horiz tail airfoil observations

Wonderful explanation, Francisco - thanks for taking the time to write
all of that out!

I still scratch my head as to why the Thomas book recommends such a
large Cl range for the horizontal tail, though. His example for a 15m
ship with some pretty common dimensions winds up with a tail Cl range
of around 0.67 to -0.73 at a static stability margin of -0.05 (pg. 136
to 139 of the Thomas book).

And regarding the positive lift on the tail: Your explanation makes
sense in light of the (basic) modelling I've done of spanwise lift
distribution... However the wing airfoil still exhibits a negative
(i.e. nose down) pitching moment. So something needs to counteract
that force - especially because positive lift from the tail would
amplify the nose-down trend. Are you saying that the CG is
sufficiently far aft that it provides the "counterbalancing force", to
put it in layman's terms? I hate to keep repeating his name (but his
book is the most comprehensive one that I've read) - however, Thomas
talks about "aft CG" a lot, and in his measurements you never see
anything further aft than about 50% of the MAC. And as a result of all
of this, doesn't a positive-lifting tail then limit your forward CG
position?

Thanks again, take care,

--Noel
P.S. My R/C gliders were so much easier - just move the battery (CG)
around until the plane was pitch-neutral with 0 tail trim! :-P

noel.wade wrote:

And regarding the positive lift on the tail: Your explanation makes
sense in light of the (basic) modelling I've done of spanwise lift
distribution... However the wing airfoil still exhibits a negative
(i.e. nose down) pitching moment. So something needs to counteract
that force - especially because positive lift from the tail would
amplify the nose-down trend. Are you saying that the CG is
sufficiently far aft that it provides the "counterbalancing force", to
put it in layman's terms? I hate to keep repeating his name (but his
book is the most comprehensive one that I've read) - however, Thomas
talks about "aft CG" a lot, and in his measurements you never see
anything further aft than about 50% of the MAC. And as a result of all
of this, doesn't a positive-lifting tail then limit your forward CG
position?

I think I had it backwards before - according to Thomas, the stabilizer
must provide upward lift when the wing is operating at a high lift
coefficient (like thermalling), and a downward load at a low coefficient
of lift (like cruising). This is on page 133 of my edition, in the
"Longitudinal trim in unaccelerated flight" portion.

--
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

noel.wade wrote:

P.S. My R/C gliders were so much easier - just move the battery (CG)
around until the plane was pitch-neutral with 0 tail trim! :-P

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

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 ;-)

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

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...

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

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