Effect on CG of tail dolly left on in flight

Every aircraft is different, even within the same type, depending on actual (hopefully recent) weighing to determine weight and balance numbers. See pdf of a Grob 103 loading chart at '...noss.ws/ssc/Grob_WB.pdf' which shows the effect of an 18 pound (big) tail dolly. Basically, not a problem with two pilots, but if flown solo and the pilot is less than 177 pounds, then yes, the CG is aft of allowable limits.

NG

Ok - I will bite - My first glider was a first production run Std Cirrus
(#57) which has the all flying "will bite if provoked" elevator.

There are conflicting demands with such a design.
The pivot point has to be close to the aerodynamic neutral point, and spar.
Then for flutter reasons you want the CG to be forward of this
point.(Only a tiny bit, but it is there - remove the trim spring and the
elevator goes full up)

Now when you pull up, the nose of the elevator goes down and there is a
light resistance from the trim spring. The elevator may as well be on a
gimbal aerodynamically, there is no air load to neutralise the elevator
So now lets assume you are pulling some G. The harder you pull , the
larger the nose down rotational moment on the elevator. This is opposed
by a static spring. As the G increases, so the stick force needed to
hold a specific AOA on the elevator decreases.
Can be quite exciting if you are not prepared for it.

Did I mention that by definition you have no decalage in this design so
it will spin like a top if you ask it to.

Solution is - in order - to ensure the trim spring is good,stabilise
your forearm, keep adequate speed and use very light control inputs.
This will reward with accurate responses and no surprises. Get too slow
in turbulent air, particularly at low bank angles and the only question
is when, not if, it will depart. Close to stall angle, gust induces
inadvertent up elevator - which is quite powerful, and self augmenting
and there she goes. The wing has less than a degree of washout so when
it goes it drops a wing decisively. Unload the wing with the same
powerful elevator and she is flying again instantly, with hardly any
loss of altitude. If corrective action is not instantaneous though a
full spin will develop very rapidly.

Bruce


On 2012/09/04 7:31 PM, BobW wrote:
On 9/4/2012 10:13 AM, JohnDeRosa wrote:
I had been told, anecdotally, that having maximum aft CG based on PIC
weight is "best" for glider performance. I don't hope to understand the
aerodynamics of why this is so (but it would make an interesting read in
"Soaring").

It has to do with the theoretical minimization of trim drag, a
conceptually simple idea with "the usual" aerodynamically and
mathematically complex details!
- - - - - -

My point is that if a glider was at max aft CG, and then you add a tail
dolly, the glider could be out of CG and you could have a major
problem on
your hands. If at max aft CG the problem would be, I assume, the glider
pitching violently upwards as soon as sufficient speed for lift was
attained. Yes/no?

"No (though with "the usual caveats)," regarding the sentence
immeedjutly ahead of the question. Not necessarily "violently", and
maybe not at all, depending (on Joe Pilot, the plane, etc.).

The aft CG limit is not definitionally defined (by the FAA, anyway, so
far as I'm aware), though something called the "neutral point" IS. The
N.P. is definitionally the point at which the aerodynamic center (the
point through which the plane's total lift forces effectively act) and
the plane's CG coincide on the pitch axis. AT that point, the plane is
definitionally/mathematically neutrally stable. If the CG is aft of the
aerodynamic center, the plane is definitionally/mathematically -
*genuinely* complex stuff! - unstable. But what it actually DOES in the
real world isn't subject to "easy definition"...

Textbooks could be - and have been - written about this concept. Whole
college courses, in fact. Shoot - some mathematically gifted folks make
specialized *careers* in it! :-)

What the preceding means to a glider's designer, is that a decision
needs to be made about where the aft CG will be declared in the POH. I'd
bet Real Money most designers choose to place the aftmost allowable CG
"somewhat forward" of the aerodynamic center, for "perceived handling"
reasons. Over the years, the FAA has sometimes insisted on the FAA's
opinions being adhered to in "the handling regard". (The 1-35 springs
immediately to mind...)

