Which has the best L:D ?

[Terry S]

Hi all, Im looking to make an easy to fly aircraft and have decided on either a gogallo wing or parafoil. Which would give the best L:D ?
What should the wing loadings be for a 2m size model ?

Thanks, Terry

[Ollie]

The L/D is mainly because of high aspect ratio. What can you increase aspect ratio with, gogallo or with parafoil? Make the wing loading lowest and (with more ballast) make the strength highest.

[jay flay tway]

Not sure how true the below is or if you know it already but...

I think aspect ratio is not a maximized variable, but rather idealized variable. I mean, there is some ideal number, it's not "as high as possible". There's an ideal number for every case, mostly depending on size (because of the non-relativeness of air molecules...Reynolds numbers stuff)
I think generally easy-to-fly and efficiency are at odds. A rogallo wing is not very efficient, especially at bigger sizes (the bigger, the higher the ideal aspect ratio) and the parafoil has a lot of drag drom the strings but is much more stable, I think.
I'd go with a parafoil with one or two spars to reduce the number of strings necessary and choose the aspect ratio according to the size (2m). Rough guess...about 10 to one or so??

I guess you've ruled out a standard (maybe Clark Y or something) airfoil with mucho polyhedral and long fuse-to-wing distance?

[vintage1]

Actually I think that all other things being equal, the higher the aspect ratio the better the L/D.

BUT you run into structural problems.

So as is normal, its all a compromise.

If you look at most high performance aircraft optimised for efficiency (U2, airliners, sailplanes) they are all long thin wings.

[jay flay tway]

But when you get small enough, you need a certain chord distance for efficiency, don't you? Because the size of air molecules is fixed, there is a certain amount of time/distance needed to get them to follow the airfoil.
I Forget where I read it but it said something like the ideal aspect ratio is not "as high as possible"; there is an ideal number, given the size of the plane. The ideal might very well be impractical though, as you say.
I think another factor is parasitic drag, which is a much higher proportion at model sizes so a very long thin wing at some point will lose more due to the parasitic drag, then it will gain in efficiency.

Not sure if this is right though. My knowledge is picked up in bits and pieces, not much from formal training.

[Tom Harper]

jay,

Easy to fly and best L/D are not compatible. A well constructed wing with a Clark Y section and an aspect ratio between 5:1 and 7:1 will yield all the performance you can use.

[Ollie]

It depends on the type (power or gliding).

In gliding, performance depends on thermal, slope (low or high wind) by low sinking rate. In gliding, performance depends on L/D with high wind airspeed covering land speed. Aerobatic planes, don't use either gogallo wing or parafoil.

[BMatthews]

Rogallo and parafoil wings rely too much on the cut of the material and airflow for their shape to ever be anything but draggy. The one exception would be if you can make a wing using the design techniques from windsurfers where you use very low stretch materials, battens and a high tension hauling system to highly preload the mast (leading edge spar) and wing skin. A lot of sewing and experimentation but if you could do it then you'll produce a decent flying model with a stable airfoil. You would also need to use the same mylar and low stretch Dacron that are used by the sail makers. Something like ripstop nylon or any of the regular model coverings stretches far too much for such use.

To give you an idea of what I mean by preloading the wing (sail) when I was windsurfing I was using a 6:1 downhaul pulley system to tension the sails with about 300 lbs of downhaul force. That force is what curves the mast over into the shape you see on windsurfers. Normally they are straight and quite resistent to flexing. To do the same on a sailwing model would probably require you to "inhaul" the wing skins with about 30 to 40 lbs of tension to achieve the required stability of shape.

Far easier to stick with normal built up wood wings.

[jay flay tway]

Sorry, I don't see how L/D and flying ease are not compatible.

I think one of the main factors of "easy to fly" is flying speed. An optimized L/D (allowing for strength and other factors of course) would allow more efficiency and thus a slower flying speed. The airfoil is another story and I'd go for a heavily undercambered one for most efficiency at slow speed.
Another one is slow reactions to pilot input, which is also aided (for yaw and roll anyway) by a long wing.
Dihedral makes the plane stable and able to fly on R/E only, both of which are desirable and both of which can be done with a long wing.

[Ollie]

"Sorry, I don't see how L/D and flying ease are not compatible."

I agree with you. Your thinking is sorry. With studying aerodynamics, your thinking will be much improved.

The two (parafoil and gogallo) don't get stable with dihedral! It is just other the way around with negative dihedral in a parafoil. Both (parafoil and gogallo) get stability in yaw, roll and pitch from very, very low CG's. The shifting CG control comes from roll and pitch, not turning with yaw. Both have very, very high drag because drag limits L/D compared with better designs. Both crash with negative G's maneuvers. They both crash with deep stalling. Look out!

[Tom Harper]

Jay,

"Sorry, I don't see how L/D and flying ease are not compatible."

What I said was a bit different. You are proposing the BEST L/D as yielding an easy to fly model. The two wing variables are section and planform. A high aspect ratio wing with an undercambered section is not easy to control. Long wings are dificult to move around.

Stability and control are not functions of best L/D. An airplane that is not efficient has a very broad operating range of consistent characteristics. It has broader bandwidth than a highly efficient model. I have built some very nice flying deltas with flat plate sections.

For a stable airplane I would use a reflex section. I have used the Eiffel 401 section on flying planks. It will fly without a horizontal stabilizer. Combine that with a 5:1 aspect ratio planform and a half span tail moment arm and you should have a model that will perform consistently over a wide range of operating situations.

I think the ability to perform consistently over a wide range of operating situations is the definition of easy to fly.

[jay flay tway]

I just realized that I contradicted myself. I forgot that we were talking about Rogallos and parafoils.
I think there is a reason that a lot of beginner planes have an undercambered wing. It slows down the dives and reduces stall speed.
This is complex stuff. I would like to know a lot more but we probably disagree less than it seems because it's hard to make the points clear.

[Guille]

Buddies;
1- L/D and easy to fly are not relatee at all...
2- Rogallo wing flyies thanks to a vortice latice generated along all LE... it consumes a lot of energy. Best L is obtained thanks to AOA
3- Parafoil flyies thanks a "airfoil effect"... mean: a wing. Best L is obtained thanks to aspect ratio + chamber

Conclusions is up to you: mine is CHOOSE THE PARAFOIL!!!!
Regards
Guille