How efficient are our tailplanes?

Hi folks,

In order to get the best performance from the gliders
we fly we need the C of G near the aft limit.

How much difference does this actually make?

Or is it just perception ?

For example, If I add 60kgs of ballast to my Discus,
I should put 2kgs in the fin tank. How much drag penalty
will I incur by not doing so? Do I lose more by adding
tailballast & sacrificing stability than I actually
gain in reduced drag?

(Hope this thread doesn't go on as long as the ballasted
pull-up one :-))

Kevin

Hi kevin,
No numbers, just some observations. After his first flight in a Duo Discus, a
friend of mine said,
"This thing won't climb or cruise".

To which, I asked, "How much water do you have in the tail ?"

He replied, "Nothing"

To which, I said, "Fill it up"

After his second flight in the Duo Discus (with water in the tail) My friend
said, " This is a damned GOOD machine.

End of my observstions concerning tail ballast in a ship with 2 pilots of 200+
lbs. each
:)


JJ Sinclair

Hi kevin,
No numbers, just some observations. After his first flight in a Duo Discus, a

One of our motorglider pilots pulled many pounds out
of the glider nose and liked the results.

When I flew my 172 for 400+ hours in 2 years, I noticed
that certain C.G.s were better. All the way aft legal
was too much, I think because the forward trim was so
much that the angle of elevator to trim created a lot
of drag. But 3/4 aft worked VERY well. It was
amazing how much faster it cruised and how much easier
it was to to trim and to flare with 3/4 aft CG.

I flew a Twin Commander with a forward CG problem.
I didn't notice until I got to fly a different Commander,
and the controls were light. It seems this one still
had the old radios left in as ballast in the aft.
It was a totally different feel.

Beware of old weight and balance sheets. The actual W&B
may be quite different but the owner may be reluctant
to reweigh it, since many aircraft lose quite a lot of
useful load when officially reweighed.

On 23 Oct 2003 11:48:25 GMT, Kevin Neave
wrote:

Hi folks,

In order to get the best performance from the gliders
we fly we need the C of G near the aft limit.


Well. maybe.

How much difference does this actually make?

As Robert Ehrlich pointed out in a earlier post at some point in the
normal operating range of speeds you probably want to organise zero
lift on the tail. Moving the C of G will move this point.

Or is it just perception ?

For example, If I add 60kgs of ballast to my Discus,
I should put 2kgs in the fin tank. How much drag penalty
will I incur by not doing so? Do I lose more by adding
tailballast & sacrificing stability than I actually
gain in reduced drag?

It is possible to calculate the induced drag produced by the tail
giving you 2 Kg of lift in either direction.
I'll let someone else do that this time.

You don't want to sacrifice so much stability that maintaining accrate
attitude becomes difficult or takes too much concentration.

I wasn't a believer in tail tanks until I flew an ASW20B with one.

Our BD4 goes noticeably faster with someone in the backseat or heaps
of baggage there and doesn't become unstable.

Mike Borgelt

Derrick Steed wrote:

Recommendation for keeping the discus sion short:
1. don't put ballast in wings

I never do, since the 2 times I tried it, I had to
dump it during the 5 first minutes of flight in order
to stay aloft and avoid landing back.

2. put as much ballast as possible in fin tank

I would never do that, since I am at the lower weight
limit (62.5 kg (myself) + 7 kg (parachute) + .5 kg (various
things) = 70 kg)

3. fly

As often as I can

At 09:30 24 October 2003, Robert Ehrlich wrote:
Derrick Steed wrote:

Recommendation for keeping the discus sion short:
1. don't put ballast in wings

I never do, since the 2 times I tried it, I had to
dump it during the 5 first minutes of flight in order
to stay aloft and avoid landing back.

2. put as much ballast as possible in fin tank

I would never do that, since I am at the lower weight
limit (62.5 kg (myself) + 7 kg (parachute) + .5 kg
(various
things) = 70 kg)

3. fly

As often as I can



I think (hope?) Derrick was joking ;-)

With a full fin & no ballast I'd be 60kg or so below
min cockpit weight!!

this has been referenced before from the DG web site,
but it's not long so I'll post it here..



The Optimum C/G of Sailplanes

A Caution from Wilhelm Dirks

In the 'classic' aerodynamic theory of airplanes the
wings generate lift and the tail plane generates stability.
Because curved wing profiles are used, the aerodynamic
moment generated by the wings, which tries to push
the nose down and has to be countered by the tail plane.


