Airfoil type for slower flights

[Boiii]

I have just built my first DIY rc plane. I have learned how to do simple manuers but I think that for my flying area and my skills it is too fast. So I want to make a new wing with someting like depron. Now I am looking more to undercambered airfoil, because somewhere I read that air does not like to travel around slow moving thick wing.
Does the undercambered wing reduces the flight time significintly because of more thrust neded to overcome the drag? And could you please post pictures how you have made undercambered wings?

 

[mikedkelly]

With depron, it is much easier to build flat-bottomed wings, so I would stick with that. If you try to curve the bottom side, you will probably end up creating something that either stalls easily or is out of trim in roll due to inconsistencies in construction. If you want a slower model, your variables are to increase wing area and/or change the airfoil to one with a higher lift coefficient. With a flat-bottomed wing, you can do that by making the wing thicker. You aren't going to match airfoil shape to the millimeter of the wind tunnel specs, so I wouldn't put too much effort into trying to get the benefit of camber effects. A larger wing will produce more drag, but since you will be flying slower, the drag and power required will be the same as your faster model. If you were to fly it at the same speed as the faster model, it would indeed use more power.

If you want to try your hand at a wing with a thin, highly cambered airfoil, here is an article by FliteTest. Simple Airfoil: Under Camber | Flite Test

 

[JetCrafts]

use a camber or semi camber wing for slower flyers

 

[Tench745]

Undercambered wings produce more lift at slow speeds and stall at a higher angle of attack than flat-bottomed airfoils. FT did a couple undercambered wings on the mini Scout, Old Fogey, and Speedster.
Very thick wings also produce more lift at slow speeds. Both wings produce higher drag than standard flat-bottomed airfoils.
Between the lower stall speed and higher drag, you might get the same flight time but it will depend on your specific setup.

The most sure-fire way to make a plane that will fly slowly is to make it light. Or more specifically, light wing-loading makes a plane fly slow. The more wing area you have the slower it needs to be going to get in the air. If you want to make it fly slow put a big wing on it and make it as light as you possibly can.

 

[quorneng]

Boiii
As Tench745 points out a plane's minimum flying speed is a function of its Wing Loading (wing area divided by the planes weight).
Changing the wing section does nothing to reduce the wing loading but can make it fly a bit slower but it will alter the way it flies. At a slower speed the control surfaces become less effective so it is likely to be a bit harder to control.

The least complex solution to reduce the wing loading is to simply reduce the plane's weight but keep everything else the same. It will still fly the same but slower. It is easier to say than to do however electric has a heavy battery so simply using a smaller capacity lighter one, like a 1500 mAh 3s instead of a 2200 mAh. It will be 30% lighter. The power will be the same but the flight time will be reduced.

Increasing the wing area, providing you don't just add lots of weight will allow it to fly slower but you are also altering the aerodynamic relationship between the wing and tail. At some point the plane will become unflyable. The least complex way to increase the area is to keep the same wing section but with a modest (20%?) increase in span. The basic handling will not change much but remember increasing the span also increases the bending forces on the wing so it may simply break unless you add some strength at the central portion of the wing.

Building 'light and strong' aerodynamic structures is a fascinating and complex subject. There is nothing wrong with 'trial and error experimentation' to find a solution that works for you.

 

[Monte.C]

Undercambered wings produce more lift at slow speeds and stall at a higher angle of attack than flat-bottomed airfoils. FT did a couple undercambered wings on the mini Scout, Old Fogey, and Speedster.
Very thick wings also produce more lift at slow speeds. Both wings produce higher drag than standard flat-bottomed airfoils.
Between the lower stall speed and higher drag, you might get the same flight time but it will depend on your specific setup.

The most sure-fire way to make a plane that will fly slowly is to make it light. Or more specifically, light wing-loading makes a plane fly slow. The more wing area you have the slower it needs to be going to get in the air. If you want to make it fly slow put a big wing on it and make it as light as you possibly can.

Well said.

 

[Tench745]

The least complex solution to reduce the wing loading is to simply reduce the plane's weight but keep everything else the same. It will still fly the same but slower. It is easier to say than to do however electric has a heavy battery so simply using a smaller capacity lighter one, like a 1500 mAh 3s instead of a 2200 mAh. It will be 30% lighter. The power will be the same but the flight time will be reduced.

Building 'light and strong' aerodynamic structures is a fascinating and complex subject. There is nothing wrong with 'trial and error experimentation' to find a solution that works for you.

