rmh

The short answer is yes
There are no tricks or secrets involved
Best yet , the results may be better than the ones you find in endless charts.
A sharp pencil , a ruler (a scale) and a couple of French Curves and you have all the tools you need
Oh yes an eraser is a good idea.
First decide what the finished model (this is all about model aircraft ) will be
How large is it to be?
How much will it realistically weigh.?
Will it have to go fast ?
will it have to be very maneuverable?
OK think about those questions
Now , can you pick n choose the materials you will need or are you stuck with a limited type of materials.
Airfoils are not really critical for most sport type model airplanes so as we pick n choose this airfoil , remember some compromises may be needed and in the end , the results won't depend on some "exact" line.
Let's start by sorting out the basics :
Thick wings can be stronger than thin ones . Pretty obvious .
Thick wings TYPICALLY can operate over a wide range of angles - called Angles of Attack- (AOA)
Why? because the shape is less critical.
think of a rolling pin as the least critical shape and a flat sheet as the most critical
somewhere in the middle -is a shape we can use -if we streamline it a bit .
Thin wings can offer less drag and do other neat stuff but lets pick a decent compromise for the first effort.
Lets assume we want to build a sporty flyer about 4 pound and about 50 "span- no scale model but a nice looks like a model type.
some really rough rule of thumb says well 500 square inches is pretty good for 4 pounds
This gives a wing loading of about 15 ounces for a squarefoot.
At that weight the model will fly easily .
A good shape for our use then will be oh- 10"chord by 50" span (500 sq inches)
Wow! we are ready to pick and design the airfoil .
Where do we start Let's look at how thick should it be
experience shows that a variety of materials can be used in this size wing and get good results . balsa - Obechee- pine (yes) fibreglass but lets make it easy and figure on mainly balsa and some spruce for the top and bottom of a main spar located about 1/3 of the airfoil ( chord)
This is like designing a bridge and so the engineers call the spar design a beam and the the weight it can carry the beam loading.
Thru trial and error we have foundthat on this type of a wing ,in this type of application - a beam about 1 to 1.5" thick is a good working range
This means the airfoil will be 10% to 15% as thick as it is long.
Let's compromise and go for about about the center of that range or 13%
Now for the curves - the critical part of the drawing !!
Not really critical! we have already done the critical part!
So ...
The drawing should give us lines which are easy to accurately build.
The leading edge is best if it is a part of a circle and this then blends to the spar and then continues in two straight lines which meet at the rear.
That's all?
Not really
This is where the curve comes in
Let's make the airfoil symmetrical- really this allows you to make panels which can be matched up easily to check for differencs (accidents) in building
so draw a straight line 10" long.
This is the centerline of the airfoil . now measure back 1/3 from he leading edge and place the spar ( 1.3" thick)centered on the line.
Leading edge shape ?
draw a 1/2" diameter circle at the leading edge centered on the line
Now grab the French curve and juggle it around till yo u get a nice blend from the leading edge to the center of the spar
flip it over and draw the bottom line.
Using the ruler (a straightedge) draw straight lines from the top and bottom of the spar to the trailing edge.
Now again with the french curve, lay it over one of these straight lines and blend from the top of the spar into the straight line as smoothly as possible
We actually want the aft section of the airfoil to be straight lines!
Why?
easier to build and duplicate and the shape is as good as any .
You may wnt to slightly seperate the lines at the trailing edge to permit the aileron to be stronger (thicker )
a sharp trailing edge gains you nothing on this model.
Ther you have it
"my first airfoil"
and for this application as good as any and better than some
It will be good for slow speeds or aerobatics and on the thinner side (11%) goes like stink ( many Quicky 500's were exactly this shape")
My old "quicky"weighed three pounds with a K&B40 and I always had to add ballast to race (rule) It also flew very slowly if I wanted i to and I used it to practice new maneuvers . vertical eights -spins whatever ' perfect manners in a great aerobatic trainer!

richg99

This posting is very helpful. I am in the process of building my third SPAD. I am enlarging the wing span to handle the weight better. I did have some confusion with your numbers...it appeared to me the wing loading (500 square inches / 4 lbs.) is closer to 18 oz per sq foot, instead of 15 ??? Is that so?

If so, my 46 x 12 inch wing will come out to 17.75 oz per sq foot. I am assuming my plane will end up at 4.25 lbs. but I don't really know that yet.

Thanks for posting this. All of the calculations can be a bit baffling for a newcomer.

re...Switching pounds to ounces and square inches to square feet ...I haven't done that for a lot of years. I wish "they" had gone to the metric system when my kids were in school. How much easier it seems than our own archaic measurement methods. Rich

rmh

I could have goofed - I did it in my head
here we go 500sq in / 144=3.47
4 pounds x 16 ozs=64 ozs
64/3.47=18.44 ozs per sq ft.
I transposed the 3.4 to 4.3! darn it .
Still a decnt loading for a 500sq in sport model

My ERATIX weighs 3.75 lbs all up and has over 600 squares

SpeedBoy

Very good info Dick !!

