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-   -   How to calaculate wing tube size needed? (http://www.rcuniverse.com/forum/aerodynamics-76/7897157-how-calaculate-wing-tube-size-needed.html)

rcguy! 08-28-2008 08:34 PM

How to calaculate wing tube size needed?
 
Folks,
I'm designing a 2 piece wing that will use 2 wing tubes to join each panel. The main tube at the high point of the airfoil and a rear tube typical where a rear spar would be located. Anyway, is there a online calculator that would help with the sizing of these? It is going to be a rather large model that will weight @ 35 pounds at takeoff and have @ 1900 square inches of wing area in a 90" wing span. Airfoil will be almost symmetrical at 14% thickness. 27.7" root and 16.4" tip cord. The wing will then bolt to the fuse. I want 15g's minimum load factor or 535 pounds. 20g's would be better at 700 pounds load.

A 2" diameter aluminum tube front and a 1 1/4" tube for the rear one? Remember it will take all the stress right at the centerline.

Anyone out here that can help?
Thank you!

Dave

OffroadBEAR 08-28-2008 09:25 PM

RE: How to calaculate wing tube size needed?
 
First, you need to know the thickness of the wing section at the spar location (chordwise). Your wing section will determine the maximum tube diameter, but the spanwise extension of the tube into the wings will also limit your tube diameter. Easy answer, go with the largest that you can fit in the wing. Your tube will have to be just a little bit aft of the main spar. I usually do some quick hand calcs, I really don't know of any online calculators. There are a lot of parameters that are used to size structural members. I have made a spreadsheet for calculating main spar size and wing tube size for my own design projects if you're interested in using that. I can shot it to you. Let me know.

Paul

rcguy! 08-29-2008 05:05 AM

RE: How to calaculate wing tube size needed?
 
Yes please send it. The wing section is @14% with a 27.7" root cord as stated. That works out to a height of @3 7/8". Top and bottom spars will be 1/4 by 1/2 spruce. Root rib will be 1/4" lite ply with a doubler at the tube locations to distribute the stress. The wing sleeves will fix into this root rib and 2 others that will support the retract units. I plan on filling in with balsa between the spars and the sleeves from the root rib out to the first retract rib (1/4" lite ply) to handle the compression loads. A 2 1/2" tube would be fine. 3" probably too big.
Thanks

Dave

OffroadBEAR 08-29-2008 07:29 AM

RE: How to calaculate wing tube size needed?
 
Okay, I'm at work now, so give me an e-mail address and I will send it when I get home.

So, you're going to have a ply root rib. Are you going to have any more plywood ribs outboard of the wing root? Locate the wing tube just aft of the main spar with a few plywood wing ribs and you will be fine, no need to fill in between the two with balsa. If you're using 1/4" plywood for the high stress wing ribs, adding balsa will be pointless.

If you know your wing rib spacing, you can calculate the approximate reaction force of the wing tube at each rib, then use that to determine the material thickness need to not fail from the bearing stress due to the wing tube.

OffroadBEAR 08-29-2008 06:46 PM

RE: How to calaculate wing tube size needed?
 
You've got an e-mail. I hope that helps you. If you have any questions, don't hesitate to ask. If you have any other airframe questions, let me know.

Paul

rcguy! 08-29-2008 07:09 PM

RE: How to calaculate wing tube size needed?
 

Quote:

ORIGINAL: OffroadBEAR

You've got an e-mail. I hope that helps you. If you have any questions, don't hesitate to ask. If you have any other airframe questions, let me know.

Paul
Paul,
WOW! Thank You! Your email explained it nicely. I MUCH appreciate it! I'll stay in touch as the design develops. Looks like a 2" tube/joiner at .035 wall would be plenty strong enough. I'll also have a smaller diameter rear tube spar/joiner helping out, maybe 1 1/4" .035 wall. Landing loads from less than ideal skill would be distributed well with 2 spar/joiners tubes.
Would you happen to know the Youngs Mod for a carbon tube? :-))

Dave

OffroadBEAR 08-29-2008 08:02 PM

RE: How to calaculate wing tube size needed?
 
