iron eagel

I am now finishing the wings for a build I am working on and have a few questions regarding wing tip design, and its interaction with the aileron.
I have included a couple of pictures to illustrate what I am talking about.
The first is the entire wing with the flap and aileron installed.
The second is the wing with the aileron extending beyond the wing with no wingtip installed.
The third is with a wingtip that extends beyond the aileron.

My ideas:
Where this is an aerobatic airplane, my original thought was to extend the wingtip only as far as needed to match the length of the aileron. I would thin down the airfoil as much as possible as it goes from the wing to the end of the wingtip. That in my mind would not reduce the roll rate or increase coupling and should work.
But I feel that an aileron that is extended out as far as the wingtip, is more prone to flutter. So I am thinking that bringing the wingtip out a bit more past the aileron to keep the aileron out of the tip vortices might be a better Idea.

Now for my questions:
Is an aileron that extends to the wingtip more prone to flutter?
If the wingtip is extended beyond the aileron is it less prone to flutter?
If I extend the wingtips beyond the aileron how much will it dampen the roll rate?
What type of effect could this have on coupling?

Lnewqban

Iron angel:

I do not know it, but we can reason together, using what we know.

What happens at the extreme of the wings is a little uncertain, since the air on the bottom surface would follow the shortest path to the lowest pressure area it can find on the top surface.
While trying to do that, it would be dragged by the friction of the wing's skink.
Closer to the wing's root, that shortest path is perpendicular to the span; but closer to the wing's tip, that path becomes more parallel to the span.

I believe that if we could sit at the wing's tip, we would see air flowing out of the tip in an angle pointing to the tail, curling upwards and being left behind, looking like a spiral.
That flow may be more or less turbulent, pushing and hitting the wing and the aileron, inducing vibration and flutter, if it is too bad.

Now your questions:

"Is an aileron that extends to the wingtip more prone to flutter?"
"If the wingtip is extended beyond the aileron is it less prone to flutter?"
I believe the wingtip will not shield the aileron; in fact, the turbulence will hit the aileron first, regardless of the presence of the wingtip.
It seems that the sole reason of the wingtip is to protect the aileron, linkage and servo from mechanical accidental forces.

"If I extend the wingtips beyond the aileron how much will it dampen the roll rate?"
It may dampen a little, due to increased drag while the wing rotates; but how much surface the wingtip can add to the wing?

"What type of effect could this have on coupling?"
I do not know, but I don't see a link with ay type of coupling.

Let's just wait for words of wisdom from more experienced posters.

Regards!

BadSplice

My Feedback: (1)

My understanding is that having the wingtip extend past the ailerons will actually Increase roll rate slightly rather than dampen it. Like Inewqban said, as you get out towards the wingtip you see the air moving more parallel to the wing than across it. When the aileron is deflected, the air that would flow along it and spill off the end runs into the wingtip, and some of that is forced back over the end of the aileron.

This could be a gross oversimplication, or just plain wrong, but I remember picking this idea up somewhere and it made sense to me.

MX240

My Feedback: (4)

Years ago I bought a large (at that time) stunt model with a 70CC gas engine. After I had bought it, there was word around that this model had a bad repetition of the ailerons fluttering. I had read somewhere that extending the wingtips even with the ailerons would lessen this effect. The wingtips were suppose to be squared off. I carved a round block wingtip that went even with the end of the ailerons and years of flying never had a problem.
That is all I know. )
Jim

MTK

IE

First off, one should understand that flutter is a speed phenomenon and any control surface that is wide and twisty, flimsy or thin and flexible, inadequately secured, connected via a poorly done or spongy linkage, and driven by a weak servo will be more prone to flutter regardless of where the tip is positioned. Surface width tends to reduce the natural frequency needed to cause flutter....that is, lower speed is required. However, width allows for easier, stiffer construction which will reduce the opportunity to flutter. The surface need not be in the turbulent vortex at the tips to flutter.

To mitigate surface flutter (and this includes the wing BTW) make the surface stiff, use a stiffer control rod, stronger servo, etc.

