The Why of Clark-Y
 

Wikipedia has this to say about the prevalence of the Clark-Y airfoil in model airplanes:

"The Clark Y has found tremendous favour for the construction of model aircraft, thanks to the flight performance that the section offers at medium Reynolds number airflows. Application on model aircraft is very wide, ranging from free-flight gliders through to multi-engined radio control scale models.

The Clark Y is appealing thanks to its high camber, which produces a very good lift-to-drag ratio on comparatively lightweight balsa models, and for its near-horizontal lower surface, which aids in the accurate construction of wings on plans mounted on a flat construction board.

Young and inexperienced modellers are thus able to build model aircraft which provide a good flight performance due entirely to its aerofoil shape. The benign stalling characteristics of the airfoil are another aid, as this allows an inexperienced radio-control model hobbyist a better possibility of recovering from a stall in flight, to the benefit of the model.

The depth of the section lends itself to easier wing repair, as the modeller will often then have better access to balsa structural elements. In addition, the Clark Y section's depth also permits the flush installation of what are considered "standard" sized R/C servo motors within a wing for actuation of ailerons, flaps etc., without an excessive reduction in performance."

RE: The Why of Clark-Y
 

In short, using the Clark-Y can make a wing easier for a novice to build and make the model easier for a novice to fly. But just how much easier to build and how much easier to fly? For most WWI aircraft the Clark-Y airfoil is painfully, obviously out of scale, the exception being the "fat" airfoils of the cantilever Fokkers, for example, the DVI, DrI, DVII, and DVIII. These generally don't look too bad with a Clark-Y. But most other WWI aircraft had a much thinner airfoil, sometimes razor thin. A lot is made of the problem caused on "smaller models" by the effects of the Reynold's Number ( http://en.wikipedia.org/wiki/Reynolds_number ). But what sort of a factor is this in reality? Is it a significant factor on the larger 1/4 and 1/3 scale WWI models which are quickly becoming the norm?

And most importantly what other airfoils ofter a good compromise between scale appearance and reasonable performance? In my opinion, the Mick Reeve's WWI models have proven that a completely scale airfoil is a viable option on larger models.

RE: The Why of Clark-Y
 

Back before I got back into RC I considered building an Ultralight airplane. It was the wife who suggested I get back into RC instead. I'm glad she did I can have more airplanes this way! ;) It may sound strange but I found a lot of Ultralights use a modified Clark - Y airfoil probably for the same reasons.

RE: The Why of Clark-Y
 

I do agree though for scale airplanes their are times the Clark-Y just doesn't look right.

RE: The Why of Clark-Y
 

Reynold's Numbers are the conversion factor for air molecules. You can't scale the air. We talk about it in reverse; the air tunnel guys build small and dream big; we get stuck making big to small. That is where it gets tough. 1/4, 1/3, 1,2 scale planes get large enough where the numbers start becoming more reasonable. Think of it like one of those guru people who sleep on the nail beds. When their body is large they get to distribute their weight over more points. GI Joe size and he has been pungy stuck. The nails dont change size only the person.
Clark Y and its competition USA35B( Cub) were the first step away from the WW1 style undercamber which have a very narrow control range; the stall is ugly and the top speed is limited on how much down trim you can add. The Y could do every thing better. A Cub is about the same size as a N11; which one performs better? The next advancement would be the M-6 which is faster still and does better upside down stuff with a good stall, and then you go symmetrical. When we replicate a scale plane we replicate the problems too; taking the out of scale is just turning it into Twilight Zone physics wise. Dont think the WW1 guys would not have wanted a Pitts Special; they would have been unstoppable against any of the WW1 planes.

RE: The Why of Clark-Y
 

Surely though there must be some nice compromise airfoils for WWI models, for example, some of the Göttingen airfoils:

http://www.ae.illinois.edu/m-selig/a...ots/goe238.gif

http://www.ae.illinois.edu/m-selig/a...atabase.html#G

Also what's the relationship between a thinner airfoil and the necessary angle of attack? My impression is that the thinner the airfoil, the greater the AofA.

RE: The Why of Clark-Y
 

Also, I've heard of guys "shaving" the tops of the BUSA ribs to get a more scale profile with no ill effects.

RE: The Why of Clark-Y
 

The Russian aircraft of WW2 predominantly used the Clark Y, so that is easy for those scale models. The problem with the WW1 airfoils is for the most part they just were not very good. As proven by the direction Fokker went. Thickness has many advantages other than just structural integrity. Those heavily cambered thin airfoils used by the allies really created a lot of drag. Couple that with flying wires and 2 wings. I am sure there are several cheat airfoils that could be used for scale, allied, WW 1 models. The main thing I believe would be to eliminate some camber and try to get a little thicker.

RE: The Why of Clark-Y
 

That's a nice analogy. Smaller model = fewer air molecules pushing it up.

