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Old Jun 24, 2011, 01:06 PM
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United States, WI, Merrill
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Sig Kadet Mark II electric conversion?

Hi Gang
I live on the Wisconsin River above Merrill, WI. I have been out of RC for almost 20 years but still have my old trusty Sig Kadet Mark II.
I'd like to convert it to electric power, but as an absolute nubie to electric power lingo, I can't figure out what motor/ESC/battery combination might work.

The plane is probably about a pound heavier than it should be from glue and fuel, and it's still on the big Floats for water and snow flying. It weighs 7 pounds right now with the floats, Fox 45 gas engine, and gas tank.

What would it take to pull this old dog out of the water and into the air?

Thanks
John in Merrill

PS a fellow RC Groupie has an Infinite 3025-770 for sale. Would that do the job?
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Old Jun 24, 2011, 01:46 PM
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a) 1st choice, buy a new plane that has been designed for electric (same size and weighs about 2 lbs less). It cost about $250.00 or less to get a new one flying good.

b) If you decide to keep the plane (a bit heavy), you would need an 60A-80A speed controller, 6s lipo to have enough power, a motor weighs around 250 - 320g, and 14 to 15" prop to generate enough thrust. These would cost around $150.00 (battery included).

Vien
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Old Jun 24, 2011, 02:58 PM
Jack
USA, ME, Ellsworth
Joined May 2008
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"..1st choice, buy a new plane that has been designed for electric..."

What?! And waste a perfectly good airplane that is a great flier? Not at my house!

John,

Over on the Glow to Electric Conversions forum there have bee numerous threads on converting that plane. Here are links to a few of them:

http://www.rcgroups.com/forums/showthread.php?t=1012419

http://www.rcgroups.com/forums/showthread.php?t=1224043

http://www.rcgroups.com/forums/showthread.php?t=1406434

If you browse those threads you'll see a lot of info on motors that worked and also some of the details on how to get the motor mounted to match the cowl and also to get the CG right.

I converted a SIG Rascal 40 and weight distribution is different enough in an electric power system that I used a two ounces heavier than was really needed brushless out runner in order to get more weight on the nose.

The guys over there that have done the conversions will have all kinds of great hints for getting it right on the first try.

If you use the Search this forum link from that forum, search the thread titles for the work "kadet" with a date range of Any Date set you'll see a bunch of threads on the various Kadet variants.

Jack
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Old Jun 24, 2011, 03:49 PM
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If you could keep the weight below 5 lbs, ~ 200g motor + 4s-4000 lipo + 60A ESC would be plenty. Many people like to run small prop fast, but larger diameter prop (pay attention to ground clearance) would give you much higher thrust efficiency --> more thrust for the same power consumption (lower kv motor). So I prefer 12" or 13" prop for this plane with 700 - 800kv motor.

Due to weight differences between the nitro gas and the brushless motor, CG will be affected.
a) Move the battery as close to the firewall as possible.
b) Use a larger motor
c) Use a larger battery pack for longer flight time.

This is what I have in mind:
_ Motor: Infinite A3025-770 kv ($40)
_ ESC: Hobbywing Pentium 60A ($40) or Mystery 60A - 80A ($20-$30)
_ Battery: Blue Lipo or Sky Lipo 4s, 4000-5000mAh, 20-30C ($30 - $40)
_ Prop: APC 12 x 8 or 13 x 6.5 on 4s would give you around 600w.

There you go, a powerful electric system for around $100.00. Depend on how you fly, flight time would be around 15 - 25 mins per charge.
Vien
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Old Jun 24, 2011, 06:25 PM
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Australia, ACT, Kambah
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Great advice from Jack. I have an electric LT-25 Kadet, just a little bigger in span and wing area but a bit lighter than yours at about 70oz. I wouldn't worry too much about yours weighing 6lb - there is plenty of history of LT-25s flying really well at around that weight, on power systems not as good as we have access to today.

Mine has flown well on as little as 240W (4S, ~400Kv, 13x10) and is very sporty on 480W (6S, 410Kv, 12x10).The 6S are only 2200mAH, but I get easily 10 minutes using a bit over half the pack, and at only 24A max, everything is understressed.