Not off the top of my head. The thing with carbon fiber structures is that its mechanical properties depend on thickness, resin type, fiber strength, weave type, and number of layers. I can try to find a strength or yield stress for it, but unless you are either wanting to know the wing maximum deflection or are taking the finite element method approach, it won't do much good. Its in the spreadsheet because I had originally intended to add maximum wingtip deflection to the spreadsheet, but I never got that far.

rcguy! 08-29-2008 08:48 PM

RE: How to calaculate wing tube size needed?
 

Quote:

ORIGINAL: OffroadBEAR

Not off the top of my head. The thing with carbon fiber structures is that its mechanical properties depend on thickness, resin type, fiber strength, weave type, and number of layers. I can try to find a strength or yield stress for it, but unless you are either wanting to know the wing maximum deflection or are taking the finite element method approach, it won't do much good. Its in the spreadsheet because I had originally intended to add maximum wingtip deflection to the spreadsheet, but I never got that far.
Paul,
Don't bother. Seems aluminum tube will be fine. Saving a few ounces at great expense on the project isn't a good return on investment. Now if it turns out to be a lead sled...:-((
Dave

CrateCruncher 08-30-2008 12:02 PM

RE: How to calaculate wing tube size needed?
 
Sounds like you have your answer already but I just want to mention a couple of observations from the details given:
1) 15 g's amounts to a 15 ft radius pullout at constant 56mph [a=(VxV/r)+g] - is that enough?
2) There will be an abrupt change in wing stiffness at the rib bay where the tubes end resulting in stress concentration if not addressed. It can be reduced by reinforcing the spars in the first bay or two outboard of the tubes to smooth the transition in stiffness.

I'm just trying to add value to your conversation guys. I don't even know if this is a trainer or a low wing pattern ship or what power you intend. Those details will make a difference. Crate.

rcguy! 08-30-2008 12:53 PM

RE: How to calaculate wing tube size needed?
 

Quote:

ORIGINAL: CrateCruncher

Sounds like you have your answer already but I just want to mention a couple of observations from the details given:
1) 15 g's amounts to a 15 ft radius pullout at constant 56mph [a=(VxV/r)+g] - is that enough?
2) There will be an abrupt change in wing stiffness at the rib bay where the tubes end resulting in stress concentration if not addressed. It can be reduced by reinforcing the spars in the first bay or two outboard of the tubes to smooth the transition in stiffness.

I'm just trying to add value to your conversation guys. I don't even know if this is a trainer or a low wing pattern ship or what power you intend. Those details will make a difference. Crate.
Yes additional load paths are being considered via spar caps. FWIW... 180MPH top speed, no 3D of course. Desinn at 20's is well covered with the preliminary main spar tube being investigated. I'll post determinations when finalized. Please keep the inputs coming!

Dave

CrateCruncher 08-30-2008 02:29 PM

RE: How to calaculate wing tube size needed?
 
180mph? Now that's interesting! Need some more details about power, airfoil, wing plan etc. If your planning 180mph, 15-20g's definately won't be enough.

rcguy! 08-31-2008 12:09 AM

RE: How to calaculate wing tube size needed?
 

Quote:

ORIGINAL: CrateCruncher

180mph? Now that's interesting! Need some more details about power, airfoil, wing plan etc. If your planning 180mph, 15-20g's definately won't be enough.
Really?? Please elaborate as to why a spar rated at 2 times safety factor using 20 G's isn't enough?

Dave

BMatthews 08-31-2008 02:19 AM

RE: How to calaculate wing tube size needed?
 
That's a pretty high speed and a rather thick airfoil on quite a big wing. Is this intended for a 1/4 scale unlimited racer by any chance? Because that's about the only way you'll see that sort of speed from a model of that sort of size.

I would also suggest that even if the spreadsheet says .035 is enough that you double it at least over the first portion to .063 or so. The reason being that if you can hold the tube in a perfect circle shape it may well hold that much load. But if the bending forces cause a small dimple at a wing joint that dimple can lead to a catastrophic failure at that point. Going for a thicker wall will not only make the tube stronger but will support the tube's shape far better at these shear plane joints.

rcguy! 08-31-2008 07:21 AM

RE: How to calaculate wing tube size needed?
 

