Aerodynamics Discuss the physics of flight revolving around the aerodynamics and design of aircraft.

washout or different airfoils?

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Old 05-30-2016, 03:33 AM
  #1  
fillofillo
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Default washout or different airfoils?

Hello everybody, i'm making an Hellcat 1,60 m.
I have a question about the wing.

I'm looking for a model easy to fly at low speed.
I need an advice on these 2 options:

1) Naca 2415 (or 2417) on root, and Naca 2415 (or 2417) on tip with 2 degree washout



or


2) Naca 2415 (or 2417) on root and Naca 0016 (symmetrical) on tip

I'm oriented on the second option, i'm making foam wings, so it is easy to put 2 different airfoils.

What do you think?

Thx everybody!
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Old 05-30-2016, 09:26 AM
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Silver Lotus
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Maybe 2417 for the roots with maybe 2412 at the tips with washout.
The model is big enough for flaps so use all the flap area (scale) you can get for landing. Let the flaps automaticaly add more effective washout.
I would not build any largish WW2 fighter without flaps.
Control high speed tip stall by fiddling with CG and elevator throw.
My old Royal Bearcat would snap over on its back in level flight with a sudden application of "full up"--which was about right, for me.
Just my two bits.
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Old 05-30-2016, 12:51 PM
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fillofillo
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thx for your suggestion!
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Old 05-30-2016, 12:54 PM
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All Day Dan
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Unless you have the CAD software that can do it, it would be very difficult to design the internal structures you will need if you use different airfoils for the root and tip. I always use the same airfoils and have had no problem with the flight and landing characteristics of my planes. Dan.
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Old 05-30-2016, 02:53 PM
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fillofillo
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i cut foam wing. Where the washout should start? From the root to the rib or somewere in the middle?
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Old 05-31-2016, 08:28 AM
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Silver Lotus
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If you are using washout, I'd start at the folding wing break. To keep the rigging of the flaps and ailerons managable.
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Old 06-01-2016, 02:44 AM
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da Rock
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Foam wings don't require CAD drawings for every rib and neither does sandwiching balsa rib blanks between templates and sanding the stack to shape.

Back when Eppler's airfoils were coming out and "new" airfoils were the rage, I used to make wings for my test mule glider to see how the modern profiles flew. Rubber band on wings had their benefits. Going from a root E205 to a tip E198 was no problem at all for example. And didn't add even a minute to the building time. During the couple of years I did those tests, I'd knock out a wing a week. They were 2m of course. The one step that took the most time was covering. No extra time at all to plot out each rib. You only need three templates for those poly wings, root, break, and tip. All the in-between airfoils show up when you sand the stack of blanks. No computations needed.
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Old 06-01-2016, 11:08 AM
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Chad Veich
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If you are interested in lofting the intermediate ribs of a wing with different root and tip airfoils it is not hard to do. Yes, a lofting program can do it for you in seconds and a CAD program can help you get it done faster but it can be done "by hand" the old fashioned way with a bit of patience. Below is a link to a tutorial that I did on the process if anyone is interested.

Let's Loft!
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Old 12-06-2016, 09:00 PM
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ibuild
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I would use the same airfoil at root and tip and with washout at the tip, because it is a simple, safe and proven thing to do.

If you use a symmetrical airfoil at the tip without washout that might not come out as it should, the NACA 0016 have poorer stall capacity than the 2415 so you should actually end up with more washout to prevent tip stall at takeoff and landing than with just 2415.

If you want to benefit from the use of different airfoils at the tip and root the characteristics of the tip airfoil should be the same or better stall capacity and at the same time produce less lift than the root, that is a quite complex thing to achieve and you will likely still end up with at least some washout at the tip if you want it all to be "correct".

Tryggve
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Old 12-08-2016, 12:32 AM
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I wanted to try and make this a little more understandable so I had a look at the Hellcat 3-view and I did some very rough calculations to use as an example, i think if anyone is interested in what difference different airfoils do in relation to each other that deserves more than just an opinion.

I measured the tip and inner section at the fold break of the wing as suggested so in my example I have just taken in to account the outer part of the wing, obviously if you want to foam-cut the complete wing half in one go then the airfoil or washout progression will be all the way from root to tip and the numbers will be a little different - anyway this is just a rough example to provide some understanding or background for my previous stated opinion.

