schafe3624

I have been working on an electric C-130 for 2+ years now. I've been afraid it was too heavy. I am at the point that it is ready to test fly. The calculators on the internet say it will barely fly. I'm hoping to get an expert answer.

Here is what I know:
- Total weight with batteries is 12lb 8oz.
- Wing span is 67"
- Wing area is about 400 sq in
- I am using Turnigy 1460KV outrunner motors. Master Airscrew 3 bladed 8X6 props. (8" is the maximum diameter I can use.) 3 cell lipos. This combination produces 380 - 400 peak watts.

I cannot change prop diameter. Turnigy offers a 1250KV motor rated for 4 cells. 1460 X 11.1 = 16206 RPM. 1250 X 14.8 = 18500 RPM. I can do this by making the nacelles longer. It will be heavier with the larger motors, batteries and ESCs.

I might be able to switch to inrunner motors. They are about 3 times heavier than my current motors but produce way more RPM. I think there is a point where a prop goes so fast it starts producing less thrust per RPM or something like that.

At this point any suggestions are welcome.

radfordc

My Feedback: (14)

As a comparison, a nice 90 size warbird will fly well at 12 lbs with about 700-800 sq in of wing. You're plane is significantly heavier and will need lots of power and will have to fly very fast.

The rule of thumb for fast electric planes is about 100-150 watts/lb. You will need at least 1200 watts and better with 1500.

Suggestion? Take lots of pictures before the first flight.

Jennifer Curtis

If each motor is runnnig 400 watts (maybe they
are SK3-3530-1460) you should have enough
power with four to fly the plane. The issue
will be wing area. If you have 400 square inches
on each wing panel for a total of 800 square
inches, your plane will have a wing loading of
36 oz per square foot. This will be fast but
flyable.

If the total for both wings is 400 square inches,
you are in trouble. The plane may fly, but
it will be so fast that you will need to take off
of pavement, and will need a very long runway
to get up speed and slow down when landing.
You will need good eyes too, because it will
get out of sight really fast.

The full size c-130 has a long thin wing, aspect
ratio about 10:1, so my guess is that your total
area is 400 square inches. You need to put the
plane on a diet.

Use smaller batteries. If you have four large
batteries like 3200 mAh, replace them with smaller
ones like 1000mAh.

If you have cargo doors, remove the servos and tape
them shut.

If you have retracts, remove them. Use foam
wheels, only three.


Use thin electric props from APC. You can
use 8" two blade with a higher pitch. You will
need the higher pitch to get the speed it will
take to fly.

I have a hard time imagining 12.5 pounds
in the small space of a 67 inch plane, unless
it is made of heavy wood, and heavily painted
and detailed, and has a lot of lead in the nose.

I have a 65 inch plane that weighs 4 pounds.
If I quadrupled the entire power system, it would
weigh about 12 pounds, but it would probably
have several times as much power as it should.
Perhaps you should weigh the plane without
any of the power system (or just subtract the
known weights from the total) and re-design
it using smaller components.

Maybe you should check your scale before
doing any of the things mentioned above.

Jenny

aeajr

My Feedback: (2)

OK. let's analyze what you have.

C130
http://en.wikipedia.org/wiki/Lockheed_C-130_Hercules

So I presume you have 4 engines on your model, each of about 400 watts. Did you test them with a wattmeter or are you working from some spec sheet as to what you think the motors are producing? You MUST test them.

Assuming you can hit the 400 watts/motor that would be 1600 watts.

Plane weighs 12.5 pounds so you are running at about 128 watts/pound. You have PLENTY of power if your motors are pulling 400 watts.


As to the wing. Weight is not the issue, it is wing loading.

You say your plane is 12.5 pounds or 200 ounces.

400 square inches = 2.77 sq ft.

So your wing loading is 200/2.77 = 72 oz/sq ft. Now, I don't fly these types of planes but that seems VERY high wing loading to me. You are going to have to fly this plane pretty fast to generate enough lift to support that wing loading.

