I'll bite.... WHY, exactly, does a 74" wing snap better than a 78" wing? Is the snap in the stall, or in the rotation? Where does wing area/planform come into play... or leading edge radius? And yes, I do know that I've asked the million dollar question. (FWIW, I've got some great slo-mo video of a 45 inverted downline negative snap with my 3D.) Personally, I feel the answer lies in the power; how much kinetic energy is the model carrying when it ceases to actually fly, compared to it's drag, which is not totally dependent on wing span. Most current 2M planes with the 160DZ will snap way better than an Atlanta with an OS61 (pre-Hanno). The Atlanta (Aurora knockoff) is much smaller than 2M. So, what's the deal? (If you survive this one, we can discuss upline/downline 'snaps')

Just when you thought you had gotten off lightly, Mike!

Brian

Everything comes into play Brian. Taper, thickness, tip sweep, tip shape to a small extent, location of high point, balance (critical), thickness of trailing edges, airfoil shape, wing loading, tail volume coefficients, and one of the most important but usually overlooked aspects, mass.

You can't be that specific and that general at the same time.

LE radius has an effect on snap entry especially. But more important is the shape of the airfoil from the LE to the high point of the wing. If it's fat and rounded, it won't break as fast as a sharper airfoil. Hoever, as another general rule, sharp LEs tend to fly better at faster speeds. So a snap tends to be fast and violent. If you take the same plane and slow it down too much, which might be an excellent speed for another model, the snap might mush or wallow. There's a million ways to do snaps. Everyone has in thier mind what a snap looks like, and it's usually different.

Wing span is just one part of the total equation, which is why I said "all other things being equal". Same airfoil, taper, high point, balance, wing loading, the one with the shorter wing will snap slightly cleaner simply due to mass and newtons laws of motion.

Ok let's talk about wing rake. Again using a general set of parameters, more rake in a wing makes it more stable in a roll, and is effective dihedral. However, it has some issues with turbulent air, which is exaggerated by a sharp LE. What else is there involved? How much is too much and where does it begin to induce serious roll coupling at hard angles?

This is where planform comes into play. Why are all of the current designs using less total rake but more taper?

Also talking about snaps, the tail areas and moments come into play BIG TIME. the larger the stab, the harder it is to snap (for instance) what you have to find is exactly enough stab to make it stable enough to fly straight, but agile enough to break into a stalled condition and recover quickly. Any more and the scale shifts. What effect does stab thickness have if any?

Just sumpin to chew on.

-Mike

Ahh one more thing about LE radius and snaps.

A smaller LE radius will allow the wing to snap at lower angles of attack. this can be good or bad, depending on the degree and how it's done. What that also allows is for a plane to break into the snap more shallow and recover cleaner....in theory. It also makes the wing a tad unstable in certain conditions, and that depends on a number of other things as well.


-Mike

Where is Dick Hanson on this discussion? Dick, you out there?

sc

Similar questiom.Where is Dean Papas in the discussion?

Probably trying to keep poop off thier shoes

Because this whole subject has as many different answers as it has questions. But somebody has to try...

-Mike

Xeretai Niko,
Kitaxo kai yelao!
Both for the rest of us on the forum, and because my Greek is household rather than technical: I will switch to English.
There are way too many design issues for us to talk about in just one forum, and when I can't really answer a question completely, I choose to keep my mouth (keyboard?) shut.
But you called me out, so here I am. I promise not to give you the answer to "Life, the Universe, and Everything" in fifty words or less.

Pattern design is a tricky business with only a few good rules, and lots of conflicting theories.
On top of that, the behavior of airplanes is very non-linear: even a amall aesthetic change to an existing airplane will sometimes produce large changes in how it flies.
and other times, dramatic changes accomplish nothing.
At best, the experienced designers have collected more "rules of thumb" about what not to do.
After that, we are all slaves to fashion, and will spend lots of time trying to trim out the designs we drew just because they were pretty.

There are Pattern designs with the stab very close to inline with the wing,
and those where the stab is much higher and "out of the dirty air" as the proponents of this school of thought say.
There are examples of both kinds of design that fly very very well, and there really is more than one way to solve the design problem.
If you are eager to learn it all, then you are going to have to build a few airplanes, or at least one with all plug-in feathers, and a simple fuselage with many wing and stab tube holes!
In the meanwhile, you might choose to pay a flattering compliment to a proven designer, and steal ... okay, borrow the "numbers" of a successful design.
The wings, tail, and thrust-line placement of a proven design can provide a skeleton around which you can draw something you like.
Then start making small changes and see what happens.
Here's an interesting one for you: the Oxalys is obviously a very successful design, but all of the ones I have seen needed lots of rudder into UP elevator mix in order to fly properly. Why? It turns out that the canaliser (or "flow straightener in English) greatly helps this problem on some designs, and does nothing for others.
Why not start out with the Oxalys skeleton and see what changes with a different fuselage shape.
My pet theory is that the Oxalys suffers from a strange airflow when it is yawed: the air separates around the tall canopy and then turns toward the belly aft of the canopy.
In my opinion, this is the cause of the strong pull to the wheels with rudder, and a fuselage shaped more like a fish will not suffer this problem. So will one with a spine that stops the cross-flow.

