How Much Load for a Load Test?

In responding to the anonymous poster's load test anxiety, I let the
facts run away from me.

Here is the test question:
You have supported a 1000 lb airplane upside down, and wish to apply a
4g load test.
How much weight do you add to the wings etc.?

..
..
..
..
Hands up, all who said 3000 lb.
Correct!
The airframe is already
supporting its own (1g) weight. :-)

Brian W

On Jul 3, 1:54*pm, Brian Whatcott wrote:

"Bob" wrote in message
...
On Jul 3, 1:54 pm, Brian Whatcott wrote:

Brian Whatcott wrote:
In responding to the anonymous poster's load test anxiety, I let the
facts run away from me.

Here is the test question:
You have supported a 1000 lb airplane upside down, and wish to apply a
4g load test.

I'm afraid I found the allegedly simple question confusing because it
makes some unstated, but important, assumptions:

Is this a test of the fuselage or the wings?

Is the airplane supported from the ground by way of the fuselage or the
wings (and if the latter, where on the wings)?

And is this supposed to be a positive or negative load test?

How much weight do you add to the wings etc.?

Depends:

If you put it upside down and proceed to add weight to the wings, then if
the plane is supported by way of the fuselage, most of the plane's weight
appears to be directly supported by the ground supports. Only the wing's
weight (rather light) would seem to be stressing/sheering the wing spars.
So 4000 lbs minus wing_weight would seem to be needed to be added for a
_positive_ 4g load on the wings.

But if the plane is supported by way of the wing tips (say), the plane's
full weight is indeed stressing/sheering the spars. Then you'd have to
add only 3000 lb weight to the _fuselage_ - but this would probably be
something like a _negative_ 4g load test. But I believe not a very
accurate one.

Hands up, all who said 3000 lb.
Correct!
The airframe is already
supporting its own (1g) weight. :-)

Just let me know where you think my reasoning is wrong and why yours is
correct. I'm thinking maybe you composed the post a tad too quickly. ;-)

On Jul 3, 1:54*pm, Brian Whatcott wrote:

How much weight do you add to the wings etc.?

What's the non-lifting weight?

On Jul 3, 3:54*pm, Brian Whatcott wrote:
In responding to the anonymous poster's load test anxiety, I let the
facts run away from me.

Here is the test question:
You have supported a 1000 lb airplane upside down, and wish to apply a
4g load test.
How much weight do you add to the wings etc.?

.
.
.
.
Hands up, all who said 3000 lb.
* * * * * * * * * * * * * * * * *Correct!
* * * * * * * * * * * * * * * * * * * * * * *The airframe is already
supporting its own (1g) weight. * :-)

Brian W

Glad someone raised this topic. Too lazy to do it myself. Assuming
a LSA gross wt. aircraft (1320 lbs.) with a wing that weighs 200 lbs.
and you want to test the wing for 4 g's. 4 possible
scenarios.....The first three I have seen in "reputable"
publications...you know...the ones you have to PAY for. The 4th one
is the only other one I could think of. I think we've decided #1 is
wrong. Turning the plane upside down and supporting the frame on the
fuselage. How many bags of manure do you pile on the wing to test.
Assumptions: each bag of manure weighs 1 lb.

Scenario #1 1320 x 4 ( 5280 )
Scenario #2 1320 - 200 x 4 ( 4480 )
Scenario #3 1320 - 200 x 4 + 200 ( 4680 )
Scenario #4 1320 x 4 - 200 ( 5080 )

Neal F.

Glad someone raised this topic. Too lazy to do it myself. Assuming
a LSA gross wt. aircraft (1320 lbs.) with a wing that weighs 200 lbs.
and you want to test the wing for 4 g's. 4 possible
scenarios.....The first three I have seen in "reputable"
publications...you know...the ones you have to PAY for. The 4th one
is the only other one I could think of. I think we've decided #1 is
wrong. Turning the plane upside down and supporting the frame on the
fuselage. How many bags of manure do you pile on the wing to test.
Assumptions: each bag of manure weighs 1 lb.

Scenario #1 1320 x 4 ( 5280 )
Scenario #2 1320 - 200 x 4 ( 4480 )
Scenario #3 1320 - 200 x 4 + 200 ( 4680 )
Scenario #4 1320 x 4 - 200 ( 5080 )
-----------------------------------------------------------------------
OR....

1320 -200 =1120 x 4 = 4480 -200 = 4280

AC weight - wing weight cause wing is supported by the air in real flying so
G's will not load the spar, then multiply by G's _then_;
subtract the weight of the wing, since gravity is pulling on the wing in
the load test, so you can take that many bags bags off.
(since gravity is doing 200 pounds of the work for you)

One fact that should be mentioned is the location of the bags. They should
be placed outwards along the wing in the approximate distribution of lift.
Also, place them centered front to back along the ribs to represent the
center of lift for your airfoil at about an angle of attack that would be
necessary to pull that many G's. When in doubt, rearward would the way to
go. My reason for wanting to do this is to see if the wing takes on extra
twist trailing edge which could lead to a very nasty early tip stall.

