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The Why of Clark-Y

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Old 02-15-2013, 12:08 PM
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Default RE: The Why of Clark-Y

Funny, that DR1 I was playing with was blown up to 1/4 scale from a Guillows kit! I think it had some problems in design. No biggie, it was given to me, then I in turn gave it away. I am currently getting ready to buld a new DR1 as I lost mine due to radio failure a couple of years ago. I am going to make it more scale, but I will leave the airfoil alone as it will be a throw around model, and I like the way they fly stock.
I think you can cheat a little on your wing, and there are certainly ways to have the wires made to stay in place for transport. Things like having the wings slide on tubes to a center section over the fuselage, leaving most of the rigging in place. Also, the stretchy necklace material you can get from the crafts store make good rigging as well. Not functional, but easy to attach and detach for the look.
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Old 02-15-2013, 04:36 PM
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Well, my challenge didn't take too long for you guys to answer. There were glimmers in nearly every post. I'm no history expert, but as a lifelong modeler and an aero engineer, the pioneer era of aviation development has always fascinated me. Here is my construct for how we got to thin airfoils and later thick ones:

1. Early aviation pioneers of the 1800's and beginning 1900's (Pilcher, Lillenthal, Langley, Wrights, etc) looked to birds for guidance in their designs, especially since birds were the only successful game in town. And birds all seem to have quite thin wings.

2. Birds fly fine with thin airfoils because of the low Reynolds Numbers in which they operate.

3. The Wright brothers built a wind tunnel to measure lift and drag properties of candidate airfoils. Due to limitations in their tunnel, they made measurements at extremely low Reynolds Numbers, about the same regime in which birds fly. To make matters worse, the Wrights didn't yet know that testing at low Reynolds Numbers didn't necessarily translate to full sized prototypes operating at higher speeds. So based on their testing they failed to discover the benefits of thicker airfoil sections.

4. Following the lead of the early successful flyers, others used the same thin airfoils. They never realized the penalty in lift and drag that came with the thin airfoils.

5. By about 1914 a really substantial wind tunnel had been constructed at the University of Gottengen in Germany. It featured a much larger test section and much higher test speeds. In a short time scientists discovered the benefits of thicker airfoils. That's why many of the early thick airfoils are named, "Gottengen 123", etc. German designers such as Pfalz and Junkers quickly took advantage of this new technology. We see this in many of the WWI Fokkers and the Junkers monoplanes. There is an interesting paper given by Dr. Prandtl in the year 1920 that illustrates just how far the Germans had advanced during the WWI years. http://ntrs.nasa.gov/archive/nasa/ca...1993080860.pdf

6. Many designers stubbornly stuck with the thin airfoils, even into the 1930's. The British didn't really get into thick airfoils until the Hurricanes and Spitfires. Many American designers were equally unwilling to change.

7. So here we are today. Nearly everybody has embraced the thick airfoils. But only a few seem to realize that thick is of no particular benefit at very low speeds and small sizes. Certainly the birds haven't gotten the message ! But guys who fly hand launch gliders, Wakefield gliders and micro RC are doing just fine with thin airfoils because of the speed and size regime in which they fly.

8. Will a model of a WWI plane fly OK with its thin, original airfoil ? Yes it will, providing it's still down in the Reynolds range where fat airfoils don't help much. You may see some modest ill effects with the thin airfoils. More power required to fly, poor aileron response due to early airflow separation on the upper wing surface, etc. But if you are willing to put up with the small stuff, you can go ahead and use the thin airfoil.

One last comment and I'll shut up. BobH made the observation that most of the full size WWI airplanes were poor flyers. That's true. I help out occasionally with a large collection of WWI replicas. The two guys who fly them (I don't) tell me that they are mostly poor in the stability and control department. The DVII is perhaps the best of the bunch, but even it has slow ailerons. Most of the problems seem to be lateral/directional. Adverse yaw and slow roll response are pretty much universal among that genre. But that's mostly due to the tiny vertical stabilizers or the fact that some verticals are all-moving with no fixed fin.

Dick
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Old 02-15-2013, 11:20 PM
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Default RE: The Why of Clark-Y


ORIGINAL: otrcman



2. Birds fly fine with thin airfoils because of the low Reynolds Numbers in which they operate.

But they are able to adjust the shape of the airfoil and the wing plan as they fly... so a stall will probably never happen with a bird. They will feel it coming on and adjust accordingly.

Cheers,

Hugh
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Old 02-16-2013, 07:54 AM
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[/quote]

But they are able to adjust the shape of the airfoil and the wing plan as they fly... so a stall will probably never happen with a bird. They will feel it coming on and adjust accordingly.

