horiz tail airfoil observations

Hi Dave,

Hey...........now that you mention it, the elevator of that Krokus had
a bent down tab along the whole trailing edge, it was about .5" wide. I
don't quite recall if the lower surface of the elevator was flat or
curved tho......................there were 6 Blue Angels screaming by
and I was slightly distracted!

Cheers,
Brad


wrote:
Eric Greenwell wrote:
Udo wrote:
As Udo pointed out, this is how the designer meets the requirement for
increasing "up elevator" force as speed increases. While this has a
safety advantage, the truly determined performance oriented pilot will
sometimes remove the undercamber to reduce drag. I've never wanted to do
it, because I want the safety advantage and I'm concerned the weight of
filler material might make the elevator flutter. It would take some
paperwork to make it legal, too.

--

Eric,
In this case the elevator and the shape is not just for safety but
also to maximize the performance, the airfoil was design as a
complete working unit. If there is a compromise it must be very small.
If you fly with the most optimum C of G there is very little elevator
deflection for the normal climb and speed range in a steady state and
if there is, let say -2 to + 2 deg of defection, I can tell you there
is no measurable drag penalty.

As I understand it, the drag penalty is not from the elevator deflection
(some of which would be required anyway), but because the airfoil is not
optimum for the lift (down force) it is producing; i.e., the undercamber
is on the side of the airfoil producing lift. There is always some drag
from the elevator, even with the control surface undeflected, because of
the lift (down force) it is producing.

--
Eric Greenwell - Washington State, USA
Change "netto" to "net" to email me directly

"Transponders in Sailplanes" on the Soaring Safety Foundation website
www.soaringsafety.org/prevention/articles.html

"A Guide to Self-launching Sailplane Operation" at www.motorglider.org

Hi Eric - I don't think this is correct. The prototype V2c I flew had a
trailing-edge tab bent down, explained to me as required for
appropriate
stick force gradient, as the elevator undersurface had no camber.
The production V2C tails added back the camber at a minor performance
penalty. Some well-known competition pilots in years back (not just
Karl)
did remove elevator camber for reduced drag, frightening the flutter
experts.

Hope that helps with the mystery,
Best Regards, Dave

PS: Jud, come out of hiding and explain it better to us
engineer-wanabees...

J. Nieuwenhuize wrote:
Papa3 schreef:
Not quite along the quantitative lines we've been following, but it is
pretty amazing to see how little the average pilot pays attention to cg
and its effect on performance in their common flight attitudes. Ask
some of the stockier pilots in your club to "trim for 55kts" (or some
similar, reasonable speed) and leave the trim there. On landing,
take a look at the elevator. I would wager at least one in two is at
or near full up deflection. Assuming this puts them at the outside
edge of the Cl ranges discussed, that's an awful lot of downforce
being produced (1/2RhoV2ClS IIRC). Aside from the "negative lift",
what's the typical induced drag that goes along with this? I'm
assuming it's pretty high given the relatively low aspect ratio,
especially of older models...

P3

Not quite correct actually; induced drag is proportional to the squared
lift coefficient and inversely proportional to the aspect ratio.
Knowing that the lift coefficient of your stabilizer is always lower
(main wing stalls first) induced drag is fairly low and certainly lower
than the weight penalty of a heavier tail. Also bear in mind that while
thermalling a glider you're flying at a relatively moderate angle of
attack, not at stall speed. (At the Discus for example you're flying
about 30% above stall speed in a thermal) This is different in
landing...

Nevertheless I usually fly at the back end of the cg-range; mainly
because of the difficulty to achieve "natural" ballast ;-)

Aha! Now things make much more sense!

Being one of those "stockier" types I find a fairly different
experience in the 304C that some of us fly. For me, full back trim
results in about 50 kts (nominal landing speed), and thermalling beyond
about 30 degrees of bank seems to massively increase the sink rate.
However, in an L33 full back trim flies about 5 kts slower and it loves
steep banks with me. Other pilots in the 304 (at the rearward end of
the CG range) report performance much more like I get with the L33. It
must be that the elevator design is different...

mattm wrote:
Being one of those "stockier" types I find a fairly different
experience in the 304C that some of us fly. For me, full back trim
results in about 50 kts (nominal landing speed), and thermalling beyond
about 30 degrees of bank seems to massively increase the sink rate.
However, in an L33 full back trim flies about 5 kts slower and it loves
steep banks with me. Other pilots in the 304 (at the rearward end of
the CG range) report performance much more like I get with the L33. It
must be that the elevator design is different...

