Largest flat-plate wing?

[xtian999]

I'm interested in using profile-style (a la shocky) construction on a glider. What is the largest feasible flat plate wing? I saw a graph of airfoil efficiencies and the flat plate did well at low Reynolds numbers. So if speeds are low, a flat wing is not bad and also simple, light, and inexpensive. Using 3 mm as the thickness, how big could it be before the heavier full airfoil became necessary?

[JetPlaneFlyer]

I dont think aerodynamic efficiency would limit wing span... A flat plate will generate lift at any span (though less efficiently than a 'proper' wing section) ... What will be the limiting factor is structural strength. A thin flat plate will be very flexible and weak... you could reinforce it with carbon or whatever but it would still be much weaker than a thicker wing section. The maximum span would therefore depend on how much stiffening you were adding, weight of model (heavy model needs stronger wing) and the type of flying you wanted to do (high 'G' manouvres needs strong wing)... there is no simple answer.

[Tom Harper]

xtian,

I have used flat plate sections on 14 sq ft (delta) models with good results. Lift is the same as any other airfoil - ridgidity (as stated above), drag and stall characteristics are the problems.

[Ollie]

Start with a 8.5" x 11" bond paper glider sheet about 0.0025" thick. The paper glider is about 7" span, about 9" root chord and 5" tip chord. The wing area is about 49 square inches. Now make the thickness two paper layers of 2 x 0.0025" = 0.005". For maximum scale factor about 0.125"/0.005" = 25. However, the paper is stiffer than foam so that make the scale factor about 10. If you keep the weight low, try a span of 70", root chord of 90", tip chord of 50" and wing area of 4900 square inches (34 square foot). Try for a wing loading under 2 oz. per square foot. Try for total weight under of 4.25 pounds.

[xtian999]

Great replies. Thank you. I've got some ideas re: structural stiffening/strengthening that will be tested. Does anyone have the equation that relates chord, speed, Re number, and efficiency?

[xtian999]

The graph is on Jeff Raskin's 'better airfoil' site but I can't read the numbers. Oh well, just have to build one and see.

[Ollie]

"Does anyone have the equation that relates chord, speed, Re number, and efficiency?"

It takes many equations and airfoil polars to answer your question. You need a whole book of equations. I recommend you to read "Model Aircraft Aerodynamics" by Martin Simons.

One kind of efficiency is the glide slope. Glide slope is lift divided by drag (L/D). In a glide, the weight vector is the sum of lift vector and drag vector.

L/D= Cl/(Cdo + Cdi + Cdp). So you must apply the L/D equation for each air speed.

You find the air speed from another equation. V= [ air density x wing area/(weight x Cl)]^2.

You use the air speed and wing chord to calucate Re equation.

Then look up the Cdo airfoil results for the Re and Cl.

Then calucate Cdi ~ (Cl^2)/( pi x wing aspect ratio).

Or use a program such as PC-Soar.
See:
http://my.athenet.net/~atkron95/pcsoar.htm

[Ollie]

There's more:
In the L/D equation, I didn't explain the Cdp (coefficient of drag for parasitic parts) such as fuselage, tail, prop, landing gear, interferenece drag between wing and fuselage, etc, etc, etc.

With low aspect ratio, the Cdi drag is very high compared to other drag coefficients. That's why sailplanes use high aspect ratio to improve L/D. Airfoils have low Cdo and that's why sailplanes use airfoils with low Cdo to improve L/D. Sailplanes use streamline fuselages, small tails with long arm moment, no landing gear, no prop, etc to lower Cdp.

With a trade off between low Cdi vs low Cdo, it depends on size. The Cdo depends on Re, chord and air speed Vs Cdi depends on aspect ratio.

There is no one equation that defines a design because the design has many purposes like simplicty, cost, size, glide slope, sinking speed, air speed, etc. A good design melds many porposes.

Quote from Don Stackhouse:

Design Philosophy

by Don Stackhouse

There have been some rather heated discussions lately on the R/C Soaring Exchange about the relative merits of computer analysis versus old cut-and-try methods in the aerodynamic design of wings. Similar threads have argued over the shape of the "ideal" planform, the "best" airfoil, the "optimum" aspect ratio, and the validity of modified airfoils or of blending from one root airfoil into a different tip airfoil. All this controversy reminds me of one of my favorite stories:

The Four Blind Men and the Elephant (an old Hindu parable)

One day four blind men encountered an elephant for the first time. They approached it cautiously, but with great curiosity. The first one grabbed hold of the trunk and declared "Aha! An elephant is just like a snake!" The second found an ear and replied "No, an elephant is exactly like a tent." The third bumped into a leg and decided the elephant was just like a tree, and the fourth caught the tail and maintained that the elephant was just like a rope. They all went home arguing, each steadfastly insisting that he was right and the other three were wrong.

