Winglet airfoil?

[jpolenek]

I'm new to the canard pusher world and intend to order Cozy4 plans in the next month or two. In the meantime, I'm learning about the design by going over the Open-EZ drawings.

Looking at this design, it appears that the winglets have an airfoil shape that produces lift in the inboard direction. I assume this is common to the Cozy4 and similar designs.

Why would the winglet be designed to produce lift inboard? Doesn't this create unnecessary stresses where the wing meets the winglet, as well as additional induced drag when compared to a symmetrical shape? Is there some aerodynamic purpose/advantage with the airfoil?

Joe

[Jon Matcho]

I understand the winglets are (or are similar to) Whitcomb winglets and they actually provide some thrust. There's a summary of some of the information here: http://www.cozybuilders.org/Oshkosh_Presentations/Nats_OSH2005_Presentation.pdf

 

I wonder why they're canted inward a couple degrees instead of outward, but not thinking to change anything about them...

[raiki]

My understanding is that because they are cambered inward, the low pressure zone above the wing meets the low pressure zone inboard of the winglet, reducing the vortex of the tip. I think in aerodynamacistsssssisisisis talk they increase span efficiency or some jargon like that.

 

Also on the bottom they are cambered out so that the high pressure zone inboard of them meets the high pressure zone under the wing, for the same reason.

 

I am pretty sure that is why they have a cambered airfoil rather than a more usual symetrical airfoil for stabiliser surfaces.

 

All up I just consider them Burt Magic, thats aeronautical black magic.

[Wayne Hicks]

They are whitcomb winglets, not rutan black magic. All Rutan did was add trim tabs (rudders). Do a search on whitcomb winglets and read, read, read. You'll find that these winglets (1) add lift that would otherwise be accomplished by lengthening the wing; (2) the lift provides thrust in the forward direction; and (3) reduce tip vortices (drag).

[jpolenek]

How can winglets produce "thrust", i.e. a forward force. In ground school we learned that an airfoil produces a force normal to the chord which can be split into a vertical lift component acting up and an induced drag component acting back. (This should be true for any airfoil.) There's no angle of attack that can produce a forward component of that force. So if the winglet is basically an airfoil on its end, why should it be able to do this?

 

Joe

[argoldman]

How can winglets produce "thrust", i.e. a forward force. In ground school we learned that an airfoil produces a force normal to the chord which can be split into a vertical lift component acting up and an induced drag component acting back. (This should be true for any airfoil.) There's no angle of attack that can produce a forward component of that force. So if the winglet is basically an airfoil on its end, why should it be able to do this?

 

Joe

Because there are two winglets, rigidly connected to each other and producing lift opposite "lift" toward each other, one explanation might be the same force vector that exists when you squeeze a watermelon seed.

 

Slurp

[Wayne Hicks]

Because the lift vector is in the forward direction. Please find and read the NACA reports. Search through google. You will find it. Or, go to the other forum and read it there. Or, search through the Cozy email archives. It's alllll there, I promise.

[Ron Springer]

How can winglets produce "thrust", i.e. a forward force. In ground school we learned that an airfoil produces a force normal to the chord which can be split into a vertical lift component acting up and an induced drag component acting back. (This should be true for any airfoil.) There's no angle of attack that can produce a forward component of that force. So if the winglet is basically an airfoil on its end, why should it be able to do this?

 

Joe

Lift is defined normal to the velocity vector, not normal to the chord.

 

I will also make an attempt to explain the winglet thrust in simple terms ...

 

The velocity vector out at the winglet has two components. The main component has the magnitude of the velocity of the plane pointing straight aft and opposite to the direction of flight. The second smaller component is pointed inward (above the wing, outward below it). That component is due to the tip vortex. Try to visualize the vortex rotating around the wingtip from the bottom to the top and how the small component of velocity is created. So, even if the winglet points straight forward, it is still at a small angle of attack.

 

If the velocity points mostly aft but slightly inboard, then the lift acts normal to it, which is mostly straight to the side but also slightly forward. The small forward component of winglet lift is a thrust component. It probably isn't much thrust, but it is greater than zero!

