Why does dihedral/ polyhedral help sink rate in tight thermal turns?

[Flyingwingbat1]

I've heard about it helping the middle part of the wing do its job more effectively, but not much more. What's the mathematical basis for the reasoning? My intuition says that the dihedral--and polyhedral, especially-- increases the AOA of the inner wingtip (which has the lowest airspeed), helping to even out the lift distribution across the wing, lessening induced drag as a result. The same result is achieved (less efficiently) in "flat wing" sailplanes by using a bit of opposite aileron deflection to prevent overbanking, or so I've read.

But I know that intuition is bad sometimes; what's the answer (or answers)?

[BMatthews]

My own experience through personal observation leads me to consider that flat winged models are overall more efficient than polyhedral models in any mode. Outboard dihedral joints cause turbulence and turbulence causes drag pretty much every time.

[MCarlton]

I've attached a diagram to explain the way I see an aircraft in a turn. I tend to agree that a flat wing is more efficient, because of what I wrongly call lift vectors. I assume that lift is generated at right angles to the wing surface. If you draw this out, you see that the outer panel of a poly wing generates its lift at an angle of more than 45 degrees, so contributing more in a horizontal sense than a vertical one. The inner wing, although generating lift more vertically, is moving more slowly and thus the overall lift is lower than a flat wing.

I don't know the answer scientifically, just my assumptions and guesses!

The only thing I would say is that we have stuck with polyhedral largely because when RC came about, we didn't have equipment which could be installed in wings, nor did we have multi channel receivers and high capacity flight packs, so those earlier models had to make do with only one steering channel and back then, that meant rudder, being an evolution from a free flight trim tab.

If polyhedral was ultimately more efficient, we would see more of it than we do in the full size aviation world.

I'm not saying you are wrong though wingbat, I'll leave it to an expert to give a definitive response!

[John O'Sullivan]

I guess I have to defer to my Guru - Mark Drela (MIT - Daedelus Full size and Apogee, Aegea, SuperGee and Supra Glider R/C designs which have more or less revolutionized R/C sailplane design in recent years). Here is a man who is not just a theorist but proves his theory with exceptional practical performance.
He uses substantially more dihedral in his designs than was previously considered. This has led to a more stable tracking aircraft. This extra built in stability more than makes up for the constant control input and attention required for flat winged models , which will squirrel off into an unexpected spiral if you don't excercise constant attention.

A little extra dihedral is better than too little.
John

[MCarlton]

Agreed John, but thats getting into the realm of controllability and stability rather than outright efficiency, I'm assuming the original poster is talking about theoretical differences with all other factors being equal. Also, and I don't know if this is applicable in this context, but are not the models you are describing DLG aircraft? So principally very light weight, low inertia aircraft which I can quite imagine need a little help with stability to make flying a little more relaxed. I don't know if that philosophy would migrate into larger models.

A bowling ball and a ping pong ball are the same shape, but blow hard on them both when they are rolling and one is easily displaced, the other isn't, thats a sort of analogy for what I mean, a small light DLG will get knocked about, turbulence, wind, thermals all make for a model which feels like it is "perched" on the air, whereas a larger model feels more "locked in".

[BMatthews]

From my reading of Mark Drela's writings I found him to be a highly practical and pragmatic designer. He seeks aerodynamic efficiency but balances that off against making the model very stable but controllable when out at distances where you're having to fly in instinct instead of observation. At those times ultimate efficiency does little for you if your controll inputs are pushing the craft into inefficient attitudes. His use of extra dihedral and other factors to make the model more "free flight like" would seem to be a recognition of this practical side.

[shaneyee]

What is the angle needed for a polyhedral wing? I understand that from the front, a wings should have a circular arc as the ideal and a polyhedral would then look like part of a regular polygon but how tight is the radius of the circle?


Shane

[ciurpita]

If polyhedral was ultimately more efficient, we would see more of it than we do in the full size aviation world.
but doesn't the composite wing flex of the most efficient full-size gliders provide elliptical dihedral? and isn't elliptical dihedral what polyhedral is approximating?

[MarkusN]

It's really a matter of efficiency vs. controllability. And in a plane that has to be contolled by a human pilot, often in bumpy air, be it behind the stick or behind the transmitter, controllability weighs heavily in the equation.

[vintage1]

The back half of a F1 grid is full of cars that passed their wind tunnel tests in a straight line with flying colors, but proved to be so uncontrollable in a cross wind, or following another car, or going round bumpy corners , that they might as well have not bothered.

Performance optimised to one thing only, can severely reduce overall performance.

