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f1boss(at)gmail.com
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 Posted: Wed Nov 08, 2006 9:10 am Post subject: Gross weight -- change to more OR less? |
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Time: 11:16:31 PM PST US
From: "Rob Prior (rv7)" <rv7(at)b4.ca (rv7(at)b4.ca)>
Subject: Re: MDRA rule change
On 20:41:18 2006-11-07 "Jim Jewell" < jjewell(at)telus.net (jjewell(at)telus.net)> wrote:
| Quote: | A month or two ago I registered my RV6-a with the MOT. The application
included a requested gross weight which I set at 1775 lb.
|
| Quote: | >What criteria did you use to arrive at this new gross weight? Did you do
>any structural analysis of the airframe to confirm that it will handle the
>increased gross weight? What will be your aerobatic gross weight? Are you
>increasing that as well? What analysis have you done to support those
>numbers? Has anyone done analysis to show that the RV-6 will operate
>safely at 1775 gross and CAR/FAR Part 23 limits?
|
Come on -- lighten up! It's a 125LB increase!
Van's has done the static testing to the limit of the wing structure, so there really isn't a reason to do that again. They say it is good for XGs at XXXX weight, and I believe them. If you want to fly at XXXX + XXX, then your G limits are reduced, and your maneuvering speed goes up. There is a bit more to it then this, but that's the Reader's Digest version. As an example, you can look in the C-172 POH and see that the ship is certified in 2 categories by using different weights and CG location limits; one category allows for higher G loading and a smaller CG range, but at lower weights, while the other limits the G with increased weight and an increase in the CG range. I'll bet your RV6 can be approved using 2 different loading schedules by using the C-172 POH as an example, along with the wing loading limits provided by Van's.
I seem to recall that the RV4 specs call out a max aerobatic weight vs a MTOW, so that would suggest that Van already uses a schedule that more or less follows what Cessna uses.
I will suggest that it is possible to load the ship so it is within the capabilities of the wing at a max of 2Gs, but the gear might not take the load. As an example, 1650 MTOW x 6G = 9900. So, if I load 'er up to 9900/2 or 4950LBS (GEEZ!), the wings will be within limits at 2Gs. At such weights you will need to consider the floor structure, but as far as I know, the whole ship is good for 6G at the factory spec'd weight...except for the gear, so that makes this an exercise that cannot actually be accomplished. So much for a non-stop flight to Hawaii! You'll have to stop at least once... 
Using the above calcs show that the 1775 lb ship will be good for 5.6G instead of 6G, and still be within the design limits of the wing. Most pilots would not have any problem staying within those limits.
While the FAR 23 limits are a good design criteria to follow, the experimental world is not required to follow those criteria. Transport Canada CAN add licensing requirements for the pilot if the wing loading is above about 20 lbs/sq ft.
I would not, however, exceed the CG limits set by Van's.
| Quote: | Will the onus be on me to go through a structural testing regime to
prove the airframe is up to the task.
|
| Quote: | >No offense intended, but if you haven't gone through the structural
>testing, or know of someone who has, or perhaps have the blessing in
>writing from Van's Aircraft, I would go so far as to say you are foolhardy
>to request the increased gross weight in the first place. Why not just
>request the book gross weight, and load it up with whatever you want to?
>It's no safer.
|
Nor is it any more dangerous if you do some 3rd grade math -- see my statement above. When you load outside the manufacturer's spec'd MTOW, your insurance is, of course, null and void. And as far as I know, the 'manufacturer' is the builder.
| Quote: | >Please keep in mind that when building an RV-6 as per the plans, the kit
>manufacturer's engineering analysis limits the airframe to a specific gross
>weight. If you have made structural changes to the airframe to raise that
>gross weight, you are no longer building a Van's RV-6. You are building a
>Jim Jewell Mk. 1, and Transport Canada has every right to request your
>engineering justification for every structural piece on the aircraft. If
>you haven't made structural changes, you're taking a risk with yourself and
>your passengers.
|
| Quote: | >Many RV-6's have been built and registered at gross weights over the Van's
>limit. It doesn't make it safe.
|
Nor does it make it immediately unsafe -- again, see above. Additionally, FAR 23 allows for a flight program to prove the strength of aircraft structures -- calculations are not the only method of proving structural abilities. I will also suggest that such MTOW increases have been used for quite some time, and we have not heard of pilots landing their planes with damage to the structure from such loading, so it could follow that such weight increases would be allowed under FAR 23....which does not apply anyway.
But, if you have data to back up your statements, by all means we had better have a look at such.