First of all what happens to the stall point (what Angle Of Attack the airfoil will stall) when the wing chord with a given airfoil gets smaller towards the tip is that the stall capacity decreases, with the 2415 airfoil on the Hellcat, the tip will stall about ~2* before the inner fold/break section does. That is a problem because it means that when the wing is at it's limits of how much AOA it can take the tip will stall first, and it will likely occur at the least wanted time of flight - close to the ground during a takeoff or at low altitude in the final last 90* turn towards the landing strip.
(((the sound of 6 months dedicated work reduced to fit in a relatively small plastic bag)))

So the solution to this as we all know you find in the Hellcat design itself, what did they do to make this airplane safe and aerodynamic effective; most importantly they lowered the AOA of the tip chord so that it matches the stall characteristics of the root perfectly. This does not just benefit the safety from tip stall on the airplane but it also increases it's aerodynamic efficiency at slow speed or in a hard turn making it fly cleaner and better.

If we wanted to make this wing without washout would it be possible? The answer is yes it would, but would it be better? I think at this point you have to ask yourself exactly why you would want to make it without the washout, what is the motive for doing this differently than the professors and scientists did when they designed this airplane?

Is it to get better performance? Is it to make it less complicated to build, or is it to make it more aerobatic capable like inverted flight? Note that this is originally designed as a fighter plane that should be capable of high speeds and all sorts of extreme maneuvers - not a Sunday flyer, however I doubt you would ever see it flying inverted for long periods of time or making rolls at a perfectly straight line.

If you want to use a symmetric airfoil at the tip like NACA 00xx we can do that, if you have 2415 at the root and tip - the tip chord will need about 2* washout to match the characteristics of the inner section. If you have 0015 (symmetric with the same thickness) at the tip you need about 4* washout, with NACA 0016 to 0018 about 2* washout (the same as with 2415 but it starts to get much thicker than the root), if you have NACA 0020 at the tip then you don't need the washout anymore. Now you have to ask yourself; will it be better to use a 20% thick airfoil at the tip chord than a 15% thick airfoil at the tip chord? The answer from an aerodynamic point of view is no, "but" - it is possible.

If we extend that example to use the same 24 series airfoil at the tip instead of the symmetric with 2415 at the inner section you need about 1 to 1,5* washout with 2416 at the tip and with 2417 you might get away with no washout at all. But from an aerodynamic point of view you want the tip airfoil to be as thin as possible compared to the root so we are heading in the wrong direction considering that, the reason for this is that at same time as the stall capacity decreases when the wing chord gets smaller - the drag increases rapidly.

So the contradictory elements that makes up the compromise is that you ideally don't want the tip airfoil to be any thicker than absolutely necessary because it's already rapidly increasing it's drag as it gets smaller and with a thicker airfoil you kinda multiply that factor. If you make the tip airfoil thinner which would solve the drag issue you have another problem to solve because of the decreasing stall capacity as the wing chord gets smaller and if you make the airfoil thinner you kinda multiply that factor.

My conclusion is back at if you want the scale model airplane to fly as good as possible in a variety of situations (just like the original) then using a proven airfoil (whatever that is) at the root and tip with about 2* washout is a very good compromise, if you need the wing to be without washout for whatever reason that may be it is possible - but it might result in a less effective and less pretty flying model airplane.

However, as a summary of my investigation of this; if the starting point is that the root airfoil or inner section is NACA 2415, the closest alternatives as I see it would be to use 2415 at the tip with 2* washout or 2417 at the tip with no washout, or something in between (that is with no safety margin). I just think this keeps the compromise within reasonable limits - whatever that is.

Finally after a huge pretentious looking post like this I just wants to say that I'm just an ordinary bloke like everybody else and I might be wrong just like everybody else, I don't want you to trust me on this - I want you to trust yourself.

