I think the main mistake we made was that we tapered our wing a little too much and kept the airfoil constant. So the Reynolds number of the root of the wing was almost twice that of the outside of the wing with the same foil. So as we got close to the stall point the lower Re section stalled slightly before the root and that is why we had to add washout in the second wing to fix the problem. Also the tip was flying at, if I remember right, an Re of about 80,000 which was too low for the E432 (?) airfoil we were using.

Sound right?

Dick I have 9 foamies from the Shock fliers to 6 of my own design. I went brushless and Lipoly only a few months ago (my old Nimh packs finally got too tired - kinda like me) and finally got a model that could get close to the hi-alpha stuff. I don't fly them well enough under those conditions to notice what you are talking about (cold and wind and bad heart keep me inside in the winter) but I am looking forward to playing with it. I don't care much for hovering but the aerodynamics of high angle of attack fully separated flight is very interesting.

I wouldn't care to fly in the Osprey - doesn't that just look scarey. I am sure that when everything works it is great but that wing is sooooo little and when you are hovering all of those little bits of linkage have to work just right or big torque roll. I don't care for helicopters either. Wings is where it is at.

The university programs are an effort to make/allow the students put the theory to practical use. Dick you miss the whole point (on purpose I believe) of theory. It lets you understand what you see and it keeps people (and you) from having to redo every possible variation in a hands on approach every time you want to design an airplane. It also stops you from mis-interpreting what you see, like the results that let people believe swinging a dead cat about your head will cure warts. What you tend to call hands on is what an aero engineer would call a wind tunnel test. Consider that a high resolution aluminum and steel model can cost many hundreds of thousands of dollars and you can see why a manager would expect his aero types to apply a little theoretical and practical knowledge to its design. Not the same magnitude of course - but I certainly didn't want my heart surgeon using me as his hands on experience when he put in my valve. I would perfer he went to med school to prove he was at least book smart and had operated on 50 or so pigs to get the hands on problems ironed out. OK - extreme example - but there is a great deal of satsifaction in knowing why what you are seeing happens. It isn't too hard, I know lots of pretty dumb engineers that work in the field successfully (my bosses seemed to imply that I fit in that category).


I enjoy the comments about the flying of the competitions. I wish we had something like that when I went to college. The Purdue Aeromodellers would make up our own contests to get some competition going, unfortunately limited to CL and indoor freeflight stuff. I would have drooled all over the floor at the prospect of being able to work in a design like the guys do today.

The closest thing I had was a 15 pound or so Senior Telemaster with a geared AstroFlight 40 in it. Very slow and stately and the takeoffs took the engire length of our paved runway. But it chugged away. It gives promise of what could be done with a little design work and finesse.

Sounds right to me, little bits can go a long way.

I am not a graduate engineer - --
Just too interested in other things.
Maybe I misinterpret some of what I see in aerodynamics -
I look at it all as a batting average.

As long as I get the results I am after -
Does it really matter how I do it?
As for dumb engineers -
My ol flying buddy -who was Prof of Engineering- confided that he felt 10% of the graduates were really capable - and not from failure of their academic studies .
It is unfortunately true in many vocations.

Once, when I was asked what my BA in economics qualified me for, I answered that it qualified me to be a graduate student in economics.

I really see both sides here. As a graduate aeronautical engineer I have a healthy respect for the body of theoretical knowledge that illuminates the field. And since most of my career was in systems test and development, and flight testing, I believe strongly in the experimental approach.

The fact is that truth resides in a melding of the two. Theory not based on, or substantiated by testing is not particularly productive, and random experimentation not illuminated by theory often leads to invalid conclusions. But when the two approaches are combined, the results can be quite significant.

Keep it up, guys. I find it quite entertaining and sometimes I even learn something.

