hexonxonx

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i see a variety of wing thickness and also pizza box flyers or flat wing type.what is the benifit of a thick airfoil .im working on a delta and have ended up with a very thin wing,not much curve on the top of the wing. any info?

rmh

little light plane- ? foamy?
thick is NO advantage
actually the opposite
lightest thinnest which is still STIFF-is best

The whole reason for "thick is make the friggen thin less sensitive to pitch -also stronger
All of the carefully worked out airfoils are simply for ONE FINITE application - change speed and AOA and it all turns sour.
But what the heck - do what you like -It's modeling
no one gets killed -------------

Gringo Flyer

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I'm no aero pro but a thick airfoil creates a lot of drag. It also creates a lot of lift. Thats why lots profiles and 3D planes have them. Slow flying bc of the high drag and lift. Another effect of the thick airfoil is that the plane dives a lot slower. Which I guess is an advantage in a 3D type plane.

rmh

lots of drag and less lift --but a broader range of AOA
dives about th same -if wing area is about the same -

flywilly

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Glad to see you're still keeping an eye on us, Dick. Or is it your dog who's watching...
Sorry you left the kit business (not to mention the R & D).
Regards,
Will Bartlett

Gringo Flyer

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Dick, it would seem to me that a thicker airfoil would mean that there is a bigger curve in the foil and hence more room behind it that would have low pressure and that would in turn create more lift. (not sure I explained that well) Like I said I am no pro in airfoil design but it would seem to be that would be the case. Also, I cant remember who told me the slower dive thing but I am almost positive I read it on a post. But it would have to dive somewhat slower if for nothing more than the thicker airfoil creates more drag.

rmh

Will, that's Libby watching --
I will be reading and hear a slight noise -look down and there is that fixed gaze --
Gringo - do not confuse our model foils with the text book efficiency generalizations for airfoils developed for man carrying machines - .
The foil in the sizes we use really means little -unles *** isdeveloped for a given load and speed.
for rip **** tear around flying - square edges and flat foils work fine -
What makes the big differences?
wing loadings and power -

hexonxonx

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thanks folks, i geuss ill continue with the build .it sounds as if this ship may still fly. when i get it done i will post pics in s.p.a.d.

Jimmbbo

Can't resist throwing my $.02 in...

Thick airfoils do create more lift and drag (both induced and parasite), and are structurally stronger than thinner sections. In FS airplanes, the section chose becomes important depending on the mission the airplane is designed to accomplish (as can be seen when comparing an A-10 wing to a 757..)

Since most of our models have power loadings (lb/hp) that are astronomical when compared to FS airplanes, drag is easily overcome by our overpowered engines and is not a practical issue, and I agree with Dick that wing loading and thrust/power are the relevant numbers.... Which is why you can see flying model doghouses, pizza boxes and lawnmowers... Put enough power on just about anything, and it'll fly

Cheers!

Jim

adam_one

Hi,
It has been found that at Re <20,000 (like many indoor R/C models and foamies), a thin flat plate actually generated more max lift than a Clark Y airfoil.
However, a properly designed airfoil optimised for that operating condition would probably outperform both of those.

One reason is that the air molecules don't "see" a thin plate airfoil the same way we see it.
Some air approaching the wing goes over the top and some goes underneath.
The point where the air splits near the wing's LE is called the stagnation point.
As the air approaches the LE of a wing with a positive AOA, it starts sensing the high pressure under and the low pressure above and some of the air just below the LE manages to sneak over the top.
This causes the shifting of the stagnation point to a spot somewhat below what we consider the wing's LE. As the air molecules hooks its way back around the LE and over the top, it rounds the shape as it goes, so the upper surface flow gets a long, curved path from the stagnation point to the wing's TE.
The result is that the air molecules "see" a thin plate airfoil with a positive AOA as it was a cambered airfoil.

rmh

Thats the beauty of the flate plate!
Simple , easy to make and very effective.
The Leading Edge Sneak --is simply caused by slight pressure "bubble" being formed -as you noted .
The easy way out is --over the top
Nature abhores unbalance in pressure -so when in doubt -just look for the easiest path- that's where the air will go .