I have flown undercambered wings in my old timers. Boy, in calm air they float forever..one gust and stall, and they lose 20' in an instant..same goes for thin long wings..except they tip stall as well..in an overall contest scenario you do NOT want ultimately to sacrifice penetration - to track from one thermal to another, or a vicious tipstall - which loses more height than you have gained, for a low sink rate. In bumpy air, you need controllability more than anything. In light air, well its a different story.

[John O'Sullivan]

MCARLTON:
I was not just referring to small DLG models. Mark Drela uses this setup in his 3.4 metre Supra which, with derivatives, has virtually taken over the international F3J class.
Mark theoretically optimises his designs and is a aeronautics professor at MIT.
Take a look at some of his designs and design theory at the Charles River site
http://www.charlesriverrc.org/articles.htm
and also his full size achievements http://web.mit.edu/aeroastro/news/magazine/aeroastro-no3/2006drela.html

As for dihedral vs flat wings, each has its application. An aerobatic model works best with little or no dihedral,while a large sailplane at 2000 ft+ and way downwind would benefit from stabilizing dihedral. I personally feel that wing with a moderate amount of dihedral has no noticeable less efficiency than a flat wing.
John

[MCarlton]

As for dihedral vs flat wings, each has its application. An aerobatic model works best with little or no dihedral,while a large sailplane at 2000 ft+ and way downwind would benefit from stabilizing dihedral. I personally feel that wing with a moderate amount of dihedral has no noticeable less efficiency than a flat wing.

Agreed, and I am not saying Mark Drela is wrong, I am nowhere near his knowledge and understanding, all I was saying is that the original post did not ask if a flat wing or polyhedral wing is "better" or more stable, or anything else, it was purely a question about pure aerodynamic theory, is a dihedral wing more efficient in a turn than a flat wing. I think the jury is out from what I can see and certainly in our world we are unlikely to see any difference, given the factors involved like controllability, stability and so on. What I was getting at was if one was to design a sailplane for absolute maximum efficiency, then would it have polyhedral?

After all, an Albatross doesn't does it. Sure, it gets round the flight control problem with an flight computer that would make NASA weak at the knees, but given the thousands of years of evolution that went into it, if polyhedral had been the most efficient layout aerodynamically, would we not have seen it in nature more often?

but doesn't the composite wing flex of the most efficient full-size gliders provide elliptical dihedral? and isn't elliptical dihedral what polyhedral is approximating?

The composite wing flex is largely a product of materials science, in that a structure rigid enough not to bend is likely to be either too heavy or too rigid. That flex helps the structure absorb the bumbs and turbulence that would otherwise crack a rigid structure. It also stops it from breaking the pilots teeth as the shocks would then be passed into the body, giving a pretty uncomfortable ride.

Performance optimised to one thing only, can severely reduce overall performance

Exactly, I was once told that the best designer is often the one who makes the best compromises, every aircraft, indeed every product, is a result of trying to acheive efficiency and performance in every arena whilst mitigating against the downsides.

Example, I have no doubt that Airbus et al could design a more fuel efficient, faster airframe, but they have to take into account all those passengers, crew, food, fuel, turnaround times, runway sizes, terminal sizes, emmissions targets, noise regulations, structural limitations, cost efficiency and market appeal that all conspire to make the ultimately efficient airframe useless for purpose without compromising to acheive an acceptable result in all those areas

What would an airframe would look like if all other factors bar aerodynamic efficiency were removed?

[kcaldwel]

A lot of the latest high performance sailplanes seem to have polyhedral. Here is the DG-100:

Kevin

[Flyingwingbat1]

The question I asked was in regards to a TIGHT thermal turn, with the resulting significant difference in airspeed from the inside tip to outside tip, sort of the proverbial "turning on a wingtip", if you will. Thanks for the responses so far.

[Brandano]

My gut feeling is that the wingtip of the outer, faster wing will contribute more to the centripetal force needed to turn the glider than the tip of the slower wing. This one will have its lift vector aligned more to the vertical, and will mainly keep supporting the weight of the glider. Essentially, the outer, faster wing will be naturally loaded more than the wing inside the turn, and that might help a bit with the overall drag and stall behavior. Naturally, that's only my impression, and not really supported by any research, so disregard it if it seems too silly.

[Thomas B]

A lot of the latest high performance sailplanes seem to have polyhedral. Here is the DG-100:

Kevin

That is more about the flex in the composite wing than designed in polyhedral.

Once you get past old vintage sailplanes like the Minimoa and the Petrel, there are basically no full size sailplanes with built in polyhedral. Lots of the composite ones gain some elliptical dihedral from wing flex when the wing it loaded up with some G.

[MCarlton]

The kicked up tips on that DG are more to do with reducing pilot effort to maintain a thermal turn than they are to do with efficiency once in that turn. Unlike us, the full size sailplane pilot has to hold those controls against their aerodynamic forces, for extended periods, whilst bumping around and keeping an eye on his instruments, other aircraft and whilst trying to pour coffee and eat hob nobs.

