Discus 2 Optimal CG location

[Dave Springford]

Can anyone advise the optimal CG location for a D2?

If the 2a and 2b have different optimal locations, I would like to know both.

TIA,

Dave

[[email protected]]

On Sunday, June 24, 2018 at 9:05:51 PM UTC+2, Dave Springford wrote:
> Can anyone advise the optimal CG location for a D2?
>
> If the 2a and 2b have different optimal locations, I would like to know both.
>
> TIA,
>
> Dave

Optimum is where ever you like it best.

I fly mine full aft for the better handling, but it glides better a bit further forward.

[[email protected]]

Hi Dave,

In normal conditions, I fly mine at station -383 which is what Knauff said was optimal when I bought it in 2002. In strong conditions, where I plan on a lot of high speed cruising, I run at stations -355– -365 to minimize the amount of nosedown trim.

Regards,

Bif “�H7�

[[email protected]]

Il giorno domenica 24 giugno 2018 21:05:51 UTC+2, Dave Springford ha scritto:
> Can anyone advise the optimal CG location for a D2?
>
> If the 2a and 2b have different optimal locations, I would like to know both.
>
> TIA,
>
> Dave

Not a long experience. I fly my Discus 2b from about 70 hours.
Not so much but in my opinion it fly better with not much backward load.
I fly mine about half the CG exursion (320-330mm).
One time I try it with more tail load but the glider became unstable and I
need to fly it faster in thermal. I really don't like it.
With a neutral CG in very weak condition I fly it at 42 kg/mq and I thermal it
at about 90-95 km/h without problem.
With avarage/good condition I fly it at about 45 kg/mq and I thermal at 105-110 km/h or more depending on how tight I need to turn.
No experience with more load (short runway).

[[email protected]]

I fly my 2B about 90% aft empty, but often don't add much tail water when ballasted. Often no tail water, especially in wave. Lead and a brass tail wheel helps with that.

I've found the WB a little unusual though as you need to measure the arms. They are a little different to the manual. I wondered if the wheel is supposed to be jacked somehow during weighing. Does anyone have any ideas about that?

[Bruce Hoult]

On Monday, June 25, 2018 at 1:19:00 PM UTC-7, wrote:
> I fly my 2B about 90% aft empty, but often don't add much tail water when ballasted. Often no tail water, especially in wave. Lead and a brass tail wheel helps with that.
>
> I've found the WB a little unusual though as you need to measure the arms.. They are a little different to the manual. I wondered if the wheel is supposed to be jacked somehow during weighing. Does anyone have any ideas about that?

Are you measuring with the tail on the ground? The glider should definitely be in something resembling flying attitude, though how that is defined for any particular glider is outside the scope of general W&B instructions. The instructions for any particular glider will be in its maintenance manual.

[Charlie M. (UH & 002 owner/pilot)]

Agreed.

Some ships may be measured with main and tail wheels on the ground, but need a level on the tail boom (with spacers) and THEN measure weights.

Not sure?
Contact those that know, discuss with your local A&P.

Many ways to set up the fuselage, some may measure off structure (spar, wing leading edge, etc.) for data points, some measure "out in space"...,,,,
Read and understand, otherwise, ask a LOT of questions.

[Charlie M. (UH & 002 owner/pilot)]

Oh, and optimal for whom?

Some like mid range, some can deal with/like aft CG.......what is good for you may suck for another......

What is your goal?
Straight high speed cruising, tight gusty thermaling, away from stall/spin, wet, dry, etc.....

I have no clue on a Discus, but others will want to know your "idea" of optimal......

[Charlie M. (UH & 002 owner/pilot)]

Sooooo.....your aft heavy bits drag the CG aft.....thus may be no need for aft water ballast normally.

I am a bit below "pilot weight" in my normal glass single seaters.
The owner ballasts to his weight, up to me to keep within CG limits.
Most of this is in ASW/ASG ships.