Another data point...

I'm a 304C pilot who lost a substantial amount of ballast about a year
and a half ago. I now fly close to the rear CG limit and the ship
climbs amazingly well with a 45% bank in thermals.

I thermal around 50-52 knots dry and about 60-62 knots wet (about 20
pounds under max gross).

I was a little surprised by the difference that 50 pounds less in the
cockpit made, but I'm rather happy with the results.

Jeremy

Thanks. I have to think this over a bit - it's been quite a while
since I played with these formulas :-)

Couple of questions below:

J. Nieuwenhuize wrote:

Not quite correct actually; induced drag is proportional to the squared
lift coefficient and inversely proportional to the aspect ratio.
Knowing that the lift coefficient of your stabilizer is always lower
(main wing stalls first) ...

Okay with the first point (relationship of induced drag to CL and Di)
and proportionally much smaller contribution of tail vs. wing.

induced drag is fairly low and certainly lower than the weight penalty of a heavier tail.

Are you suggesting that a tail with a higher aspect ratio would be, by
definition, heavier or talking about the tactic of putting additional
weight in the tail to move the CG?

Good stuff.

P3

Matt -

More than likely, the issue is that your weight is closer to gross and
closer to the forward end of the CG envelope in the 304C. Both of
these things would favor a higher stalling speed and poorer steep-turn
performance (because the wings and tail are more heavily loaded in a
turn, and because the slow flight also necessitates more trim/elevator
deflection - resulting in increased drag).

Take care,

--Noel

mattm wrote:
Being one of those "stockier" types I find a fairly different
experience in the 304C that some of us fly. For me, full back trim
results in about 50 kts (nominal landing speed), and thermalling beyond
about 30 degrees of bank seems to massively increase the sink rate.
However, in an L33 full back trim flies about 5 kts slower and it loves
steep banks with me. Other pilots in the 304 (at the rearward end of
the CG range) report performance much more like I get with the L33. It
must be that the elevator design is different...

Papa3 schreef:

induced drag is fairly low and certainly lower than the weight penalty of a heavier tail.

Are you suggesting that a tail with a higher aspect ratio would be, by
definition, heavier or talking about the tactic of putting additional
weight in the tail to move the CG?

Good stuff.

P3

Heavier construction. Heavier stabilizer means larger moment of
inertia, higher torsional stiffness of the tail... leading to maybe 4
or 6 times as much "extra" construction weight as only the extra
stabilizer weight. And you have to correct that with even more wing
surface ;-)

J. Nieuwenhuize wrote:
Papa3 schreef:

induced drag is fairly low and certainly lower than the weight penalty of a heavier tail.

Are you suggesting that a tail with a higher aspect ratio would be, by
definition, heavier or talking about the tactic of putting additional
weight in the tail to move the CG?

Good stuff.

P3

Heavier construction. Heavier stabilizer means larger moment of
inertia, higher torsional stiffness of the tail... leading to maybe 4
or 6 times as much "extra" construction weight as only the extra
stabilizer weight. And you have to correct that with even more wing
surface ;-)

Okay. To summarize your comments, the induced drag created by a
stabilizer, even one operating at it's maximum (negative) Cl is
relatively insignificant to the overall system efficiency. Did I get
that right? Further, the structural considerations involved in
building a higher aspect ratio tail would more than negate any slight
decrease in drag. Also correct?

Ahh, engineering compromises...

I'd still be interested to see the numbers in terms of total drag on a
given elevator operating at basically neutral trim vs. max up elevator.
I guess I could sit down and do this, but it would mean pulling out
some old text books that are awfully dusty right now :-)

P3

*sigh* I had a very eloquent rant that I tried to post yesterday; but
for some reason it isn't showing up. I don't have the heart to try to
reconstruct the entire rant, so I'll summarize:

It seems there only two types of aircraft design books/articles:

1) Those that use ballpark figures and rely on historical examples of
existing designs

--OR--

2) Those designed for engineers, with accurate but very complicated
equations in Engineering notation that are indecipherable by the
layman.

Is it so hard to bridge the gap, for those of us that can't decode long
strings of Greek letters into practical terms?