--
Cheers!
Mark
[quote][b]
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rv7(at)b4.ca
Guest
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| Posted: Wed Nov 08, 2006 11:21 am Post subject: Gross weight -- change to more OR less? |
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On 2:24:53 2006-11-08 "Jim Jewell" <jjewell(at)telus.net> wrote:
| Quote: |
As far as the blessing letter from Van goes, a copy of one exists and
is being sent to me to use in defence of my request should I desire
to go forward with the 1750 LB propposed gross weight.
|
I was unaware of this... If indeed Van endorses it, then i'm sure he's done
the analysis to support it... The liability issues would be horrendous if
he hadn't. I'd like to see the wording of his letter, though... What
modifications to G-limits does he make, for both normal and aerobatic
category? What's the new aerobatic weight?
On 5:42:29 2006-11-08 khorton01(at)rogers.com wrote:
| Quote: | This has not been noticed by Transport
Canada, but I fear that an insurance company could use this to claim
that the pilot was not qualified to fly the aircraft, and thus they did
not have to pay out.
|
My further concern to that is that an insurance company could easily say
that you flew your aircraft at a gross weight higher than the manufacturer
recommended, and refuse to pay out... Regardless of what Transport says
about legality. A Vans RV is a known quantity to an insurer, it's expected
to have certain performance and usage characteristics that the insurance
company is comfortable with. When you make changes to the design (and
increasing gross weight is a change to design), the aircraft no longer fits
their metrics, and strictly speaking is no longer a Vans RV.
On 9:09:31 2006-11-08 "Mark Frederick" <f1boss(at)gmail.com> wrote:
| Quote: | Come on -- lighten up! It's a 125LB increase!
|
Only 125lb, you say? There are RV-6's in Canada registered at 2000lb
gross. This was before Van issued or alluded to any letters, too. At what
point is an increase "reasonable"?
When you analyze the structure of a light aircraft from nose to tail, you
will learn that this is *not* a trivial change. It's not as simple as
saying "add 10% to gross weight, knock 10% off G limits". That 125lb...
Where will it go on the airframe? Will you increase your baggage capacity
from 60 lb to 120lb? If so, you need to cut in *half* the G-limit on your
baggage compartment floor. Will you add fuel to the wing by increasing the
tank by one bay? You'll reduce the load on the wing spar at the wing root,
but you'll increase the shear load on the skins outboard of the existing
tank, *and* you'll move the stress concentration caused by the tank ending
out one bay along the wing. Was the spar taper designed to take that?
People assume that the weight will be just "spread around". If it could be
uniformly spread around the airframe as a thin film of excess weight then
you could indeed say that 10% more weight equals 10% less G-limit. There
would never be an issue, and our gross weight and G-limits could be shown
as a line on a graph and not a hard number limit. But realistically the
weight isn't applied that way. The airframe components don't weigh any
different, it's all the additional "stuff" that people add that makes the
difference. It gets applied in discrete locations, that need to be
considered individually to determine the overall effect on safety and
performance.
| Quote: | Nor is it any more dangerous if you do some 3rd grade math -- see my
statement above. When you load outside the manufacturer's spec'd
MTOW, your insurance is, of course, null and void. And as far as I
know, the 'manufacturer' is the builder.
|
Personally I prefer to fly airplanes that weren't built using 3rd grade
math. You're right, in Canada the builder is classified as the
manufacturer, but i'm sure the insurer will still consider the manufacturer
of the kit when they choose whether to insure you. If you showed up on an
insurer's doorstep with a "john doe special", two seat, weight and
performance like an RV, worth $75K US, do you pay the same rate as someone
with a built-to-plans (structurally, anyway) RV, of which there are
thousands flying? This is something I don't know the answer to... It just
seems silly to me that an insurer would accept that their risk is the same
whether you build a "known quantity" or a "one-off" design.
-Rob
PS - I was unlucky in that the airframe structures course I took at
university used the RV-6 as their design example the year *after* I took
the course. My class used a Murphy Rebel. The analysis is pretty much the
same on either aircraft though, and the things you need to consider are
significant. On the plus side, I know how to make the airframe of a Murphy
Rebel fully aerobatic, but it's not knowledge that i'm likely to put to
use. 
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pbesing(at)yahoo.com
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| Posted: Thu Nov 09, 2006 7:03 am Post subject: Gross weight -- change to more OR less? |
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|
When I set my GW on my -6A I spoke with an unnamed person at Van's...Their response was something like this:
"On the record, you should set your Max Gross at xxxx (can't remember, 1400 maybe?)"
