Electric Aircraft - The Thread

That's a very clever idea. 

I don't think the new "structural Tesla batteries" will be able to remove ribs completely, but it eliminates a lot of battery-pack assembly weight -- important in reaching 400 kilowatt hours per kilogram that is expected to happen for these cheap electric airplane conversions.   But it even becomes economical for jump ships even before the 400 kWh threshold;

I'd imagine that electric Otter/Caravan conversions would just simply put them inside the now-unused wing tanks, with sufficient heat conduction to airframe, to do simple air cooling of the batteries, thanks to the newly invented easily-heatsinkable battery ends (where 70% of battery heat is focussed).

Here's what 130 kilowatt hours looks like now, with the new structural heatsinkable Tesla 4680-format batteries:

This 130kWh structural battery pack outputs almost a megawatt -- over 1000 horsepower -- in only 8cm thick (excluding cooling/heatsink plates).

It'd easily fit inside the wings of the airplanes for electric airplane conversions!   There'd be about 2000 horsepower available if you used both Caravan wings.  Or even 4000 horsepower, if you used four of these size-equivalents.  You don't need that mucho horsepower anyway -- a bit extra horsepower is just needed to haul the battery weight around, then you don't lose much passenger capacity in the cabin.

See....the extra horsepowerage minimizes the passenger loss.  Who cares if you discharge this battery by 25%-50% per 13500 foot skydive for a fast 7 or 8 minute ride; if the recharge takes only 12-15 minutes on the ground between loads for a 10-passenger / 5-tandem electric caravan that's shockingly cheap to fly.  With minor further battery improvements (2030-2040), you might get by with no passenger reduction if you only need short jump lifts.

The inconvenience of a 12-to-15-minute battery recharge between loads is a minor price to pay for a $10-to-$15 jump ticket that's still profitable for a DZO (for dropzones in cheap-electricity states, or airports with their own adjacent solar farm).

19 minutes ago, mdrejhon said:

I'd imagine that electric Otter/Caravan conversions would just simply put them inside the now-unused wing tanks, with sufficient heat conduction to airframe, to do simple air cooling of the batteries, thanks to the newly invented easily-heatsinkable battery ends.

The first Tesla, the Roadster, was a converted Lotus. Elon Musk knew the value of having an electric design from scratch rather than a conversion - so while I think conversions would be how the transition happens, the REAL gains will be from a new design. The Model S to the original Roadster.

NASA's X-47 Maxwell actually explores what's needed for new designs while using very cheap LFP cells rather than lithium-ion, so it's expected that they can achieve more than 50% reduction in required energy by reducing structural weight and using distributed propulsion.

EDIT: LFP cells also have a higher discharge rate than lithium-ion, but a lower energy density. Elon and Tesla are already seriously trying to make them work for the Model 3 (it would increase their profit margins massively). LFP cells also happen to have twice the lifespan of lithium-ion in terms of cycles so it's even better for a jump plane with lots of cycles.

Yes, a cleansheet design is better, though the first electric jump plane will probably be a cheap Caravan/Otter conversion kit (compared to turbine), circa 2030-2035.

1000+ horsepower electric motors are now being invented that are lighter than a 600 horsepower airplane engine + associateds (differential gears, fuel piping, etc).  Electric motors are direct propeller drive and modern inverters are miniaturized (like wallwarts vs iPhone chargers).

Thanks to horsepower, battery weight haulage is no problem at the Caravan scale, if the airplane frame can support the weight and enough capacity to jump altitude in under 10 minutes using only 25-50% battery + fast 12 minute charge between loads.  

It’s now all within the envelope of circa 2030 DZ economics. “Do I buy a new plane, or do a turbine re-engine, or do an electric conversion?  Those electric conversion kit prices are tempting...”

6 minutes ago, mdrejhon said:

Thanks to horsepower, battery weight haulage is no problem,

Not that I disagee that an electric jumpship is a great idea, you keep on saying that more power allows for more weight to be carried. That is not correct. Increasing the power of an aircraft does not increase its MTOW.

28 minutes ago, gowlerk said:

Not that I disagee that an electric jumpship is a great idea, you keep on saying that more power allows for more weight to be carried. That is not correct. Increasing the power of an aircraft does not increase its MTOW.

