where is the best place to locate fins on a flying wing?

[Joel K. Scholz]

I am rebuilding a wing that was destroyed due to wing flutter. There was some discussion that I had the fins located in a bad place on the wing. Most wings seem to have fins ( if there are any at all) on the wing tips. Is this the best case scenario for all wings. I can see how they might prevent tip losses and increase directional stability on a swept wing, but what drawbacks are there to having them placed where I had them?. Aside from lower drag, what is the argument for no fins ? I prefer them if for no other reason is to aid in orientation when flying. This plane is designed as a motor glider and will use an EH 212 airfoil.

[nmasters]

Without stating my own opinion I'd like to mention that the similar threads listed below the Quick Reply box has a good discussion of this very topic that you may find helpfull.

[JetPlaneFlyer]

The effectivness of a fin is a combination of it's area multiplied by the moment arm between the fin and the aircraft's CG. I think the 'problem' with the fins where you have them is that they look to be only just behind the CG so their effectiveness will be limited.

Having said this providing the model had no problems with directional stability then they must be working 'good enough' so no real need to change them.
Tip mounted fins, being further behind the CG have a greater moment arm, so you could make them smaller (less drag) and still have the same overall fin effect. As you mention tip fins may also have some benificial wing tip end plate effect. These issues are only important if you are trying to extract tha last few fractions of a % of performance, for a sport model aesthetics and practicality are at least as important.

I'm no expert but I believe that finless swept wings (model and full size) usually, if not always, have quite marginal directional stability. I understand that this was the case for some of the Horton designs.

Steve

[biber]

The only reason to go without any fins would be the cool looking.
For performance winglets will be the best option in most of all applications.
They may, as JPF stated, be smaller because of beeing further behind the CG.
You asked for any drawbacks and there are some indeed.
Badly designed wing/winglet combinations can produce tip stall tendencies.
Winglets will contribute to the effective dihedral.
If you have lots of sweep, that alone gives some dihedral effect whenever the wing generates lift,
with the winglets adding to that, some anhedral in the base wing may be required to compensate for it.
Winglets increase mass at the tip, that may effect the fluttering.
Winglets tend to break off of the tip.

I for one, though seeing these issues, choose the winglet option all the time.

http://home.arcor.de/andre.kubasik/mubu/jpg/mi02.jpg

biber

[Joel K. Scholz]

Beautiful plane Biber

[JetPlaneFlyer]

Beautiful plane Biber

I'll second that :) a very graceful and efficient looking model... Plans available?

[biber]

No real plans, but all basic information needed to build one should be in the following thread:
http://www.rcgroups.com/forums/showthread.php?t=534401
In #8 e.g. you'll find the planform data.

I thought the picture would be just right to show, that winglets can be real tiny and yet work.

biber

[Texas Buzzard]

As already posted: Effectiveness of a fin is a function of the Area of the Fin and how far it is behind the C of G. Some more highly swept wings can be happy with the fins (2) out at the wing tips.

As I see the two fins on your wing I am reminded of the F-22. I built an F-22 with scale fins - they are positioned like yours. They work just fine. I have a Delta with one big fin positioned on the center line, it works too.

In my opinion the person who mentioned that wings have the fins out at the wing tip was well intentioned but not experienced empirically. To answer your question for your wing......you could move the fins to the tips and fly it several times for your answer.

***But remember that the more centrally located fins are in the propwash. Being near the center of the wing those fins would have more "muscle". While the fins out at the tips have to rely on nothing but Airspeed.

**** I would design the wing just as you did and ignore any suggestions to use tip fins. Fins in the blast of air from the prop will be more effective. Here is a good-flying F-22 with fins near the centerline.

DUTCH ROLL ???? How do you know your fin is TOO SMALL?
If the Fin is too Small, when you trim for straight ahead glide the plane will "hunt" or swing from side to side as if the "pilot" is bumping the rudder from side to side. Cure - enlarge the fin. Maybe your fins are too small. Look at the F-22 - - - big fins in the propwash!

[JetPlaneFlyer]

Texas Buzzard,
Prop wash will not help the fins to be more effective in terms of directional stability because the propwash is aligned with the fuselage (and fin) axis not the direction of flight. For a fin to work it must have a angle of attack generated by the yaw of the aircraft from it's direction of travel... Prop wash will always be aligned with the fin so provides no correcting angle of attack in a yaw.
Prop wash will make the rudder more effective but it wont help directional stability... in fact it could make it worse.

Probably a mute point anyway because the fins on the model in question cannot be positioned 'in the prop wash' as there is no structure aft of the prop to locate a fin on :rolleyes: To position a fin forward of the prop (in the props 'in-wash') would mean that the fin was located roughly on the CG and therefore it would be ineffective. And anyway.. it's a motor-glider and it needs to be directionally stable when in glide mode as well as power mode ;)

Steve

[2phar]

Joel,
Other than the flutter, how did she fly. Fix whats broke first.
I agree with not changing them unless bad yaw stability.
Actually "benificial wing tip end plate effect" is a myth. No such thing.
By definition, wingtips leak. No way around it.(pun intended)
Its just not true that finless swept wings usually have quite marginal directional stability.
Some of the Horten wing problems resulted from not enough twist.
Hang gliders have no fins and they are extremely yaw stable.
Dutch roll cannot ALLways be corrected by bigger fins.
It's not lack of yaw stab. on a Boeing 727 that causes the Dutch roll. (did you ever see the size of the fin on a 727?).
Rather, it's a roll yaw couple problem.
The best way to get ALL three axis stability on a swept flying wing is to add geometric twist-- washout.
Looks like you might have some in your second picture.
happy landings, chris

