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Help building the GP P-51D electric

Old 11-24-2008, 05:43 PM
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beavenx5
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Default Help building the GP P-51D electric

I have been flying planes for a little over a year now. I followed a pretty standard pattern at the beginning: started with a HZ Supercub followed soon after by the PZ T-28 Trojan (great foam plane) that I still love flying. Earlier this summer, I wanted to "build" nicer blasa planes: I started by assembling a ARF Richmodel P-38 Lightning that I modified slightly to accomodate larger batteries and then later in August got a ARF E-flite Taylorcraft that I converted to a floatplane with the help of a few guys here. Up to here, the modifications and motor/battery selection was pretty straight forward...

Now, I am about half way through building my first Kit airplane: the Great Planes P-51D Mustang (kit #GPMA0175). Unfortunately, this plane was designed for a 40 size glow engine and this is how the plans and instructions were written. There is very few information available from GP or on this forum on how to convert it to electric. Since this is my first build (and obviously first electric conversion) I have a few questions:

1- I have a Turnigy 42-60 600Kv motor, 60amps ESC, and 5S 4900 lipos (18.5v) and 5S 2600 A123 (16.5v) batteries. I will try MAS 12X6, 12X9 and 13X8.5 props to get close to 850watts. I estimate the plane final weight to be around 6lbs. What do you think of this set-up?

2- I will start assembling the fuselage soon and was wandering where is the best place to install the battery hatch. Probably where the fuel tank is supposed to be installed... but what is the best way to get the battery in/out of the plane? make the canopy open? make the air scoop removable? What would work best?

3- Engine mount? How do I install the outrunner? Should I start with the glow engine mount and use this as a starting point for the motor mount or should I build one from scatch or should I buy a stick mount?

Any other tricks I should look for?

The instructions / plans from Great planes are pretty good and building the rudder and wings have been pretty good. I am just not looking forward for the covering . Just covering my foam floats for the Taylorcraft was a struggle...

Here is a link for the GP P-51D instruction manual if needed:
http://manuals.hobbico.com/gpm/gpma0175-manual.pdf

Sorry for the long post and thanks for your help.
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Old 11-25-2008, 04:20 PM
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Dereck
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[quote=beavenx5;510520]

Now, I am about half way through building my first Kit airplane: the Great Planes P-51D Mustang (kit #GPMA0175). Unfortunately, this plane was designed for a 40 size glow engine and this is how the plans and instructions were written. There is very few information available from GP or on this forum on how to convert it to electric. Since this is my first build (and obviously first electric conversion) I have a few questions:

1- I have a Turnigy 42-60 600Kv motor, 60amps ESC, and 5S 4900 lipos (18.5v) and 5S 2600 A123 (16.5v) batteries. I will try MAS 12X6, 12X9 and 13X8.5 props to get close to 850watts. I estimate the plane final weight to be around 6lbs. What do you think of this set-up?

Won't be lacking in power! You could perhaps even get away with less, but as long as you appreciate what the throttle's for, a little extra isn't a bad thing.

2- I will start assembling the fuselage soon and was wandering where is the best place to install the battery hatch. Probably where the fuel tank is supposed to be installed... but what is the best way to get the battery in/out of the plane? make the canopy open? make the air scoop removable? What would work best?

A long tale. I've often fancied this kit myself - my first GP electrocution was a CAP 232, 46 sized, around nine years ago. The P51 offers a good mix of scale looks and aerobatic performance, in a size that fits the car (bigger ain't better if you can't carry it to your flying site!). I don't recall reading of a conversion of your model, but may I suggest you do a little forum reading on Sig's Four Star 40 conversions? It's about the most converted kit ever, all your questions above will be answered to some degree and it's not that much different in size from your 51, though it's different in shape and complexity.

Structural conversion? My CAP ended up with my framework between the kit's cowling, canopy and rudder. Everything inbetween was different, though the same shape on the outside - saved somewhere around 3/4lb in AUW.

My MO with a kit electrocution is to build the wings and tail first - in your case ,fit the retracts if you're going to. Get as much of the model assembled first, bolt in the big expensive and heavy stuff - motor, RC gear - and try to avoid making too much commitment. Now see how the balance issues are shaping up. It might be okay - the Four Star has a fairly short nose, but seems to balance up just fine, some kits aren't so accomodating and yours is pretty much virgin territory. If it isn't, much better if you can work on a blank canvas than start messing with a nearly built, massively tail heavy and somewhat impractical model.

You only have to read ready-made reviews to learn how tedious it can be to have a hugely tail-heavy model with its battery rammed up front out of the way of the battery hatch, and the fuselage servos down under the tailplane - you don't want to end up in that line of work.

