RV-6 Loading (was [Wing] Consumer satisfaction?)

[Jim Oke]

Since I am hoping to do the paperwork for a final on my RV-6A in a few days,
I have more than a passing interest in this.

The terms normal, utility, and acrobatic (as they are used in describing
light aircraft) are defined in great detail in FAR 23 - which can be seen at
http://www.tisco.com/aviation/FAA/far-23/index.shtml

The essential points are:
Normal category - max G +3.8, stalls only, "normal" maneuvers less than 60
degs bank
Utility category - max G + 4.4, stalls and spins, "normal" maneuvers >60
degs but < 90 degs bank
Acrobatic category - max G + 6.0, no restrictions (except those shown by flt
test, etc.)



in Van's does not actually say in the builder's manual that 1650 lbs (well,
1600 for a -6 and 1650 for a -6A - can't say why) corresponds to the
"utility" category - or at least I cannot find any such reference. The 6 G
limit for aerobatic category is mentioned however. If 1650 lbs is for
"utility category" at 4.4 G then a slightly higher gross weight at a reduced
max G load should be practicable.

An alternative which some builders have used is 1750 lbs for "normal
category" operation. The normal category designation implies the lower G
limit, restricts the aircraft from spins, and also (by Van's advice) moves
the CG aft limit .6 inches forward (28% vice 29% of MAC).

The intent (as I see it) is that a properly designed aircraft can fall into
all three categories depending on how it is loaded, and this is how I intend
to propose my aircraft's W&B limits be defined.

Jim Oke
Winnipeg, MB
RV-6A C-GKGZ (getting close)








----- Original Message -----
From: "George McNutt" <gmcnutt at intergate.ca>
To: <wing at vansairforce.org>
Sent: Wednesday, June 11, 2003 6:19 PM
Subject: RE: [Wing] Consumer satisfaction?


>
>
> -----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.
>
>
>
>
>
>
>
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