If you can comfortably fit a 12" prop, then I'd target a motor around 650-700Kv for 4S and 450-500Kv for 6S. Even at 6lb, that will still give you well over 80W/lb and a well matched efficient system that will get your Kadet up and away very well. Conveniently, many motors in that Kv range tend to weigh around 10oz, so that will help balance the Fox 45 you're removing.

If you have to go with a smaller prop, then you'll need a bit higher Kv - If you take a look here, you'll find a really easy to use tool that lets you trade off maximum prop sizes, cell count and Kv without needing to know all sorts of motor details.

You can then use the compare feature on Hobby King's website to find motors that match your desired Kv and weight.

6s might sound a bit extreme, but with cheap ESCs like the Plush60 that handle higher voltage, the cost disadvantage of higher voltage has gone away, and the electrical efficiency benefits are compelling.

But first things first, figure out what diameter prop you're happy with (enough ground clearance) and work from there.
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Old Jun 24, 2011, 06:44 PM
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I am not so sure about 240w mentioned above. The plane could fly/cruise with 240w but would be very difficult to take off with 240w of power (0 wind speed and super long runway).
It doesn't hurt to have more power. On a high wing trainer, I use at least 120 w/lb x 5 lbs = 600w. I need to consider some emergency situations that need more power.

I had a 6lbs trainer that setup with 400w and 700w power system. The difference was like night and day. It was NOT fun at all to fly with the 400w.

Vien
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Old Jun 24, 2011, 07:39 PM
Jack
USA, ME, Ellsworth
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I don't think he was suggesting that it was enough for everything, just that the SIG Kadet has a clean and slippery wing and fuselage and that 240W would keep it comfortably above stall speeds.

My SIG Rascal 40 was the same way, it was almost hard to land because of it's low drag and long glide. The Rascal 40 flew at about 7-1/2 pounds and had a 625W power system. That would get it off the ground in 30-40 feet or so and it was immediately throttled back to climb out. On a day with light winds it could fly the pattern on 215 Watts or less than 30 Watts per pound.

Depending on the kind of winds you fly in having a heavier plane is not a handicap as long as you have enough power. The Rascal 40 could handle winds up to 20-25 MPH with 625 Watts and it had enough power to do all the basic aerobatics, fly inverted, etc. And I would expect John could get similar performance from his Kadet II with a power system that was around 80 to 100 Watts per pound.

I used a Torque 4014-570 motor, a 5S A123 pack (equivalent to using 4S LiPO), and a APC 14 x 10 TE prop. That 10 oz. motor have been capable of up to 800 Watts with more battery cells and a bigger prop but at 83 Watts per pound it had all the power it needed.

Jack
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Old Jun 24, 2011, 08:00 PM
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Agreed with what you said.
When choosing a power system, we need to consider what it needs at take off. IMO, the mentioned power system in post #4 would give John plenty of power, light weight, and very reasonable in price.
The Torque 4014 is a good motor, but the motor itself is close to $100.00
John is building a light weight fuse and hopefully we get the plane ready to flight for less than 5 lbs. At 120 w/lbs (600w), I bet it will take off in less than 30ft, and will be fun. Dont you agree?

Vien


Note: I used the exact same power system with APC 14x7 prop on a 3D plane (weighs 3.8 lbs). Take off in less than 3 ft.
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Old Jun 24, 2011, 09:22 PM
Jack
USA, ME, Ellsworth
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Quote:
Originally Posted by vienquach View Post
Agreed with what you said.
When choosing a power system, we need to consider what it needs at take off. IMO, the mentioned power system in post #4 would give John plenty of power, light weight, and very reasonable in price.
Oh, I agree with that. I wasn't trying to express any disagreement. As always, there are many motors in the right power range and at quite a range in prices too.

Quote:
The Torque 4014 is a good motor, but the motor itself is close to $100.00
You're right about expensive, I bought it second hand but not used for $70 and don't regret it now that I've used it some. The Torque motors line is exclusive to one seller and they are not a "bargain basement" or high volume type seller. They stress and sell mostly RTF planes and do that with an emphasis more on quality and performance than prices. And some people are OK with that.

Quote:
John is building a light weight fuse and hopefully we get the plane ready to flight for less than 5 lbs. At 120 w/lbs (600w), I bet it will take off in less than 30ft, and will be fun. Dont you agree?
He is building lighter? I thought he was reviving his old fuelie and that it was going to be heavier. If the structure is sound I'm OK with planes like that. Sometimes fuel soaked old balsa cannot be saved though...

