This is expected and should be a standard expectation by all manufactures. Unfortunately, this is not always the case. Especially when highly engineered - wind tunnel tested foil designs are accessible thanx to people like Dr. Eppler.

What is there about this airfoil which provides an improvement for models of this size?
I ask, because I find that if models of this size - have a structurally good wing-- the really relevant factor is simply wing loading .
a simple shape such as the old 2412 -or even a sym foil of 12 %-- are effectively the same .
The flying speed of these model aircraft is so varied - that I simply can't see how one can develop a superior foil for such broad parameters of use.
That this airfoil will be better at a specific point (aoa and speed )-I can understand
If someone really can explain this - please steop up .
On gliders - where parameters ar much tighter - i understand how shape can be more critical -but not on models such as described here.

At this time, I can't locate all the details. The airfoils used are Selig 8036 and 8037 airfoils, with the 8036 at the root and the 8037 at the tip. While the various Top Flite Gold Edition warbirds have intended maximum design weights, we've had people tell us of models that we'd consider grossly overweight still have good handling. These particular airfoils tend to not have particularly-bad stall characteristics...in fact on the tapered wings of the warbirds, tip-stalling is much-reduced over airfoils used in previous models.

One of the .60-.90 size models calculates out to a little over 32 oz/sq. ft. wing loading (713 sq in area, 10 lbs max weight). In the past, this was considered rather high for that size model, yet it handles very nicely with a very reasonable stall speed. Even grossly overloaded to about 13-14 pounds, the stall was reasonable, given that it was at a higher airspeed due to the weight.

Yes, the airfoil change from root to tip may be the main reason for good handling, but it works and works well. That's why it's used on most of the warbirds (the only ones that don't have that airfoil were designed before they were developed).

------ Actually downthrust does come into play in engineering equations in terms of pitching moment. All wings produce a forward pitching moment downwards because of the reaction to downthurst from the back of the wing. This is one of the reasons you need a tail stuck out on a long boom behind you to fly straight. Or a canard out front to keep the nose of the plane from dropping. This is the reason some of the early homebuilt canards ran into trouble in heavy rain. The rain stalled the canards (too small in area) and the nose pitched down in an uncontrollable dive. ----------

Well the pitching moment is due to the integrated pressures above and below the wing. As a result of those pressure fields the air as it leaves the wing will flow down and we have downwash, but, the wing is reacting to the forces that are applied on it. Those forces (as banktoturn indicated) can come from only one thing, the pressure differential. What happens first is you get differential pressures and as a result you get downwash. Just follow the time history of a molecule from front to back.

------ So the question is then: How does a flying wing work? No tail out back, no canard up front. Stability and control in a flying wing is a real design challenge. It has a great deal to do with airfoil design. --------

Actually it is quite easy, put the CG at 20 percent and reflex the elevons. I have several little electrics that are flat plate and follow that advice. Stability is only the relationship of the CG to the Neutral Point of the thing flying, in a flying wing it is located at 25 percent of the mean aerodynamic chord. The pressure differential does give a nose down moment about the CG which is at 20 percent. Counter that with some reflex in the airfoil or elevons and it is a done deal. Nothing harder than that.

------ I think the practical solution to my question is exactly as described by banktoturn. Just pick an airfoil that works. I am just curious about scaling. Wish I could find that missing article. ------

I try to push the concept that the vast majority of model designs that have been flying over the last 40 years are a vast source of knowledge. It is better than having a wind tunnel test data set. Pick something that is working very well and copy it whether a flying wing, canard or pattern ship. The evolution into today's RC airplanes has produced a fine bunch of flying machines. Copy and enjoy.

In terms of Reynolds number effect on airfoils its effects can be kinda summarized by looking at what flies. Dicks (and mine) flat airfoil which work really fine at small sizes and lots of power, symmetrical airfoils of 10-18 percent thickness for a 60 size model. The big 30-40 percent scale monsters can use the full scale airfoil. In general the only downside of not using a perfect airfoil for the given Reynolds number is that the airplane might be draggier and the stall might be off. In our models - who cares, as Dick said, just add a little power and power through it. A lot of the 3D maneuvers are done with a fully separated wing.

The Selig airfoils are a good example of airfoils optimized for model use. Think about it, we go to dentists and doctors that get really picky about what they do and the medicine they give you. They don't just stand back and throw a hand full of different pills and say that it doesn't matter, just take the ones that hit you. Aerodynamics is a science which includes the very low speed through the multi-sonic speeds - including the speeds our models fly at. The science can and will predict airfoil behavior and performance if you have the right tools. There is no need to dumb down the science by saying that some airfoils or model sizes don't fly according to the "Laws of Aerodynamics". Not true, what is true is that we may not understand the laws governing the flow dynamics in question.