Last edited by ibuild; 12-08-2016 at 10:46 PM.
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Old 12-09-2016, 04:49 AM
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FWIW, 70's era pattern plane designers asked this sort of question a lot. One of the more influential ones was Jim Kirkland who designed the highly successful Intruder. His experiments indicated that it's the shape of the leading edge more than anything else that determines the stall characteristics of a wing. So his practice was to choose an airfoil (a NACA 0015 for the Intruder) and just keep the LE profile as he tapered the wing. That effectively gave him a more blunt leading edge and a thicker percentage airfoil out toward the tips. That may not be the most technical engineering ever done on an RC plane, but it did work. His designs flew very well and are still pilot friendly with no bad habits even by today's standards.
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Old 12-09-2016, 11:30 AM
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Silver Lotus
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Originally Posted by ibuild View Post
Finally after a huge pretentious looking post like this I just wants to say that I'm just an ordinary bloke like everybody else and I might be wrong just like everybody else, I don't want you to trust me on this - I want you to trust yourself.
Just stick a semi-symmetrical airfoil on it with, thinner at the tips (for the scale look) with flaps and a bit of washout on the tips.
And just learn how to fly it. The wing will always do its best, the pilot is the big variable.
Drilling down to the point--learn how to land it properly using full size airplane techniques adapted to the particular airplane/ flying field.
There is precious little info out there on how any of the commonly used airfoils will act when scaled down to the Re numbers that .60 size models fly at on landing.
So unless you can find hard wind tunnel info (from a reputable wind tunnel--and you won't-- current fully characterized wind tunnels consider operating Re numbers lower than about 10 million as beneath their dignity) on airfoils in the 100,000 Re range there is no point in trying to get overly technical on powered model wing design.
You have nothing to base anything on. But if it makes you feel better tho, go for it.
If you're an average to below average pilot, you'll foul up the landing approach sooner or later, and re-kit it, super trick airfoil or not.
Just my 2 bits.
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Old 12-10-2016, 11:23 AM
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My perspective is from using the polar images from the airfoils and not real experience (though I have some of that too), anyway I just thought I mention that I thought of this today and what I found on the NACA 2415 is that it has a very much optimal stalling characteristic so it just seems very difficult to find a tip airfoil that actually have a better stall capacity than this. I did find however that the NACA 2315 should be slightly better at the tip with just a little more gentle stall curve that stretches the stall point just a little further.

Then I thought that if the aim was to make the wing more similar to an aerobatic aircraft, perhaps it would be an alternative to turn things around a bit. Because if you had the symmetrical NACA 0016 at the inner section and the stall superior NACA 2315 at the tip, that seems to make a perfect match with no washout.

Any thoughts of that?
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Old 12-10-2016, 03:28 PM
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"My perspective is from using the polar images from the airfoils................."

You asked if there are any thoughts on that idea. My thought is, as Silver Lotus said, that the published data on these airfoils is not particularly representative of the real world for our model size wings. As a matter of fact, the polars can be wildly inaccurate.

To help understand the effect of Reynolds number, a simple analogy may be helpful. Consider a wing the size of the Spruce Goose. Imagine that it had that same 2415 airfoil that you mentioned. What would be the radius of curvature of the upper surface of the wing ? Huge is the word that comes to mind. Now, take that same airfoil and build a wing with a 6" chord. What would be the radius of curvature of the upper surface now ? So the question is: how well does the airflow follow the curve of the Spruce Goose airfoil versus the 6" wing, in spite of them being the same airfoil ? It's a sure thing that the air will flow much further along the upper surface of the Spruce Goose wing before separation (think: stall) as compared to the little 6" wing.

This analogy is not entirely accurate, but it helps to grasp the effect of size on any given airfoil. Unfortunately for us, most wind tunnel testing is done to predict the performance of large airfoils, not small ones.

Dick

Last edited by otrcman; 12-10-2016 at 03:30 PM. Reason: spelling
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Old 12-10-2016, 10:18 PM
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Alright, that is some mighty positive feedback in this thread.

So what you are saying basically is that using the same analyzing tools that the professors and scientists use to make the airfoils, and that is "the very basis" for making today's high performance model airplanes like an F3B competition glider - is of no use. And that whatever airfoil you have on your wing at tip or root or if you have washout or not is of no consequence?

If you think the airfoils we discuss to learn about this is entirely wrong for use at the low Re range from about 100k/150k to about 300k/500k, then how about suggesting exactly what airfoils we should be discussing?