I can't help thinking that the reason that the Clark-Y is so well established in model airplanes is that for most of the history of scale modeling the models were pretty small. But now that 1/4 scale is starting to be the "norm" (and 1/3 scale quite common) for WWI models, do we really still need the Clark-Y?

RE: The Why of Clark-Y
 

An idiot can build a Clark Y wing; I know, I have built many. Building an undercambered wing scares people; they invest time they dont want to worry. Clark Y will always be king just because of that. It is still going to fly better even if the better is not all that better. It is the airfoil they learned how to fly on. If your hand cutting ribs, half the rib is done with a straight edge. All it does is separate the men from the boys. Cessna airplanes hold their value not because they are better planes than others; its because 99% of modern pilots learned how to fly in one and they are too scared to move to something different. Not bad planes but a 172 will sell for equal or more than a M20C-E Mooney on average even though the Mooney is faster and has retractable gear. The Y is the comfort food of airfoils.

RE: The Why of Clark-Y
 

A close cousin of the Clark-Y is the USA-35B. That's the airfoil that is used on the J-3 Cub.

Just my $.02

Bob

RE: The Why of Clark-Y
 

Here's something to chew on:

Don't hand launch gliders and most small rubber models use thin, undercambered airfoils ? If a Clark Y or USA-35B is better, why don't the highly competitive HLG guys use the thicker airfoils ? And how did full scale designers of the WWI era get started using thin airfoils in the first place ? What about the Wright Brothers, didn't they use a wind tunnel to develop their thin airfoils ?

Hint: These questions will lead you to an important insight for your WWI models.

Dick Fischer

RE: The Why of Clark-Y
 

A couple of things spring to mind here.

WWI aircraft had to climb, usually as fast as they could. Models of WWI airplanes are flown at a low altitude, (several hundred feet), and don't have to go far. Also, we usually use our throttles a lot more than in 1918. So our "mission profile" is rather different to that of the original. We also understand how to improve on the original's flight characteristics. For example, the ailerons on a Clark Y wing usually work pretty well. Those on an RAF 15 wing usually need a boot full of rudder too. Which one is easier for a weekend warrior to fly?

Martin

RE: The Why of Clark-Y
 

I think there is a certain amount of apples to oranges when comparing thick to thin airfoils. If you really want to take it to an extreme take a look at some of the ultra-lightweight microfilm models and their single surface undercambered airfoils. Thick airfoils existed before WWI. At least one of the Antoinette monoplanes had a (relatively) thick airfoil. While there is no denying that more modern airfoils have generally speaking better performance the thin undercambered airfoils worked well given the materials and structural science of the day. They were flown at a different range of airspeeds. I have seen a thin airfoil on a scale Demoiselle (spelling?) that was capable of flying very slowly with just a .40 size motor. An undercambered airfoil isn't going to do a good job of screaming around the sky like a Yak or a Mustang but if you are building a pioneer aircraft that is going to put around at a scale like speed they work pretty good. The fact that some of their structure is external of the wing skins in the form of struts or bracing wires can be intimidating but it is possible to cheat a bit with materials that are now available such as carbon fiber.
Regarding the Clark Y, I'm not that sure that all of the airfoils labeled as such really are true Clark Y airfoils. It is pretty easy to call a flat bottomed airfoil with a rounded nose a Clark Y. As TFF notes it is a comfortable label. Abufletcher describes that it is easy to build, it's also easy to draw. It was the first airfoil I ever learned to draw. As I was rambling a thought occurred to me related to being comfortable with the flat bottom air foil as it relates to high fashion. You can be comfortable or you can look good and you look marvelous.

RE: The Why of Clark-Y
 

I would say that the focus may not be so much on thin, but the extreme camber. Maybe I need to get current on the latest airfoils, but , for example, the Eppler 203 is a good glider section and it is fairly symmetrical till the "tail" drops.
I believe the Fokker DVII in particular was a better design than just about anything else, because of the wing. The ability to be strong enough without all of the draggy wires. It seems the first major improvements coming out of the great war was the elimination of drag mainly. The thicker airfoil just allowed this to happen until fast forward to now, we have the materials to build a thin wing, strung enough to support itself in a monoplane configuration.

RE: The Why of Clark-Y
 

By the end of the war most fighter missions were above 10,000ft, more like 15,000. They were just learning what altitude sickness was. That is one reason the BMW powered Fokker DVIIs were so tough; high altitude engine. Dogfighting was always an altitude loosing proposition. Climbing made you a sitting duck. SE5s, SPAD and DVIIs were gun platforms and relatively fast; climb high and dive on the unsuspecting. Stuck in a dogfight Camels and DrIs could brawl, but the faster planes if the pilot was smart would dive away to fight another day or far enough away to make a safe climb.

RE: The Why of Clark-Y
 

I read somewhere an article relating to the performance of the RAF 15 Airfoil (used on many Allied WWI aircraft) and it's performance at various Reynolds numbers.

