The_Pipefather

I have some questions about tapered (non-constant chord) wings. I did a search, but if some of these have already been "asked & answered" please excuse.

1) Why is a tapered wing more efficient than a constant chord wing?

2) I have read here that a tapered wing seems to "think" it has dihedral, why is this so, and is there a rule of thumb that says "so much taper approximately equals so many degrees of dihedral" ?

3) Is a tapered wing more susceptible to a tip stall than a straight wing of the same area? How to get around this?

4) What is the amount of taper from root to tip that will achieve the max. possible efficiency while being resistant to tip stalls?

5) I have noticed that pattern planes almost always have a tapered wing. Is this a case of form over function, or does it result in any advantages such as the above, and also more "precision" ?

B.L.E.

Less energy is consumed in wingtip vortices. Actually, swept wings act as if they had dihedral, whether or not they are tapered. You can use washout at the wingtips. An elliptical wing has the maximum efficiency. (Supermarine Spitfire) Everything on a pattern plane is function. Snap rolls involve intentional tip stalls.

Washout on a constant chord wing mimics the elliptical lift distribution of an elliptical wing at cruising speed and reduces cruising drag.

banktoturn

Pipefather,

A tapered wing is more efficient than an untapered wing in that
it suffers from less induced drag. Induced drag is generally only
important when the wing is operating at a high coefficient of lift.
For example, when a pylon plane is in a high-G turn, induced drag
is very important. For a plane with reasonable wing loading, induced
drag is not a huge deal for most flying.

Induced drag is minimum when the lift is distributed elliptically along
the span of the wing. A tapered wing can give a decent approximation
of an elliptical lift distribution, but can also be more subject to tip
stall. Keeping the chord at the tip above about 60% of the chort at
the root can avoid a really nasty tendency toward tip stall, and still
give some induced drag reduction.

In addition to possibly allowing intentional tip stalls, tapered wings
can allow rolls to be faster, which is one reason you might see them
on pattern planes.

banktoturn

rmh

Pattern planes have tapered wings because they look nifty- Period
Pattern planes are NOT form follows function models
Tho some think that -- they are simply a very slowwly evolved art form.
There are some really silly features in some pattern designs ( large noses to slow down lines (?!)
The part which makes em work well - for the most part is they are lightly loaded and have good power to weight--otherwise -they will not fly the patterns
it is that simple.
I f you have built a few hundred and designed many which have won nationally and placed internationally - let's hear your arguments to the contrary.
They are - fun to fly -and for the most part easier than most scale designs but a rectangular wing on a Pattern plane works just fine.
Lowering the wing loadings cures 99.99% of any of the problems with planform on a good aerobatic model

The_Pipefather

That clears up some things.

But I dont understand why a tapered wing can act as if it had dihedral. Could someone please explain? Also, is this effect really significant, in light of the fact that banktoturn and dick say that the efficiency advantages of a tapered wing are not really that significant?

My application involves designing a normal sport plane with a straight (i.e. non-dihedral) wing. If it turns out that a tapered wing can give me the self-correcting effect of dihedral (while both upright and inverted) I want to use it.

BWooster

I have never heard that taper acts like dihedral.

I have heard that sweep can substitute dihedral: 1 degree dihedral equals 7 degrees of sweep.

rmh

tapered wings --when a taper is in the leading edge is the same as what is called swept leading edge
so -- if you tilt the wing up (increase or decrease AOA past zero) - the wing is effectively adding dihedral
cut a piece of paper and tilt it -looking at it straight on --you will see it
The sweep operating at increased AOA simply adds more span wise flow
air ALWAYS takes the path of least resistance -
no formulas required - just look to see which is easiest path for the air to flow and almost any question about flow answers itself

B.L.E.

Swept wings, (like transport jets) have a yaw-roll coupling due to one wing having more projected wingspan than the other when the plane is yawed.

