I'm new to the hobby. I've met a group at a local flying field that I'm convinced will help keep me on track. I have sooo many questions.

I've recently made 2 deals here on this site. The first one was for a realitively large glider- a Sailaire, which isn't going to be built until I can confidently fly the one currently being built on my bench- a 2 meter Easy Eagle. And the second was for a JR Tx/Rx: both of these I believe will keep me busy for a while......

The questions I have are with regard to flight characteristics of polyhedral wings. Stable as hell- that part I understand. Most of the glider models with polys (sans ailerons) I've seen sport some very large vertical stabs or vees. As I grow with this sport, I will want to "graduate" to an aileron model. If I constructed a wing with ailerons using the models original wingplan without the polyhedral (well, maybe I would be wise to include some dihedral), would I also have to consider a revised empennage?- with smaller vertical control surfaces? Or are there other factors I should be considering?

I read somewhere that putting ailerons on the polyhedral wing of a floater was a waste of time.

Comments welcomed.

You can do what you're talking about just fine. Generally the poly models have some small amount of dihedral in the center joint. So if you just build the wings flat on either side and keep the center dihedral then you're off to the races.

I think the problems you heard about were from people trying to put the ailerons onto the original poly wing. That just doesn't work very well as the two things fight each other. Use full poly or use airlerons but don't try to mix them. It's not the poly as such but rather the overall dihedral angles involved. Many aileron models use the poly format these days but the angles are very shallow compared to the Rudder-Elevator poly angles.

AND...... You're right in that the vertical area will be much too large on most designs for flat wing conversion. You may easily end up with a spirally unstable model that feels like it's flying on a tightrope all the time and wants to fall off to either side all by itself. This effect is much more noticable as you move the Center of Gravity back to get the wing flying more efficiently. The easy solution is to remove the HUGE rudder that was needed for the poly model and replace it with one that's about 1/2 or even a bit less in area. Just rehinge the new surface onto the original fin. In fact if you plan ahead you could use the pinned hinges and just replace the small wires with a long single wire that can be zipped out to let you put the new surface on easily.

I went through this with an aileron two meter I built a few years back. It was never "just right" so I built a new sheet extra large fin and rudder onto it for testing. As I moved the CG back from the original too far forward point I found the model got very spirally unstable to the point where I could barely hold it level. To counter this I would cut off 1/2 inch from the top of the sheet surface. This made the model more stable and groovy in the turns. I kept cutting down the vertical and moving the CoG back until I noticed it was adverse yawing in the turns and turn entries. I patched on the last piece to restore the nice turn entries and eliminate the last bit of adverse yaw and side slipping in the turns and called it good. That gave me a model that entered turns nicely and had very little tendency to wind in during tight thermal turns. And the need for "top" aileron in all but the steepest turns was almost totally eliminated.

I'm convinced that many commercial designs using ailerons that have a reputation for needing a lot of "top aileron" are in actuallity suffering from too much vertical fin size.

This testing was all very empiracal but it should help you for when you want to try this. The spiral instability issue is well know in the Free Flight area that I also dabble in and it was this background that gave me the idea for the fin testing above. I'm glad I took the time because it answered a lot of questions for me.

Like the horizontal tail can be sized using the tail volume coefficient the vertical can be sized using the same coeficient but instead of the wing area it uses the projected wing dihedral side area and fuselage side areas. If you can find out the numbers for a whole list of designs and fly a few of them to see what number is a good number to have you could predesign the modified rudder to match the new wing. But since this is a very empiracal number I think it's just as easy to make an all sheet test rudder and hack it away until it flies to your taste and then build up a nice built up version and stick that on.

Long and wordy but there's a lot of concepts at work here. Hope this helps.

And switch airfoils to something more zoomy when the time comes for that new wing.

And isn't it fun to learn and experiment.....

