Facet Opel

The book by Torenbeek is probably more than you are looking for at this point. It does have many good references and does quantify several aspects of design that others may not even mention. But it is a relatively advanced text and despite a lot of illustrations and graphs, does tend to be rather dry. I have it and used it on numerous occassions but it may not be for the one time designer.

Thanks. Good to know!

also, the typesetting and the quality of the graphs are less than those of Raymer's book.

Re Facet Opal
I have just joined up and this is my first post.
I would like to reactivate this thread as it seems to have lanquished for over a year.
I have always been interested and facinated in flying wings (I flew hang gliders for about 20 years starting in Feb 1976).
I knew a bit about the Opel, I have have some news clippings and had seen a short TV feature or video ages ago on this plane, and had also met Scott once at a fly-in also ages ago. I have a one quarter scale RC kit of this plane also stored under lots of dust in loft of my garage. Scott Winton was also the designer of the Sapphire ultralight that I have been flying for about the last four years (now with a 50 HP Rotax, which I cruise at 80 and 90 knots, using 12.3 litres per hour, ave). I previously had a little ultralight Hiperlight bi-plane (a US design) for several years (also with a Rotax 503).

I was wrapped to learn on this forum a lot more about Scot's Opal. To me, I always considered it a purpose built (to break records) and new it was a very expensive (hi-tech materials) aircraft, which had a very small cockpit for a very small pilot (NB: I am over 100 kg). Consequently, the design as is, would be as usefull as an everyday plan, as a formula one racing car would be for normal on road use.

However, I would love to build one (one day) which would be a much more "utilitarian" version of the Opal, suitable for "every day" use.

What I have in mind is as follows:-

-Larger cockpit.

-Larger wing for more lift and shorter take off run (and to meet the minimum wing loading regulations under the Australian 95.10 ultralight category, (ie a category with much less ownerous regulatory building requirements, NB: These reg's were designed indirectly to keep the max speed down by imposing a light wing loading requirement, but a light wing loading with a simetrical section combined with a low frontal area (ie low aspect ratio) should, with 50HP result is a quite fast aircraft anyway.)

-Thicker wing section (probably 12 or 15%) to keep the spar light and the strength up.

-The larger wing also gives a larger chord, giving better pitch stability, and with a deeper spa giving more strength and reduction in spa cap material/weight/cost. There is also the possibility of retracting the main gear into the bigger deeper wing space.http://www.homebuiltairplanes.com/fo...le%20happy.gif

-Wider landing gear, for obvious reasons.

-More user friendly wing section (the so called Bi-Convex section seems down right scary).

-Simetrical wing section if possible, giving more AoA and drag for easier landings and approaches, probably laminar flow type with thickest point at say 50%, provided this can be done without making it too unstable in pitch. My Sapphire has an all flying tail, is sort of neutrally stable (or am I just used to it?), but goes divergent rather exponentially if I let the stick go, even if nicely trimmed, but I love crisp controls and this doesn't worry me unless I need both hands to hold a map, but I manage.

-Constant chord (same as the the orig) for ease of design and construction, and for known safe flying characteristics.

-Rotax 503, 50HP (cause' I have a couple of these already), should give good power to weight for a light sigle seater.

-Inboard Fin/Rudder (as per the orig) for asthetics, but larger, for good yaw stability/authority. I know that wing tip fin/rudder/fences would improve low speed LD, but high drag is helpfull when landing, and the 50HP engine (brute force) and low wing loading should overcome the low LD/high drag on take off.

-I like foam ribs (probably hot wired) as they can be mass produced easily, and thin "stressed plywood skins", cause they are stiff in torsion and easy to buy and install, plus they can share some of the spar loads.

-Main spa using wood (probably box section, but possibly an I beam), cause they are farly cheap easy to work with (and very very easy to taper in both directions so as to to keep the weight down on the outboard sections).