Quote:

ORIGINAL: BMatthews

That's a pretty high speed and a rather thick airfoil on quite a big wing. Is this intended for a 1/4 scale unlimited racer by any chance? Because that's about the only way you'll see that sort of speed from a model of that sort of size.

I would also suggest that even if the spreadsheet says .035 is enough that you double it at least over the first portion to .063 or so. The reason being that if you can hold the tube in a perfect circle shape it may well hold that much load. But if the bending forces cause a small dimple at a wing joint that dimple can lead to a catastrophic failure at that point. Going for a thicker wall will not only make the tube stronger but will support the tube's shape far better at these shear plane joints.
Thanks for the input. The calcs say an 2" diameter tube .035" wall at 35# gross takeoff weight at 20's gives a 9.5 safety factor. (below 1.0 will result in failure). An .049" wall will give a 13.0 safety factor. So.....Theoretically....a 2" tube at .049" wall will fail at 260 G's at gross take off weight.
Yes an .049" wall would be best with very little weight penalty and great peace of mind.

For comparison, a BVM KingCat at 35 pounds gross take off weight can do 225 MPH and uses a 1 1/4" diameter by .035 wall aluminum main spar tube.

Dave

OffroadBEAR 08-31-2008 11:44 AM

RE: How to calaculate wing tube size needed?
 
All very good points.

Just for reference, your pullup radius at 20 gs and 180 mph should be in the ballpark of 114 ft. Here is the equation

R= (V^2)/(g*(n-1)) (V is in ft/s, g is the acceleration due to gravity, or 32.2 ft/s^2, n is the load factor, or g loading).

If you have an idea for a pullup radius (i wouldn't know where to start for your purpose), you can back out the load factor. Good luck.

Paul

BMatthews 08-31-2008 02:27 PM

RE: How to calaculate wing tube size needed?
 
As modelers parked on the ground it's all but impossible to "feel" the G loads our models expereince. A few extra grams of material isn't a big deal on a model of that size. The "confirmation" of the tube on the BVM is also nice to see. So I'd still opt for the .049 thickness just because I could.

BruceTharpe 09-01-2008 09:39 AM

RE: How to calaculate wing tube size needed?
 
Hi Paul, I would be interested in that spreadsheet calculator as well. I've always wanted to know the proper size of wing tubes for models, beyond just copying a similar bird.

OffroadBEAR 09-03-2008 08:21 PM

RE: How to calaculate wing tube size needed?
 
Sure thing Mr. Tharpe. I wish I could add files that aren't JPG, TXT, or GIF, but I can't, so I'll send it to you via e-mail.

BMatthews has a good point about keeping shape and the tube dimpling. Depending on the alloy and temper, thin wall aluminum tube can be easily crushed or dented, so transportation can compromise the structural integrity of the tube (also it can become deformed with regular use).

As for the equation I posted. That is the equation for the pull-up radius of an airplane based on velocity and G-loading. "V^2" means velocity squared, the "^2" means "squared." If anyone wants any more nifty equations, I'll be happy to post them. There is also an equation for turn radius for those of you pylon racers.

iron eagel 09-03-2008 09:33 PM

RE: How to calaculate wing tube size needed?
 
Quote:

ORIGINAL: OffroadBEAR

Sure thing Mr. Tharpe. I wish I could add files that aren't JPG, TXT, or GIF, but I can't, so I'll send it to you via e-mail.

BMatthews has a good point about keeping shape and the tube dimpling. Depending on the alloy and temper, thin wall aluminum tube can be easily crushed or dented, so transportation can compromise the structural integrity of the tube (also it can become deformed with regular use).

As for the equation I posted. That is the equation for the pull-up radius of an airplane based on velocity and G-loading. "V^2" means velocity squared, the "^2" means "squared." If anyone wants any more nifty equations, I'll be happy to post them. There is also an equation for turn radius for those of you pylon racers.
If you would be so kind.
I have some but it is always nice to have more...


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