Glider Handling Question

[Roy B.]

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

[Bruce Hoult]

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 :-)

[Roy B.]

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

[Tango Whisky]

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.

[firsys]

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

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

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.

[Roy B.]

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

[Steve Leonard[_2_]]

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

[Eric]

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

[[email protected]]

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

[WAVEGURU]

Has this change in handling actually been quantified, or is it just a perception?

Boggs

[Jonathon May[_2_]]

At 18:26 23 September 2016, Waveguru wrote:
>Has this change in handling actually been quantified, or is it just a
>perception?
>
>Boggs
>
That's fairly easy ,it's well documented that the spin characteristics is
effected
by the tail weight.
So
You can easily test that with a ship that has a fin tank ,full tank ,high
tow and
time 45degree left to right ,then test the ease of entering a spin.
Then drop the tail ballast and repeat.
With careful planning you could probably do it from 1 high tow.
It sounds like a good winter protect.

[Matt Herron Jr.]

I like the "crotch theory" best. We should develop special soaring pants with a "U" shaped stiffener to insure full aileron deflection is possible with the stick all the way back. This would also give us that cowboy swagger after landing...

[Scott Williams]

I agree with Steven L,
The differential forces required to roll a wing must be easily of the order of 5-20 kg, making a increased tail load of "5kg" influential.
after all total aerodynamic drag of most gliders in level flight at slow speeds is somewhere around 20 lbs isn't it?

Max weight divided by numerical L/D at best L/D speed?

I.E. std Cirrus 728 lbs divided by 36 = 20.2 lbs drag (no water)

interesting topic!!

[Paul Agnew]

https://sourceforge.net/projects/xflr5/

Who wants to download XFLR5 to test some hypotheses?

Random thoughts:

We tend to envision Center of Gravity as moving exactly along the design longitudinal axis and ignore any vertical component to the actual balance point of an aircraft. Just like larger helicopters have to calculate lateral CG position as well as longitudinal, perhaps we need to think about the actual CG in relation in three dimensions to explore whether CG affects roll rate (roll moment).

True CG location not only moves fore and aft, but up and down depending on where weight is added or moved. We've been taught that it isn't important. Imagine adding weight well above or below the longitudinal axis; let's say you installed an O2 tank or added nose weights. The CG's relation to the center of rotation would change and could affect the roll moment. How much, I can't say, but I'm trying to figure it out.

Paul A.
Jupiter, FL

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

From what I've seen on multiple gliders, nose weight is very close to the 3 dimensional CG, fixed tail weight is similar.
Water weight in the tail has a small part (maybe 35%?) below the total CG, thus changes the total CG a tiny bit although 2 dimensional CG is still valid.

Is it really worth it?, I don't know.

No, I'm not an engineer, but I have some reasonable background.

[[email protected]]

Hello, I'm the author of the article and I also don't know. I just referred the engineer's comment about the roll-rate I (roughly) measured during the test flight.

Nice to know someone's actually reading my articles ;-)

all the best,

Aldo Cernezzi




Il giorno mercoledì 14 settembre 2016 14:04:30 UTC+2, Roy B. ha scritto:
> 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

[[email protected]]

On Tuesday, September 27, 2016 at 8:14:18 AM UTC-6, wrote:
> Hello, I'm the author of the article and I also don't know. I just referred the engineer's comment about the roll-rate I (roughly) measured during the test flight.
>
> Nice to know someone's actually reading my articles ;-)
>
> all the best,
>
> Aldo Cernezzi
>
>
>
>
> Il giorno mercoledì 14 settembre 2016 14:04:30 UTC+2, Roy B. ha scritto:
> > 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

Aldo,
Keep the faith! I always read your articles. They are excellent and quite comprehensive, so please continue writing them. I am very sure that they are also read by glider pilots all over the world. We can all benefit by the knowledge and insight you provide.

Thanks again - Renny

Renny Rozzoni
Abuquerque, NM

[Roy B.]

On Saturday, September 24, 2016 at 6:31:36 PM UTC-4, Charlie M. (UH & 002 owner/pilot) wrote:
> From what I've seen on multiple gliders, nose weight is very close to the 3 dimensional CG, fixed tail weight is similar.
> Water weight in the tail has a small part (maybe 35%?) below the total CG, thus changes the total CG a tiny bit although 2 dimensional CG is still valid.
>
> Is it really worth it?, I don't know.
>
> No, I'm not an engineer, but I have some reasonable background.

Hello Aldo! Your articles are always the first things I read! Your aircraft reviews are fair and informative. ROY