Damping of roll rate is a function of aileron area percent: if you use aileron area that is small compared to the wing (less that 10%) ROLL RATE WILL BE SLUGGISH OR WELL DAMPED FOR AN AEROBATIC MODEL (hit caps by mistake...I'm not yelling). Around 15% is about right for nice response without being touchy. I have settled on 17% in my aerobatic designs because I love the response....reaction only when commanded. I use the best servos available on ailerons with lots of torque but moderate speed

Ailerons have negligible effect on coupling. Dihedral has profound effect on roll coupling. You will need to get dihedral right for your specific wing placement vertically on the fuse side and in relation to your thrust datum line.

Hope this helps

Matt Kebabjian

CrateCruncher

My Feedback: (1)

To me the three things that are the most common causes of aileron flutter are: very high speed, springy control linkages, and heavy ailerons. Try to reduce those three things and I don't think you'll have a flutter problem. I've flown reasonably fast planes with full span ailerons and never had a flutter problem. However one thing I do like about having the tip extend past the aileron to the full chord is that you have a quick visual reference for the neutral position. Sometimes ailerons can develop a warp over time that isn't obvious with a full span aileron.

As far as roll rate, two things to consider that determine how fast a design is going to roll: inertia, and aerodynamic resistance. If you twist a detached wing in your hand you can feel the aerodynamic resistance as you try to roll faster. Having a slightly longer aileron will make a slightly faster roll for a given deflection but probably not noticeable. I think deflecting the shorter aileron further will more than compensate.

These are just one guys humble thoughts. I look forward to hearing about the maiden.

BMatthews

Extending the aileron like that is probably going to have no noticable effect on the flying of the model. But it would be a fantastic place to attach a forward extension arm that carries a mass ballance to help move the CG of the aileron surface forward and reduce the risk of flutter.

iron eagel

First of all, thanks for the replies to my questions.
I was wondering what effects that wingtip design, style, or shape had regarding overall performance of an aircraft. As I was finishing up the ailerons I began to consider how I should do the wingtips. My knowledge of aerodynamics is limited, and very limited as far as what you may label the “Macro� view. Perhaps at times I obsess about what are very minimal issues as far as the overall design, where I tend to look at the “Macro� view in each aspect of the overall design. As I have read your replies it has given me a better understanding of just how much span wise flow enters into the picture, but it has also made me wonder about exactly what I did when I decided to re-size the ailerons. My original question was just focusing on how the wingtip affected the flow of air around it not taking into consideration that it was attached to the wing. I for some reason, had gotten it into my mind that wingtip vortices are a part of what may cause flutter, if you had already taken the time to minimize the flex of the aileron, and the slop in the control linkage. I perhaps should have given you a bit better idea of what I am working with and what I was thinking about.
My ailerons are fairly rigid, they have a carbon fiber trailing edge, are fully sheathed, and they will be covered in carbon veil as well.
As far as the slop in the control linkage the ailerons are going to be linked to the servo by a pull/pull setup, so I can move the mass of the servo into the wing root to keep the outboard mass of the wing down. Using the pull/pull should keep the slop to a minimum, and using a Hi Tec HS 425 bb servo that has 57 oz of torque at 6 volts I should have a strong enough servo. While it is a bit heavier servo than I had original intended to use at 1.6 oz as opposed to the .75 oz ones (I had thought of using), the Hi Tec servos have a know history as opposed to the new chap servos that are lighter. (I decided that I did not want to use this plane as a test bed for a cheap servo.)
Hopefully the ailerons should not be all that heavier than the stock un-sheathed slab ailerons that are normally covered with plastic film, that this kit was original designed for. The ailerons have also been cut down from the original strip type setup that they were originally, for the addition of flaps to play with STOL type of flight as well as mixing them with elevator as done in CL stunt. By doing this I know I have really cut done on the ailerons effectiveness and anticipate that I may have to increase the throw to get them to work at slower speeds.
Now after reading your replies, I have begun to consider just what the effect of the reduced aileron surface will be. I have included a shot of the airplane in question to give you a feel for what I have done to it overall. As far as the wing I have added fairings to the fuselage to blend the wing root into the fuselage. With that I have added roughly 8 square inches of surface to the wing root, and with the wingtip I will be adding anywhere from 10-15 square inches of area to the wing at the tip. The wing as originally designed had roughly 355 inches of area on each side, and the aileron area accounted for 66 inches of that. My ailerons have roughly 33 inches of area so I am fairly sure I will have to increase the maximum throw for performance, where the aileron area has been decreased to just under 10% of the wing area. The reason the ailerons are extended beyond the end of the wing at this point was to make up for the area of the wing which my fairings took up. My solution originally was to just extend the wingtip to either match or extend beyond the aileron to make up for the ailerons and flaps having been shifted outboard by addition of the fairings. I would like to build the wingtips to reduce drag and if possible enhance performance. This airplane is not been modified with speed to be the goal, but rather to reduce drag to improve duration of flight, while trying to keep to a target weight that is as light as the original stock model would be. I am using a .55 engine instead of the specified .46, and do not want to have to run a larger fuel tank (which I would like to shift a bit more aft than the original design called for). I was just wondering what the optimum style of wingtip might be.
The reason I had asked about flutter, roll rate, was if there was any way the wingtip could be designed as to improve issues with either of these (or if they were even issues to be considered). The question about coupling goes back to an original discussion about this project when someone said the flat wingtips were intended to “improve coupling� a remark that someone from Sig had made when questioned about the design of the wing of this plane, in response to my comment about the blunt wingtips “not looking right�.
Although, Bruce’s suggestion regarding counter balance on the aileron has me to thinking about adding counters to the ailerons.
Any thoughts about that???
This plane has been for the most part an experiment with bashing a kit I had into a fun fly that would be more for pattern flying than 3d. I know I really trashed the roll control in hover by adding the flaps but was not overly concerned about that part of the flight envelope, but now I am a bit concerned about the roll response during normal flight.
I have include a picture of the progress thus far to give you a better idea of how the plane has been modified thus far.