I'm a computer professional, so I'd like to think I'm decent with math
- but even 3d-graphics-programming has only required a solid grasp of
algebra, trigonometry, and matrix math. The calculus and short-handed
equations in many technical articles might as well be modern art on the
page, for all I can tell. Many factors are often not defined by the
author - who assumes the reader knows what they mean; even those
targetted at "first time" designers!

In terms of this tail issue, for example, is it really too hard to put
it in terms like... "At speed ____ your design would have to pull a Cl
of ___, requiring an angle of attack of ____. With the airfoil chosen,
the coefficient of moment in this situation is ____. Applying equation
_____________ to that and the Center-of-Gravity at ___, you end up with
a total pitching force of ____. This must be counter-balanced by the
tail producing an equal and opposite amount of force. Given the wing
downwash effects and angle of incidence, the horizontal stabilizer is
flying at an angle of attack of ____. So to provide enough force, the
coefficient of lift must be ____ and/or the tail area must be ____
(assuming no elevator deflection). "

I mean, am I missing something; or can't you put it into those simple
and direct terms? I guess I've left out is the stability margin - but
that's got to be something you can factor into the above process,
right? Surely such a direct-calculation approach would require
iterative design to find the optimal solution in all flight regimes -
but even that is better for the amateur designer than an inverse
solution that cannot be solved by the average joe!

Somebody please feel free to step up and slap me if I'm way off base
here.... I've got a good wing design, a good fuselage, and a good
vertical tail; all with numbers that I can calculate and verify - but
I've been wrestling with this horizontal tail issue for a week and its
really getting to me!

Thanks, take care,

--Noel

noel.wade wrote:
*sigh* I had a very eloquent rant that I tried to post yesterday; but
for some reason it isn't showing up. I don't have the heart to try to
reconstruct the entire rant, so I'll summarize:

It seems there only two types of aircraft design books/articles:

1) Those that use ballpark figures and rely on historical examples of
existing designs

--OR--

2) Those designed for engineers, with accurate but very complicated
equations in Engineering notation that are indecipherable by the
layman.

Is it so hard to bridge the gap, for those of us that can't decode long
strings of Greek letters into practical terms?

I'm a computer professional, so I'd like to think I'm decent with math
- but even 3d-graphics-programming has only required a solid grasp of
algebra, trigonometry, and matrix math. The calculus and short-handed
equations in many technical articles might as well be modern art on the
page, for all I can tell. Many factors are often not defined by the
author - who assumes the reader knows what they mean; even those
targetted at "first time" designers!

In terms of this tail issue, for example, is it really too hard to put
it in terms like... "At speed ____ your design would have to pull a Cl
of ___, requiring an angle of attack of ____. With the airfoil chosen,
the coefficient of moment in this situation is ____. Applying equation
_____________ to that and the Center-of-Gravity at ___, you end up with
a total pitching force of ____. This must be counter-balanced by the
tail producing an equal and opposite amount of force. Given the wing
downwash effects and angle of incidence, the horizontal stabilizer is
flying at an angle of attack of ____. So to provide enough force, the
coefficient of lift must be ____ and/or the tail area must be ____
(assuming no elevator deflection). "

I mean, am I missing something; or can't you put it into those simple
and direct terms? I guess I've left out is the stability margin - but
that's got to be something you can factor into the above process,
right? Surely such a direct-calculation approach would require
iterative design to find the optimal solution in all flight regimes -
but even that is better for the amateur designer than an inverse
solution that cannot be solved by the average joe!

Somebody please feel free to step up and slap me if I'm way off base
here.... I've got a good wing design, a good fuselage, and a good
vertical tail; all with numbers that I can calculate and verify - but
I've been wrestling with this horizontal tail issue for a week and its
really getting to me!

Thanks, take care,

--Noel

I have wrestle with that my self.
I had no background in it when I started and still my knowledge is very
narrow. But over 25 years I have bulldoze my way through. 17 years ago
things started slowly changing for me, aside for rudimentary formulas.
With the advent of ACAD and aerodynamic software as well as the
internet, things started to fall into place. To day I use a 2D and a
3D software. Both of them are commercial programs.
Combined with subscriptions to Technical Soaring and other publications
I slowly started to make sense of it.
The results were, two projects that were limited to changing airfoils.
My new project starts from scratch. For it to be fine tuned
a rely on the 3D software, as well as what is out there on the flight
line.
Udo
PS. Go to the Glider Tech Group, a Yahoo group. I just listed a file
comparing the
DU13.7-86 vs. the FX71-150/30 for two speeds with values that are
appropriate for those speeds