Facet Opel

Awesome, Norm. Thank you!

I'm sure I'll be able to relate the page numbers in that edition to mine. Shouldn't be more than a few pages off, I would think.

pfeww, looking through all these corrections, I can tell you that I'm glad that I purchased the original German language edition....

Ja aber die von uns, die nicht Deutsches sprechen, würde ein wenig eine rauhe Zeit mit dem haben.

Originally posted by Topaz
Ja aber die von uns, die nicht Deutsches sprechen, würde ein wenig eine rauhe Zeit mit dem haben.

quite so... especially so because the German used in that book is of a rather arcane style, typical for an academic mathematician from the pre-WWII era.

I think that is one of the advantages of growing up in The Netherlands: you get a lot of exposure to non-Dutch languages: Currently I use English, German and Dutch on a daily basis, and to a lesser extend some French as well.

--- topic break ---

Back to Facet Opal: have you noticed how the landing gear puts the wing at a rather large initial angle of attack? I guess that goes with using such a low aspect ratio?

Use any larger aspect ratio, and you end up with a CG-range that is unpractically small. Very unsafe.

Just an idea of mine: what about reducing the AR further, but leave the area intact. In that case the MAC would get longer and the CG-range gets somewhat larger too.
Now, to off-set the disadvantages of lower aspect ratios, why not put the two rudders at the wing tips, turning them into winglets. This would increase the _effective_ AR, hence reduce span load, hence reduce stall speed...

I have other ideas that center around creating a low aspect ratio flying wing (low part count anyone?)

What do you think?

Heh. I used Altavista's Babelfish for my little bit 'o German. You have the advantage on me in terms of languages!

The photos that Pylon500 posted were with the aircraft 'empty' and it's sitting back on the tail skid, lacking the pilot's weight to keep the nose down on the nosewheel. That may be why it looks like the airplane is so nose-up.

I've done a bit of research on the low aspect ratio question for a flying plank project of my own. From what I've read, as long as your aspect ratio is above approximately four, the airfoil section characteristics dominate the wing's behavior in terms of angle-of-attack for a given CL. That isn't a 'hard' transition point, but it's a good rule of thumb.

Orion and I were just discussing vertical stabilizer placement earlier in this thread, to no firm conclusion. Putting them out at the tips definitely improves the effective aspect ratio without sacrificing wing chord as you say, even if you're just using them for endplate effect rather than designing 'real' winglets. The down-side(s) seem to be that such an installation is heavier, more prone to flutter (being out on the tips of a relatively flexible structure), and possibly less effective at large yaw angles for a flying plank installation. I'm not saying it can't be done successfully - check out the SB13 sailplane or the powered versions of Al Backstrom's Flying Plank, for examples.

I'm leaning towards inboard verticals myself of late, much like the Facet Opal. The installation is lighter and since my airplane would have removable wings, the verticals could remain on the center wing section and I wouldn't have to break the rudder control system when removing the wings.

Orion is probably quite correct in that personal preferences end up being the driver on this question. The 'technical' pros and cons come out more or less even, IMHO.

Back on the angle-of-attack question, from the numbers I've run so far and seeing what others such as Winton and Debreyer have done, flying planks generally similar to the Facet Opal tend to have a lot of wing area compared to 'conventional' aircraft, and such aircraft tend to use that part of the induced drag equation to produce good climb rates - and also reasonable angles-of-attack - for a given flight condition, rather than aspect ratio. Supposedly this configuration naturally pitches down when entering an upward gust, so the ride is still pretty good as long as you don't get rediculously low with the wing loading. You can tolerate a fairly low aspect ratio and still get good performance numbers, if you're carrying that extra area. You take a hit on parasite drag from the extra wing, but then an airfoil is pretty 'clean', relatively speaking, and you're not carrying around a full fuselage or tail assembly, either.

Topaz,

you summarize a lot of what I have been thinking of late.

I see you are familiar with Debreyer as well, so it might be interesting to point out a striking similarity between Facet Opal and Pelican: both planes were solely flown by their respective designers. There's no third-party account of flight behaviour. Just a thought to put all information on the web in perspective...