What is there about this airfoil which provides an improvement for models of this size?
I ask, because I find that if models of this size - have a structurally good wing-- the really relevant factor is simply wing loading .
a simple shape such as the old 2412 -or even a sym foil of 12 %-- are effectively the same .
The flying speed of these model aircraft is so varied - that I simply can't see how one can develop a superior foil for such broad parameters of use.
That this airfoil will be better at a specific point (aoa and speed )-I can understand
If someone really can explain this - please steop up .
On gliders - where parameters ar much tighter - i understand how shape can be more critical -but not on models such as described here.

Dick the increase in goodness might just be a little but in a field such as aerodynamics where we have developed pretty good airfoils as of this date and where any performance increase, even a percent or two, is good, then someone with the proper tools can design an airfoil that is better. I don't know if we could say they are superior like it was twice as good or something but it can be just a little better and might make the difference in winning and coming in second.

Yes most airfoils are optimized for a single point but they don't rapidly fall apart at speeds around that point either. The difference between using an airfoil that is optimized for high lift gliders and a nice symmetrical airfoil when used on a pattern ship is pretty obvious, it would be hard to fly a pattern with the high lift glider airfoil. We could then say the symmetrical airfoil is superior. If we are trying to wring the last bit of goodness out of an airfoil then the differences between two symmetrical airfoils might be difficult to determine by just test flying but might be evident in a wind tunnel test. You could say one is slightly better but not superior in the sense of really better.

On a pattern ship we have moved from 15 -18 percent thick airfoils used in the 60s and 70s to the very thin sections used in today's airplanes. We could say the modern airfoils are superior for our present pattern use. But a slight iteration on the present thin section might produce at best a slightly better airfoil, but doubtful that it would be superior.

I do understand that -
I should say that -on a model such as these overloaded scale warbirds tend to be - the ONLY real difference in performance I have found-- wingloading -
sure-- speed increases of only a tiny bit fixes lots of weight problems ( v squared times weight )
but the landing - consider the landing speeds
add flaps OR the best airfoil in the world - when that pride and joy rotates in for a 3 point or a wheel landing - the slower the touch down speed - the less likely it is to shed the entire undercarriage - especially on grass fields .
If all one really wants is straffing run performance - pile on the weight - -just use a hard surface runway - 1000 ft long.
I know not everyone likes aerobatics - a loop is wild stuff for some setups . But power off performance is a reality of flying and no amount of trick airfoil design will fix that.

You are absolutely right, weight is a major downer in airplanes of any kind and the effects of it are pretty easy to determine. My lead sleds fly pretty badly but are good for golf clubs and can't be broken. Landing speeds go up and maneuverability goes down.

But if you have two airplanes of similar light weights, low wing loadings, high power loadings etc. then you might also see the differences between a carefully selected airfoil and a WAG airfoil. Being careful about the selection just makes sure a good design isn't hurt by a bad choice in airfoils and is something that is easily remedied in the design process. After all it's no harder to build and cover a good airfoil that a bad one.

Plus you have the mental good feeling knowing that you have done everything the best you can, and that will make the airplane fly better. Airplanes we don't really like usually don't get our best piloting efforts.

I will have to keep looking for the Grail--
So far -the factors which I really noticed meant anything --were structural stiffness (very important except for variable dihedral - that autobending is a plus on most stuff ) and loading .
My pattern planes wer the first tip off - I watched -and built models with very thick foils and -very thin foils - th first thing I really noticed was that the very thin foils had no more tendency to "tip stall" (a misnomer if ever there was one) than th thick ones .
The razor shaped leading edges offered no advantage on snaps -
progressive airfoils again ---could not see any difference - so I simply said "what the hell" and made em as light and as thin as I could manage -without having em break.
This was a case where the result also lightened the wing loading -and it all just got better - better penetration (fly at low AOA-)
better power to weight
slower approach speeds .
On the dumb flat foils- the situation got even more interesting - Even with CG at 50% of MAC the planes fly just fine except for the really pronounced trim shift when inverted .
I sure wouldn't want any of this on a plane I had to fly in but I probably would do no worse than some of the widow makers on the commercial market --
A Caravan got a pilot ltwo weeks ago here - in the classic cold damp weather - ground observers saw the plane tilt a wing then wag the wings - then go in -under power -very experienced pilot- but the super lift wing became super bad -super fast- in ice creating conditions - I am not making this up.
Think this is an isolated occurrance?
check it out.

