Facet Opel

The Facet Opel got my attention some years ago, as an optimum design for single seat fun flying. Also, Mitchell of Mitchell Wing fame apparently had a flying wing design intended for Cleveland racing. Al Backstrom did his Flying Plank. Such an aircraft might be built with empty weight of ~200 lbs.

I'd be curious about how Scott Winton dealt with stability & control aspects as noted in the video. Thoughts?

Got any more details about the engine and landing gear of the Facet Opel?

And, on viewing the video again, I'm impressed that FO seems to work well even with bar-stool landing gear on a grass strip. As an alternative, one can image a sort-wing version of a Fauvel AV-36 flying wing, with the vertical stabilizers also housing outrigger wheels.

Originally Posted by Bart

...I'd be curious about how Scott Winton dealt with stability & control aspects as noted in the video. Thoughts?

Run a search on "Flying Plank" or "Tailless" in this forum and you'll see that we've discussed the issue quite a bit, especially in this thread and one called "Affecting Thrust/Propeller", starting about here. Flying planks have plusses and minuses, just like any other configuration. If you can live with the liabilities, there's a lot to be said for them.

Originally Posted by Bart

...And, on viewing the video again, I'm impressed that FO seems to work well even with bar-stool landing gear on a grass strip....

I'm not sure that I'd call that kind of 'wobbling' and dragging a wingtip in the grass "acceptable" ground behavior. Personally, I'd either go single main/single nosewheel with outriggers such as you suggest, or simply retract conventional tricycle mains into the wing on longer legs.

Still, it's a very nice little airplane and with appropriate pilot training and care in the design phase, would make a very nice little single-seater sportplane, IMHO.

for a production aircraft this would indeed not have been an acceptable ground behaviour. Having said that, Facet Opal was designed and build for a singular purpose: breaking records. In that context, Facet Opal was all that the designer hoped it to be.

Originally Posted by h_zwakenberg

for a production aircraft this would indeed not have been an acceptable ground behaviour. Having said that, Facet Opal was designed and build for a singular purpose: breaking records. In that context, Facet Opal was all that the designer hoped it to be.

Reportedly, the landing gear had only a ~20" width, but a wider & more stable stance would not seem to be difficult or heavy.

This was a very nimble aircraft, a cross between the proverbial homesick angle and a bat out of hell. 2,000 nautical mile range is also extraordinary, opening all sorts of possibilities if the plane were not demanding to fly. Anybody here have any insights as to its handling characteristics?

Originally Posted by Bart

Reportedly, the landing gear had only a ~20" width, but a wider & more stable stance would not seem to be difficult or heavy.

This was a very nimble aircraft, a cross between the proverbial homesick angle and a bat out of hell. 2,000 nautical mile range is also extraordinary, opening all sorts of possibilities if the plane were not demanding to fly. Anybody here have any insights as to its handling characteristics?

a wider landing gear would have been much more difficult to make retractible. If you look at the available pictures earlier in this thread you might even discern a fairing bulb on the fuselage when the gear is retracted, indicating that the main gear could only be retracted into the fuselage partially.

As to handling characteristics: we've discussed that early in this thread. The anecdotal evidence from here and there seems to indicate that handling at low speed was more difficult than at high speed, where Facet Opal apparently flew quite well.

Originally Posted by Bart

...if the plane were not demanding to fly. Anybody here have any insights as to its handling characteristics?

None specific to the Facet Opel, but for flying planks in general, you can expect them to be very lightly damped dynamically, although the long chord on the FO would mitigate that somewhat.

All flying planks have a characteristic at low airspeeds where pulling back rapidly on the stick increases angle of attack, but causes the airplane to actually sink in altitude, since the "up" elevator/elevon is acting like a "reverse flap" and reducing wing lift more than the increase in angle of attack might cause lift to increase. This can cause a premature (and heavy) touchdown if it happens during the flare to landing.

In most designs this seems to be a transient effect, much more pronounced if you make rapid control movements. Training the pilot to make smooth, slow-control-movement approaches and flares will mitigate the problem somewhat or completely, depending on the specific design. There's really nothing you can do aerodynamically to eliminate the behavior, short of separating the elevator from the wing, as was done on the Genesis sailplane. The resulting design is then something of a hybrid between a flying plank and a conventional airplane - stability is still provided almost entirely by the wing, and control by the separate elevator. In a pure-plank like the Facet Opel, the only way to "fix" the problem is with pilot training.

Originally Posted by Topaz

...There's really nothing you can do aerodynamically to eliminate the behavior, short of separating the elevator from the wing, as was done on the Genesis sailplane. The resulting design is then something of a hybrid between a flying plank and a conventional airplane - stability is still provided almost entirely by the wing, and control by the separate elevator. In a pure-plank like the Facet Opel, the only way to "fix" the problem is with pilot training.

Interesting that you mention this, as I've just been reading about Don Mitchell's "ailevons" on his flying wing designs such as the U2 and his planned but apparently never built Goodyear racer. Such "ailevons" are mounted behind and somewhat below the trailing edge of the wing, with a gap for air to come up from the bottom of the wing and then over the top of the "ailevon." This reportedly prevents stalling or backwash on the upper surface of the trailing edge of the wing, perhaps allowing higher angle of attack while maintaining control.

So, what are your thoughts on using such a gapped "ailevon" on a flying plank like the Facet Opel?* Would such device likely cure or lessen the mush problems you noted?


