Reflex in wing profiles

Hi Guys

I'm planning my next foil design and I was wondering if anyone has experimented with wing profiles with a reflex trailing edge?

I know that reflexed trailing edges are meant to decrease pitch moments in wings and increase stability and are often used in single wing airplanes like hang gliders. Alpine foil talk about a reflex profile and it looks like the Taaroa sword might have a bit of reflex in the trailing edge as well?

Profile



Zoom on trailing edge showing normal Eppler 817 vs. Reflex tip

Hi, cool idea, but I doubt it would work. The rear wing is there mainly to correct for pitch changes in the main wing. The rear wing is quite large and far behind the main wing, giving it an incredible moment of inertia, compared to typical aeroplanes, never mind tail less hang gliders. Unless you plan to move away from using tail wing configuration, I doubt the reflex would have a noticable effect.

But maybe if you want to try a radical very low aspect ratio single wing foil (no rear wing) then reflex could work. But I think you would have to add alot of reflex.

But I think reflex will start to affect the foil's performance a lot. If you look at high performance hang gliders like the Atos, it has no reflex but uses a small tail wing for pitch stability.

But that is just my opinion. I could be wrong?

Excellent explanations Christoff !

Thanks for the replies guys.

I'll try and read up on it a bit more, but stick with the conventional "hooked" trailing edge of the Eppler profiles for now.

My favourite foilbaord broke this holiday. It broke at the rear of the fuselage while riding (hitting the sand bank) and the rear wing broke off. The rest was still all OK. I did not notice it and carried on riding, but obviously could not keep the foil up. It was impossible with no rear wing, but it felt like it may be possible with a lot of tweak and a lot of practice.

So I decided to make a foil with rather small rear wing stabiliser and mount it close to the main wing. It was horrible. It is difficult, but still very much rideable. So I thought that maybe it would be a bit more stable at high speed. It could be that the trim was out, but it was a bit more difficult at higher speed. I think I will stay with a reasonable size rear wing well spaced out. It makes riding so much more comfortable, and is not a problem in big swell or waves.

Hi Christoff

Pity about your board-but an interesting result from trying the smaller wing - my guess is that there must be a trade off between rear stab wing size and fuselage length - small stab/long fuselage or large stab/short fuselage, it seems from your testing that stability suffers with a shortish fuselage.

My intention in the original post was not to remove the rear wing, but just find out if there were any benefits from a reflexed, or I guess just "flattened" trailing edge - perhaps reduced drag, decreased stall tendency or improved pitch stability?

On a seperate note, I built myself a second "high wind" main wing - 435cm^2 projected, and it needs a bit more board speed and maybe a knot or 2 more wind, but still lifts off and maintains just fine.

phezulu1 a écrit:

- my guess is that there must be a trade off between rear stab wing size and fuselage length - small stab/long fuselage or large stab/short fuselage

There is a formula called in french "volume de sta"b that says excatly that. Have a look in the aeronautics basics.
For instance, the manufacturer Spotz has a the shorter fuselage on the market AND the bigger stab (compared to the main wing).

Does any one know what the advantages of a larger rear wing with shorter fuselage is versus using a smaller rear wing with longer fuselage to get the same effect? Maybe it has been discussed on the French side, but I cannot find it?

From what I can imagine:

A shorter fuselage with bigger wing:
- more solid fuselage= less likely to vibrate/ resonate
- larger wing= less likely to stall or be affected badly by turbulence

A longer fuselage with smaller wing
- less area = less drag
- less downwind lift counteracting the main wings upwards lift making a more efficeint foil

I cannot think of other obvious reasons, except that maybe the force might match the moment of the main wing better at speed if the size is of a certain proportion.

Does anyone have any ideas? (or a link to a previous discussion on the subject)?

it is also easier to push on the rear wing if it is under your feet during a jibe. theoritically at least

I have been making lots of wing the last few days, and would really like to obtain a fast, glide feeling foil, but that is still very stable. To get the stability much better, I want to move my centre of gravity as far forward as possible which is the classic way of getting an aeroplane more stable and this seems to work brilliantly.

But to do this, one needs a strong pitch up moment so that the forces are in equilibrium with your weight further forward. To do this, the rear wing needs to create a downwards force, which is probably the configuration on all foilboards.

But now the front wing andd rear wing are creating lift forces in opposite directions, and one needs much more lift from the front wing to counter the rear wing's downward force. The front wing need to be bigger than it otherwise needs to be, and both wing create more induced drag than what they would if they did not create lift forces that go to waste.

Maybe using relfex could give a bit of pitch up moment without needing so much rear wing downward force? But I find that the foils are quite sensitive to the rear wing configuration when you are looking for a well balanced ride across a wide speed range and it might take a while to get a good result if experimenting with reflex.

Hi Christoff

That's sort of the way I was thinking at the beginning of the thread.
Have a go at downloading and playing with XFLR5 - you can put together your own foil sections and generate the polars for them - it's a bit of a tricky program to learn, but works quite well. That way you can see how much pitch moment you get. For the pitch moment equation
Pitching moment [ Mc/4 ] = Cm × ½ X Density X V² × Planform area, I doubt whether the changes in Cm from reflexed wings will be enough to compensate for the velocity squared term - but haven't actually had a go at working it out - I could be wrong.

I've also wondered if swept back wings might offer some advantages in stability and drage reduction - I haven't investigated properly yet.

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