Dick you really need to instrument the airplanes to really know what the changes you have made resulted in doing. Visual sight and trying to remember just isn't going to hack it.

What was a very experienced pilot doing flying in icing conditions. Good grief. You put ANY airfoil in the same conditions and it will come down It is an interesting story but has nothing to do with making a model airplane fly better based on sound engineering judgment and tests.

Nothing special about a flat airfoil. When put at an angle of attack it developes a flow field around it with a neutral point and the whole bit just like any airfoil section. They don't disobey any of the rules as we know them. There hasn't been a lot of engineering work on them since they are basically useless for full scale work.

The weight will effect the angle of attack you need to fly at but not the thickness. Symmetrical airfoils from 8-18 percent have pretty well much the same CLalpha curve, thin does have lower drag which does indeed provide for better penetration.

Being a musician thru formal training from 7 years old - I am familiar with sound engineering and testing-------.

My point on the Caravan was that even well designed setups like the Caravan, often fall into the same deadly trap.
When you trade off an inefficient tho basically more forgiving setup --for higher efficiency- there is always a trade off .
In this case, slight reshaping of airfoil and wing loading increase spells real problems.
On the "heavy" model - where the wing loading is allowed to go too high - the trade off is rapid loss of controlled flight at lower speeds and in dicey conditions - such as cross wing landings etc..
my sound judment and testing procedures are just not along the same lines you learned.
This is probably the worst pun of the year --------------

I was reading earlier in this forum about laminar and turbulant flows. Why exactly would a turbulant flow be better than a laminar flow and how does the separation effect the characteristics of the airplane?

Turbulent flow refers to a very thin turbulent layer that hugs the airfoil skin. This "rough" layer helps the smooth laminar flow above it stick to the airfoils shape better.

A true 100% laminar flow that hugs the airfoil shape for it's whole chord is the ultimate but it's very hard to maintain under the entire operating range of the airfoil. At high speeds it's not that hard to accomplish but as the speed drops, the lift coefficient goes up and the air has to "work harder" to follow the shape it will tend to separate in a big bubble on the upper mid to rear areas. If it re-attaches before it comes off the trailing edge then some of the damage is minimized but the model will typically be considered as "mushy" while the bubble is there. Dropping the nose a bit and speeding up will make the bubble collapse and return the feel to "slippery and responsive". If the bubble does not re-attach then it's considered as a stall. Laminar flows typically transitiion from smooth to bubbled or stalled very quickly and this gives the model a reputation for being a snap roller or nasty staller.

Using an airfoil that promotes a thin turbulent layer either with shape, construction design or with turbulator strips will force a thin turbulent layer to form before the separation bubble can form. If done right this turbulent layer will greatly delay the separation bubble formation and delay and soften the stall charactaristics. Yes it is a bit more drag than a true laminar flow but most folks consider it a good tradeoff.

Reading through this thread several times I seem to notice some things that might serve to settle the question. From Ben’s responses consider the following:

Several things stand out.

1. The vast majority of R/C models fly quite well regardless of whether the airfoil is scientifically designed or just a WAG.

2. An airplane designed for specific competitive criteria may gain a small advantage if the airfoil is based on actual test data rather than intuition.

3. The difference between a carefully designed airfoil and a WAG is generally small enough that it can’t be noticed by just observing flight.

When designing a model aircraft, even for competitive purposes, airfoil selection is probably the least critical factor. Such factors as wing and power loading, wing and tail planform and location, moment arms and force layout, cg, fuselage shape and cross section, etc., all have significant effect on the airplanes ability to perform the required maneuvers, and present the largest possibility of significant improvement.

Even in serious competition, the winner is not always the one with the best design, but skill and consistency play a very significant part. Some of the many hours spent doing scientific design might yield more results if spent in practice.

So if you are a serious competitor, pull out all stops and spend all the time and effort you want to design the perfect machine. But for most of us who just like to play with model airplanes, put together something that looks good, and doesn’t vary too much from other successful designs, and go have fun.

Right -except - why do you always exclude WAG airfoils as simply a piece of uninformed , puttering- when the WAG may actually be as good as it gets?
a very good friend of mine once worked as a wind tunnel model maker (Convair) and he told me that a fair amount of the "carefully designed /calculated etc., foils were simply --WAGS. based on past experience
The bosses liked the term "engineered" better than "a wild assed guess."

Lou's summary is tremendous - in a few words he put the relative importance of everything in perspective. Well done.