A polyhedral aircraft achieves roll control through yaw to roll coupling. When the rudder yaws the plane the angle of attack of one side of the wing increases and the other side decreases. The difference in angle of attack produces a difference of lift between the two sides which causes the plane to roll. The roll causes the plane to turn. Ailerons also produce a difference in lift from side to side but they also produce a difference in drag from side to side causing adverse yaw. The adverse yaw must be compensated by coordinated rudder deflection. It takes more pilot concentration and visual feedback to produce a coordinated turn with ailerons. A wing without dihedral or poly hedral is not spirally stable at any angle of bank The more dihedral or polyhedral the larger the angle of bank can be without loosing spiral stability. Not only are polyhedral planes easier to thermal near the limits of vision but they take much less pilot work load to fly. The main advantage of ailerons is that they can be made to roll the plane without yawing it by rudder. Wings with both dihedral and ailerons work extremely well if adverse yaw is avoided. The combination of uncompensated adverse yaw and dihedral can overcome aileron roll effect enough to prevent turning but with compensation beautiful turns can be made.

Lots of good points Ollie but I have had different results in a couple of areas.

First, I'm not sure how much dihedral you consider "enough" but the aileron 2 meter design I did those tests on had about 2 inches under each panel. Certainly a totally flat wing would be less stable and a pain to fly. But the 2 inches WHEN THE FIN AND RUDDER WERE THE CORRECT SIZE was enough dihedral to see the model correct itself from banks up to 5 or so degrees. Before I had the vertical area matched it showed the whole range of bad traits you are talking about from an extreme of falling off even from level flight, to needing generous top aileron as the first bits were cut off, to being stable and actually needing a tiny bit of pro aileron for light turns to maintain the bank. I shoudl add that I was using coupled rudder and ailerons for this model. The key here, in my experience, was carefully tailoring the vertical tail volume coefficient via the fin trimming to the amount of dihedral. A method that has been used in Free Flight designs with great success in the past by many noted designers I should add. This isn't my own new idea.

Knowing what I know from that model I'd have no qualms about changing the vertical tail on any model that I thought could be improved. Or in the case of a V tail model I'd look at adjusting the V angle to provide more or less vertical volume as needed.

And secondly, neither of us mentioned the use of lots of differential in the controls to help control the adverse yaw. But even with that I agree with you that some positve rudder must be used in coordination to avoid those sloppy looking side slips in the turn entries.

Other than that Shredz, Ollie makes a very good point about the greater difficulty in controlling the model at great distances. You've got to be at the point where you can fly by instinct as much as visual feedback. And the workload is much easier with a poly design.

If or when you get around to making this "flat" wing you'll want to keep it in close for quite a while. It really is like learning all over again. In many ways it would be nice to have two models during this transition. The old reliable poly model for sharpening up your thermalling and the new aileron bird for the excitment of the new challenge. That way you can have the smooth, sleek fuselage to go with the zoomy fast airfoil section on the wing. The best of both worlds.

I know that the inherent stability of the poly wing would save me at sight limit distances, which is why I would build any floater model to the plan- I'm still a newbie, but I'm a solid thinker and I love to have my mind working on the next target. The ideas that I have would be used after I've learned what the model can do...and not do.

I'd like to build a second set of wings for the day that I want something different (but still familiar) to fly. And the added bonus of it being somewhat of my own design....sort of.....from ideas obtained by direct observation- BM: like your experimentation process of altering free-flight models which lead you to conclusions, from which you could theorize, with confidence, what was occuring and how to develop your model further- a process I rather like. But when I consider the intended design of both planes- floaters- and both of them have rather large vertical stabs and rudders (not to mention big horiz stabs) when compared to aileron equipped planes of similar size, I surmized that I'd be dealing with a few more mods to the empennage make this idea "fly".....properly... which is confirmed by your observations with your modded 2m. It seems my 2m will be a similar project of discovery....someday. I'd like to know more about the relationship of vertical fin area vs dihedral (the span has to be in there somewhere..)....perhaps a publication?