Point being that handling doesn't "change stepwise" as the CG passes
some arbitrary point. Understand, the handling may not change
*linearly*. It's possible (for example) perceived larger (more difficult
for the pilot to easily/instinctively/intuitively deal with) handling
differences will occur per unit-of-movement of the CG position as the CG
nears the aerodynamic center. Though this isn't necessarily a given,
it's yet another reason to pay Real Test Pilots. :-)
- - - - - -


Would this event be compounded by having a CG hook?

Entirely possible, and arguably likely...
- - - - - -

Mitigated by a nose
hook (until release from tow)? Elevator authority? Trim setting?

Yup, yup, yup...and, of course, the devil is always in the details.
- - - - - -

Have we any St'd Cirrus drivers willing to share 0'beer thirty tales of
how their ship's all-flying-stabilator early models transition from
positively/neutrally stable in pitch to "something abbie-normal"? By
that I mean most of us are used to having to apply increasing back force
in order to generate increasing G-load, but I've been told (never flown
one) by more than one "reasonably technical" St'd Cirrus driver they've
experienced having to REDUCE aft stick force at some speeds/CG's in
order to NOT continue to generate increasing G-loads on
higher-speed-of-entry pullups. None of these pilots admitted to flying
with an out-of-aft-range CG.

Certainly, over the decades, the FAA has changed their opinions on what
handling standards must be met in order to obtain an Approved Type
Certificate. (F'r'example think powerplane spin criteria...)

Bob W.

--
Bruce Greeff
T59D #1771

On Tuesday, September 4, 2012 12:31:40 PM UTC-5, BobW wrote:
On 9/4/2012 10:13 AM, JohnDeRosa wrote:

I had been told, anecdotally, that having maximum aft CG based on PIC

weight is "best" for glider performance. I don't hope to understand the

aerodynamics of why this is so (but it would make an interesting read in

"Soaring").



It has to do with the theoretical minimization of trim drag, a conceptually

simple idea with "the usual" aerodynamically and mathematically complex details!

- - - - - -



My point is that if a glider was at max aft CG, and then you add a tail

dolly, the glider could be out of CG and you could have a major problem on

your hands. If at max aft CG the problem would be, I assume, the glider

pitching violently upwards as soon as sufficient speed for lift was

attained. Yes/no?



"No (though with "the usual caveats)," regarding the sentence immeedjutly

ahead of the question. Not necessarily "violently", and maybe not at all,

depending (on Joe Pilot, the plane, etc.).



The aft CG limit is not definitionally defined (by the FAA, anyway, so far as

I'm aware), though something called the "neutral point" IS. The N.P. is

definitionally the point at which the aerodynamic center (the point through

which the plane's total lift forces effectively act) and the plane's CG

coincide on the pitch axis. AT that point, the plane is

definitionally/mathematically neutrally stable. If the CG is aft of the

aerodynamic center, the plane is definitionally/mathematically - *genuinely*

complex stuff! - unstable. But what it actually DOES in the real world isn't

subject to "easy definition"...



Textbooks could be - and have been - written about this concept. Whole college

courses, in fact. Shoot - some mathematically gifted folks make specialized

*careers* in it! :-)



What the preceding means to a glider's designer, is that a decision needs to

be made about where the aft CG will be declared in the POH. I'd bet Real Money

most designers choose to place the aftmost allowable CG "somewhat forward" of

the aerodynamic center, for "perceived handling" reasons. Over the years, the

FAA has sometimes insisted on the FAA's opinions being adhered to in "the

handling regard". (The 1-35 springs immediately to mind...)



Point being that handling doesn't "change stepwise" as the CG passes some

arbitrary point. Understand, the handling may not change *linearly*. It's

possible (for example) perceived larger (more difficult for the pilot to

easily/instinctively/intuitively deal with) handling differences will occur

per unit-of-movement of the CG position as the CG nears the aerodynamic

center. Though this isn't necessarily a given, it's yet another reason to pay

Real Test Pilots. :-)

- - - - - -





Would this event be compounded by having a CG hook?



Entirely possible, and arguably likely...

- - - - - -



Mitigated by a nose

hook (until release from tow)? Elevator authority? Trim setting?