Dick - I consider you my friend (if ever we are at the same place let's meet - I will want a discount if I can ever afford your models) and have worked in a wind tunnel (aero rep running the test) and can tell you that at Mcair we didn't waste money on WAGs or SWAGs. Maybe in 1912 but when we went into the F-15 tests with an airfoil the group of guys in charge of it had worked with programs, experience, other test results and a great deal of intuition to come up with a proposal for the wing airfoils. They had to sell it to the management with pressure curves and expected results. It was a carefully considered engineering approach.

Dick we only exclude WAGs when they are presented as being better than a carefully scientifically designed approach. The guy who says his WAG is better than the engineered airfoil is like a lightening rod, he is going to get the attention of everyone who has ever done it as a science and as a living.

It would be similiar to a dentist hearing that a guy used a dremel tool and epoxy to fix his tooth. It might work but gives you the willies.

One of the shorter term instrumentation "engineers" at Lockheed on the L-1011 saw the fuselage sans wing for the first article.
The cutout for the wing carrythru is definitely not what one would expect.. It resembles an NACA 23012 airfoil, inverted!
The fat part is on the bottom!
This weenie wrote an official letter to the company president, saying Lockheed's aero guys had put the wing on upside down!
He knew this because he had a pilot's license and flew a Cessna 152.
He wasn't there much longer..
Was sent off to Lockheed Missiles, working on a submarine.. we always wondered if he'd ever tried to open a hatch underwater to see what would happen.
Anyway, the changes in wing profile from root to tip on the L-1011 are quite extreme, going from "upside down" at the root to lifting at the pylon to symmetrical at the tip.. all figured to contribute to the full flight regime of the airplane, doing specific jobs at some speeds, and not interfering at others.
The DC-10/MD-11 has an extreme example of the changes in profile and incidences root to tip. for the same reasons.
Any full-scale plane with its large operating envelope is a flying compromise of parts, all intended to work well, but not interfere when they're not needed..
Interference costs money to the customer, and he don't like that!
We spent many many hours fine tuning the aerodynamics of the plane, for 1 or 2 drag counts, which translate into millions of dollars of fuel -not bought- by the customer over the life span of the plane.
SWAG when used with experience, as Lou mentions, is what anyone at the upper end of the performance envelope uses when looking for that last itty-bitty bit of performance, in full-scale and toy airplanes.
For the bank-and-yank crowd, just add more motor!

Dick, and the rest of you, I think you're missing the point that anyone that cruises through a few plans and considers what they are looking at is hardly capable of making a true WAG any longer. Rather what they draw will be a distillation of what they have seen before modified to suit what they think they need.

If you do a search around here for a couple of recent threads where some very new modellers are trying out their design skills for the first time you'll see that the airfoils they WAG are crude in the extreme. It's not that they are any less smart than the rest of us but they just have not had their pool of knowledge seasoned with some successful background spices. I'm sure that like myself you and Ben and Paul all had a lot of reading and plan studying of the model mags under our belts before the pencil ever hit that first piece of paper. So a WAG is just not possible under those circumstances. Rather a suitable airfoil that will perform "well enough" is almost always going to be the result.... at least for our model purposes.

So what you are considering to be a WAG is actually a sausage of history wrapped up in a pastry of inspiration with a dab of edcuated guess sprinkled on top.... sorry for the food parables but I'm eating breaky as I type this....

I guess I am just a bit touchy about some things -
I get the strong impression that some feel any airfoil not developed thru wind tunnel and computer work ups -are just not worth much.
I don't have any full scale experience in designing airfoils - none zip zero nada.
That said -I really don't see much correlation with the foils we need .
I have plodded thru a motley collection of textbooks tho -and must admit -they are 50 years old for the most part.
Some times I tempted to say " if my airfoil is only good enough - let's see one which performs better"- but that serves no one any good.
As a background --I have designed a lot of pneumatic circuits- some patented innovations -not just improvements on patents - so I do understand a wee bit about how air operates .
best yet the designs were and still are used- intact since 1970 -
So when I say SWAG- I mean a guess based on a lot of thought - but -it is still just a guess.
I will keep on guessing .
The Lockheed engineer story has many parallels -I am sure -
We had a engineer , who was asked to design a table for the tooling shop -for a two ton press.
He had one made which would support two tons ---

Topic is interesting but I do an off topic answer:


Lars radioed of temperature rise...and tried to land soon after that.

He had 70 hours logged on a high performance AC.

Planes tailwheel was almost at the same level as the mainwheels..how you supposed to land that kinda craft ?

Estimated landing speed was 90 mph....what if he was coming in at 95 mph and throttled back and torque got him...elevator and rudder were really small.

How was the Vmax Probe cooled anyway ?

-

I am working on a 50 hp powered sorta same kinda pusher as Lars with more wing and slightly shorter ( by 2 feet ).