Cooling holes - I'd fit them around the motor mount. You will also have to get your motor wires through the firewall - there may also be room to mount your ESC under the hood someplace, which is good for air cooling it. THen you just have to run the battery leads through the firewall.

Hatch - again, structurally dependent. You should try to build in a hatch long enough to easily remove your LiPo for charging, of course. Easy way out is if you can run a hatch between kit formers - if you have to go someplace else, you'll have to develop new former shapes. I've ended up with large hatches and only had to add 1/4 x 1/8" spruce reinforcing strips alongside the inner edges of the new big hole in the frame.

Please don't get suckered into using the wing as a battery hatch. That's so last century ( I love that expression! ). Plus, imagine if you had to get at the pack quickly ...

There might be a way you can use the 51's fuselage shape just aft of the wing to build in an unobtrusive cooling outlet. WHatever, inside the fuselage, build in a sloping light balsa ramp to persuade hot internal air to leave and external air to stay out there - don't just assume a hole in the bottom will do the job, it won't.

3- Engine mount? How do I install the outrunner? Should I start with the glow engine mount and use this as a starting point for the motor mount or should I build one from scatch or should I buy a stick mount?

Motor mounting - lay your motor on your plan with your spinner putting the prop in the right place, see how much gap there is between the back of the motor and the kit firewall. Hobby Lobby, IIRC, sell tubular stand-off mountings that might be the easiest route, otherwise make up a Birch ply stand-off wooden mounting box to suit. The glow kit mount is unlikely to be any use with an outrunner. Try these:
http://www.hobby-lobby.com/motor-mount.htm

Any other tricks I should look for?

The instructions / plans from Great planes are pretty good and building the rudder and wings have been pretty good. I am just not looking forward for the covering . Just covering my foam floats for the Taylorcraft was a struggle...

Covering is one of those things that ain't quick, despite what MoneyKote International tells you. You have to be able to sit down with little distraction and work at it steadily. There's a couple of good books around on how to 'film cover too - Flying Models has a great selection of books and should sell some on covering. Maybe there's some websites on the subject too, but make sure you feel happy with the author first.

GP do good aerodynamics, their kit engineering is good, so they build pretty well. The instructions are worth following, once you get around the legal cop-out pages (best end of a hobby knife to hold, etc!) Shame no-one at GP understands weight, their models do end up a little chunky - I'd buy a spare cowl and canopy, use the kit as a set of templates, treat the plans kindly and ThiefBay the kit when I'd finished.

Look up a 'real' P51, the restored ones are usually spotless and gleaming, which suits a "Monokote scale" model finish, and try to replicate a 'real one' - makes your effort truly yours if the finished model doesn't look just like the kit box lid. Hey, winter's here, not like you've got a lot of flying to do right now?

Good luck, take some photos (and if you'd like to see yourself in a real International model aircraft mag, I have a contact there too )

Regards
Dereck
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Old 11-25-2008, 06:33 PM
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beavenx5
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Thanks Dereck for taking the time to answer all the questions in detail. You brought up a few good points I had not though of like proper venting of the battery compartment...

I am glad that my motor will fit the project good and it looks like the motor mount will be the easy part with nylon tubes and spacer.

From what I read in your post, it sounds like the P-51 has a long heavy tail and CG might be the challenge here with this kit conversion. I don't understand why Great Planes sells this kit as a "convertible" model without giving you more instructions on how to do it.

Thinking out lound here: my outrunner weight 275g, almost half of an OS46 glow engine (490g) and my larger lipo 5S 4900mAh weight 625g compared with a 10oz fuel tank at roughly 350g. Total weight is comparable but I will have to mount the battery a little forward of where the fuel tank is supposed to go to get the same CG. I can also mount the 2 tail servos a few inches more toward the nose.

I will post pics of the project as it comes along.
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Old 12-11-2008, 02:09 AM
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Hey beavenx5, sounds like you have one heck of a project going. I know the last post has a few weeks on it (catching up after work distracted me, what was I thinking) but I thought what I learned converting a House of Balsa Super Decathlon (yes, the build thread STILL isn't finished) might help. My Decathlon wound up tail heavy mostly due to the length of the battery I used (GP 3S 2100mah). I placed it just behind the firewall/eng mount area. It extended past where the tank would normally be. I was forced to add three oz.s of lead to the nose It really hurts because I had it easily under the recomended AUW. I could have bought another two batteries that were shorter and then wired them together, but didn't want to spend the money. At any rate, thats my two cents. Good luck and share some pictures of your project!
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Old 12-11-2008, 02:55 AM
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Okay, in today's (rapidly failing!) consumer driven society, this might seem odd, but there's a cheap trick that can take care of a tail-heavy model, though it doesn't do a lot for a plain old heavy one.

