RE: GIANT SPAD DESIGN! (update w/ pix)  
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RE: GIANT SPAD DESIGN! (update w/ pix) - 11/23/2006 4:56:22 PM   
Villa



Posts: 872
Joined: 8/12/2003
From: Wilson, NC,
Status: online 		Thanks for the correction ChrisSpad. It is SPADTOTHEBONE. To Pilots12: I agree with what ChrisSpad mentioned about the spar being too weak. I hesitated to mention it before since I am not an expert and this is all a learning process. By adding another piece at the top of the spar you will just about double the strength of the spar. You spar will then have a crossection of an I-Beam which is a very efficient beam. Now that I have opened my mouth let me go further, but do remember I am not an expert. I do acrobatics with my planes and I try to cause them to fail in the air (away from everyone else). All my current SPADS use a double spar, like the DOMINATOR in SPADTOTHEBONE.COM. You may not want to do acrobatics with your plane, but if you lose control (screw up) you may be doing acrobatics trying to recover. At the ends of your center yardstick you have two short pieces to make a LAP JOINT with the outer yardsticks. You need to make a LAP JOINT also with the bottom and top members of your I-Beam crossection. Otherwise you are wasting the top and bottom pieces at the ends. You are doing a good job. Keep us informed.

(in reply to CTH)
       Post #: 26

RE: GIANT SPAD DESIGN! (update w/ pix) - 11/23/2006 5:40:55 PM   
Pilots12


 

Posts: 86
Joined: 7/6/2006
From: Northglenn, CO, USA
Status: offline 		Thanks for the info guys. Office Depot ran out of year sticks, so thats all I have for now. I'm going to add more to the top of the spar to make it the "I" shape like Villas talking about, and maybe even add anotehr spar about an inch apart.

The two sheets of balsa in the middle are going to be where the fuse sections bolt onto the wing. They tested at over 25lbs and didnt snap so that should hold up just fine for that

(in reply to Villa)
       Post #: 27

RE: GIANT SPAD DESIGN! (update w/ pix) - 11/23/2006 6:27:47 PM   
ChrisSpad



Posts: 1523
Joined: 4/19/2002
From: Cordova, TN,
Status: offline 		the issue is this: when a plane is climbing, it weighs more than it does staticly. When pilots do aerobatics, they reach 7 or 8 g's - fighters sometimes reach 9+G's. What is a G? the force of gravity. Iow, when a pilot pulls a 5 g turn, if he weighs 200 lbs, it feels like he weighs 1000 lbs. 5G is not an unreal number for an r/c airplane - even one that you fly gently. On a plane that size, i'd not use yardsticks at all. They're just not strong enough. On something that size, you really need something heavier - like 1/4"x1 1 1/2" lathing.

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       Post #: 28

RE: GIANT SPAD DESIGN! (update w/ pix) - 11/23/2006 8:10:06 PM   
Pilots12


 

Posts: 86
Joined: 7/6/2006
From: Northglenn, CO, USA
Status: offline 		Yeah I realize what G's are how how many G's you get when flying. But just climbing your not ever going to reach 5 G's... I fly Cessna and with smooth climbs, your still at 1 G. I know that theres more force on the wing when your climbing but its pushing up, not G's. I'm pretty confident that with double yard stick spars, stand up in the I shape, and not pulling aerobatics this wing should hold up just fine. Plus only being 16 and having a crappy job cooking chicken, I dont have much money to put into this. I dont want to spend more than $200 on this and thats already $70 to get the servos needed for this. Its a cheap build just for the fun of it, if it flies, sweet I'm sure all the guys at the field will gather aroung to take a look at it. If the wing snaps, oh ****, I'll learn something new to use on the next project. I'll try to finish the wing up ASAP and post pix of it before i glue it down and then I'll see hwat you guys think of it. But I feel pretty good about it holding up.

(in reply to CTH)
       Post #: 29

RE: GIANT SPAD DESIGN! (update w/ pix) - 11/23/2006 10:02:01 PM   
ChrisSpad



Posts: 1523
Joined: 4/19/2002
From: Cordova, TN,
Status: offline 		just hope that if it does snap, that the 20 lb lawn dart you've just launched isn't aimed at anything or anyone, or 200 bucks will be pocket change compared to the damage you'll be liable for.