Ollie: The Tx/Rx combo I'll be using has an aileron differential setup as well as the mixing needed to coordinate the rudder and ailerons. I guess from a purist point of view, it's cheating :devious: , but, as I wrote earlier, I'd be flying this plane without ailerons until I had learned enough with it. I figure building a full-house wing with wingroot dihedral would be a good learning experience. The Sailaire, with quite a pronounced polyhedral, would be thermaling machine. The tail on that thing has huge surfaces and unless I was set to re-invent the wheel, I'd best leave it the way it was designed.......well, maybe spoilers.....I've heard this plane doesn't like to come down sometimes.

Thank you both for your time and the information. You've shown me the idea isn't way out there, but I have to give this some more consideration.

Ted

Here is my take on vertical tail area.

Free flight models often have very small vertical tails and lots of polyhedral. The generous polyhedral keeps them from spiralling in when the plane is banked by lift on one side of the wing. The small vertical tail puts them on the verge of yaw instability (dutch roll) but, allows the model to follow lift rather than just boring through it.

R/C thermal soarers need larger vertical tails to give them the necessary yaw control authority without very large, drag producing, rudder deflections. Most R/C thermal soarers would benefit from vertical tails of higher aspect ratio so that the induced drag oassociated with rudder deflection was less and the yaw control authority could be achieved with less vertical tail area. Generous polyhedral produces stronger yaw to roll coupling. This reduces the necessary yaw angles and reduces the drag associated with a yawed fuselage and with a highly deflected rudder. The extra parasitic drag of a large vertical tail has to be ballanced against the drag reduction associated with maneuvering. Where this balance should fall depends on how much time the plane spends maneuvering versus the total flight time. Not an easy question to find a neat answer to.

You can learn a lot by studying the designs of Dr. Mark Drela and reading the Ask Joe and Don section of the DJ Aerotech web site. See:
http://www.charlesriverrc.org/articles.htm
http://www.djaerotech.com/dj_askjd/askdesign.html
http://www.polecataero.com/ (go to articles)
http://www.monkeytumble.com/hlg/supergee.htm

Dr. Drela's designs are the most thoroughly engineered, refined, structurally superior and aerodynamically superior that I have seen in over 35 years of R/C modeling. I don't believe I am overstating my judgement. There is a treasure house of information in his designs.

Here is an item I picked up on the RCSE summary this morning:

"Jim,

I got to fly Dr. Drela's Bubble Dancer last summer. The first flight was
off a hand toss. I circled around and came back. All felt fine. The next
throw was a little harder. I caught a small thermal about 10' off the
ground. Mark said to go ahead and treat it like a hand launch, which I did.
There is something a little unnerving about flying someone else's 3M plane
10' off the ground, but I put it into a tight core and up it went. That
flight lasted about 10 minutes.

The appeal for me was a very easy handling big plane that goes up on just
about any type of lift. It also flew slowly very well, nice for landings.
In fact, I don't recall ever throwing spoilers as it was just crawling in.

It is an extremely well thought out design, beautiful to see. It is a
builders plane, but anyone would appreciate the detailing and smart
engineering put into the structure.

The only downside I saw was coming back from downwind. There was a slight
(7-8 mph breeze) and it was the first time I've flown a plane that was too
light. This is a nice problem to have as long as you can ballast easily.
With a couple clicks of down it moves, but nothing like a camber changed
wing. Once again to be fair, this was totally unballasted and it was my
first time flying it.

I have a Bird of Time that I would say is the closest feel in flight. I
watched Mark fly off the winch later in the day. The wings flex, but he was
zooming pretty nicely off the top. Not monster zooms, but you don't really
need that for this plane. It has a great L/D just floating along. You can
burn plenty of minutes just cruising around.

JE
--
Erickson Architects
John R. Erickson, AIA"

Here is another:

" Date: Sun, 15 Dec 2002 05:42:32 +0000
From: "Jeff Nibler" <[email protected]>
Subject: Re: [RCSE] Bubble Dancer

Yes. To add to the comments others have made... it could be a sport flyer or
a RES competition ship. It's capable of full pedal zooms. It's wingtips
are extraordinarily light... far lighter then any composite RES wingtip,
which makes for great yaw response. The main idea behind the plane is an
extremely light wing loading (to go up in minimal lift) with a fast thin
airfoil (to range out). This allows it to have a fantastic L/D, yet if you
put the nose down, it will move out nicely without a gigantic loss in L/D.
You get the benefits of a floater and a led sled all in one."

