aerodynamics the fin question

[thunderstreak.keith]

do we need a fin and why? I know it stops the rolling effect due to torque (i think) but why? think back to when you made your first paper dart. fold in the top corners to the centre, fold in half and fold doewn the the outside flaps to form wings and away to go. it flew, most of the tim anyway. then you ripped a peice at the tail end and lifted it to form a fin, in my experience it didnt fly any any further. yet full size deltas, and bi planes etc. all had a fin. when the wright bros designed theirs all i can see is that the all moving vertical tail pieces were to steer the plane. ( i have a rubber power scale job i did on this so might take that off and do some experiments) why after making paper finless deltas did they decide to put fin(s) on. interesting point i think, must have had a brainstorm this morning.

[The Wright Stuff]

The primary reason for having a fin is simply for stability in yaw. Think about viewing the model from the top: any slight yaw anticlockwise relative to the airflow will cause a net force on the right hand surface of the fin, providing a clockwise moment which brings the nose back in line with the airflow.

There is an effect in roll, too, but that is a secondary effect, and is more complicated, but all aircraft will have roll-yaw coupling to some extent...

TWS

[TheFlyingCrust]

The American stealth bombers have no fin (vertical stabiliser) but to enable them to fly they have an airbrake, in the form of a split aileron, on each wingtip to provide yaw control and it needs to be under computer control.

With a paper aeroplane the "fuselage" part provides yaw stability. Providing a fin just transfers some side area from under the wing to the top.

Ian

[Gary Manuel]

There is a good series of about 11 short videos on Youtube by RCBatics.

They explain the fundamentals of flight and move onto more detail as the series progresses.

Here's a link to the first one. I have no association with the group.

[Bob Cotsford]

the basic paper dart has a 'fuselage' which is another triangle, deeper at the rear, providing 'fin' or vertical surface area behind both the cofg and cp. The slightly more complex common paper aeroplane design uses a strip of paper folded into a V section going back, which is essentially a V tail.

It doesn't matter for straight and level flight whether the vertical surface is on top or bottom of the model. best if it's at the back though! Anyone remember the KK Caprice, a glider with the fin under the tail?

[Colin Leighfield]

I had a Caprice, great flyer. Also had the Contest Kits Inchworm A2 glider, which had the same under-fin. It was a practical solution I think to the need to incorporate a tip-up tailplane for de-thermalising. (Memories of smoking wicks and serious fire-risks)! I have bought the parts from Belair to build a new one and replace the one I wrote off about 1960, I always get there eventually.

I suppose the under-fin could be slightly more efficient through being out of any blanketing effects from the wing on a steep tow or bungee launch as well?

I think you should consider taking the fin off the Mustfire and giving it a try, that would be interesting(!) When Martin Baker built the MB2 fighter they decided that the keel area of the rear fuselage was enough to make a fin unnecessary, so didn't fit one. It didn't take them long to change their minds and put one on! You can see the before and after in photos, I'm amazed it lasted long enough for them to have the opportunity to correct the mistake!

[Phil 9]

does the shape of fin matter.

why did so many ww2 aircraft have a curve shape to the fin. with cost and speed of manufacture a priority why did they not use a square shape

[Cuban8]

Birds and the extinct Pterosaurs managed without a vertical tail surface. I suppose their brains perform(ed) in a similar fashion to a FBW computer and corrected for yaw via tail and wings - watch a modern seagull continuously twisting its tail feathers, for example.

Surprising that as far as I'm aware, natural selection has never resulted in any flying creature having a vertical control surface (although some Pterosaurs did have very large spoon shaped vertical bills that may have assisted with yaw control when moving their head from side to side). I remember having the fin of an aerobatic sloper knocked off after a mid-air collision - total loss of control even though ailerons and elevator were still working!

[Mark Kettle 1]

Yes 'Thunderstreak Keith' fin or fins add stability and balance.

Any long and slender body moving through the air like a fuselage tends to want to move sideways - broadside trying to find its centre of balance. If you throw a stick even with a lump a lead one end it will yaw a little, however add some 'fletchings' like fins like on a archers arrow it will straighten up and its flight will be much better.

