The Neutral Stabilizer

[Richard Miller]

Dave,

This is mainly for you, but anyone can charge in and have at it.

I've been working on the design of a [truly] light-wind soarer and come face to face with the idea of the Pilot/Plane Parity. What happens when it shifts into (-) territory and the pilot outweighs the glider by, say, a factor of two?

This brought me presently to the idea of using weight shift. I saw that at one exteme, say the Std. Class sailplane, a small [aerodynamic] force is used on a long moment arm to achieve that holiest of holies, longitudinal command. At the other, a large force, the pilot's mass, on a short moment arm, the inch or two I imagined would be necessary to effect necessary changes in pitch.

I had to mull that over, kind of meditate on it, and in the end I was satisfied that the glider would be responsive and do what the C.G. dictated.

Then I began to think about the horizontal surface. It would have no hinges nor horns nor wires nor turnbuckles, which is all to the good when you're intent on a *really* light glider. And it would have no flight loads, just trail along back there, a symmetrical section at zero angle of attack. Hmmm.

Then I thought of the vertical surface. That's what it does, providing it's *just* large enough to suppress dutch roll at V/min.
Would something similar be the case with the horizontal? A kind of longitudinal wandering until the surface was *just* big enough?

I'd appreciate any reflections on this.

-Richard

[Dave Robelen]

Hello Richard,
Sorry about the sluggish response. My server went down as I was typing it yesterday.
At any rate, I would expect the horizontal to have some load almost all of the time in order for your glider to be speed stable. It would be preferable with the weight shift only to have the tail rigged with a slight "up", andthen you would wiggle to maintain trim at that setting. Too little decalage and the glider would be so tricky to manage that you could barely find the trim point. I woukdalso plan on the occasional gust load, even though you are only interested in flying in smooth air. Ifthere is lift present, you might find "bumps".
Good luck, Dave

[Richard Miller]

Dave,

I've read and am digesting what you've written. Meanwhile, since yesterday, have continued to try to rationalize the business of a weight-shift glider and the role of the horizontal surface.

Begin with a pilot in a pod with a boom sticking out behind it. The parasol wing is swept forward so that the CL of the wing is above the C.G. of the pilot+plane. By virtue of the 30 or so vertical inches between the two, the pendulum stability that provides, this apparatus can be made to glide in a stable manner.

Equilibrium flight, but only in totally equilibrium air [and I do NOT contemplating flying in such air]. Along comes a little gust. It arrives at the front part of the wing, the area ahead of the CL, and causes a perturbation. The pendulum stability is sufficient to damp this, maybe three oscillations in as many, or a few more seconds.

Confession Time: You classically trained aerodynamicists must understand, and have quantified and given nomenclature to all this of which I am ignorant. I go ahead with the conviction we can patch up that part.

Next, a 2X gust, one the pendulum stability cannot damp. So we stick a bit of horizontal surface on that boom, enough so that the acceptable no of perturbations in the acceptable period of time result.

Next, we decide on the maximum gust case we intend to fly in and add enough horizontal surface to take care of that. What I see now is that the horizontal surface is going to fly along in neutral until the gust comes along, then take a load, or a sequence of opposing loads, until things quiet down.

This is Richard moving out where the ice is thin, wondering, and very pleased to have you there, Dave, to take me serious enough to respond to these postings.

-Richard

[Dave Robelen]

Hello Richard,
If I understand your description, you are conemplating the concept of esentially a flying wing with a suspended weight, and then adding the stabilizer as sort of a safety device. Have I got it right so far?
By using an airfoil that has a neutral pitching moment of the sort that are common to flying wings, that might well work out. The typical airfoil that has a generous lift coefficient for slow gliding also has a generous nosedown pitching moment, and this is where the stabilizer would be rigged with a download to keep you in trim. Either case would be sized to give an adequate damping performance in those gusts. The other issue with the weight ratios you mentiond is that you will have to be able to stay very steady in flight to avoid undesireable trim shifts. There is also an element of safety in the case where the tail is down loaded in that if things went to pot and a dive started, the machine would automatically begin nosing up as the speed built.
That flying model is looking better all of the time.
Regards, Dave

[Richard Miller]

Dave,

Confession time. The design of *this* glider - neutral stabilizer - is not the other glider. I'm a design junkie and skip around, like getting a row of pawns to the 8th rank and making them all queens.