Much obliged, Ollie. I'll have a look.

I really like the idea of a higher aspect vert stab.


Please allow for my analogy....
The way I've been thinking of this is like a car driving across an icey incline where too much incline would cause the car to slide sideways. The only thing resisting the slide would be the traction (via friction) provided by the tires on the surface: lateral resistance. As the incline is increased and traction is reduced, a gentle sideways drift would occur. At the point of sliding the stability of this drift would depend on the weight distribution between the front and back of the car- weight distribution, and its effect on this lateral resistance, would play a large role in how the slide would develop. Over simplistic when compared to the flight dynamics involved in this topic, (I know...comparing a non-dihedralled fin/rudderless plane in a banking turn to a car on an incline with lousy tires? ).....but a wing with dihedral has a lateral profile as does the vert stab: again, lateral resistance. There's lots of room to refine this comparison- it happens I have a somewhat limited understanding of aerodynamics, but lots of driving time ....something that I'm sure will change as I build a few and twist sticks.

Q1a & 1b) If a plane with a reduced dihedral and original empennage (when compared to it's original design) is banked, will it display the spiralling characteristics (yaw/roll instability mentioned by BM)? If it does, will reducing the size of the V stab correct it?

Ollie, you've mentioned aspect ratio of the V stab. Being a sailor, I've experienced the difference between high and low aspect ratio sails of a similar square footage- high aspect ratio sails generate more power per square foot. I assume that a high aspect V fin of a similar area will provide more stability for the tail of the plane..? It looks as though I might be visiting this area of thought for a while...

For now I'm going to pore over the Drela design links you've provided. There goes my morning..

Cheers! Ted

Shredz,

Yaw stability and spiral stability are two different things. Yaw stability is the tendency to return to non yawed flight after a gust or control input causes the aircraft to yaw. The bigger the vertical tail and the longer the tail moment arm, the greater the yaw stability. Spiral stabilitity is the tendency to recover from a roll. The greater the effective dihedral the greater the spiral stability. Too little vertical tail area and too much dihedral result in an oscillation called a Dutch roll which is a combination of lots of spiral stability and not enough yaw stability.

You really need to stop wasting energy trying to construct analogies and spend your effort reading an easy aerodynamics text like Model Aircraft Aerodynamics by Martin Simons. This will give you the fundamental concepts and foundation of understsnding you can build on.

Yes, that's what I was saying. You wouldn't want to just take the dihedral out of the poly wing and fly the model as is other than that one mod.

I had the chance to fly a Dodgson Windsong back when I was flying a lot of glider time. It was just accepted that the Windsong needs a lot of "top" aileron to keep it from tightening the turn up into a spiral. With what I learned from that 2 meter design I'm pretty sure the Windsong would have benifited from a fin and rudder of about 10 or 15 % less area.

As for the high aspect tail surfaces you may get away with it on the fin and rudder but watch out for the stabilizer. I've seen many mentions in design article that say you don't want the aspect ratio of the tail surfaces to be close to the wing aspect ratio. They said the lower efficiencies of the narrow surfaces (lower Reynolds numbers) could cause premature tail surface stalling. In effect you wouldn't be getting your money's worth out of the tail and you could actually generate more drag due to the surfaces stalling at reduced throws. I'm remembering this from a long ways back so all this may not be 100% but the bottom line was that you didn't want the tail surfaces to be anywhere near the same aspect ratio as the wing. The general rule was that it should only be about .6 or .7 of the wing's aspect ratio. In fact I believe this may have been in Simon's book.

One way to gain some psuedo aspect ratio for the find and rudder is to use a T tail. The stabilizer acts as an end plate for the fin and effectively increases the vertical area by 10% or so as I recall.