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       Post #: 30

RE: GIANT SPAD DESIGN! (update w/ pix) - 11/23/2006 11:48:21 PM   
MrGreenSpeed



Posts: 622
Joined: 12/31/2002
From: Lakewood, NY, USA
Status: offline 		If Your going to take the time to build it, the extra time to build the wing correctly will be worth it and shouldn't add to the cost of the build that much.
Better to be safe than sorry.
Just My .o2
Jeff

(in reply to ChrisSpad)
       Post #: 31

RE: GIANT SPAD DESIGN! (update w/ pix) - 11/24/2006 12:08:09 AM   
Pilots12


 

Posts: 86
Joined: 7/6/2006
From: Northglenn, CO, USA
Status: offline 		What else should I do then to reinforce it other than 2 spars, each in the I shape and ribs?

(in reply to CTH)
       Post #: 32

RE: GIANT SPAD DESIGN! (update w/ pix) - 11/24/2006 12:15:43 AM   
Chunky C


 

Posts: 68
Joined: 12/26/2002
From: Muncie, IN, USA
Status: offline 		What else can you do? For starters, you should stagger the joint so that the the horizontal bits and the vertical bits don't all break at the same place. Second, 1 inch of overlap is way too little.



A heavier spar as chrispad suggested earilier is great advice.

We're not ganging up on ya, we're all just thinking of the people and property that will be around when that wing fails.

cc

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Chunky C

(in reply to CTH)
       Post #: 33

RE: GIANT SPAD DESIGN! (update w/ pix) - 11/24/2006 12:29:00 AM   
CTH


 

Posts: 17
Joined: 7/4/2006
From: , KY, USA
Status: offline 		Pilots12 are you planning on using standard size servos? I'll give ya some if you are. I want to see this plane in the air! PM me with what you need.

(in reply to Chunky C)
       Post #: 34

RE: GIANT SPAD DESIGN! (update w/ pix) - 11/24/2006 1:03:10 AM   
Pilots12


 

Posts: 86
Joined: 7/6/2006
From: Northglenn, CO, USA
Status: offline 		Alright I ll take the short lap joints off and add longer ones, and for the 2nd spar u'll just use two back to back.

CTH, I'm planning on using 7 standard servos.

I've never flown anything of this size before, only 40 and 60 size planes. Would a standard servo be enough for the elevator for something this size and weight?

< Message edited by Pilots12 -- 11/24/2006 1:23:03 AM >

(in reply to CTH)
       Post #: 35

RE: GIANT SPAD DESIGN! (update w/ pix) - 11/24/2006 3:36:11 AM   
batchelc


 

Posts: 176
Joined: 11/8/2004
From: Wichita , KS, USA
Status: offline 		Pilot12:

The spar is no place to save time, money or effort.

I would start by laying the yard sticks horizontal instead of vertical (you may have allready done this). One at the top and one at the bottom. You can add a shear web between them with another yard stick or similar material. Ideally you would find material that has a vertical grain for the shear web. The stresses actually act about 45 degrees to vertical.

You can join the spars at the center several ways. The easiest may be to go and buy some cheap fiberglass resin and fiberglass cloth (~10 for the resin and ~5-10 for the cloth). Unidirectional cloth would be best. Walmart and home depot both sell the bondo brand of resin/cloth. You want the fibers of the cloth running lengthwise on the spar. 3-4 layers should be plenty top and bottom. I would make the cloth 25-30% of the wingspan in length.

If you lay a piece of waxed paper down, put your cloth down and pour your resin down. Then put another piece on top. now spread the resin around to saturate the cloth. You can cut this sandwich with scissors (old but sharp). Peel off one side and stick to your spar. Then peel the other side off. Make sure to work out any air bubbles.

Clean up with acetone. You will want to wear disposable gloves (can be found the in the pharmacy section of some stores).

You can build the whole spar on the building board and then glue it into the wing. Anyone have any cheaper ideas for joining the spars?

I would be happy to answer any questions you have.