As to low aspect ratio advantages:
- a larger chord allows for some serious hull-wing blending, reducing hull-wing interference drag.
- in a pusher application, these blends will create a lot of room, some of which could be used to house an advanced silencing system.
- with a set level of workmanship, the waviness of a larger chord will be smaller, relatively speaking. The waviness will be a smaller percentage of chord, meaning that it's easier to build closer to design specs.
- a larger chord/short span wing is inherently more stiff than a wing with a larger AR, that's why I feel that real winglets will not create a significant flutter risk. A low AR wing is stiff to begin with, a lightweigt carbon winglet will itself not flutter and will hardly put energy into the wing at frequencies where a low AR wing will flutter...

just my 2 €-cents

bye
Hans

Originally posted by h_zwakenberg
As to low aspect ratio advantages: ...

Yeah pretty much. Any structure can flutter though, it's just whether or not that flight condition exists inside the 'regular' flight envelope of the aircraft. It's interesting to see how various people 'weight' particular design considerations, isn't it?

I agree with you in regards to the quality of information on the web. I'd been under the impression that at least two or three additional Pelicans had been built by people other than the designer, but I'm not sure where I got that information. Probably here, although a quick scan of the page only revealed two or three still under construction here in the US.

For me, knowing that the Facet Opal had some bad flying characteristics (as we've learned in this thread) is softened by having some understanding of why that might have been the case - possibly a bad airfoil selection, or rather, one that was chosen for record-breaking instead of good handling.

Marc

another low AR idea that crossed my mind:

if you divert from the plank configuration and instead use a tapered wing, (assuming identical span and AR) and if you'd at least have a straight trailing edge.... then you'd automatically have increased the lever with which the elevons do their thing. That must be good for both control and stability...

If you take that idea to its extreme, you end up with a delta, which is also an interesting configuration in terms of speed potential, part count and CG-range.


Actually, my idea of an ultra light would be a modification of the Pelican/Vampyr, that would include:
- straight trailing edge, with an outer panel taper ratio as it is right now
- modify the control geometry to include elevons in in outer panel and a speedbrake/flap 'around' the MAC chord position (trim neutral deployement). The current elevator would not be included.
- remove the rudders and instead design & include real winglets.
- the fuselage would be shaped more like a sailplanes front fuselage pod.
- in Europe we can have ultra lights with retracts. Weight/space/money-permitting, I would design them in...

just a few thoughts... It would create a plane akin Pelican, with only a slight weight increase and a significant performance improvement...

bye
Hans

Originally posted by h_zwakenberg
... if you divert from the plank configuration and instead use a tapered wing, (assuming identical span and AR) and if you'd at least have a straight trailing edge.... then you'd automatically have increased the lever with which the elevons do their thing. That must be good for both control and stability...

True, but then you're also introducing sweep into the equation, and, having read Tailless Aircraft..., I'm sure you're aware of all the potential pitfalls in that. I'm sure it can be done - it has been done - but I do believe that, at the moment, any kind of small, general aviation swept flying wing is probably beyond my abilities. I'm sure I could work up to it, but the added risk isn't worth it for my current needs.

Your mileage may vary, of course.

Originally posted by Topaz
True, but then you're also introducing sweep into the equation, and, having read Tailless Aircraft..., I'm sure you're aware of all the potential pitfalls in that. I'm sure it can be done - it has been done - but I do believe that, at the moment, any kind of small, general aviation swept flying wing is probably beyond my abilities. I'm sure I could work up to it, but the added risk isn't worth it for my current needs.

Your mileage may vary, of course.

I understand your concerns about sweep, but you will no doubt have read their evaluation of the SB-13, where it reads that they thought the designers to have been too conservative with sweep. In their opinion, a bit more sweep could have reduced the size of the winglets significantly.
I don't know. The Braunsweig students had computers at their disposal, so I reckon that this trade study was done all right...

Yes, I read through that with interest. Nickel in particular is so concerned about induced drag (unsurprising given his sailplane background) that I wonder sometimes if he doesn't have a touch of the Horten's "sacrifice flying qualities for L/D" mania. The SB-13 is already constant chord, so how do you prevent tip stall if you sweep the thing even more? Inverse taper?!?! It could also be done with the twist distribution, but then you're making the airplane even more of a 'point' design, and that's not a good thing for a sailplane (or any airplane for that matter).