Perhaps from a structural/regulatory perspective, yes.

But many aircraft can’t even reach the airframe’s MTOW without a turbine engine upgrade.  So many old jumpships out there that are being generally run below airframe MTOW rating.... the MTOW can be downrated by an older, less powerful engine already extant on the aircraft, and other variables like runway lengths and air density.

While electric doesn’t fix all, it’s no longer hard to max out an approved airframe MTOW rating with electric without reducing passenger capacity much (for favourable conditions, low altitude with great runway).  

You only need enough battery for one trip to jump altitude with a reserve margin, and that’s all the battery weight you need.  By 2030, strategic battery sizing may be lighter than the avgas it replaces.  The point is horsepowerage is no longer the limiting factor.  The electric supercars now can do 1300 horsepower with a battery now lighter than the weight of the avgas of a full caravan fuelup!!  

But that can be skipped, and the battery right-sized for one or two loads at 50% or 25% discharge per full-throttle load, and heat is no longer an issue with the low internal resistance and new heatsink-friendlier battery heat-radiating design (just heat-conduct to airframe, little extra cooling-related wright).  Just a little more time and the weight math checks out.

Those dropzones with old engines, by 2030, will be deciding between turbine refreshes versus cheap electric conversion kits (and tolerating the 12-minute recharge between loads).

31 minutes ago, mdrejhon said:

The point is horsepowerage is no longer the limiting factor.

It never has been. More power will give you faster turn times. Turbines and even turbos will allow you to keep the climb rate up where the air is thin.

31 minutes ago, mdrejhon said:

But many aircraft can’t even reach the airframe’s MTOW without a turbine engine upgrade.

I'm not sure about this. Can you provide an example? A piston powered aircraft is generally loaded to the MTOW, or as close to it as possible in a skydive operation. A more powerful aircraft like say a c-130 will be limited by the space for jumpers rather than the weight limits.

AFAIK, the certification standard for MTOW includes performance requirements.... to a certain point.

Please correct me if I am wrong, but...

We've got skydiving planes that use original engines and original propellers, versus upgraded engines and upgraded propellers -- that kind of slightly affects MTOW approval, up to the approved structural limits of the airframe. 

When an airplane is refurbished with completely upgraded engines (e.g. turbine upgrade + better propeller on a formerly piston aircraft), it can remove the performance-derived MTOW downrating if the airframe structural MTOW is not the limiting factor.

There are Twin Otters with piston engines, and Twin Otters with turbine engines upgrades paid for by dropzones.  And propeller design have improved over the lifetime of the Twin Otter.

The piston engined Twin Otter is underpowered relative to the Twin Otter structural ratings. It is a very strong airframe designed for rough bush landings.  In actual practice, dropzones usually load the turbine Twin Otters a little bit more without needing longer runway + climb faster. 

Yes, some MTOW downratings are internal/unofficial (DZO-mandated) because of their airport-specific risks ("It climbs like a grandma on hot days, I'm nervous of this squirmy jump load to keep plane safe during takeoff"), that are then removed when they do their turbine upgrade ("Go safely nuts on the throttle, up to the marked redline, we have tandems to push!"). 

Your DZ's maintenance department may thus also request a recommended MTOW, to save maintenance costs.  See?   A redlined piston engine will need maintenance sooner than a redlined turbine engine.  And a redlined electric needs even less maintenance than a redlined turbine engine.  By 2030, dropzones will probably have a choice between a turbine upgrade or an electrific upgrade.  FAA is already working on regulations on such upgrades, thanks to the Magnix work. 

I have frequently jumped out of many Twin Otters of different engine powers, at multiple dropzones in multiple countries. Dropzones pilots generally babies them very differently.

Anything else?

16 minutes ago, mdrejhon said:

The piston engined Twin Otter is underpowered relative to the Twin Otter structural ratings.

17 minutes ago, mdrejhon said:

Please correct me if I am wrong but I have jumped out of both types of Twin Otters.