I have designed more than just a few really good airplanes over the years, and still have more questions that answers !
Later I will attempt to answer your question about taper ratios.

Yia,
Dean Pappas

Wow, I'm on the same page with Dean! Cool!

Funny, but that's exactly how I designed my first plane. I took some basic measurements that I knew were sound, drew a straight line on a piece of paper....made tick marks where I wanted specific measures, and connected the dots. And that's where it started. By now it's really changed a lot, but I had to start somewhere.

Refining is an ongoing process, and you're never actually "done". That's one thing I think people don't quite get about pattern designs, is that they are not static. They are dynamic. Even the gurus are just more experienced and have just simply tried more things over the years. Some work, and as Dean mentioned, some do NOT work....but on a different design, the thing that didn't work may be just the ticket. And that's the problem. You can't just change wings on a plane and expect it to fix everything. You have to change or at least slightly tweak almost everything. The better the plane flies in actual testing, the less tweaking is needed. But we are a bunch of perfectionists, seeking perfection knowing full well it's unattainable. But along the way some darn fine pattern planes get made!!!

I have a question, one that I'm pondering right now. How does the thickness of a vertical fin/rudder come into play? It seems in general that a thicker one may be better for some of the current designs, but I'm wondering to what degree. Of course, I have about 5 different fins and rudders made to play with and see for myself

Yeah....I'm just the crazy one that posts this stuff knowing how many people are either not interested or completely disagree. I just hope it helps somebody out there.

-M

Hi Dean,

It was a great pleasure to read your answer to my question. I used to read your articles in Model Builder magazine during the very early stages of my involvement with R/C.

Now that I'm older I still have lots of questions, that I hope to be answered here.

Actually I don't own an Oxalys, I used to have the smaller version, so it is not possible to test the model. I thought that the T canaliser's role was to direct the airflow towards the tail and make the rudder more effective. I did not know that there were pitch coupling issues with that model.

To be honest I like the designs that have the canopy towards the engine cowl, like Oxalys, Integral, Evolis etc. in older designs the canopy used to be further back. Do you think that Oxalys' tendencies are due to a wrong location of the canopy?

Regards,
Nick

Wow, much has been discussed since I last chimed in, and I am definitely out-classed with the likes of Dean Pappas and Mike Hester in here. Having not done much design myself, most of my opinions are based on theory, some mine, some borrowed, and some learned just by observation. I just felt like chiming in on a couple of things that were mentioned, but not expanded on much. First is the thought that some have that the horizontal stabilizer needs to be out of line with the "dirty" air coming off the back of the wing. Now from what I've seen of airfoils in wind tunnels, there isn't much turbulence on the back side of the wing as long as the ailerons are not deflected, especially symmetrical airfoils. If the air becomes turbulent while travelling across the wing, it kills lift. On most modern pattern designs the ailerons begin so far out on the wings that only the last inch or two of the stab would be affected by the turbulent air coming off the wings. Therefore, my feeling is that having the horizontal stab in line with the wing is not necessarily a bad thing, but whether or not it is needed there is another question, and has a lot more to do with the vertical placement of the wing on the fuselage. The next thought I wanted to throw out there is in regards to Mike's comment regarding vertical stabilizer thickness. My feelings on the vertical stab were seeded by Bryan Hebert and a conversation we were having as I compared my Genesis to his Shinden, and he got me thinking on the subject of the vertical stab's airfoil. He feels, and I agree after much pondering, that many designers overlook the importance of the fact that the vertical stab is a flying surface, which means that at certain attitudes of flight it will act just like a wing, generating lift (knife-edge flight, snaps, spins, or lateral lift anytime rudder is applied). He pointed out that one of the flaws of the Genesis was the extremely thick leading edge on the stab that then has a constant taper back to the trailing edge. If you were to turn this stab on it's side and imagine it as a wing, well, let's just say that dog won't hunt. Now, if viewed as just part of the fuselage, it looks just like it is part of the airfoil created by the fuse, but unfortunately the stab sticks out into the air all by itself, and will not be very effective under certain conditions. Now, that being said, in my opinion the thickness of the vertical stab is not that big of an issue, as long as it has an effective airfoil, and affects asthetics more than flight. As Mike correctly pointed out earlier a thick leading edge on a wing will decrease the angle of attack needed for the stall as you enter a snap. This principle can be applied to any airfoil. In other words, the vertical stabilizer with a very thick leading edge will stall easier, especially when you don't want it to (slow roll, snap roll). When an airplane is performing a snap roll, or a spin, only the wing is stalled, and the horizontal and vertical stabilizers are flying the airplane. If you lose control (stall) on either one of these surfaces during the maneuver you either end up with an ugly (sometimes zeroed) maneuver, or even worse a very expensive pile of splinters (or glass/composite materials). OK, my wine is starting to kick in and I can't keep a decent train of thought anymore, so I'll stop here, and leave these opinions to be eaten alive by Dean and Mike. Have at it, guys!