* as I've seen this spelled both Opal and Opel, its worthwhile on Google to use both spellings

You must have been reading an old article. Later he changed to the more common term for an all flying surface "stabilator". There is constructive interference going on between the wing and stablator but the momentary lift decrease wouldn't be affected much, if at all. A tail, any tail, produces a pitch increase as a secondary effect of its own down force. The longer the tail is the less that downforce has to be. When you move the tail right up the the trailing edge of the wing it has to make a lot of down force to effect a pitch change. This down force is seen as a reduction of lift that occurs before the pitch change that will eventually result in a net lift gain although not as much as if the tail were farther back. An aft slotted wing (Hugo Junkers called it 'doppelflugel') dose stall a bit later and produce more lift than an equivalent reflexed airfoil but also higher drag at high speed.

It's well known that the way to make a smooth landing in a plank is to hold off until the plane loses energy and settles to the ground on its own. Even pilots who know this still can't resist the urge to pull back and try for a spot landing. The result is often a bouncing ride down the runway. If you saw the video of the new AV-36 in France last year you noticed that he busted the landing wheel.

Originally Posted by Bart

...So, what are your thoughts on using such a gapped "ailevon" on a flying plank like the Facet Opel?* Would such device likely cure or lessen the mush problems you noted?...

As Norman said, no, the effect would still be to reduce the lift of the main wing before it creates an increase in angle of attack. The lift reduction is immediate upon moving the control surface, while the pitch change takes some small but finite amount of time to accomplish. The difference in time between the two "results" is where the problem occurs. Once the pitch change "catches up" with the lift reduction, the effects usually cancel out and you start developing a correct-sense change in lift, as the pilot commanded.

A separate "Junkers-style" elevon would still influence the pressure field on the main wing, unless they were separated by a significant fraction of the wing chord, which would be awkward structurally. Putting the surface out on "outriggers" that long would create such a moment arm that it might twist the wing enough to create a control reversal.

Originally Posted by Norman

...It's well known that the way to make a smooth landing in a plank is to hold off until the plane loses energy and settles to the ground on its own. Even pilots who know this still can't resist the urge to pull back and try for a spot landing. The result is often a bouncing ride down the runway. If you saw the video of the new AV-36 in France last year you noticed that he busted the landing wheel.

Part of the problem is that sailplane pilots are routinely trained to make spot landings on every flight, as practice for landing out. It conflicts with the correct way to land a flying plank. A powered-aircraft pilot might have an easier transition to a flying plank, as a result.

Originally Posted by Bart

So, what are your thoughts on using such a gapped "ailevon" on a flying plank like the Facet Opel?*

BTW several other flying wings used external airfoil flaps. The Horton "Wingless" was suposed to fly as a low AR "wing but the retractable outboard panels were never tested in flight

I wonder how a plane such as a Facet Opel would fly with vortex generators, tubercles, etc.. Reportedly, tubercles have shown considerable promise with wind turbine blades.

And, on the TWITT website archives, I found information saying Facet Opel was not designed by Scott Winton, but rather Winton deviated somewhat from the designer's plan. Reportedly, the vertical fins were designed to extend below the wing, though Winton did not include such in his construct. This is the only reference I've found suggesting another designer than Winton. That said, his Facet Opel vertical fins do seem a bit small.

Originally Posted by Bart

I wonder how a plane such as a Facet Opel would fly with vortex generators, tubercles, etc.. Reportedly, tubercles have shown considerable promise with wind turbine blades.

Depends upon what you're trying to "fix". If you do the design right, you don't need any of those things.

Originally Posted by Bart

And, on the TWITT website archives, I found information saying Facet Opel was not designed by Scott Winton, but rather Winton deviated somewhat from the designer's plan. Reportedly, the vertical fins were designed to extend below the wing, though Winton did not include such in his construct. This is the only reference I've found suggesting another designer than Winton....

That's the first I've heard of it, too. I've always seen the design attributed to Winton, exclusively.

I ran into a paper someplace recently on the outboard horizontal stab. It's supposed to reduce induced drag a great deal. I don't know if that's relative to the original span or the increased span, though. The paper also indicated that root bending moment wasn't greater than normal, and that, although I'm skeptical, maximum twisting forces on the wing were no more than normal, though obviously they'd extend over more area. According to the paper, if you threw in handling properties, putting the stab two chord lengths back was best, but if you were interested in minimizing induced drag, it was better to put them much closer. Not sure I remember, half a chord length? I also don't remember if they were measuring from quarter chord point or someplace else.

Originally Posted by DaveK

Kind of late in the game on this thread, but thought I would throw in an idea I've played with to see what you all think. Take the basic plank or tapered flying wing with verticals out by the tips as has been discussed, but add small horizontal stabilizers out past the tips. I've attached a sketch of the Opel with this superimposed to give you an idea of what I'm talking about.
The models I played with awhile back seemed to fly alright, except I found that if the verticals were on the small size you could induce a weird yawing tumble with a hard rudder doublet.
I played with this because I thought you could have the basic benefits of a plank, but with a bit wider cg range. But in reality I just thought it might be fun.

Dave

Originally Posted by h_zwakenberg

Why would you want to avoid the vortex generation altogether? After all, this is what gives delta's a large part of their lift at high angles of attack. Doing away with that will result in a much larger wing area.

What would be the result of adding Whitcomb winglets to the tips of a delta wing? I had thought that this would give you the advantages of a delta wing with an increase in effective aspect ratio.