The flight of a model sailplane is a complex phenomenon, each portion of the model seeing its own unique set of conditions at any given time, yet still having an influence on all of the other parts of the model at the same time. In addition, we expect our models to perform well at a wide variety of operating points within the overall flight envelope. To help us achieve this aim we have available a large database of experimental data and theoretical analysis tools, plus the insight garnered from all of our own experiences and the experiences of others, and the results of actual tests of the model. The results depend on how well we use all of this information together to reach the final design.

If we get hung-up on one parameter, or one design technique, or one phase of the design process, we automatically give ourselves a case of "tunnel vision". There is no single airfoil, aspect ratio, planform, tail size or type, etc., that is optimum at all flight conditions for even a single model, much less a variety of models.

While it is true that the section at the mid-span of a wing of two wildly different tip and root airfoils may have or may not have any of the characteristics of its parents, it is also just as risky to believe that the airfoil that is optimum for the conditions at the root will be equally appropriate at the tip. Ideally you should study the sections at a variety of points along the wing, as well as local chord, twist, flow characteristics, et cetera.

The effects of what is happening at the tail, along the fuselage, along the span of the wing, all influence each other in different ways at different flight conditions, with corresponding effects on the overall control, stability and performance of the model.

None of our design tools is perfect. None of our data is completely reliable at all conditions, and some of it isn't very reliable at any condition. In my experience the best approach is to use all of the available tools and data to the fullest extent possible, then look for the consensus forming between the different approaches. This way the strong points of the different approaches can compensate for their individual shortcomings. The first '93 Monarch hlg went through 150 hours of computer work, PLUS six fuselages, seven tails and eight wings before we froze the design.

If you try to build an elephant with only a tree, or only a rope, a snake, or a tent, your result will almost certainly fall short of your expectations. Likewise, the model that was designed with only theoretical methods, or only past experience, will probably not be the best design possible. Only by using all of the available tools as cooperative members of a team effort can you achieve a design that is more than just the "sum of its parts".

[vintage1]

So true Ollie..so true.

I have always been fascinated by the aerodynamics of birds, and also the hypotheses that their wings are subject to 'iterative evolution' - that is, that for every bird type existence, many subtle and minor variations have naturally occurred, until what remains is pretty much optimum for that birds lifestyle..

So we have extremely efficient soarers (gulls) and less efficient soarers but ones that can turn tighter in thermals and slopes..(raptors, large) ones that can hover (humming birds and the Kestrel) ones that are very fast in short bursts (pigeons) ones that are fast in a dive (peregrine) others that are very efficient on LONG migrations (geese) and some that never get of the ground (ostriches)

Its all a compromise..there is no overall 'best' bird at all..

[yoyoML]

You find the air speed from another equation. V= [ air density x wing area/(weight x Cl)]^2.


This should be V=[ 2 * weight / ( air density * wing area * CL ) ] ^ 1/2.

[JetPlaneFlyer]

This should be V=[ 2 * weight / ( air density * wing area * CL ) ] ^ 1/2.

I agree :)


I think all this is however sort of missing the original point... a flat plate wing is never going to be the most efficient for typical outdoor RC models... If however simplicity and ease of build are the prime design criteria then a flat plate may be worth considering, subject to structural strength/stiffness considerations... If however you want to get all technical and start looking at Re numbers, Cl, Cd induced drag, parasitic drag... etc. etc. then a flat plate is not for you.

[Ollie]

Thanks for correcting my equation, yoyoML!

[vintage1]

I don't think a flat plate is the best wing for anything at all..its just good enough to fly reasonably well if you don't care that much.

Structurally, it's also a disaster..hard to make a big one at all..though a CURVED plate a la slow stick, can be stunningly good at low airspeed, and stiff enough.

