Very low aspect ratio planes?

[Vigilant1]

So, the only major advantage I see is perhaps ability to crash in a narrow zone.

For this purpose, as implemented in the Facetmobile, the advantages over a more conventional (higher AR) design would be:
1) A relatively large wing area with low structural empty weight. This allows for a low stall speed (despite the lack of high lift devices and possibly a lower Clmaxl).
2) A large passenger area volume for the weight. This allows for extensive crush area without impingement of the occupied cabin area.

But, yes, a plane with a higher AR can have a stall speed just as low as a low AR plane, and will likely have a higher glide ratio (which has its own safety value).

 

[poormansairforce]

This is just nonsensical now, sorry.
A key takeaway: Use a well maintained 4stroke and reduce the likelihood of any of this in the first place.

Why is that nonsensical? See the video above. And why do people think we are going to pancake it in from up high? As rotax618 pointed out you keep your speed up until flaring and then hold it off as long as possible bleeding off speed until touchdown. As Wainfan pointed out in his presentation this is a logical maneuver in an emergency. Horizontal speed is the rule until you don't want it anymore. And there are no guarantees with your engine but I guess I need to put my thinking hat on since you obviously have much more insight on low AR than I do.

 

[BBerson]

1) A relatively large wing area with low structural empty weight. This allows for a low stall speed (despite the lack of high lift devices and possibly a lower Clmaxl).

I don't know if low structural weight has been proven. But I do remember Barnaby saying that the Facetmobile concept wasn't ideal for an ultralight. The Facetmobile is more optimised for high speed and perhaps a large speed range, but not abnormally low speed for the wing area, I think.

 

[Arfang]

The PAV report by Mr. Wainfan puts the L/D of the conceptual airplane above 10.5 and according to the same report, the FMX-4 has a L/D slightly above 7. In both cases it should be enough to not ''pancake'' your way down, or am I misunderstanding the issue here?

 

[rotax618]

Dispute it all you want but seeing is believing, it really isn’t possible to discuss LAR on this forum without someone saying “impossible” in spite of the video evidence To the contrary.

 

[pictsidhe]

I wanted to build an ultralight Facetmobile. Right up to the point where I ran the numbers...
Would probably make an interesting LSA, though.

 

[Vigilant1]

I wanted to build an ultralight Facetmobile. Right up to the point where I ran the numbers...
Would probably make an interesting LSA, though.

It would. The PAV study makes a good case for the practicality of the form, for the right application (just like many other layouts. Horses for courses.)

 

[cluttonfred]

The inverse relationship between structural weight and aspect ratio is simple engineering fact. Pick a weight and a wing loading, run the numbers for a constant-chord wing of AR 8/6/4/2/1 using a common airfoil. With each drop in AR, the wing weight per unit area will go down as the cantilevers get shorter and the spars get thicker. Once you get down to something like the Facetmobile or Arup or Flying Founder both of those effects are very pronounced, plus you start being able to combine some of the functions fuselage and wing in the same structure.

I don't know if low structural weight has been proven. But I do remember Barnaby saying that the Facetmobile concept wasn't ideal for an ultralight. The Facetmobile is more optimised for high speed and perhaps a large speed range, but not abnormally low speed for the wing area, I think.

 

[Speedboat100]

A steep descent can be an asset for the class of aircraft that we are discussing, it is not a problem for gyros and being able to land and takeoff from a small patch of land is very usefull. As can be seen from the video of the Arup S2, an undercarriage with a long travel is an asset.
I imagine the technique for power off landing would require maintaining sufficient forward speed to have the energy for a flare at high AOA before touchdown.

Yes it sorta slams on the field at Part 103 speeds.

 

[BBerson]

Once you get down to something like the Facetmobile or Arup or Flying Founder both of those effects are very pronounced, plus you start being able to combine some of the functions fuselage and wing in the same structure.

The Facetmobile is 370 pounds from aluminum tubing and fabric and typical ultralight engine.
How is that lighter than a typical ultralight?

 

[rotax618]

The Facetmobile was also 214sqft, its the wing loading that makes for a successful LAR airplane. Obviously there are no ”free lunches” everything is a trade off, lowering the aspect ratio ( increasing the span loading) increases the induced drag, the two main compensatory things that can be done is to decrease the wing loading and select a planform that generates a stable vortex at high alpha to arrest the descent for a very slow landing. There are other benefits in LAR - lighter structure, short deep light spars, large internal volume, Stall and spin resistance, compact size for storage.
The ideal and probably most successful LAR airplanes are as a light single/2 seat seat sportsplane

 

[rotax618]

The UFO is no match for a 300HP Cub at the Lonestar competition but for it’s 12’ wingspan and 50HP it has a pretty short takeoff roll.