Rockets and bombs require fins for the same reason. Therefore by adding any fin or even a tailplane to one end moves the neutral point forward and if this neutral point is by chance on a aeroplane should be near the the wings aerodynamic centre then balance and stability are coming together and hat-presto it fly's.

I always remember whilst training to fly hang gliders the instructor asked what makes the glider fly with the 'pointy end' going first, and forwards, well all sorts of answers were coming out from the trainees, none of us got it right, it travels 'pointy end' first because there is more area of wing behind the central of gravity than the front and I think that's the same as your paper dart Keith.

Cheers Keith - great question

Mark

Edited By Mark Kettle 1 on 28/05/2014 13:37:02

[Colin Leighfield]

There was a tradition of using particular fin shapes for brand identity in many cases, De Havilland, Bristol and Hawker all did that. Also I'm sure that some designers did consider aesthetics in many cases. Not every plane is beautiful, but efficient aerodynamics often tend to go with elegant shapes and with the fin being so prominent the extra cost and complication of making it curvaceous is probably pretty small beer compared with the total cost and trouble of everything else.

As far as the issue of flying creatures is concerned, it's probably the same as us walking. Our two legged stance is completely unstable, but our brain working through our senses continually translates where we are heading and co-ordinates that with all of the other data from our bodies and surroundings to create the combination of stability, direction and speed that we are aiming for. That's why we don't need outriggers and the reason that birds etc. don't need fins is the same,their brains are continually sensing and correcting deviations from horizontal and vertical flight path.

That is why modern computer systems make the B2 a practical proposition, whereas the XB35 and YB49 were too erratic in flight direction to provide a stable bombing platform and were fatally dangerous in some flight conditions. (Ask poor Glenn Edwards and his crew). It's the same reason that the Luftwaffe rejected the Horten design as a practical fighter, much as we love it, because it's directional instability made it an erratic gun platform and it couldn't hold its' aim on target. A fin is such a simple thing and adds so little drag, why leave it out? Unless you are looking for trouble, that is! I'll leave it to Bob, he's braver than me.

[Paul Jefferies]

"does the shape of fin matter.?"

Yes and no......... Firstly, it must be big enough to do it's job but the actual outline is not that important, witness the wide variety seen, from the swept forward Beechcraft of yesteryear to the many swept back designs and just about everything in between. However the angle of the rudder hinge line IS quite important because if the hinge line is swept back, as the rudder deflects there is a small amount of up elevator effect. This is not necessarily undesirable as most aircraft drop their nose in a turn so this effect may reduce that to some extent though probably not by much. Overall though, I think the shape doesn't matter and is chosen so it looks right aesthetically.....

Paul

[The Wright Stuff]

Another contributing factor to the presence and shape of the fin is most likely the traditional requirement for a rudder. The fin makes a convenient structure to hang it on, and there is no doubt an aerodynamic consideration when designing the shape of the fin, in order to generate smooth air flow over the rudder.

Early aircraft in general suffered from adverse yaw as speeds began to increase (just as models do) and hence a rudder with good authority would have been important. Once the general tailplane layout is established - I guess there is little incentive to vary it. Variations such as T-tailed aircraft were (I understand) originally used to place the elevator further from the wing turbulence (I would be interested to find out what the first T-tail plane was)...

Cheers, interesting light-hearted discussion!

TWS

[Ady Hayward]

I often fly my venerable "Klingberg Wing" powered flying wing glider. This has a swept wing with a small ply keel under the centre section for a launch hook and hand hold. Its yaw stability is very good despite it having no fins at all.

In this case the yaw is controlled by the wing sweep and as a wing tip advances forwards the wing presents more frontal area thus lift and drag than the other side where the tip is retreating. The result is the drag pulls the advanced tip back until equilibrium is reached and the wing resumes straight flight.

There is some slight yawing action seen at times when lift is about and the flying wing flies through a thermal and the disrupted air causes the drag of each wing panel to vary but in every case it straightens out quite quickly.

A B52 lost its fin many years ago and managed to fly home and land safe. One of the findings was that the wing sweep kept the yaw under control allowing the pilot to use differential throttle to control yaw and effects of the roll spoilers on the wing.

