New topic old subject - Primary Force Electric

[Steve Dwyer]

If anyone has the electric conversion PDF plan for the Primary Force I sure would appreciate you sending me the file.
Built a PF gasser in 2015 and have always loved the plane, friends are all converting to electric and I'd like build one.
I did reach out to Mickey but have not received a response. Your help will be appreciated.

Thanks,

Steve

[John Rist]

I suspect one of Brodak's conversion kits will work.

[Mike_Ostella]

I used a Tom Morris nose ring electric mount. I notched the fuse to accept the mount. I replaced the hardwood mounts with spruce to save weight. Filled with blocks, then carved and sanded to shape. I'm in the process of painting the plane now.

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[Steve Dwyer]

Mike and John,

Thanks for the responses.

Interesting nose concept on the PF, I like the recesses to house the electric hardware and the breathable encapsulated motor housing to accept a flush spinner.

I sent my post to find if someone had the plan changes Mickey Pratt had made to make his PF design electric compatible. I had come across several inputs by others recommending lightening holes in the fuse and tail surfaces. Others were discussing forward CG and the need to move the 4C battery back into the wing. No one wants to build an unstable plane, was hoping there was some hard fast experience and recommendations I could follow.

I plan to keep my existing OS FP 40 powered PF as is and build a new electric version.

Steve

[Mike_Ostella]

I'll keep you posted. I should have it flying in a few weeks. I have lots of photos of the building process if you are interested. I just have to find them
Cheers

[Steve Dwyer]

Mike,

Definitely would like to see the build photos. Do you have any plans for the size motor, prop. and battery? What are you basing your flight time and lap to be?  Do you expect the total weight to end up in the mid 40s?

Steve

[Mike_Ostella]

Hi Steve, 
Since the plane is about Nobler size,  I will be using a cobra 2820/12 970 kv motor.  I will give a K&R timer a try on this one.  Battery will be a 4s 2800 25c thunder power. I'll probably start with 60ft lines and try to get close to 5.2 for a lap time. An Apc 11x5.5 ought to do the trick but I have been experimenting with falcon and xoar wood props. I found 55 photos of the building progress. I could put the best ones in a zip file and e mail them to you. I may have to get someone to help me figure out how to do it. Here are a few.

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[Mike_Ostella]

I have been asked some questions about the e conversion of the PF. The maple motor mounts were replaced with hard balsa. The motor mount from Tom Morris is 1/4"thick and it's made of 6 ply plywood.  Its hard as a rock. The supplied ply doublers are 1/8" ply, 5 ply , nice hard stuff.  Im Undecided about whether 1/8 is necessary for electric power. you might consider 1/16" ply for your project. The areas in front and back of the mount are filled with balsa blocks,  carved and sanded to shape and covered with glass cloth. Here are a few more photos. Hope this helps.

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[Mike_Ostella]

Painting finished except for touching up a few spots and clear coat.

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[Steve Dwyer]

Mike,

Thanks for the build update, I'm impressed with the cowl and the airbrush work to create the canopy, you are creative. I have a couple questions:
Can you weigh the plane with and without the electric hardware following painting?  Did you maintain the dimension per plan of the nose length from the leading edge to the tip of the fuse? Same question for the dimension from the TE to the rear?
Are the colors on the wing painted or Monokote? It looks like you covered the fuse with carbon fiber veil, if so can you call out the product and is it doped on? You also mentioned using fiberglass cloth on the cowl parts? Same question on it's source and is it doped on or resin and hardener? I've always tissue/dope covered the hard surfaces and I'm curious about your covering.
I'm in agreement with your selection of the motor and battery, did you anticipate the final weight comparable to a Nobler in the mid 40 oz. range?
Will in your experience the motor cool enough with air entering behind the spinner? Are either sides of  the cowl cheeks removable?
Please continue to send additional build photos mostly associated with the nose and electric hardware installation. I very much like what you have done, once flown kindly forward a report as well.

Thanks much.

Steve
 

[Ken Culbertson]

Will in your experience the motor cool enough with air entering behind the spinner?

I am impressed as well.  Almost want to re-do the nose on my Twister...almost.  I am running the 2820/12 with a full cowl and have no heat problems.  As long as there is a spinner gap around 1/16" there will be enough for the motor to vent and the open rear will probably help.  I am a bit concerned that there is not enough clearance around the front half though.  That motor is going to be spinning at 8-9k rpm and there might not be enough space.