I had looked at swept flying wings when I started working on tailless airplanes (don't we all? ), and found the compromises unteneble for a two-seat general aviation-class airplane. Especially so for one with a relatively small engine such as I would like to use. It could probably be done with a fairly large engine, but that's outside my budget.

You could probably get away with a very small amount of sweep (say, less than ten degrees), but you'd have to watch the taper and twist very carefully, and you'd lose most of the effects of the sweep. Tip stall in (edit: swept) flying wings terrifies me. It's too efficient a way of shedding wings and lives.

wing tip stall cán be analyzed, so there's no point in being affraid. Cautious, yes, but it's an enemy we know, hence we can defeat.

Powered airplanes can get away with being less allround than sailplanes.
With my current powered airplane flying, I use a 'flying by the numbers' method, which results in a limited number of operational configurations:
- take-off & landing
- long range cruise
- high speed cruise

So it's actually a 3-point optimisation problem.

In the X-Plane simulator I have been playing around with a configurion described in the Nickes book, that uses three control surfaces per wing. To describe this crude design, I could list the following:
- flying wing derived from a blended fuselage/wing delta design, the derivation being the addition of outer wing panels to the delta.
- the fuselage section is derived from a sailplane's front pod.
- pusher engine / single seater
- winglets
- three control surfaces per wing.

X-Plane shows that this configuration is indeed quite stable. Flap deployment creates virtually no trim change. I'll see if I can shoot a screen shot and include it in a next posting...

Originally posted by Topaz

Back on the angle-of-attack question...

You can tolerate a fairly low aspect ratio and still get good performance numbers, if you're carrying that extra area. You take a hit on parasite drag from the extra wing, but then an airfoil is pretty 'clean', relatively speaking, and you're not carrying around a full fuselage or tail assembly, either.

Flying wings built with little or no incidence are notorious for their inability to rotate for takeoff, especially if the thrust line is a little high. The Delt-air 250 tested fine in the wind tunnel but couldn't pull the nose off the ground under full power, even well beyond the minimum flying speed. When the pilot cut power the nose down moment disappeared and it jumped into the air.

Your observation about wetted area is basically what I've thought for some time. i. e. You're just trading all that non working skin on the fuselage and empennage for working skin on the wing. Although the wing is substantially bigger on a tailless than on a conventional of the same weight the total surface isn't realy much greater. Is it? If you could somehow change the pitching moment without affecting CL or CD you could then use a smaller wing and possibly have less wetted ar

This small screenshot shows the control configuration:
- the inner wing control surface is a flap
- the middle wing control surface is an aileron
- the outer wing control surface is an elevator

There is a linkage between the middle and outer surface: the middle surface is mixed with the outer surface and is also used as an elevator. When used as an elevator, the outer surface makes the largest movement.

With this configuration, when pulling the stick rearwards, you induce a twist in the wing that increases from inboard to outboard. The faster you fly (stick further forward) the smaller the wing twist becomes.
The inner control surfaces is arranged 'around' MAC in such a way that the forces from the inside of MAC and outside of MAC are roughly equal, resulting in a deployement with almost no pitch trim change....

What do you think about this config?

Originally posted by Norman
Flying wings built with little or no incidence are notorious for their inability to rotate for takeoff, especially if the thrust line is a little high. The Delt-air 250 tested fine in the wind tunnel but couldn't pull the nose off the ground under full power, even well beyond the minimum flying speed. When the pilot cut power the nose down moment disappeared and it jumped into the air.

Your observation about wetted area is basically what I've thought for some time. i. e. You're just trading all that non working skin on the fuselage and empennage for working skin on the wing. Although the wing is substantially bigger on a tailless than on a conventional of the same weight the total surface isn't realy much greater. Is it? If you could somehow change the pitching moment without affecting CL or CD you could then use a smaller wing and possibly have less wetted ar

Total wingarea divided by total wetted area is indeed one of the major drivers of efficiency. In one of my aero books, a comparison is made between the L/D-ratio of a 747 and an Avro Vulcan delta bomber.
Intiutively, being a former sailplane pilot, I would have guessed the 747 to have the better L/D by a large margin. Sailplane pilots are raised to think that high AR equals high LD....
Alas, the 747 and the Vulcan are almost identical in this respect, both have an L/D of around 17. This means that a plane with a low AR wing can have a decent L/D, as long as non-wing wetted surface is reduced as much as possible.