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

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!!

Most contest pilots trim their sailplanes to the most
rearward permissible position.

I have found that many sailplanes don't like the full aft CG position. My
ASH-25 handles best at about 75% of the allowable range and the LS-6 is about
80%, I'm told the Discus likes to be about 92%.
If you go too far aft with the CG, you will find yourself trimming forward,
when you drop the flaps and enter a thermal.
I use the tail tank to find my best CG position, dump a little until she feels
right and then after flight, dump the tail water into a bucket and weigh it.
That is the weight that I add (in lead) to my tail. Then when I fly with water
in the wings, the tail water only balances the added weight of the water in the
wings.
I restrict the tail dump line, so that it will dump proportionally (at the same
rate as the wings) and this allows me to dump a part of my water and still
maintain my ideal CG position.


JJ Sinclair

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?

Jim Marske did a study of the Genesis and found that the elevator on the
tail is about 15% of the drag. you can see the article on our Marske Flying
Wings Site.
-mat

--
Marske Flying Wings
http://www.continuo.com/marske