banktoturn

adam_one,

This relationship between Reynold's number and the 'best' thickness for maximum lift is a pretty fundamental one, and worth understanding. As air flows from the leading edge over a typical airfoil, the pressure goes down, roughly until the thickest point of the airfoil, and then starts going back up, or 'recovering', as the airfoil guys would say. During the pressure recovery part of the flow, the air is flowing from a lower pressure to a higher pressure, which makes it much more likely to separate. The higher the Reynold's number, the more likely the air is to acheive pressure recovery without separating. Thicker airfoils have a bigger variation in pressure over the chord, and so the pressure recovery is more extreme. For this reason, wings operating at lower Reynold's numbers cannot be as thick as wings operating at higher Reynold's numbers without separating. This is why 'good' low Reynold's number airfoils are thinner, and tend to resemble flat plates, and also why flat plates compare more and more favorably to traditional airfoils as the Reynold's number gets small. Having said that, it is the thickness that relates to Reynold's number, not the flatness. For a given thickness, the optimum airfoil shape will not be a plate with constant thickness (i.e. a flat plate), at least in terms of lift and drag. If aerobatic requirements dictate that a wing be stallable on command, then a flat plate may have some favorable characteristics, although those could be acheived by reducing the leading edge radius as well. If, in addition to the ability to stall on command, you want high drag, and don't need high lift, then the flat plate starts to look really good, especially since it is so simple to make. We shouldn't overstate this by saying that flat plates are just as good as traditional airfoils for low Reynold's numbers, even though they are obviously perfectly good choices for lots of planes.

banktoturn

adam_one

I agree, but in case of an extremely low Re along with a rather thin flat plate wing, the air molecules "reshape" the plate into a cambered airfoil (at a positive AOA).
As the Re gets smaller, the required thickness gets smaller, but there are also drawbacks for being too thin e.g., the max thickness point has to move forward and its distribution along the airfoil becomes critical as well.
If you want an airfoil for an extremely low Re that also has a wide speed range, the job gets rather tricky.
I think designing really good airfoils for extremely low Re is one of the most difficult challenges in the model aerodynamics.

banktoturn

adam_one,

I understand how the 'reshaping' can happen, but it is not unique to airfoils that are thin or flat, and it is not the entire explanation of the performance characteristics of thin airfoils.

Low Re airfoils are tricky, mostly because the body of knowledge we would like to take advantage of is all aimed at higher Re airfoils, and hence often not applicable. Beyond that, it is tricky to optimize an airfoil for any set of operating conditions, whether it is low Re, high Mach number, etc.

banktoturn

LouW

Well said. You are particularly on the mark when you said, “it is tricky to optimize an airfoil for any set of operating conditions”. Since the vast majority of data are for Re’s well beyond anything we usually design, dick Hanson’s cut and try approach with a little intuition thrown in is about as good as any.

I just have one observation. While a flat plate works fine for a flip-flop type aerobat (it’s symmetrical you know), A slight leading edge camber will result in a more efficient airfoil at moderate lift coefficients. It will allow a little higher angle of attack before stalling and has the added advantage of providing more stiffness to the wing.

adam_one

banktoturn,
Well, you are very much welcome to provide us with the entire explanation.

Yes of course, but since most of the design software and wind tunnel data available are aimed at much higher Re's, it makes the optimisation of an airfoil for extreme low Re even more tricky and difficult to achieve. So, maybe the simplest solution is by trial and error as Dick Hanson has done.

rmh

I rely on simply watching -- and on the small 1/4" thick foam thingies - they fly pretty darn flat at say -10-15 mph
In fact -I can not detct any AOA as they fly by --and I can fly em 5 ft from me .
as soon as you get below say 10 mph -- the required AOA becomes noticable
as you slow to say 5 mph (walking speed) - you get a lot of sink OR you can power neatly into vectored thrust flight.
I have tried squezing LE to lesser thickness. but of course -there is no way to see any change in performance .
I really don't know how low the AOA really is at say-- 30 mph and I daresay any calculation at this would be very chancy as I would bet the thing is actually bobbing along and never holding a fixed AOA
Indoor gliders and powered stuff -such as microfilm ,fly in pretty still air and at walking speed which is more or less, a small speed envelope and I can see where a foil could likely be optomized for these parameters.
These little thingies I am playing with have a very broad speed envelope -I have had em reverse/ flex /whatever in a high speed dive and on anything else -an explosion of parts would ensue--
But The medium is so cheap and easy to work with - a max failure will only cost lunch money to replace.

