Facet Opel

Adding any type of tip device to a low aspect ratio wing should be of some help however the extent of any performance improvement will most likely be a function of the actual tip device, the wing planform and of course, wing load.

For instance, the original configuration of the Rhor 2-175 was that of a plain delta however, according to Walt Mooney there were plans to eventually incorporate tip devices. He indicated that the change was to aid the craft's climb rate and possibly, to slightly increase the top end, although he did admit that the latter might not be all too significant due to the low planform load at which the aircraft operated at.

Orion:

I posted the question because of the quote I saw in this thread about delta wings developing most of their lift by vortex generation. The winglets as are used on the Varieze help reduce wing tip vortices and so I was wondering if that implies that they would be counter productive on a delta wing. I assume that they would actually help out a delta wing a lot and would help counter some of their "bad" low aspect ratio characteristics. They might even help reduce the chance of tip stalls on swept wings by helping contain the airflow within the wing area.

I'm assuming that whatever the wing loading (I had been thinking of a delta wing in an ultralight application when I considered winglets) winglets would help make a delta wing more efficient but at lighter wing loadings the tip loss is less and therefore the benefit is also reduced.

In level flight, delta wings act more like conventional planforms in that the lift generated is as with conventional sections and shapes. It is at higher angles of attack as in slow flight or during energetic maneuvering where the deltas get an additional benefit which comes form the formation of the leading edge vortex. The magnitude of that benefit however is a function of the planform load and angle of attack - the full vortex usually forms only at higher angles so for something like an ultralight I'd guess that there would really be no benefit. But I haven't looked at the low end of the spectrum so don't know for sure.

I became interested in the Facet Opel flying-wing some years ago, after looking/researching Backstrom's Plank. Seems to me the simplest/easiest to build Flying Wing most closely resembling the attractive Facet Opel would be a Pelican with a 17% constant-chord wing, with no dihedral and no twist. 20hp 4-stroke watercooled Paramotor would power it. (Pelican 3-view drawing can be found at : Charles Fauvel and his flying wings see also: pelican and: pelican

This would be cheap & simple to build, the 17% wing would be strong, and it would motor along on 10hp. Ballistic-chute of course! I've made a simple cardboard model, and it certainly looks very attractive. Looking for comments.

The Nest of Dragons section of the Nurflugel site is old. Keon has his own URL now but I don't think anything has been added to the Pelican material. TWITT has some more information but it's not very recent either. As far as I know the Pelican, and the later version of it called the Vampire, is an orphaned design. But I'm often wrong

I have seen the Pelican and have been very interested in it's apparent simplicity and lightness. I have been unable to find much specific information on it though. I have never heard of the Vampire verison and will have to look into that. Any sites you can suggest?

I wrote an email to someone who supposedly could sell Pelican plans but never received any response.

No matter what the exact details of the original were, I would think that a successful version could be redesigned from scratch. The Facet Opal (or Opel, I guess) also indicates that some very good range and altitide numbers can be achieved on low power with a light airframe and low aspect ratio.

I like the low aspect ratio and thick spar of the Pelican. If the performance is acceptable with the low aspect ratio the wing structure could have impressive strength at these proportions.

For the Opel, I would change the airfoil and solve whatever the problem was with the spar structure (I had read that high heat from the engine damaged the composite structure). There are various accounts of what happened. It's very unfortunate. I think he could have gone on to make some impressive planes.

I would also try to place the pilot on or very near the c.g location for a plane like this. I think that anything else would make changing pilots very difficult without a lot of wasteful ballast on a very short moment arm to compensate.

How did I miss this?

YouTube - Scotty Winton Opel (Australia) **FLYING WING**

YouTube - Scotty Winton Opel (Australia) **FLYING WING**
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Indeed.
Looks like the same video that's posted 2 pages earlier in this thread.
The Torsional Resonance Search Conundrum strikes again.

I say good day Sir........

Always fun to watch that.

Well, since we cannot seem to get much information about the Facet Opel, but do have some pretty knowledgeable people here, why not come up with some sort of well laid-out generic approximation?

Opel tells us approximate size, weight, and power to achieve spectacular performance. It is said to be a STOL design into the bargain.

How about a somewhat larger, more powerful version which would get similar or better performance due to similar wing loading, but better power to weight ratio and/or more efficient fuel consumption via 4 stroke engine? Engine suggestions?

We could have a more stable airfoil than Opel (said to be symmetric). Suggestions? Longer chord for better pitch stability?

How about extending the vertical fins below the wing, for better long. stability at low speed, and to fair and house outrigger wheels?

Or, vertical fins at the wing tips, for winglet effect?

Materials: Opel is said to have been mostly carbon fiber. While an excellent material, it's simply too expensive and too difficult for a homebuilder to make in a reliably durable way. How about vinylester and fiberglass instead? Suggestions?

Opel landing gear was too short coupled and twitchy. How much would the stance need to be enlarged to overcome this, yet still have good STOL capacity on fields similar to that Winton used in the video?