There is and never has been a piston engine Twin Otter. Every aircraft has a weight that it is certified to take. There are many reasons for that, and the length of a take off roll is not one of the main ones. What is a main factor is how the aircraft will handle when the engines fail. In an emergency situation with no power there needs to be a reasonable chance that the pilot can keep the aircraft in control and perform a no power landing rather than a crash. MTOW numbers are hard limits normally independent of STC approved upgrades to the propulsion system.

But I still think that electric jumpships are something that will happen.

Ooops, that was the Otter (the Garrett upgrade by Texas Turbine Conversions), not the Twin Otter.  Memory slipped there, it's been a long time.  Doh.  

Texas Turbines advertises "Our conversion will allow your airplane to haul more, take-off shorter, climb faster, cruise farther, and burn less fuel.".

Haul more!  Just what you need for battery weight.

They do Twin Otters and Caravan upgrades too that defacto allows you to milk your airframe's approved MTOW. 

Much of the points stand -- including unofficial DZ-specific MTOW downratings (whether be load quirks, power limits due to maintenance department edict or DZO edict, and mudane factors like that) -- like moving the redline marker lower and mandating the pilot not to exceed that mark, forcing loading decisions on extreme days (hot, winds, etc). 

The horsepower differences of the various horsepower upgrdes (turbine-replace-turbine upgrades) that are made to underpowered turbine-engined Twin Otters turning them into 900hp Super Otters.  Like the ones at Skydive Perris, of which I frequently jumped bigways from.  Electrics can redline more frequently than turbines can before it needs maintenance, so that's another bonus, too. 

Terminological issue too -- when I say MTOW increases, I mean removal of power downratings / MTOW downratings (including the unofficial DZ specific ones) and being able to push all the way to the approved MTOW without reducing manifest by 1 or 2 skydivers on hot-weather days due to bad specific air density for an older engine whose maintenance department recommended a lower redline for.

In BEV research, it was found that strategic fast-discharge/charge cycling of battery doesn't really wear the battery any differently from slower charge of that specific battery cycle, as long as it's within the 80%-to-30% SoC (battery bar) envelope and within thermal criteria.  Fit one skydive load in that SoC envelope, and quickly becomes a sealed deal economically.

Both the Texas Turbine upgrade & the Magnix electric upgrades includes a more efficient propeller and what appears to be stronger engine mounts.

In other words, Texas Turbines makes legal MTOW easier with less risk

Imagine, the electric equivalent of a Texas Turbines Upgrade (maybe they'll get into that market too, purchasing electric motors from vendors like Magnix, if they want to franchise them later this decade!)

It can make it comfortable to move the DZO/maintenance-prescribed lowered voluntary redline marker back up higher.  Giving pilots more pillow-comfortable freedom with frequently safely reaching FAA approved MTOW happily going near vertical, with tons of stall safety margin, sooner on the runway, on worse summer days, without as much maintenance wear-tear and without an early trip to the Tucson boneyard.  

Electric Deadheading / Repositioning Operations

The Harbour Air electric tests have a removable in-cabin battery that will later be integrated into the airframe (presumably the wings). 

Now, it was discussed among electric airplane engineers that an optional removable in-cabin battery can be used only for deadheading/repositioning the airplane across the states (NYC-to-California), in operational practice, you only need one jumpship load of built-in battery in the wings. 

The ruggedized deadheading battery modules are modularized to hand-liftable (~30-60lbs each) out of the airplane door, and can be stored at the dropzone (at ~50% storage SoC) that airplane is leased/loaned/stored at, until the electric airplane needs to be moved to a different airport/dropzone. 

The thought exercise, is that optional extender batteries just looks like small strap-down cargo boxes -- like upcoming bigger aviation-standardized many-kWh sized versions of electric camping batteries - some of them have high power AC outlets built into them + high-amperage car booster outputs.   With the new Tesla 4680s, it is increasing possible to fit almost 10 kWh in a 50 pound "suitcase" small enough to fit under skydiving bench seats and be strapped down during repositioning flights.  And by 2030, more capacity is possible. 

