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Context: By "slow-flying" I refer to ultralights that can be classed as "motorfloaters" - similar wing loading to a hang-glider, stall speed below 40km/h, cruise speed not much above.

I'm interested in designing something of similar capabilities to M. Sandlin's Bloop ( http://m-sandlin.info/bloop/bloop.htm ) where the plan is to get into the air as cheaply as possible and then "fly too slowly/lightly to really be able to hurt yourself". (See also Ron Wheeler's Skyscout, more traditional ultralight design but manages an empty weight of 55kg, MTOW 135kg and a stalling speed of 32km/h, and and first flew in 1974 - http://all-aero.com/index.php/54-planes-p-q-e-r-s/9873-skycraft-scout- .)

The Bloop's designer chose a biplane layout as an easy way to achieve high wing area and structural strength for a given span, with cheap/light materials. The wings are approx 9m^2 each, with a span of 7.6m. (Due to diminishing aerodynamic returns on additional wings, I'm not sure whether to treat this as a total wing area of 18m^2m, or less - I have seen some sites suggest *1.5 instead of *2 as a good estimator.)

Given deltas also offer a large wing area and good structural strength, I am curious: Assuming similar wing construction - rigid metal frame, foam LE supports, stiff fabric covering - what differences in flight performance could be expected if an otherwise-similar craft flew with a single delta wing, of equal total area but lesser span? For example's sake, let's say a span of 5m.

Off the bat, I would expect induced drag to be much higher due to vastly lower aspect ratio, but I'm unsure to what degree that may be balanced by the removal of an extra wing's worth of profile drag, interference between the two wings, and parasite drag from struts and bracing wires etc. Plus, possible improvements in lift coefficient for same AoA range as I would think the wing would operate at much higher Reynolds numbers due to greater average chord length.

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  • $\begingroup$ In all honesty there are a lot of questions in here. Furthermore you seem to be concerned with the structure and rib design when you should be wondering if a slender wing will fly at all at the speeds you can expect of an ultralight. If you can refine this down a bit or at least make clear what is the main question, you'll get a better response. Meanwhile, look into the effect of wingspan on lift to get an idea on why cropped deltas are usually high-speed propositions. $\endgroup$ – AEhere supports Monica Jan 28 at 10:25
  • $\begingroup$ You mean the sort of wing on a flexwing microlight like this: en.wikipedia.org/wiki/Ultralight_trike#/media/… $\endgroup$ – Dave Gremlin Jan 28 at 21:53
  • $\begingroup$ You should clarify whether you're referring to a Rogallo-derived wing, as on classic "delta" hang gliders, or a rigid delta wing like a Voorhees or Dyke Delta homebuilt aircraft. They're very different in terms of flying characteristics. $\endgroup$ – Zeiss Ikon Jan 29 at 12:05
  • $\begingroup$ @AEhere Thanks, I've edited as you suggested. $\endgroup$ – Thomas Bullock Jan 29 at 12:05
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    $\begingroup$ @ZeissIkon Yes, apologies, I've amended my question to specify a rigid, non-Rogallo wing. The Verhees Delta is very similar in appearance to what I'm picturing, but this would be far lighter (120-130kg MTOW) and with a tiny paramotor-class engine (8-15kW range) whereas the Verhees is more of a "proper aeroplane". :) $\endgroup$ – Thomas Bullock Jan 29 at 12:09
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One major issue with a delta wing was discovered in the early days of supersonic fighters: delta wings don't handle well at low speeds. At high angle of attack, they (like any highly swept planform) exhibit strong "Dutch roll" in which roll and yaw are coupled in an oscillation, and because they stall at very high angles of attack the Dutch roll comes on when control authority is at its lowest.

Your plan, to fly just above the stall at very light wing loading, will require control surfaces that make up a very large percentage of the total wing area, effectively compromising lifting area. You'll also need a very large vertical fin to maintain stability, and much/most of that will need to be movable for yaw control.

Generally, the main reason Rogallo-derived designs have maintained dominance in hang gliders is because they overcome most of these failings of rigid deltas, and can be made lighter, store more compactly, and cost less to construct as well.

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  • $\begingroup$ That's interesting and a shame - I've found deltas my preferred planform for slow-speed in RC flying, but I guess it's one of those things that vary with scale. :( What are the characteristics of the non-rigid, Rogallo-style deltas that help minimize these issues? I know very little about behaviour of non-rigid wings. $\endgroup$ – Thomas Bullock Jan 29 at 12:41
  • $\begingroup$ @ThomasBullock That's an interesting question; you may want to see if you can turn it into a fully formed question on this site. This site doesn't work like forums, where one question can lead to another and be answered right there. Instead, it asks that we ask one solid question at a time, when then gathers a bunch of answers. Follow-on questions are meant to be separate. This helps keep things focused, and especially helps internet searchers who may ask the same question as you. $\endgroup$ – Wayne Conrad Jan 29 at 12:45
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    $\begingroup$ @WayneConrad Thanks. New here, and, as you guessed, more used to forums! I'll make a separate question. $\endgroup$ – Thomas Bullock Jan 29 at 12:53
  • $\begingroup$ @Thomas - the main factor I've found (also with models) to improve stability of deltas is that the fin needs to extend well behind the trailing edge of the wing. This would cause a problem on a full size aircraft, as you already need an excessively long nose gear to achieve enough AOA to take off. $\endgroup$ – Robin Bennett Jun 3 at 12:55
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The main (low speed) characteristic of a delta is that when it stalls, the lift doesn't drop sharply. Instead the drag continues to rise, and with enough power you can fly at extremely high angles of attack. With the power off, you can descend very steeply, with the nose high and very little airspeed.

This is a lot of fun in a model with enough power to accelerate vertically out of a stall, but in a microlight it would be just as dangerous as a normal stall.

A side effect of this is that landings often result in large flare angles, and take off require more angle than for a conventional plane. This means you need very long landing gear, and the pilot's view may be restricted.

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    $\begingroup$ "and the pilot's view may be restricted" Hence the Concorde's droop nose, if I'm not mistaken. $\endgroup$ – a CVn Jun 3 at 13:13

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