I looked around to see if I could find a similar aircraft to yours.

http://www.kitcutters.com/Pal/PalC130Herky.htm Their C130 has about 750 sq inches of wing and comes in at 11 pounds, or less than half the wing loading of your plane. around 33 oz/sq ft.

So, yes, yours is HEAVY.

OldScaleGuy

My Feedback: (2)

What no one has mentioned is the multiplying factor by using multiple engines, or motors in this case. I have flown many twins and four engine bombers over the years. I don't know that there is scientific data to back this up but just the same as you increase the number of power plants you multiply the power by more than 1 to 1. In other words a twin engine plane flies as if it has the power of three engines. A four engine plane will have the power of approx. 6 engines. Any one else with multi-engine experience will understand this. As far as the wing loading goes, if it flies well who cares what the numbers yield. Make sure you have plenty of ground speed to lift off, a take off run does not mean it has to go airborne. Hard pavement will also help in building up ground speed. On final, keep the flying speed up at least until you know your plane and how it will react at slower flying speeds which you can determine at higher altitudes. Good luck, let us know how it goes for you.

radfordc

My Feedback: (14)

I don't think lack of power will be the issue for this plane. Maybe this short video will give us an idea:

https://youtu.be/y57VHMX39n8

<iframe width="560" height="315" src="https://www.youtube.com/embed/y57VHMX39n8" frameborder="0" allowfullscreen></iframe>

radfordc

My Feedback: (14)

Here is an article on "wing cubic loading" and why it is a good indicator of flyability. http://www.theampeer.org/M1-outrunne...unners.htm#CWL

Here is a table of "normal" WCL's for some types of models:
Type of Aircraft Wing Cube Loading Factors
Gliders 4
Trainers 6
Sport Aerobatic 9
Pattern 11
Racers 12
Scale 10-15

WCL is calculated by dividing the weight in oz by the wing area in cu. ft raised to the 1/5 power: For this C-130 it looks like this: WCL factor = 200 oz. / ((400 sq.in. / 144 sq.in.)^1.5)

Reducing the wing area side of the equation yields this: 400/144 = 2.78 sq. ft. 2.78 raised to the 1.5 power = 4.64

Dividing 4.64 in 200 oz gives a WCL of 43. As shown above this is about 3 times heavier than you would want for a heavy scale model.

aeajr

My Feedback: (2)

The question that started this thread was a request for an opinion about the weight of the plane. We have been expressing our opinion. No one will know how it flies but indications are, based on wing loading and WCL is that this plane is going to be very heavy for the size of the wing and will not likely fly anything like the full scale plane or the typical RC model plane. It will probably get off the ground but only at a fairly high speed. Landing will have to be very hot.

Probably best to find a way to lighten the plane. It must be made out of maple.

schafe3624

Thanks for all the feedback. Here is where I went wrong. I want a fully scale plane. I started with a 3d file of a c-130 that is intended for use in a video game. From there I modified the plane in cad designing the ribs, landing gear mounts, nacelles and such. I then printed it on a 3d printer. I realized early on that balsa is way lighter than plastic even when I printed the thinnest layers possible. Balsa is also way stronger than the thinnest layers of printed plastic. My printer prints a .4mm string. 1/16 inch balsa actually measures 1.9mm. Even though the balsa is almost 5x thicker it is still lighter.

I changed my my original plan replacing much of the plastic with balsa. I stayed with plastic for the detailed areas like nacelles, main lg housing, and intersections of the elevator and rudder. I also stayed with plastic for the front and main lg mounts. I replaced the plastic nose with a vacuformed nose.

The wing and control surfaces are almost all balsa. The fuse is plastic and balsa. Before I ruin the wing with a test flight, I think I better rebuild the fuse. I will print the bulkheads and then trace them onto light ply and cut them out of that. I will try to vacuform other parts. I will also redesign the lg mounts to eliminate a lot of plastic. Check back in a year or so to see how much weight I can eliminate.