A DZ be able to load up the whole cabin full of these, strapdowns-permitting.  A few hundred kWh worth of cheap human-carryable modules!   (Remember: lithium batteries is quickly hurtling to $62/kWh by 2030, and $30/kWh by 2040.  Even if you double/triple that for module assembly and profit margin, this becomes affordable to dropzone electric aviation by 2030s-2040s)

Suitcase sized supersized equivalents of cellphone battery banks.  With lift handles on them metal boxes with rubber corner bumpers.  To resist bumps of loading/unloading/storage, with strap attachments for the existing floorbottoms of existing airplanes -- treating the extender batteries like cargo.  Flexible daisychainable connectors allow you to attach a variable number of modules to the airplane's built-in supplemental-power outlet in the cabin to supplement the in-wing battery. 

And you can load as few/as many extender batteries you want up to the cabin space + cabin load limits + MTOW -- at projected 300-400 kWh/kg plus weight of battery pack casings -- almost megawatt-hour worth of battery for an electric Twin Otter for repositioning, and still be below MTOW.

If theoretically approved for use during skydiving operations, you might be able to do 30,000 feet HALO operations with fewer skydivers + some extender batteries strapped down under the bench seat.  But they'll likely mainly be used for long-haul repositioning.  

And when the deadheading batteries are not in use, they can also conveniently double as backup batteries during DZ power outages or boost other turibne Otters replacing existing battery wagons.

There's no need for long-haul deadheading batteries during regular commercial operations. 

It's a brilliantly simple idea -- strap-in batteries for passenger cabin only used for longhaul deadheading operations -- and right-sizing the in-wing battery for your revenue service (i.e. one jump load discharging 25-50% of a battery that can be replenished in about 12-15 minutes between loads using a fast-charger installed at an airplane dropzone).

The borrower dropzone would just hand-remove the normal looking cargo boxes (extension batteries) set them aside in the hanger.  The borrower dropzone just trickle-recharges them from the hanger's regular AC outlets at 1875 watts (115V/15A) to finish charging them within 3-4 days (the common time period of a bigway/boogie/record attempt).  Then put the battery-cargo back into the electric jump plane for the repositioning flight back to Skydive Perris, with enough weight leftover for three or four DZ-employee passengers and their luggage.  Longer flights might need one quick-charging stop at some intermediate dropzone, but that's not the end of the world.

So yes, in year 2035, Skydive Perris can still loan their future electric Twin Otter or electric Caravan to Skydive Spaceland or Skydive Deland's big boogie or record attempt.

1 hour ago, mdrejhon said:

"Our conversion will allow your airplane to haul more, take-off shorter, climb faster, cruise farther, and burn less fuel.".

It allows you to haul more because the Garret/Honeywell turbine that replaces the R-1340 radial engine weights 383 lbs versus 930 lbs. for the geared 9 cylinder P&W. It has nothing to do with the power.

Yes.  True.

The weight of a 750 horsepower electric Magnix airplane motor is only 297 pounds, so there’s still extra battery haulage opportunity of an electric conversion.

In some respects 750hp electric outperforms 900hp turbine because of more instant spool-up torque.  Much briefer brake hold-down for a leaping start on short runways, it’s more immediately apparent to the pilot the motor confidently and safely leaped to full power, in an uncannily smooth near-instantaneous spinup.  This can be useful in getting you out of tight situations, the ability to instantly rev for a go-around.

The horsepower per pound is also improving on the electric motor front.

So, still extra battery haulage without sacrificing much passenger load (if any) for right-sizing wing battery to regular skydiving ops.

—

BTW, for other readers, there is now a 100 watt charger for a specific model of a cellphone — which injects 50% back into the battery in just a few minutes and a full battery charge in 17 minutes.  It tapers wattage safely based on charge progress and temperature monitoring.

Tesla uses massive parallelized charging like that, like thousands of separate fast chargers for each cell in each battery pack — just by plugging in one Supercharger connector. It’s incredible engineering of concurrency to speed-charge, and this can also be done to future electric aircraft needing frequent cycles, like jump planes. (Charge controllers are often built into cells or modules)

A 50% refuel in under 10mins, so the 12-15min estimate are cheaper/more realistic, given well-engineered battery design optimized for gentler ultrafast shallow-cycle charging from a future aftermarket truck megawatt recharger retrofitted to recharge future jump planes.  

Realistically, like partially filled wing tanks, it increasingly appears unnecessary to perm-install more battery than needed for one jump lift for a 25-50% discharge.  