Someone jump in and smack me down if I am wrong but I was told that the newer Cobra motors create their own airflow internally and with a front mount the motor would push air out the spinner gap.  I have run mine with and without "airflow" spinners an there wasn't any difference that I could see.  Keeping the ESC cool is #1 priority and it looks like yours will be exposed so no problem there.

I use the "poor man's" method of measuring motor temperature.  If you can touch it and not get burned, it is not too hot.

Good Luck - Ken

[Steve Dwyer]

Jim,

I'm trying to dial in on the electric selection before beginning the build, it'll be my first electric.

Based on a 500 sq in wing and mid 40ish oz. total weight the Cobra 2820/12 970 KV motor would seem the correct size. What size is your battery, is it a 3 or 4 cell?

Can you advise what your complete plane with battery weighs?

Thanks,

Steve

[ericrule]

Hi Steve;

When I owned RSM we did all of Mike's laser cutting for the P Force. Converting the model to electric is dirt simple and I know Mike Hanson, the new owner of RSM, still has the AutoCAD file so with a phone call to Mike Pratt permission can be obtained to run up a kit for you.

As far as power goes the Cobra motor you listed has a 960 kv so it will require using a 4 cell lipo battery. Anything in the range of 31-3400 mAh will give you a 5-1/2 minute motor run and still leave you with between 25-30% battery capacity left. That will make your batteries last a long while. My personal feeling is to use the same size motor but in the 1100 kv range as that will allow you to use a 3 cell battery. Same size as the 4 cell battery but 1/3rd less weight. Either motor will work fine for that size and weight of model but with the 3 cell battery you get about 3 oz less  weight so your "horse power to weight ratio" is much better.

[Steve Dwyer]

Eric,

Thanks for your response, back in 2015 I bought a Primary Force from Mickey that you cut. I powered it with a OS FP 40, it's still flying today and probably one of the nicest models I've ever flown.

I'm going to take the challenge of building this one light, my earlier "over doped" and "over built" came in heaver than others were posting at the time but it still performed well, as a result it needed to fly a little faster than I would have liked.

Others now flying the PF with the 970 kv and 4 cell configuration have reported good results. I won't silk the opens and tissue the hards this time, a quick cover of Kote and some spray on Krylon will hopefully keep it in the 25 oz. range.

Steve

[Steve Dwyer]

Mike and Ken,

I removed the OS MAX 40 FP and tank from my Primary Force so I could compare to the bare weight to your models. My PF is 4 years old and has some repairs bringing it to 30.1oz with wheels and spinner placed on the scale.
The electric motor, battery and controller I've ordered will total 14.2 oz adding this to the bare model I'll be at 44.3 oz. can you tell me how this compares to your planes?

Steve

[Mike_Ostella]

My primary Force is ready for clear. Came out to weigh 21 ounces without any hardware or power components.  It should easily come in at around 46 oz ready to push the button which should be fine.

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[Steve Dwyer]

Very nice Mike, I almost have to doubt you will reach 46 oz. given that you are at 20 oz. now. Unless I've read the power options weight you planned for incorrectly you shouldn't be over and additional 16 oz. Adding for the gear, elevator and clear coat say another 3 oz. you shouldn't even hit 40 total, that's impressive and with quality finishing as well. Good job, and great color scheme.
Let us know how the first flight goes.

Steve

[Steve Dwyer]

Mike,
I have looked over tom Morris' site for the nose ring electric mount you referred to in a photo. Can you direct me to the part before I call Tom.
Steve

[Mike_Ostella]

Hi Steve , : here is the nose ring I used.   https://stunthanger.com/hobby/product_info.php?products_id=197
No changes to the nose or  tail moment were made. flying season is over here so I'm slowing down on this project before I make a mistake but I'll keep you posted.
Mike O.

[Steve Dwyer]

Where can I purchase a collet style prop adaptor with a 5mm inner and 1/4" outer? It seems all the 5mm adaptors come with 5/16" outers threads. I want to use a 1/4" prop.
Also who offers a 1 1/2" vented metal standard spinner (not pusher style)?