I'm doing many what you explained on my project , I didn't understand all the calculations you mentioned though .

What I did was a reduction on a stock airfoil (5/8" at the highest point of the first rib) I'm tinking just on speed and make faster this model , that's all . Here is where my questions come , does exist a way to calculate the % of the lift that I'm sacrificing and the % on speed that it should gain ???

This are the specs.

Wing Span .......... 42 1/2"
Overall Length ..... 33 1/4"
Wing Area............450 Sq in.
Flying weight........ 4 lbs. ( aprox.)

The highest point on the first rib measures 1 1/16" and on the last rib 5/8".

bogbeagle

TLAR

topspeed

This is just the way Gustave Eiffel did his foils I assume...but he also built a windtunnel to test them.

I guess when you have a good grasp on these lift coefficients etc you can just tune the foil thickness by heart ( like making a good sounding violin ).

I used to do wing foils with french curves before there was a computer..I traced the foil from an existing foil. Very easy indeed...and planes flew nicely ( not so well the ones that I hastily drew with french curves after looking at the photos from existing planes...not recommending this approach ).

But I am sure that a good mathematically gifted dude with a gift to draw well can do the job and define a well flying AC with his own foil. This needs the understanding of the speeds, weights and liftcoefficients needed for the project and basic understanding how foils with low reynolds area actually perform ( bubbles etc ). Wing geometry has a lot to do here too ( and lotsa MOJO and SWAG ).

Easier would be getting a Profili program.

pimmnz

With that section, which looks symmetric, you cannot 'loose' any lift. The lift the wing will generate will always be similar to the weight of the model, in straight flight. Thinning the section will have little discernible effect on speed. If you want to go fast, loose weight. Less weight means less induced drag, and in accelerated (turning) flight, much less drag. If you want to go faster, then pay attention to all other forms of aerodynamic (parasitic) drag. Increasing power will not have as much effect as drag reduction, but in combination with less weight and a cleaned up airframe will show good results. Actually, you can't 'loose lift' with any wing section, the whole thing is determined by the weight and speed of the aircraft the wing is trying to fly.
Evan, WB #12.

Sandmann_AU

Great post.. lots of info there. Maybe you can explain something to me. I've built a couple of kits that use separate upper & lower spars that're located by notches in the wing ribs. One kit (a high wing trainer) used cheer webs between the spars all the way to the tip, on both sides. The other kit (GP Super Skybolt) used them on both sides for about 2/3 of the wing's length where the front ones stopped. The back ones continued on a little way more and then also stopped. Both manuals emphasised the importance of making sure the sheer webs were well affixed to the spars, but neither explained what they were for, and I'm hoping you can? I figured they're to stop the spars twisting? Or maybe to create a single large box section spar without the weight of a solid one?

banktoturn

A spar is essentially a beam, in the sense that its main job is to bear a bending load. Imagine a beam consisting of top and bottom members, like your planes' spars. If the beam is rigidly fixed at its 'root', and you try to bend it, this will tend to cause the two members to move relative to each other along their lengths. This motion is called a shearing motion, and the shear webs are intended to resist it. If the upper and lower members of such a beam are prevented from shearing relative to each other, the beam is prevented from bending. This leads to the term 'shear web'.

banktoturn

Sandmann_AU

Ahh that makes perfect sense. Thanks

Ben Lanterman

If you want to take a look at what other folks have done in the past go to

http://www.ae.uiuc.edu/m-selig/ads/coord_database.html

There is a list of 1550 airfoils including some for FAI and Quicky 500 pylon racing, soaring, tailess, general aviation, high lift, plain old free flight, RC hand launched gliders, Rutan's Defiant canard, human powered airplane, propellors, P-51, PT-40 trainer, Ultra-Sport 1000, low reynolds numbers big heavy lifters, and so on and so on.

There is also a list of links to view airfoils, and the TraCFoil airfoil program to plot ribs for models and a lot of good other stuff.

It is pretty impressive what has been done to make things easy and fairly precise. It all depends on what you want the airplane to do. There is no one method. Dick's approach will work for a general flying around airplane. If you want to optimize a design then you need to go to UIUC Database and do some research. Along with the names of airfoils you can print out the profiles of them and there is usually a notation of what the airfoil was used for.

Good luck,

Ben

topspeed

http://www.raes.org.uk/cms/uploaded/..._aerofoils.pdf

There is a 1977 study of human powered AC foil.

rmh

The foils are of course good for low speeds much like the Wright Bros foils
the obvious proof is that the fully deployed wing enhancers on large commercial aircraft are similar for landing setups.
For man carrying aircraft - look at DrPaul McCready's approach
he didn't fuss with airfoils (he did use a low speed foil) -as much as he fussed with the REAL problem-strength to weight and power to weight .
After all- that's what the search for manned flight has been about for many years.