In electric aircraft, the power system seems to be the most limiting factor in keeping things light. If you use a large motor you need a large enough battery and ESC to match. But, if you can keep your motor size to the bare minimum, now you can downsize the battery and the ESC, and cut some significant weight.

Let's do a quick bit of math to illustrate this.

You can estimate stall speed (in miles per hour) by taking the square root of the wing loading (in oz/square foot) and multiply that by five.

Let's use the FT mini Scout as a quick example.
The scout has about 1 square foot of wing area. We're ignoring some cutouts here for easier math.
24" wingspan x 6" chord = 144 square inches = 1 square foot.

My scout weighs just over 6 ounces with a 600mah 2s, so it has a wing loading of 6oz/sqft.
If we take the square root of 6 we get 2.45. Multiply that by 5, and you get an approximate stall speed of 12.25 miles per hour.

Now, imagine I stuck a 2000mah 2s on there instead. It weighs 2.6oz more. That would up my wing loading to 8.6 oz/sqft. making the stall speed about 14.7mph.

 

[mikedkelly]

You can estimate stall speed (in miles per hour) by taking the square root of the wing loading (in oz/square foot) and multiply that by five.

Nice! I'm writing that one down.

 

[checkerboardflyer]

An airfoil that you may be interested in is the Kline Fogelman. It’s a stacked, flat plate design. Easy to build and capable of slow flight characteristics if that is what you are looking for. And it will fly fast as well. The YouTube video shows a Depron model called the Buschtrottel by a German designer, Thomas Buchwald. Plans for the model are a free download. The model has a Kline Fogelman airfoil. More resources for foam board RC modelers on my blog: https://foamboardflyers.com

 

[Hoomi]

Check out the wings on both the Simple Scout and Simple Storch. Both will fly slow and stable. The plans are available online for free, and you can either build the planes as designed, or adapt the wings to your own build.

FT SIMPLE SCOUT BUILD | Flite Test

FT Simple Storch - Build | Flite Test

 

[SSgt Duramax]

In addition, if you do want a full clark y airfoil, you could do one with full length ailerons, but when you hook the ailerons up have them angled down slightly like extended flaps. Or you could just do flaperons and fly with flaps half way the whole time.

 

[Matthewdupreez]

An airfoil that you may be interested in is the Kline Fogelman. It’s a stacked, flat plate design. Easy to build and capable of slow flight characteristics if that is what you are looking for. And it will fly fast as well. The YouTube video shows a Depron model called the Buschtrottel by a German designer, Thomas Buchwald. Plans for the model are a free download. The model has a Kline Fogelman airfoil. More resources for foam board RC modelers on my blog: https://foamboardflyers.com

This almost looks like a timber...

 

[Boiii]

If the wingloading is low . Which airfoil should I choose thin or thick.

 

[Tench745]

If the wingloading is low . Which airfoil should I choose thin or thick.

A thin airfoil will generally produce less lift at a given airspeed, but will also usually have less drag, so the plane may have a higher top speed.
A thick airfoil generally produces more lift at a given airspeed, but also more drag so the plane probably won't have as high a top speed.
With a high drag profile you generally need a larger prop with less pitch, and for a more slippery plane you can increase pitch and decrease diameter.
It's kind-of a tradeoff of what your equipment can do and what mission you want to achieve. Without doing a bunch of math the simplest thing to do is just try one and see how you like it.

 

[quorneng]

As general rule the smaller plane the thinner the wings need to be. The viscosity of air is a constant which means the boundary layer between the moving air and the wing surface is proportionally thicker on a small wing making it behave 'thicker' than it physically is. The smaller the wing its actual section becomes progressively less important. Small planes fly surprisingly well with simple 'flat plate' wings.
There are a number of factors that come into play but it helps to explain why a full size glider can achieve a glide ratio of 60:1 at 50 mph.
A 2m model glider is hard put to achieve even 25:1 and flying at only 20 mph.
For planes bigger is better as far as aerodynamics go.

 

[Inq]

If I were to want better than 2 significant digits of accuracy, I'd turn to someone that wins contests with his air foils on model gliders versus trying to use full-scale foils like NACA Laminar flow foils, NACA 4-digit, or even Clark-Y airfoils. His glider foils have been proven to work (and win) at the lower Reynold's Numbers of models - https://www.mh-aerotools.de/airfoils/index.htm

But I'd certainly do agree, if your build technique won't create better than 2 digits of accuracy, you'd be wasting your time anyway. Stick with a Clark-Y. Now for those of us using 3D printers for our wing foils, 3 digits is no problem as well as things like building in wash-out twist. - Press Enter-button and wing spits out.

VBR,
Inq