Tail Load and Dynamic Stability Re-visited

[i278]

This was brought up briefly in a rather infamous thread, and I'd like to fill-in a rather glaring hole in my knowledge.

The short story: Miper has posted in the past that a tail download must always be present to ensure adequate longitudinal dynamic stability. (My apologies if I've misinterpreted or misquoted)

The short question: Why?

The longer question: If static stability is present (neg slope on the dCm/dalpha curve for all configs), then shouldn't dyn stab depend on the relationship of the initial forcing function (crudely based on the slope of the stat stab curve? and, hence, tail volume?) to the damping coef (crudely based on what? a 'non-dimensional' tail arm?) If this is at all close to reality, I don't see how the absolute value of tail load (positive, neagtive, or zero) has any bearing on the matter.

It's obvious by now that I don't have the Perkins & Hage text. My current books have lots of material on stat stab determination (the easy stuff), but only a cursory overview of dyn stab, complete with those helpful little graphs. (Oh look, the divergent case diverges! How useful!) Would P&H sort me out on material like this?

Always learning...

Regards

 

[Sparweb]

As the originator of the thread that got derailed, I never intended to discuss this topic, but I thought that I would contribute because I have personally created and observed "divergent phugoid motion" in an aircraft. It was a glider model, 8 foot wing-span, and in an attempt to increase its stability, I actually decreased it.
Once launched from the tow line, its alternating nosedive/climb/nosedive flight brought about a very violent end to the experiment. It was a big surprise because the behaviour didn't show up when the glider was hand-launched.

My education in dynamic stability was about as in depth as yours, i278, and I've not needed to expand it, yet. But if it comes up, the first place I'd look is in Roskam's series of books. Part 7 deals with stability and control issues. To read Roskam usually requires a pencil and paper beside the book at all times, so it's always slow going. That's my experience, at least, from getting through Part 1. Roskam always is relating his info to real, working aircraft, not "theoretical" models. Other members may have better, or more economical, references.

There are also military standards that are available on the internet from ASSIST that deal with the issue. Some are publicly available, some are not. Try MIL-A-8###, MIL-F-8###, etc.

STF

 

[jetmaker]

just an interesting aside.

If the aircraft CG is behind the wing center of pressure, then the horizontal stab must be providing upwards lift. The problem here is that you can stall the airplane, and never be able to recover from the stall as the aircraft no longer has the natural tendency to pitch over.

jetmaker

 

[Sparweb]

Funny how some topics can turn back on themselves, but the pilot of the Piaggio that started my interest in 3-wing aircraft was killed a few years back by exactly the kind of accident jetmaker described. Ferry flight in a Seneca (I think), and just ran out of elevator on take-off.


STF

 

[edmacs]

Hi all,

Well, Jetmaker said that having the wing CG behind the aircraft a.c. (i´m sorry if i happen to misunderstood!).

However i think that if a plane is designed so that the wing stall´s before the horizontal empennage there is no problem in having the wing cg behing the aircraft aerodynamic center.

Actually, for performance it´s quite good to have upward load on the empennage!

Please, correct me if i´m wrong!

Thanks,
Edmar

 

[O320]

Stabilitywise a canard aircraft is nothing but a normal aircraft with an extreme rearward cg,(using the fwd wing for reference) and a corresponding huge tail volume. (Main Wing) So flying in a stable manner with all surfaces lifting upwards is not impossible.

 

[Sparweb]

i278,

In case you're still looking for Perkins & Hage, you could try digging it up on Alibris.com. Lots of used and out-of-print textbooks for aero eng junkies like us.


Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

 

[i278]

SparWeb,

Thanks for the lead! There's nothing like curling up in the easy chair with an old text or NACA report, pencil and calculator in hand.

Regards

 

[Sparweb]

You forgot to mention the spouse, standing in the background, shaking her head in disbelief...


Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout