Spar Strength on a 108" C-130
 

I am building a 108" C-130 from scratch. This bird will have retracts, brakes, fowler flaps, functioning ramp/door, cargo rails, and will be used to airdrop pallets, Jeeps, ect. I am quite a ways into the build and have several questions.
1. This aircraft will have a modified flat bottom airfoil, a wingspan of 108", a chord of 12.5" and about 2.5" of camber. I don't want to overbuild this thing but am worried about the overall strength as I have never done a complete scratch build. It goes without saying but i have no intentions of pulling high g`s in this bird.

My initial thought is to make a 1 piece detachable wing using 2 parallel boxed spars, one 3" aft and one 9" aft. This is very similar to the actual aircraft except it uses I-beam spars and the outer wings detach outboard of the #2 and 3 engines. I would love to set up mine to detach the same way but don`t want to create a weak point or add the excess weigh of aluminum tubes or carbon rods. I was planning to use 3/8x1/4 or 1/2x1/4 hardwood for the upper and lower parts of the spar and 1/8 balsa for sheer web. I am hoping that someone with a little more experience could give me some insight, i would estimate that the fuselage will weigh between 8 and 10 lbs. fully sheeted with radio and servos. all of the fuel/batteries will be in the wing aft of each engine or in the aft nacelle, this should help with wing flex a bit. This brings me to my second question

2. Power plant: First of all i need to decide how much thrust i will need, I planed to use 4x .25 or .35 glow motors, however i am terrified of syncing and keeping 4 glow motors running. I also thought of using 4 electrics for reliability but have ZERO experience with them. Thoughts?

I took a picture this morning while cleaning the garage to show where i am athttp://www.rcuniverse.com/forum/atta...mentid=1924647

[QUOTE=jordanorville;11623756]I am building a 108" C-130 from scratch. This bird will have retracts, brakes, fowler flaps, functioning ramp/door, cargo rails, and will be used to airdrop pallets, Jeeps, ect. I am quite a ways into the build and have several questions.
1. This aircraft will have a modified flat bottom airfoil, a wingspan of 108", a chord of 12.5" and about 2.5" of camber. I don't want to overbuild this thing but am worried about the overall strength as I have never done a complete scratch build. It goes without saying but i have no intentions of pulling high g`s in this bird.

My initial thought is to make a 1 piece detachable wing using 2 parallel boxed spars, one 3" aft and one 9" aft. This is very similar to the actual aircraft except it uses I-beam spars and the outer wings detach outboard of the #2 and 3 engines. I would love to set up mine to detach the same way but don`t want to create a weak point or add the excess weigh of aluminum tubes or carbon rods. I was planning to use 3/8x1/4 or 1/2x1/4 hardwood for the upper and lower parts of the spar and 1/8 balsa for sheer web. I am hoping that someone with a little more experience could give me some insight, i would estimate that the fuselage will weigh between 8 and 10 lbs. fully sheeted with radio and servos. all of the fuel/batteries will be in the wing aft of each engine or in the aft nacelle, this should help with wing flex a bit. This brings me to my second question

2. Power plant: First of all i need to decide how much thrust i will need, I planed to use 4x .25 or .35 glow motors, however i am terrified of syncing and keeping 4 glow motors running. I also thought of using 4 electrics for reliability but have ZERO experience with them. Thoughts?

How about this, use 1/2" x 1/4" strait grain spruce upper and lower spar caps, but span wise taper them from 1/2' thick through the center section to 1/8" thick at the tips. This would allow you have the strength you need for flight and allow you to lose weight as you move towards the tips and the load diminishes. I would also glue in 1/4" x 1/8" hardwood verticals throughout it's length and then sheet both sides of the spar with medium density 1/16" thick balsa sheeting as webs.

Bob

That sounds like it would work well bob, my biggest fear is to not build the wing strong enough. It is good to know that I would have over built rather than under built.

What is your estimated worst case all up weight going to be?

Bob

i dont think it will weigh more that 15lbs but have not weighed it. i can weigh what i have when i get home tonight or in the morning.

If your airplane comes in that light then you have nothing to worry about with a spar as I stated, that is not very much weight for something that large. keep in mind that the more weight you add to your airframe, the more weight you will need to add in structure to support itself. I think your on the right track with what you have done with everything so far.

Bob

back on track

He didn't say 10 lbs. He said 15 lbs. and believe it or not, one can build an airframe lighter from balsa, and plywood then from all composites. just saying...

Bob

still the same

Foam core wings and balsa sheeting done right is another very strong solution with low weight benefits.

Bob

humm :p

Just food for thought...

Bob

:cool:

just not worth ;)

:cool:

ya back on track....

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:cool:

I love the double deer eating popcorn. But my all time favorite is the Panda.

Bob's spar will work fine in this model.

For the pull apart feature of the wing, no need for "I" beam subspars. Let me suggest carbon tubes. The carbon tubes I use are 7/8" OD x 1/32" wall x 30" long, for a 2 meter wing (78"). Very light and strong, even for a 6HP powerplant and 16 lb weight.

For this project, a pair of 5/8"-3/4" OD tubes x 1/32" wall x 24" long is plenty of strength but assumes the pulling apart happens in the middle of the 108" span. The tube sockets will need to be built inside Bob's main spar and inside the shorter subspar at the 9" station of the chord.

Fifteen pounds is achievable but I'd hazard a guess that the OP may not have the experience to build light and strong. Some of the suggestions we are making should help in the light and strong area (I've been at this for 45 years myself just like Bob). But even at 18 lbs, if the wing area is around 1500 squares, this plane will fly superbly.

To the OP, let me suggest that you add some more carbon tubing longerons on the fuse, all around. 1/4" x .020" wall will give great strength and should help reduce the balsa longeron breaks when you handle the model. With added longerons, 1/16" sheeting is all you need. And if you plan to tissue and nitrate dope the covering and paint for finish, even better....

On my 103" C-130 I used two 1/4 X 1/4 hardwood spars on the top and two on the bottom. They extend out just past the outboard engines.
http://www.rcuniverse.com/forum/atta...mentid=1928436

Now this is good information!

Bob

I can't resist ... How much does it weigh?

Great looking ship Aaron!

Look at a realistic all up weight.
Remember that the Wings in flight must support this times the number of "G"s contemplated.
We tend to fly RC planes aggressively, compared to full size.
Is 10G's reasonable?

The stupidest fight ever. No one has any realistic info. It's like my sisters fighting, when they were 12.

Girls, you are both pretty