Thread Tools
Nov 28, 2019, 11:31 AM
Registered User
Thread OP
Discussion

King Kobra electrification question


I have a King Kobra which I have procured a Rimfire 80 as power.

The distance between the X plate and the kit firewall is about 1.63 inches when the prop flange is flush with the cowl opening.

Is it feasible to use some thick wall aluminum tubing with long bolts through them as stand offs? Will they flex too much because of the torque?

Is it better to create a "box" to space the motor out from the firewall (I see this often in other planes)?

Thanks for the feedback and ideas.
Sign up now
to remove ads between posts
Nov 28, 2019, 03:51 PM
Registered User
For that distance bolts and tubing would be ideal.

A box really comes into its own if you install it through the firewall and make it big enough that the battery can fit into it, so the battery's as far forward as possible.
Nov 28, 2019, 08:23 PM
Registered User
scirocco's Avatar
Quote:
Originally Posted by abenn
For that distance bolts and tubing would be ideal.

A box really comes into its own if you install it through the firewall and make it big enough that the battery can fit into it, so the battery's as far forward as possible.
Through, of course. Light bulb moment there! I hope I can remember it.
Nov 29, 2019, 12:40 AM
Registered User
vollrathd's Avatar
Quote:
Originally Posted by Ivanstein
I have a King Kobra which I have procured a Rimfire 80 as power.

The distance between the X plate and the kit firewall is about 1.63 inches when the prop flange is flush with the cowl opening.

Is it feasible to use some thick wall aluminum tubing with long bolts through them as stand offs? Will they flex too much because of the torque?

Is it better to create a "box" to space the motor out from the firewall (I see this often in other planes)?

Thanks for the feedback and ideas.
FYI, I would not use aluminum tubing for this sort of thing, unless they are around 1/2 inch in diameter.

Take a run over to your local well stocked Ace Hardware store. Ace stocks aluminum spacers in various diameters in various lengths. You can stack a couple of them if needed. And, a hack saw works well to fine tune their length. Just run four long screws through the motor mount, the spacers, the fire wall, and secure them with locking type nuts with nylon inserts.

I've run motors exceeding 3000 Watts with this setup, with no issues. IE my last setup used eight 1/2 inch diameter one inch long spacers with 10-32 steel screws for the four corners. Be sure to provide flat washers on the inside of the fire wall though. Blind nuts work just fine. but blue lock tite them.

Several years ago, I had two of the motor mount screws come out in flight, the third was half backed out, the fourth was finger tight. That was on a 4500 Watt electric motor that turns a 22X10 prop at 7300 RPM! It's lock tited now!

REF
https://www.mcmaster.com/standard-unthreaded-spacers
Last edited by vollrathd; Nov 29, 2019 at 12:48 AM.
Nov 29, 2019, 09:40 AM
Registered User
Thread OP
Quote:
Originally Posted by vollrathd
FYI, I would not use aluminum tubing for this sort of thing, unless they are around 1/2 inch in diameter.

Take a run over to your local well stocked Ace Hardware store. Ace stocks aluminum spacers in various diameters in various lengths. You can stack a couple of them if needed. And, a hack saw works well to fine tune their length. Just run four long screws through the motor mount, the spacers, the fire wall, and secure them with locking type nuts with nylon inserts.

I've run motors exceeding 3000 Watts with this setup, with no issues. IE my last setup used eight 1/2 inch diameter one inch long spacers with 10-32 steel screws for the four corners. Be sure to provide flat washers on the inside of the fire wall though. Blind nuts work just fine. but blue lock tite them.

Several years ago, I had two of the motor mount screws come out in flight, the third was half backed out, the fourth was finger tight. That was on a 4500 Watt electric motor that turns a 22X10 prop at 7300 RPM! It's lock tited now!

REF
https://www.mcmaster.com/standard-unthreaded-spacers
Yes, I suppose "spacers" would have been a better word choice. I was figuring on 3/8 O.D. or better.

I also thought about threading the I.D. and bolting directly to that from inside the firewall to secure the spacers, then from the front to secure the motor. I am not dead set on this yet, as I suspect the average aluminium spacer is probably some form of 1000 series stuff that isn't great for structural applications.