[kcaldwel]

That is more about the flex in the composite wing than designed in polyhedral.

Once you get past old vintage sailplanes like the Minimoa and the Petrel, there are basically no full size sailplanes with built in polyhedral. Lots of the composite ones gain some elliptical dihedral from wing flex when the wing it loaded up with some G.

Uhmm, did you look at the photo I posted? That is designed in polyhedral in one of the latest sailplane designs. There are a couple other new sailplanes that are similar. That is not flex you are seeing in that photo.

Kevin

[Thomas B]

Uhmm, did you look at the photo I posted? That is designed in polyhedral in one of the latest sailplane designs. There are a couple other new sailplanes that are similar. That is not flex you are seeing in that photo.

Kevin

Forgot about the kicked up outer tips on the DG-1000. In any case, it is pretty rare. I was speaking to polyhedral wings like our common model sailplanes that have 4 wing panels of around the same size.

I would call what the extended DG-1000 has a non planar wingtip...technically a type of polyhedral, but a special case.

I would not be surprized if it was as much about about wingtip ground clearance on the DG-1000 when ballasted up with water as it is about polyhedral for better turns.

[MarkusN]

I would not be surprized if it was as much about about wingtip ground clearance on the DG-1000 when ballasted up with water as it is about polyhedral for better turns.
Good point. As it affects maximum acceptable crosswind for landing that's a feature that can affect the usability factor of a design a great deal.

[kcaldwel]

There are a few of the new gliders with subtle polyhedral at the tips. I doubt it is for ground clearance, since these carbon fiber spar gliders are far stiffer than the old unlimited type sailplanes like the Nimbus 3, and they didn't put polyhedral in those.

It seems to have something to do with blending into the winglets.

http://www.hph.cz/304c_technical.php?&lang=en
http://www.hph.cz/images_big/304_Shark_vykres.jpg

Kevin

[Texas Buzzard]

This was posted earlier on this thread. "It's really a matter of efficiency vs. controllability".

Let's say a pilot with just minimal experience wants to fly a sailplane. First he needs a stable A/C. He will tend to give way too many commands and this contribute to extra drag which can produce sink in dead air....right?

So the fewer control commands we use the better the ship will be streamlined with the incident air. Since we would like to be as effecient as we can, we help the ship by staying off the sticks.

Back to the question; The effeciencey GAINED by using a wing of low or little dihedral is Minimal to the Novice Pilot. Why? Well the Novice will find his low dihedraled plane will seem to out of control to him thus he will ruin the "efficiency" of the super sailplane. "Flat wings" are for Experienced Pilots

The slight effeciency of a "flat wing" is LOST if a beginner is flying! The Beginner needs a plane THAT WILL FLY ITSELF.

The "OLD BUZZARD" the guy ( Thornberg )who designed the World Champion Sail plane used Polydihedral. That plane was "The Bird of Time". He still flies a Carl Golberg "Gentle Lady" ( Polydihedral) I fly a G.L. even Now.

When you hear that a "flat wing" is "better", check out that flier's experience. The Dodgson designs of the 1990s were for the Contest Pilots - they had the low dihedral wings, and were excellent for a contest pilot.

But the average to novice pilot will fly better if his plane will fly itself for a while. The tiny bit of EFFICIENCY GAINED by using "flat wings" is not worth it to the average pilot. Be happy, smile at some dihedral. That's it.
HOWZ ZATT?

[biber]

I would not be surprized if it was as much about about wingtip ground clearance on the DG-1000 when ballasted up with water as it is about polyhedral for better turns.It is to a far extent a ground clearance issue on the DG 1000.
My Club owns one.
There is a small wheel right inboard of the intersection, so that the plugged in wingtips need no wheels.
(Would be more wheels otherwise to pay for and this way the forces the wheel exerts on the wing don't stress the wing-wingtip connection.)
The DG 1000 stands on a high undercarriage and with one wing tip lying in the grass,
you need the extra dihedral in the wing extension to get the little wheel on the ground and clear the extension.