One might need more weight in partial battery than equivalent partial avgas, but the weight envelope freed by lighter powerful motor enables that without much sacrifice (if any) to jumper capacity, and the charging ends up still much cheaper than avgas for many airports. 

Assuming they can pull off the upcoming near-term economics, the numbers starts to check out, surprisingly so — cheaper jump tickers and bigger DZO profits is something rare simultaneously.

(Accidental double post removed)

This hasn't been brought up in a long time, but I just wanted to add that IMO only 30 minutes total fuel is required if you are taking off and landing at the same airport--not 30 minutes in addition to the anticipated flight time.

Quote

 

§ 91.151 Fuel requirements for flight in VFR conditions.
(a) No person may begin a flight in an airplane under VFR conditions unless (considering wind and forecast weather conditions) there is enough fuel to fly to the first point of intended landing and, assuming normal cruising speed -

(1) During the day, to fly after that for at least 30 minutes; or

(2) At night, to fly after that for at least 45 minutes.

 

https://www.law.cornell.edu/cfr/text/14/91.151

1 hour ago, nwt said:

This hasn't been brought up in a long time, but I just wanted to add that IMO only 30 minutes total fuel is required if you are taking off and landing at the same airport--not 30 minutes in addition to the anticipated flight time.

Maybe so, I'm not going to argue one way or the other over the legalities. But I would never send an aircraft up without adequate fuel to fly the load plus enough to get to an alternate strip in case something closes the field, plus a reserve on top of that.

4 hours ago, nwt said:

This hasn't been brought up in a long time, but I just wanted to add that IMO only 30 minutes total fuel is required if you are taking off and landing at the same airport--not 30 minutes in addition to the anticipated flight time.

 

Do you have any backup on that?

Because that's not what I was taught. 

"To the first intended point of landing" includes what ever flight is planned between takeoff & landing, even if it's at the same point.

For example, a sightseeing flight, planning on covering a set distance (in a known amount of time) is required to have the fuel for that intended flight plus reserves. 

A load carrying jumpers is required to have the fuel for the planned flight (climb to altitude) plus reserves.

Just because the flight ends at the point that it originates doesn't change the fact that the flight plan includes time & distance covered.

3 hours ago, gowlerk said:

Maybe so, I'm not going to argue one way or the other over the legalities. But I would never send an aircraft up without adequate fuel to fly the load plus enough to get to an alternate strip in case something closes the field, plus a reserve on top of that.

I wouldn't argue against that.

1 hour ago, wolfriverjoe said:

Do you have any backup on that?

Because that's not what I was taught. 

"To the first intended point of landing" includes what ever flight is planned between takeoff & landing, even if it's at the same point.
 

It doesn't say that. So unless you have some other reference such as an FAA interpretation or something, I don't see why we should believe it means that.

For a typical skydiving flight, you're at the first intended point of landing for the entire flight. I could see that maybe being different for a sightseeing flight.

Also, battery reserve conveniently can double as the same “reduced battery wear and tear” SoC charge cycle and battery sizing planning.  Skydives never going below 30-50% battery bar.
 

Whatever is below 30% battery can be designed to be enough power for level flight for 30 minutes.  Also, it can potentially do some regenerative (gliding downwards recharging battery a bit) as battery motors typically cam double as generators.

Interesting idea.

Having the reserve being a part of the battery charge that ordinarily would be used anyway.
Hard on the battery if you use it, but it shouldn't ever be needed.
Reminds me a bit of the old "emergency military power" thing in some of the old warbirds.
There was a 'protected area' of the throttle travel. If the pilot pushed that far, it broke a safety wire. The engine would need a comprehensive inspection (teardown? I don't remember all the details) after use, but it was there if really needed. 

The 'regen on descent' part is probably not quite as useful.

Jump planes come pretty much straight down. Throttles to idle, props to beta (not talking about pistons, which have shock cooling concerns). 

Normal flight ops have descents far more of a 'cruise descent' with reduced power and a gentle descent. 

But it could prove somewhat useful in a 'run out of battery at altitude' situation. Do a 'regen descent' and have a bit of power available for use on approach and in the pattern.

