FTA: “The project aims to demonstrate up to 30% improved fuel efficiency for a typical 250-nautical-mile regional turboprop mission”
30% improvement makes much more sense.
This isn't like a hybrid car. It's a parallel hybrid, where the gas engine is just big enough for efficient cruise at altitude, and the electric motor/generator provides extra power for takeoff and ascent (or go-around power), and then charges slowly during cruise if needed.
This means that the battery is quite small and light, having only enough charge to take off and get to altitude.
I suspect that this system probably improves safety as well, if architected properly. If one or both of the gas engines fail, so long as they are not seized, that electric motor can still provide some power for diversion.
Isn't that exactly what hybrid cars (e.g. Prius) are? Extremely efficient gas engine for highway cruising, but insufficient for acceleration, which is aided by electric motors?
Both the engine and motors are used at all speeds. Particularly during highway acceleration the entire assembly rotates in the same direction.
And what if you need two go-arounds?
On an airplane, most of the energy in cruise is spent overcoming parasitic drag, not induced drag. It’s spent pushing the airmass out of the way as it moves forward, not creating lift to stay aloft.
For that reason, a change in weight does not significantly change cruise fuel usage.
Weight is still precious, but that’s because airplanes’ load are more often weight constrained than volume constrained, and capital and operating costs are such that you want to maximize the load.
If you're recharging the batteries for extra go-arounds during landing, they are as dead weight as the fuel you would otherwise reserve for that purpose. And if you have 30% more efficient engines, meaning less fuel and smaller engines, it's possible you could come out ahead, weight-wise.
> what if you need two go-arounds
I assume that a go-around requires less sustained power output than a full climb from takeoff, so you will probably get more than one go-around anyway, and we don't know how much over-capacity they're designing for. In any case, any design will require tradeoffs in safety, and having more engine-out capabilities might improve safety enough to overcome the higher risk with go-arounds.
Not saying this project is will work out or that you're even wrong necessarily (this could be the equivalent of a concept car for Pratt & Whitney).
A plane doesn’t have this luxury and needs predictable output. The fossil fuel engine either needs a sacrificial “overboost” mode for emergencies (at the cost of wear/long-term longevity), or has to be sized for full power at the ultimate cost of efficiency.
Perhaps it's not all negative: the electric portion could give a pilot a bit more glide than the gas portion dies.
Like a big-boy prius.
I do wonder if prop engines can act as "windmills" (similar to turbine engines, which often in accidents still have been found to provide a bit of hydraulic power), which means regenerative braking could be used instead of speedbrakes.
No.
Source 1: PE = mgh
Source 2: am pilot
- 13000 * 9.81 * 1000 = 127.5 MJ, to reach your altitude
- 0.5 * 13000 * (77)^2 = 38.5 MJ, to accelerate to your climbing speed.
Total: 127 + 38.5 = 166 MJ, or about 46.11 kWh
For a go around, re-accelerating from 1.3 * stall speed (85 knots / 44 m/s) to your climbing speed, and going to your missed approach altitude of 1000 m, you will need:
- 13000 * 9.81 * 1000 = 127.5 MJ, to reach your altitude
- 0.5 * 13000 * (77^2 - 44^2) = 26 MJ to accelerate back to your climbing speed.
Total: 127 + 26 = 153 MJ, or about 42.5 kWh
Also, PE = mgh is probably an not a great formula for energy cost of takeoff/go-around, as there are probably large costs it ignores (gravity loss, less efficient engine use, maybe less efficient turbines?).
For your source 2 I have no rebuttal so will have to defer to you, but would ask for an explanation.
an IFR missed approach can have you climb quite high, especially in areas with serious terrain. Example: https://aeronav.faa.gov/d-tpp/2607/00346IZLZ17R.PDF airport is at 4400 feet over sea level, but missed approach says: climb to 13,000. Also, some go arounds will lead you to have to divert to an alternate airport, getting there may require climbing high to clear terrain or gaining required engine efficiency to fly the distance.
> And the mass is less since you've expended fuel
In our theoretical aircraft with batteries, mass is the same.
> You also retain some kinetic energy but I assume that is closer to a negligible effect.
Negligible indeed.
so, unless the pilot is fighting weather it would make sense that equal throttle levels and equal pitch plans in equal weather conditions would require less and less fuel burn until the tanks are empty.
Interesting, thanks.
> In our theoretical aircraft with batteries, mass is the same.
The fuel that's expended during cruise reduces the mass.