My main worry was deflection from the motor torque. If it works for a three kilowatt system, this should be fine.
Nov 29, 2019, 01:00 PM
Registered User
I would use the tubing or spacers as a sleeve, with the bolt all the way through. Threads cut into aluminium can easily strip.
Nov 29, 2019, 09:42 PM
Registered User
Thread OP
Quote:
Originally Posted by abenn
I would use the tubing or spacers as a sleeve, with the bolt all the way through. Threads cut into aluminium can easily strip.
Well, 95% of the HiLoks installed on a commercial airliner are a titanium HiLok with an aluminum nut. Undoubtedly, threading into 6000 series O temper aluminum isn't a good idea. However, some 7075 extruded tubing should be fine.

That part isn't a big worry to me. And, if it does strip, a helicoil would make up the difference.

Thanks for your input though, I appreciate all I can get.
Nov 30, 2019, 11:27 AM
Arrowhead
Are you converting a built plane from fuel to electric? Or building the kit and modifying it as you go?

I picked a used King Kobra a few months ago, and also plane to convert it. Would love to see pics of your conversion.
Nov 30, 2019, 01:14 PM
Registered User
Thread OP
Quote:
Originally Posted by CF105
Are you converting a built plane from fuel to electric? Or building the kit and modifying it as you go?

I picked a used King Kobra a few months ago, and also plane to convert it. Would love to see pics of your conversion.
Building it as I go.
Rimfire 80, 6S lipo
Retracts

Going to give the FrSky Neuron a try. According to ecalc, this motor on a 23x8 prop should pull around 75A or so.

Not sure though. This is my first time assembling an electric without canned instructions.
Nov 30, 2019, 08:14 PM
Registered User
vollrathd's Avatar
Quote:
Originally Posted by Ivanstein
Building it as I go.
Rimfire 80, 6S lipo
Retracts

Going to give the FrSky Neuron a try. According to ecalc, this motor on a 23x8 prop should pull around 75A or so.

Not sure though. This is my first time assembling an electric without canned instructions.
Uh Oh
I'm assuming you have access to one of those "wattmeters"? If not, it's like driving an old car without a working gas gauge. Sooner or later, it is going to cost you.

With a Wattmeter, you can quickly determine if your power setup is running within the limits of the motor/ESC/battery pack, without burning up anything. Another option is one of those AC and DC clamp on ammeters.

Note that most clamp on ammeters are AC only, not useful for our DC battery powered models.

FYI, those clamp on ammeters have a jaw that simply clamps around ONE of the battery leads ie, no adapter is required. These units have a lot of other uses, like measuring the starter/alternator current on your car or truck, tracking down a obscure battery drain on an old car while the engine is not running and so on.

I've had one of these for many years. The newer unit has 0 - 60 and 0 - 600 Ampere ranges, both AC and DC (Battery power)
https://www.sears.com/uni-t-ut216c-a...blockType=G16#

Also have one of these. This unit has a 0 - 2 Amp, 0 - 20 Amp, and 0 - 100 Amp AC and DC ranges. This meter is quite useful, in that it has the usual voltage ranges, resistance ranges. And it has something called "NCV", where it "Beeps" and flashes an LED if you hold its closed jaws near a live 120 or 240 VAC conductor.
https://www.amazon.com/Uni-T-B4Q094-.../dp/B00O1Q2HOQ

Yikes!
I was just fooling around with that UT210E unit with its NCV function, holding it near an old electric water type radiator heater under my computer table. The whole danged thing was HOT! I measured 75 Volts AC between the radiator and earth ground. Just finished rewiring it for a three wire with ground power cord.
Last edited by vollrathd; Nov 30, 2019 at 09:12 PM.
Nov 30, 2019, 08:46 PM
Registered User
Thread OP
Quote:
Originally Posted by vollrathd
Uh Oh
I'm assuming you have access to one of those "wattmeters"? If not, it's like driving an old car without a working gas gauge. Sooner or later, it is going to cost you.

With a Wattmeter, you can quickly determine if your power setup is running within the limits of the motor/ESC/battery pack, without burning up anything. Another option is one of those AC and DC clamp on ammeters.

