Choosing an Airfoil

[David Pentland]

OK I borrowed a set of plans for the 1/4 scale Concept Fleet biplane and reduced them to 48" for electric power. Now the 1/4 scale kit has a semi symmetrical airfoil about 14% thick. I also have the Concept electric fleet plans and it has a Clark Y airfoil. So my question would be which airfoil to go with, and why?
I have seen several of the 1/4 scale size fly and they just fly super. They are sedate and seem to fly at a scale speed. So I'd be tempted to keep the same airfoil on my smaller version. I understand the semi symmetrical airfoil will need a little more power to fly than the Clark Y, but why is there an unwritten rule that all electrics must have a Clark Y type airfoil?

[harveyweewax]

why is there an unwritten rule that all electrics must have a Clark Y type airfoil?


I never heard of that rule before :confused: I suspect the reason you see a Clark Y (flat bottomed) on many models (gas or electric) is because the flight characteristics are pretty good overall and it is easy to build. That doesn't mean it is the best choice. The choice of airfoil is dependent upon the misson the plane is expected to perform and the reynolds number. I suspect the semi symetrical airfoil would be just fine however you might have to play with the incidence anglles.

Regards,

Sven

[JetPlaneFlyer]

There is no single 'rule' about what airfoil to use.
Clark-Y airfoild are popular because they are: easy to build, suited to all forms of construction, and have good safe all round flying charecteristics.
A Clark-Y is probably a decent choice for most sport models providing you are not looking for strong inverted performance or outright speed.
If you want the model to fly really well inverted then you need a airfoil that's closer to symmetrical than the Clark-Y, but it's all a compromise and with airfoils that are close to symmetrical or fully symmetrical then 'upright' performance is somewhat comprimised in that stall speed and therefore take off and landing speed will be increased.

One guideline to bear in mind is that thick airfoils dont work well on small scale models, so you may want to think about going with something thinner than 14% regardless of the 'shape' of the airfoil.

[vintage1]

One guideline to bear in mind is that thick airfoils dont work well on small scale models, so you may want to think about going with something thinner than 14% regardless of the 'shape' of the airfoil.

What is wrong with thick ones?

[mnowell129]

The Clark Y is "semi-symmetrical", that is, cambered.
Airfoils are either cambered or not cambered.
So what is a "flat bottomed" airfoil? The only airfoil you could legitimately call "flat bottomed" would have a zero leading edge radius ( i.e. be sharp) have a flat lower surface that was also the chord line. This would be a lousy airfoil.
Shouldn't the real discussion be 1) what camber do I need in my airfoil? and 2) what thickness do I need in my airfoil?

David's original question is hard to answer because in the comparison between a 14% semi-symmetrical airfoil (unnamed) and a 12% semi-symmetrical airfoil (if it's a real Clark Y) we don't know the camber percentage of airfoil number 1. If it is only .1% cambered then this is different from 4% cambered.

[mnowell129]

What is wrong with thick ones?
Nothing at some reynolds numbers. A lot with other reynolds numbers. At very low reynolds numbers the viscous forces dominate and form and parasite drag are your enemies. Thickness doesn't work at these Re. Ergo indoor free flight airplanes tend to have near zero thickness distributions.

[MCarlton]

David's original question is hard to answer because in the comparison between a 14% semi-symmetrical airfoil (unnamed) and a 12% semi-symmetrical airfoil (if it's a real Clark Y) we don't know the camber percentage of airfoil number 1. If it is only .1% cambered then this is different from 4% cambered

I'm guessing that what he was driving at was that is there any reason why a lot of the models he has seen have a section like ClarkY and some, like the 1/4 scale job, have a less cambered section, as I am making a guess that what he describes as "semi Symmetrical" has less camber than the Clark Y.

I know the question of "flat bottom" being inaccurate does the rounds here and there and maybe technically valid, but I am guessing that most of us call as section flat bottomed when the line between the main spar and the trailing edge is a straight line. So technically it isn't an aerodynamic term but it IS what it looks like and for most of us thats a useful enough parameter to use.

I for one don't tend to talk in specific aerodynamic speech when I am describing a model, I remember seeing on the info panel on the box of my Middle Phase 2 that the section was described as "10% Flat Bottom" and if thats a good enough description for Chris Foss to use, then I'm happy to use it.