-Regarding lack of elevator authority to enable rotation for take off, why not just vary the height of the nose wheel to give greater AoA for take off. If appropriate, the nose could be extended very rapidly "after" getting plenty of air speed, to ensure a crisp ground departure (to avoid any wheel barowing on the front wheel). If the front gear is normally raked forward say at 45 degrees, an eccentric cam could be rotated to move the gear leg back a few degrees to raise the nose, easy!
This seems a really simple solution to me, or is there something I am missing here???

To sum up I like the concept of using relatively cheap materials, easy to use construction materials and simple construction methods together with the low parts count associated with flying plank type wing.

This should make it quick, cheap and easy and thus painless to build, and enable proof of concept without costing the earth.

Plus one could always then build a higher tech "mark two" version later, as it is always easier the second time round.

Now all I need is time and and some motivation to do it. But, although I do enjoy building, (and I have done other projects), I enjoy flying a hundred times more.

Please forgive me for being long winded, but I do have a passion for the Opal, and also unique designs/concepts that embody simplicity!

I hope this inspires some more input and discussion on this thread.

PS I tried to contact Lachlan Conroy by phone to get a copy of his Video, but he has moved. Does anyone know his current phone number?

Greg

The Facet Opel has a close association with Al Blackstrom Plank wing glider developed and built in the 50,s Blackstrom was a professor of aeronautics at a number of universitys in the USA during his career. Like all flying wing the majority chose the Abrial reflexed airfoil developed by a genius in France in the 1930 Mr Abria.

A number of Blackstrom planks were built and flow and two were built in Australia. They were totally stable and very successful through the flight speed range. Over the last 15 years I have collected data on Abrial , Fauvel and Jim Maske work. Maske booklet is well worth reading as inside 30 pages Maske tells you how to wade through getting a flying wing built and in the air.Makse papers will save a designer 4 years study and probably $20,000 worth of airframe building and research. Maske was a self made man with flying wings and got his money where his mouth was and test flew all his aeroplanes. But he was also very aware of the Abrial airfoil that Fauvel wrongly called his own. Of course they are still flying to-day. Anyone interested in Maske work should contact me as I dont believe his papers are available anymore.

I also have a set of master drawings for the last single seat VW powered Fauvel wing The AV-60. This aircraft was develooped by George Jacqumin a veteran aircraft designer who worked most of his life with Lockheed and Kelly Johnston. The Fauvel AV-60 was a very nice little aircraft and I have origional film footage of its first flight in Texas. The drawings were almost lost but I believe I have the only set remaning. Fauvel of course used Abrial remarkable airfoils which of couse were used by so many flying wing designers.

DR. Ramer Horten also did his work with the Horten wings. Dr. Horten before his death made the maths available for his wing and I have that data as well. Over the last 7 years I have done extensive research into reflexed airfoil due to the fact that I built up and test flew a MIgnet tandem wing aircraft which is a nice aeroplane. However the later models never used Abrial airfoils but a 1937 model did. Mignet chose to modify NACA 34 series airfoils for his aircraft except the last two production aircraft the Balerit etc., used a modified Abrial airfoil with a cuff and truly remarkable results.

For those of you who want to design a flying wing. DONT GO THE LONG WAY HOME. The Abrial airfoils are the secret and the cuffed version are truly passive and stable right through the speed range and are really magic.

thanks mignet

Originally Posted by mignet

Mignet chose to modify NACA 34 series airfoils for his aircraft except the last two production aircraft the Balerit etc., used a modified Abrial airfoil with a cuff and truly remarkable results.

I found an Abrial 17 airfoil profile (picture below), but what is a "cuff" as mentioned above?

A cuff is generally a more-rounded leading edge (often 'drooping' a little) applied retroactively to an airfoil with a relatively sharp nose. In the case of an airfoil, it's done to improve the stall characteristics and maximum lift. On wings, they're sometimes used over part of the span to improve stall behavior (edit 1/8/07: removed "resistance", replaced with "behavior") on a design that otherwise is having problems in that department. The designer can either be looking to modify the airfoil in that area, or generate a vortex (at the sharp spanwise discontinuity of the cuff) to help maintain attached flow in some region of the wing.