Note that most clamp on ammeters are AC only, not useful for our DC battery powered models.

FYI, those clamp on ammeters have a jaw that simply clamps around ONE of the battery leads ie, no adapter is required. These units have a lot of other uses, like measuring the starter/alternator current on your car or truck, tracking down a obscure battery drain on an old car while the engine is not running and so on.

I've had one of these for many years. The newer unit has 0 - 60 and 0 - 600 Ampere ranges, both AC and DC (Battery power)
https://www.sears.com/uni-t-ut216c-a...blockType=G16#

Also have one of these. This unit has a 0 - 2 Amp, 0 - 20 Amp, and 0 - 100 Amp AC and DC ranges. This meter is quite useful, in that it has the usual voltage ranges, resistance ranges. And it has something called "NCV", where it "Beeps" and flashes an LED if you hold its closed jaws near a live 120 or 240 VAC conductor.
https://www.amazon.com/Uni-T-B4Q094-.../dp/B00O1Q2HOQ
The Neuron ESC has a high accuracy current meter included. Telemetry will tell me current. I think it's within 125 mA. Close enough to tell if something will be smoking or not.

If that doesn't work, I have a 150A telemetry sensor. I have used it in other models and it is accurate within 2% or so.

As a last resort, I can dig back into my tools from when I was an auto mechanic and use the alternator out put current attachment to my multi meter.

I am not at all unfamiliar with electronics, circuitry, and the like. However, I am suspicious of the manufacturer specifications as I think they are a bit optimistic on most things hobby related. (Most LiPos are only good for about 40% of their advertised C rating...) I also get the idea that ecalc has a lot of fudge factor, but seems a good place to start..

I think my biggest quandry with this is getting my desired thrust with a short prop. The original plans say an 11" prop, and most motors for a 60 size ship are low enough KV that they won't turn an 11" prop fast enough to make the desired thrust.
Dec 01, 2019, 11:21 AM
Arrowhead
Well the Rimfire 80 should be plenty, although as you pointed out you’ll need a bigger prop. You can also look at using a 3 or 4 blade to reduce diameter, but as you add blades you lose efficiency. Everything is a trade off...

If you want more kv, maybe look at the E-Flite Power 60B. Also runs on 6S and certainly hauls my 20cc size P-47 around nicely.
Dec 01, 2019, 12:27 PM
Registered User
Thread OP
Quote:
Originally Posted by CF105
Well the Rimfire 80 should be plenty, although as you pointed out you’ll need a bigger prop. You can also look at using a 3 or 4 blade to reduce diameter, but as you add blades you lose efficiency. Everything is a trade off...

If you want more kv, maybe look at the E-Flite Power 60B. Also runs on 6S and certainly hauls my 20cc size P-47 around nicely.
Yeah, I can use more blades. Efficiency loss won't be horrible, and if I get the thrust to weight ratio ecalc says, I should have my hands (thumbs?) on a rocket ship.
Dec 01, 2019, 04:21 PM
Registered User
scirocco's Avatar
If diameter is constrained below 13", the Rimfire 60 is a better bet than the 80. Same size & weight and thus same power handling capability, but will give the rpm on 6S to make an 11"prop viable.
Dec 01, 2019, 04:40 PM
Registered User
Thread OP
Quote:
Originally Posted by scirocco
If diameter is constrained below 13", the Rimfire 60 is a better bet than the 80. Same size & weight and thus same power handling capability, but will give the rpm on 6S to make an 11"prop viable.
Yeah, that may be a better motor. However, I couldn't beat the deal on the 80, and I have it on the bench waiting. If it's really out of whack, I can look for a 60.


Quick Reply
Message:

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Question Guillows Super Cub 95 Electrification Question(s) Thermalin Parkflyers 2 Dec 19, 2017 01:18 AM
Help! Evolution 15cc in 60size king kobra jdm85 Engines 8 Jul 03, 2016 08:24 AM
Discussion Sig King Kobra pathfinder Classic Pattern Flying 6 Jan 24, 2016 10:27 AM
Discussion Sig King Kobra E-conversion questions CoronaL Glow to Electric Conversions 2 Aug 29, 2008 04:40 PM