Cheers,

Hugh
[/quote]


That's true, Hugh. Birds are marvelous devices that we are yet to match in flying ability. Some birds even have automatic feathers on their leading edges which pop out when their angle of attack reaches a point where stall is imminent.

But if thick airfoils were of any advantage at low Reynolds Numbers, wouldn't at least some birds have them ? If they had thicker wings, it sure would make for more space for additional muscle tissue and bones.

The stall with a thin airfoil isn't necessarily any worse than for a thick airfoil. I've flown some full scale 1920's era thin wing airplanes (Tiger Moth, Travelair, for instance) that have very benign stall characteristics. And I've flown some thick wing airplanes that have terrible stall characteristics.

Dick
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Old 02-17-2013, 07:04 AM
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Default RE: The Why of Clark-Y

ORIGINAL: otrcman
BobH made the observation that most of the full size WWI airplanes were poor flyers. That's true. I help out occasionally with a large collection of WWI replicas. The two guys who fly them (I don't) tell me that they are mostly poor in the stability and control department. The DVII is perhaps the best of the bunch, but even it has slow ailerons. Most of the problems seem to be lateral/directional. Adverse yaw and slow roll response are pretty much universal among that genre. But that's mostly due to the tiny vertical stabilizers or the fact that some verticals are all-moving with no fixed fin.
I, for one, don't want a model that flies BETTER than the original. I don't want a RE-ENGINEERED version. I'll never fly a full-scale replica...the closest I'll ever get is a scale-flying miniature. And if the original had adverse yaw, then I want to experience what it was like to fly with adverse yaw (and "a boot full of rudder").
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Old 02-17-2013, 07:26 AM
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ORIGINAL: abufletcher

ORIGINAL: otrcman
BobH made the observation that most of the full size WWI airplanes were poor flyers. That's true. I help out occasionally with a large collection of WWI replicas. The two guys who fly them (I don't) tell me that they are mostly poor in the stability and control department. The DVII is perhaps the best of the bunch, but even it has slow ailerons. Most of the problems seem to be lateral/directional. Adverse yaw and slow roll response are pretty much universal among that genre. But that's mostly due to the tiny vertical stabilizers or the fact that some verticals are all-moving with no fixed fin.
I, for one, don't want a model that flies BETTER than the original. I don't want a RE-ENGINEERED version. I'll never fly a full-scale replica...the closest I'll ever get is a scale-flying miniature. And if the original had adverse yaw, then I want to experience what it was like to fly with adverse yaw (and ''a boot full of rudder'').
The problem here is that unless some cheats are added, it will fly worse because it is smaller. One thing we do not have to worry about, is the marginal power some of these aircraft had. Our powerplants for the models are great.
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Old 02-17-2013, 07:38 AM
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Default RE: The Why of Clark-Y

ORIGINAL: vertical grimmace
One thing we do not have to worry about, is the marginal power some of these aircraft had. Our powerplants for the models are great.
Do you mean marginal in terms of power or marginal in terms of quality? I'm generally in favor of "scale power" but I will admit that the reliability of modern model engines is very welcome.
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Old 02-17-2013, 05:38 PM
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Default RE: The Why of Clark-Y

I think he means the "Get You Out of Trouble" margin of power we add on as a matter of course.

For instance, the BUSA 1/6 "scale" models (Pup, Ne-17 and Morse) all had OS40 Fourstrokes in the Protoypes and probably flew fairly scale with this power plant. But a 6 - 7 Lb Bipe with only a 40? I would want at least a 50 in mine... Just for that little bit more oomph that might be required to get me up over the tope of that loop (realising that the Full Size would have had to dive into one to have enough momentum).

The reliability of todays engines is also welcome.

Cheers,

Hugh
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Old 02-18-2013, 01:14 PM
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Default RE: The Why of Clark-Y

I meant marginal in terms of power. Quality may have been an issue as well. Power always effects design and then tactics. Richtofen wanting more power to keep up with the later Allied designs. As world war 2 was described as a "The war of engines" . That is when they reached their pinnacle it seems.
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Old 02-18-2013, 02:55 PM
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Default RE: The Why of Clark-Y

ORIGINAL: abufletcher
The Clark Y is appealing thanks to.............. its near-horizontal lower surface, which aids in the accurate construction of wings on plans mounted on a flat construction board.......
I would say that easiness of construction due to the flat bottom surface is the only reason of the popularity of the Clark-Y airfoil for model airplanes.

With the power to weight ratio of our models, even a brick flies; for high performance sailplanes, more sophisticated airfoils are better.

