edit 1 spelling
  Edit 2 maths!
« Last Edit: July 30, 2020, 07:44:07 AM by Robmoff » Logged

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Nigel Monk
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« Reply #4 on: July 30, 2020, 06:09:23 PM »

Bear in mind that IC engine power is measured at the prop, i.e. it's the output power, whereas electric motor power is generally measured at input by multiplying voltage and current to give Watts. Small brushed motors are fairly inefficient so I would aim to increase the 45W and put a smaller prop on if it is over-powered, then use a timer. Don't run any battery down to flat to stop the motor or you'll wreck the battery. Supercapacitors would tolerate that but they'll be too heavy at the size you'll need.
LFP have higher energy density but lower power density than the common LiPo's (which are LMnCo) but I'm not aware of any available at small model scale (I may be out  of touch). LFP tend to be wound into a metal can like dry cells. LFP are inherently safe and don't suffer thermal runaway because there is no native Lithium at the cathode in the charged state. In a LiPo, there is typically 50% which can react if exposed to the moisture in the atmosphere. (Just thought someone might be wondering.)

A geared brushed speed (sic) 400 will cope with maximum 10A so on 2S LiPo at between 7 - 8V gives potentially 70-80W input. I suspect that will be more than enough because the one time we managed to trim my late father's Ladybird, the Merlin was de-compressed almost as far as possible. Over-powering it makes it loop uncontrollably, but we have/had no trimming skills. I still have one in my garage if you would like an auw.

HTH
Nigel
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Tim Hooper
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« Reply #5 on: July 31, 2020, 04:29:33 PM »

I've got a freeflight electric Ladybird, that I bought as a built airframe, that's still in the trimming stage.

A little outrunner up front (mounted on springs and ball link to offer variable thrustline), allied to a small 2S li-po, with a KP timer to keep everything under control power-wise.

The project has been stalled since last summer, and needs completing sometime!

The big issue with the Ladybird is that hard-to-adjust tailplane, so I settled on a tiny screwjack using a 2mm bolt to contol its incidence.
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Nigel Monk
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« Reply #6 on: August 01, 2020, 05:39:02 AM »

That's smart, Tim, both ideas. I might borrow the tailplane adjustment method.
Do you find the springs can hold the motor rigid against any prop imbalance?

This particular Ladybird is mine. I moulded soft balsa around a glass to make the cowling more Beaver-like than baked-beans-can-like.
I'd forgotten the last developments. I think I even sent a pic to AM about it being radio assist.
It went from uncontrolled looping with a Merlin that would either scream or stall on launching from fuel surge, I think, with nothing in between, to being under-powered with the Mills .75 shown. The Mills unwound it's compression screw or was under-compressed with the lock shown. It would struggle across the filed barely flying and land out before I could turn it. I should replace the motor.
AUW is 451g or 16 oz. I guess that's getting a bit heavy for a Mills .75 ?
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