Once again, this is not my personal discovery. It's known as the vertical tail volume coeffiecient. I remember a chart was drawn up for a number of popular designs of the day (early 90's) and the coeficients were compared to the handling charactaristics. It was noted that there was a "sweet spot" in the coeffient range with models on either side being noted for strange handling quirks while turning. It was this info in conjunction with my free flight background that got me thinking about how my design was behaving and the possibility of making it better. But it's an imaginary number that has no meaning aside from comparison with other similar designs (ie: poly or flat winged) and using actual flight testing to place meaning to the number.

And yeah, you're reaching a bit with those analogies...... I got lost trying to follow it and figure out how it relates to the flying...... it didn't...... sorry.



Ollie-

About the yaw and spiral stbility thing. During my testing I saw both ends of the spectrum. When the vertical area was too large the model wouldn't even fly straight and level without constant correction. Left to itself it would diverge into a spiral dive. As I cut the fin away this tendency was reduced until I went too far and now the model had very poor yaw recovery and showed large amounts of adverse yaw when rolling into turns despite the large amounts of differential aileron I was using. So I would have to suggest that the two are related as opposite ends of a single effect. Adding the last couple of bits back onto the fin restored the yaw recovery traits without ruining the behaviour while turning.

I also found that these traits responded strongly to minor movements of the Center of Gravity. The more reaward the balance the worse the spiral instability.

BTW. Not all free flight designs had marginally small fin areas. The Goldberg Clipper is one model that has a very bad tendency to fall off into a spiral and not recover. I've seen this happen and it isn't a pretty sight. The cure was to not get greedy with moving the balance point too far aft. The later Clipper Mk2 with poly added to the tips was able to tolerate a much farther back balance point and still be spirally stable. Less vertical tail volume.

When I tried that experiment I had no idea just how closely tied together the balance point and vertical areas were. And how much even a few percent change in area would affect the flying charactaristics of a design. It was certainly educational to say the least. I was literally able to move the model's flying traits from Jekeyll to Hyde and back again with about a 15% change in surface area or a 5% shift in balance point positioning.

Hey, look what I found.

http://members.cox.net/evdesign/pages/database.html

OK so the table of design info isn't readable (unless you guess a lot) But the chart of vertical tail volume to dihedral angles does show some relation. It's a grasp but if you imagine a straight line through the median it does rise with the increase in dihedral.

And here is a site that has the formulas for both the volume calculations. It's gratifying to my testing results that the center of gravity is used for calculating the vertical volume as well as the horizontal.

http://www.eaa62.org/technotes/tail.htm

I feel vindicated......

Gotta go look for spiral instability nowl..... I'll be back.

No sweat about my analogy - I guess the point I was trying to make there was I got what you guys were saying. I guess what I was trying to get across got lost........but I know I could explain it better if I thought of it long enough

But, in my mind- from what both of you have said, I've got the picture and have a grasp of- albeit in it's rawest form- the concepts involved in making changes like that. And that's why I came here to post.....

Thanks again to ya!


Ted

PS Those are killer links.

Hi, it's me again. Yeah, the pest....

Found this in a question of the day thing at DJAerotech. Pretty well sums up what I found (and it agrees with my findings so it must be right ). And since I'm sure you guys are tired of reading my ramblings sit back and digest this.....

Now let's talk about "Spiral stability" and "Dutch roll". Spiral stability is the tendency of a model to roll out of a turn by itself while the controls are centered. As should be apparent from the above, this is usually near neutral or even slightly negative for most aircraft (i.e.: they want to steepen up the bank angle in a turn). Dutch roll is a tendency to oscillate side-to-side in both yaw and roll, something like a falling leaf. On full scale aircraft it's an excellent way to promote a demand for additional airsick bags from the rear seat passengers. Spiral stability and dutch roll are intimately connected, both controlled by the balance between dihedral and vertical fin. Too much fin or not enough dihedral and you get spiral instability; too much dihedral and/or not enough fin and you get dutch roll. It's very difficult to find that exact balance where you don't get some of one or the other.