[Wayne Hicks]

Andy Amendala wrote an excellent winglet article. Here it is in its entirety.

 

I figured I would take the time to educate you on some of the functions the winglets perform and how they do so. I'm not sure what you know about aerodynamics so if I sound like I'm patronizing your knowledge, *please* don't take it that way, I simply wish to inform. I thought you'd be surprised to find out just how much those giant fins you have on your Long-EZ are doing out there.

 

First off, let me say that you are 100% correct in your assumptions about the forces applied to the wing structure on the whole. The resultant forces of the winglets do resolve to a tension in the high-pressure surface of the wing (the lower side obviously), and of course if we're in tension on the bottom side, the top side of the structure must be feeling some compressive load. I'll give you an explanation of why this is the case and the function of the winglets as a whole. I'll try to be as detailed as I can but keep it relatively simple as well.

 

First and foremost, the reason the wing feels those forces is simply because the winglets on the Long-EZ produce lift. In fact, they produce a tremendous amount of it. I haven't done the precise lift calculations but cursory calculations indicate right off that you'd have a hard time accelerating your Long-EZ on the ground to a high enough speed to even lift off if you had a symmetric airfoil on the winglet (as opposed to the lifting airfoil we have). So the winglets on the Long-EZ pull inward toward the fuselage really hard. Much harder than you might imagine... Take notice of the substantial amount of glass applied when attaching them to the wing.

 

So let's get technical... Why all this force pulling inwards from the creation of lift? Why do we have these structures as opposed to just a longer main wing? Why are they angled inwards toward the wing? Why are they canted toward the nose? And so on.

 

 

The higher pressure air under a wing wants to spill around the wingtip to try to fill in the low pressure area on top of the wing. This flow results in a tip vortex trailing aft from the wingtip, like a horizontal tornado. You can see these vortices at the wingtips of a jet fighter during a high-lift maneuver in sufficiently humid air, or at the tips of an airliner's flaps during a landing approach in wet weather. The energy extracted continuously from the aircraft to make the air swirl like that is a direct result of the creation of lift and is dubbed 'induced drag'. These vortices are at their worst when we're trying to make lots of lift with relatively little airflow. This means that slow flight (low speed, low mass flow, high lift coefficient) is one of the worst cases. This also means that the intensity of the tip vortices will be highest at these kinds of flight conditions. The higher the intensity of these vortices, the higher the induced drag on the aircraft, and thus, a greater amount of wasted energy. If you trace back how your airplane is really flying, you get to one source of energy, the fuel in your tanks. Extracting every ounce of energy from that fuel in every respect is a challenge of aircraft design. So the more energy we waste on things like wingtip drag, the less energy for the airplane to use for other means. I won't go into it here but you can read about a coefficient that you can calculate that will tell you in general how efficient your aircraft is... this is known as the Oswald Efficiency Factor.

 

So back to winglets specifically, there are generally two families of winglets you'll find on aircraft today. Simply put, lacking many specifics of course, one family has the production of lift as one of its primary jobs, and the other does not. The winglet style on the Long-EZ is of the lift-producing family and was designed by Richard T. Whitcomb. Our winglets are hence called Whitcomb Winglets. A small historical fact, the first aircraft to ever fly with these winglets was Varieze N4EZ.

 

So we need to talk about "helix angle". If you understand the pitch of a prop, you're already familiar with it. Helix angle is one way to measure how far something rotates compared to how far it travels forward in the same time. The blade angle of a propeller blade is nearly the same (minus its efficiency effects and local angle of attack) as its helix angle. A wingtip vortex has a helix angle as well. This angle will be nearly parallel to the airplane's direction of flight when induced drag is low, but twist up into increasingly greater angles relative to the flight direction as we slow down or pull more "G".

 

If we have a significant amount of induced drag, and a correspondingly stronger tip vortex, then the flow at the wingtip will not be parallel to it, but rather at an inward angle on the top and an outward angle on the bottom. This is where the winglets come in.