[xtian999]

All this reminds me of a a story: A man asks another man if he knows the time. So the other man starts to build a watch... Lol
You guys are the best. I am impressed/intimidated by some of the information given above.
Going by Raskin's article, "The lift we can get from different kinds of airfoils changes dramatically at different Reynolds numbers. This chart will give you a feel for how differently they can behave. At very low Re, a flat plate is best, at higher Re a curved plate is best, and finally a "conventional" airfoil is best."
Unfortunately, I could not make out the numbers on the chart. I doubt if it takes an entire book of equations to redraw the line, but it certainly may take that to fully understand why a flat plate is superior.
But that's not my goal. I saw the flatness as a challenge. Strengthening it w/o adding thickness or weight. An exercise in elegant design with an admittedly ungainly subject. Anyone can buy or build a modern sailplane wing that is stiff, efficient, and beautiful. The shockfly guys could do 3D with balsa and doped paper too. However, flat foam has advantages in cost, durability, and ease of construction. If Raskin is correct, it may also be better at certain speeds and Re values.
Remember when the Skipper told Gilligan that flying was impossible so Gilligan fell? Well, after this baby flys, you guys can tell me why it can't. Lol
Seriously though, thank you for your help and knowlege. I'm learning quite a bit through the boards here and the guys who are willing to answer my stupid questions.

[xtian999]

The famed Raskin article here: http://jef.raskincenter.org/published/airfoil.html

[Sparky Paul]

The Re's where flat works well are in the very low-speed regime.
Indoor fliers use the single surfaced wings most efficiently.
Some of the simpler foamies use single surface curved wings.. but don't expect any performance that can't be matched by conventional designs better, outside.

[Ollie]

Butterfly wings have about 7,000 Re.
Indoor rubber powered might have .000059" mylar film wings ~10,000 Re.
Rubber powered Coup d'Hiver have wings ~ 30,000 Re.
Large R/C thermal soaring sailplanes have wing with Re ~100,000.
Multi-task R/C sailplanes have Re ~400,000.
Pylon racing R/C models have Re ~1,000,000 root chord and ~500,000 tip chord.
Hang gliders (full scale) have Re ~200,000 at tip and ~ 600,0000 at root.
Large R/C DS sailplanes (250 MPH) have wings with ~3,000,000 Re.

At alitude of sea level, the RE= 6080 x chord in feet x air speed of feet per second.

A thin, flat airfoil is good for Re ~ 20,000 or lower. A thin, cambered airfoil is good for above Re ~ 10,000. An airfoil with ~ 8% to ~12% thickness is good for Re ~100,000.

[JetPlaneFlyer]

Those figures illustrate the issue very well Ollie.... A flat plate wing according to Raskin works efficiently at very low Re... but these low Re values are reserved for insects and indoor microfilm covered models... A flat plate is not going to work with anything like the efficiency or a conventional or even a cambered plate airfoil for an outdoor RC model. It will also, if anything, be more costly to build a large flat plate wing because you are going to have to use exotic materials (e.g. carbon) to stiffen it up. For a given stiffness it will also be heavier than a conventional wing.

Bottom line; for larger models the flat plate really has nothing at all going for it... This is not to say it wont fly because done properly it will, but it wont be as efficient as a conventional airfoil, also it's operating envelope will be narrower and the stall will come sooner.

[ghoti]

This might be of interest(?): http://mypage.yhti.net/~dmcdnld/pizzabox.htm

[RCAV8R13]

This one is pretty big.
RCA

[BMatthews]

I think a large part of the issue is being missed in the quest for the "best" airfoil. But "best" is a matter of determining your parameters.

A flat plate has the least drag at very low angles of attack and low lift coefficients. As these factors rise the flat plate suffers from a very early separation and creates a lot of drag as a result. So why would someone use such an airfoil?

Much of 3D flying these days involves transitioning from hovering or near hovering to normal flight and back again. For these fast transitions a flat plate can be a good option even at mid sized model applications because it causes the model to brake to a stop quickly and without a lot of pesky flying towards the nose of the model when a massive elevator input is made. It sort of pivots and STOPS RIGHT NOW! If that suits the style of flying that you're trying to achieve then go for it.

Structural considerations will mean that you can only use a psuedo flat plate or you'll need to rely on significant numbers of bracing wires or struts. It'll be pretty much impossible to make a wing for a 4 foot model that is only 1/4 inch thick have the sort of strength AND RIGIDITY that you desire and still stay light. Unless some carbon skin and supportive core materials are involved. You'd have to do your homework on that one. Even at 1/2 inch thick I don't think you're going to use "normal" model methods or materials. That monster one in RCSV8R13's post is amazing but I'll bet it weighs less than 2 lbs and uses a lot of carbon tubing or similar.