Regards,
Paul

da Rock

From the looks of that wing, I'd suggest that your roll response is going to be very good. It will result more from the short wingspan than anything else.

What was the original span? and your new span?

iron eagel

Thanks DA,

The original span was 51.5 the updated would be between 53-56 inches.
The kit was A Sig Somethin Extra.

da Rock

Don't sweat the wingtips having anything at all to do with coupling. They don't. As already mentioned.

If you want to obsess over something, think about what your very short aspect ratio wing is going to do to induced drag when the elevators try to pitch the sucker. Long ago CL combat flyers discovered that low ARs worked like airbrakes when pitched. Pull up or down and the suckers slowed down like the engine had sucked air. You may wind up with an elevator that you will not want to use very fast. Unfortunately, there aren't any formulas to predict what you'll see when you fly it. It'll be something that'll show itself when you're at medium speed and higher. 3D flying doesn't see it at all. On the other hand, low ARs do affect stall speed. The lower the AR, the lower the AOA possible before stall and the lower the max lift possible and the higher the induced drag.

da Rock

OK, I've see them fly. They're decent, really decent. Halving the aileron area isn't going to cut the response in half, because you're keeping the part that has the most leverage.

Don't worry about balancing the ailerons until you find a reason for that. Although it does sound like you plan to fly the plane more like a pattern plane and keep the speed up more than not, flutter shouldn't be a concern unless you really plan to fly it really fast. And that thick wing isn't going to go along with that plan.

da Rock

You mentioned cutting drag to get more speed.

The ones I've seen have a load of crap on the empennage as "stiffeners". If you want to cut the drag on that airplane, doing that stiffening a better way will really cut the drag. Get some airfoiled aluminum tubing and use it for those struts. It's easier to do, quicker to build, stronger, and really, really reduces the drag. You simply flatten the ends of the tubing, bend that flat to the angle needed, drill a hole for the bolt, and bolt the suckers through the stab. You wind up with an extremely clean structure. And being airfoiled instead of round, is about 10X less drag.

iron eagel

Despite how the plane looks I do not intend to fly it fast, I just wanted to clean it up a bit and lose some of the flat sides of the fuselage. Like you said those thick wings will really limit that aspect. Hopefully I have not trashed a nice design with my modifications just tailored it a bit more to how I intend to fly it. It also has a hard point on the cooling duct to mount payloads such as a camera pod, or bomb drop for contest. I Figured that the SSE would be a good fun fly for me considering I had been given the kit as a present. Besides it will not look much like any other SSE at the field.

da Rock

Looks like you're doing really nice work. Is that your paint base coat on the fuselage?