The conclusion seemed to be that for the sort of airspeeds that matter for WWI full size and scale models, this particular Airfoils was a good all rounder.

I will see if I can find the document.

Cheers,

Hugh

RE: The Why of Clark-Y
 

My gut feeling is that we WWI modelers could use scale or near scale airfoils with relative safety. But not if we want a model that flies BETTER than the original.

RE: The Why of Clark-Y
 

mgnostic mentioned an Antoinette with a fat wing. This was the "replica" built for "Magnificent Men" as they were unwilling to put up with the flight characteristics of the original.

The new indoor models work very well with moulded foam wings because of the low Reynolds numbers. The microfilm guys have been using single surface wings for years with simple arcs or logarithmic curvature and about 5% camber, (IIRC). This is the extreme end of "thin" airfoils.

Almost any airfoil will work for basic flight if the wing loading is low enough. Raise the wing loading and/or push the flight envelope, and specific airfoils start to work better. I agree with Don regarding the suitability for scale WWI airfoils for our models - the challenge is to building them strong enough. That's a large reason for the fat wings we see on BUSA planes. They are thick enough to be cantilevered, an important consideration for many people.

Martin

RE: The Why of Clark-Y
 

Hello,
I want to say something about it.
The Antoinette had not a Clark Y profile, not the original and not the replica.
Before the World War I there was no Clark Y wing, <span class="hps">developed</span> <span class="hps">1922.
</span>The thickness of the ribs does have a large impact on the aerodynamic and Clark Y <span class="hps">is a</span> <span class="hps">profile</span> <span class="hps">that</span> <span class="hps">is ideal</span> <span class="hps">for all aircrafts.</span>

Greetings matz

RE: The Why of Clark-Y
 

The flight charastics of WWI planes were abismal compared to later aircraft. Ask any one who has flown the full scale.

A sharp leading edge leads to a quick and pronounced stall often resulting in an unrecoverable spin. The undercamber design was copied from observing bird wings. Where else was one to look for working examples back then?

Fokker did get it right on the DVI and the DVII. A thick airfoil, plenty of lift and no unsuspected stalls or spins. Those two conditions killed many low time pilots of the day.

RE: The Why of Clark-Y
 

I've heard this comment about the sharp edge inducing stalls but don't understand why this would be the case. As for the undercamber being originally copied from bird wings, I have my doubts about this type of story. I think the early builders were a lot more methodical than this...and in fact, most of the early ideas about airfoils came from experience with the kites and gliders (manned and unmanned) that were the precursors to powered flight. Originally, these "kite wings" were in essence just "curved single surfaces" which eventually developed into "curved double surfaces."

Regarding the use of "fat wings" to avoid needing functional rigging, my feeling is that, if you don't want to have rigging, you shouldn't be modeling aircraft that needed it. ;) That's sort of like me wanting to build WW2 stuff but complaining about retracts.

RE: The Why of Clark-Y
 

With undercambered wings you can make a lot of lift at slow speeds, but the center of lift moves around more than it does on modern airfoils. As it moves around the CG is either in the right or wrong place, something hard to control. Their limited knowledge and horsepower it was the only way and they were damn smart to figure it out. If engines had been 20s-30s quality the answer would have been different. It is one of those world history culmination of Industrial Revolution and Enlightenment.

RE: The Why of Clark-Y
 

The elimination of flying wires was not mentioned because they are a pain to model, but because they had a negative effect on drag. (on the full scale) Another thing that we see with Fokker was the EV/DVIII. With the parasol wing being fully sheeted, thus ushering in skin strength, which creates more strength as well.

Interesting for me this last year, I flew a Triplane model with a scale like cambered section. Only having experience with the Balsa usa airfoil on my WW1 models, I was surprised to find how ineffective the ailerons were. The stock BUSA models can be flown around all day with just ailerons, but that camber forces you to use rudder otherwise they will not turn, so I found.
This is good marketing as modelers do want easy to build and fly planes. Of course we can cheat with mixing as well if we want now. The lesson I learned though was that heavily cambered wings need rudder to turn. In fact it seems that you have to lead with the rudder.

RE: The Why of Clark-Y
 

I just wanted to point out a quirk of WWI modeling. Can you imagine the flak I'd get if I showed up at a WW2 scale fly-in with a 1/4 scale Spitfire with non-retractable landing gear?!? [X(] Well, IMHO, using a fat wing and non-functional rigging on a WWI model is the same sort of "scale sin." It's something I hate about my own EIII. And part of the "hidden agenda" of this thread is to help me think about how I might replace it down the road...if the model survives the "remaidening" process.

As far as ineffectual ailerons on a scale DrI, I think that probably has more to do with how fat the BUSA ailerons are rather than the undercamber of the wing. The LE of the ailerons on my fat-wing EIII is close to 3/4 inch. The scale ailerons on a DrI were made of simple steel tubing, i.e. they were essentially flat.