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Discus 2 Optimal CG location



 
 
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  #11  
Old June 26th 18, 12:01 AM posted to rec.aviation.soaring
Dave Springford
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Default Discus 2 Optimal CG location

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.

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  #12  
Old June 26th 18, 12:22 AM posted to rec.aviation.soaring
Bruce Hoult
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Default Discus 2 Optimal CG location

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?
  #13  
Old June 26th 18, 12:50 AM posted to rec.aviation.soaring
[email protected]
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Default Discus 2 Optimal CG location

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 ...
  #14  
Old June 26th 18, 02:32 AM posted to rec.aviation.soaring
Michael Opitz
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Default Discus 2 Optimal CG location

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

  #15  
Old June 26th 18, 04:36 AM posted to rec.aviation.soaring
Ben Coleman
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Default Discus 2 Optimal CG location

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
  #16  
Old June 26th 18, 05:16 AM posted to rec.aviation.soaring
[email protected]
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Default Discus 2 Optimal CG location

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.

  #17  
Old June 26th 18, 05:19 AM posted to rec.aviation.soaring
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Default Discus 2 Optimal CG location

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.
  #18  
Old June 26th 18, 08:08 AM posted to rec.aviation.soaring
[email protected]
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Default Discus 2 Optimal CG location

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.
  #19  
Old June 26th 18, 04:16 PM posted to rec.aviation.soaring
Dan Marotta
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Default Discus 2 Optimal CG location

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.* Therefore,
if the main gear is taller, the tail will simply be blocked up higher
for the proper level.* 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
  #20  
Old June 26th 18, 04:40 PM posted to rec.aviation.soaring
[email protected]
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Posts: 451
Default Discus 2 Optimal CG location

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
 




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