"Off the record, I personally know of many RV's flying around at 1700-1800 pounds with no problem...just keep it in CG, and don't fly aerobatics at those weights"
So, like Mark said, don't exceed the CG...YOU are the manufacturer, you can set your GW wherever you like. I personally used 1650 I think which was a 200 pounds or so more. I have flown it at that, but the issue comes with an aft CG when you burn off fuel. Do what you like with the weight, but evaluate the scenarios carefully when burning off fuel, because RV's can get out of CG really easily if you exceed those weights by much after you burn off fuel.
Paul Besing
Mark Frederick <f1boss(at)gmail.com> wrote:
[quote]Time: 11:16:31 PM PST US
From: "Rob Prior (rv7)" <rv7(at)b4.ca (rv7(at)b4.ca)>
Subject: Re: MDRA rule change
On 20:41:18 2006-11-07 "Jim Jewell" < jjewell(at)telus.net (jjewell(at)telus.net)> wrote:
| Quote: | A month or two ago I registered my RV6-a with the MOT. The application
included a requested gross weight which I set at 1775 lb.
|
| Quote: | >What criteria did you use to arrive at this new gross weight? Did you do
>any structural analysis of the airframe to confirm that it will handle the
>increased gross weight? What will be your aerobatic gross weight? Are you
>increasing that as well? What analysis have you done to support those
>numbers? Has anyone done analysis to show that the RV-6 will operate
>safely at 1775 gross and CAR/FAR Part 23 limits?
|
Come on -- lighten up! It's a 125LB increase!
Van's has done the static testing to the limit of the wing structure, so there really isn't a reason to do that again. They say it is good for XGs at XXXX weight, and I believe them. If you want to fly at XXXX + XXX, then your G limits are reduced, and your maneuvering speed goes up. There is a bit more to it then this, but that's the Reader's Digest version. As an example, you can look in the C-172 POH and see that the ship is certified in 2 categories by using different weights and CG location limits; one category allows for higher G loading and a smaller CG range, but at lower weights, while the other limits the G with increased weight and an increase in the CG range. I'll bet your RV6 can be approved using 2 different loading schedules by using the C-172 POH as an example, along with the wing loading limits provided by Van's.
I seem to recall that the RV4 specs call out a max aerobatic weight vs a MTOW, so that would suggest that Van already uses a schedule that more or less follows what Cessna uses.
I will suggest that it is possible to load the ship so it is within the capabilities of the wing at a max of 2Gs, but the gear might not take the load. As an example, 1650 MTOW x 6G = 9900. So, if I load 'er up to 9900/2 or 4950LBS (GEEZ!), the wings will be within limits at 2Gs. At such weights you will need to consider the floor structure, but as far as I know, the whole ship is good for 6G at the factory spec'd weight...except for the gear, so that makes this an exercise that cannot actually be accomplished. So much for a non-stop flight to Hawaii! You'll have to stop at least once...
Using the above calcs show that the 1775 lb ship will be good for 5.6G instead of 6G, and still be within the design limits of the wing. Most pilots would not have any problem staying within those limits.
While the FAR 23 limits are a good design criteria to follow, the experimental world is not required to follow those criteria. Transport Canada CAN add licensing requirements for the pilot if the wing loading is above about 20 lbs/sq ft.
I would not, however, exceed the CG limits set by Van's.
| Quote: | Will the onus be on me to go through a structural testing regime to
prove the airframe is up to the task.
|
| Quote: | >No offense intended, but if you haven't gone through the structural
>testing, or know of someone who has, or perhaps have the blessing in
>writing from Van's Aircraft, I would go so far as to say you are foolhardy
>to request the increased gross weight in the first place. Why not just
>request the book gross weight, and load it up with whatever you want to?
>It's no safer.
|
Nor is it any more dangerous if you do some 3rd grade math -- see my statement above. When you load outside the manufacturer's spec'd MTOW, your insurance is, of course, null and void. And as far as I know, the 'manufacturer' is the builder.
[quote]>Please keep in mind that when building an RV-6 as per the plans, the kit
>manufacturer's engineering analysis limits the airframe to a specific gross
>weight. If you have made structural changes to the airframe to raise that
>gross weight, you are no longer building a Van's RV-6. You are building a
>Jim Jewell Mk. 1, and Transport Canada has every right to request your
>engineering justification for every structural piece on the aircraft. If
>you haven't made structural changes, you're taking a risk with yourself and
>your [quote][b]
| | - The Matronics RV-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?RV-List |
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