The S.E. I flew a year or so ago had a roll rate that you couldn't see, much less time. You won't miss that aileron area you turned into flap area. And you might be able to actually time the roll rate. But don't count on it.

iron eagel

I put my stiffeners in internally on the stab and rudder. (1/8 inch carbon fiber tubes)

iron eagel

Thanks for the compliment!
Yes that is the base coat for the finish, the base coat bonds carbon fiber veil to the sheathing (1/32 inch balsa). I am still up in the air about the finish paint other than the colors being white, red, orange and yellow.
The major portion of the plane will be white with the other colors being accents.

da Rock

Red, orange, yellow............ hoo hah.......... my favorite color!

rmh

The mass balance shown here reduced the CG on the aileron to aprox 20% of chord .
Worthwhile.
The Dalotel has slashed tips which can and do change roll couple - I once used carved foam blocks to simulate the slash tips, to see if it really could affect roll couple
On my planes which had aspect ratios of 5.5-1 , they did.
A good aerobatic setup in the size of the subject , needs about 15% wing area in aileron and up to 30 degrees throw actually, large, long, barndoor ailerons work the best here. On tiny models , I go up to 40% aileron (honest) sounds absurd unless you already fly very small , ultra light stuff. ( 6 ounces or less) Then you have seen this.
The wood grain direction shown in the photos may cause a problem unless there is atually a cross grain beneath
I found that it is impossible to make an aileron too light - too flimsy was easy but ram rod stiff and a CG well forward of 25% very tricky
Here is one setup which works well

Lnewqban

Hanson:

What / how is, in your experience, the effect of slashed wingtips over roll coupling?

Regards!

MX240

My Feedback: (4)

Iron Eagle,
Ha, Here is a picture of my Sig Something Extra's wing tip. Why did I carve a block of balsa wood and glue it onto the wingtip??? Because those fat wingtips were so darn UGLY. Had nothing to do with flutter.
If you want that plane to do good stall turns you had better add a half to three quarters inch more to the rudder.
Nice plane in it's day...but I like my Funtana 100 better now to play around with.
Jim

rmh

If you need a bit more dihedral- the slashed tips may fix things It did when I played with it.
I never tried inverted slashed tips tho they should work
but look odd
Getting the slash correct -is easy if you have a band saw and can jig the wing for each pass. otherwise it will be a cut n sand trial.

da Rock

Dick, would you say that how much effect the slashed tips have on roll couple isn't exactly predictable?

rmh

Nothing is really predictablein amount of effect
But if the plane has a mild adverse roll , the slash tips may fix it

There is not a lot of angled area to work with but it does help. the big reason being that it has a pretty long lever arm.
Solving proverse/adverse roll on a precise aerobatic setup , often requires very slight changes
changing cg and or shifting wing and stab to lower their vertical relationships can also work but it is a "little bit" kind of a fix
the biggest fix is still shifting CG to match speed. (sounds impossible?)
futzing with engine offset is often a problem causer rather than a problem fixer
All said n done - go for least possible weight and most power -to allow the flying all to done at lowest possible angles
This, plus the lowest possible aspect ratio- all aids in making a really good, precise flyer.
The total dihedral in my wing panels (aerobatic setup), is typically the angle on the lower surface of the wing panels due to thickness taper ONLY
so, the plane is pretty darn nuetral stable in roll. This plus LOTS of well placed fuselage side area.
In this case a slashed tip can easily make a difference but on a X Ray or a Seniorita -something having a lot of dihedral for openers , you likely will see no difference. I never even bothered trying changing tips on these types

iron eagel

Thanks guys you have given me a lot of info to work with.
Dick, I see you caught the direction of the wood grain. Yes, the underlying structure is cross grain to that (running the length of the aileron). I used the stock Sig aileron applied a 1/16 inch carbon fiber rod for the trailing edge, and then shaped them into an airfoil shape (tapered to the trailing edge) and then sheathed them cross grain with 1/32 balsa. The whole assembly will be covered with carbon fiber veil applied with dope, much as we used to do silkspan.
I was just wondering how much effect wingtip design had as far as model aircraft go. Granted where we are talking about a rather small area on a model any effect should be minimal, I just wondered if there was any way to optimize it for overall performance. Where this airplane was an aerobatic design it seems to have minimal dihedral so that is not much of an issue.
I am thinking of adding counters to the ailerons just to minimize servo loading. After reading what you said you did to get the CG point at 20% of MAC using counters it seems like it is well worth the effort. Bruce had already mentioned that that might be the way to go and after having you suggest it as well that seems to be the best thing to do.
Thanks!