Murphey

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I'm starting a scratch building project and want to test the strength of the wing spar. I plan on building a "Test Spar" consisting of the upper and lower spars and shear webing in the middle. How would I test the spar for strength?

Murphey (As in Murphey's Law)

cruzmissile

my method would consist of having the "test" spar supported above ground. I would then attach a cord or rope to the middle and have a system to put weights on it. Kinda like lifting weights at 5lb increments. And at the point it starts to crack would be the amount of force, or pounds, needed to break the wing. Or at least that's what they do to full scale aircraft. With the construction methods used to build model planes now, and the amount of force they exert, I think the wing would not break anywhere in the middle. I would only suspect that if the two halves are not fused together correctly then the wings would break at the middle. But you should have no problems with a 2 spar, webing design.

BMatthews

Sorry cruzmissle but using a cord to hang the weights onto is limiting the test to a point load rather than a proper distributed load.

To properley test the spar first you'll have to arrange so that as it bends it can't twist. This function is handled in the wing by the ribs and leading and trailing edges. But in your test there's none of that there so you'll have to arrange for some form of fences to hold it vertical to close tolerance. Or you may find it's easier to add some simple retangular ribs and leading and trailing edges to do the same thing.

To load test the spar in a meaningful way similar to the aerodynamic loading on the wing you need to distribute the load evenly all along the spar. In full sized aircraft before fancy test jigs it was common to mount a wing upside down and load it up with layer upon layer of sandbags in an even thickness all over the wing. I've seen some old photos that make you a real believer in the strength of simple structures done right. With the sandbag method the load you place on the wing will be directly applicable to your model. Double the weight that the single panel supports and then divide it by the weight of your finished model and that give you the G loading that it will withstand. For obvious reasons it's good to derate that spec for a safety margin to come up with a safe G load limit.

If you can't do the sand bag idea then molded patio paver stones may be an option. Buy them, do the test and then return them saying that the color was wrong or something. (sneaky, aren't I.... )

But in the end most folks don't bother since we overbuild too excess.

I helped test a vacumn bagged glider wing one time. I was the dumb weight that got to stand on the root of the test wing panel. We set it so there was about 8 or 10 inches sitting in the core bed upside down and the bottom core bed was placed over the lower root. A piece of plywood finished my perch. Then the started adding scuba lead weights along the wing. They got to about 60 or 70 lbs before they levered me off the bench and the test halted. I learned later that they clamped the same test wing down properley and stacked up about 120 lbs of weights with lots of bend but no sign of failure. The test stopped there because they didn't have enough weights but they added a "bit more" by pushing down with their hands and it still just flexed. I have no doubt that if they blocked up the wings on the finished model one of us could easily have stood on the center section with no damage.

Murphey

My Feedback: (11)

Thanks for the info Bruce! I'm enlarging a plane by 150% and didn't want to just make a bigger spar but one appropriate for the loading. I think the sand bag concept will work. But just out of curosity, any idea how many +G's are exerted on a sport flyer? That's probably an off the wall question as it depends on who and how it's flown....but generally!

Murphey (As in Murphey's Law)

BMatthews

Not a clue, but I do seem to remember a magazine article that went into just this topic lots of years ago. Don't shoot me if I'm quoting wrong but I seem to remember 8 to 10 G's for a highly aerobatic model in a full elevator high speed pullout.

It all depends on the speed and the radius of the pullout. It's been too many years since my high school physics class but I do know there's a formula for it. You just have to be good at guessing the max speed and realistic about how tight your model will be able to turn when you hit that full up elevator.

cappio777

My Feedback: (2)

The formula for calculating the G forces its easy, whats hard is to measure the exact turning radius of the model so there is a lot of fudge factor involved. But if you want to go at it here is the formula.

Centripetal force=mass of the model(kilograms)*(Velocity squared(meters/sec)/radius of path(meters)).

The number you get from this gets divided by the gravitational acceleration and that tells you the amount of Gs

#G=Centripetal Force/Gravitationa accel.

Dont confuse mass with weight, many people use them interchangeably. a person doesnt weight 100 kg....their weight would be mass*gravitational acceleration=100kg*9.81(m/sec_sq). Hope this helps.

cruzmissile

could you make that out into an example equation. Like the plane weights X amount going at X MPH gives you X g's on wing load?

Tall Paul

Here's a composited image of my Somethin' Extra in a loop...
Level flight speed about 70 mph

Cactus.

My Feedback: (1)

heres me testing a prototype wing and getting the strenght a little bit wrong.

cappio777

My Feedback: (2)

ok, using TallPaul picture that depicts an 18 feet radius and the aircraft flying at 70 mph lets assume the aircraft weights 6lbs. In reality the airplane weights 6 lbf (pounds force). Since weight is a measure of force we need to divided by the gravitational acceleration to get mass:

mass ----> 6lbf * (1lbm/lbf 32.2 ft per sec_sq)= (0.186 lbm/ft per sec_sq)

now convert velocity into feet per second;

70mph*(5280 ft/mile)*(hr/3600 sec)=102.67 feet per second

Fc=Centripetal Force=(mass * velocity_squared)/ Radius of path.

Fc = ((0.186 lbm/ft per sec_sq) * (102.67 fps)^2)/18 ft = 108.9 lbf << this is the load experienced by the wing area>>

Now we calculate the Gs

Gs = Fc / Gc -----> 108.9 lbf / (32.2 ft per sec_sq) = 3.4 Gs

English Units suck.........easier done with standard units (Kilograms and meters)

William Robison

My Feedback: (3)

In a manuever the Czech pilots call a headache, and we use their word "Lomcevak" as its name, a full size aerobatic plane can exceed 16Gs.

When I was still subjecting my Diablo airplane to such things, the centrifugal force inside the plane (not centripetal) was sufficient to push the fuel tank into the engine mounting screws hard enough, compressing the foam rubber surrounding it, to punch holes in the front wall of the tank.

When I tried to fill up for the next flight fuel ran out the bottom of the plane.

That was probably somewhere around the same 16Gs on my poor little airplane.

Bill.

cappio777

My Feedback: (2)

Centrifugal or center fleeing force is only an ilusion from the perspective of an object (person) inside the inertial frame being study (airplane). In reality the aircraft is exerting the force on the person as it is changing direction (accelerating) in a turn. Therefore in the case of the fuel tank, the aircraft exerted the force on the tank.
Basically its the same force in magnitude but in technical (physics) terms centrifugal doesnt exist.
Now 16 Gs .....I dont know about that

Soar Head

My Feedback: (5)

some Aero professor from MIT published plans for a glider with a strong spar. He detailed his spar design, and testing procedures. He tested one spar to destruction.

There's a lot of reading, but it should prove useful for you.

http://www.charlesriverrc.org/articl..._allegro2m.htm

Johng

My Feedback: (4)

It's still not unheard of in the kit-plane world of EAA to do the same thing - at least up to the limit load if you still plan to fly that wing. I know I've seen a pic of cement bags all over a wing in Sport Aviation. Of course it would be more accurate to distribute the weight according to the lift distribution of the wing, but level loading will be conservative.

And yes, the spar designs of the MIT professor are really advanced - and take full advantage of the strength of CF materials. The Prof's name is Mark Drela and he has designed a series of gliders that are outstanding. He has posted around here with username "drela". The link to his project has much good info.