And you're right, it will be fun. When my Rascal was taking off in 30-40 feet that was usually off and flying before I got it to full throttle the way I roll the throttle on.

Quote:
Note: I used the exact same power system with APC 14x7 prop on a 3D plane (weighs 3.8 lbs). Take off in less than 3 ft.
That sounds like a good way to avoid tire wear.

Jack
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Old Jun 25, 2011, 03:18 AM
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54 w/lb goes a long way!

Quote:
Originally Posted by vienquach View Post
I am not so sure about 240w mentioned above. The plane could fly/cruise with 240w but would be very difficult to take off with 240w of power (0 wind speed and super long runway).
It doesn't hurt to have more power. On a high wing trainer, I use at least 120 w/lb x 5 lbs = 600w. I need to consider some emergency situations that need more power.

I had a 6lbs trainer that setup with 400w and 700w power system. The difference was like night and day. It was NOT fun at all to fly with the 400w.

Vien
I can assure you it flew just fine on 240W in, as measured by Astro Whattmeter. Actual rpm was close to predicted. I made sure I had enough pitch speed by using a 13x10 prop. Takeoff was short maybe 20-30 metres on close mown but bumpy grass and climbout was quick, not 45 degrees, but very positive, never felt like it was struggling. Enough power for loops without a dive, and didn't fall out over the top. The first takeoff was my initial biggest worry, but it was very surprisingly good.

At that power level my motor (Aveox 1409/3Y geared 3.33:1) is about 85% efficient, so predicted power to the prop was about 205W.

I'm not saying it isn't more fun on 400W plus, but I am saying it flew on 240W like a lot of .40 powered trainers I've seen.

Regarding fly/cruise, you'd be surprised how little power is actually required. I always measure how much I put back into my packs, and have my timer set for 10 minutes. Worst case, If I fly a mix of aerobatics and full throttle low passes, I use about 1800mAH, which averages out to 1.8Ah divided by 1/6 hour, or 10.8A, probably ~ 220W. BUT, if I'm cruising around doing aeros for half the flight, plus a few touch and goes, I only use around 1Ah, or 6A average, probably about ~140W. Actual cruise power would be way under 100W, probably closer to 70.

There's nothing wrong with more power, but I believe it's important not to give the wrong impression that you NEED a minimum 100W/lb. I think we do electric beginners a big disservice by that, especially on glow conversions where the original acft design did not really contemplate the size props we like to use. If we use the prop size that is practical and use sensible Kv x cell count, we can get an effective conversion with good run time and low stress (=cheap) on all the components.

The hardest part is indeed landing - I'm sure the wings are full of floatium
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Old Jun 25, 2011, 09:14 AM
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United States, WI, Merrill
Joined Jun 2011
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New Fuselage

Hi Guys I can't begin to thank you all enough for the help in getting my Kadet back into the air. I was thinking it would be reasonable to get the whole old kadet with huge and heavy 32" GEE BEE floats running for water and snow use. It was 7 pounds with floats, glo engine, and tank and old heavy reciever and battery. When I looked hard at the extra weight of the multi repaired and fuel soaked old fuselage, I remembered that I had purchased a new Kadet to replace the busted up wing on the first one. I have the second fuselage built up and just need to finish tail feathers. The current wing and unfinished fuselage come in at just 32 oz. I am sure I can lighten the plane with some round or oblong cutouts on the back half and build a much lighter tail than original and still be near 32 oz. Landing gear and wheels, motor, and batteries will be the main weight additions. I am pretty sure it will be under or near 5 lb AUW. I will give up on the heavy floats for now, ( I got back into this sport just a month ago with the purchase of a Great Planes Seawind rimfire 37" wing seaplane that is still waiting for low winds and no rain). There are probably much lighter float solutions than what I have on that old fuselage if I decide to go that way again.

Thank you so much for the dialog on this project. I did not know where to start. Just looking at the tons of electric motors it is hard for me to picture what will or won't work and why. Now I have a much clearer idea, but if I could bother you all one more time, I can't picture using a 14" or even 13'' prop. Is that going to be almost necessary? The old Fox 45 is running a 10" prop, but I am not going to go back to fuel.

