Where is your CG with the tail dolly attached?

I recently used SeeG to find out what would happen to my CG if I took
off with the tail dolly still attached, and I thought I would share my
results.

I fly a Ventus C, and I wanted to know if I would still be within
normal operating limits and would have enough elevator authority to
keep from ballooning/stalling on take-off (or possibly on release from
tow). The tail dolly weighs about 9.5 lbs according to the bathroom
scale, and my tape measure showed the moment arm from the leading edge
to the estimated center of mass of the installed dolly to be about 145
inches.

SeeG ( http://www.glideplan.com/ ) shows my normal, dry center of
gravity to be 57% aft. I duplicated my glider in the program, then
added a moment arm called Tail Dolly. I input 9.5 lbs for the mass
and 145 inches for the arm and found that my CG shifted aft to 88% of
limits. This is a little over 30% change in CG position. OK in my
case, but it might be alarming for those that fly with the CG a little
farther aft. I ran the numbers with the airplane full of water and it
went from 58% aft to 81% aft, a little less of an impact as one might
expect from the overall increase in mass.

I would be curious to hear if others have done this and what the
numbers came out to.

Matt Jr.

On Sun, 17 Apr 2011 11:26:44 -0700, Matt Herron Jr. wrote:

I recently used SeeG to find out what would happen to my CG if I took
off with the tail dolly still attached, and I thought I would share my
results.

I fly a Ventus C, and I wanted to know if I would still be within normal
operating limits and would have enough elevator authority to keep from
ballooning/stalling on take-off (or possibly on release from tow). The
tail dolly weighs about 9.5 lbs according to the bathroom scale, and my
tape measure showed the moment arm from the leading edge to the
estimated center of mass of the installed dolly to be about 145 inches.

SeeG ( http://www.glideplan.com/ ) shows my normal, dry center of
gravity to be 57% aft. I duplicated my glider in the program, then
added a moment arm called Tail Dolly. I input 9.5 lbs for the mass and
145 inches for the arm and found that my CG shifted aft to 88% of
limits. This is a little over 30% change in CG position. OK in my
case, but it might be alarming for those that fly with the CG a little
farther aft. I ran the numbers with the airplane full of water and it
went from 58% aft to 81% aft, a little less of an impact as one might
expect from the overall increase in mass.

I would be curious to hear if others have done this and what the numbers
came out to.

Matt Jr.

Many years ago, my father was instructing at a beginners course for about
7 folks, running a midweek operation with the 'minimally' experienced
crew, a professional winch driver at the other end of the field and a
mornings worth of training for the course members on running a launch
point. Some way into the week, just after lunch, my dad was sat in the
back of a K7 with a student up front, the ground crew did the usual and a
launch was initiated, my dad felt something was wrong immediately, but
not wanting to worry the student just decided to fly this circuit
himself, he says the glider felt very 'light', and there was a trembling
that felt like mild turbulence coming through the elevator, he decided to
fly a very gently circuit and landed without issue...

Getting out of the glider he discovered a large tyre still sitting on the
horizontal stabiliser... he describes it as a truck tyre. ... nobody
dared to think about calculating where the C of G might have been., also
the circuit flys over the center of a busy local village, if the tyre had
'departed' there, things might have been bad on the ground.

Suffice to say the course got a very detailed run down of proper tie down
and launching procedures for the rest of the afternoon, and my dad got a
lesson in expecting the unexpected of the inexperienced.

Luck, and the forgiving nature of the venerable K-7 probably allows my
dad to tell a decent tale in the clubhouse in the evenings, it could have
been different.

On Apr 17, 11:26*am, "Matt Herron Jr." wrote:
I recently used SeeG to find out what would happen to my CG if I took
off with the tail dolly still attached, and I thought I would share my
results.

I fly a Ventus C, and I wanted to know if I would still be within
normal operating limits and would have enough elevator authority to
keep from ballooning/stalling on take-off (or possibly on release from
tow). *The tail dolly weighs about 9.5 lbs according to the bathroom
scale, and my tape measure showed the moment arm from the leading edge
to the estimated center of mass of the installed dolly to be about 145
inches.

SeeG (http://www.glideplan.com/) shows my normal, dry center of
gravity to be 57% aft. *I duplicated my glider in the program, then
added a moment arm called Tail Dolly. *I input 9.5 lbs for the mass
and 145 inches for the arm and found that my CG shifted aft to 88% of
limits. *This is a little over 30% change in CG position. *OK in my
case, but it might be alarming for those that fly with the CG a little
farther aft. *I ran the numbers with the airplane full of water and it
went from 58% aft to 81% aft, a little less of an impact as one might
expect from the overall increase in mass.

I would be curious to hear if others have done this and what the
numbers came out to.

Matt Jr.

Since my fully ballasted CG is at the aft limit of the envelope I know
for sure that the addition of the tail dolly will put it well aft of
the aft limit. However the ASW-28 aft limit is variable with flight
mass, probably to protect from the tail tank failing to dump, so that
says nothing about whether it would be controllable.