Basically, a cuff is a fix.

That Abrial 17 is an odd-looking beast! Very sharp leading edge, very forward-loaded, and a hard break on the upper surface where it goes reflexed. Mignet, how is this airfoil better than other, more modern choices? It would seem to me to be very draggy and potentially have a nasty stall.

Other than reflex, there's no 'magic' of which I'm aware for flying plank airfoils. The critical parameter is that the pitching moment be positive. How positive a value needed is a function of the wing shape, placement, and (therefore) how much static stability you want. Other than that, all the usual needs apply - pick the camber and thickness distribution that gives the lift and drag characteristics you need at the design CL, and a gentle stall (although that can be saved somewhat with the wing design).

Yes the basic Abrial is a standard reflexed airfoil in to-days world. However it's the amount of reflex you put in the airfoil that really matters. This of course comes back to the mean camber line. Abrial of course had no computers in his day, but he managed to hit the jackpot with centre of pressure stability using a fluid tank. Jim Makse also discovered that even in the computer age, it tells the full story. Makse modified all airfoils the Abrial the 23112,and others all to a certain reflex percentage to make them work in situation like the tumbling of a flying wing. Maske work was so complete that he could demonstrate a tumble knowing that he could recover.

The lift bubble on any airfoil as you would all know moves rapidly forward when you increase the angle of attack on the wing from 0 to plus 4 deg.,and then develops some stability in the later part of the range. The Abrials have almost a complete stable centre of pressure from 0 to plus 12 deg if there reflex is optomised to 75% of chord.The percentage can be taken further but the you run into an airfoil profile that has construction problems and the drag curve goes wrong.

Having bent my brain around this for the last 9 years I find that computers dont really give you a practical answer. A fluid tank does. I have proved that my-self.

The cuff on a reflexed airfoil is a vital piece of the nose profile and mean camber line profiling and the two must be sorted out together. Cuffed airfoils are not new. Cessna used one on the last 80,s model Cessna 172 and 182. The flaps had a 4 position range and I can remember several times doing parachute drops just how well they worked with an overloaded aeroplane back at the stall speed. Providing you eliminated yaw you could get the aeroplane to hang on well below the 50kt mark. That was not possibe with a non cuffed airfoil. However the single most important part of a cuff is to safe guard yourself especially with a flying wing a situation where you could experience partial wing stall due to GIO ( Gust induced Osolation) and turbulence.

With GIO at low speeds in a flying wing a partial wing stall can roll you upside down very quickly. In the Mignet Fleas a partial stalled front wing can be fatal if you dont read the signs and you don't have a cuffed airfoil. In fact fatal crashes with Mignet aeroplane due to wing stalling are still around due to the fact that even though the Fleas have reflexed airfoils on front and back wings with good stability its not enought. Most Fleas carry a rear flap to stop the back wing from generating to much lift should the front wing carrying 75% of the load have a stalled condition. The result is the aircraft stands on it's nose. The time for this to happen is point 6 of a second.
However producion certified Mignet aircraft had cuffed Abrial modified airfoils and the problem was eliminated.
You have to remember that there are very few fully flight tested reflexed airfoils around. NACA at Langley never flight tested the 2300 series reflexed airfoils. The data has also proven to not be that accurate. George Spratt did a great deal of work on the 23112 airfoil and provided a new set of flight tested figures. But Al Blackstrom wouldnt buy into the 2300 series airfoil and Maske modified the mean camber line which is a must to suite your design and he had great success. Makse aso twike the 4300 series NACA airfoils and in fact married several together to get the result he wanted.
Mignet

Originally Posted by mignet

...You have to remember that there are very few fully flight tested reflexed airfoils around. ...

