Two seater CG question

Anyone know why the Duo (and supposedly other Doppelsetzers) have the same minimum front seat weight with or without a person in the back seat?

The rear seat moment arm has about 1/4th the effect compared to the front, so I would think a 200 pounder in the rear would reduce the front min weight by 50 pounds - nicht wahr?

Maybe the numbers are based on a super cautious approach for situations like an instructor jumping out when the student solos and forgetting to recompute the cg?

Karl,

I really believe this is just reflective of the manufacturer's preference to compose some simple weight and balance rules, rather than trust the unwashed and unlearned pilot to do any computations. Unless you ballast the wings, the loading rule is simple: something like 70kg minimum (depends on the individual aircraft) on the front seat , and a max of 110 kg on either seat. Easy to remember. Not totally representative of the physics involved.

Of course this leads to some strange situations. If you have a 110kg guy in the back and a 69 kg person in the front, the glider isn't legal to fly (ignoring the front seat trim weights). But add 1 more kg in the front, and you can put something like 8 liters in the tail!

Some other German manufacturers like this same approach -- rules instead of computations.

IIRC The POH for the Janus C does actually show the computations for reducing the front seat min pilot weight based on the weight of the back seat pilot.

I agree with the previous post that many manufactures may believe in the KISS standard to keep pilots from flying with the CG aft of limits.

BillT

On Wednesday, June 4, 2014 12:52:29 PM UTC+12, Karl Striedieck wrote:
Anyone know why the Duo (and supposedly other Doppelsetzers) have the same minimum front seat weight with or without a person in the back seat?

Yeah, I think it's just a CYA thing.

Even if the CofG works out the same with heavy in the back and light in the front, the 2nd moment will be different which might affect things such as spin recovery. I'd have thought it would make it *easier* to recover, but it will certainly be different.

Our DG1000s also have 68 kg placarded front seat minimum regardless of back seat, but I don't go putting weights in the front unless the student looks under 50 kilos (I'm 120ish). And I'm still invariably trimming back from the 2/3 forward takeoff position once settled into the tow.

In theory the moments say down to about 40 in front should be ok, but I'm not quite that game

I don't know, but maybe it could be an issue with the moment of
inertia? That might affect the spin behavior, and is a parameter
separate from CG.

My personal guess, is that there is no reason for the rule. It may be
like the reply that the British Tax office once gave to a question
about their tax rules: "There is no reason for this rule, it is simply
our policy"


At 02:52 04 June 2014, Bruce Hoult wrote:
On Wednesday, June 4, 2014 12:52:29 PM UTC+12, Karl
Striedieck wrote:
Anyone know why the Duo (and supposedly other
Doppelsetzers) have the
sam=
e minimum front seat weight with or without a person in the back
seat?

On Tuesday, June 3, 2014 6:52:29 PM UTC-6, Karl Striedieck wrote:
Anyone know why the Duo (and supposedly other Doppelsetzers) have the same minimum front seat weight with or without a person in the back seat?



The rear seat moment arm has about 1/4th the effect compared to the front, so I would think a 200 pounder in the rear would reduce the front min weight by 50 pounds - nicht wahr?



Maybe the numbers are based on a super cautious approach for situations like an instructor jumping out when the student solos and forgetting to recompute the cg?

I believe it specified in JAR-22 as the design minimum-minimum seat load, just as 110kg was specified as the design minimum-maximum seat load. Not based on CG limits. I recall a conversation about flying a G-103 from the rear seat with the front seat empty and still being with CG limits.

That said, you can read all about it in CS-22 (formerly JAR-22)

http://easa.europa.eu/system/files/d...20Amdt%202.pdf

or from the link here
http://easa.europa.eu/document-libra...22-amendment-2

See paras 22.21 through 22.31.

Frank Whiteley

Hi Karl, I think the reason is that EASA airworthiness requirements
stipulate a maximum allowable figure for the minimum cockpit load.
Manufacturers make sure that their glider complies with this and then
placard that maximum figure instead of bothering to work out the actual
figure for all cases. Some years back I took delivery of a new DG500, it
was placarded minimum front cockpit load 75 Kg (I think that was the
figure, it was a while ago). On weighing, and checking the C of G limits
in the flight manual, we found that a big pilot would actually be within
limits flying solo in the rear cockpit!

At 00:52 04 June 2014, Karl Striedieck wrote:
Anyone know why the Duo (and supposedly other Doppelsetzers) have the
same
minimum front seat weight with or without a person in the back seat?

The rear seat moment arm has about 1/4th the effect compared to the
front,
so I would think a 200 pounder in the rear would reduce the front min
weight by 50 pounds - nicht wahr?

Maybe the numbers are based on a super cautious approach for situations
like an instructor jumping out when the student solos and forgetting to
recompute the cg?

All computations aside (and most of them are probably very correct), why
not just use the recommended minimum weight from the manufacturer as an
actual minimum?

Spoke to a pilot the other day who didn't, because his computations showed
less was also possible and ballast weights were not readily available. Got
into an unintentional spin that flattened out rapidly and from which he
recovered at only 300 feet, just enough for a flare and off-field landing.

My point: stay well on the safe side, and don't make a habit out of
undershooting the minimum recommended weight limits... Just because it is
possible to do things, doesn't mean you have to. ;-)

At 00:52 04 June 2014, Karl Striedieck wrote:
Anyone know why the Duo (and supposedly other Doppelsetzers) have the
same
minimum front seat weight with or without a person in the back seat?

The rear seat moment arm has about 1/4th the effect compared to the
front,
so I would think a 200 pounder in the rear would reduce the front min
weight by 50 pounds - nicht wahr?

Maybe the numbers are based on a super cautious approach for situations
like an instructor jumping out when the student solos and forgetting to
recompute the cg?




My guess....The European High Command, EASA, has a strict definition for
the format of the placard that MUST go in the cockpit. This has a space for
only one configuration. If Shempp were to alter this it would make the
aircraft unsafe to fly in Europe.

Jim

Instructions for reducing min front seat load when flying with
both seats occupied are in the manuals for the newer Duo X,
Arcus and through optional TN 295-29 for Janus.
Maybe it's also in the revision 9 for Duo FM, but the download
link on S-H website doesn't work. Ask S-H, and they will
probably solve it.
Regarding blaming EASA, I think it's actually the opposite,
german LBA had very restrictive view on complicated load
instructions, but EASA seems to accept this.
Also the ASK 21 has a corresponding TN nr 35, where the
reduction is allowed in the german and UK-flagged manual
pages, but not in the US-flagged. So maybe the FAA is to blame?

Bengt



At 07:55 04 June 2014, Jim White wrote:
At 00:52 04 June 2014, Karl Striedieck wrote:
Anyone know why the Duo (and supposedly other
Doppelsetzers) have the
same
minimum front seat weight with or without a person in the
back seat?

The rear seat moment arm has about 1/4th the effect
compared to the
front,
so I would think a 200 pounder in the rear would reduce the
front min
weight by 50 pounds - nicht wahr?

Maybe the numbers are based on a super cautious approach
for situations
like an instructor jumping out when the student solos and
forgetting to
recompute the cg?




My guess....The European High Command, EASA, has a strict
definition for
the format of the placard that MUST go in the cockpit. This has
a space for
only one configuration. If Shempp were to alter this it would
make the
aircraft unsafe to fly in Europe.

Jim