Being a model forum - practical experience is likely more easily absorbed by the hobby flier.
I see the formulas posted -which are relevant to those who use this type communication in a daily basis.
Theory is very important -in any technical field. I do understand this .
The formulas and charts given may be very applicable but for the purposes of this forum - communication about hands on testing and fixes relevant to hobby models seems to be a more workable approach in passing on info.
Not being a graduate from any University , I do however , find myself defined by a number of State and Federal Courts as an expert in a number of technical matters.
These involve both mechanical engineering and safety control systems for machinery.
I mention this only because I get repeated referrences of having a non technical background .
When I do my model experiments - I do try to prove -or disprove a question.
Sometimes the experiments fail .
Fail or succeed - I stay with em till I find out why .
That's what I believe modelling is all about.
I once taught theory to professional repairmen- to help them understand trouble shooting of involved equipment.
Watching the eyes of the students , it was evident that theory worked only for a few .
The desire to learn quickly turns off if the communication becomes difficult.
So if my comments seem very non technical - please bear with me .

Dick,

You may find some solace in this quote from no lesser a source than Albert Einstein:

"Imagination is more important than knowledge"

My take is that if you get the right answer then you used the right method. No man in his right mind would try to argue that point. However, considering these Forums, well----------.

As the instigator of this thread-----my thanks to all.

That was easy for Albert E. to say, he had absolutely no lack of either (including an incomprable computing brain) - he had hair problems but otherwise - pretty good stuff.

What Einstein did do was try to fit theory to fact.

Econ and History majors - the things that make dad's eyes roll up to the heavens! - and no I am not going to apologize to the econ and history majors in the group.

I thot Einstein tried to find a theory to explain theretofor unexplained phenomena.
The bad hair was from constantly scratching his head whilst cogitating the mystery of tip stall.

Alrightly here. If that was me you were calling out in the initial post, thanks. Nice to have my input valued. If not, lemme go fix this cramp in my arm. I think it's stuck over my shoulder in patting position.

Anyway, I saw this headline and avoided it. I knew dust would be flying and I figure I'd let you all beat on each other and then come in for the prize when you all got tired out. So, I read this thread once when it was about half as big, and just got into it again. lemme just put some stuff out there and then respond to a few things I've read. This is a huge post, so go get something to drink.

First let's try to define a tips stall as one that originates at or near the wing tip. I don't think it needs to be confined to that area, but let's say a tip stall starts at the tip.

I think a true "tip-stall" is really only likely to occur on long wings. \
]\
=]\]

Ahh geez, just spilled a some water on my keyboard. Maybe not so much on the something to drink.

What I was saying; I think a true "tip-stall" is really only likely to occur on long wings. Just becuase of the distances involved there is more likely to be localized stalling rather than a stall-front that bursts over the wing. On most sport plane wings, there is asymetric stall, but not so much from the tip.

One thing that is quite common is the tendency for one side to stall completely, and then the other side. This is what causes that falling-leaf motion on planes that aren't tuned for 3-d, and some that are. It's often called wing rock. I'm not sure of the dynamics, but I've seen an excellent photo from a smoke tunnel showing a model with hershey-bar wings where one side is fully 100% stalled and the other isn't. A review of the test read that the stall condition would oscillate from side to side and is a common phenomena. This explains perfectly the behaviour of my old Stinger when I tried to force it into high-alpha flight. Wasn't ever going to happen as long as that oscillation was going on. I think half-wing stall is what occurs to most sport planes whether wing-rocking or not. I think you could define a tip stall as a specific case of the more-general "snap-roll" term, where a snap-roll is any roll that uses wing stalling. I need to get that wind tunnel book and scan the picture in. LouW has it right as far as his tuff tufted flights.

The thing is, not that wings don't stall - like when Dick pops his foamies into an elevator manuever. It's that they stall evenly. That's where Dick's enlarged tail surfaces come in to play. (post 22) Not so much in maintaining near zero yaw, but just in forcing the wing to hold a stalled angle of attack instead of a transient partial stall, and probably some motion damping as well. The larger surface would do both better.

