Lazair ultralight series II used biplane props to absorb more power from the engine....so my question is what really is a biplane propeller and are they efficient in comparison to a four bladed prop?
It is essentially two propellers stack on top of each other. You can see it in this picture of a Lazair.
Many have asked why Ultraflight opted for double propellers stacked in a biplane fashion. This is a very unique setup and draws lots of attention. Apparently when Ultraflight changed over to the Rotax 185 they tried several different propellers. The problem was the 185’s had a woodruff key slot in the crankshaft and would snap the crank whenever the engine coughed or kicked back on start up. This was quite a serious problem. After much deliberation it was determined that the rotational inertia of the propellers was too much for the crank to handle.
Dale could not locate a two bladed prop light enough to fit the need. However, since they had been producing a lot of the early lazairs with Pioneer engines fitted with a single plastic prop, he had a ton left over that fit the pioneers. He found that these could be “doubled up” and help to produce the extra thrust that the Rotax 185 was capable of producing over the 5.5 hp Pioneer engines.
It appears to be a quick way to gain more thrust from the props without increasing prop mass too much.
Depending on the pitch and blade airfoil, a biplane propeller as used on the Lazair might be more or less efficient than a conventional four-blade propeller the same diameter. The interference between blades causes additional drag (which absorbs power without creating additional thrust), but the turbulence from each blade affects the following one in a conventional four-blade design, with similar effects, especially in a flatter-pitch design or at low airspeed.
The potential advantage of the biplane propeller is that, presuming the hubs have six or eight torque transmission bolts (or even four, though that arrangement is less versatile), the blades may be arranged with the forward propeller directly ahead of the aft (true biplane propeller), or one or two holes leading or trailing in rotation. Without wind tunnel testing, it's impossible to be certain, but I'd expect an arrangement with the forward propeller trailing the aft by a fraction of a revolution to be most free of interference effects and hence most efficient, quite possibly more efficient than a four blade design with the same diameter, pitch, and blade area.
A biplane propeller is one prop stacked above another, similar to a biplane wing.
This looks to be very inefficient. The gap between stacked blades looks like about one chord at the tip and a quarter chord at the root. This should cause problems with the high pressure under the upper blade interfering with the low pressure above the lower blade. At minimum the blades should be spaced farther apart.
Here is an analysis of biplane wing efficiency relative to separation as a percent of chord. Efficiency is 10.5% higher when the gap/chord is 1.5 as compared to 0.75. It doesn't go to 0.25 but it seems that would be much worse as the 1.5-1.25 gap drop in lift/drag is 3% and the 1-0.75 drop is 4.2% at 4%. This configuration is likely losing 6%-18% compared to a gap at 1.5 chord. The .075 gap/chord entry in the table looks like a typo.
The reference shows biplane efficiency is lower than a similar monoplane, though not completely clear if wing area, span or chord are held constant for that comparison. It also states that positive stagger (upper wing ahead of lower) by 0.4 chord gives 5% better efficiency, so rotating the upper prop 0.4 chord would be beneficial compared to the vertical stacking shown.