 

[rotax618]

The landing roll in the video isn’t as short as the UFO usually has as David was getting used the this version’s as a taildragger. The aircraft has been converted back to tricycle which allows for higher alpha on landing.

 

[erkki67]

A similar design with a straight LE was the AP10 with similar performance figures!

 

[nestofdragons]

One of the things that attracts me to this layout is the fact that in an emergency I can use the full CL to get my landing speed as low as possible. Yes I'm going to total the plane but I'm probably going to walk away in most cases.

In the talk of Barnaby Wainfan on youtube he shows a graph about speed at crashlanding and survival-posibility. He mentions that he wanted to be able to land at a certain speed to be more sure about the walking away part. That is what i also like in the Flying Flea concept. In panic you pull as reaction the stick to your stomack, that gets you in a not-really-stall situation and you start a parachutal descent (you land nearly vertical at a speed of a WW2 parachute) ... and walk away.

 

[nestofdragons]

Dear Barnaby,
Few flying wings seem to have flown with landing flaps.

Have any deltas or low aspect-ratio flying wings flown with flaps ... specifically split flaps under the centre of lift?

The SWIFT hangglider has huge inboard flaps When using them you needed to push the stick forward to keep the speed high enough for safe landing. Those flaps were very very very usefull in many flight situations during flight. Even in thermals.

The Me163 had a special flap for landing.

 

[Arfang]

In the talk of Barnaby Wainfan on youtube he shows a graph about speed at crashlanding and survival-posibility. He mentions that he wanted to be able to land at a certain speed to be more sure about the walking away part. That is what i also like in the Flying Flea concept. In panic you pull as reaction the stick to your stomack, that gets you in a not-really-stall situation and you start a parachutal descent (you land nearly vertical at a speed of a WW2 parachute) ... and walk away.

For those like me who didn't know that there is a video of Mr Wainfan explaining the LAR/Facetmobile concept in more details:

Thank you, nestofdragons and Mr Wainfan.

 

[BrianW]

Would you be willing to pull a Cessna up to 45 degree AOA to try and get 1.85 CL while 20 ft off the ground? At 30 mph there wouldn't be any authority from the tail to go there and even if you could it would probably snap roll.

The inheritance of training in a C-150 conducted by a WW2 RAF pilot instructor (then based east of Dallas), was the edict that one never push, but always pull in the landing phase.
This finally resulted in a flattened tail hold down eye bolt after several years, which was replaced when its thickness was down to 50% at the contact point, and on one occasion the request to conduct my weekly landing at Tulsa Intl adjacent to the tower (presumably so they could enjoy the fireworks show ). True, I finally learned to ALWAYS land at 20 degrees flap which places the nose down, and the tail out of the contact zone.
Brian W

 

[Traskel]

My word of advise: do not draw. Build mock-up and test yourself before drawing. I did a few tests in the past and i was surprised that some things i thought to be good were in fact very very bad.
You can read about the tests here: My own prone tests - Nest of Dragons
I took the Horten HXb as source of inspiration.
My best result was this:
View attachment 103288
I could hold this position several hours if needed.
The secret of the good comfort of the "seat" was shoulder support. My chin was even free. No support there. And that takes a lot of uneasy feeling away.

I agree that modeling positions is best to begin. That said I believe that a GP motorcycle racing position is preferred. Less prone so not as easy to streamline the cabin but much more comfortable , especially while using upper and lower body inputs and safer. Also improved field of view with less neck strain and greatly improved balance and recovery.

Human proprioception, (brains awareness of where the body parts are in space and relative to each other), and equilibrium are seriously compromised beyond 40.degrees from vertical. Fighter pilots sit comparatively upright for that reason based on much testing.

Comfort to lay about reading a book is very different than the signal processing needed for awarness through mult-G maneuvers.

Additionally, Grab CAD has some talented modelers who have created articulating ergonomic models. Free...

 

[danmoser]

(snip) ..Swift pulls down its flexible, fabric, centre tail for landing. (snip)

Incorrect.
The SWIFT hang glider uses rigid sandwich core composite construction on all control surfaces, mounted on hinges... not flexible fabric.
Actuating the inboard flaps produces a slight nose-up pitching moment, which can easily be counteracted by shifting pilot weight forward and/or elevon control input.
In fact, SWIFT is an acronym; Swept Wing with Inboard Flap for Trim
My assertions can be verified by contacting Dr. Steve Morris, the designer.
I absolutely LOVED flying my SWIFT.. but hated the storage, transport, and assembly characteristics.
I sold it, but wish I hadn't.. adding a small electric propulsion unit to fly it as an ultralight rather than a hang glider seems like a great way to go.