I will post some pictures. I think it looks great.

schafe3624

You can see all the plastic in the last photo. The wing is balsa except for the nacelles and the center section.

aeajr

My Feedback: (2)

Are you sure you gave us the correct total wing area? If you gave us the area of one wing half rather than the entire wing that would cut the wing loading in half and now you have an aircraft within an acceptable range. Please confirm you provided the total wing area.

littlera

I build a 120 size Sportster that came in at about 13 pounds. I was disappointed in this high weight, which gave a wing loading 1/2 of what you are looking at. I have to take off and land at high speed - I calculated a stall speed of 27 mph. It is a hand full. Flies fine at altitude.

Your wing will have a higher lift coefficient than my symmetrical wing, but you will still be looking at a stall speed of at LEAST 35 mph. I think it will be un-flyable in practical terms.

radfordc

My Feedback: (14)

I think he is close. 67" span with tapered 10:1 aspect ratio gives about a 6" chord. 67 x 6 = 402 sq. in.

schafe3624

I am very sure about wing area.

Jennifer Curtis

How much does it weigh without the batteries, speed controls,
motors and wheels. I'm thinking that rather than scrap the
fuselage, you can take all the heavy junk out and make a
slope soarer out of it. Are there any slopes in Iowa?

Jenny

schafe3624

not really

schafe3624

I guess the next question is - how much weight should i get rid of? The short answer is "as much as possible." What is the better answer?

aeajr

My Feedback: (2)

I would suggest you try to shed at least 1/3 of the weight and, if possible nearly half the weight. What did you do to make it so heavy?

The alternative is to to increase the wing area, a lot. I am baffled how you got so out of balance between weight and wing area. What did you use to build this?



Again I refer you to a working model of the same design. Their wing has almost twice the area of yours and their model is 1.5 pounds lighter.


http://www.kitcutters.com/Pal/PalC130Herky.htm

Their C130 has about 750 sq inches of wing and comes in at 11 pounds, or less than half the wing loading of your plane. around 33 oz/sq ft.


Are you sure you gave us the total wing area and not the wing are of just one wing half?

Quorneng

schafe3624
You have obviously put a lot of work into this.
This does pose the question do you have the experience in flying RC to handle a multi engined a scale plane?

You ask how much weight to get rid of? But to get a confident answer you need to know how much power you really have by using a Watt meter. The Turnigy 3530 is a good motor but there is no way of knowing if your particular prop is absorbing 400W unless you measure it.
Once you know the actual power available (and the current drawn doing it) you can work out what capacity (and thus weight) battery is appropriate.
The motor and battery then represent the majority of the 'fixed' weight that you have to live with.

You then decide what wing loading is sensible. The advice so far suggests for the size you really need to halve your current figure. Your target then is to get the airframe weight down so when it is added to the weight of the 'electrics' the total is not too far off the 6lb mark!

Jennifer Curtis

Six pounds should be the max for the
wing area you have.

Jenny

schafe3624

I did measure the watts with a watt meter - 380 to 400. I don't remember the current draw at full throttle. Since the wing is complete I am keeping that as is with the exception of grinding out some plastic from the center. I will redo the fuse using mostly balsa with vacuformed pieces for the detail areas.

I guess where I went wrong was expecting to be able to use so many pieces that came straight from my 3d printer. I also made the lg sections bigger than they likely needed to be with denser infill settings on the printer.

On to version 3 of my build - less plastic more balsa.

aeajr

My Feedback: (2)

We tend to overbuild or "beef up" and end up adding a lot of unnecessary weight.

schafe3624

What I really wanted was an arf c-130. There does not seem to be one available. I had seen the kits suggested by aeajr. I should have bought one of those kits. I am over $1200 so far on this project not including the 3d printer or the filament I used. My wife does not know this fact. The only thing she knows is that it has been over 2 years and I still have nothing to fly.

aeajr

My Feedback: (2)

C130 ARFs

http://www.astramodel.cz/cz/katalog/...rf-p10133.html

http://rcextreme-hobby.com/c130info.html

http://www.leetshop.com/Product/GWS-...S-EDP300-Q.htm