John
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Old Jun 25, 2011, 09:54 AM
Jack
USA, ME, Ellsworth
Joined May 2008
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SIG still has all the info on the Kadet II available on line if you had not seen it or can use it:

http://www.sigmfg.com/IndexText/SIGRC49.html

And the basic specs and setup info is here:

http://www.sigmfg.com/BuildManuals/S...detMkIISP.html

But they leave the prop choice up to the guy that chooses the engine and that makes a lot of sense. If you look at a glow engine prop size chart for a .40 to .45 fuel engine it shows a 11 x 6 as an ideal size and the 10 to 12 range at practical. And you can certainly use those sizes with an electric motor too. The electric props are a little thinner and lighter because they run in an environment where there is much less inherent vibration and shock.

With an electric motor the prop choosing drill is to get a prop that will pull close to the maximum rated Amps at full throttle. The voltage, current draw, and propeller load will all balance to a point where the motor is producing enough power but not overheating. You can get a feel for what you have and the limits with a little static testing. A 10-15 second full throttle run, then stopping and a couple of finger touches to check for heat in the motor base and on the ESC and you'll have entered a whole new world of prepping a plane for flight.

And when you fly it you'll be amazed at the difference between electric and fuel flying. The power band will be much more linear across the RPM range and you can use any spot in the range at will. You'll never again worry about "losing the fire" from playing with the throttle.

So if a 11 x 6 or so is what you have now it will probably work. As always the old "less diameter and more pitch or more diameter and less pitch game" can be played until you find the one you like the best.

Here is a list of what's available in the APC Thin
Electric line:

http://www.apcprop.com/SearchResults.asp?Cat=24

Jack
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Old Jun 25, 2011, 11:04 AM
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14" prop might not be necessary and pushing the motor limit (Infinite A3025-770) if WOT is used a lot.

If ground clearance is not a problem, the APC thin electric 12 x 8 prop would give you plenty of speed and thrust. The APC thin electric 13 x 6.5 gives you more thrust but slower flight speed due to lower pitch (ground clearance issue ??).

I did a static thrust measurement with the Infinite A3025-770 several months ago:

_ Blue Lipo 4s-4000 30C, APC 14x7E prop, 125 oz thrust @ 53A current draw, 127 F motor temp.

The 12x8 would give about 75-80oz of static thrust at approximately 35A. Motor runs barely warm.

Vien
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Old Jun 27, 2011, 04:00 PM
Speed cost money.
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With nitro the props were very much related to the displacement of tjhe motor, not to what was best for your airframe.

Electric motors come in a variaty of KV values. Think of the KV as the gearing. The higher the number the higher the gearing thus the smaller prop it can use.

Now back the the kadet. It is not a speed plane and has a slow flight speed requirement.

Thus a lower gear will be best for that airframe.

1) Pick a prop size that will have acceptable ground clearance. (bigger the better in this case)

2) pick a motor that is rated to 700-800 watts. (your going to need the weight to make up for the original design using a heavy nitro motor.

3) choose the batteries and ESC setup that will give you about 600 watts of power WOT

Install it all check and adjust CG and enjoy.

Reasoning.

1) larger prop will be more effecient in that airframe
2) Larger motor will fix weight issues and run cooler on hot days
3) 600 watts will give you room to play.. (Floats will most likely be fine also)

I personally like the idea of a 6s battery setup. Since so many smaller planes use 3s with 6s you can just use 2 packs in series. I like long flights so something around
4000MAH at 6S would be a great starting point IMHO

You will find with a larger prop the the electric plane can take off with a really short runway. For scale T.O. you will have to use 1/2 throttle.

Once in the air if you just want to float around that big prop will be so quite. You will probably get tired before the batteries are run down.

Highflier

PS. Brushless electric motors are everybit as powerfull as a Nitro. If you can do it with Nitro you can do it with Electric
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Old Jun 27, 2011, 04:27 PM
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Check out this 4s-5000 pack, plenty of flight time and still quite heavy enough to balance the plane (assume new built fuse).
http://www.hobbypartz.com/77p-sl5000-4s1p-20c-4444.html

The purchased motor with APC 13 x 6.5 should give the plane at least 550-660w of power on 4s lipo.
If you were closer, I could help you setup the plane and do the test flight with the new power system.

Vien
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