NiMh batteries!

My Four Star 40 flew great on a set of 16 Sanyo 3000 NiMh, though the battery did weigh some 34 ounces - a lot more than the 4S 3700 lipo I have flown it on. But no-one ever suggested to me that it was boring to watch

APart from nailing a tail-heavy model - and some are so bad here regardless of how much you alter the structure - they have other attributes worth bearing in mind.

One - you can have any shape battery you need. Soldering up packs is not hard as long as you have a hefty soldering iron. I did my packs with an 80W Weller, both end to end and tabbed. E-t-E is not for the faint hearted though...

Two - they're safe enough to recharge inside the model. I soon learned that was good with my GP CAP 232 - the layout defied even me coming up with a long enough hatch to get its flight pack out, so I quit on that line and re-charged one pack inside the model.

To some extent, A123 offer many of those qualities. it's also getting harder to find good, 45A capable sub C sized cells - Sanyo appear to have disappeared.

Another idea I saw on a 65" Hawker Tempest (or was it a Typhoon?) lately published in a UK mag. That type has a real short nose for an inline engined aircraft, so what the designer did was mount two 4S Lipos, connected as 4S 2P, vertically on the front face of the firewall, either side of the motor mount. Can't get weight much further forwards than that! How he got to them to charge, I leave to the imagination!

But do something like that, and you have the interior free to move the fuselage servos forwards of the CG - every little helps.

Hope that helps

Dereck
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Old 12-11-2008, 03:50 AM
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beavenx5
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My building is going well. I am only a few hours away from my first CG tests. The fuse is now fisnished and I now need to assemble it together and finish building the scoop under the wing.
I ended up building a battery hatch in the nose from F-2T to and including the cockpit. This should allow me to install my huge 5S 4900mAh lipo all to way against the firewall. I also plan on installing the servos as much in front as possible (almost at the CG point) and install 2 ailerons servos (instead of one for use of flaperons) inside the wing close to the leading edge. This is at least 2 inch forward of the intended location.
I hope these small modifications will help me attain the desired CG without adding lead to the nose.
My second option would be to use my 2 5S A123 batteries in parallel instead of the lipo to add some usefull weight in the front. My A123 batteries are build in a compact triangle form that will fit perpenticular in the fuselage against the firewall.
Even if I am only a few hours away, I don't think I will have time to get there in the project before my vacation next week and then the Christmass Holiday. To be continued in January....
Thanks for the help. I will keep you posted.
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Old 12-12-2008, 04:26 AM
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I just did my first unscientific center of gravity test: So far the CG is about two inches forward of what it should be. Looking good...


I am still missing the following items on the plane:
  • Plastic halft cowl
  • Prop and spinner
  • cockpit and pilot
  • 4 servos (to be installed closed to the CG anyway)
  • a few bits and pieces to hook-up the stabilisator, rudder and elevator
  • small section of the scoop under the wing.
  • Covering
The main parts were already in the fuselage where they belong: motor, ESC, UBEC, battery

I truly beleive I will be able to get the CG right without having to add any lead or heavier A123 batteries in parallel (one 5S A123 2600mAh or 5S 4900mAh lipo should be enough weight).

Look at my large battery hatch: should work well also

I am quite happy with the result so far....
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Old 12-12-2008, 03:31 PM
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Sounds like you're doing good so far - but don't underestimate the power of covering material and a few little bits and pieces down the tail end to shift the CG.

There's another unwritten rule of aeromodelling that goes something like:
'The moment you mention 'Scale', it all gets twice as hard and takes four times as long'

Regards

D
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Old 08-27-2009, 02:40 AM
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Beaven,

Did you ever manage to finish your P51. I would like to see how it all turned out. I was thinking of converting the same plane to electric. I built the nitro version and it flies so well that I bought a second kit to build as electric. I couldn't find any threads on the conversion so I posted one up on RCGroups. Turfbuster was good enough to let me know about this one.
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Old 08-27-2009, 02:56 AM
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Hi stinker,
Unfortunately, I have been extremey busy at work and I keep most of my free time for family activities. Unfortunately, my P-51 build has not moved forward since last December. I have less than 10 hours of flight time since May on my other planes...

Most of the electric conversion is done but I don't know yet how it performes. Let me know if you have any questions or would like more pictures.
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