Oh, and as far as I can remember, the answer to "Life, the Universe, and Everything," aka the "Ultimate Question" is without a doubt 42. I've thought about it for quite a while, and it's 42.

If you are really keen of trying something else that is different -but works -- start with a lot more power -
a 22-24" prop - on a light spark ignition2 stroke setup
these are now extremely smooth runners and the big prop really makes speed control easier
you just dial in speed at will - slower or faster --

Dick, how about diesel? From my understading, diesel has a very consistent torque curve, allowing larger props to be swung. Similiar to electric dont you think? larger props, slower rpms, more consistent throttle response, no need for special tuning.. etc

your thouhts?

Chuck

the diesel engines used as model engines run on ether and oil and really lousy as throttlable powerplants -
I tried a conversion years back and the inability to control point of ignition made it not suitable .(compression controls ignition)
Glow engines are a step up from these and tho the nitro "softens " the critical nature of the glo engine - it is still -in my book NOT the ideal engine
the new generation of spark ignitions is really getting good and on alky and a tiny splash of nitro - are extremely smooth runners . The problem--is thay use lots of fuel -compared to gasoline .
a lightly stressed , large displacement, mildly tuned, two stroke , spark ignition is -in my book the best possible setup.
It is all a compromise -- Don't compare these to converted weedeaters --
If you have not actually ran these and flown them -don't be too quick to dismiss em .
40cc engines seem about right for the current 5 k weight limit-I did a few models with this and I thought they were terrific - others said they were grossly overpowered when they tried em - but that was -at the time -the objective. Noise and vibes wer also a problem -I think I am close to correcting these last two problems - A new ignition system is on the horizon and the carbon fibre exhausts are working extremely well.
By the way Ed Skorepa is doing the carbon fibrework (I won't touch that stuff) the ignition is a Czech design- the engine a lightly modified ZDZ rear intake /rear exhaust.
OR you can go with electric motors - The power curve of these is the direction we are trying for.

i would love to go electric but the batts are killers.

Chuck

Much more expensive than practical and the planes have to be lighter .

Dean,

But we already KNOW the answer to "Life, the Universe, and Everything". It's 42. But what is the QUESTION??!!

I just couldn't help myself.

Jon Lowe

PS, for those of you that don't get it, read "Hitchhikers Guide to the Galaxy". Don't Panic!

Some may disagree with part of this statement.
For me it's more practical to run the E.
The money I have invested in my E ship is the same as setting up a ship with a 160 DZ.

But to each his own.

The planes do have to be lighter though.

Now, keeping with the thread. Here's a design idea. I've mentioned this before somewhere. Yes the E planes need to be built lighter. Fair? Probably not IMO. Takeoff weights have to be lighter for E than they do for glow (not trying to start a discussion on this as it's already been covered). Therefore in the wind the glow plane can penetrate better. (I fly in a windy place) So flying slower with the E and being lighter isn't an advantage at times. So I have had the idea of mounting a fuel tank in my E plane and adding 16 to 20 ounces of fuel. It's gotta be legal right?
I may implement this in a future project when I get a little lighter. May even bring it to the nats.. That'll be interesting to hear what they have to say. LOL

C

One thing I learned attending my first Nat’s this past summer is that the rules are not always black and white (even when they are clearly written!) One must also consider the intent or spirit under which the rule was created.

Interesting proposal, but I don’t think that would fly…wrong kind of fuel for your powerplant of choice!

Who sa it neds to be fuel, you could load the plane up with a pressureized container of fluid, then, upon completion of the flight.. purge the tank. land, weigh, and be under 11

just a thought.

Chuck

Whats the advantage to carrying up more weight? I have always felt like a 10lb plane was easier to fly than 11, (or 11.5 and 12.5 wet weight). No matter what the wind condition. Takes less power to fly etc. Plus the lighter plane will slow much better and respond to throttle quicker than the heavier plane.

I have been patiently waiting for someone to come up with a gasser thats available and has a smooth idle, so much so that someone will take up the challenge do the R&D and carry it out; like Jason Shulman did for electric pattern.

Ive seen the ZDZ 40 in a plane, and the MVV 26 in a plane. Im really curious to see the ZDZ 40 F3A if they ever release it.

The advantage IMO is in the wind. If it isn't windy, I wouldn't want the extra weight. I find that I don't want an 10lb electric. I'd rather have it 12 lbs in the wind.
If you watch Troy Newman fly his Ys powered Pinnacle in the wind (as he fly's quite fast) he doesn't get bumped around. It cuts through the air. Not that his plane is heavy, because I doubt it is. But the slow flight of the electrics tend to get bumped around IMO.
Add a little ballast and you cut through the bumps a little better.
Of course once I go down from a 22x12 to a smaller prop I may think different. Just haven't done it yet.

I have seen people with glow that have a very very large tank and fill it completely on windy days. Landing with over a half tank still.
Same concept really.

C

Why not just fly faster?

Mainly because I like the performance of the 22x12 on my motor. I will be testing some other props soon though on my new ship as I may like a smaller prop better.

[8D]