normal/utility category

[AINut]

I know it's back to basics, but I still don't quite have this one down.
A Mustang II is rated at 1600 lbs in the utility category, but no spec
is given for the normal category. Normal cat will allow a higher weight
capability, AIUI. Is the difference between the two based solely on how
hard you G-load the plane during flight? What about landing weights or
if you prang it pretty hard? (Of course, none of US ever do that!) 8-).

Thanks.

[plasticguy]

"AINut" >
> A Mustang II is rated at 1600 lbs in the utility category, but no spec is
> given for the normal category. Normal cat will allow a higher weight
> capability, AIUI. Is the difference between the two based solely on how
> hard you G-load the plane during flight? What about landing weights or if
> you prang it pretty hard? (Of course, none of US ever do that!) 8-).


A few things. There is no "Normal" or "Utility" category
in the Amateur Built / Experimental class of aircraft. Those two
categorys apply to normally certificated aircraft.

Am/EXP aircraft operating envelopes are defined by the builder
during the test period. You may be safe flying at a higher
weight than 1600 lbs but there other considerations. How does the
airplane slow fly at a higher weight, is the approach speed adversely
affected, how does it stall, spin and all that other stuff. Are the
brakes adequate, is the prop correctly pitched to accelerate the new
mass. How is the climb rate..... Pulling G's is only a small part of the
picture.

Airplanes are systems, as such, messing with one data point cannot be done
without
evaluating the entire package for changes........

Scott.

[Ron Wanttaja]

On Sat, 15 Jan 2005 09:18:38 -0600, AINut > wrote:

>I know it's back to basics, but I still don't quite have this one down.
> A Mustang II is rated at 1600 lbs in the utility category, but no spec
>is given for the normal category. Normal cat will allow a higher weight
>capability, AIUI. Is the difference between the two based solely on how
>hard you G-load the plane during flight? What about landing weights or
>if you prang it pretty hard? (Of course, none of US ever do that!) 8-).

Mustang IIs are not in the utility category. They are in the Experimental
category, where there are no requirements for structural integrity.

With that said, homebuilt designers often claim their designs meet normal,
utility, or acrobatic category load limits. What that really *means* is solely
up to them. The general assumption is that the aircraft meets the required
limit loads... +3.8 for normal category, 4.4 for utility, or 6.0 for acrobatic.

So an airplane that is designed for a utility class rating at 1600 pounds should
be able to withstand about 1850 pounds if the G is limited to the normal
category limits (4.4/3.8 x 1600)

Whether a designer's claim that the aircraft meets normal, utility, or aerobatic
limits also include the 1.5x safety factor, the requirements whose levels are
also based on load factors, or the landing gear requirements (of which some
allow weights less than gross weight) is solely up to them.

Finnish regulations require ALL aircraft comply with FAR 23 requirements. Back
in the '70s, a Fly Baby underwent a full Part 23 structural testing regimen:

http://www.bowersflybaby.com/safety/Finnish_Load_Report.pdf

Ron Wanttaja

[Ron Natalie]

AINut wrote:
> I know it's back to basics, but I still don't quite have this one down.
> A Mustang II is rated at 1600 lbs in the utility category, but no spec
> is given for the normal category. Normal cat will allow a higher weight
> capability, AIUI. Is the difference between the two based solely on how
> hard you G-load the plane during flight? What about landing weights or
> if you prang it pretty hard? (Of course, none of US ever do that!) 8-).

The difference between the normal and utility category is primarily
G-loading. You're wrong in thinking that every design however has
a higher normal category gross weight than it's utility category
gross weight. There are a number of things that go into determining
gross weight of which the G loading is only one of them. For example,
while the 172 has a 2-seat, lower gross utility category envelope within
the normal category envelope, the 152 is certificated solely with
Utility category limits.

[[email protected]]

> So an airplane that is designed for a utility class rating at
> 1600 pounds should be able to withstand about 1850 pounds if
> the G is limited to the normal category limits (4.4/3.8 x 1600)

Ron, I have the greatest respect for your posts, but this is not quite
right. When calculating bending moments for sizing the wing spar root,
the weight of the wing should not be included. The wing, in essence,
is self supporting and only the fuselage (and everything in it) is
being lifted by the wing.

So for a 1600 pound gross airplane, let's say the wing weighs 200 lbs.
Moving from utility class to normal category yields the following:
1600 - 200 = 1400 lbs
1400 x 4.4/3.8 = 1621 lbs
1621 + 200 = 1821 lbs new gross weight (not 1850 lbs)

BUT WAIT! If the aircraft has fuel in the wings rather than a fuselage
tank, the weight of the fuel is also not included when calculating spar
bending moments (because the fuel is in the self-supporting wing). If
we assume the fuel also weighs 200 lbs, our calculations for a 1600 lb
airplane might look like this:

1600 - 200 - 200 = 1200 lbs
1200 x 4.4/3.8 = 1389.5 lbs
1389 + 400 = 1789.5 lbs new gross weight

Now we are seeing significant difference from the suggested 1850 lbs.
Even these calculations are an over-simplifcation. My point is, one
has to be careful about making engineering assessments without
reviewing the original stress report.

[Ron Wanttaja]

On 20 Jan 2005 13:47:43 -0800, wrote:

>> So an airplane that is designed for a utility class rating at
>> 1600 pounds should be able to withstand about 1850 pounds if
>> the G is limited to the normal category limits (4.4/3.8 x 1600)
>
>Ron, I have the greatest respect for your posts, but this is not quite
>right. ...
>
>Now we are seeing significant difference from the suggested 1850 lbs.
>Even these calculations are an over-simplifcation. My point is, one
>has to be careful about making engineering assessments without
>reviewing the original stress report.

You are undoubtedly right...I was just shooting for a ballpark figure, and
obviously it wasn't the right approach in this situation. Thanks! This one's a
keeper....

Ron Wanttaja