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       Post #: 36

RE: GIANT SPAD DESIGN! (update w/ pix) - 11/24/2006 3:54:15 PM   
Villa



Posts: 872
Joined: 8/12/2003
From: Wilson, NC,
Status: online 		Hi Pilot12. I don't fly giant scale so I cannot advice you on whether a standard servo will work on the elevator. I was hoping that someone with giant scale experience would answer you. Since none have, here is some of my experience. Earlier I referd you to my Flying Lawn-mower. It uses a 46FX engine and weighs about 6.5 pounds. The elevator is in two halfs which also serves as ailerons. Is is called an elevon. Each half has a servo and an electronic mixer does the magic (I don't have computer radios). I started with standard servos but they were being overpowered and one failed and I had a crash. I changed to the next size stronger. I think they are Futaba 3010. This is two servos on a 46 size plane weighing only 6.5 pounds. I assume your elevator will be one piece. You are planning 7 servos which sounds like you may be using two servos on your one piece elevator. Remember this: if one of the two servos fails ( I have had many servos fail) the other servo will not be able to overcome the resulting drag/jam and you will be without an elevator.

Pilot12, you are 16 years and I am 70. So what. You have received a lot of negative input to your project. I sure hope you don't get discouraged. I'm a retired design engineer. I love to design my planes, especially on the computer. You seem to enjoy that also. Don't let any of this discourage you. Why don't you build a few more proven designs and get more experience under your belt. The Canard plane you see to the left is one I designed and fly every day. The detailed drawings are on this site at

http://www.rcuniverse.com/forum/m_3580825/anchors_3580825/mpage_1/key_canard/anchor/tm.htm#3580825

I tend to build a little stronger that most. Maybe it is because I have the advantage of over 100 crashes in more that 30 years. Hope to hear more from you.

(in reply to batchelc)
       Post #: 37

RE: GIANT SPAD DESIGN! (update w/ pix) - 11/24/2006 5:00:09 PM   
Omaha_RC_Flyer



Posts: 489
Joined: 3/7/2004
From: Omaha, NE, USA
Status: offline
quote:

ORIGINAL: Pilots12

What else should I do then to reinforce it other than 2 spars, each in the I shape and ribs?


Not to rain on your parade, or discourage your enthusiasm, but I would strongly consider replacing your spar, with one like I built for my Triple DPS. http://www.rcuniverse.com/forum/fb.asp?m=3585156 I used three pieces of 1/4" pine 74" long, epoxied together, to make it. I would hate to see all your work, end up crashing, or hurting someone, due to a spar failure. The yardsticks will not cut it, especially the way you have them pieced together. My Triple DPS is 19.5 lbs., yardsticks for a spar, was completely out of the question, when I designed it.

Later,
Omaha

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(in reply to Pilots12)
       Post #: 38

RE: GIANT SPAD DESIGN! (update w/ pix) - 11/24/2006 5:36:37 PM   
Pilots12


 

Posts: 86
Joined: 7/6/2006
From: Northglenn, CO, USA
Status: offline 		What about two double spars like this from yardsticks....


(in reply to Omaha_RC_Flyer)
       Post #: 39

RE: GIANT SPAD DESIGN! (update w/ pix) - 11/24/2006 6:04:39 PM   
batchelc


 

Posts: 176
Joined: 11/8/2004
From: Wichita , KS, USA
Status: offline 		Your idea of adding a second shear web will add some strength, but not in the area thats needed. See my other drawing. The tension and compression loads are the concern. If you were to do anything you could add more mass on the horizontals, not the vertical.

You should build a test spar for some real-world experience. Just build the 2 piece spar and hang some weights on either side. You can hang the weights 1/2 way out to be conservative. For a 20# model you should build to 1.5 times the design load. So for 3.8 G's (same as your Cessna) that's 5.7g's or 114#. Divide by 2 to get 57# per side. Now make sure that the spar holds that for 3 seconds. You can hang dumb-bell weights or similar. Make sure to hang the same on each side.

Now you can see that this is quite a load for some yard sticks. By contrast a 5# model would only have 28.5#. Divide by 2 for 14.25# per side.

If the wing is going to fail it is likely that your joint at the root will fail.

You may want to look at your local home improvement store and go with metal or good pine/spruce like someone suggested. You may find strip wood in the trim section.

Chris


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(in reply to Pilots12)