Great Planes F-15 Conversion - RC Groups
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Nov 14, 2006, 12:20 PM
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Great Planes F-15 Conversion


I want to get some input from you conversion gurus. I've got an older Great Planes F-15 kit that I got from the kids a while ago and they're really bugging me to build it this winter.

Now that I'm into e-power, I was wondering if this plane would make a good conversion.

Model specs are:
Wingspan: 47 in.
Wing area: 615 sq. in
Weight: 6-6.5 lbs
Engine: 46 2-stroke

After looking through the manual and the plans, there are areas that could be lightened without a lot of work. The only area that really concerns me, is the nose. It's made of blocks of balsa carved to shape. There aren't any provisions for cooling and the front end is pretty tight.

So the the question becomes, what would be a good motor and battery combination? Like everyone else, I'm looking for a good turn of speed and would like a minimum of 700 watts of power, its gotta fly like a jet! Or is this conversion not worth while?

Any thoughts?? TIA!

Cheers!
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Nov 14, 2006, 03:03 PM
Recruiter - 60 Amp Club
Ed Lyerly's Avatar
imodel,
Have a good read through this thread.
https://www.rcgroups.com/forums/showthread.php?t=481567
You can do must anything electric these days, however, by giving up the ability to turn a large prop, you are really giving up the biggest advantage electric power has to offer .... the ability to turn a more efficient (larger) prop. You will also need 1200 or so watts for it to fly like a glow powered prop jet.
So, IMHO ..... this is not a really good subject for E power.
Don't get me wrong, it can be done ..... but read through Brad's thread and see how he has had to persevere. Most people are not willing to go to these lenghts.
Ed
Nov 14, 2006, 03:38 PM
Registered User
Ed, thanks for the link. I subscribed to that thread a while ago and have been following his trials and tribulations. That was one reason I posted the question. I think you're right about the amount of watts needed to get the performance I desire. I was thinking that it's a smaller airframe, therefore maybe smaller problems. Thanks for your input!

Cheers!
Nov 14, 2006, 05:39 PM
Registered User
I encountered the same problem on the nose of my Great Planes Learjet 40. I am actually working on the nose now. I am replacing the plastic cowl with wood blocks for durability and look. In order to cool the motor, I am making a scoop on the bottom out of clear plastic. It should be barely noticible in flight and you don't really need a bit one at the speed these planes fly. The air goes in the scoop and into the fuselage too cool the batteries. It will exit on the bottom of the fuselage.

GP kits are heavy. But if you replace some of the sheeting with contest balsa you will get the plane out to a reasonable weight.

I think you would have a fun project on your hands!

For power I am using a Neu Motors Ork 1910-2Y on 8s - about 1200 what at 40 amps.

Cheers,
Dave
Nov 14, 2006, 06:25 PM
Registered User
Interesting thoughts Dave. What size prop will you be using? Both models are about the same, though the Lear is sleeker.
Nov 14, 2006, 11:05 PM
Registered User
Sorry, I made a mistake, I meant 9S

10*9 APC, 1210 watts in, 120 mph pitch speed, 175 oz thrust 14100 rpm

On 8S,

11*9 APC, 1150 watts in, 103 mph pitch speed, 173 oz thrust 12170 rpm

10S is also possible but I am worried about weight. The Learjet is heavier than the F15 with all those plastic parts and also less wing area. But the glow guys are flying it at 8+ pounds. I replaced a lot of wood with contest and used Hitec 225MG servos for the elev/rudder. Old Rohm air retracts. I am hoping to have the full airframe framed up by next weekend so I will be able to get a better weight estimate by then.

There are probably some better motor choices in the Neu Motor ORK line. They are fairly reasonable priced though. You can even get some with internal fans which will also help with the cooling problem.

There isn't much propjet electrics I've seen except for the F16 Ed pointed out. Its definately doable though!

Good Luck,
Dave
Oct 19, 2007, 12:25 AM
AND FOR MY NEXT TRICK....!
jodini's Avatar
Well, I think it's time to try to convert this model!

I bought mine from a guy for cheap so I'm not emotionally attached to this model, therefore I'm willing to take the chance.

Here's where I'm at....I have a E-flite 46 installed. 4 Cell Li-po installed. Prop is a 13x8.

I'm not much into measuring KV's, watts, and all that other tech stuff, but this set-up has worked great in my Something Extra weighing 4.5 pounds. This plane weighs about 6.5 pounds but I'm not expecting to hang on the prop...HA!