On most conventional aircraft the correct ratio of keel area needs to be maintained to ensure yaw stability hence the use of fins at the rear of the aircraft. The shape has been often used by manufacturers to identify their brand and shows that within certain bounds the shape has little detrimental effect.

[John Olsen 1]

Even an arrow needs fletchings in both planes in order to be stable. It is not the centre of the area (either side for yaw or plan for the pitch plane) that matters, it is the centre of pressure. One point about this is that the centre of pressure will usually move forward as the angle of attack increases. If the centre of pressure is behind the centre of gravity, the object will be stable in that plane. If the centre of pressure is ahead of the COG, then it will not be stable. (the centres of pressure for pitch and yaw need not coincide, but both should be behind the CoG.)

There is usually some side area ahead of and around the CoG, so therefore it is generally necessary to provide some side area behind to move the centre of pressure far enough back to give yaw stability. By providing a decent moment arm, eg the rear fuselage, the amount of area that needs to be added is reduced.

John

[thunderstreak.keith]

Didn`t that german proto-type jet also not have a fin? it also looks very similar in design to the stealth bomber, uncanny when the yanks got hold of it after the 2nd ww. sorry i cant think what it was called at the moment. Flying crust, you say that the stealth has stability due to computer controlled split ailerons. surely this german flying wing didnt have computers on board at the time? I understand that a fin is necessary, but thought it would be interesting discussion on why? my theory being that stability can be controlled by ailerons (flying crust ty) so iff a model is fitted with a gyro gismo then surely that would work when coupled with wing tip ailerons? very interesting comments so far anyway

[Colin Leighfield]

There's loads of information coming out here, all of which is correct. The German jet that Thunderstreak.k refers to is most likely the one that I mentioned, the Horton HoIX. As I said there, what is not widely reported is that the Luftwaffe rejected it as a fighter because it proved to be an unstable gun platform. It was like the Northrop XB35 and YB49, we all love them, but they proved to be tactically useless and dangerous at times.

It's absolutely right to say that if you have efficient gyros and associated control set-up, you wouldn't need a fin because elevons would provide continual inputs to stabilise the flight path, which is what happens with the B2 stealth bomber. Fins and tailplanes are designed to provide a degree of auto stability and eliminate the need for a continuous level of control input from the pilot that would be impossible. Modern electronics and miniaturisation techniques make all things possible, it all depends on what your objectives and interests are really, I suppose.

[thunderstreak.keith]

apologies colin, I must have missed your post with, as you say, reading and trying to digest everything else that has been said. so basically, unless its a flying wing then the fin is a must. I wonder if the wright bros. realised this or whether it was just a thought about "how are we going to steer this thing?" stabilisation coming from the 2 fore planes at the front. As far as I know (i may be wrong,usually am) there wernt any ailerons as such. not sure if they had wing warping then, the 2 vertical fins at the rear being the steering mechanism. The2 props also facing aft would have no effect on the wings or foreplane and torque would have been reduced with them contra rotating. Not sure now why I started this thread lol I suppose I needed to get some braincells working again flight theory was the first thing that came to mind. thanks to everyone whos taken an interest in this, made good reading

[Colin Leighfield]

No problems mate. Don't forget that the rudders on the Wright flyer function as fins as well. You could actually call them "all-flying fins" in today's language. Also it used wing warping. In truth, deforming the whole wing that way is more efficient than using ailerons and was in another way the thinking that Barnes Wallis used when he dreamed up the "Swallow". It is just that the idea as originally applied needed a very flexible structure which also needed extensive bracing to have any strength and lacked engineering practicality.

Barnes Wallis designed the Swallow without fins. His thinking was way ahead of its' time, because he said that whatever attitude an aeroplane (aerodyne) was in, whether going straight on, turning, going up or down etc. there was an optimum shape for it. His view therefore was that the most efficient plane was one that could change its' shape to suit whatever it wanted to do. It was this thinking that lead to swing wings as with the F111, Tornado etc, but those are a gross simplification of his thinking. His Swallow idea could be practical now with current technology.