With dihedral, yaw increases the angle of attack of one wing and reduces the angle of attack of the other wing. This is easier to visualize if you imagine a wing with extreme polyhedral, so the wing tips are verticle. What happens to the AOA of these wingtips when the plane is yawed?

mesae

Dihedral effect is a change in rolling moment as a result of a sideslip. There is a dihedral angle for any airplane that will result in zero change in rolling moment with sideslip. This is what most aerobatic models want, and the geometric angle depends on the position of the wing on the fuselage, among other things. Non-neutral dihedral effect will ALWAYS be assymetrical (opposite) between upright and inverted.

Wing leading edge sweep works the same upright or inverted, assuming the wing also has "neutral" dihedral effect, and affects mainly yaw, not roll. The Giles series are good examples of this. Zero yaw-roll coupling (zero bank stability), yet strong yaw stability, without discernible dutch-roll, and with no penalty against knife-edge performance either. Really groovy. The tradeoff is relatively poor high-alpha performance, compared with Edges (you have to work more during harriers). Not bad, mind you, just not quite as good as planes with less LE sweep. Exceptional at traditional precision aerobatics, especially rolling maneuvers (ever try a 32 point roll? The plane begs you to try it.), and can be made passable 3D ships if kept very light.

The efficiency advantage of a tapered or elliptical wing are not significant for models. After all, who cares about efficiency, unless you are going for duration or distance? To make a constant chord wing more efficient (approximate elliptical lift distribution that banktoturn mentioned), it must be washed out. To make an elliptical or taperd wing less efficient, give it wash out, since the elliptical, or approximately elliptical lift distribution applies only when those planforms are un-twisted. But they are often twisted anyway, at least on non-aerobatic models, to make them handle better at stall, which is a safety consideration and usually overrides a small efficiency loss due to the twist. But aerobatic airplanes, at least ones that will spend a lot of time upside down, do not want wing twist at all, since even though it will make stall more gentle during a positive stall, it will make it vicious when negatively stalled.

Tapered or elliptical wings generally have less drag, roll faster and snap roll better than equivalent area and aspect ratio constant chord wings.

Gremlin Castle

My Feedback: (14)

I would have to agree with you about tapered wings and pattern ships. My personal preference is wing shapes other than constant chord but only for the esthetics.
I believe that William T. Piper was the one who made the statement that below 200 mph the constant chord wing would compete nicely with other wing shapes.
Tom Cassutt seemed to verify that with his Cassutt racers also.
Performance aside,I will continue to build tapered wings just because I can.

mesae

Even Piper eventually semi-tapered the Arrow's wing. When they did that, cruise speed, efficiency and roll handling improved. Seems to agree with the theory.

However, if you find the right amount of washout to give a constant-chord wing at cruise, it really does make a difference. The trick is finding the best compromise twist angle. Work is being done on variable twist ailerons called twisterons, which so far makes a few percent difference in drag a cruise, but much bigger difference in turning flight.

Dick, or anyone: How many constant-chord winged pattern planes are in the winner's circle? It's not just convention that they are scarce. To make a constant-chord wing "efficient", you have to build in washout. This works fine for cruisers and even some racers that are always posively loaded like the Cassut (Cassut was a fanatic about weight, incidentally). But when you fly a washed-out constant-chord wing inverted, that twist becomes wash-in, and becomes a large liability in induced drag, and tip stall characteristics. So, say it makes no difference if you wish, but for aerobatic airplanes that are to be used inverted a lot, I choose tapered, or even semi-elliptical like the Cap 10B. What a gorgeous airplane!