[[email protected]]

On Tuesday, 26 June 2018 09:29:27 UTC+12, Bruce Hoult wrote:
> On Monday, June 25, 2018 at 1:19:00 PM UTC-7, wrote:
> > I fly my 2B about 90% aft empty, but often don't add much tail water when ballasted. Often no tail water, especially in wave. Lead and a brass tail wheel helps with that.
> >
> > I've found the WB a little unusual though as you need to measure the arms. They are a little different to the manual. I wondered if the wheel is supposed to be jacked somehow during weighing. Does anyone have any ideas about that?
>
> Are you measuring with the tail on the ground? The glider should definitely be in something resembling flying attitude, though how that is defined for any particular glider is outside the scope of general W&B instructions. The instructions for any particular glider will be in its maintenance manual.

Of course it is weighed in the correct SH specified attitude.

[Dave Springford]

Thanks Bif, That is what I am looking for. My club just bought a 2b and I'm trying to sort out the weight and balance for it.

[Bruce Hoult]

On Monday, June 25, 2018 at 3:35:29 PM UTC-7, wrote:
> On Tuesday, 26 June 2018 09:29:27 UTC+12, Bruce Hoult wrote:
> > On Monday, June 25, 2018 at 1:19:00 PM UTC-7, wrote:
> > > I fly my 2B about 90% aft empty, but often don't add much tail water when ballasted. Often no tail water, especially in wave. Lead and a brass tail wheel helps with that.
> > >
> > > I've found the WB a little unusual though as you need to measure the arms. They are a little different to the manual. I wondered if the wheel is supposed to be jacked somehow during weighing. Does anyone have any ideas about that?
> >
> > Are you measuring with the tail on the ground? The glider should definitely be in something resembling flying attitude, though how that is defined for any particular glider is outside the scope of general W&B instructions.. The instructions for any particular glider will be in its maintenance manual.
>
> Of course it is weighed in the correct SH specified attitude.

Then I didn't understand your "I wondered if the wheel is supposed to be jacked somehow during weighing". What does it mean?

[[email protected]]

On Tuesday, 26 June 2018 11:22:10 UTC+12, Bruce Hoult wrote:
> On Monday, June 25, 2018 at 3:35:29 PM UTC-7, wrote:
> > On Tuesday, 26 June 2018 09:29:27 UTC+12, Bruce Hoult wrote:
> > > On Monday, June 25, 2018 at 1:19:00 PM UTC-7, wrote:
> > > > I fly my 2B about 90% aft empty, but often don't add much tail water when ballasted. Often no tail water, especially in wave. Lead and a brass tail wheel helps with that.
> > > >
> > > > I've found the WB a little unusual though as you need to measure the arms. They are a little different to the manual. I wondered if the wheel is supposed to be jacked somehow during weighing. Does anyone have any ideas about that?
> > >
> > > Are you measuring with the tail on the ground? The glider should definitely be in something resembling flying attitude, though how that is defined for any particular glider is outside the scope of general W&B instructions. The instructions for any particular glider will be in its maintenance manual.
> >
> > Of course it is weighed in the correct SH specified attitude.
>
> Then I didn't understand your "I wondered if the wheel is supposed to be jacked somehow during weighing". What does it mean?

It is sprung, so the question is whether the wheel measurement varies depending on the extension of the landing gear, state of the springs etc. The springs were also upgraded as part of the 2001 landing gear AD.


I haven't looked into it, but my wheel measurements are slightly different from the manual.


I have a recollection of someone suggesting the main wheel should be chocked to full extension. Whether that makes a difference ...

[Michael Opitz]

At 23:01 25 June 2018, Dave Springford wrote:
>Thanks Bif, That is what I am looking for. My club just bought a 2b
and
>I'm trying to sort out the weight and balance for it.
>
>
Placing a spare battery in the in the tail battery compartment (or
not) seems like something that might work for a club operation.
Lighter pilots fly with no tail battery, while heavier pilots fly with it
installed. (You will have to run the numbers for your specific bird,
as I have no idea what your empty weight is, whether it's had tail
boom repairs, or even if the original owner opted to order it with
a tail battery compartment installed, etc.)

When I first bought my D-2b, that's what Tilo told me to do, and
he suggested that I'd be pretty close to where I wanted to be with
a seat load of ~210#, plus tail battery installed. (I had asked him
to set it up specifically just for me W&B wise before delivery.)

It's a club. There will have to be compromises.

Not numerically detailed like Bif's answer, but it will be hard to nail
down his numbers if there are multiple pilots of different weights
that are going to fly it in a club environment.