The relatively modern analyzing tools we have today is not the same as reality, but it gives and "indication" - a prediction of what you can expect from your wing design. This is what these tools are made for and they are absolutely necessary to any aircraft designer that have something just a little more complex in mind than an ugly stick, that is if you are designing a full size aircraft or a model airplane.

If you are to spend - lets say 2 years scratch building a scale model airplane, you want to have a good starting point and something that is likely to work like it should, yes?

"You need a wind tunnel and make about a 1000 test models before you can make your model airplane and know what you're doing" - don't give me that crap, it's bollocks.

Thanks for the interest and positive feedback, makes it all worth it...
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Old 12-10-2016, 10:33 PM
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I think his point was that at the sizes we work with, the data from full scale testing doesn't apply. I've read comments from several different pattern plane designers that said airfoil selection isn't nearly as big a deal in models as it is in full size planes. The thickness matters, as does the leading edge radius. Everything else might matter a little, but not nearly as much as it would if we were building fighter jets or passenger planes.
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Old 12-11-2016, 06:25 AM
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ibuild
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Ok, I apologize if I have offended anyone, sometimes great passion makes fire but I do respect everyone's opinion. I always try my best to contribute with something positive but I guess it doesn't always turn out that way.

Thanks
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Old 12-11-2016, 09:37 AM
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Silver Lotus
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Originally Posted by ibuild View Post
"You need a wind tunnel and make about a 1000 test models before you can make your model airplane and know what you're doing" - don't give me that crap, it's bollocks.
Until somebody expends some time and effort (money) to develop a series of airfoils in the powered model sizes, it is hit (not much of a hit,really) and miss. Even if you were to have known good airfoils it wouldn't guarantee success when installed on an airplane.
In reality, it doesn't matter much.
Model airplanes all fly pretty bad compared to the big stuff--which is what makes it challenging.
--
At a company training class, a couple weeks ago, the instructor said " CFD gives you the wrong answer to the right question, and the wind tunnel gives you the right answer to the wrong question".
That's why even now it takes more than a few development iterations to get a full scale airplane wing/fuselage/nacelle/flaps combination nailed down.
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Old 12-11-2016, 10:03 AM
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Chad Veich
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Originally Posted by jester_s1 View Post
I've read comments from several different pattern plane designers that said airfoil selection isn't nearly as big a deal in models as it is in full size planes.
Having built and flown quite a few scale models of my own design I totally concur with this statement. At least when it comes to the warbird types that I generally build and fly. It is my opinion that building light and accurate are far more important than trying to find some magic airfoil. I am a very strong proponent for wash-out on heavily loaded scale types as well and rarely design an airplane without it.
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Old 12-11-2016, 10:35 AM
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In ibuild's defense, I should state up front that my post #14 was incomplete. I started to include "the rest of the story", but decided that the post was already longer than desirable and stopped with the first half of the story. If you'll bear with me, I'll try to round out my thoughts now.

Measuring airfoil characteristics by use of wind tunnels goes back at least to the days of the Wright Brothers. This was the the era of cut-and-try, in which candidate shapes were built mostly by eye and then performance checked in a wind tunnel. Unfortunately, the small size and low speed of the Wright's tunnel wasn't accurate in predicting the performance of their larger and faster full scale aircraft.

Ludwig Prandtl (in the WWI time period) began to explain the scaling effects of size and speed, and opened our eyes to the need to test in the regime that the full size airplane would be flying.

As research marched on between WWI and WWII, people gained much greater understanding of airfoil technology, always with an eye to gaining efficiency and going faster. Then came the challenges of transonic flight and supersonic flight. Newer wind tunnels were optimized for the problems of the day in full scale aeronautics, leaving our model airplane regime far behind.

In more recent times, Computational Fluid Dynamics has come along to augment experimental wind tunnel testing. CFD isn't perfect, but it does facilitate study of airfoil shapes much less expensively than construction and operation of a dedicated wind tunnel just for the benefit of science in the model airplane regime.

Only in recent times have some world class engineers (Dr. Michael Selig immediately comes to mind) come along to combine their talents and interests with tools like CFD and wind tunnel in their study of low speed aerodynamics.