Oh, and in case anyone was wondering, I'm not really green with a big alien head.[&:]

Gremlin Castle

My Feedback: (14)

I really was not wondering, but think how much more interesting life could be if you were a green alien. You could live and fly for free in places like Roswell.
Back to straight wings. they pretty much dropped out of the pattern scene by the early 60s after the Taurus and Orion type planes became the front runners. As I said earlier, I will continue to build other than straight wings simply because of esthetics.
As for the Cassutt I helped in the construction of two of them back in the early 60s. I also found out what a accelerated stall felt like at close to 200mph.

rmh

Well -I have done many pattern designs and 3D designs and th tapered wing is mostly for eye appeal
sure the tapered wing has a lot going for it - but for low speeds and quick response - that dumb looking 3.5-1 aspect ratio hershey bar is tops .
I am looking at a model where I can fly slow - fast accelerate like crazy - snap fast at ANY speed and this shape -over and over --wins.
Many equate the straight wing with th 20% thick 3D thingies - but make that wing a constant 10% setup and look out - it goes fast and you can hit a full -HARD up elevator "wall" which stops on a dime and will then allow fast roll from a stopped hover .
I am doing a Cassut for aerobatics -here is the powerplant /prop spinner n exhaust all 12 lbs of it
size is 50% scale which is only 7.5'span and 8' overall fuselage BUT 2300 square inches should be -32 lbs thereabouts -

B.L.E.

Another advantage of a tapered wing that has not yet been dicussed here is that there is less bending load on the wing's spar because most of the wing area is near the fusilage. This allows a lighter and/or stronger structure than a constant chord wing of the same wingspan and area.

Gremlin Castle

My Feedback: (14)

Well one thing for sure is that the strip ailerons will be scale. The full size ones that we built used a old fashion roller skate wheel for the tailwheel so at least finding a tailwheel should not be a problem. The main gear used J-3 Cub wheel that had about 3" cut out of the hubs and re welded to use a narrower width tire.

The_Pipefather

Whoa that's almost an overload of information here.

mesae if I understand correctly you say that a tapered wing without any washout gives the best neutral performance, upright and inverted; is that right?

What about a wing that has its trailing edge tapered and LE straight (a la Edge 540) ? Why is that done?

mesae

Elliptical planforms are most efficient but they are harder to build and a true ellipse gives no warning prior to stall because the whole span stalls at the same time (if coordinated). Most designers don't consider them worth the effort since they are almost always modified from a true ellipse anyway, for safety.

I write this with trepidation so I will try to carefully qualify my answer: In general, an un-twisted tapered wing will have less induced drag than either the same wing twisted, or a constant-chord wing with the same area and aspect ratio, all with the same airfoil. If you give an aerobatic model a two-to one thrust to weight ratio, you are not likely to notice this difference in induced drag. But the tapered planform also (in general) gives better snap roll handling, which is good or bad, depending on mission.

An un-washed-out constant-chord wing will have more induced drag than the same wing with a twist that provides elliptical lift distribution with span. It's sort of like fooling the air into thinking the wing has an elliptical planform, but it only works when positively loaded. Too much twist will negate the drag reduction. In summary, wash-out can be used either to reduce the drag of a constant-chord wing, or to improve upright stall characteristics of tapered or elliptical wings, or some compromise between the two, usually favoring stall characteristics. All this assumes positive loading. Highly aerobatic wings should not in general be twisted.

The folks at Zivko claimed the straight leading edge gave the Edge exceptional high-alpha performance i.e. higher deck angle before stall.

rmh

I built a few small Cassutts to test out my thoughts on the planform. . these were 300 & 400 sq in foam models . and because the setups are are so quick and inexpensive to do , it was easy to do comparable sized stuff with elliptical and tapered and straight LE Straight TE etc..
The structure requirements were easy to take care of -using flying wires and carbon fibre strips and rods and tubes
Anyway---the power off glide of the Cassutts was not great -comparatively speaking but the overall performance was the same - fast snaps , good high AOA slow speed stuff .
I have studied the Cassutt quite extensively and it looks like the basic layout -which has been subject to extensive interpretation, will work as well as any of the popular aerobats . A local friend built a Cassutt - I could not fit in the darn thing -
I have at the moment - a 33% EDGE-a 27% 260 a a28% YAK54 etc., and have done Zlins (by the armload ) including the Z50 for the TOC in 1977(?) I forget -
the upshot of all of this is pretty sobering .
The wingloading and power loadings make more difference than all of the rest of the characteristics put together .

