Facet Opel

OOPS, forgot how to post multiple images.
Arthur.

Thanks Pylon500 for those pictures....! Very enlightening...

Both fins appear to sport an air intake on their roots. Any idea what they were for, perhaps ram air intake for high altitude flights? Kirk wrote to me that Scott employed supplemental oxygen for his altitude flights, but seeing those intakes, this might not be the entire story....

Originally posted by Topaz
Some of the stuff in Nickel & Wohlfart's book suggest that separate surfaces might be desirable in a flying plank, and I was wondering what the Opel's designer was using.

since you're using 'Nickel & Wohlfahrt', I think you should be aware of an error in the English translation, where figure 2.7.5 reads:

Xc=l(0)/4+2P(1+2Z)/(1+Z)


However, in the original German publication the formula in figure 2.7.5 reads:

Xc = l(0)/4 + 2/3 ( (1+2Z) / (1+Z) ) * P

meaning that the English translation seems to be lacking the term 2/3 in the middle and the term * P at the end.

I don't have the English edition myself, so that I can't compare all formulae. Comparing notes with another flying wing afficionado, we discovered this discrepancy...

Originally posted by Norman
A constant chord plank can use either system because the longitudinal distance from the CG to the trailing edge is constant along the span. Forward swept planks like Marske and Fauvel have the pitch control at the center and ailerons at the tip because the tip doesn't provide any moment arm.

Marske doesn't use any washout because the slight forward sweep has the same effect, the root stals first

on a plank it must be a good idea to have the elevons as far outboard as possible, as this would automatically create a wash-out angle at low speeds / high Cl (the elevons are deflected upward in that configuration). The danger of wing tip stall is reduced that way.

Originally posted by pylon500
AT LAST, I found one of my photos! with a small amount of 'bi-convex' in the section

I believe "bi-convex" refers to the shape of the mean line. Regardless of what the airfoil section looks like if the mean line crosses the chord line at some aft location and then curves up toward the trailing edge it's reflexed. Depending on the airfoil thickness and amount of reflex it may not be obvious to a casual observer but the stability influence of the reflexed mean line is there nonetheless. Note that a reflexed section with a pitch control surface is not necessarily a good thing because if the mean line of the elevon is curved it will have the same effect as a trim tab pushing the TE down. The ARUP series of low AR flying wings used the M-6 airfoil and at high speeds they had a pitch problem. At low speeds they were stabile but at high speed they took off toward the ground and required some muscle to fly fast. This change in moment with speed is not a characteristic of the M-6 airfoil, it's the result of the elevators having camber. In tailed aircraft this phenomena isn't a major problem but in a flying wing, especially a planck, it can really mess up your control harmony

Originally posted by h_zwakenberg
since you're using 'Nickel & Wohlfahrt', I think you should be aware of an error in the English translation, ... in the original German publication the formula in figure 2.7.5 reads:

Xc = l(0)/4 + 2/3 ( (1+2Z) / (1+Z) ) * P

THANK YOU!!!!!

I'd noted that the formula was wrong, by running their example numbers through it. There's a nice matrix of information about the Horten gliders in the book, and I couldn't get the printed formula for the C-point to get the results shown in their examples.

VERY frustrating! I'm not enough of a mathematician to re-derive the formula from the information they give, so I was in the dark until you posted this.

Edit: I just ran the Horten examples from the book through this updated formula and the results match the C-point in the examples exactly. Definitely the correct formula! Thanks again.

Originally posted by pylon500
The impression I got was NO reflex at all!
I do note however, that the elevons are hanging UP slightly in the photo, which could mean either, over mass balanced controls (not sure) or a spring trim system (more likely) showing that it actually flew with the elevons held up a little.
Hey I found the other photos!
These were all taken in my earlier days of photography were if I saw something of interest, I just shot it, without thought of contrast or background clutter!
Enjoy,
Arthur.

Thank you, Arthur!

They do the job for me, regardless of 'artistic' merit. Yours are the first shots I've seen that reveal some of the more interesting details of the aircraft.

I'm with Norm that "biconvex" implies some degree of reflex in the camber line, but you're closer to the source, of course.

If Scott Winton was really using a straight symmetrical airfoil, then the elevons being rigged slightly up is no surprise whatsoever. Doing so would introduce some 'effective' reflex to the wing, and add some stability by allowing trimmed flight with the CG farther forward of the neutral point. From earlier comments, however, it sounds like he was running near-neutral stability.

That 'end on' view of the wing does look like it's a pure symmetrical airfoil, but of course it's impossible to be sure from just a photo.

Originally posted by Topaz

I'd noted that the formula was wrong, by running their example numbers through it. There's a nice matrix of information about the Horten gliders in the book, and I couldn't get the printed formula for the C-point to get the results shown in their examples.

I have the complete errata for "Tailless Aircraft in Theory and practice. I didn't mention it before because when you referred to some text by page number in a previous post I couldn't find it in my book so I ASSUMED that the print to order shop that's publishing it now had probably included the corrections and that's why the page numbers are off. I'll see if I can find the e-mails from Serge

One minor little correction (and it may nor really apply here but FYI): A bi-convex section classically refers to a simple airfoil shape that consists of two circular arcs (top and bottom), either joined at two sharp points fore and aft, or joined at a point at the aft end and simple small radius at the front.

A convex airfoil uses an arc on the top surface and a flat line on the bottom. The terminology is often used in the design of supersonic wings, usually in the preliminary design stages where you are working at defining the basic planform and a baseline structure. The bi-convex geometry can be easily adjusted (radius of arc) to deliver a variety of structural depths without significantly affecting the performance aspects of the projects.

The biconvex design works well since in the transonic and supersonic regions you're really not interested in any particular camber line, short of an eventual certain level of refinement of the root to tip distribution.

The Facet seems to use a more conventional symmetrical section with a forward shifted maximum thickness. The maximum thickness of a biconvex section is always at 50%.

Whoa, a thousand posts? I need to get a life.

Originally posted by Norman
I have the complete errata for "Tailless Aircraft in Theory and practice." ... I'll see if I can find the e-mails from Serge

Wow, I'd definitely love to get my hands on a copy of that.

My book is the english translation, but apparently no corrections have been made.

Marc

Originally posted by orion
Whoa, a thousand posts? I need to get a life.

Don't you dare!

Originally posted by orion
Whoa, a thousand posts? I need to get a life.

I love my computer because most of my friends live in it

Originally posted by Topaz
Wow, I'd definitely love to get my hands on a copy of that.

The errata is spread out over three e-mails. I'll put them together and post it latter. Most of the corrections are verbal stuff listed by page number. I think the on demand printer is using a different format than the AIAA edition so text isn't always on the same page. The math corrections are listed both by page number and section number so the math corrections should be okay for either printer's edition

Here ya go. Hope it makes sence for the new eddition