horiz tail airfoil observations

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

*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