For slow-flying ultralights, how would a delta wing compare to a biplane layout of equal wing area?

Question

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.

Answer 1

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.

Answer 2

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.