While I agree completely with the statement above, you're using some very odd, non-standard terminology in the rest of your post, and I'm having a tough time following your thinking. None of the following is intended to be argumentative - I'm just trying to fit what you're saying into what I've already learned.

"lift bubble"? "Gust induced Osolation" [sic]?

When you say "reflex percentage," do you mean 'percentage of the chordwise length that is reflexed' or 'how deeply the reflexed portion of the camber line is curved,' or something else? The only way I can see the specifics of airfoil reflex pertaining to a behavior such as longitudinal "tumbling" is how they relate to the overall static and dynamic stability of the airplane, and that's tied more intimately with the wing planform and the influence of the fuselage pod (if any). As I understand it for flying planks, one can look at the amount of reflex as either setting the no-control-deflection trim speed for a given static margin (if the airfoil design is the variable) or setting the static margin (via wing or CG placement) if the airfoil and design trim speed are fixed. Dynamic effects are even more removed. How is some particular shape of the camber line and thickness distribution a unique 'fix' to an issue that seems, to me, to be more tightly bound to the overall planform and configuration?

What exactly is it that you've "proved" with a fluid tank, and how exactly are the usual computer methods (X-Foil, Profil, etc) deficient in the case of reflexed airfoils? I've used X-Foil quite a bit - I like it, within its limitations. Are you accounting for Reynold's number effects?

It also seems like we're using two different definitions of "cuff" in this context. Adding a cuff or drooped nose to a sharp-nosed airfoil such as the Abrial 17 posted earlier is naturally going to improve the stall characteristics, which should have been handled properly by the original designer in the first place. A similar situation is the source of Harrry Riblett's modification to the NACA 6-series airfoils. But unless I'm reading you wrong, you may mean something else by "cuff" in this contextl, and specifcally with regard to reflexed airfoils. How is this different from simply having a more appropriate forward camber line, such as Riblett has done? Adding a "cuff" in the conventional meaning would do that, fixing a poorly designed airfoil's bad one.

Thats correct. % of chord is what matters. 75% of chord. ie 1500 m.m. = 1125 m.m.
This percentage you can play around with all day on a computer but the proof of the pudding is when you test a physical version of the airfoil in a fluid take at speed. It will show you what a computer wont show you and that is longitudional stability.

The lift bubble may be an old term but it doesn't matter to much. What matters is how stable the lift bubble is at the centre of pressure.

The fluid experiment is one where you can propel a physical airfoil sample in scale or full size in a fluid and it will tell you if the airfoil is truly stable through the speed range. Its a basic test but !00% accurate and has proven to be the salvation of many flying wing designers.

Dr. Ramer Horten whom I usto write to used this type of test way back in 1939 to determine just how stable an airfoil is at speed in a fluid. When you cuff an airfoil it means you push the mean camber line well forward and the result is an under camber in the bottom of the D section which looks like a cuff.

mignet.

The Abrial airfoils aren't sharp nosed airfoils at all. They are very even nice rounded airfoils at the D section and that makes them very stable. If you look at the Fauvel airfoils ( which are Abrial airfoils) you will find they are very nice rounded shapes.
mignet

Originally Posted by mignet

...It will show you what a computer wont show you and that is longitudional stability....

Well, Mignet, either we're still using very different terminology or we'll just have to agree to disagree on this one. My recollection from my education and every resource I have now shows longitudinal stability of an aircraft - regardless of configuration - to be a function of the overall planform and CG position, not a particular airfoil or camber shape.

On flying planks, a reflexed airfoil is required to balance the nose-down moment of the CG being appropriately ahead of the neutral point of the overall configuration. A swept flying wing tailors the spanwise lift distribution to accomplish the same end - a nose-up moment balancing the weight-induced moment provided by the CG position. A conventional airplane does the same with a horizontal tail, which also contributes to the overall planform and so the neutral point of the entire aircraft. The amount of reflex on a flying plank; the sweep, twist, and taper distribution on a swept flying wing; and the size and incidence of a horizontal tail on the conventional airplane all are set by the moment required at the design speed to result in no-control-deflection trim at that speed and CG position.