26 minutes ago, wolfriverjoe said:

The 'regen on descent' part is probably not quite as useful.

The energy used to fight gravity to go up will be available on descent. It would be the same as Mexican overdrive in a truck. Coming down faster will just make it available in a shorter period of time. As long as the batteries can take a charge fast enough that is. I have no real knowledge of that.

8 hours ago, wolfriverjoe said:

Interesting idea.

Having the reserve being a part of the battery charge that ordinarily would be used anyway.
Hard on the battery if you use it, but it shouldn't ever be needed.

You meant “wouldn’t be used anyway”, but with that correction, you are correct.  Yes, the battery reserve can double as SoC management AND a rarely used reserve.

Regen is usually a freebie feature (no extra weight) in proper EV design, since inverters are bidirectional capable in some vehicles.  Might as well have it handy on your descent, it’s extra landing / go-around power.  

An electric skydiving plane that’s not in a huge hurry (slower manifest) can just descend at optimal regen glide, which might be a specific shallowness or steepness.  Could maybe be a 20% time-save in battery charging wait.  Turning a 12-minute charge wait into 10-minute charge between loads, in theory.  Even 5% or 10% would be 5% or 10% cash savings in electricity per jump!

Who knows, maybe a dollar more profit per jumper... Split the profit with the dropzone pilot as a quid pro quo to incentivize regen descents.  The slower descend time also allows a shorter between-loads charge time.

There likely be a “sweet spot” balance, on time-cost and electricity-profit, for a specific propeller’s efficiency and flight stall safety (aka maximum regen capacity).

Beyond a certain descent rate, is just wasted gravity when regen exceeds motor/battery limits and propeller efficiency given airplane propellers are not as big as wind turbines.

Although the fast steep-glide airflow compared to ground wind makes up for a lot of that deficit, the rules of the exponential curve (Double wind speed creating 8x+ power, quadruple wind speed creating 64x+ power...).  Dive hard enough and you may match that truck megawatt charger on the ground you’d normally use to recharge the plane with.  Yes, thanks to the law of exponents, sheer speed can make a (custom electric-regen-optimized blade-tiltable) Twin Otter propeller generate as much power as a full size wind turbine!

At some sufficient steep descent rate, the regen may exceed charge controller capacity and the motor simply throttle speed by its own sheer increased magnetic resistance (from throttled regen current flow), or electronics auto-idling the propeller to keep regen envelopes.

Optimal regen glide angles will be part of a DZO profit planning of the electric 2030s/2040s when some slower days pushes profit-per-jumper instead of jumpers-per-day. 

Who can say no to converting time into free electricity? (when profitable enough).

10 hours ago, mdrejhon said:

You meant “wouldn’t be used anyway”, but with that correction, you are correct.  Yes, the battery reserve can double as SoC management AND a rarely used reserve.

Regen is usually a freebie feature (no extra weight) in proper EV design, since inverters are bidirectional capable in some vehicles.  Might as well have it handy on your descent, it’s extra landing / go-around power...

 

...There likely be a “sweet spot” balance, on time-cost and electricity-profit, for a specific propeller’s efficiency and flight stall safety (aka maximum regen capacity)...

...Optimal regen glide angles will be part of a DZO profit planning of the electric 2030s/2040s when some slower days pushes profit-per-jumper instead of jumpers-per-day. 

Who can say no to converting time into free electricity? (when profitable enough).

Oh duh. Yes, "wouldn't". 

And I think you are absolutely right about that 'optimal descent angle (speed)' It won't take too much effort to calculate that. The couple of electric and hybrid cars I've ridden in (never driven one) have all sorts of displays on that stuff. Push the nose over further and further until the 'charge meter' peaks.  It will likely be pretty steep. 

The idea that a couple minutes added to the descent will end up saving both time and money on the ground will appeal to smarter operators. 

To my point above, descents in passenger carrying planes are usually not steep at all. Shallow enough that some power is required to maintain speed. A 'no power' descent is steep enough to become 'less than ideal' for many passengers. Even a normal landing pattern has some power on all the way down. 
Airplanes use the same pattern we do, and for a typical light single engine, the power is pulled back on downwind opposite the spot of intended landing, usually to 1500 RPM or so. Pulling the power all the way to idle at that point makes for a much tighter and shorter pattern. 
 