So the next theoretical question would be - if you know you were daft
enough to takeoff with the tail dolly on, does dumping ballast make it
worse or better? Does your CG move forward or aft if you dump ballast?
Should you dump just long enough to clear the tail tank but not the
wing tanks?

Easier to take it off before flying, and that's one thing anyone
hooking up a glider should check for.

Andy

On Apr 17, 11:26*am, "Matt Herron Jr." wrote:

SeeG (http://www.glideplan.com/) shows my normal, dry center of
gravity to be 57% aft...

What is that 57% relative to? Are we talking about % MAC, % of
approved CG range, or % of something else?

Thanks, Bob K.

On Apr 18, 10:34*am, Andy wrote:
On Apr 17, 11:26*am, "Matt Herron Jr." wrote:





I recently used SeeG to find out what would happen to my CG if I took
off with the tail dolly still attached, and I thought I would share my
results.

I fly a Ventus C, and I wanted to know if I would still be within
normal operating limits and would have enough elevator authority to
keep from ballooning/stalling on take-off (or possibly on release from
tow). *The tail dolly weighs about 9.5 lbs according to the bathroom
scale, and my tape measure showed the moment arm from the leading edge
to the estimated center of mass of the installed dolly to be about 145
inches.

SeeG (http://www.glideplan.com/) shows my normal, dry center of
gravity to be 57% aft. *I duplicated my glider in the program, then
added a moment arm called Tail Dolly. *I input 9.5 lbs for the mass
and 145 inches for the arm and found that my CG shifted aft to 88% of
limits. *This is a little over 30% change in CG position. *OK in my
case, but it might be alarming for those that fly with the CG a little
farther aft. *I ran the numbers with the airplane full of water and it
went from 58% aft to 81% aft, a little less of an impact as one might
expect from the overall increase in mass.

I would be curious to hear if others have done this and what the
numbers came out to.

Matt Jr.

Since my fully ballasted CG is at the aft limit of the envelope I know
for sure that the addition of the tail dolly will put it well aft of
the aft limit. *However the ASW-28 aft limit is variable with flight
mass, probably to protect from the tail tank failing to dump, *so that
says nothing about whether it would be controllable.

So the next theoretical question would be - if you know you were daft
enough to takeoff with the tail dolly on, does dumping ballast make it
worse or better? Does your CG move forward or aft if you dump ballast?
Should you dump just long enough to clear the tail tank but not the
wing tanks?

Easier to take it off before flying, and that's one thing anyone
hooking up a glider should check for.

Andy

My CG doesn't move more than 1-3% with water because I have a tail
tank. But the increased mass of the glider when wet means that the
dolly will have less of an effect on the CG. When wet, the dolly
moves the CG back to 81%. When dry, the CG moves back to 88%.

Matt

On Apr 18, 3:30*pm, Bob Kuykendall wrote:
On Apr 17, 11:26*am, "Matt Herron Jr." wrote:

SeeG (http://www.glideplan.com/) shows my normal, dry center of
gravity to be 57% aft...

What is that 57% relative to? Are we talking about % MAC, % of
approved CG range, or % of something else?

Thanks, Bob K.

approved CG range

On Apr 17, 11:26*am, "Matt Herron Jr." wrote:
I recently used SeeG to find out what would happen to my CG if I took
off with the tail dolly still attached, and I thought I would share my
results.

I fly a Ventus C, and I wanted to know if I would still be within
normal operating limits and would have enough elevator authority to
keep from ballooning/stalling on take-off (or possibly on release from
tow). *The tail dolly weighs about 9.5 lbs according to the bathroom
scale, and my tape measure showed the moment arm from the leading edge
to the estimated center of mass of the installed dolly to be about 145
inches.

SeeG (http://www.glideplan.com/) shows my normal, dry center of
gravity to be 57% aft. *I duplicated my glider in the program, then
added a moment arm called Tail Dolly. *I input 9.5 lbs for the mass
and 145 inches for the arm and found that my CG shifted aft to 88% of
limits. *This is a little over 30% change in CG position. *OK in my
case, but it might be alarming for those that fly with the CG a little
farther aft. *I ran the numbers with the airplane full of water and it
went from 58% aft to 81% aft, a little less of an impact as one might
expect from the overall increase in mass.

I would be curious to hear if others have done this and what the
numbers came out to.

Matt Jr.

Mine would be well within CG limits with ME in it, But first, I'd
have to duct tape it on, because it's one of those funny Glob things
that is just a wheel on a post. It would be very slightly aft of the
allowable range with a 152 pound pilot in it.

On Apr 19, 8:08*pm, "Matt Herron Jr." wrote:
approved CG range

That would not seem to be a useful description for any glider where
the CG limits are flight mass dependent. Some people have been
surprised to find that is true for the glider they fly.

Andy