One the subject of tail effect, there is most certainly an effect of loosing tail control surface effectiveness that either results in enough yaw to cause a tip-stall or snap or just results in an unresponsive airplane that does it's own thing, often including roll, near the ground. This latter is really just a loss of dirctional control, but is often called a snap, because the pilot "lost" it and the plane did it's own thing into the ground. Lemme get into this a little more, and I even get to bring SAE into it.

Let's talk about the first case. A lot of SAE planes have long (too long) high aspect ratio wings. They are prone to the dreaded tip stall. At least when they had a planform area limit in the rules, they also didn't want the engine way out in front, with the cowling eating up valuable area. They also had rather small horizontal tails for the same reason. That means a critical balancing act in terms of CG location. Once backed into this corner, minimal tail weight was needed. So, with a larger vertical fin weighing more, to save weight, the verticals were pretty skimpy. So, go get heavy, run down the strip and pull up. Now you are at max CL with a twitchy wing and not enough vertical, and maybe a questionable CG anyway. whoop-de-do! A little yaw goes a long way in that situation.

I have noted with some of my flying buddies that planes with too-long wings but plenty of vertical did a lot better as far as tip-stalls and lack of them. The good teams design with controlability as the priority, the questionable teams design with lifting capacity as a priority and control as an afterthought. That's the lesson of the competition. First you need to design an airplane, THEN it needs to carry weight. Not the other way around. This fits Dick's supposition in post 67 to a T, but that's not the only effect for sure.

Now for the loss of tail response that "looks" like a tip stall but isn't. I have had the opportunity to fly some scale gliders, which tend to be some of the fastest movers in the soaring community. The problem with these is the same as with many scale planes, the size of the tail is a bit too small for carefree flying. I have a 100" Pilatus B-4 which is an excellent plane. Really aerobatic, rolls faster than once a second. It also will tip stall when asked and you don't need to demand it. It will readily do so. However, I have seen it at altitude during aerobatics and it takes at least 75 feet of altitude for the speed to come up, the yaw to go away and the plane to start flying again. It's quite a ride. Now, it also does something else that isn't a tip stall, but some might think so.

I've been flying it in the landing pattern, at a speed where it will keep flying in straight & level flight, with some reserve over stall speed even. However, when manuevering and pulling the stick, there comes a point where the wing is still flying, but the glider isn't responding. The tail surfaces have lost effect - at least enough to recover from a turn onto base or final. The plane flys, and being in a bank the upside wing is going faster. Without responding to the tail, the up wing goes further up and the plane wraps itself into a windmill turn. it's not a snap roll, in my opinion. It's still going forward, wiothout a lot of pitch. It's just doing it's own thing until it whumps into the ground. The way around this is to get on a stable straight in approach from higher up. Then the tail can just trim the glide instead of being asked to power the attitude of the glider past it's limits. The Pilatus model is tough enough, and has nice breakaway features that allowed me to learn this lesson a couple times with hardly any damage, but a couple of scuffs. So that's a possible pseudo-tip stall as mentioned in post 29

As for the down-going aileron causing tip-stall. It's a consideration, but in practice I have hardly ever seen it, having been to SAE for at least 10 of the events. Mostly the wing is going over before the aileron input and the lack of countering rudder doesn't help.

It's also worth notiing that FAR 23 doesn't necessarily apply to all general aviation airplanes - at least not the version you are reading. An airplane design only has to comply with the version of FAR 23 that was in effect when the design was certified. So, the Cessna 182 only need to comply with a FAR from the early 50's. [X(]

As for Bruce's wish that SAE would allow a group of modellers into the competition, wish no more. I believe they would. They wouldn't compete for place or prizes, but would be allowed to participate. I think they call that "competing Hors Concurrs" in the rules, which may be latin (or French?) for "for the hellofit". I don't have the rules in front of me but as a former rules chairman I think this has been allowed for ages, but never done that I know of. So go to sae.org, download the rules, and get to building. I have thought of this myself - where I'd sponsor my own prize ( when I had a good paying, permanent job) where I'd build a plane and give an award to any team that could beat me. 'Course I never had time to build such a plane. I do love flying them in the competition though.