TexBuzz, the lower workload is not alone to the advantage of the beginner.
Any gliderpilot interested in the overall performance of the man machine system wants a plane
that requires minimum attention in order to free as much mental recources for navigational, tactical, strategical duties.
And not to forget, on a flight of 5 hours or more in a one seater, you need to drink, eat and certainly will have to take a leak sometime.
To speak from the experience I have made myself, it's a pleasure to fly the modern ships with the pre-bent wings.
Compared to that, some of the lower dihedral very straight winged older gliders of the last three decades are a real PITA.

biber

[Thomas B]

It is to a far extent a ground clearance issue on the DG 1000.
My Club owns one.
There is a small wheel right inboard of the intersection, so that the plugged in wingtips need no wheels.
(Would be more wheels otherwise to pay for and this way the forces the wheel exerts on the wing don't stress the wing-wingtip connection.)
The DG 1000 stands on a high undercarriage and with one wing tip lying in the grass,
you need the extra dihedral in the wing extension to get the little wheel on the ground and clear the extension.

biber

I was pretty sure that I had read exactly that reason somewhere, that the non planar wingtip was about better ground handling

[Texas Buzzard]

The poster asked WHY IS THE SINK RATE IMPROVED WITH DIHEDRAL ..OR.. SOMETHING TO THAT EFFECT.

THE QUESTION SHOWS A LACK OF UNDERSTANDING.

If the SINK RATE is improved when he flies a ship with polydihedral as opposed to one with maybe 2 degrees of dihedral: the improvement is due to the pilot not contenually making corrections with the sticks.

When the sticks are moved some induced drag will slow the plane. The lift of a wing is greater at a higher speed.

Polydihedral DOES NOT improve aerodynamic lift. But it is freindly to novice pilots. Why? The plane will fly itself.

[Thomas B]

Polydihedral DOES NOT improve aerodynamic lift. But it is freindly to novice pilots. Why? The plane will fly itself.

It does improve sink rate if the model slips into and out of turns with less control deflection and less slipping/skidding. Reducing the drag caused by these does indeed reduce the sink rate of the model, in tight turns.

His specific question is in the title to the thread...."Why does dihedral/ polyhedral help sink rate in tight thermal turns?"

No one is saying that dihedral/polyhedral reduces sink rate in steady undisturbed flight

[philami]

The difference between a flat wing and polyhedral configuration is mostly a question of rotation time. Polyhedral rotates faster. In essence this means the glider will rotate and return to the thermal more effeciently. This is why in freeflight thermaling flat wings have no chance against moderate dihedral or polyhedral. Keep in mind that in freeflight the model is always obedient to the dynamic forces and relationship of being in a thermal and out of it. There are many significant differences difficult to sence in rc because we induce controls on the model most of the time. This is why the freeflight has a built in left turn (usually in the Norther Hemisphere) with some stabalizer tilt to reduce the wings aerofoil from occilating and shedding lift.
The fundimental scenario is this (using freeflight as a example): The thermal is lower in density then the air outside of it. Now then, there exist a 'buffer zone' at the edge of the thermal that is losing heat (and increasing in density) just as the outside air bears it's buffer against the thermal's side by reducing both temperature and density of that thermal. The denser out of thermal air is compressing the thermal until it eventually dissipate the thermal. In freeflight this means as the model penetrates this 'buffer zone' the wings temporarly lose lift. The rudder, however, is still fully functional and working. The rudder in not functioning for lift and is functioning by the air pressure of forward movement against it on the model's rudder as it flys forward through the air. Thus as the wing lose lift and the rudder kicks in it's full function this will rotate the model left --- because the wing momentarily has little air 'grip.' This interaction returns the freeflight back into the thermal. The time factor, just seconds, is such you can not rotate a rc at the same rapidity of a freeflight because your response time can not match the instantanious rotation time of a freeflight. The rc aerofoil function would return to the wing there by becoming more resistant to turning by the time we react to a 's' dip that had occured. In rc you will go much further away from the thermal then a freeflight would. Add the resulting altitude loss because of the added rc 'return' time. It's my opinion not a whole bunch of us can not visually see this comparison difference. One of the reasons is in both within the thermal and outside of it turbulances exist. The freeflight knows the difference between turbulance and thermal exit. In turbulance the wing's aerodynamic lift factor in fully functioning at varying degrees at all times. This is not so when the wing's aerofoil exits the thermal's 'buffer zone.' There would be a 's' shape down dip at thermal exit. Since we are on the ground it's impossible, except for a good guess, to differentiate between turbulance and 'buffer zone' penetration. In a full size sailplane that is piloted the pilot would hear the variometer (lift sencing) and physically 'feel' the difference between turbulance and 'buffer zone' penetration. On the ground, without electronic lift sensors on the model we have no seriously accurate way of knowing which had occured. In a full size sailplane the pilot would feel the turbulance as wing stress occilations. Would feel the whole sailplane 's' dipping, minus the wing stress occilations, and therefore know he had penetrated the edge of the thermal.
This I'm telling you will stand the test of time. Only experience and understanding the dynamics involved in soaring can lead to better thermaling time. You must develope a deep understanding the fundamentals of this. I'm sure better discriptions do exist. I do appreciate your observations more then you can realize.

Thought you'd like to know --- every one of our comments adds to the over all of our hobby.