RO

[Ben Coleman]

Wheel will be a bit different location if spring is different strength or deformed (and/or glider is different empty weight) but correct measurement of arms and moments makes it irrelevant.

Cheers Ben

[[email protected]]

On Tuesday, 26 June 2018 15:36:52 UTC+12, Ben Coleman wrote:
> Wheel will be a bit different location if spring is different strength or deformed (and/or glider is different empty weight) but correct measurement of arms and moments makes it irrelevant.
>
> Cheers Ben

Yes, that is our conclusion too. I have heard some people chock the landing gear extension somehow.

FWIW, we have measurements of 140 and 4285. The SH manual says 128 and 4278 (128+4150).

The 12mm makes a significant difference to the CG calculation - 90% aft vs 84% aft with the factory arms.

The lesson is to be a little careful of the factory arms, even in a glider with no damage history.

The 90% aft value ties in well with Bif's value above and it does feel good for thermalling, but you quickly start wishing for less aft weight when the trim is fully forward and you still need plenty of forward stick pressure when running.

[[email protected]]

On Tuesday, 26 June 2018 16:16:40 UTC+12, wrote:
> On Tuesday, 26 June 2018 15:36:52 UTC+12, Ben Coleman wrote:
> > Wheel will be a bit different location if spring is different strength or deformed (and/or glider is different empty weight) but correct measurement of arms and moments makes it irrelevant.
> >
> > Cheers Ben
>
> Yes, that is our conclusion too. I have heard some people chock the landing gear extension somehow.
>
> FWIW, we have measurements of 140 and 4285. The SH manual says 128 and 4278 (128+4150).
>
> The 12mm makes a significant difference to the CG calculation - 90% aft vs 84% aft with the factory arms.
>
> The lesson is to be a little careful of the factory arms, even in a glider with no damage history.
>
> The 90% aft value ties in well with Bif's value above and it does feel good for thermalling, but you quickly start wishing for less aft weight when the trim is fully forward and you still need plenty of forward stick pressure when running.

I should add that the MM uses the same arms for loaded and empty CG calculations.

[[email protected]]

When I picked up a Discus 2c from the factory in 2005 and asked for advice about the optimal C of G for performance I was told about 15% forward of the aft limit and that further after "not only is it less safe but the climb will suffer" - which surprised me at the time but I stuck to it over the years wet and dry. I guess the same will apply to the 2a and 2b as the wing profile is the same.

[Dan Marotta]

Not familiar with Discus W&B, but I'll bet the gear height has nothing
to do with it since the instructions in the maintenance manual probably
give instructions for leveling the ship prior to weighing.Â* Therefore,
if the main gear is taller, the tail will simply be blocked up higher
for the proper level.Â* At least that's the way it's been done for all of
my previous gliders, none of them were SH.

On 6/25/2018 5:50 PM, wrote:
> On Tuesday, 26 June 2018 11:22:10 UTC+12, Bruce Hoult wrote:
>> On Monday, June 25, 2018 at 3:35:29 PM UTC-7, wrote:
>>> On Tuesday, 26 June 2018 09:29:27 UTC+12, Bruce Hoult wrote:
>>>> On Monday, June 25, 2018 at 1:19:00 PM UTC-7, wrote:
>>>>> I fly my 2B about 90% aft empty, but often don't add much tail water when ballasted. Often no tail water, especially in wave. Lead and a brass tail wheel helps with that.
>>>>>
>>>>> I've found the WB a little unusual though as you need to measure the arms. They are a little different to the manual. I wondered if the wheel is supposed to be jacked somehow during weighing. Does anyone have any ideas about that?
>>>> Are you measuring with the tail on the ground? The glider should definitely be in something resembling flying attitude, though how that is defined for any particular glider is outside the scope of general W&B instructions. The instructions for any particular glider will be in its maintenance manual.
>>> Of course it is weighed in the correct SH specified attitude.
>> Then I didn't understand your "I wondered if the wheel is supposed to be jacked somehow during weighing". What does it mean?
> It is sprung, so the question is whether the wheel measurement varies depending on the extension of the landing gear, state of the springs etc. The springs were also upgraded as part of the 2001 landing gear AD.
>
>
> I haven't looked into it, but my wheel measurements are slightly different from the manual.
>
>
> I have a recollection of someone suggesting the main wheel should be chocked to full extension. Whether that makes a difference ...

