Crashj007

I am converting a gas Flyboy C-119 kit to electric. The challenge is the kit was designed for two .20 nitro engines and is pretty heavy. Airframe only weight is six pounds. I am using online software to estimate performance and select motors and batteries and I run into a problem right away. The booms and fuselage take away some of the wing area, so I'm not sure what to use, gross or net area.
Specifically, the wingspan is 59", root chord is 9.5", tip is 5.0", so roughly 428 sq ins gross. The fuse is 5.0" wide, the nacelles are each 3.5", so the actual area nets out to about 340 sq ins.
It makes a lot of difference in the predictions of flight capability, between brick and fast but flyable.
So when you are at the ragged edge, do you figure area that close in selecting motors, props, and batteries.
Any thoughts on this puzzle?

JetPlaneFlyer

Convention is to include wing area that's 'inside' the fuselage and nacelles.

Crashj007

I suspected as much, but I was not sure that the reason is to simplify or is it realistic.

kyleservicetech

With about 400 square inches of wing, or 2.77 square foot wing area, and 6 pounds for the airframe alone, that is going to be a very heavy model. You might be pushing 8 pounds plus for the total weight. That comes out to something like 46 ounces per square foot. That might be OK for a giant scale model, but for a smaller 59 inch wingspan model, flying it is going to be interesting to say the least. And, landing it might have some issues in dropping a wing and ouch.

Nice thing about Electric power, you can select a power system that is capable of getting your model off the ground. IMHO, consider $$$$ motors that have a known quality and will perform to their specifications. These electric power systems, properly setup, can also exceed the performance of a similar sized glow engine.

It's known in www.wattflyer.com that kyleservicetechs motor of choice is the Hacker series. They do perform to specs, and will handle the power levels that are advertised.

Something to watch for on these twin prop setups, often you are limited in prop size due to physical clearance on the model. Electric motors are usually designed to turn big diameter props at comparatively low RPMS, to get maximum thrust for the buck (watts).

The Hacker motors, such as their A40 series have a number of different copper windings for different performance with a given battery and propeller. If you can fit a big enough diameter prop on one, those Hacker A40's can push near 1000 watts each. http://www.aero-model.com/Hacker-Brushless-A40-10S.aspx, or http://www.aero-model.com/Hacker-Brushless-A40-12S.aspx. Those two motors require a prop on the order of 14 inches in diameter to get their full watts rating.

In the real world, those two A40 Hackers will put out about a combined 1400 watts. If your model comes out to 8 pounds, that is some 170 watts per pound, a power level that should get your model off the ground without problems. Flying it, as previously indicated, should be interesting.

FYI, a fellow club member has a giant scale F7F model that he wants to electrify. That thing is going to weigh in at some 35 pounds, or 70 ounces per square foot of wing. I convinced the guy that electric may not be the way to go.

If you've not found it, www.motocalc.com might give some clues on your setup.

Nice thing about electric power, no fuel slopping all over your model, and properly setup and flown, much lower chance of a "Dead Engine" in flight, with the instant result of difficult to fly model.

JetPlaneFlyer

A little of both I think. fuselages etch do contribute a little to lift so including the 'enclosed wing area' within the fuselage makes some allowance for that effect.

Either way your C119 does sound like a 'lead sled'.. did these kits ever actually fly with i.c. power?

Crashj007

Kyleservicetech, I've come to the same conclusion, the model is pretty much unflyable given the weight vs wing area. Thanks for the advice on Hacker motors. I still intend to go through the design effort and put the plane together but not include flight motors, batteries or radio. The maximum prop diameter is 10.5", limited by clearance between the fuselage and the booms. Thanks for the link to Motocalc, I had not found that one yet. I'll check it out.
Jetplaneflyer, I begin to wonder myself if any amount of power would make this plane practical.
I was a loadmaster on C-119s out of Homestead so the model is special to me. I think I'll just set it up for display.

kyleservicetech

Uh oh
Check out the Hacker A40-12S motor on a 10X8 prop with a five cell Lipo on www.motocalc.com. That motor will turn the 10X8 at somewhere around 11,000 RPM, pulling 800 watts each or so. This motor is rated for four to six LiPo cells. Two of those will give you the power, but that wing loading???

BTW, motocalc is free for 30 days, then $39. It will give you a realistic view of your models flight abilities, with stalling speed, flying speed, motor power, RPM, current, volts, and a whole lot of other stuff.

DennyV

Crashj007

Motocalc says this model is a brick. It can't find a suitable battery and motor combination unless I cut the weight to 4 lbs. I can't figure out how to set up my plan for batteries which was to put 4S 2200MAh out in each boom with another in the fuselage with all of them in parallel. Of course, that is 1.7 lbs right there. Add another pound for motors, ESCs and props and the all up weight would be 8.7 pounds, maybe 9.0 lbs with retracts, and such.
Not going to be practical, you think?
Thanks for the help.

kyleservicetech

I agree, not to practical.

What did motocalc indicate the stalling speed would be? If it's much over 30 MPH, that would be a good lead brick.

Crashj007

Over 40 MPH.
"and she rolls and rolls and rrooollls . . . "

kyleservicetech

Wow
40 MPH, that plane would be a good display model only