Other suggestions on how to take best advantage of Opel's virtues, yet design out its vices?

Originally Posted by Bart

Well, since we cannot seem to get much information about the Facet Opel, but do have some pretty knowledgeable people here, why not come up with some sort of well laid-out generic approximation?

I've rather wondered why someone hasn't done that already, frankly. Realize, however, that this task will be designing an airplane from scratch that just happens to look a lot like the Facet Opel. You still have to do all of the design work.

Originally Posted by Bart

Opel tells us approximate size, weight, and power to achieve spectacular performance. It is said to be a STOL design into the bargain.

There's nothing particularly magical about the airplane's performance. It gets that the old-fashioned way: Small, low drag airframe, very low weight, reasonably good power for the weight and size. No different than the Quickie or any other small, light, clean airplane.

Any claims of STOL performance must be taken in context. Flying Planks make lousy STOL airplanes, and for its size and power-to-weight ratio, the Facet Opel was no exception. Compared to a 747 it's definitely STOL, but not compared to, say, a Kitfox.

Originally Posted by Bart

How about a somewhat larger, more powerful version which would get similar or better performance due to similar wing loading, but better power to weight ratio and/or more efficient fuel consumption via 4 stroke engine? Engine suggestions?

Any of the usual engine selections would apply. You figure out your weight and the power-to-weight you want, and there's the power you need. Hit the engine catalogs and pick one you can afford. The usual story. If it were me, I'd keep it the size it already is and pick a half-VW. About 36hp, a little less than the Opel. Heavier than the two-stroke that he was using, but then I wouldn't be looking to break records, either, and we've got plenty of nice, long, paved runways here in SoCal.

Originally Posted by Bart

We could have a more stable airfoil than Opel (said to be symmetric). Suggestions? Longer chord for better pitch stability?

The aircraft already has a fairly long chord for its size, which probably helps its flying characteristics (and particularly the dynamic pitch stability characteristics - damping), so I don't see that there's much to be gained by making that any bigger (by scale) than it is already.

As for the airfoil, it needs a reflexed section. Without getting into a lot of design detail (as in 'calculating the numbers'), picking a particular airfoil is pointless. Some of the more moderately reflexed Eppler 300 series might be applicable, but that's as detailed as I'm willing to go without running the numbers. Airfoil selection on a flying plank has all the usual considerations you see in conventional airplanes, plus you choose the amount of reflex to set elevator (elevon) zero-deflection trim point at your design static margin and speed. The latter is rather like choosing the angle of incidence of the horizontal tail on a conventional airplane. Mythology to the contrary, a reflexed airfoil does not create stability. It allows you to put the CG ahead of the Neutral Point (which fact creates stability) and not have to hold back on the stick all the time to keep yourself from becoming a lawn dart. Balance of forces, just like a conventional airplane.

Originally Posted by Bart

How about extending the vertical fins below the wing, for better long. stability at low speed, and to fair and house outrigger wheels?

Or, vertical fins at the wing tips, for winglet effect?

I personally prefer the latter, set well aft, but either way would probably work.

Originally Posted by Bart

Materials: Opel is said to have been mostly carbon fiber. While an excellent material, it's simply too expensive and too difficult for a homebuilder to make in a reliably durable way. How about vinylester and fiberglass instead? Suggestions?

Build it with whatever you're comfortable working in. The material selection doesn't really matter. If it were me, I'd go Rutan-style fiberglass and full-depth foam. But that's me. An equally valid case could be made for all-wood construction, or all-metal, or any combination thereof.

Originally Posted by Bart

Opel landing gear was too short coupled and twitchy. How much would the stance need to be enlarged to overcome this, yet still have good STOL capacity on fields similar to that Winton used in the video?

Well, I'm a glider nut, so if it were just for me I'd do a retractable bicycle undercarriage with a nosewheel, a main just a short ways aft of the CG, and outriggers in the bottom of tip-mounted vertical tails. I like the simplicity and low weight. The other option is to retract conventional tricycle-gear mains into the wing surface. Might get pitch changes (possibly severe) upon extension, though, which I suspect is why Winton didn't do it.

Originally Posted by Bart

Other suggestions on how to take best advantage of Opel's virtues, yet design out its vices?

Yeah. Make sure you don't duplicate the drive-shaft and structure problem that killed Steve Winton. It was described early on in this thread, I believe.

Thanks, Topaz, we're off to a good start.

Per my earlier comments and your timely responses, I'd vote for a bicycle centerline landing gear, with outrigger wheels buried in lower extensions of the vertical fins or winglets. Don Mitchell's Goodyear racer planform comes to mind. Google for it. On the TWITT website, a guy named Lindsay Olen* from OZ wrote in last March, and said he was the designer of Opel, but that Scott Winton had altered the plan by deleting the vertical fins below the wing. (This may have left the craft with inadequate low-speed yaw stability. Too bad, as the lower fins might have been a great place to put and fair outrigger wheels.