--
Dan, 5J

[[email protected]]

The handbook should provide instructions for "leveling" (getting the correct angle) the fuselage by raising the tail and putting a bubble level on the tailcone with a ## by ## wedge. You can calculate the desired angle and use a bubble protractor instead, or one of the new electronic versions.

After weighing and measuring everything, you may hop in the cockpit and/or fill the ballast tanks to get a weighing at that configuration. You'll have to check the level again: even if the landing gear isn't sprung, the main wheel tire will deform more than the tail wheel tire (if you have one of those; life with a tail skid is mostly a hassle with this one exception!). Obviously this is at least a two-person job: one to strap into the cockpit and one to level the fuselage again by (almost certainly) lowering the tail slightly. Blocking the landing gear may prevent sagging on the springs but won't stop the tire from deforming slightly. No, I haven't done the math recently to figure out how much difference it makes but the last glider I did had a sprung landing gear so we just rechecked the fuselage angle anyway.

One thing you don't always think of: water sloshing around in the ballast tanks can shift the CG more than you think with slight changes in the angle of the fuselage. And if you have older flexible tanks, make sure they're slid down against the spar (hold the wing trailing edge down about 45 degrees during assembly and you'll hear the bags slide).

Or just don't worry about the ballast. The tanks will likely be forward of the likely CG anyway and should shift the CG in that direction. That assumes your CG is not already close to the forward limit, that is.

Chip Bearden

[Michael Opitz]

At 04:16 26 June 2018, wrote:
>On Tuesday, 26 June 2018 15:36:52 UTC+12, Ben Coleman
wrote:
>> Wheel will be a bit different location if spring is different strength
or
>deformed (and/or glider is different empty weight) but correct
measurement
>of arms and moments makes it irrelevant.
>>
>> Cheers Ben
>
>Yes, that is our conclusion too. I have heard some people chock
the landing
>gear extension somehow.
>
>FWIW, we have measurements of 140 and 4285. The SH manual
says 128 and 4278
>(128+4150).
>
>The 12mm makes a significant difference to the CG calculation -
90% aft vs
>84% aft with the factory arms.
>
>The lesson is to be a little careful of the factory arms, even in a
glider
>with no damage history.
>
>The 90% aft value ties in well with Bif's value above and it does
feel good
>for thermalling, but you quickly start wishing for less aft weight
when the
>trim is fully forward and you still need plenty of forward stick
pressure
>when running.
>
>
The factory set my D-2b up using the 4.4/100 aft fuselage blocking,
and the 128/4150 measurements. Mine has the 5" larger main
wheel option, as well as the tail wheel instead of the stock skid. If
these options have different arms from stock, S-H did not use them.
S-H set it up for a minimum seat load of ~190 Lbs (87 Kg) with
the empty C/G point at 681 mm (with an allowed range of 617 -
700 mm). At the time, I weighed ~210 Lbs with chute, so I was
about 20 Lbs above the minimum seat load by their calculations,
and that is where they thought it would be optimum for me to race
at. They did say that in their opinion, the D-2b did not like to be
too close to the aft limit in order to get the most out of it. I will
say that I used the handbook table to calculate the amount of tail
water to add when I filled the wing tanks. I figured that it would
put me close to the aft C/G limit. I flew it like that, and did not like
that aft C/G location at all.

RO

[[email protected]]

Hi Dave,

good practice is to trim a standard class glider at a speed of 120-130kph with elevator in neutral position without water ballast. This keeps elevator deflection low in the relevant speed ranges. It usually results in a mid-range to slightly aft C/G position at your take-off weight.

Such a setup can easily be achieved with a few test flights and the help of a folding rule, adding ballast as required. You will need to stay within allowed C/G ranges, of course.

I'm flying an LS8, but what I heard from Discus 2 pilots at various competitions is in line with what RO said. The D2 has a reputation of rather poor handling with aft C/G locations.

Best regards,
Christoph

[Dave Springford]

Thanks Michael and Christoph,

Good information.