I'd like to try some rules changes that would have students thinking a little more. How about making them carry qualifying weight - 8lbs cargo - of a close trimmed grass runway. One round, just to see that the plane can handle more than the often imagined flat-slate runway.

Also, give a bonus to a plane that can land AND STOP within the 400 foot box. Now that's a STOL competition!.

On the last page we have a discussion of formal education, engineers, and "theory".

I share frustration with bad or even "average" engineers. I suppose I sound egotistical but I hate being lumped in with that group. Especially when looking for a job these past couple years. THe industry has grown around people that can do one thing well, and there is a limited call for people that can take on various challenges and work thru them. My experience is filled with a wide variety of things, which I think is an asset, but probably turns off a company that just needs someone to sit in their cube and check that the right widget has left-handed threads and the left widget has right-handed threads, or write C code all day or whatever. I hear engineer, and I am often pretty cynical until I see what they can do, and I are one.

As for theory, the use of the word sometimes denotes a bit of uncertainty, or conjecture, but I wanted to be sure, so I looked it up with Webster's online.

Theory:
1 : the analysis of a set of facts in their relation to one another
2 : abstract thought : SPECULATION
3 : the general or abstract principles of a body of fact, a science, or an art <music theory>
4 a : a belief, policy, or procedure proposed or followed as the basis of action <her method is based on the theory that all children want to learn> b : an ideal or hypothetical set of facts, principles, or circumstances -- often used in the phrase in theory <in theory, we have always advocated freedom for all>
5 : a plausible or scientifically acceptable general principle or body of principles offered to explain phenomena <wave theory of light>
6 a : a hypothesis assumed for the sake of argument or investigation b : an unproved assumption : CONJECTURE c : a body of theorems presenting a concise systematic view of a subject <theory of equations>

So, are we talking about number one or number 6? If number one, I'm fine with that, because what we learn in aero engineers school is fact. This is stuff that can go out and be proven time & time again. There is no question that it works.

If we are talking about definition number 2 or 6, then I have a problem. And it seems at least some folks are. They refer to the difference between theory and hands-on reality, or that the rules of flight seem to be flexible or that "they" can't explain how a bumblee flies. The difference between theory and reality is almost always user error. People that learn about Bernoulli's equation and what it means, or what they think it means, and then treat that like it's all of aerodynamics. And when they can't explain something they see they say - ohh, theory and reality don't match. No, that really means they don't have the whole story, but want to shift the blame.

I guess you could call it theory in the conjecture (#6) sense of the word in that the user is supposing that what they know should explain what they see. And instead of looking at their own knowledge or interpretation - they just parrot some tired old variation of : "that book learnin is nice, but it doesn't always apply to the real world" Actually it does, they just haven't learned the whole of it. Or they didn't account for something in reality with their "theory" That's not a problem with the book, it's a problem with the accounting.

And, it's mildly insulting to put forth an absolutely rock solid explanation of something or another based on well earned education to have it completely discounted in favor of something that really is conjecture by someone who hasn't taken the time to understand what I'm talking about. You know who you are [X(] But here I'm ranting I expect to win an argument on it's merits, not on the fact that I have a diploma. I don't bring it up when making a point. But sometimes, it's like were talking a different language, and somehow it's my fault that someone else doesn't follow.

So as not to end on a gripe, I'l address the Einstein quote: ""Imagination is more important than knowledge". THat is a great line. If you consider being open-minded as a form of imagination, that's the key to being able to design. It's one of the stumbling blocks of new engineers, to be able to come up with(imagine) an idea and get it on paper - then using education and facts go back and analyze it and revise it to something workable. Then when you are pretty sure you aren't going to waste someone's time, go build one and test it.


Holy cow, it's after midnight. That's enough for me...