According to the late Andy Lennon, the Eppler 193, used in powered models, is superior and more suitable for our low Re's:

http://library.propdesigner.co.uk/ht...teristics.html
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Old 02-18-2013, 03:19 PM
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ORIGINAL: Lnewqban

ORIGINAL: abufletcher
The Clark Y is appealing thanks to.............. its near-horizontal lower surface, which aids in the accurate construction of wings on plans mounted on a flat construction board.......
I would say that easiness of construction due to the flat bottom surface is the only reason of the popularity of the Clark-Y airfoil for model airplanes.

With the power to weight ratio of our models, even a brick flies; for high performance sailplanes, more sophisticated airfoils are better.

According to the late Andy Lennon, the Eppler 193, used in powered models, is superior and more suitable for our low Re's:

http://library.propdesigner.co.uk/ht...teristics.html
It would seem that extreme accuracy in construction is needed as well to utilize the shapes and realize the performance gain. Hence, vacuum bagged foam cores or hollow molded composite construction. Certainly not an undertaking seen here. (with ww 1)
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Old 02-18-2013, 03:31 PM
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Default RE: The Why of Clark-Y

I would guess that accuracy in rigging is more critical to the success of a WWI biplane than the choice of airfoil.
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Old 02-18-2013, 04:20 PM
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Default RE: The Why of Clark-Y

If I may interject some information for consideration, I built a 1/4 scale Albatros D.Va with scale undercambered airfoil, (very thin and sharp LE) about the time Proctor came out with their kit. It it is a joy to fly and performs very predictably. Ailerons are very responsive but still requires a bit of rudder.

The second plane, a 1/4 scale Roland C.II, also with scale airfoil (thicker undercambered foil with a more blunt LE) , originally flew with noticeable adverse yaw and required substantial differential aileron throws (very little down and max up) and a lot of rudder to perform well. Now flies more as it should - my impressions this far.

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Old 02-18-2013, 07:55 PM
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ORIGINAL: vertical grimmace


ORIGINAL: abufletcher



I, for one, don't want a model that flies BETTER than the original. I don't want a RE-ENGINEERED version. I'll never fly a full-scale replica...the closest I'll ever get is a scale-flying miniature. And if the original had adverse yaw, then I want to experience what it was like to fly with adverse yaw (and ''a boot full of rudder'').
The problem here is that unless some cheats are added, it will fly worse because it is smaller. One thing we do not have to worry about, is the marginal power some of these aircraft had. Our powerplants for the models are great.


There seems to be a popular misconception that real WWI airplanes were generally poor performers. In fact, most (excepting the very early planes) are real hot rods. From a standing start on takeoff, planes like Camels and Nieuport 28's accelerate like dragsters. The impression of low performance comes because they don't go awfully fast. With two wings and low RPM motors they definitely are limited in top speed, but for takeoff and climb they are very impressive. Remember, many of the fighters patrolled at altitudes above 15,000 feet. Try that in your full size Cherokee or Cessna 172 !

I occasionally work as a ground crew member for launching and retrieving full size WWI replicas. It's always a thrill to watch them jump off the starting line, tear down the runway for a few seconds, and then lift off in a surprisingly steep climb.

Power to weight ratio is a good indicator of an airplane's takeoff and climb performance. The lower the weight for any given power the better the performance will be. Consider the power to weight ratios of some of the WWI planes. A cursory search of the internet shows power to weight figures between 7.7 lb/hp (Nieuport 28) and 11 lb/hp (Camel). Compare this to some modern light planes (Cherokee Warrior, Cessna 172, etc.) which are in the 14 to 15 lb/hp range. Only the later 150 hp Citabria approaches the WWI fighter range at 11 lb/hp.

How do you simulate the rapid takeoff and climb, but low top speed with a model ? Mostly, you would use a large diameter propellor with very low pitch.

Dick
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Old 02-19-2013, 05:55 AM
  #40  
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Default RE: The Why of Clark-Y

"How do you simulate the rapid takeoff and climb, but low top speed with a model ? Mostly, you would use a large diameter propellor with very low pitch."

And THAT'S why geared electrics are a good match to short nosed WWI - a little heavier but able to swing bigger props.
(OK, now I'll duck as Don throws fuel tanks and glowplugs at me!)

Dick, a nice summary of early airfoil development.

Martin
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Old 02-19-2013, 07:06 AM
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Default RE: The Why of Clark-Y

I admit that I still tend to think of "electrics" in terms of little whiny motors with shiny black plastic propellers held on with rubber bands. But the large motors with a hub that accommodate a large wooden propeller aren't QUITE as offensive.
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Old 02-19-2013, 07:59 AM
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ORIGINAL: abufletcher

I admit that I still tend to think of ''electrics'' in terms of little whiny motors with shiny black plastic propellers held on with rubber bands. But the large motors with a hub that accommodate a large wooden propeller aren't QUITE as offensive.

