Water explanation, please.....

Discussion related to the Condor...

Moderators: Uros, OXO, GR

TimKuijpers
Posts: 3202
Joined: Sun Apr 24, 2005 12:33 am
Location: The Netherlands
Contact:

Postby TimKuijpers » Wed Nov 12, 2008 1:56 pm

I'll explain the theory of C/G, and how this not works in Condor.

If you fly 180km/h in std class, you will need to push quite hard.
This means you have a lot of lift, and therefor drag, on your horizontal stabilizer.
If you put the center of gravity forward, you will need less lift on your stabilizer, as compared to the first situation, there is a little nose down moment by the weight.
Therefor you will have less drag, and you will perform better in that situation.

But then again, during slow flying it is completely opposite, you will have less drag when you have the C/G a bit to the tail.
Either way you have to find a compromis between thermalling and gliding performance.


But in condor (I was told) that C/G changes does nothing else but change the stick/trim range.
If you put the C/G back, you will be able to pull the stick further, back, and less forward.
And performance wise it will not matter in condor.
Think positive, flaps negative.

User avatar
OXO
Condor Team
Posts: 4536
Joined: Mon Feb 13, 2006 2:08 am

Postby OXO » Wed Nov 12, 2008 2:03 pm

Tim, nearly right, but you forgot that the tail generates lift in the downwards direction, so you need to move the cg backward for less drag!
OXO,
Condor Team
Official facebook page

TimKuijpers
Posts: 3202
Joined: Sun Apr 24, 2005 12:33 am
Location: The Netherlands
Contact:

Postby TimKuijpers » Wed Nov 12, 2008 3:13 pm

And what if you fly very fast Chris?
Think positive, flaps negative.

Bulau
Posts: 539
Joined: Wed May 28, 2008 12:23 am
Location: Ontario, Canada
Contact:

Postby Bulau » Wed Nov 12, 2008 3:18 pm

Fiddeflygare wrote:...i dont see how you would get less drag with the trim in neutral position? On a real life high performance glider and even most older gliders of glass/carbonfiber construction the trim only affects the stick as for example in my clubs LS8. This means that you would get the same drag having trimmed the aircraft to the speed you want as if you have the trim in neutral and push the stick forward in order to maintain your desired speed. Since you don´t have a trim rudder on the elevators.

I'm with you on this Fredrik. Even in the case of a trim tab, all the tab is doing is deflecting the control surface. My understanding of trim is that it is there for pilot comfort. Yet, in the other flight sim I use, IL-2, there is no shortage of advice to the tune that being "in trim" reduces drag.

How is a bungee or spring pushing the stick forward any different than the pilot pushing the stick forward? How is a trim tab deflecting a control surface any different than the pilot moving the stick and deflecting the same control surface?
Image
Image

TimKuijpers
Posts: 3202
Joined: Sun Apr 24, 2005 12:33 am
Location: The Netherlands
Contact:

Postby TimKuijpers » Wed Nov 12, 2008 3:23 pm

If you change the center of gravity, and therefor in thermals have to pull less.
You are obviously producing less lift, and thus less drag.

What is it that you don't get, the first, or the 2nd line?
Think positive, flaps negative.

User avatar
OXO
Condor Team
Posts: 4536
Joined: Mon Feb 13, 2006 2:08 am

Postby OXO » Wed Nov 12, 2008 3:24 pm

TimKuijpers wrote:And what if you fly very fast Chris?


more -ve lift, more drag.. why?
OXO,
Condor Team
Official facebook page

Wiley
Posts: 173
Joined: Thu Jul 03, 2008 12:40 am
Location: Georgia, USA

Postby Wiley » Wed Nov 12, 2008 3:50 pm

I guess I should have prefaced my remark by saying that my observations apply only to Condor and not to RL. As my RL experience is nil.

One of the things that made me adjust the CG was that in online play when I went to autopilot the plane would pitch significantly indicating to me that there was a disparity between the center of lift on the wings and the elevator. The lift surfaces were not working together. With the use of the CG slider my trim setting is now close to zero as opposed to greater than 50%. And now when I go on auto the plane makes only a small pitch movement, if any.