For engine, rather than 1/2 VW but more than twice the power for the same or less weight, a BMW boxer oil-cooled motorcycle engine. ~90 hp, ~140 lbs, superb quality and low specific fuel consumption, altitude compensated electronic fuel injection means carrying less fuel weight for a given range.

The Facet Opel is said to have been only ~242 lbs. empty weight. I'll bet using plywood and foam/fiberglass instead of carbon fiber, a proportionately similar weight could be achieved, at much lower cost. Strojnik said plywood was pound for pound the best choice, at least for wings etc. which are not compound curves. A somewhat larger and more comfortable plane than Opel could be built, with similar or better performance, for under 300 lbs.

I agree about the propeller shaft through the spar idea being not so hot. A pusher arrangement more like Varieze is simpler and better.

* Here's the text of Olen's note as reported in TWITT: "I have studied flying wings for many years and built numerous flying models. Scot built and flew the Facet Opal from a design that I drew for an admirer of his for "the simplest flying wing that can be built as an ultralight." Scott altered the design by not installing the underwing fins and shortening the udercart without raising the thrust line to give adequate ground clearance to the parking strut. This design is actually a STOL aircraft, using vortex lift at low speeds. Details of my designs were always freely available but I have yet to receive any recognition.. Cheers, Lin." I emailed him, and got TWITT to email him, too. So far, no response.

Originally Posted by Bart

...Per my earlier comments and your timely responses, I'd vote for a bicycle centerline landing gear, with outrigger wheels buried in lower extensions of the vertical fins or winglets. ...Too bad, as the lower fins might have been a great place to put and fair outrigger wheels.

I skimmed back through the first half of this thread, and Orion makes a compelling case for keeping the verticals somewhat inboard. If they're just inside the quarter-span, they still would be useful for outriggers.

If you have any aspirations towards selling the design, figure out a way to tuck conventional tricycle gear into the fuselage. Only glider nuts like me seem willing to give mono-wheel or bicycle gear a chance.

Originally Posted by Bart

...For engine, rather than 1/2 VW but more than twice the power for the same or less weight, a BMW boxer oil-cooled motorcycle engine. ~90 hp, ~140 lbs, superb quality and low specific fuel consumption, altitude compensated electronic fuel injection means carrying less fuel weight for a given range.

I suppose. That engine option has been discussed occasionally here. The need to develop a PSRU for it usually stops the speculation pretty quickly.

Originally Posted by Bart

...The Facet Opel is said to have been only ~242 lbs. empty weight. I'll bet using plywood and foam/fiberglass instead of carbon fiber, a proportionately similar weight could be achieved, at much lower cost. Strojnik said plywood was pound for pound the best choice, at least for wings etc. which are not compound curves. A somewhat larger and more comfortable plane than Opel could be built, with similar or better performance, for under 300 lbs.

You're not going to beat carbon fiber for stiffness-to-weight, so anything else will end up heavier. However, that doesn't at all mean "too heavy." As I said before, you can build a safe and light structure out of any of the common aircraft construction methods.

Originally Posted by Bart

...I agree about the propeller shaft through the spar idea being not so hot. A pusher arrangement more like Varieze is simpler and better.

The engine in the Opel was where it was for CG reasons. The problem with the crash was that the rear bearing for the shaft was mounted in the rear spar, and when the bearing failed the heat generated softened the composite structure to the point where it failed. As was pointed out, it wasn't this failure that killed Steve Winton, it was the tree he ran into on the roll-out. The situation could've been avoided any number of ways, including not mounting the bearing to the spar and building the spart out of something that won't fail when heated. It was a freak accident. Hard to plan for, but at least the rest of us have the benefit of that experience to que our thinking moving forward.

Originally Posted by Bart

...* Here's the text of Olen's note as reported in TWITT: "I have studied flying wings for many years and built numerous flying models. Scot built and flew the Facet Opal from a design that I drew for an admirer of his for "the simplest flying wing that can be built as an ultralight." Scott altered the design by not installing the underwing fins and shortening the udercart without raising the thrust line to give adequate ground clearance to the parking strut. This design is actually a STOL aircraft, using vortex lift at low speeds. Details of my designs were always freely available but I have yet to receive any recognition.. Cheers, Lin." I emailed him, and got TWITT to email him, too. So far, no response.

I can't comment on his claim, other than to say it's counter to every other source I've heard. In fact, another person earlier in this thread who claimed to have been around the airport at the time the Opel was being tested says the airplane was originally flown with no verticals at all, which makes it rather odd that Olen didn't mention that interim step.

Originally Posted by Topaz

You're not going to beat carbon fiber for stiffness-to-weight, so anything else will end up heavier.

Sure you can, the next step up in stiffness to weight is boron fiber. Now you're talking big bucks

I'll wait for the real carbon fiber. You know, diamond filaments...