[biber]

Actually "benificial wing tip end plate effect" is a myth. No such thing.
By definition, wingtips leak. No way around it.(pun intended)Of course winglets, done correctly, can take away some off the induced drag.
After all you have increased the wings length of the winglet and the leaking tip is now the winglets upper end. :p
Its just not true that finless swept wings usually have quite marginal directional stability.
Some of the Horten wing problems resulted from not enough twist.
Hang gliders have no fins and they are extremely yaw stable.Yeah, and the giant amount of washout can kill performance over a braod range of speeds quite effectively. :rolleyes:
What L/D do those very stable hanggliders have? ;)

biber

[JetPlaneFlyer]

Joel,
Hang gliders have no fins and they are extremely yaw stable.

I believe is mainly/entirely due to the 'billowing upward' of the fabric wing which gives a fin like effect. The rigid wing hang gliders do have tip fins: http://www.aeros.com.ua/structure/rw/index_en.php (obviously no one told them they dont work and are unnecassary) I think if you are to compare rigid wing models to hang gliders then you should compare them to rigid wing hang gliders ;)


Actually "benificial wing tip end plate effect" is a myth. No such thing.
By definition, wingtips leak. No way around it.(pun intended)

Maybe you should inform Boeing, Airbus, and every other commercial airliner manufacturer of this fact because they are all using wing tip devices on their aircraft :rolleyes: I fully appreciate that the benefits only manifest themselves in certain flight regimes however to insist that the benefits are a "myth" is ridiculous.


Dutch roll cannot ALLways be corrected by bigger fins.
It's not lack of yaw stab. on a Boeing 727 that causes the Dutch roll. (did you ever see the size of the fin on a 727?).
Rather, it's a roll yaw couple problem.

Dutch roll is indeed a yaw roll couple issue but it 'could' theoretically be fixed with a larger fin or ventral fin... Sometimes however the aircraft may have such strong lateral stability (like a swept wing airliner that needs dihedral to allow the engines to clear the deck) that the fins required to prevent Dutch Roll would be so large that they would add excessive weight and drag... So they solve the problem with gyro yaw damping instead.


The best way to get ALL three axis stability on a swept flying wing is to add geometric twist-- washout.

As Biber already said; excessive wing twist is very inefficient. It would also result in excessive static margin and undesirable flying characteristics. Far more effective to use 'just enough' wing twist for pitch trim then add any additional yaw stability with fins... which is why pretty much all current high performance tailless swept wings do it that way

[2phar]

dudes,
Whitcomb winglets which is what they put on the tips of airliners and biz jets are NOT end plates.
they are toed out with approximately 7 degrees so as to unstall them in their local flow. yes they reduce drag a bit.
the rigid wing hang gliders have a few degrees of twist and their best L/D is hurt most by having their dirty pilot and structure hanging out in the wind. (this pilot not dirty)

jetplane: the most common rigid these days is called Atos. it has no fins anywhere.
"excessive wing twist is very inefficient"- i agree.
with their terrible handicap of a human body hanging out in the breeze their getting 20 to 1 glide at around 33mph.
cheers, chris

[biber]

On the current full size gliders there is not even close to 7 degrees toe out.
Mostly the they are installed close to their zero lift angle and get imposed a circulation by the wings circulation whenever the wing does lift.
So even an endplate can become a winglet, if it is high enough.

biber

[2phar]

biber,
i assume by zero lift angle what you really mean is longitudinaly i.e. not toed at all.
there is absolutely no flow anywhere near the wingtip which is longitudinal.
the pressure difference tween top and bottom surfaces drives a rolling mass causing
decidedly inboard flow above the wingtip.
the slower the airplane flies, the more inboard this becomes.
theirfore whichever angle you choose for the tipfin, its going to be wrong for either
the low speed or the high speed regime.
the fins on sailplanes are surly for higher speed help (running).
I've examined them closley, and i'm not sure their right.
but then i'm a little off myself.
chris

[biber]

Again speaking for modern fullsize gliders only:
Yes, the inboard flow imposes a CL on the WL.
It is installed so that it works close to Cl=0 when the wing does too.
Take the direction of zero lift as what it is, the alpha, where the airfoil does not lift.
For the winglet airfoils on current glider designs that will be about -2° or so.
That's about what you see on them as toe out.
From an aerodynamics standpoint that is in effect zero toe out.

The design goals of fullsize glider winglets are set by a certain cruisespeed above which a drag penalty can be accepted.
The more you are short of that speed, the more performance you gain by the winglets.
This break even speed is mostly chosen to be about 100 mph in todays gliders, or say a Cl arround 0.3.
It is the point, where the friction drag of the added winglets is just compensated by the decrease in induced drag they account for.
In order to shift that break even point to a higher speed (or down to a lower Cl),
you have to live with slightly less drag reduction by winglets in the regimes where they actually help.
That's just a compromise you have to make to get a broad positve effect instead of a one working point optimisation.
You buy less disadvantage above break even speed by accepting less advantage below that speed.
No glider pilot needs a winglet, that makes the plane extremely efficient in thermaling,
when it will kill all performence if he might decide to speed up even slightly.

So they are a compromise but still, it wouldn't be justified to call them to be not right.

biber

[2phar]

I wonder if anyone has tried articulating them?