The design of *this* glider has been going ahead on the UltraLight Soaring page of yahoo egroups, which I invite you to join to see my recent posting there: PENDULUM STABILITY. It's very easy to subscribe. Just fill in a few menu items. And it could well open some doors to other realms.

-Richard

[Dave Robelen]

Hello Richard,
I will catch up with the Yahoo group as soon as things slow down just a trifle. I have given some more thought to your concept of forward sweep on a hang glider with a "neutral loaded stabilizer", and it is kind of scary. By using forward sweep, you are shortening the restoring forces from the wing considerably, and the inertia of you hanging beneath this could generate some potent upsetting motions if things get busy. Perhaps I would have a better feel for the concept if I were to visit the other site, right now I am a bit in the dark.
Regards, Dave

[Richard Miller]

Dave,

I've come to my senses, more or less. I finally managed to rationalize the matter to my satisfaction. It is that pendulum stability as a possible means of longitudinal trim, provided a suitable vertical distance between the CL and the CG, comes increasingly into play as the weight of the glider as a proportion of that of the pilot diminishes. As a figure of merit, I aim at a glider half my weight.

There remains, however, a null point, when the pendulum line is common to the longitudinal axis. No effect, none until there is some angular difference, as pitch up or pitch down, then it's going to vary with the sine of the angle, or something like that. I'm not qualified to do the math.

That null area, be it ever so small, must be filled in with aerodynamic force, those up and down on the horizontal surface. I now see and acknowledge that. The good news, from the *very* light glider point of view, is that the size of that surface, its moment arm, the degree of decalage, all factors in weight and drag, *can* be reduced. At least some.

I now envision a horizontal surface of adequate size with a slight negative chamber. That strikes me as the most economical way to handle the matter.

The sweepforward is essential to get the CL over the [pilot's] CG in a *very* light glider, especially with a boom and cruciform tail that weight only a few pounds. Of the complications from that I know little, not having thought *that* matter through.

I'll appreciate you counsel. Do remember that this is intended to be a one-speed glider, that for minimum sink, or best LD, both of which will probably be within 0.003 mph of each other, and to be flown in ultralight conditions.

-Richard

[Dave Robelen]

Hello Richard,
I do not envy you the structural design challenge of keeping that swept forward wing from diverging at the tips. Are you going with rigging? I am glad to hear that you are moving toward a seperate horizontal tail. I have used reverse camber in several models to good effect when I did not have direct pitch control and a rapid recovery from disturbances was necessary. Another thought about the forward sweep. It will have the effect of providing negative dihedral compared to a straight wing. Of course with the CG so low, thee will probably be ample deihedral effect.
Regards, Dave

[Richard Miller]

Dave,

*This* design, the one being discussed in this Thread, is a light-wind soarer, intended to be *very* light, as a pair of wing panels weighing maybe 40 or 45 lbs. Thus, a CF D-tube and model-airplane-type construction aft of that. Strut bracing.

So, the tips cannot diverge outward, and not upward, and insomuch as I'll use an adequate drag spar, neither forward nor rearward.

I just, overnight, completed a small balsa model. Wing span = 14 inches; lifting stab about 31% of wing area; short-coupled, like 3+ inch TMA; level of the stab about an inch below that of the wing. Horizontal mounted on a balsa dowel; goes into a length of alum tube, rotates.

Results: 1) The C.G. falls well after of the wing 1/4-chord point; and 2) there's plenty of trim force; it recovers right away, so I could possible increase the incidence of the tail; 3) the effect of tail tilt is dramatic: positive and instantaneous. That coupled with using the wing halves differentially should result in a really tight turn.

So, on to a 6-ft RC model. Meanwhile - I seem to be hot right now - I see a very simple way to incorporate Spratt's free- [in my case spring-] wing idea. That will reduce loads all round, reduce weight maybe a little, keep the pilot on an even keel. Mr Spratt had SUCH a good idea, and the world just went on fixing the wing as firmly to the fuselage as possible. !Aye de mi!

-Richard

[Dave Robelen]

Hello Richard,
I applaud your willingness to stick with a goal that is well off of the beaten path. Incorporating some of Spratt's work sounds like a real inspiration.
The use of scale models to develop a design is near and dear to me, this was my primary source of income for numerous years at NASA prior to my retirement. There have been several occasions where I followed the same path in developing a complex R/C model. First the simple glider, then on to more costly methods.
Good luck, Dave