 

If we park a lifting surface in the middle of this angled air flow, it will develop lift perpendicular to the angled air flow. The resulting lift will be angled forward, and the forward component of that lift will be producing thrust. The lifting surface (the winglet) will also be producing drag of its own, including both parasite and induced drag. So essentially, the winglets on the Long-EZ are producing lift not only due to their high-pressure-on-the-outside airfoil, but also due to the energy they are harnessing from the tip vortices. So the winglets, being an effective wall in the middle of the tip vortices, don't just waste the energy there, they utilize it for lift and thrust and in the end, you have a highly diminished vortex trailing behind the aircraft and that means lower induced drag at the tips.

 

But recall I said that the winglet makes drag of its own too... If the drag the winglet produces is less than the forward component of its lift, then there will be a net thrust applied from the winglet to the aircraft. Yes, your Long-EZ winglets actually provide some thrust to the aircraft! This thrust actually represents some of the energy in the tip vortex, harvested from the vortex by the winglet and given back to the aircraft. That's it. That's all there is to it, quite simple really.

 

Ok, now the catch... How do we maximize that thrust? This is where it gets complicated. Let me quickly define a couple of geometry terms I'll refer to. When I say "toe-in", I'm referring to the angle of the leading edge of the winglet with respect to the absolute tip of the Long-EZ's nose. So if you stared at the winglets from the FRONT of the airplane, the more of the "outside" of the winglet you can see, the greater the toe-in angle. I'll also refer to winglet "cant". The "cant" I'm referring to is the tilt inwards of the winglets toward the wing. If you look at a Long-EZ, you'll notice its winglets tilt inwards slightly (the top of the winglets point each other).

 

If you increase the angle of attack of the winglet by increasing the toe-in angle, then it makes more lift force (which should theoretically increase the forward component of that lift), but it also makes more drag of course. Depending on the specific situation, this could increase, decrease, or not change the net thrust of the winglet. It's going to depend on a lot of factors, including the flight condition.

 

The last item is particularly critical. Because the amount of induced drag, and the helix angle of the vortex decrease as you increase airspeed, the energy available for "harvesting" by the winglet decreases as you fly faster. Meanwhile, the parasite drag of the winglet is increasing. Eventually you get to a point where the total drag of the winglet is equal to the forward component of its lift, and at that point the winglet produces zero thrust. This is called the "crossover velocity". At airspeeds higher than the crossover velocity, the winglet adds to the aircraft's total drag and you'd be better off without it.

 

Thankfully, we don't have to worry about most of this with the Long-EZ since the aircraft is already superbly aerodynamically engineered. I just thought you'd find it informative. So I covered why the high pressure side is on the outside, and what the toe-in does for lift, but what about twist and cant?

 

The process of "unwinding" the tip vortex that the winglets perform is accomplished both because they are a physical wall in the way of the vortex, but also due to the effective aerodynamic twist of the airfoil. The orientation of the upper and lower winglets provide effective aerodynamic twist to assist in this function. I'll leave this alone unless you desire details.

 

As far as the "inward-cant" of the winglets is concerned, when you think about it, you might think they're detrimental to the design to some degree. If lift is created perpendicular to the airfoil body, and the winglets on the Long-EZ are canted slightly inwards, don't we end up with a slight portion of that lift pointing towards the ground (I.e. adding to the weight of the aircraft)? Yes, we do. However, it is entirely negligible, it's that small. Burt ran me through some quick calculations a ways back just to show me how negligible it is. So why do they point inwards at all then? They reduce the effective dihedral of the wing.

 

You know of course that the Long-EZ main wings have sweep to them and, duh, they have winglets. Adding wing sweep and a winglet to a wing both make the wing feel as if it has dihedral. Since they don't actually have dihedral physically, we call it 'effective dihedral'. When Burt designed the Long-EZ with a larger wing, he needed wingtip clearance for crosswind landings... Because of this, he needed to do away with the anhedral design of the Varieze. Think of the consequence... The Varieze's effective dihedral from both adding the winglet and from sweeping the wing is counteracted by the anhedral in the main wing surface. Reducing this anhedral to zero, as was done on the Long-EZ for tip clearance, would obviously bring the effective dihedral back up and make the craft more stable, however, more difficult to turn. So to reduce this effect as much as possible, Burt canted the winglets on the Long-EZ inward slightly.