So how does spiral stability work? Imagine an aircraft that has just dropped a wing in response to a gust. It hasn't really started turning yet, so there isn't any centrifugal force of significance at this point. The lift vector's horizontal component starts pulling to one side, and its reduced upward component allows the model to begin to descend. In effect, the model looks like it's sliding down along the slope of the tilted wing. At this point a couple of things happen. The sideways motion acting on the dihedral of the wing causes a yaw that increases in angle of attack on the down wing panel, and decreases the angle of attack on the other one. This causes a difference in lift between the two, that tries to roll the aircraft back to level. On high wing models, the yaw can cause a difference in air pressure on the sides of the fuselage, which increases the lift of the down wing panel, and decreases the lift of the other panel. On low wing aircraft the opposite can occur. This is why sometimes a high wing aircraft can get away with less dihedral than an equivalent low wing design.

Meanwhile, the fin and rudder are also reacting to this yaw. They attempt to yaw the aircraft back in line with the new airflow direction, even while the dihedral effects of the wing are trying to roll it back the other way. If the fin and rudder, plus the overbanking tendencies that take hold as the aircraft begins to turn, are powerful enough, their corrections can cancel out the dihedral's attempts to roll the aircraft back towards level flight. The aircraft stays banked, and may even tighten up into an eventual "graveyard spiral", the classic example of spiral instability. On the other hand, if the dihedral effects dominate, then the aircraft will tend to roll back towards level flight. However, if the fin/rudder effects are unable to zero out the yaw as as the bank angle comes back to zero, then the aircraft will continue rolling into a bank in the other direction. This can lead to a chain reaction, with the yaw and roll effects chasing each other back and forth like a dog chasing its tail. This is our old nemesis, "dutch roll".

For trainers, it might seem that shading the balance in favor of more spiral stability might be desireable, for that often advertised (but seldom delivered) ability to roll out level if the controls are released. Unfortunately, the ensuing dutch roll is likely to drive beginners crazy! The exception might be very lightly loaded trainers with very long tail moments (like our new 2-meter Chrysalis), where the curvature of the airflow in the turns can be used to help the spiral stability without getting into dutch roll problems. This generally isn't as effective on aircraft with higher wing loadings because the turning radius is larger for a given bank angle, which reduces the curvature of the airflow. Of course the tail moment must not be TOO long, since too much of a good thing there can make consistent, tight turns very difficult. Some of our competitors' products with unusually long tail moments have been observed to have some trouble in that regard. Everything in an aircraft design influences everything else. If you go overboard trying to fix one problem, you're likely to create new problems in other areas.

In any case, too much spiral stability is not necessarily a good thing, because it can make it more difficult to do smooth turns. Somewhere close to neutral spiral stability is generally accepted as the most desireable setup. On thermal soarers we spend a huge amount of effort in both the engineering and test phase to get a design that will go around and around in perfect circles all day long with an absolute minimum amount of control inputs.

From stability.html at DJ Aerotech


..... and now back to your regualarly scheduled programming....

That's about it. This was part of a larger post about pitch and yaw stability. You can download the whole series of questions on a lot of different topics HERE and the name of the file that has the stability stuff in it is "stability.html".

There was a bit on winglets and their effect on spiral stability but I clipped it out of the stuff for this post as it didn't relate to "normal" layouts. It's in the full meal deal though.

Ollie,
Could you suggest a book on the topic? I'd love to explain things in terms you'd understand

Bruce,
It appears you are in my area. I'd like to have a closer look at the Chrysalis HLG and the 2m. Which shop would be the best one to see them?

Ted

I did suggest a book in post #9.

Good luck finding much for good gliders around here. I think you'll find that you're pretty much stuck with mail order and reviews from online.

The best advice I can offer is to get in touch with the Okalla Hawks club and join up. This is the only group that is hosting contests in the area and is flying high zoot contest type models. Most of the park flying folks are content with fairly simple poly designs. Not that there's anything wrong with that as the realities of flying out of local parks don't require super high performance. But if you have aspirations to fly higher performance types then it's the Hawks for you.

Thanks all for all your input!

Ollie, Thanks. I guess I missed your reference to "Model Aircraft Aerodynamics" by Martin Simons: it will be in my possesion as soon as I track down a copy.

This site is a wealth of knowlege and experience..... thanks for the education.

T