How do you measure G?

[chris willis]

There is no problem with the foam wing or spars, all that was being asked was how to measure G, as people are probably aware this is about one of my designs, the wheel and retract unit remove a big chunk of the wing from front to back, as its a prototype wing loads are being looked at, i have one full spar, one half length spar and a smaller l/e spar, I am perfectly happy with the construction on mine, the construction design that i use on my foam wings is very strong, had some very heavy landings with my tiffie and not moved anything on the wing, as for fibreglass, i have seen retracts torn out of them, I would opt for decent foam wings any day myself.

[Simon Wright]

Agree that there is nothing what soever wrong with foam and veneer wings , particularly when the construction includes main and sub spars tto distribute loads. Landing loads are a really difficult to calculate and an empirical approach is typically the mose successful.

As I said earlier in the waffle, don't try to redesign the wheel, find a design that you know works well and use it!

[Ken Bones]

There is no problem with the foam wing or spars, all that was being asked was how to measure G, as people are probably aware this is about one of my designs, the wheel and retract unit remove a big chunk of the wing from front to back, as its a prototype wing loads are being looked at, i have one full spar, one half length spar and a smaller l/e spar, I am perfectly happy with the construction on mine, the construction design that i use on my foam wings is very strong, had some very heavy landings with my tiffie and not moved anything on the wing, as for fibreglass, i have seen retracts torn out of them, I would opt for decent foam wings any day myself.

As Chris has said, I am sure that the wings we have for the Yaks are fine, I was just a bit curious as to how G could be measured on a model. Sureley just having figures from a formula is not a lot of good if we are talking toy airplanes.
My Greenly that has now had 100s of flights ( some of them were vary scary with a big glider on tow with a not so good glider pilot!) has a foam wing with no spar, just a bandage round the middle. That wing is still 100% after about 5 years of use.
I had some VARY heavy landings with my Tiffie
Ken.

[Simon Wright]

Done the maths (well excel spreadsheet did actually)

S&L - V M/S 40.75 36 32 24 20 16 14
S&L - V KMH 146.7 129.6 115.2 86.4 72 57.6 50.4
S&L - V MPH 90 79.5 70.67 53. 44.1 35.331 30.9
Cl 0.12 0.15 0.19 0.40 0.49 0.77 1.00
M Nm -23 -9 -5 -2 -2 -2 -1
Re Wing K 1195. 1055.88 938.56 703.92 586.6 469.28 410.62

If we assume 3:1 speed range 30mph stall, 90 mph max
1m2 area 42cm mean chord, cl max 1 cl @vmax = 0.12

Max flight G= 1/0.12= 8.33 G say 10G for safety.

So 27lb model at 10g has maximum design lift of 270lbs at vmax.

Wing section 22" root 11" tip x 13.5% thick gives a root bending moment of 3170 lb/inch, shear of 135lb and cap load of 1757lb. Using spruce spars top and bottom with half to quarter spar width balsa shear between you would need 1" wide x 0.35" thick at the root, tapering to 0.6" wide 12" out and 0.25" wide 24" out. then tapering to nothing at the tips.

This assumes the spars take all the loads. A veneer and glass epoxy skn will add further strength or allow reductions in spar size.

trust that helps. May be worth asking Andy if he could double check the structural calcs? Like him its been a long time

[Ken Bones]

So are you saying that the Model Yak wing has to supprt 270 lbs?

Ken

[Simon Wright]

Yes, assuming it weighs 27lbs and is designed to pull 10g!

As the wing is the lifting surface you can actually deduct its own weight from the calculation. it only really supports the fuselage etc so if the wing itself weighs 6lbs the 10 g load reduces to 27 -6 = 21 x 10g = 210 lbs.

This is normally ignored as it provides another safety margin.

[Ken Bones]

Ok, so how would you support the wing and add 270lbs to it to see if the wing could take that weight?

Ken

[chris willis]

Is that 270lbs of weight spread across the entire wing, or one spot.
Chris

[Rob Buckley]

Ok, so how would you support the wing and add 270lbs to it to see if the wing could take that weight?

Ken

Click the link on my earlier post & scroll down to 'Onex Wing Static Load Tests' Load Case 19. Basically, put the wing upside down on a workmate or similar & ballast up with sandbags.

Is that 270lbs of weight spread across the entire wing, or one spot.
Chris

Click the link on my earlier post & scroll down to 'Onex Wing Static Load Tests' Load Case 19. The weight is spread out across the entire wing, the most weight at the root & tapering to nothing at the tip.

