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

[Rob Prior]

Somehow my email program lost George's last reply, so i'll answer 
George's questions at the bottom of Jim's email...

Jim Oke wrote:

> 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.

You're right, there's no "explicit" wording that says that.  I inferred 
it from the website, where it says:

"The RVs have a design stress limit of +6 and -3Gs at aerobatic gross 
weights: 1375 lbs for the RV-4 and RV-6/6A, and 1600 lbs for the 
RV-7/7A, and RV-8/8A. This equates to an ultimate or failure limit of +9 
and -4.5 Gs."

I *assumed* that the if the RV's were Aerobatic at 1375, that they must 
meet the next category down (Utility) at gross, but that may not be 
correct.  But if they're actually Normal Category at gross weight, it 
argues even more heavily for not increasing your Gross Weight.

> 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.

And you're 100% correct.  But you will need to *show* that these limits 
are safe in each category.  To do that, you'll need to consider where 
the extra weight can go, and that those locations can take the extra 
weight.  You'll also need to show show what the limits are in each category.

> ----- Original Message -----
>From: "George McNutt" <gmcnutt at intergate.ca>
>
>I am a little ignorant here Rob, are you an Aeronautical Engineer?

Why yes, I am.  At least I was trained as one at University, i've never 
found a job working as one (not for lack of offers, there just haven't 
been any in the Vancouver area and I don't really want to move).

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

The numbers I gave were intended as an example, but I would expect Vans' 
mission profile to consist of the advertised specs/performance from the 
website, plus some numbers not mentioned there but mentioned in the 
builder's manual.  For example, I didn't know that the RV-6 manual 
specifies an occupant load of 225lb per seat... We used ICAO standards 
for analysis when I was in university, which amazingly enough assume 
170lb for a "standard male."  But even I break that limit by 5-10lb, 
depending on what i'm wearing and what I had for dinner the night before.

Specs:
http://www.vansaircraft.com/public/rv-6spe.htm
http://www.vansaircraft.com/public/rv-6per.htm

>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.

I'll guess you've got 160-ish HP, which would suggest a service ceiling 
of 16,300 at 1600lb.  So 300fpm at 15000' and 1550lb sounds about right. 
  Service ceiling is defined as the point where climb rate equals 
100fpm, so you were pretty close to it.  Don't be surprised if your 
service ceiling drops by as much as 2000' if you're flying at 1750lb, 
though.

>>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!

Exactly.  And i'd bet that there are more airplanes flying around like 
this than people would admit to, just because it's one heck of a lot 
easier than jumping through hoops to meet the requirements.

As an aside:  Most people know that an airplane will fly just fine over 
gross if you fly it smoothly and expect some pitch sensitivity.  When I 
did my flight test, we took off 50-100lb. over gross, because I was told 
to do all my planning for a 170lb passenger (ICAO Standard Male, 
right?).  That's all the room I had, once the plane was fuelled and 
loaded.  Then the examiner showed up who must have weighed *at least* 220lb.

>more than utility category "G" limits. However the regulations also
>allow
>builders to set a higher weight.

That's true, and I never argued that point.  All I wanted to point out 
was that choosing this number shouldn't be done arbitrarily (as in the 
case of an RV-6 with 3968lb Gross, for example).  You need to show (or 
at least be really comfortable with someone else's choice) that 
increasing that limit is safe.  Personally, i'd want to do the 
calculations myself.  I wouldn't want to do it "because others were 
allowed to."

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

That comes from Vans' numbers on his website, although it's "reading 
between the lines" to get it.  The website states that the Aerobatic 
limits are 1375lb at 6G.  It also states that Gross is 1600lb (1600-1650 
*recommended* in the builder's manual).  I *assumed* that since the 
airplane didn't meet Aerobatic Category at gross weight, that the intent 
must have been for it to meet Utility Category.

The RV's may well be safe for 5.2G at gross (1600-1650), and Van just 
doesn't publish the numbers.  But since we don't *know* those numbers, 
we have to work with the standards that the plane is designed to, and 
those standards only have three categories.

I can't find the reference right now, but i'm sure i've seen it written 
somewhere that the RV's were designed to meet or exceed FAR Part 23 
requirements (for certificated aircraft).

>>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)

I'll try...

"Chapter 549 Amateur-Built Aircraft
549.5 Construction
(b) Methods of fabrication and assembly, and workmanship shall be 
appropriate and should conform to accepted aviation standard practices."
http://www.tc.gc.ca/aviation/REGSERV/CARAC/CARS/cars/549/toc.htm

In this case, the "accepted aviation standard practise" is to follow the 
requirements of US FAR Part 23, or in Canada, Chapter 523:

"Chapter 523 Normal, Utility, Aerobatic And Commuter Category 
Aeroplanes"  Specifically Subsections C and D.
http://www.tc.gc.ca/aviation/REGSERV/CARAC/CARS/html_e/doc/nav-523e.htm

Now, that's a really short answer to your question.  But it's what 98% 
of designers do, because it simplifies greatly the process of bringing a 
design to market.  If you can tell people that your design meets the 
requirements of the standard that all aircraft in the last 50 years were 
built to, chances are it's going to be a reasonably safe design.

Hopefully that's enough for a free lunch... If it comes with a ride to 
Chilliwack, i'll buy the gas... 8-)

>  (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.

The problem here is that you need to *show* the Minister that you meet 
these requirements.  How do you *show* that your homebuilt is safe for 
flight?  Typically, you show that you have built a kit to the 
specifications of a respected and/or qualified designer.

In the case of changing gross weight limits, you have to show that 
someone has done the analysis to show that the limits are safe.  Hence 
the requirement for a letter from Van's, as the designer, in the past. 
I would expect (but i'm not positive) that if you were able to show with 
appropriate calculations that the increase in gross weight coupled with 
a reduced G limit didn't violate any loading conditions, that you could 
have sidestepped the letter from Van's.  But those would have to be 
pretty thorough analyses to convince Transport.

-RB4