Glider Handling Question

In this month's Gliding International article on the Ventus 3, Aldo Cernezzi writes, "According to Berd Weber, I flew with a relatively forward CoG, which may have an impact on roll speed". This got me to thinking of my own anecdotal experiences with very forward CGs having a negative impact on roll rate, and I think that a forward CG does degrade the roll rate. But I can't figure out why that happens. Can anybody explain it in simple terms?
ROY

On Wednesday, September 14, 2016 at 3:04:30 PM UTC+3, Roy B. wrote:
In this month's Gliding International article on the Ventus 3, Aldo Cernezzi writes, "According to Berd Weber, I flew with a relatively forward CoG, which may have an impact on roll speed". This got me to thinking of my own anecdotal experiences with very forward CGs having a negative impact on roll rate, and I think that a forward CG does degrade the roll rate. But I can't figure out why that happens. Can anybody explain it in simple terms?
ROY

I don't know, but I've found that in some gliders lacking in roll rate (in particular the original Janus) it seemed to help to unload the wings. e.g. pull up a bit, then push forward to reduced G and roll at the same time.

This also ties in conveniently with reducing from cruise speed to thermaling speed :-)

On Wednesday, September 14, 2016 at 8:24:00 AM UTC-4, Bruce Hoult wrote:
On Wednesday, September 14, 2016 at 3:04:30 PM UTC+3, Roy B. wrote:
In this month's Gliding International article on the Ventus 3, Aldo Cernezzi writes, "According to Berd Weber, I flew with a relatively forward CoG, which may have an impact on roll speed". This got me to thinking of my own anecdotal experiences with very forward CGs having a negative impact on roll rate, and I think that a forward CG does degrade the roll rate. But I can't figure out why that happens. Can anybody explain it in simple terms?
ROY

I don't know, but I've found that in some gliders lacking in roll rate (in particular the original Janus) it seemed to help to unload the wings. e.g.. pull up a bit, then push forward to reduced G and roll at the same time.

This also ties in conveniently with reducing from cruise speed to thermaling speed :-)

Bruce: I understand that. Unloading the wings reduces the roll moment of inertia (think of why a tightrope walker uses a heavy long bar instead of a short light one) and also impacts the wing torque problem that happens when (on a long span glider) the wing twists against the aileron deflection. But I can't understand why forward CG impacts roll rate at all - yet I am sure that it does. ROY

Le mercredi 14 septembre 2016 15:08:15 UTC+2, Roy B. a écritÂ*:

Bruce: I understand that. Unloading the wings reduces the roll moment of inertia

No it doesn't. Moment of inertia is not related to airflow or gravitation.

On Wednesday, September 14, 2016 at 8:04:30 AM UTC-4, Roy B. wrote:
In this month's Gliding International article on the Ventus 3, Aldo Cernezzi writes, "According to Berd Weber, I flew with a relatively forward CoG, which may have an impact on roll speed". This got me to thinking of my own anecdotal experiences with very forward CGs having a negative impact on roll rate, and I think that a forward CG does degrade the roll rate. But I can't figure out why that happens. Can anybody explain it in simple terms?
ROY

A famous scientist once said" to understand a phenomenom properly,
one must make measurements"
Apropos in this case; I hazard a guess that moving the CG aft has
reduced the circling A o A; this increases aileron effectiveness.
We need a careful experiment.

JMF

I will say, from various model experience, an aft CG can quicken roll rate, but at times, faster than the pilot can respond in variable/ gusty situations.

Why?, not quite sure.

I will let others give a reasonable explanation on this.

Worst case for me, swapping from a heavy pilot to a light pilot (me), CG was BEHIND the aft limit for me.......by "quite a bit".......,, Owner/towpilot commented, "spin much?!!!!"....... As I spun off into thermal pulls.

Yes, got my attention.
Yes, I was a bit above ground.
No, I didn't hurt anything.

On Wednesday, September 14, 2016 at 8:04:30 AM UTC-4, Roy B. wrote:
In this month's Gliding International article on the Ventus 3, Aldo Cernezzi writes, "According to Berd Weber, I flew with a relatively forward CoG, which may have an impact on roll speed". This got me to thinking of my own anecdotal experiences with very forward CGs having a negative impact on roll rate, and I think that a forward CG does degrade the roll rate. But I can't figure out why that happens. Can anybody explain it in simple terms?
ROY

Well, no answer to my initial question which means 1) nobody knows or 2) nobody cares. Or, could be, both. So, I'll advance my own theory: A change in CG also changes and relocates the center of lift (CL). As CL moves forward on the wing away from the ailerons, they become less responsive. As it moves back toward the ailerons they are more responsive. Any other ideas?

ROY

How about this?

With more forward CG, for the same flight weight and speed, the stick is further back, causing it to be closer to your crotch, so you can deflect the stick less to the sides, thus adversely affecting roll rate.

Don't like that? Maybe this.

With more forward CG, for the same flight speed, the stick is further back. Creates more downforce by the tail, which means the wing has to make more lift and operate at higher angle of attack for the same flight speed (total upforce require by the wing is increased, even though weight is the same, and steady, 1 g gliding flight because the tail is pushing down harder if the CG is more forward). At the higher angle of attack, there is a shift in the differential lift created by deflecting the ailerons because the wing itself is operating at a higher angle of attack. Adverse yaw is increased, and overall roll rate diminishes slightly.

Unloading the wing by pushing the stick forward greatly reduces adverse yaw, so the net result is you can get a higher roll rate with a sailplane if the wing is unloaded and you are flying slow. At least, that last part has been my experience with my Nimbus 3.

Steve Leonard

Can't see the higher wing lift being the issue. The increase in tail
download is only going to be of the order of 5kg so about 1% of extra
wing lift required. That will equate to a tiny increase in AOA.

On Thu, 22 Sep 2016 12:54:50 -0700 (PDT), Steve Leonard
wrote:

How about this?

With more forward CG, for the same flight weight and speed, the stick is further back, causing it to be closer to your crotch, so you can deflect the stick less to the sides, thus adversely affecting roll rate.

Don't like that? Maybe this.

With more forward CG, for the same flight speed, the stick is further back. Creates more downforce by the tail, which means the wing has to make more lift and operate at higher angle of attack for the same flight speed (total upforce require by the wing is increased, even though weight is the same, and steady, 1 g gliding flight because the tail is pushing down harder if the CG is more forward). At the higher angle of attack, there is a shift in the differential lift created by deflecting the ailerons because the wing itself is operating at a higher angle of attack. Adverse yaw is increased, and overall roll rate diminishes slightly.

Unloading the wing by pushing the stick forward greatly reduces adverse yaw, so the net result is you can get a higher roll rate with a sailplane if the wing is unloaded and you are flying slow. At least, that last part has been my experience with my Nimbus 3.

Steve Leonard

On Wednesday, September 14, 2016 at 8:04:30 AM UTC-4, Roy B. wrote:
In this month's Gliding International article on the Ventus 3, Aldo Cernezzi writes, "According to Berd Weber, I flew with a relatively forward CoG, which may have an impact on roll speed". This got me to thinking of my own anecdotal experiences with very forward CGs having a negative impact on roll rate, and I think that a forward CG does degrade the roll rate. But I can't figure out why that happens. Can anybody explain it in simple terms?
ROY

Faster control response in pitch and yaw is due to reduced stability that comes with aft CG.
Roll stability does not change with CG.
One possible affect of aft CG would be quicker yaw response and yaw/roll coupling due to dihedral in the wing increasing the roll rate. Gliders with lots of outer dihedral would be most affected by this.
Just one theory.
UH