Ben Lanterman

The Wright Brother's airfoil wasn't all that great. It was for the time, 1903, but no one has seen fit to use it since. This site

http://www.dreesecode.com/primer/airfoil1.html

has a good fairly simple interesting analysis.

Here is a illustration out of it showing the point of separation between laminar and non-laminar flows. The Wright's were the worst.

Ben

rmh

Now for model airplane use - how would you evaluate them?-

To rephrase
If you were to select a couple for a sporty little plane such I did in the sublect thread which one of those would you pick
and why-
PS the Wright Bros airfoil is almost a dead ringer for current,indoor microfilm competition models

iron eagel

Dick where you said, "Let's make the airfoil symmetrical".

I would say that you would want the P-51 root pictured above, it looks the most symmetrical to me...
None of those pictured above are real "Symmetrical" from my stand point all appear to have some camber or a lot.
However I suspect that would have a fairly sharp stall in a model...

The airfoil you described sounds a lot more rounded on the leading edge with a longer taper, than the ones pictured above.
It would have a lot gentler stall.
I think I would go for something more along the lines of a Selig 8036 where it performs well at either low or high r#'s...

Now for your application would you taper it toward the tip (as far as cord or would you stick to a constant cord)?

rmh

That p51 airfoils is pretty
Now build it
The theoretical is no good unless one can execute it per the drawing

For those that think this is a easy one - build it and get it right both sides and identical
I have been building for a looooog time and "getin it right" is not easy for many builders.
The benifits of a less exotic shape often outweigh the speed/weight specific airfoil.

A rather blunt 15% foil again symmetrical,are that you can easily rock a panel on a flat bench - compare side to side etc., and enjoy the benefit of a wing whare you can't figure why one panel always drops out - due to errors . A you note big fat airfoils are gentle - not efficient -but gentle and THAT has a lot going for it. As for taper ?
not for the raw novice but having done tapers of many types - most of the benefit in the model is appearance


For really good fast aerobatic a 3-1 -4-1 aspect ratio rectangle is awfully hard to beat.
stable, excellent stall character-
etc..
Not efficient ?
yes - so what - they really work well
Even some really excellent full scale kit designers stick with the simple layout (Ken Rand designs) sometimes derided as the Hershey Bar wing
for the critics I say: -k em.
The proof is in the results.

iron eagel

Dick,
Yea the P-51 is a fun wing to build flat and straight!

As far as building a symmetrical airfoil wing I cut a pair (or several to get it right) patterns out of hard stock, then drill holes and bolt them together and sand and flip them around top and bottom to get the best symmetry. As far stock for the patterns, I like to use Formica/Masonite to make patterns out of, then I cut a stack of blanks (balsa) to cut the ribs from, bolt them together with the patterns on the outside of the stack and then sand the blanks to shape.
I then use the bolt holes to run rods through (which are supported by a frame) to keep the wing panel flat when I build it.


Keeping the shape simple is hands down the best way to approach it.
I was just wondering how you progress form the airfoil to the actual wing itself as a design consideration.

What I gather from this is you are saying that to design an airfoil from scratch is not all that hard and mostly depends on where you want the sweet spot as far as performance???

rmh

yes- something which can be done in a vriety of materials and with a minimum o f extras such as jugging and the result will be accurate -

I have built from kits and scratch built using jigs aligned with drill rod etc., and also doing hotwire foam - (literally thousands of those ) and done some kits which provided NO method of aligning (some very well cut Japanese kits fom the past).
First efforts have enough pitfalls without trying to make some shape which is not easily checked as you go.
Double tapers with semi symmetrical foils ,easily twist when sheeting -unless one knows that alighnment is necessary in this step.
Also the fancy reflexed foils or contineous flowing curves can easily change as the final assembly is sanded
The sad part is : all of the subtle curves which DO make a real difference on over 100 mph full scale are of no value on a very small sub 100 mph model.
Keep it light keep it straight and stiff are the real requirements the foil can one of many.
Hoping that some superior airfoil will save the design is just wistful thinking. Here is a building exercise Irecently did in3mm and 5mm foam -with a spruce topped spar ready to fly the wing is 8 ounces =600 squares and very simple to do.
It's for a Cassut aerobat with a 15 EFlite motor all up flying weight should be 2-2.25 pounds.
At the other end of the scale , scratch built 40% Caps Idid for local friends.