Going back now to the Original Poster's question, I would suggest that if you want to taper from one root airfoil to a different tip airfoil to improve stall behavior, then Dr. Selig's work would be much more useful than reliance on wind tunnel data from the 1920's or 1930's. As an example, it's my understanding that the root and tip airfoils for the Top Flite P-47 were recommended by Selig with the specific intent of making a model with gentle stall characteristics.

If you prefer to use one of the early airfoils such as 2415 or even Clark Y, then the time honored wing twist method will probably give your more predictable results. Twist is pretty much a slam-dunk solution to wing drop in the stall. And don't be bashful about a little more twist. About 20 years ago I built a Comet 54" Taylorcraft for SAM 1/2A scale competition. I covered it with silk and, unfortunately, the wing developed a nasty warp on one side. I tried all the usual tricks to remove the excessive twist, but to no avail. In desperation I finally warped the opposite wing to match, assuming that the model was a throw-away at that point. But the model flew very nicely and went on to win many contests. And, boy, was it stall proof. How much was the twist ? 10 degrees. Go figure.

Dick
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Old 12-25-2016, 04:20 AM
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As it is a scale model, have a look at the full size sections. from the website
The Incomplete Guide to Airfoil Usage
http://m-selig.ae.illinois.edu/ads/aircraft.html


Grumman G-50 F6F Hellcat root NACA 23015.6 tip NACA 23009
Why not go with those sections? And add 2 degrees washout for safe handling. I have no idea if the full size had washout. Check your scale drawing, and if it did add an extra one degree.
BTW, the NACA 5 digit series have a lower pitching moment than the 4 digit series which reduces the download needed on the tail. If the scale tail size is small that might matter.

Full size aircraft like these always have sections that taper in thickness like this. A model with a similar tapered thickness will look better. Just be sure not to give the tip a sharp leading edge: round off the LE radius.

Last edited by alasdair; 12-25-2016 at 04:26 AM.
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Old 12-25-2016, 06:29 AM
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A good many designers have tried just using the full scale plane's airfoil on a serious scale model, and the results are generally pretty bad. Going to even 50% scale changes the way the wing interacts with the air so much that it will make the original design's airfoil completely wrong. Generally, models need thicker airfoils and rounder leading edges to retain the same flying characteristics. In addition, they need bigger tails and a much higher power to weight ratio because of aerodynamic efficiency losses. What works in full scale does not work in models.
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Old 12-25-2016, 07:46 AM
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Thanks Jester. I do believe that it's called the Reynolds number that I know what the Reynolds number is. Dan.
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Old 12-25-2016, 09:08 AM
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Reynolds number surely has an effect, but that is dealt with for instance by the LE radius. I think most of the problems with scale models come from their excessive weight. For instance, you need the bigger tail to have enough damping with the big moments of inertia. Try alasdair's spreadsheet to calculate a scale-like weight, probably you would be surprised. At least I had this surprise when I tried a few models just in the simulator. They were beasts at their real weights but became kittens at a scale-like (far lower) weight (mass).

As to power-to-weight ratio, more drag at model-size Reynolds numbers is probably the least of the problems. Actually, the more wing loading the more P/W ratio is needed. Still most models today are over-powered, what substantially contributes to excessive weight. Just see what alasdair wrote in the other thread.

And retaining the same flying characteristics as the full-size original has is just about impossible on a model, again due to its weight. What alasdair's spreadsheet calculates is a reasonable compromise between scale-like speed and scale-like movements. The resulting model weight is so low that it's rarely seen on actual models, meaning many scale models can't fly scale-like anyway.

Years ago we had a thread here at RCU about a scale-like flying DC-3 model. It turned out that about the only thing that made the model flyable at low speed were flaps. I would recommend flaps to the OP here as well. (Split flaps are simple.) And flaps as well as washout were effective only if wing loading was reasonably low in the experiments back then.

Thin airfoils were not a problem, indeed they are more efficient than thick ones at low Re numbers. The thicker ones with the round leading edge begin to stall at quite low AoA but they don't lose lift. Their drag increases to very high values, though. That could be accepted at the wing root to have a lighter structure, but tapering to thinner airfoil with less incidence (washout) to the tip is a good compromise.

By the way, there are even reliable wind tunnel data at model Re numbers for a few airfoils, for instance NACA 2415 (even with flaps).

Last edited by UStik; 12-26-2016 at 11:05 PM.
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