B.L.E.

Most full scale airplanes land with flaps down. These flaps, in addition to providing a lower stall speed and extra drag so that the plane can have a steeper glide slope, also effectively give the wing tips washout when employed. The ability to reflex barndoor style ailerons up also acts as temporary washout, allowing a surprise free landing. Tip stalls are deadly when the plane is low and slow, i.e. during a landing approach.

mesae

I agree with all of that. Except that I would tongue-in-cheek add the condition that unintentional tip-stalls can be deadly. Low level snap rolls are cool, and relatively safe when done by an expert aerobatic pilot at an airshow or contest.

I lost count long ago of all the models I have seen lost to unintentional tip-stalls/partial snaps/incipient spins. Almost always, the pilot does not fully understand stalls, and especially what causes tip-stalls. It's amazing how much difference a correct understanding of aerodynamics can have. Often the difference between crash and no crash. Every now and then a gentleman will set himself apart by freely admitting that he just plain screwed up, instead of trying to blame it on the radio, or the design of the airplane, etc.

Here's a scary story: I met a guy who stalled/spun/crashed a KR-2 and lived to tell about it. He was maimed pretty badly by the accident. It was clear to me from his description of the accident that he didn't understand spin recovery very well; "I kept moving the controls around but it wouldn't recover". Now for the really scary part: The one and only time I met the guy, he was building another KR to replace the one he had crashed. He hadn't gone back and studied and did not seem to have learned a thing from his accident. He didn't seem to know anything about emergency spin recovery, or the contribution that rudder coordination (or un-coordination) makes to spin entry. I don't know what ever happened to him; it was many years ago.

rmh

I recall that spin recovery is not required anymore for a private ticket .
My brother -who has had a license for years , once discussed with me what had been the extent of his ground school and flight training.
I was appalled.
Basically the info was related to the type of craft he was flying and some theory (?) which was related -somewhat .
I couldn't decide if they were simply afraid of scaring him off or if the pressure was on the instructors to ONLY deal with situations which may occur if the pilot follows the rules for that particular craft.
A number of my close friends are excellent pilots - commercial -private- aerobatic -ex military etc..
The general opinion I get, is that the basic training can only go so far -then the fledgling must take the bull by the horns (and avoiding the bull s h i t) and carefully expand his abilities.
Either that -or he gets a "real wake up call"------

mesae

The official reason spin training is not required anymore, except for CFIs, is that more people were getting killed doing intential spins in training, than were getting killed doing accidental spins. That makes sense anyway. If you kill off the pilots who can't handle spins, while they are still in training, (along with their apparently unqualified CFIs), then they can't have spin accidents anymore.

rmh

thinnin the herd----

B.L.E.

I have heard stories of pilots bailing out of planes that were in a spin only to have the plane recover on it's own and fly in lazy circles until the fuel was gone.

BMatthews

I'm guessing that this was an Edge like wing with a straight leading edge and swept forward trailing edge? If so then you effectively have some forward sweep angle at the 25% chord line. AFAIK forward sweep tends to delay the stall at the tips.


And it always will. Especially at our sizes and weights. The shape, sweep and other stuff are bandaids. But I think it has more meaning in full sized stuff as they operate over a far narrower achievable range of wing loadings. The full sized stuff generally tries to keep the wings as small as possible for less cruise drag but with a nod of the head to landing speed range. But since they are all designing to a rather narrow range of requirements is it any wonder they all come out much the same shape and size for the given mission? We only see a big move away from this rather narrow path when the mission alters radically like with the U-2 or the recent two decades of aerobatic planes.


I've seen a lot of that when flying gliders. The pilots that learned on polyhedral RES models just cannot seem to get it into their heads that you can't take a model flying low and slow and throw a bunch of aileron into it for a last minute steep turn. The model dorks in and cartwheels and they are totally befuddled as too what happened. A wild range of theories results but the one real cause is never uttered.