We're not all aerodynamicists and we don't need to be, I'm not being a luddite about this, but to be honest, it can get a little wearing when a simple question gets bogged down in theory that in a lot of cases just bamboozles and confuses the poster and in some cases seems to be aimed at making them appear to be foolish for asking an "over simplistic and scientifically invalid" question, thats enough to put people off and stop them asking questions, I think we need to answer questions in the spirit in which they are put. I know this is a Modelling Science forum, but that is a broad church between those who want to know in basic form why or how something works, and those who are really tuned into and expert in advanced aerodynamic theory. There's room for us all in here.

After all, which of us can say we build so accurately that when we build a model with the latest whizz bang GFH7464 7/14/3 that we are creating anything like an accurate wing?

To my mind and for "sport" models, it goes like this for the most part;

"Flat Bottom"

Easy to build as the ribs pin down flat on a building board, decent lift, moderate drag, flies well but not so good for aerobatics

"Semi Symmetrical"

Less camber, meaning inverted performance is a bit better and the section is a bit less "lifty" so potentially easier to trim out throttle responses

"Symmetrical"

No Camber, thus no lift at 0 degrees incidence or AOA. Good for aerobatics if you want to do a lot of inverted as at 0 degrees incidence, the wing doesn't mind which way up it is.

In answer to the original post;

In most sport models, especially those with a built up wing, if out and out aerobatics are not the idea, then a lot of designers plump for a "flat bottomed" section simply because it is easier to build at a smaller size, as you do not need jigging tabs, alignment tabs and the like, which are a fiddle at small sizes. You also get more ribs from a sheet of balsa in my experience, so thats another consideration.

On an electric biplane of 48" span, it depends on what you want, if you want an easy to build wing which will do most of what you need except perform inverted very well and need a bit more down thrust, then go with the Clark Y type section.

If you want a bit more inverted performance at the expense of a little more complicated building and less "float" on landing, then go for the "Semi Symmetrical"

As far as thickness is concerned, then I don't think 14% will be much of an issue at this size. A 48" span biplane is not a small model really, so it won't really be operating at a very low Re. number.

indoor free flight airplanes tend to have near zero thickness distributions

If you are talking indoor duration I would agree perhaps, but then Reynolds numbers are similarly affected by airspeed are they not? Indoor duration we are looking at being aloft for the longest time at the lowest speed possible, thus viscosity is a relatively bigger deal, but with a moderately loaded (maybe 14-16oz/ft2?) and paced (35-50mph?) model, I'd say we are in a "safe" area.

[mnowell129]

Point well taken. Apologies for hijacking the thread with a pet peeve.
The Clark y is flat from the 30% station aft. This does make it easier to build as the ribs can be pinned down, although the LE is up in the air.
My mild frustration is that an airfoil that is "flat bottomed", such that you can pin down the leading and trailing edges is not a clark y. As such it is more cambered than a clark y, not less.

[mnowell129]

.... then go with the Clark Y type section.
...... then go for the "Semi Symmetrical"

But my point is that the Clark Y is semi-symmetrical and to keep referring to at as flat bottom is confusing.

[JetPlaneFlyer]

Regarding my comment about thick airfoils on small scale models... I'd got it in my head that the model in question was small :rolleyes: ... At 48" span and reasonably fast flying speed a 14% airfoil should be fine.

[MCarlton]

I see your point, and I understand it, I'm not saying you are wrong in what you say, obviously a "true" flat bottom wing section would be pretty hopeless, but I think part of the problem is that modellers and the modelling industry still use the "flat bottom" descriptive for this kind of section.

I suppose I could re-write this:

......then go with the Clark Y type section.
...... then go for the "Semi Symmetrical"

As;

...then go with the Clark Y type section
...then go for the less cambered section

The Clark y is flat from the 30% station aft

Ergo, some, if not all, of the section does in fact have a flat bottom, whereas the portion of wing behind the 30% chord point of most sections which people refer to as Semi Symmetrical is a curved line, not a straight one, thus, when on a building board, it rocks, like a see saw.

What people see is that on a "semi symmetrical" section (so called) when it is on a building board at 0 degrees AOA, the trailing edge is up in the air, whereas on a flat bottom section, it is on the board.