 

So now you can run off and think about all that's happening out there on those fins. I think I've dragged you on long enough, but think about how the rudders on the Long-EZ might work given your knowledge of winglets now. They function differently than conventional rudders. Also think about what happens to roll rate if you cant the winglets outward instead of inward? Think about how changing the toe-in angle would seriously change things? Also think about my favorite modification that I still fail to agree with, cutting off the lower winglet...

 

If you think about what all those changes do, you'll better understand the function and design of the winglet. If you've got any questions, write me back privately, I'd be happy to respond however I can.

 

Safe flying!

 

Andy Amendala

Long-EZ

[Steve Innova]

So if I make my winglets big enough, will they make enough "thrust" that I can dispense with the engine altogether?

[Rydogg]

That sounds like some sort of perpetual motion contraption:scared: . Patent it before it's too late! hahahahahahahahahaha...

[Jon Matcho]

Wayne, Andy's article is excellent -- thanks for posting.

 

I was wondering about the thrust relative to the drag (whether drag was factored into the "winglets create thrust" feature at all), and am glad to know drag has been considered.

 

Regardless, some will no doubt remain in a state of disbelief. Not I. Carry on!

[Wayne Hicks]

The winglets only make thrust when they're moving forward. I think we need a supplement thrust device to start them moving forward through the fluid media of choice.

[satch]

The winglets only make thrust when they're moving forward

Pretty much thinking all is not well if winglets are moving in the other direction ....

 

Nice article, thank you!

[Ron Springer]

Good post, but I find this statement a little difficult to believe:

 

I haven't done the precise lift calculations but cursory calculations indicate right off that you'd have a hard time accelerating your Long-EZ on the ground to a high enough speed to even lift off if you had a symmetric airfoil on the winglet (as opposed to the lifting airfoil we have).

[Marc Zeitlin]

Good post, but I find this statement a little difficult to believe:

Yeah. I must have missed that statement in Andy's original message to the COZY mailing list, or else I would have said something about it, too. I sent Andy an email last night regarding that sentence, because some "cursory calculations" of my own indicated that it had to be incorrect. Here's Andy's (whittled down) response:

 

"The simple fact is: I blew a few numbers on that original post."

 

An honest response. Clearly, the winglets do NOT provide anywhere near the amount of thrust that would affect take-off (or any other segment of flight, either). For a LE/COZY, it's almost lost in the noise.

 

Andy also mentioned that there were a few minor issues with his original post, but that this claim was the big error.

[Jon Matcho]

Yeah. I must have missed that statement in Andy's original message to the COZY mailing list, or else I would have said something about it, too.

Likely story.

 

Andy also mentioned that there were a few minor issues with his original post, but that this claim was the big error.

Is "this claim" referring to the symmetrical airfoil winglets and take-off, or the notion of thrust -- however small -- produced by the setup of the winglets?

 

At this point I'm ready to put on 6-12 winglets and see about running an O-235.

[Marc Zeitlin]

Is "this claim" referring to the symmetrical airfoil winglets and take-off...

Yes.

 

or the notion of thrust -- however small -- produced by the setup of the winglets?

No.

 

At this point I'm ready to put on 6-12 winglets and see about running an O-235.

Yeah, that was the logical conclusion to the original claim regarding the MAGNITUDE of the thrust - all you'd need is something to get you started, and then the winglets push you through the air :-).

 

At any rate, there IS more thrust than drag, so the net effect is a small forward force, but it IS small.

 

Also remember that symmetric airfoils DO produce lift, else aircraft with them couldn't fly. They just do so at different AOA's and with more drag than non-symmetric airfoils.