NFOOTE

Its bad to run out of lunch money, keep the m&m's flowing!

LouW

I think we are mostly in agreement. I am willing to concede:

1. Your foamies are great fun.

2. For such cheap craft whose sole mission is to entertain, the cut-and-try design process is the only one that makes any sense. In fact, though I am able by education and experience to do engineering design, I use cut and try on most of my model aircraft. Even a typical .40 size machine is easy and cheap enough to build that it’s easier to just build it and try it. Besides, I would rather spend my hobby time gluing balsa and stuff together than putting rows of ciphers on a note pad. Even full size aircraft (some rather famous) were largely designed that way in the first half of the century of flight.

3. Since the only requirement for your foamies is to entertain, anything that will fly, and can be controlled will do. With such a loose mission, it’s realistic to assume that cg, airfoil, moments, etc. don’t matter, and under those conditions they really don’t. This is reinforced by the lack of test data in the range under consideration. If it’s fun it’s a success.

That may well be changing. With the current interest in little flying “spybots” with wingspans measured in inches rather than feet, and very sophisticated missions involving not only pre-programmed but self directed flights, there may well be extensive test data produced which will facilitate effective analytical design of little aircraft. With critical flight profiles, limited power available, and relatively high cost per copy, cg, airfoils, moments, etc. will be important.

There is nothing about being little that makes an airplane exempt from the basic laws of physics. Airplanes from your foamies to the Boeing 747 all operate subject to the same principles of flight.

rmh

Yep!
All of the principles have always existed -from the time of Adam and Eve.
The intrepretation of those principles has been varied tho--.

Most analytical data I have seen --is simply documentation of someones' earlier "cut and try".
SpyBots are really nifty -
Too damn bad that the general approach of the US design has been controlled by "whim of commitee" engineering --
Original ideas are typically poo pooed till they can be shown to actually be the result of some HMFIC's idea.
The real worry of the encroaching spy stuff is that you can't invite a plane to the Officer's club---

mithrandir

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Did anyone mention forced transition? What is the line between thick and thin? RC Pattern... 10% is thin... FAI F3B... 7% is thin... What is usuially "GOOD" is a rounded leading edge... and if the Rn is very low, then TURBULATE it. Use Zig Zag tape if you must, but even pin striping tape is usually enuff. Full size SAILPLANES are KING of EFFICIENCY... they typically run in the 15 to 17% thicknesses... although they are tending thinner as Carbon Fiber comes down in price. So the short answer is don't make it any thicker then it NEEDS to be! AERO is a science of compromises... what set of compromises result in the best solution... that question will lead you to the correct answer.
My own personal observation of how different thickness airfoils behave is this: Thick sections will stall at a LOWER Angle Of Attack then a thin section, but the thin section will stall more abruptly. I suspect this is due to the fact that air does not like flowing around curves, especially in a flow field where the pressure is increasing, ie. top of the wing behind the thick point. (Adverse Pressure Gradient) The thick section typically has MORE curvature in this case then the thin section. When the thin section does finally depart, it does however with less indication and more suddenness... (This is admittedly a subjective opinion/observation)

raptor22

In a way, all of the discussion on HOW you would produce a low RN foil 9or whether it is worth while or not) is unimportant, since none of us (I'm assuming) can make one accurately at that size, anyway.

--Alex

mithrandir

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Well... look at the cross section of a dragon fly's wing... or a butter fly.... hardly a precision structure from a uniformity standpoint. But the structures irregularity does kinda ensure a turbulent flow... these guys are not always flapping you know... I have on many occassions watched them just glide along.

Actually, depending on what you consider accurate... I have available machine shop tools (CNC mills, Lathes etc) and sufficient molding and composite skills that I could make something in the plus'r'minus range of about .005"... but I agee with you... it is not worth it!!!!

adam_one

The chart below shows how a flat plate has more max lift than a typical "thick" airfoil at Re's <40,000.

rmh

where/when/why did this chart originate?