It also seemed obvious to me that if things are not working together there is a penalty. In this case increased drag (decreased lift, speed, etc.) and at my level of expertise a tenth of a meter in lift or speed may just get me to the finish.
Image

TimKuijpers
Posts: 3202
Joined: Sun Apr 24, 2005 12:33 am
Location: The Netherlands
Contact:

Postby TimKuijpers » Wed Nov 12, 2008 4:14 pm

Ok, you got me there.
But it took me a trip to the windtunnel and it's proffessors to convince me. ;)
Knowing this I think, that in RL I have flown with a C/G too far behind.
As there were moments where dumping tailwater increased the performance tremendously.
But that also might have to do with the trimsprings, and mental aspects.
Think positive, flaps negative.

User avatar
cruiser
Posts: 1538
Joined: Mon Apr 04, 2005 9:55 pm
Location: Slovenia
Contact:

Postby cruiser » Wed Nov 12, 2008 5:07 pm

Moving CG back too much, makes the glider easier to spin and its handling twitchy. Having it too much in front, makes the glider unresponsive in thermals and worse climb performance.

I'm using a brass rear wheel to shift the cg rearward (rendering my mosquito's minimum seat weight to 90kg).

User avatar
OXO
Condor Team
Posts: 4536
Joined: Mon Feb 13, 2006 2:08 am

Postby OXO » Wed Nov 12, 2008 6:37 pm

TimKuijpers wrote:Ok, you got me there.


Go OXo, Go OXO :D :D
OXO,
Condor Team
Official facebook page

mumbles
Posts: 3
Joined: Wed Apr 05, 2006 10:57 pm
Location: Lynchburg, Virginia, United States
Contact:

Postby mumbles » Wed Nov 12, 2008 11:12 pm

The University of North Dakota has some good software trainers that explain the effect of CG on lift. It is designed for powered flight but the basics of course transfer.

http://undaerospace.com/software_trainers.asp

You have to register but it is free. Find in the aerodynamics section the Weight/CG Effects trainer.

In commercial and charter operations, operators intentionally load to the aft limits because it is more efficient for the aircraft and they get better range and fuel efficiency. But it does reduce stability.

P.S. This is my first post in the forums. Hope it is helpful and productive.
Joshua Clark
M41
N9223S
"Mumbles"

User avatar
OXO
Condor Team
Posts: 4536
Joined: Mon Feb 13, 2006 2:08 am

Postby OXO » Wed Nov 12, 2008 11:43 pm

Brilliant first post actually :)

You should see some of the junk we have to read :D :D :D
OXO,
Condor Team
Official facebook page

User avatar
Alpha Whisky
Posts: 244
Joined: Sat Jun 28, 2008 7:48 pm
Location: Scotland

Postby Alpha Whisky » Thu Nov 13, 2008 2:21 pm

OXO wrote:You should see some of the junk we have to read :D :D :D

I resemble that remark :lol:

Great link: http://www.dg-flugzeugbau.de/schwerpunkt-e.html

Don't know how well it's modelled in Condor but it explains the issues surrounding CofG in real life gliding.
ImageIf flying were the language of man, soaring would be its poetry.....and the PW5 would be William Topaz McGonagall.

User avatar
OXO
Condor Team
Posts: 4536
Joined: Mon Feb 13, 2006 2:08 am

Postby OXO » Thu Nov 13, 2008 2:26 pm

As Tim already said, only the handling effect is modelled.
OXO,
Condor Team
Official facebook page

User avatar
Alpha Whisky
Posts: 244
Joined: Sat Jun 28, 2008 7:48 pm
Location: Scotland

Postby Alpha Whisky » Thu Nov 13, 2008 2:51 pm

That's odd, because the Condor manual says

C/G bias: Here you specify the relative position of your plane's center of gravity (C of G). The influence of this setting on performance is very small. The more important effect is glider handling.


You would think if it wasn't modelled at all, the manual would say "This setting has no influence on performance".
ImageIf flying were the language of man, soaring would be its poetry.....and the PW5 would be William Topaz McGonagall.


Return to “General”

Who is online

Users browsing this forum: No registered users and 9 guests