Updated by Bill Lofton 12/21/16
If automobile EVs can regen while braking, then e-aircraft can regen while aero-braking. They can and they do.
In an automobile EV, “regen” means recharging batteries during vehicle braking by using the resistance of an electric generator instead of (or in addition to) brake pad friction to slow the vehicle. The EV’s electric traction motor also serves as generator, so no additional equipment is needed to get this “free” energy.
Actually, the energy isn’t quite free, because the motor controller must be designed to handle power from the motor. To provide power to a motor, a motor controller must invert DC volts from the battery into AC volts for the motor. To receive power from a generator, a motor controller must convert AC volts from the motor into DC volts for the battery.
Motor controller with regen capability can handle power flow in both directions:
- Normal power flow: batteries -> controller -> motor
- Regen power flow: motor -> controller -> batteries
In flight, when power is reduced and/or the aircraft is pitched forward sufficiently, the propeller will “windmill” and slow the aircraft.
More accurately, the engine (or electric motor) that resists the spinning of the propeller is the thing that slows the aircraft. If you could push a clutch pedal and disengage the propeller from the engine/motor, the propeller would do little to resist acceleration of the aircraft in a descent.
When an internal combustion engine (ICE) resists a spinning propeller, the rotational energy is converted to (wasted) heat. But when an electric motor resists a spinning propeller, the rotational energy can be converted to electrical energy and stored in the battery. (When this is happening, the motor is functioning as a generator, hence the “motor/generator” terminology that you see occasionally.)
Regen During Aircraft Descent
Yanking back the power in level flight won’t do much to recharge your battery pack. But resisting acceleration in a descent, that’s another matter. Of course, a cross-country trip with a single climb to cruising altitude, followed by hours of cruising–humor me, better batteries are coming–and then a single descent to the destination airport will have a small % recovery through regen.
Flight Profile and Regen
The opportunity for windmilling the propeller to produce increased battery state-of-charge (SOC) is highly dependent on the flight profile:
- Soaring: Many regen descents in good soaring conditions could reasonably be expected to fully charge an electric motorglider’s battery pack. In fact, you may need provisions to avoid overcharging the pack.
- Pattern work: You’ll make multiple regen descents during pattern work. Pipistrel estimates that regen contributes 13% to the energy used by their Alpha Electro primary trainer airplane in pattern work.
- Cross-country trip: Unless your cross-country flight includes soaring, the only opportunity for regen is the descent from cruising altitude to the destination airport.
Aero-Braking Regen Requires a Negative Angle of Attack (AoA) of Propeller Blades
During level flight at constant airspeed, a propeller’s blade has a positive AoA over most of the length of the blade (i.e. a net positive AoA). That’s how it generates forward thrust.
Ideally, a windmilling propeller has zero thrust, which requires a net zero AoA on the blades.
To produce aft (rearward) thrust for regen, a propeller’s blades must have a net negative AoA.
In level flight, a negative AoA can be achieved momentarily by sufficiently reducing the power setting (of an ICE or electric motor). Of course, with aft thrust and level aircraft pitch, airspeed will plummet.
To maintain airspeed, the aircraft must have sufficiently negative pitch for gravity to provide the forward force that the propeller was providing before reducing power.
Propeller blade angle of attack during aircraft descent
Similarly, to maintain desired airspeed during a descent, e.g. in a landing pattern, the pilot retards the power setting to decrease the propeller’s thrust. (Thrust is decreased because the propeller’s rotational speed is decreased.)
- aircraft airspeed,
- aircraft negative pitch angle,
- propeller rotational speed, and
- propeller blade pitch,
blade AoA will be lowered to:
- a smaller positive value,
- or some negative value.
If the propeller blades’ (net) AoA is negative, then the propeller produces a “braking” force and electric power can be generated and used to recharge batteries.
Maximizing Regen Power With Your Propeller
The steeper the descent, the more regen power (energy per time) can be obtained through aero-braking. Maximum regen power will be limited by:
- Motor controller power limit,
- Blade stall,
- FAR 14 Part 91.307, or
- Passengers screaming.
Motor controllers have a limit on the DC/AC inverting power they can handle when driving a motor and they also have a limit on the AC/DC converting they can handle when being driven by a generator. If the AC/DC conversion max power limit is reached during aero-braking, the controller will limit regen power to that value and the propeller’s negative thrust will be limited as well.
If the propeller blade negative angle of attack reaches the limit for stall, the propeller’s negative lift will have peaked and will fall off rapidly.
If the e-aircraft airspeed is low, blades will stall before reaching the motor controller AC/DC power limit. If airspeed is high, the motor controller AC/DC power limit will be reached before the blades stall.
At some e-aircraft airspeed, the blades will be at max (negative) lift/drag and the motor controller will be at the AC/DC power limit. That e-aircraft airspeed should be the best for maximizing the electrical power that can be regenerated in a descent.
Of course, if the pitch attitude for max regen is alarming (see “dive bombing”), the motor controller can be directed to limit AC/DC power to a lower level. (Some automobile EVs enable the driver to choose the degree of regen to limit the “slamming on the brakes” feeling that regen can engender. The same could be done for e-airplane regen to limit the “dive bomber” feeling.)
Courtesy of flightsim.com
And then there’s Part 91.307:
- 91.307 Parachutes and parachuting.
- (c) Unless each occupant of the aircraft is wearing an approved parachute, no pilot of a civil aircraft carrying any person (other than a crewmember) may execute any intentional maneuver that exceeds—
- (2) A nose-up or nose-down attitude of 30 degrees relative to the horizon.
Last but not least, your buddy sitting next to you might have a much smaller pitch tolerance than Part 91.307.
Propeller Design for Maximum Regen
Efficient regen requires an airfoil on both sides of the propeller blades
Just as an efficient airfoil on the forward side (front or top) of propeller blades maximizes forward thrust of a propeller for a given shaft horsepower applied to the propeller, an efficient airfoil on the aft side (back or bottom) of propeller blades maximizes aft (rearward) thrust of a propeller while the blades are experiencing negative angle of attack.
So if we want efficient regen from a propeller during aircraft descent, we need the propeller to have an efficient airfoil on the aft side (back or bottom) of its blades.
Symmetric airfoil on the propeller blade?
One solution for efficient airfoil on the aft side (back or bottom) of propeller blades would be a symmetric airfoil on the propeller blade. (Same twist along the length of the blade, but symmetric airfoil throughout the length.)
A symmetric-airfoil propeller would produce more regen than a typical propeller. This would come at some cost of increased drag during forward thrust, however, so it doesn’t necessarily follow that a symmetric-airfoil propeller is a better e-propeller.
I expect the cost/benefit analysis for higher drag during forward thrust to achieve higher regen during negative thrust to be a function of the amount of regen flight compared to cruising flight. I.e., a symmetric-airfoil blade might be better for an e-airplane used for primary training (lots of descents in the landing pattern), but not better for an e-airplane primarily used for cross-country flights (which we don’t have yet).
- John Brandon’s 8/21/12 Engine and propeller performance
- Lancair Online’s 10/16/07 Aerodynamics for Naval Aviators excerpt
propeller drag: windmilling vs stationary