Yes, I think it is a misnomer. At least in reference to one wing dropping. As others have said, if both wings stall evenly, there is no wing drop, regardless of where the stall originates on the wing panel. Technically, the actual tip NEVER stalls, under any condition. The lift goes to zero at the tip and it has a lift slope of 0 (infinite angle of attack capability). BUT, you can have the middle portion of the wing stalling before the root and in that case, the entire wing panel reaches stall almost simultaneously. If there's even 1 degree difference between left and right wing panels, autorotation sets in and over she goes!

3d aerodynamics

John - Amen

Wes - Agreed if there is zero beta. I think what we generally see in models is the airplane gets loose in yaw on the slow speed landing approaches. That encourages the asymmetry which causes the stall. Most of the tip stalls that happen in this manner end up with the airplane in the dirt. It's hardly ever seen at altitude and normal flying speeds.

Are you sure the tip never stalls? Certainly at sufficient angle of attack the whole wing will separate which by definition is a stall. Your separation at the root and middle sections are for the perfect 0 beta case. Dick's little foamys (and mine when I can get it right) go through a trasistion phase that indicates this.

I was thinking about what is needed to cause the roll after stall last night. It occurred to me that it takes very little aileron to roll a model (non 3D maneuvers). It doesn't take a lot of wing separation in an asymmetrical mode to get a nice roll into the ground.

JohnG- -nice write -

Interpretive feedback is what I really want.
Positive or negative - -does not matter to me - It all serves same purpose .
Anyone who has not waded thru a bunch of failed experiments has learned very little.
The SAE events really sound like they are a lot of fun
I hope the students get a real chance to learn and benifit from them.

John,

That pleasant feeling coursing through your body is your arm un-cramping. You were indeed included on the original post.

For further thought (reflection) a couple of more quotes from Albert Einstein:

1. "Reality is merely an illusion, albeit a very persistent one."

2. "The secret to creativity is knowing how to hide your sources."

In addition to a mind of unbelievable proportion he had a wry sense of humor, which always seemed to be rooted in the truth.

I seem to be on a 'quote' kick lately so let me add one more which I feel might offer some 'tongue in cheek' dimension to your comment:

-------"I don't bring it up when making a point. But sometimes, it's like we're talking a different language, and somehow it's my fault that someone else doesn't follow." ------


This time the author is Mark Twain-----

"In Paris they simply stared when I spoke to them in French; I never did succeed in making those idiots understand their language."


Thanks for the input.

are we still talking about stalls gentleman

Stalls - which are a reflection of the fundamental rules that run the Universe just dummied down a little because it has to do with wings and air. After all, birds and insects even some small mammals are able to fly without a whole lot of intelligence. I don't think they ever have tip stalling now that I think about it. Can you imagine something that you stick insects or seeds in the front and it flys so beautifully - amazing.

Solch ist das leben.

Wow, had to check in on this thead, having so many replies for such a simple topic. Tip stall is simply the outer portion of the wing stalling before the inboard portion, which causes you to lose roll control (because your ailerons are in the stalled portion of the wing). Washout or a different root vs. tip airfoil section is used to ensure the inboard portion of the wing stalls first- wing stalls, you still have aileron (roll) control, the airplane naturally pitches down a little, the wing starts flying again- that's it.

Maybe this simple explanation was included in the thread already, but really it's too much to wade through for such a simple topic.

Yes, at the very tippy tip of the wing, there is no lift as the wingtip vortices are slipping off, but a "tip stall" is not meant to refer to the outer most .01% of the wing. And yes, sometimes the term can be misapplied, such as when someone is banking hard at a slow speed; the wing stalls and the plane falls or spins in in the bank direction- not a tip stall, just a stall in a bad attitude.

K.I.S.S.

Yes, but the real meat of the dicussion is: how often is this a really accurate description of an asymetric (rolling) stall? Not very often. Most of the time people say "tip stall" that's not what happened. You don't need a tip stall to have an accelerated rolling stall (snap/spin entry).

Funny, John, I edited my message seconds after you posted.