Basic Formulas

[10cent]

I wondered if there was a good source for the bare bones formulas that everyone uses to start a plane design? How do you calculate your skeleton. For simplicity sake lets say top wing training/park flying models. I've had a hard time finding the "tool box" of formulas anywhere.

Here is a site that I found for wing design. It seems like a resonable approximation:
http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html

Any other sources of formulas that you guys know of? This whole thing started because I am having trouble finding any formulas for tail placement (distance from CG).

Thanks - 10Cent

[Ollie]

Dr. Drela:
"Ch = (A_hori/A_wing) * (tail_arm/avg_wing_chord)
Cv = (A_vert/A_wing) * (tail_arm/avg_wing_span )

A well-sized tail will be in the range...
Ch = 0.35 - 0.50
Cv = 0.02 - 0.035
If the Ch and/or Cv are below the minimum values, the handling will suffer."

"There is something very suspicious about this analysis. I don't have the 1980 NFFS Sympo, so I can't examine Bogart's method. According to the graph in the 1996 report, increasing the tail arm length will make the airplane more spirally unstable. This is exactly opposite to what really happens.

By far the most reliable predictor of spiral stability is Blaine Rawdon's criterion. You first compute the "B" parameter:

B = EDA x (tail_length/span) / CL

EDA is the Equivalent Dihedral Angle (in degrees), discussed by Blaine in his Modal Aviation article series "Dihedral" a while back.

Anyway, the airplane is spirally stable if B>5, and spirally unstable if B<5. A longer tail increases B, and hence increases spiral stability. The vertical tail area doesn't even appear. Good rudder/elevator RC gliders have enormous vertical tails by FF standards, and are spirally stable regardless, just as the criterion above predicts."

"
The notion that more vertical tail area causes spiral instability
is partially a myth. It is actually a secondary factor, and is
only a concern on planes with little dihedral which are spirally
unstable to begin with.

Blaine Rawdon's Plane Geometry package gives an approximate criterion
for spiral stability which matches more exact analyses quite well,
at least for typical configurations in shallow bank angles:

EDA * tail_length/span > 5 CL , with EDA in degrees

So the only sure ways to induce spiral instability are:

* reduce Equivalent Dihedral Angle
* shorten the tail arm
* stretch the span
* increase CL (fly slower)

The vertical tail area doesn't even appear in Blaine's expression,
and increasing it will have little effect on spiral instability.

During steeply-banked circling, the picture gets more fuzzy,
since the spiral and Dutch roll modes then become mingled.
The few analyses I've done indicate that adding vertical tail
area in this flight condition tends to suppress spiral (+Dutch)
instability somewhat, quite opposite to what common lore says.
This was verified when I enlarged the vertical tail on my
Allegro RES glider soon after the initial flights. The
tendency to spiral in during very steeply banked circling flight
was clearly diminished. But this may be configuration dependent,
tough to say. The Dutch roll damping and roll power naturally
improved.

The bottom line is that there is little downside to increasing
the vertical tail area on a RES glider other than a bit of extra
weight and profile drag. If the tail's vertical span is increased,
then there will be a reduction in the rudder's induced drag, so
there may actually be an overall drag reduction.

- Mark Drela"

"You should be aware that on a rudder/elevator glider, the ideal
dihedral distribution is more like a shallow circular arc or parabola, not an ellipse. The arc has a minimum induced drag penalty from the sideslip during an established turn. In contrast, the elliptical dihedral in a sideslip has the near-vertical tips loaded and unloaded too much. This not only increases induced drag, but can also easily result in local stalling on the inside tip. The outside tip can easily go to negative cl's, which will likely increase drag there as well."

I HAVE MORE FILES.


http://members.home.net/evdesign/pages/technical_articles.html

[10cent]

Ollie,

Thanx for the info. That's perfect, but I couldn't get the link to work??

Another related question, what about vertical placement of the tail. Should the horizontal surface of the elevator be in the thrust line etc. It seems that this would have a huge impact on the behavior but I haven't seen it discussed.

[phat23]

This is great for establishing the basic parameters for a simple trainer type model.
http://www.palosrc.com/instructors/basicdes.pdf

[Ollie]

http://members.cox.net/evdesign/

"Should the horizontal surface of the elevator be in the thrust line etc."
Prop wash is too complex a question for me. http://www.rcgroups.com/forums/showthread.php?t=263787