Getting ready to layout the fuse dimensions on the Primary Force.

Steve

[Steve Dwyer]

Hi Mike,

Can you tell me where you purchased the vented spinner? Is it the style with a collet style hub on the back or did you use a separate connector fitting?

Steve

[Fred Underwood]

Here are a couple of sources light weight inexpensive vented spinners.  Not exactly what you asked, sorry.

https://stunthanger.com/smf/gettin-all-amp'ed-up!/help-with-motor-overheating!/

https://hobbyking.com/en_us/turnigy-turbo-spinner-51mm-white.html

[Ken Culbertson]

Here are a couple of sources light weight inexpensive vented spinners.  Not exactly what you asked, sorry.

https://stunthanger.com/smf/gettin-all-amp'ed-up!/help-with-motor-overheating!/

https://hobbyking.com/en_us/turnigy-turbo-spinner-51mm-white.html

I have several of these spinners and they don't seem to cool much better than a standard one.  I have stopped using them.  Also they do not come in 1 3/4 or three blade but they do sound really cool.  The not cooling better might be because I use counter clockwise props and they are cut for clockwise.

Ken

[Steve Dwyer]

Fred and Ken,

Great thread recommendations on both your parts, excellent, thanks much, just what I needed! Now I can place orders and get working.

Steve

[Steve Dwyer]

After receiving my Cobra 2820-12 970kva motor I am wondering if the shaft can be reversed so the longer protruding shaft can rotate freely from the outer winding. The motor comes with an adaptor to attach to the short shafted rotating can end of the motor. If I am not clear I can send a photo or refer you to a cobra motor photo.

Steve

[Ken Culbertson]

After receiving my Cobra 2820-12 970kva motor I am wondering if the shaft can be reversed so the longer protruding shaft can rotate freely from the outer winding. The motor comes with an adaptor to attach to the short shafted rotating can end of the motor. If I am not clear I can send a photo or refer you to a cobra motor photo.

Steve

I have several of these and I am not sure what you would accomplish.  With a front mount you need the shaft for the collet and with a rear mount there is no room to move the shaft.  However, if you must, there are two set screws in the short end sort of inside the motor that you can loosen and move the shaft.  It has a retaining clip you will have to take out.  They will run that way but I don't think they were intended to.  FYI those set screws that you would loosen to change bearings make handy places to put an Allen wrench to hold the rotor in place while you tighten a stubborn collet.  (if you can reach them!) 

If you are contemplating using the rear mount on the motor as a front mount and pushing the shaft through to use a collet, it can be done but you will have to run the motor without the retaining clip and I can tell you from experience that you don't want to see what happens if a collet comes off at 10,000 rpm.  Plus, you will need a press to push the shaft through unless you are skilled in the ancient art of shaft tapping.

Ken

[Steve Dwyer]

Ken,

If you refer to Mike's response (third photo) showing his Cobra 2820 he is front mounting the motor and attaching to the longer protruding shaft. On my motor the longer shaft is attached firmly to the green end. Is this clear?

Steve

[Fred Underwood]

The green end has a large bearing and the shaft spins in the bearing.  I can't visualize the shaft being attached?  Picture?

[Steve Dwyer]

Fred,

Let me "redact" the question from my last post. When holding the green housing part of the motor, the shaft will rotate I'm getting twisted up on the front and rear mounting configurations.

I do notice, thou, when I rotate the motor there appears to be a "rolling" or detent if you will, in other words, the shaft doesn't continue to spin freely on the bearings as I would expect it to. To be clear, it's doesn't feel like a rubbing, but more of a lobe like feel in a bearing. Is this normal?

What happened to the days when there was someone down the street you could go to?

Steve

[Ken Culbertson]

Fred,

Let me "redact" the question from my last post. When holding the green housing part of the motor, the shaft will rotate I'm getting twisted up on the front and rear mounting configurations.

I do notice, thou, when I rotate the motor there appears to be a "rolling" or detent if you will, in other words, the shaft doesn't continue to spin freely on the bearings as I would expect it to. To be clear, it's doesn't feel like a rubbing, but more of a lobe like feel in a bearing. Is this normal?

What happened to the days when there was someone down the street you could go to?