Undercamber, to my mind, goes the other way, so that if the trailing edge is on the building board, the wing is actually at a negative AOA, but I could be very wrong.

Terminology is a wonderful thing if we all use it in the same way.

If I see a Boeing 747-400 flying above me and someone says "look, a jumbo jet", should I tell them they are wrong because their terminology isn't exact? It doesn't matter to them if it is not strictly speaking "accurate terminology" because that terminology is well enough known that most people would know what they were saying if they told a neighbour that a "jumbo jet" flew over their house.

I flew for years with a chap who referred to the roll controls on his models as "airelons" but everyone knew what he meant by the context in which he spoke.

[David Pentland]

Thanks guy's. I think MCarlton answered my question in post #7. There is no good reason not to go with the Semi Symmertical airfoil on this project. I like to hotdog a bit now and then so the present airfoil would suit me fine.
I did a little Motocalc on the Fleet today and the camber is 1.5%. The other plan I have the bottom of the airfoil is flat for probably 90% of the chord. Most electric designs I see use a similar airfoil. My first trainer had a similar airfoil and I have disliked it ever since.

[lincoln]

The real Clark Y is NOT going to fly like the usual flat bottomed airfoil. For most purposes, it will be a lot better. Semi symmetrical doesn't tell you anything about how it's going to fly, since it gives you no idea at all of the camber. For instance, the S4233 looks semi symmetrical but the Clark Y has just as much max lift, and is faster too. (The S4233 is nice and thick, though.) The Clark Y won't be very good inverted, though.

If you want to hot dog, maybe that semi symmetriclal would be good, and maybe it woudn't. (I'm assuming you want inverted performance, too.) Maybe it would look funny, but the Trainer 60 airfoil is fully symmetrical, but gets a lot of lift. (At least if your Reynold's number is above 100k when landing. I don't know how heavy your bipe will be.) (coordinates can probably be found on UIUC web site. I'm basing this on their wind tunnel data) It doesn't have a sharp stall, though, if you're into snap rolls. You might have to install stall strips or something to get a good snap.

[macboffin]

An airfoil is either symmetrical, or cambered.There are NO semi-symmetrical airfoils, that is a nonsense term. Might as well say that a cambered airfoil is semi-flat! Keep seeing the term used but it is incorrect.
"Symmetrical" means that each side matches the other.Cambered means that the center line of the airfoil is bent. Reflex means that it is a flattened "S" bend.Flat plate is self-explanatory. OK? If this is modelling science, lets keep it accurate!

[richard hanson]

The best airfoil for the FLEET?
EZ- build the thinnest one you can ---which is structurally up to the job.
All the quacking about best airfoils and % etc., really should center on "how do yo build the BEST WING for the model
Imagine this: if the structural requirements could be set aside ,what would the wing look like? for best performance.
Most full scale planes have shaped DICTATED by structural needsfirst - then the futzing with streamlining can be considered.
What's wrong with thick airfoils ?
nothing except they are iinefficient .
Their advantage is that they are numb to angles of attack.
I have two airfoils in my inventory I start with : one is a cylinder - - very strong, AOA is non critical but lift is lousy.
The other is a flat sheet , floppy but if stiffened - the lift is the best tho AOA gets critical as weight increases
Everything else is a compromise .
For the Fleet -if you can, do a 10% thickness-(choose the "airfoils" based on ease of construction for good strength)
Fleets can be easily built with low wingloadings . They are very rewarding models -no fudging of surface sizes needed .

[MarkusN]

your simplification has merit and hits the nail on the head, but it's a simplification.

Real world airfoils are a compromise, true, but a compromise for the flight envelope we plan for our designs.

Extremely thin wings are one-point designs. Not really good in the real world of changing flight tasks and turbulent air.

[richard hanson]

your simplification has merit and hits the nail on the head, but it's a simplification.

Real world airfoils are a compromise, true, but a compromise for the flight envelope we plan for our designs.

Extremely thin wings are one-point designs. Not really good in the real world of changing flight tasks and turbulent air.
Yes - I Know -- I do understand this stuf Lotsa practical application
on that model the 10% setup is very good

IF- that poses structural problems for you -go to 12%