For this the tail plane has to produce downward force
dependent on the airspeed and C/G. The higher the airspeed
and the more forward the C/G, a higher downward force
is produced. In a rearward C/G the tail plane can even
produce a lifting force.

Most contest pilots trim their sailplanes to the most
rearward permissible position. In theory this improves
performance, especially in circling flight one does
not have to 'pull' on the stick as much. At the 'German
Soaring Symposium' in Stuttgart a paper was presented
and discussed which showed these well known facts in
graphic detail for several sailplane types.

In the first phase it was researched just how much
the performance was affected by forward or rearward
C/G positions. The permissible C/G values are determined
by the designer. A forward C/G determines the size
of the tail plane and elevator, i.e. that sufficient
force is available in the elevator to make circling
flight possible. The aft C/G is determined so that
the airplane has satisfactory pitch stability and is
able recover from a spin. Generally the calculations
for all types showed a performance difference of 1.5
to 2 points between the foremost and rear most C/G
positions. That is a significant difference! Is it
therefore correct to fly with the rearmost C/G? Let's
think about it.


Performance Factors
The L/D of a sailplane is calculated by the ratio of
total lift to total drag. If the tail plane produces
downward force the wings have to generate more lift,
and that causes increased induced and profile drag,
reducing the L/D. In spite of this the rearmost C/G
does not necessarily produce better results. The tail
plane is not designed to produce much lift. It normally
has an almost symmetrical profile.
The wing profile is designed to produce lift and is
much better at this task. In addition the tail plane
produces a disproportionate amount of induced drag
because of it's low aspect ratio. The optimum condition
would be one in which the tail plane in slow flight
produces just enough lift to compensate for the loss
of lift of the wings in the fuselage area. That would
minimize the induced drag of the sailplane.
This condition is obtained, more or less, depending
on the design, by the distance of the tail plane from
the most rearward C/G position.


Flight Characteristics and Safety
At aft C/G stability is minimal, especially in the
roll axis, and the sailplane must be 'flown' at all
times to avoid air speed variations usually encountered
when thermaling. Depending on experience and skill,
the pilot may tire faster and his concentration can
diminish, so that the theoretical advantage is greatly
reduced. At the rearmost C/G the sailplane will enter
a spin much easier at less than the minimum airspeed
then at forward C/G, where spinning in many instances
is not even possible. This can have deadly results,
especially close to the ground. Different sailplanes
react differently, but the tendency is clear. Even
an experienced contest pilot should be very wary of
choosing the extreme rear most C/G position.


At 10:06 24 October 2003, Kevin Neave wrote:
At 09:30 24 October 2003, Robert Ehrlich wrote:
Derrick Steed wrote:

Recommendation for keeping the discus sion short:
1. don't put ballast in wings

I never do, since the 2 times I tried it, I had to
dump it during the 5 first minutes of flight in order
to stay aloft and avoid landing back.

2. put as much ballast as possible in fin tank

I would never do that, since I am at the lower weight
limit (62.5 kg (myself) + 7 kg (parachute) + .5 kg
(various
things) = 70 kg)

3. fly

As often as I can



I think (hope?) Derrick was joking ;-)

With a full fin & no ballast I'd be 60kg or so below
min cockpit weight!!

Just to tag on the ending of the DG article:

'The C/G definitely influences the flight performance
and efficiency.
A position in the forward half of the C/G envelope
produces negative results and should be avoided.
A good compromise is a C/G position about 30 to 40
% forward of the aftmost position.
Flying with the C/G at the rearmost position is endangering
your life.
Check your C/G and do a weight/balance of your sailplane,
and weigh yourself.'

John Galloway


At 13:18 24 October 2003, Mark Stevens wrote:
this has been referenced before from the DG web site,
but it's not long so I'll post it here..



The Optimum C/G of Sailplanes

A Caution from Wilhelm Dirks

In the 'classic' aerodynamic theory of airplanes the
wings generate lift and the tail plane generates stability.
Because curved wing profiles are used, the aerodynamic
moment generated by the wings, which tries to push
the nose down and has to be countered by the tail plane.


For this the tail plane has to produce downward force
dependent on the airspeed and C/G. The higher the airspeed
and the more forward the C/G, a higher downward force
is produced. In a rearward C/G the tail plane can even
produce a lifting force.