Steve

What you are feeling is normal if it is what I think it is.  All of the motors I have feel a bit different but there is going to be some resistance to the magnets when there is no power to the motor.  The 2820 is the closest to smooth and my 2826 is the worst.  My 35's are somewhere in between.  It doesn't hurt an electric to run it up for short periods without a prop.  Putting all of this crap into a plane is not as easy as the 4 bolts for an engine so I always bench run my set-up before putting it in the plane, especially if I have done any soldering or wire cutting..  What goes where will become as natural as aligning a needle valve but if this is your first shot at it double check everything.  Maybe even hook everything up but the battery and take a picture.  It is not like a engine that won't start if you get it wrong and you can keep trying.  With electronics, it is OVER the second you do it wrong.

Pretty sure the guy down the street moved.

Ken

[Steve Dwyer]

Ken,

After hitting send the thought came to mind about magnetic feel. I plan to test run the set up before installing. I agree it's best to check everything twice just like measuring twice before cutting, there's little forgiveness here.

 It appears motors are different. The Cobra has it's longer shaft protruding from the green mounting housing end which lends itself to front prop mounting. If I reverse the configuration to a rear mounting the unused protruding shaft points to the rear and leaves only the short stub shaft up front requiring the use of the Cobra supplied adapter. I notice other motors when rear-mounted have the long shaft pointing forward, I'd prefer this configuration. This is what twisted me up initially thinking I could reverse the motor internally.

The guy down the street never moved I did. He's 95 and a long time away from modeling. His family brought him out for a visit last year.

Steve

[Fred Underwood]

I think that "cogging" is the term for what you feel when rotating the motor.

Look at the Innov8tive RC site for instructive videos about replacing bearings and changing the shaft.  Shaft can be removed and reversed so that the long end is where you want it.  Cobra shafts are pressed in, not just set screwed.  Lucien, on the site suggests the 1 ton press, not the 1/2 ton.  Inexpensive at HF with a coupon. 

[Steve Dwyer]

Ken,

I received the following from RC Dude on reversing the motor shaft. This appears to be straight forward. Thanks for the help as usual.

Steve

Many people have reversed the shaft on the Cobra motors and it’s not too hard but there are a few things you’ll need and things you’ll need to know.  Basically you remove the C-clip from the shaft on the motor and you will then be able to pull the two motor halves apart.  There will be a set screw holding the shaft on the bell/magnet side of the motor near where the bolt-on prop adapter is normally installed.  Once that set screw is loose the motor shaft will still be a very tight press fit into the motor so you will need to use an arbor press, drill press, vise, or something similar to push that shaft to the desired location – leaving just enough shaft coming out that side still for the collar to be installed when finished.  The set screw can then be tightened, motor halves put back together and the collar installed to hold the motor together.  The ground out flat areas on the shaft may no longer match up with the set screw or the collar set screw but that is not usually a problem.  If you wish you can dremal in new flat spots on the shaft that line up with the new set screw locations.  If you wish you can also reverse the shaft by pushing it all the way out and flipping it around, if it looks like the existing flats will line up a bit better.

[Ken Culbertson]

Ken,

I received the following from RC Dude on reversing the motor shaft. This appears to be straight forward. Thanks for the help as usual.

Steve

Many people have reversed the shaft on the Cobra motors and it’s not too hard but there are a few things you’ll need and things you’ll need to know.  Basically you remove the C-clip from the shaft on the motor and you will then be able to pull the two motor halves apart.  There will be a set screw holding the shaft on the bell/magnet side of the motor near where the bolt-on prop adapter is normally installed.  Once that set screw is loose the motor shaft will still be a very tight press fit into the motor so you will need to use an arbor press, drill press, vise, or something similar to push that shaft to the desired location – leaving just enough shaft coming out that side still for the collar to be installed when finished.  The set screw can then be tightened, motor halves put back together and the collar installed to hold the motor together.  The ground out flat areas on the shaft may no longer match up with the set screw or the collar set screw but that is not usually a problem.  If you wish you can dremal in new flat spots on the shaft that line up with the new set screw locations.  If you wish you can also reverse the shaft by pushing it all the way out and flipping it around, if it looks like the existing flats will line up a bit better.

Glad you found this.  I would be curious though what would be gained by doing this.  All I can see is being able to use a collet on a rear mounted motor.  IMHO anything that gets you out of having to use a collet is a good thing.  Others may disagree.  The prop adapters work just fine but you do have to allow for the fact that you cannot change the prop position once mounted.