A good compromise for a stall fast and dirty sort of airfoil may be a diamond shape. A 1/4 inch diameter nose with straight lines to a one inch square spart at the 30 to 40% point and straight lines to the trailing edge would have significan't ability to design and build in the strength you want but the diamone shape and sharp'ish leading edge should provide an earlier and snappy stall for braking down to your hovering modes.

[Sparky Paul]

This one is pretty big.
RCA
.
The Reynolds Number at that point in the flight envelope is zero. :)
The lift loads on the structure are equally zero.
Don't take that plane outside... :)

[xtian999]

Thanks for all the great input. Here's what I am shooting for: 60" wingspan glider with about 4oz./ square inch wing loading. 2 channel micro radio and servos. All up weight 10 oz. Can't do an airfoil w/3mm depron, so it is flat w/flat carbon spar and some strapping tape. Less is best!

Should fly in light winds. Not much penetration, but if it goes up and stays up and comes back when I ask it to, I'll be happy.

[Ollie]

xtian999,

Sorry, you will not be happy. You don't understand enough "great input" in my opinion.

You mean 4 oz./ square foot? Then the average chord of about 1/2 of foot (6") so that glider flies best with wing coefficient of lift around 0.2. That means it flies faster than you think. The rub is the very, very narrow range of air speed flying. Either it dives or stalls outside the very small range of airspeed. It is not practical enough.

[BMatthews]

Thanks for all the great input. Here's what I am shooting for: 60" wingspan glider with about 4oz./ square inch wing loading. 2 channel micro radio and servos. All up weight 10 oz. Can't do an airfoil w/3mm depron, so it is flat w/flat carbon spar and some strapping tape. Less is best!

Should fly in light winds. Not much penetration, but if it goes up and stays up and comes back when I ask it to, I'll be happy.

At this size and weight you are going to need a proper airfoil. You won't get such an airfoil by bending up some 3mm depron. To get the efficiency you need for this wing you need to build up a wing or go for a vacuum bagged cut core with load bearing skins.

What you're describing is in the same range of operation as RC handlaunched gliders or many free flight models. You need to look at those styles of model designs for your inspiration.

If you're serious about it being a calm air only deal then look at some of the free flight models for airfoils and construction methods. The high camber airfoils and construction methods used by a lot of the FF contest models can be adapted to this size and style of RC with excellent results. But it means working with real wood and spars and coverings.

What would the launch method be? Electric motor assist or a light duty high start or a runner with a towline?

[Sparky Paul]

With a Jedelesky type airfoil, your flat foam can have some shape.
This worked on a couple of foam gliders..

[jon8889]

im building a 16foot (4.5 meter ) span Zagi type wing but its a bit more complicated than that
though it is made from large foam flat sheets i am carving out an aerofoil/ sanding/ reinforcing with big carbon fibre tubes

i hope to make it for very high aerial photography should be able to see it :-)
with retracts/ digicam/ lighting/ etc - but the foam sheeting will be carbed to an aerofoil shape which minimizes towards the wingtips
it will be covered in fiberglass and sanded / painted to reinforce this large wing
but its pretty big -
made from flat plate you could say -
im sure it would work fine if it didnt bother to carve out the aerofoil shape as im going to use a large four stroke glow engine - or twin smaller four strokes - but i really want it to work well and GLIDE allot better when i switch the engines off at 1000 foot - rather than letting the bulky model sink under flat foam shapes
its like a piece of paper sinks when you let go-
but this will take a while :-) its big give me a few months and il post some pictures
is this the biggest? 16 foot wingspan?

[BlazerB52]

My largest flat plate wing is 48 inches and AUW of plane is over 3 lbs. :eek:
Made out of 1/2" pink foam.
Funtana type plane. Power is 2915-5 motor and 3S 4000mah pack.
Flys well- but not 3D.

[Mister UHU]

With a Jedelesky type airfoil, your flat foam can have some shape.



My thoughts too.

These work really with balsa, 2 metres, even 3 metres,
I'm sure they were done long ago.

You might try a 6mm front section with a rounded LE,
and 3mm for the rear section.

The ribs can be 6mm (lazier) or more 3mm.

You would probably would need some carbon tubing for a spar or two.

Or beef up the wing with deep (ie 10mm +) 6mm Depron spars,
but carbon would be much better than Depron, as it would make a thinner wing.

You can also stabilize the wing using struts to the fuselage, eg from carbon.

60" no problem.
Just don't test fly in gale as I tried, about 30 years ago,
the foam did some very elegant curves, then snapped like a twig !
Hours of work gone in a few seconds.