In reality, it scales a bit better than 8x+ power at double wind speed, because many blades tend to become more efficient at faster speeds, up to their sweet speed. 

Some ground wind turbines typically generate 9x better power approximately, at doubled wind speeds.  Basically the cubed bonus plus the higher-windspeed efficiency bonus (overcome inertia/friction/stiction/momentum), same reasons wind turbines can't spin anymore at half wind speeds -- blades don't spin until wind speed is fast enough.  Beyond a specific point, it tends to follow the cube, but there's a particularly large bonus-above-cubed speed magnitude above minimum windspeed to spin the blades.

Most airplane propellers are designed to be efficient at fast speeds, so the regen power (in theory) should scale better than cubed powers.   

Thusly, what regens at 50mph will be more than 64x regen power at 200mph in an airplane.  And, thusly, you can turn 10 kW regen to a megawatt regen simply by slightly more than quadrupling wind speed. 

A plane will have to respect approved airspeeds though, say, ~200mph, and the whole airplane has its own terminal velocity for idled dives too.  And regen will add slight more air resistance (a few percent less terminal velocity). 

So terminal velocity or maximum approved air speed may be reached before max regen capability depending on airframe design and propeller design.  Then by all means, the existing jump plane return dives, are already at optimal regen angle -- it is quite possible.

Although I am not sure what the optimal regen pitch for propellers is; the blade pitch adjustment range might or might not need to be much wider than a regular propeller -- so electric replacement propellers optimized to also have a regen envelope -- might (or might not) need a pitch not available in existing pitch-adjustable propellers.  Efficiency for takeoff and powered flight is far more important than regen, so some compromises will happen for regen. 

If current blade geometry is hugely suboptimal for efficient regen, then a double propeller system (theoretical foldable or idled-by-default regen-optimized blades behind the regular blades) might someday later play a role, if regen outweighs the weight cost (which I doubt at this time).  Long term (cleansheet designs), possibly ultra-lightweight regen-optimized blades folded flush against fuselage could automatically latch onto the shaft when it unfolds (via a clutch mechanism that occurs only when unfolded to allow it to spin up safely after an interlock ensures the main propeller is idled), allowing the same shaft to double as a RAT (ram air turbine) with little weight penalty. 

Blade geometry, obviously, will be optimized for powered operation.  But adjustable pitch range improvements might in theory be beneficial for regen capabilities.  But for now, realistically, existing propellers will just undergo minor modifications (if any) to properly accomodate "bonus-regen" only as a secondary priority after max-efficiency powered use.  The propeller engineers of ePlanes will know better than I on regen requirements.

Regen is already accomplished on an electric airplane (Pipistrel electric trainer at Pitt Meadows, and also I think in Vancouver too) and was claimed to recover 13% of energy of the ascent.  But it didn't work as well in normal use by students, except in steep dives scary to students, where you need to use speed brakes.  BUT....steep dive regen is what you want for experienced skydiving jump planes!   13% is a lot of money savings.  Imagine in-flight manufacturing 13% of your avgas...

Regenenerative capability should be a part of any electric jump plane retrofit (Texas Turbine style) for quite obvious economic reasons...

On 8/18/2017 at 7:23 PM, riggerrob said:

A couple of companies are developing 300 horsepower diesels to replace the Continental O-470 and TIO-520 engines currently powering Cessna 206 jump planes. Development is slow, but there are thousands of privately-owned Cessna's, Mooneys, Pipers, Beechcraft, etc. that will need to convert engines over the next 20 years.

Riggerrob, you wrote this in year 2017.  Any comments on the 750hp Magnix electric conversion kit that's lighter weight and reportedly more powerful than the 900hp Garrett conversion in some aspects?  

It apparently reportedly has faster and smoother rev-up torque (good for takeoffs, go-arounds and emergency manoevers), reports are that it is very "slam-the-throttle-forward" friendly compared to avgas powered motors -- doesn't lurch or do any weird rev-releated vibrations and never sounds damaging to the motor -- it just instantly and surprisingly smoothly revs up to max power almost instantly, suddenly spooling-up much faster than any piston or turbine engine -- with no hint of needing maintenance afterwards.  