[Ken Bones]

Ok, thought i would try and put some weight on the wings tonight whilst round at my mate Daves house. Anyway over a few beers and stuff we decided that we must both weigh about 270 lbs. So we balenced the wing in the kitchen and sat on it. At this point Daves cat walked under the wing, just as it split( the wing, not the cat) and the two of us fell on the cat.
Now, Daves a big bloke and he now has really hurt his elbow.
The cat is ok though.
The wing should be back together once I get down the model shop to buy some cyano

Bonzey

[Rob Buckley]

No wonder that didn't work- if you're sitting on the wing there's a concentrated load going into the structure, not representative of flight loads at all.

If you're going to test the repaired wing using yourselves as ballast, you need to lie on it. If you lie face down, nose at the wing tip, arms back by your sides & legs bent up at the knees, that should do the trick. The added benefit is that if it breaks, you'll be able to estimate the ultimate G the wing could take by how much your nose hurts...

Keep the cat out of the way, they interfere with readings.

[Simon Wright]

Ok, thought i would try and put some weight on the wings tonight whilst round at my mate Daves house. Anyway over a few beers and stuff we decided that we must both weigh about 270 lbs. So we balenced the wing in the kitchen and sat on it. At this point Daves cat walked under the wing, just as it split( the wing, not the cat) and the two of us fell on the cat.
Now, Daves a big bloke and he now has really hurt his elbow.
The cat is ok though.
The wing should be back together once I get down the model shop to buy some cyano

Bonzey

You are joking right?

I saw your post last night but was out and couldn't answer.
The loading is best done with small bags of sand or similar, located on or at least around the spar.

The best way of holding the wing is to use the fuselage to test the dowels, wing seat and fixing bolts too. Test inverted for positive G or upright for negative G. A non symmetrical airfoil will produce less lift and consequently G when inverted.

Measuring from the root you can acheive a reasonable load distributuion with the following point loads:

from root to 11.75" from root distribute - 86lbs
from 11.75" from root to 23.5" from root distribute - 38lbs
from 23.5" from root to 35.25"from root distribute - 9lbs
from 35.25" from root to 47" from root distribute - 2lbs

I dont know where and your mate sat on the wing (and I dont mean in the kitchen ) but am not surprised that 2 x 135lb men sat on hte wing snapped it.

Si

[Andy Boylett]

Looks to me like you need for each side:
one small wife approx 86lbs
a child of approx 38lbs
a small dog approx 9lbs
and the cat, approx 2lbs

Then see if you can get them to sit still. You have an issue with the cat sat next to the dog. At leats if the wing fails the cat is on top this time .

[stewart clifford]

Hmmm but Andy where would you get a replacement cat from if you did squash it?

[Simon Wright]

Looks to me like you need for each side:
one small wife approx 86lbs
a child of approx 38lbs
a small dog approx 9lbs
and the cat, approx 2lbs

Then see if you can get them to sit still. You have an issue with the cat sat next to the dog. At leats if the wing fails the cat is on top this time .

Brilliant

I know that figures are flying everywhere but unfortunately there is no single line answer for the questions raised, aerodynamics and the forces applied during any maouvre are a dynamic condition where almost every condition affects the other. Flying flat out in a gentle dive perhaps at 80 mph is possible with a 10" pitch prop and 8000 rpm. the instant you apply up elevator to pull a loop lets say, increases lift, drag and immediately reduces velocity, Reynolds numbers fall, CL changes, prop load changes due to its relative airflow and rpm fall. Acheiving a 10g in flight load is not as easy as you may think, and in fact would require a huge excess of power.


read and digest this.
http://flysafe.raa.asn.au/groundschool/ ... etal_force

[Dave Hayfield]

Regardless of stress calculations, methods of construction are critical. I've seen metal tube mainspars fold under extreem conditions even when they are in kit built models. I don't think I have ever seen a decent 'I' secion or box section mainspar fail in the air even on very heavy models. Full size methods on light aircraft are worth looking into.

[Mike Booth]

Agreed Dave.
I watched A top quality third scale composite jet being put through some early test flights this year. In what seemed a fairly low G manoeuvre there was an audible thud' from the airframe at about 200 ft. After a normal procedure landing inspection showed compression failure of the spar in the inboard root area.
On further inspection inside I was staggered to see that the spar tubes terminated at 50mm into the doubled fuselage former.
The design was relying on spreading the load around the two formers instead of having a continuous tube spar arrangement .
What made me more surprised was the undoubted quality of the manufacturing and pattern making.
The wing spar simply wasn't up to the job and was dependent on uncompromised epoxy bonding which in this case it must of been slightly.