Ken

[Steve Dwyer]

Ken,

Now having a better understanding of the motor mounting options I can proceed with the build. This my first stab at electric power, if all goes well my next build, a Geiske Nobler will be electric as well.

Thanks again,

Steve

[Ken Culbertson]

Ken,

Now having a better understanding of the motor mounting options I can proceed with the build. This my first stab at electric power, if all goes well my next build, a Geiske Nobler will be electric as well.

Thanks again,

Steve

I have converted one.  Not much room in the nose.  If it is a "fresh" build I would make the nose 1/4" wider and a 1/2" longer.  I mounted most of my electronics in the cowl.  Great plane the Gieseke.  Lots of E-versions out there you should be able to find some good advice when the time comes.

ken

[Brent Williams]

Ken,

Now having a better understanding of the motor mounting options I can proceed with the build. This my first stab at electric power, if all goes well my next build, a Geiske Nobler will be electric as well.

Thanks again,

Steve

If you can keep the weight reasonable on your upcoming electric g-nobler, I would recommend the use of a 2814(3536) sized motor and a TP2200/4s battery.  I had the cobra 2820 and a TP 2800/4S in mine and it was nose heavy.  Lots of power, and I had a lot (30-35%) of reserve left in the battery, but there isn't much sense in carrying the extra weight, needlessly.  .02

[Ken Culbertson]

If you can keep the weight reasonable on your upcoming electric g-nobler, I would recommend the use of a 2814(3536) sized motor and a TP2200/4s battery.  I had the cobra 2820 and a TP 2800/4S in mine and it was nose heavy.  Lots of power, and I had a lot (30-35%) of reserve left in the battery, but there isn't much sense in carrying the extra weight, needlessly.  .02

Brent:

I have never tried the 2814 and I have a boatload of 2200 4s batteries with no real purpose in life. Ny Nobler has a 2826 in it and it is nose heavy but flies OK.  Which 2814 would you recommend?  I assume that it has the same fwd mounts as the 2820 and would just "pop" in.

Ken

[Brent Williams]

Brent:

I have never tried the 2814 and I have a boatload of 2200 4s batteries with no real purpose in life. Ny Nobler has a 2826 in it and it is nose heavy but flies OK.  Which 2814 would you recommend?  I assume that it has the same fwd mounts as the 2820 and would just "pop" in.

Ken

Sorry to Steve for the thread hijack, but the info is usable.

Ken, the Cobra 2826 is way, way more motor than what is needed for a Nobler sized plane.  Just dropping one size down from the 2826 to the 2820 would drop 1.16oz directly off the nose.  That and a 2200/4s should yield quite the weight savings.  My Gieseke Nobler was around 44-45oz as equipped above with an ounce+ of lead in the tail to get it to turn well.  It would accelerate effortlessly straight up during the wingover and anything vertical.

Depending on the weight of your plane, the 2814 might be on the borderline for power, but there are very successful examples of ARF Noblers/Orientals with 2814/3536 sized motors powering them. (Joe Daly's arf Nobler conversions weigh 43/44oz rtf with a Cobra 2814 850kv/Exceed 3015 910kv)

The Cobra 2814/16 1050kv weighs 107 grams (3.77 oz.) and is rated for 450 Watts of Max Continuous Power on a 4-cell Li-Po.  Good* to around a 40oz model
The Cobra 2820/12 970kv 138 grams (4.87 oz.) and is rated for 590 Watts of Max Continuous Power on a 4-cell Li-Po.  Good* up to a 54oz model
The Cobra 2826/10 930kv weighs  171 grams (6.03 oz.) and is rated for 660 Watts of Max Continuous Power on a 4-cell Li-Po.  Good* up to a 60oz model

 *Good* Using the 11 watts per ounce rule of thumb, which is another way of stating around 175 watts per pound, for those that calculate that way. 
In the RC world, they like to be 175-200 watts+ per pound for 3D flying and endless vertical performance.  We don't fly at full throttle like the 3d guys, or fly out from a dead stop in those hanging on the prop maneuvers, but the watts/ounce formula at least gives us a rough range of motor capability/power selection.  YMMV *