[Wing] Consumer satisfaction?

[George McNutt]

-----Original Message-----
From: wing-admin at vansairforce.org [mailto:wing-admin at vansairforce.org]On
Behalf Of Rob Prior
Sent: Wednesday, June 11, 2003 1:32 PM
To: wing at vansairforce.org
Subject: Re: [Wing] Consumer satisfaction?

Hi Rob

This is getting so long I am not sure how to format this so my answers to
Robs reply are logical so will use a double >> for this last reply.


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?

>> You are mixing the words normal and utility category.
>> I do not fly beyond normal category weight limits, there may be a rare
time when the aircraft
>> is loaded to gross and it is beyond utility category weight limits. When
this occurs it is
>> always operated as a normal category aircraft just like thousands of
Cessnas and Pipers.
>>


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).

>> I am a little ignorant here Rob, are you an Aeronautical Engineer?

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


>> what is Vans actual "mission profile limits" please supply a reference.

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.

>> OK here, I would get engineering evaluation if I was adding fuel tanks.

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.

>> OK with this, sample loading on p14-9 of builders instructions shows
225lb pilot and
>> 225 lb pax. No limiting weights specified.

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 are required to stay within the CofG limits for all loading
configurations be they baggage >> or whatever.
>> The 100 pound baggage limit is the structural limit of the baggage
compartment floor.


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?

>> You lost me fere?  Of course the performance decreases as weight
increases, same with
>> all aircraft, if someone finds a 15000 foot service ceiling is limiting
they should go around >> high mountains. In my case I will never know the
service ceiling of my 6A will be at 1800lb
>> because my gross weight is 1750 plus I will burn 30 lb of fuel getting up
there. What I can
>> tell you is that it is still climbing 300 FPM at 15,000 feet at 1550lb
and trues out to 160
>> knots on the estimated 45% power available at that altitude.

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?

>> In the end most people say "dont bother with the hassle of changing the
paperwork, you know it >> will carry the load, just put it in and go". Is
this change going to affect someone flying to >> Chilliwack for pie?
Probably not unless they have an insurance claim!


> 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".

>> I am curious, where does your 4.4 G at 1600-1650lb figure come from.

> 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.

>> If you show me a CAR that specifys a structural strength limit for
amateur built aircraft I
>> will buy you lunch!! (this offer is limited to Rob, can't afford to buy
everyone lunch)
>> The aircraft must be safe and designed and constructed in conformity with
C 549, which does
>> not specify structural limits. I like the word "shall" in the first
sentence of 507.06 below.

507.06, the Minister shall issue a special certificate of airworthiness in
respect of an aircraft that
  (a) meets the criteria for one of the classifications of a special
certificate
  of airworthiness specified in Chapter 507 of the Airworthiness Manual;
  (b) conforms to the applicable type design or, in the case of an
amateur-built
  aircraft, is designed and constructed in a way that ensures its
airworthiness,
  in conformity with the requirements of Chapter 549 of the Airworthiness
  Manual; and
  (c) is safe for flight.


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!!

>> Happy Building,


>> George McNutt
>> Langley, B.C.