PeterC

IMHO I believe the bottom line to design an airfoil is this. Yes, you can design any airfoil you want that you think will do the job. Flat plate to 15 0r 20% thick symetrical or lifting. However if you want a GOOD airfoil with a GOOD L/D ratio which is efficient then why not choose one which has been researched. I used to design a lot of planes in my youth including my own airfoils for gliders and freeflight. Laater in life when I gt into R//C I tried the same thing and my planes were never as good as the other guys so I started researching airfoils and I learned about Selig, Gottengen, Clark Y, and lots of others. I continued my research and built a few planes with "real" airfoils and they flew much better than my early attempts.
Sure you can design your own but if you want performance, why bother, there are so many good ones to choose from.
Peter

iron eagel

Thanks Dick!
I love the Built up foam wing.
Did you use foam for the shear webs as well? Looking at the picture they appear to be foam also.
The Cap looks very nice!

iron eagel

Peter for the most part I do agree with you as far as not trying to reinvent the wheel.
However I feel that someone that has Dick's expertise can design an airfoil that is purpose built.
I for the most part will look at the flight envelope that I hope the plane to have and pick an airfoil that has the best shape for the purpose I am looking at.
The real fun of this hobby for me is to scratch build and or tweak a design to give it my own flavor.
Keep in mind most airfoil designs are for full scale and with models while the design considerations are the same the R numbers we are dealing with are much lower so that sometimes what works in full scale aircraft does not apply to our airplanes.
As you I have designed some airfoils that did not work all that well, and then I have had a few that have outperformed some that the designer of kits had selected for how I flew.
Another thing to keep in mind is that now that we have some really lightweight composite materials to work with we can try using lower thickness wings that still have good stiffness, so some of the traditional design considerations do not apply.
Some particular types of airplane that comes to mind is those used in the pursuit of high speeds, or long duration flight, or very high stress aerobatic types.

I would like to add to this:
For the past few years I have been toying with blended wing body type of aircraft and have found that they have a whole new set of issues to deal with as far as "airfoil" design.

PeterC

Hi IE,
I totally agree with you; my point was that if you are looking for performance then there are a whole array of airfoils to choose from. (why reinvent the wheel?). For sport or general flying most anything will do. People are amazed when I take their Eagle 63 trainer and do inverted passes with it. Definitely not what the plane was intended to do with its flat bottomed airfoil. I think that particular airfoil is a good example of a home-brew design because you never see one like it on any full size aircraft except possibly ultra lites.

There are many purpose designed MODEL airfoils which have been researched and do provide performance way beyond what you or I may come up with. A proper airfoil requires lots of number crunching and research equipment (windtunnel) which most of us don't have access to. I too have played with lifting body type AC and yes, this is a whole different realm of design. I like the transitional airfoils used of some flying wing type aircraft. How do you answer what toype of airfoil did you use if evey rib is different? If I had to research them my head would explode.

Composites allow us to use airfoils which might be structurally weak or provide a level of rigidity or flexability not possible with regular structures. The air passing over a wing does not care what the wing is built of providing the finshes are equivalant. Also, composites usually allow a lighter structure so more of the load capability of the wing is available for payload.

A person can still give a plane it's own flavor but still use tried and proven designs. In my many years of engineering design (Engineering Instructor) the one thing that really stood out was that there really is very little left to design. Today, design consists of taking proven shapes and concepts and re-arranging them to suit your own purpose.
Peter

iron eagel

Peter as far as the BWB airfoils the easiest way to describe what I have done is say I use x for the root and y for the tip and they blend into one another across the span. But then you have to add the wing shape into the mix.
Profili and Cad help to do the "heavy lifting" as far as the numbers go...

I have seen a few purpose designed airfoils that work very nicely, some of which were conceived in a spar/hot tub using tin foil or foam to experiment with shapes...
There was a case of no wind tunnel needed.
There has been a couple of discussions on here about using fluids as substitution for full blow wind tunnels. (I have yet to try the fluid experiment for a wind tunnel, but the hot tub story is from a fellow club member who has a very interesting plane he has flow for the past 20 years).

One thing I do think that composites give us is the ability to look at what we can build as far as structures and their load bearing abilities. Because of a whole different approach due to the structure possible, what we can do as far as how we fill the structure allows for a whole different way for us to approach the design and its shape in general. Combining composites with vectored thrust, and computerized electronics, will allow for designs of "a different impression".

How well they will be able to fly is yet to be seen...

As far as engineering...
I still an optimist that thinks...
"There has to be a better way to do it".

PeterC

I have been trying to quote your last statement but I don't seem to be able to get just that, only the whole window.
My response to " There has to be a better way..." is: and when it happens that will be the norm. Engineerig is not blind, only cautious. It takes years for new techniques to be approved for genral use, for example plastic pipes for water distribution.
I love this hobby for exactly what you describe and the most important point is "nobody gets killed". Modelers run to the scene of the accident, full size run away from the scene of the accident.
We would be nowhere without some people making those first big steps into the unknown.
I think we have both made our points, lets let it be
Peter