[Wing] Consumer satisfaction?

[Rob Prior]

George McNutt wrote:
> To answer your questions will probably get into a great diatribe on the
> subject, however it may be interesting and it is nice to see some action on
> the wing website, so here goes. Remember, in my case I am speaking of rules
> that existed in April 2002 and there may have been subsequent changes.

I agree, this is an interesting exchange... I hope others are getting 
something out of it as well.

 > (2) I do not feel qualified to second guess the designer. Therefore I 
 > will observe his structural loading limits, especially the aerobatic
 > weight.  However the recommended gross for my RV-6A is not specific in
 > the builders manual, it states "Vans recommends a 1600-1650 pound
 > limit" but it also states that "the aircraft builder is allowed to
 > specify this [gross] weight" (Page 14-1 #2).

The first thing that comes to mind here is, if you think that Vans' 
limits for normal category are too low, and willingly fly above them, 
then why would you bother observing his aerobatic structural limits?

Still...

I think a little background may be necessary from an Aeronautical 
Engineer's perspective (which could be likened to Vans' perspective, 
although I don't claim to be speaking for them at all).

First off, an airplane is not designed with a "variable" or 
"unspecified" gross weight in mind.  An airplane is designed to meet a 
desired "mission profile".  Such a profile will contain requirements like:

  - two 170lb passengers + 100lb baggage
  - 200mph cruise
  - +4.4/-2.2 (utility category) limits
  - 1600lb gross weight

These numbers are used to design the entire structure of the aircraft. 
In the case of the RV-6, the gross weight limit was chosen to be 1600lb 
at a utility category loading of 4.4G, or 1375lb at an aerobatic loading 
of 6G.

Let's take the example of your aircraft, raising the gross weight to 
1800lb.  That's an extra 200 lb.  Where does it go?  A few places come 
to mind, let's explore what happens when you put the weight in those 
locations:

Fuel.  You could add fuel in the manner of an additional bay along the 
leading edge, or at the wingtip.  Keep in mind that this places an 
additional dead-weight shear load on the wing, either along the leading 
edge or at the tip, locations that were not designed to take these loads 
originally.

Passengers.  You could carry extra-heavy passengers.  Keep in mind that 
the seat structure was designed to carry a certain passenger load, and 
to carry it safely up to a certain G load.  Raising your passenger 
weight lowers the safety factor on your seats.  Also, an extra heavy 
passenger will likely have larger legs and torso, which may hamper 
control movement.

Baggage.  I think many owners of RV-6's will agree that the aircraft 
starts to show an aft CG problem if it's loaded near Gross with baggage. 
  Adding more baggage means that problem gets worse, and affects the 
stability and performance of the aircraft.

You could distribute the extra 200 lb throughout your aircraft.  You 
could have an extra 1/2 bay of fuel in each wing, an extra 50lb of IFR 
avionics, an extra 50lb of baggage, and an extra 50lb of passenger. 
That'll distribute the load quite evenly, right?  Well, consider that 
the stall speed creeps up, and service ceiling and range creep down, 
with an increase in gross weight.  For example, note that the service 
ceiling of a 160HP RV-6A drops from 20,500 to 16,300 when you go from 
the solo weight to the gross weight, a change of 225 lb.  How much lower 
is it at 1800 lb?

In the end, are any of these changes going to affect someone flying an 
RV from Langley to Chilliwack for pie?  Possibly not.  But keep in mind 
that by increasing your gross weight, you're cutting into your safety 
margins.  Despite all the inspections, friends looking at the 
construction, and everyone's agreement that you fly a beautiful RV-6A 
(which you do... 8-), can you say with 100% certainty that every 
structural component on your aircraft matches the plans?

> Certainly I could set gross at 1600 or 1650lb. (which one?) and always have
> more than utility category "G" limits. However the regulations also allow
> builders to set a higher weight. The beauty of the homebuilt regulations is
> that a builder can increase gross weight and fly the aircraft like a Boeing
> or Airbus which have flown millions of hours with a 2.8 "G" limit.

This is one of the points I make above... The RV-6(A) is 1600-1650lb 
gross at 4.4G (Utility Category) and 1375 at 6G (Aerobatic Category). 
So at 1600-1650, you do not have "more than utility category limits".

> How high could I have theoretically set my gross weight, CAR's limit
> homebuilt weights to 3968 pounds, and there are no structural strength
> requirements in the regulations.

This is a bit of a misconception.  The CAR's do have structural strength 
requirements, but this isn't the section where you find them.  The CAR's 
say your aircraft must be able to withstand a design load factor at a 
design load condition.  For example, Gross Weight at 4.4G (utility 
category) or Aerobatic Weight at 6G (aerobatic category).  That is the 
structural strength requirement.

Once you've designed your aircraft around this, you look to see if it's 
a high-performance aircraft or not, and that is defined using wing 
loading.  For an RV-6(A), the wing loading is 14.5 lb/ft^2, which falls 
within the limit of about 20 lb/ft^2.  This section wasn't intended as 
an opportunity to show that your aircraft could have a larger gross 
weight, it's intended to show that your aircraft falls well within the 
non-high-performance category.  Nothing more.

> It is my understanding (from a telephone conversation with Maurice Simoneau)
> that we actually do have one Canadian RV-6(A?) registered with a 3968 lb
> gross weight, I sure would have liked to witness that climb test!!

I'd also want to check that the pilot has a high-performance endorsement 
(3968/110 = 36lb/ft^2).

> Happy Building,

And Happy Flying... 8-)

> George McNutt
> Langley, B.C.

-Rob P.