I've heard many times that Great Planes build heavy and this is true on this plane too. Lots of thick balsa everywhere! But we'll see how she flies first and then go from there.

Wish me luck!
Oct 19, 2007, 02:12 PM
AND FOR MY NEXT TRICK....!
jodini's Avatar

Let the project begin!


Well, off to the dremel I went and started to cut away. Had to cut the nose off...painful! But if this thing flies half way descent it's going to be recover it anyway. I love scale planes but I can't handel flying a grey plane in the grey skies!

Anyway, weight......I'm a little over 6.5 pounds...everything balanced great with no weight added (thank goodness). I could put lighter wheels on, lighter servos, get rid of a bunch of wires are just start cutting holes in this fuse. This thing is THICK all over the place. I wouldn't mind just resanding the thing I bet I would loose a few oz! I do have two servos on the rudder which is huge over kill so I might start there.

Very little prop clearance! About 1/2 inch!

Any suggestions would be great. How about the guys that have flown this with a nitro....how do you guys feel about the power of a .46 on this bird?
Last edited by jodini; Oct 23, 2007 at 10:52 AM.
Oct 19, 2007, 03:41 PM
AND FOR MY NEXT TRICK....!
jodini's Avatar
A friend of mine in "the know" gave me a few suggestions. Since I'm redoing the nose anyway, we moved the mount up 1/2 inch which gives me plenty of clearance with my 13x8 prop.

Also he suggested that I did my scoop out the back and put a few small holes in the firewall, avoiding the drag.

And lastly, we decided not to lighten up the model till the test flight. With the motor on, it really feels like plenty of power. Won't be a screamer but that's alright for now.
Last edited by jodini; Oct 20, 2007 at 12:47 AM.
Oct 20, 2007, 08:47 AM
AND FOR MY NEXT TRICK....!
jodini's Avatar

Nose job!


Well not sure how I'm going to cool of the motor, but here's where I'm at. Any suggestions?
Oct 20, 2007, 10:10 AM
115 inches of 'hog
ScaleBrad's Avatar
You can go to a 5S with the power 46, maybe a 12x10 prop to gain some speed and ground clearance
Oct 23, 2007, 12:28 PM
Hooper, full throttle!
Tommy D's Avatar
Power 46 on 5S and a 12x8E Prop below:



Tommy D

http://tommydsblog.blogspot.com/
Oct 23, 2007, 10:49 PM
AND FOR MY NEXT TRICK....!
jodini's Avatar
So is that a good thing?????

Like I said Tommy D, I'm not much into the KV thing. Right now I'm using the 4 cell back with 4000 mah. The 5 cell will increase the weight so I'm wondering if it's worth it?

Thanks for your help.
Joe
Oct 24, 2007, 12:15 AM
Hooper, full throttle!
Tommy D's Avatar
Joe

The 12 x 8 on 4S gave me a tested 700W for a cost of 45A. Thats a HUGE difference in power.

It sounds like right now your going to come in on or around 100W per lb. Before you risk this model i stronglu suggest you put a meter on it to register Amp draw and watt output along with the battery voltage sag. Also a tach would not hurt either.

I'm sure it will fly just fine on 100W per lb but it sure will not be fast like I imagine most prop jets <wet power> to be.

Best Regards

Tommy D
Oct 24, 2007, 01:30 AM
Done it all
jodini,

I wasn't going to butt in, but you said something that attracted my attention. You said that you're not into the kv thing. I said the same, but, I did change my mind about it and now use it. This is why:

Well, this is the conversion forum, we convert nitro or gas planes to electric. So, I was thinking that all we had to do was make the same prop that sits on a nitro motor, spin the same RPM on an electric motor, and whammo, you got an electric plane that flies the same as nitro. Man this was easy, so why make a fuss about meters, kv, amps, watts. This stuff is a bunch of BS for the nerdy guys to just talk about. Jodini, I was wrong, very wrong, electric is more complecated and it does require some thinking, testing, and measureing.

The object is to get the plane to fly the same or better than nitro or gas, but, the method is different. In some setups electric requires larger props and lower RPMS. To do this right, you need to know kv of the motor and how to measure the output of different combinations, amps, watts, micro amp hours.

Yep you could shortcut the measurements and testing, and you can get a small electric motor to spin and nitro prop the same RPM as the nitro motor, however, it may only do it for 30 seconds, then burn up.

I had to give in, just like you will have to give in. Motor size (kv), watt usage, amp demand, micro amp duration, all important to make the darn electic conversion emulate the performance of nitro. Any ya know what, sometimes it just aint possible to do it.

Al


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