How Much Additional Power?

How much additional power can we expect, to charge the batteries of a flying electric aircraft, from our ‘Robinson Regenerator’?  (Let’s coin a phrase).

The Physics:  A 20 mph wind (or prop blast) provides 600watts per square meter of power.  Force.  Energy.  Wind increases by its velocity cubed (V3).  So, double that blast velocity to 40 mph, and the power shoots up to 6,400watts per square meter.  (6.4kw)

The target area of our turbine mounted into the lower third of the cowling, protruding about three inches from the cowling, is .5 square meter.  That 40mph, 6.4kw available power is then 3.2kw on the turbine.  Since this general aviation electric aircraft might travel 120mph, there will be 86.4kw of available power hitting the turbine.  We have been recapturing 16.7% of our available power on our test mule.  Not a lot.  But… That’s still @ 14.0kw coming out of our turbine, except…

Our best axial flux PMG so far maxes out at 3.8kw produced.  If we can keep it from flying apart, we can expect that power production even after figuring in drag, (?), weight (21lbs) and turbulence.  We’ll still be providing at least 3kw new energy into the battery pack.

About that drag — the cross flow fan is 80% inside the cowling, creating no drag.  The 20% out in the prop blast is fairly slippery — it is cylindrical, and it is spinning very fast.  About 600rpm. It is also faired-in with filets on each side.  It is also covered by a blister (like an eyelid) of lightweight carbon fiber that opens and closes with a (lightweight) servo and lever.  Open more, more power.  Half closed, less, closed completely, zero.  Closed completely at take off and climb to cruise. Aerodynamically, when closed, it will be like a smooth bump on the hood of a muscle car.  

Then there is the famous loss of efficiency from the turbine — wiring, rectifier, bus, MPPT controller, inverter/charger.  We’ve studied that, and it isn’t more than 12%. 

Remember, this regenerative system can also ‘remain closed’ in the cowling and only opened for emergency power, or opened only slightly to provide 12-24v for the control systems.  It enables a builder to design an aircraft with a lighter battery pack, for better performance, or a standard pack, for more range.

Drag at the mounting position in the cowling is minimal at best, and always has been.  This low pressure area is where cooling openings are placed.  Our turbine isn’t an opening — it is spinning, the prop blast ‘sees’ it as a bump.  Let’s let the foremost expert on Aerodynamic Drag explain.  (this is from the classic primer on aviation aerodynamics, H.C. “Skip” Smith’s The Illustrated Guide To Aerodynamics” ):

“Cooling drag has traditionally not been considered a large factor in holding back an airplane’s forward progress; however, reductions in the parasite drag of other parts of the airplane have been brought about by great efforts in recent years to counteract high energy costs.”  (Our electric aircraft is already reducing energy costs).  “It is very difficult to calculate accurately, and even more difficult to reduce appreciably….cowl flaps, which adjust the flow of air over the engine to eliminate unnecessarily high heat under certain flying conditions, have been one way of reducing drag”

Our spinning turbine in the lower cowling is more like a cooling flap than an obstruction to the airflow.  We can direct the ‘waste’ air back into the cowling to cool the electric motor (which is mounted in the dead spot behind the prop) and the battery pack, perhaps eliminating the need for liquid cooling of either component, trading some of the 21lbs we’ve added to our 800 pound airplane.

—Dale Robinson, 12/30/16