Now, how about the fact that conventional airplanes have a tail that is
pushing down to achieve stable flight. That "weight" has to be "lifted" by
something, and that would have to be the wing. Better get some more bags.
How many? Up close to 10% ? That is only a guess; anyone know? So 10% of
4480 is 480 more bags, right?

That puts our wing load test up to 4760.

Wait !!! Did you take into account that the fuselage contributes a
substantial percentage of lift depending on the design? If you knew how
much, you could subtract that calculated factor from the weight you are
going to have to put on the wing for the test.

There are other factors you should think about, such as extra loads placed
on the rear spar due to aerodynamic forces created by the flaps and
ailerons. Somewhere about now my head starts to hurt, so I add bit more
for the wife and kids and let it go at that

Whew! There ARE reasons why people go to school to get Aerodynamic
Engineering degrees.

More food for thought?
--
Jim in NC

From: "Morgans"
Sent: Saturday, July 04, 2009 4:28 AM
Newsgroups: rec.aviation.homebuilt
Subject: How Much Load for a Load Test?

AC weight - wing weight cause wing is supported by the air in real flying
so G's will not load the spar, then multiply by G's _then_;
subtract the weight of the wing, since gravity is pulling on the wing in
the load test, so you can take that many bags bags off.
(since gravity is doing 200 pounds of the work for you)

So, I'm flying along in flight and I roll inverted and pull through to the
vertical. The ground's coming up fast and I pull to recover. The wing's
weight is towards the nose. How hard can I pull? If I designed to this
criteria I can't pull four Gs. I didn't test to four Gs. Why do you call
this a four G wing?

Rich Isakson

"Richard Isaksom" wrote in message
...
So, I'm flying along in flight and I roll inverted and pull through to the
vertical. The ground's coming up fast and I pull to recover. The wing's
weight is towards the nose. How hard can I pull? If I designed to this
criteria I can't pull four Gs. I didn't test to four Gs. Why do you call
this a four G wing?

Rich Isakson

I am, of course, wrong here. The discussion is about testing with the wing
mounted upside down in a fixture so the wing weight would be credited as
part of the lift.

Rich Isakson

wrote:
....
Glad someone raised this topic. Too lazy to do it myself. Assuming
a LSA gross wt. aircraft (1320 lbs.) with a wing that weighs 200 lbs.
and you want to test the wing for 4 g's. 4 possible
scenarios.....The first three I have seen in "reputable"
publications...you know...the ones you have to PAY for. The 4th one
is the only other one I could think of. I think we've decided #1 is
wrong. Turning the plane upside down and supporting the frame on the
fuselage. How many bags of manure do you pile on the wing to test.
Assumptions: each bag of manure weighs 1 lb.

Scenario #1 1320 x 4 ( 5280 )
Scenario #2 1320 - 200 x 4 ( 4480 )
Scenario #3 1320 - 200 x 4 + 200 ( 4680 )
Scenario #4 1320 x 4 - 200 ( 5080 )

Neal F.


What a well-phrased question! :-)

It would be most reassuring, if one used the heaviest load, no doubt.
But you have specified a test only of the wing.
This wing must carry not only its own weight multiplied by the test
acceleration, but that of the rest of the airplane's gross weight
multiplied by the test factor.
As mounted in the test rig, it is already loaded with one wing weight,
so we should add another 3 wing weights and four weights representing
the aircraft gross less wing weight. What does that come out to, I wonder?

The load mentioned above represents
3 X 200 +
4 X (1320 - 200) =
5080 lbs.
As you can see, this is just another way of expressing your Scenario #4
This is not quite the heaviest option.....

But wait: that 3 times wing weight does not have to be reacted through
the wing attach at all.
That's one reason why fuel tanks in wings are favored - the wing lift
can react the fuel load locally, and that's a less stressful job than
holding the tank up on the end of a (more or less) long lever arm.
So we COULD allow ourselves the benefit of discounting the accelerated
load due to the wing itself, though the wing could still collapse say by
crushing or buckling, lets allow a 50% reduction.
Than we would load the wing with
3 X 200 X 50% +
4 X ( 1320 - 200 - (200 X 50%)) =
(thats an allowance for the self-support of the OTHER wing too)
total 300 + 4080 = 4380 lbs
Well, would you look at that: the smallest load of all! :-)

And that's when we start considering the merits of Barnaby Wainfan's
flying wing, made with flat panels, to boot. The WHOLE weight is locally
reacted by the wing.
How strong is THAT!

After considering the merits, you are going with the biggest number -
it's your life, after all.

Brian W