And here I thought you liked glow motors because of all the authentic castor oil goo that covers the rigging at the end of a flight ! On a full scale rotary plane such as a Camel, the engine uses about a gallon of castor oil per hour. A gallon covers a lot of area when it's spewed out into the slipstream. You can wipe the plane down after every flight, but it's still a losing proposition for all the little nooks & crannies. The build-up on the plane after a while is just like on a model. Hopelessly icky.

Dick
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Old 02-19-2013, 02:29 PM
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Default RE: The Why of Clark-Y

Its a badge of honor.
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Old 02-19-2013, 03:15 PM
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Default RE: The Why of Clark-Y

Yes, there's just something deeply wrong with a squeaky clean model.
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Old 02-19-2013, 04:08 PM
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Default RE: The Why of Clark-Y

I know we are really drifting on this thread, but I'd like to contribute just one thing regarding the glow vs electric controversy: A real WWI rotary or Mercedes or Hisso doesn't sound anything like a glo motor or an electric motor, or even like a four stroke for that matter.

If you want to hear actual recordings of WWI engines, there is a audio disc set available to purchase. It's called "Sounds Of Ghosts of The Great War". Aircraft recorded are: Fokker E.III, D.VI, D.VII, D.VIII and S.E.5a.

Available from AirCraft Records, (800) 982-7767, or www.aircraftrecords.com. The recordings were made in about 2004, using modern, high quality audio equipment and original WWI engines. Cost of the two disc set is about $25.

I have no financial interest in the company. The disc set was given to me and I just like to listen to it.

Dick

P.S. The above link includes a 30 second sample that you can hear for the price of a couple of mouse clicks. It's a rotary doing a flyby. I don't know which rotary it is.
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Old 02-19-2013, 05:49 PM
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Default RE: The Why of Clark-Y

The problem with using digital recordings is tube technology was only available. Digital is too harsh unless washed through the warm glow of tube amps. The thing with a glow engine or gas is it is a real engine. You get to be a real mechanic wrenching on your little mechanical wonder. Electrics work well; yawn.
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Old 02-19-2013, 10:11 PM
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Default RE: The Why of Clark-Y

I'm not sure there's anything else to say about the Clark-Y other than that it's inappropriate for almost all WWI models.
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Old 02-22-2013, 05:11 PM
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I am currently acquiring info on the Fokker DVI. I got some plans for a 56" model and also received the Windsock data file today. My goal is to take a BUSA DR1 kit and scratch one into a Fokker DVI. I still have my DVII plans and am basically going to morph the DR1 and the DVII into a DVI, which is what that aircraft was anyway. I have wanted a 1/4 scale DVI for a long time.

In looking at these plans for the little DVI, the designer used a Clark Y in the center section of the wing but used a scale rib section at the tip. So this wing is using 2 different airfoils. The technique her is to stack your rib material and sand them tapering down in size. Only the tip and root section were given on the plans. I am not really sure what the benefit is here other than the tip would be higher lift. The DVII style wing likes to be built upside down. having the taper on the bottom.

Maybe I am just thinking out loud here, but I was wondering of you guys thoughts on using these 2 airfoils to maybe at least give the impression at the tip that the whole wing has under camber, but then possibly deriving the benefit of good aileron control with the Clark Y? But then maybe with the camber at the tip, the ailerons would still be less effective?
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Old 02-22-2013, 05:20 PM
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ORIGINAL: vertical grimmace
I am currently acquiring info on the Fokker DVI. I got some plans for a 56'' model and also received the Windsock data file today. My goal is to take a BUSA DR1 kit and scratch one into a Fokker DVI. I still have my DVII plans and am basically going to morph the DR1 and the DVII into a DVI, which is what that aircraft was anyway.
Sounds like a reasonable plan. But if you want 100% scale outlines, I'd check EVERY SINGLE aspect of the BUSA plans. Don't assume that anything is how it should be.


The DVII style wing likes to be built upside down. having the taper on the bottom.
A truly scale DVI or DVII (and I think also DVIII) wing is "tapered" on both the top AND the bottom. In effect, this gives the wing both dihedral (the upward sloping bottom surface) and anhedral (the downward slope of the top surface). I'm not sure of the aerodynamic benefits of this.
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Old 02-22-2013, 07:31 PM
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Default RE: The Why of Clark-Y

I would think in 1/4 scale it is not going to matter all that much. What does the GTM use? I dont think it needed anything but scale. If I was to compromise, I think a Clark Y modified with just enough under camber to see it; sometimes a hint is all people need to think they see the real thing.
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