Pitch authority from the sizing and design of the control surfaces sets the forward end of the CG range, and a specified static margin sets the aft end, also regardless of the airplane's configuration.

Maybe I'm way off base on this subject (in which case I hope someone speaks up before I design something unsafe - Orion?, Billski?, Norman?) but I've read variations on the above in more than one text.

I'm guessing the Abrial 17 shape posted earlier is not representative of the versions of which you're speaking in your posts - that airfoil appears to have a terribly sharp edge at the nose, where the upper surface curves down to meet the lower.

I can't really comment on use of a fluid tunnel. I've used a small wind tunnel at Cal Poly Pomona when I was a student there some years ago, but never a fluid tunnel. How big do your test airfoils need to be for realistic Reynold's numbers?

The profile in post #111 is really crude. I had assumed it was hand drawn but that’s exactly what’s shown in the databases. So I looked up Fauvel in the Incomplete Guide to Airfoil Usage. It lists 1 Fauvel design as using an airfoil of his own design (F2) and 4 as using a Wortmann helicopter section. Marske used a 14% Fauvel section on the XM-1 but all the later planes (before Genesis) are built around NACA 5 digit sections. Genesis has a custom section designed by John Roncz and David Lednicer. I read in several places that the Fauvel sections are Abrial sections but I don’t have any information about Abrial sections except that they were the first stabile cambered airfoils. Marske describes his water experiments in his little folio “Experiments in Flying Wing Sailplanes”. It wasn’t a water tunnel it was just a tub of water that he pushed plywood profiles across. If the model goes straight it’s stabile if it pitches trim it and repeat the test. Simple and effective. He didn’t do it because it was a great method, he did it because it was cheap. Once he got in the air with the Abrial section he found out that heavily reflexed sections limit the speed range. Basically they’re low lift sections and you can’t get more lift by drooping the TE because then your +CM disappears. That’s why he used the NACA 5 sections on the later planes. The forward camber of the 5 digit sections gives good lift with about 1/7 as much pitching moment as the same amount of aft camber. To remove that last little bit of -CM0 he trimmed some material from the aft 25% of the lower surface and pushed it across his bath tub to confirm that it was stabile. The thinner 5 digit sections can be recognized by the distinctive cusp under the leading edge. Years ago I moved the point of maximum camber of a solid balsa wing forward by sanding a shallow reverse curve into the forward 15% of the lower surface. Theoretically it should have increased the Clmax but I didn’t check the sink rate so I don’t know if it worked, all I know is that it didn’t affect the distance. Saying that you have to do tunnel work to get pitching moment doesn’t sound right to me. All computer generated polars that I’ve seen show CM0.

That sharp nosed airfoil in the above picture looks like it came from one of Henri Mignet's original airfoils, which were made up by himself. The Pou-de-Ciel and some of its derivatives had that same sharp leading edge, which I think was due to nothing more than the simple construction method of the wing, which didn't use any kind of sheeting over the L.E. to smooth it out. I doubt if he had any aerodynamic reasons behind it.

Sounds about right, PT, in terms of the likely genesis of that section, and thanks, Norman, for the information. Amazing at how things are done sometimes, but I guess you can't argue with success, more or less.

I've played around quite a bit with reflexed sections in X-Foil, both with existing sections and ones of my own derivation. Found out some interesting things, and I think I now have a good airfoil for my own project that will do what I want. Certainly no "miracle", but it should do the job - nice soft stall, decent maximum lift, nice broad low-drag area, and a moderately positive CM. I may have to tweak the latter to the specific needs of my airplane, but right now I'm happy.

X-Foil gives very nice numerical output. Like any computer program, they're not any better than the routines used to calculate them and the inputs by the user, but good enough for comparisons to other airfoils and for my needs. It's not like I'm trying to set any records with my airplane.