Most contest pilots trim their sailplanes to the most
rearward permissible position. In theory this improves
performance, especially in circling flight one does
not have to 'pull' on the stick as much. At the 'German
Soaring Symposium' in Stuttgart a paper was presented
and discussed which showed these well known facts in
graphic detail for several sailplane types.

In the first phase it was researched just how much
the performance was affected by forward or rearward
C/G positions. The permissible C/G values are determined
by the designer. A forward C/G determines the size
of the tail plane and elevator, i.e. that sufficient
force is available in the elevator to make circling
flight possible. The aft C/G is determined so that
the airplane has satisfactory pitch stability and is
able recover from a spin. Generally the calculations
for all types showed a performance difference of 1.5
to 2 points between the foremost and rear most C/G
positions. That is a significant difference! Is it
therefore correct to fly with the rearmost C/G? Let's
think about it.


Performance Factors
The L/D of a sailplane is calculated by the ratio of
total lift to total drag. If the tail plane produces
downward force the wings have to generate more lift,
and that causes increased induced and profile drag,
reducing the L/D. In spite of this the rearmost C/G
does not necessarily produce better results. The tail
plane is not designed to produce much lift. It normally
has an almost symmetrical profile.
The wing profile is designed to produce lift and is
much better at this task. In addition the tail plane
produces a disproportionate amount of induced drag
because of it's low aspect ratio. The optimum condition
would be one in which the tail plane in slow flight
produces just enough lift to compensate for the loss
of lift of the wings in the fuselage area. That would
minimize the induced drag of the sailplane.
This condition is obtained, more or less, depending
on the design, by the distance of the tail plane from
the most rearward C/G position.


Flight Characteristics and Safety
At aft C/G stability is minimal, especially in the
roll axis, and the sailplane must be 'flown' at all
times to avoid air speed variations usually encountered
when thermaling. Depending on experience and skill,
the pilot may tire faster and his concentration can
diminish, so that the theoretical advantage is greatly
reduced. At the rearmost C/G the sailplane will enter
a spin much easier at less than the minimum airspeed
then at forward C/G, where spinning in many instances
is not even possible. This can have deadly results,
especially close to the ground. Different sailplanes
react differently, but the tendency is clear. Even
an experienced contest pilot should be very wary of
choosing the extreme rear most C/G position.

Would anyone out there like to suggest a figure for
how much induced drag is generated by the tailplane
of a typical glider when producing 10kgs of download?

And to reference the next article from the DG website
which rpeorts on actual Akaflieg flight testing:


'Add on after the soaring symposium Stuttgart 2000
and Braunschweig 2001:

Members of the Akaflieg Braunschweig have once again
focused on achieving an optimal center of gravity.
They have therefore completed a number of test flights
with two ASH-25 which had been carefully inspected,
set up and checked. Both planes were equipped with
highly complex measuring equipment due to the expected
marginal rate of differences between the two planes.
One was declared to be the reference plane, the other
was designed to allow altering the center of gravity
before every flight.

The test flights have been completed with various speeds
and flap settings. The results were later evaluated
by computer. The specific results are of no real interest
for us. A common pilot flies at various speeds with
a variety of flap settings during a normal long duration
flight. What was of interest to us was the average
result of the test flights. And even without the complete
set of results one thing can be noted:

The aft position of the center of gravity is not desirable
in order to achieve a really good result. The best
results were achieved about 30 - 35 % in front of the
aft limit of of the specifications for the center of
gravity. That is caused by the elevator since it is
not in a neutral (streamlined) position with a centered
C/G. Instead it has to produce lift (an up elevator
deflection) which it's not actually designed for resulting
in increased drag.

Some of our customers who want participate in competitions
ask us to carefully weigh the aircraft and set it up
at 98% aft center of gravity. Although we generally
fulfill all of our customers wishes this is one we
should not comply with. The sailplane will not fly
any better by achieving this extreme aft center of
gravity. It will simply react more nervously to pitch
and roll inputs. In fact, should the pilot loose 2
liters (1/2 gal) of water during a long flight (de-hydration!),
the sailplane has exceeded the specified limits of
C/G and control in all axes becomes very sensitive.



Please choose a center of gravity that is 30 to 35%

before the rearmost limit of the aircraft's specification.

This will grant an optimum in safety and performance.'

John Galloway



At 13:18 24 October 2003, Mark Stevens wrote:
this has been referenced before from the DG web site,
but it's not long so I'll post it here..