Now that an electric Cessna Caravan has been converted, I think it kind of changes the ballgame when a potential ePlane conversion might have better economics for jump pilots, especially future implementations with 10-15% regen recovery (refuel on descent).

Considering that with the new Tesla 4680 battery format, for the capacity required of a single jump flight, a 12 minute partial charge times (one loadful at a time) are now possible between loads!  So you don't need a full day's charge, and now you only need a much lighter battery.

The ability to design a battery size to deplete only 25% for a typical jump + hopefully FAA mandated reserve for more than 30 minutes of level flight, it is now completely unnecessary to preinstall a battery larger load than one jump + reserve. 

Multiple spare human-liftable standardized cargo batteries -- 10kWh now fits inside a suitcase smaller than checked baggage -- in human-liftable military boxes that can be strapped down under the bench seat for ferry flights such as loaning to other dropzones. or doing specialized jumps such as HALO.  Store them (and use them as hangar Powerwalls) when not in use.  With Tesla hurtling batteries to $62/kWh by 2030 and $30/kWh by 2040, the numbers starts to compete with normal conversion kits.

So many solutions are happening that ePlane conversion kits likely will quickly become as economical as the usual re-engining -- I now expect inexpensive electric jump plane conversions around 2030-ish based on what I now know.

On 10/18/2020 at 2:08 PM, mdrejhon said:

In reality, it scales a bit better than 8x+ power at double wind speed, because many blades tend to become more efficient at faster speeds, up to their sweet speed. 

Some ground wind turbines typically generate 9x better power approximately, at doubled wind speeds.  Basically the cubed bonus plus the higher-windspeed efficiency bonus (overcome inertia/friction/stiction/momentum), same reasons wind turbines can't spin anymore at half wind speeds -- blades don't spin until wind speed is fast enough.  Beyond a specific point, it tends to follow the cube, but there's a particularly large bonus-above-cubed speed magnitude above minimum windspeed to spin the blades.

Most airplane propellers are designed to be efficient at fast speeds, so the regen power (in theory) should scale better than cubed powers.   

Thusly, what regens at 50mph will be more than 64x regen power at 200mph in an airplane.  And, thusly, you can turn 10 kW regen to a megawatt regen simply by slightly more than quadrupling wind speed. 

A plane will have to respect approved airspeeds though, say, ~200mph, and the whole airplane has its own terminal velocity for idled dives too.  And regen will add slight more air resistance (a few percent less terminal velocity). 

So terminal velocity or maximum approved air speed may be reached before max regen capability depending on airframe design and propeller design.  Then by all means, the existing jump plane return dives, are already at optimal regen angle -- it is quite possible.

I think you are looking at this wrong. Wind turbine energy production scales in proportion to velocity cubed because the actual energy content of the wind scales that way. In our case, we have a fixed amount of energy available for recovery, in the form of gravitational potential. The more quickly we recover it, the more quickly it is depleted--you don't get more of it simply by burning through it faster.

Overall efficiency is going to be a summation of multiple factors. Propeller efficiency vs. speed as you mention is an important one that I don't have a good intuition for. Another one will be minimizing losses through the energy cost of flight, which would seem to favor getting the flight over with as soon as possible. Another one will be the cost of drag, which scales with the square of wind speed... though maybe this point is not distinct from my previous one.

On 10/17/2020 at 10:59 PM, mdrejhon said:

Beyond a certain descent rate, is just wasted gravity when regen exceeds motor/battery limits 

I think the amount of energy we recover is going to be sufficiently small compared to the energy we've spent, that this will not be a factor even though we are recovering it in a shorter period of time than we are spending it.

edit:

One last point: As people have mentioned, harvesting this energy will necessarily increase drag because energy is never free and it has to come from somewhere. However, this may actually work to our advantage by allowing an even faster descent. As has been mentioned, the descent is limited  by max airspeed. The consequence of that is the more drag you have, the steeper angle you can dive at to obtain that max speed, which results in a faster vertical rate.

edit 2:

Another realization: A good chunk of gravitational potential energy is going out the door on jump run.