Yup, yup, yup...and, of course, the devil is always in the details.

- - - - - -



Have we any St'd Cirrus drivers willing to share 0'beer thirty tales of how

their ship's all-flying-stabilator early models transition from

positively/neutrally stable in pitch to "something abbie-normal"? By that I

mean most of us are used to having to apply increasing back force in order to

generate increasing G-load, but I've been told (never flown one) by more than

one "reasonably technical" St'd Cirrus driver they've experienced having to

REDUCE aft stick force at some speeds/CG's in order to NOT continue to

generate increasing G-loads on higher-speed-of-entry pullups. None of these

pilots admitted to flying with an out-of-aft-range CG.



Certainly, over the decades, the FAA has changed their opinions on what

handling standards must be met in order to obtain an Approved Type

Certificate. (F'r'example think powerplane spin criteria...)



Bob W.

Bob. Thanks for the lengthy reply. Very enlightening.

As to all flying tails. When first buying a glider, I was steered away from the early DG-100's for that reason (and a CG hook). DG rapidly changed the design to a standard tail design in the DG-101 and later.

In a series of safety talks in the UK, it was said that a Puchacz flown 2 up has a certain sensitivity of stick movement to pitch change, but when the instructor gets out and leaves say a slim girl pilot to go solo in it, still with CoG in limits but much further back, the sensitivity increase by a factor of 4. It is not unusual to see such a first solo pitching up and down a bit on the downwind leg of the pattern, as the pilot is getting used to a glider with a characteristic she has never flown before – and nor has her heavy instructor.

This is second hand, but I believe it is based on good science.

Chris N.

At 18:04 05 September 2012, Chris Nicholas wrote:
In a series of safety talks in the UK, it was said that a Puchacz flown 2
u=
p has a certain sensitivity of stick movement to pitch change, but when
the=
instructor gets out and leaves say a slim girl pilot to go solo in it,
sti=
ll with CoG in limits but much further back, the sensitivity increase by
a
=
factor of 4. It is not unusual to see such a first solo pitching up and
dow=
n a bit on the downwind leg of the pattern, as the pilot is getting used
to=
a glider with a characteristic she has never flown before =96 and nor
has
=
her heavy instructor.

This is second hand, but I believe it is based on good science.

Chris N.


The Puchaz has adjustable ballast for the front seat.
An instructor who gets out and sends a light pilot on first solo
without adjusting ballast to correct the CG change is delinquent.
Instructors who are qualified to send students solo should
be well aware of the potential change in pitch sensitivity
and adjust ballast accordingly.

John F

On Saturday, September 1, 2012 3:34:25 PM UTC-7, danlj wrote:
Exactly what effect on CG is caused by leaving the tail dolly on in flight?



I've seen many comments that this will "throw the CG off", risking safe control in a stall. But... I know gliders have successfully flown with the tail dolly in place. But I doubt that there's reason to panic, especially if one keeps the airspeed up.



And... some gliders are designed to carry tailfin ballast. For example, my Ventus can carry 2.0 L (kg), 4.4 lb of water in the tail. My tail dolly weighs 4.2 kg = 9.3 lb, only twice as much, and is a couple of feet forward of the tail ballast tank.



If I can lift the tail with the elevator during takeoff, I am going to be able to fly. So I can, to be safe in that circumstance, simply complete the takeoff, fly comfortably above stall speed, fly a normal pattern, and make a main-wheel landing to avoid the unknown control behavior close to stall and to avoid running the tailwheel caster down the runway at high speed, possibly shaking the thing to death.



Why not weigh one's dolly and do the weight-and-balance calculation? Then we will understand the consequences, if this happens to us, and will not panic.



Thanks for thinking about this.

Danl J

This whole thread demonstrates the value of sharing knowledge and experience. I am sure some believe that flying with a tail dolly will likely kill you, and as such may shout 'abort', or may panic if it happens to them. Knowing that this can most likely remedied by keeping the speed up is crucial. As for shouting 'abort' this should be limited to the beginning of takeoff roll, once the glider starting to get airborne it can no longer aborting the take off.

Ramy