4 ms·

And why they are so expensive. General aviation is still running on pistons. Not because small jet engines can't be built, but because they don't get cheaper a

And why they are so expensive. General aviation is still running on pistons. Not because small jet engines can't be built, but because they don't get cheaper as they get smaller. 6-passenger bizjet sized engines seem to be the lower economic limit. Williams tried and tried. They built good small jet engines, all the way down to jetpack size, but those never got cheap.[1] There are "very light jets", but the smallest in production, the Cirrus Vision Jet, is around US$2 million. [1] https://en.wikipedia.org/wiki/Williams_International https://en.wikipedia.org/wiki/Williams_International [2] https://en.wikipedia.org/wiki/Very_light_jet https://en.wikipedia.org/wiki/Very_light_jet

hnuser123456 2y ago
I wonder, as batteries and electric (BLDC) motors get better and better, if we will find applications where electric ducted fans outperform (electric driven) propellers, since electric motors are the same complexity regardless of application.
- geocrasher 2y ago
  Ducted fans are by nature less efficient than propellers. This is one reason that the next big leap in engine efficiency may come from what are essentially unducted fans.
  - Manuel_D 2y ago
    I thought adding a shroud to propellers increased efficiency? That's why we use turbofan engines instead of turboprops.
    - consumer451 2y ago
      One of the latest designs appears to do away with the duct/shroud, for the thrust creation part of the engine. It also hopes to achieve a 20$ decrease int fuel usage. https://en.wikipedia.org/wiki/CFM_International_RISE https://en.wikipedia.org/wiki/CFM_International_RISE https://www.cfmaeroengines.com/rise/ https://www.cfmaeroengines.com/rise/
  - 00N8 2y ago
    Doesn't this vary for different cruising speed targets? I thought jets & ducted fans were more efficient above 400 kts or so, while (non ducted) propellers were more efficient below maybe 300 kts. But I'm mostly thinking in terms of turboprop vs. turbofan designs - not 100% sure if it applies the same way for electric types, although I assume it probably would.
jabl 2y ago
There are a couple companies working on 'cheap enough'(?) turbines in the GA size category. https://turb.aero/ https://turb.aero/ (latest news is from March 2023, not sure the company is still afloat?) https://www.turbotech-aero.com/ https://www.turbotech-aero.com/ Interestingly, the turbotech engines at least are recuperated engines, which is kind of unusual. But they claim it's necessary to get decent efficiency of such a small engine.
nostrademons 2y ago
Curious why the Wankel engine never took off in general aviation? A lot of the advantages of the Wankel engine seem like they'd be even more important in aviation: high power-to-weight ratio, can fit in small spaces, higher RPMs, no vibration, can use lower-octane fuel, reacts quickly to increased power demand, etc. The disadvantages - poor efficiency, poor emissions, and maintenance issues with the seals - are pretty big, but it seems like they'd be less of a problem with general aviation, where planes are used a fraction of the time that a family car would be and already have significant maintenance expectations.
- potato3732842 2y ago
  >and already have significant maintenance expectations. Exactly. It's bad enough with conservatively engineered piston engines. Adding apex seals is gonna make it a whole lot worse.
- labcomputer 2y ago
  They didn’t catch on for all the reasons you state: * High RPMs are bad in a aero engine. There are very few (no?) propellers on GA airplanes which operate above 3k RPM, so you need a reduction gearbox. That cuts into your weight savings and also reduces reliability. * Vibration isn’t a significant concern for GA airplanes. * Throttle response is not a significant concern in GA-sized reciprocating engines. * Poor efficiency is a major problem because 1 lb of extra fuel is 1 lb less payload. All airplanes are limited by takeoff weight. (The 3000 HP-class radials built by Wright and P&W at the end of WWII are some of the most efficient reciprocating engines ever built) * Maintenance is a huge concern for GA owners because labor costs $200/hr. Airplanes with 1200 TBO engines sell for a noticeable discount to airplanes with 2000 TBO engines.

hnuser123456 2y ago

I wonder, as batteries and electric (BLDC) motors get better and better, if we will find applications where electric ducted fans outperform (electric driven) propellers, since electric motors are the same complexity regardless of application.

geocrasher 2y ago
Ducted fans are by nature less efficient than propellers. This is one reason that the next big leap in engine efficiency may come from what are essentially unducted fans.
- Manuel_D 2y ago
  I thought adding a shroud to propellers increased efficiency? That's why we use turbofan engines instead of turboprops.
  - consumer451 2y ago
    One of the latest designs appears to do away with the duct/shroud, for the thrust creation part of the engine. It also hopes to achieve a 20$ decrease int fuel usage. https://en.wikipedia.org/wiki/CFM_International_RISE https://en.wikipedia.org/wiki/CFM_International_RISE https://www.cfmaeroengines.com/rise/ https://www.cfmaeroengines.com/rise/
- 00N8 2y ago
  Doesn't this vary for different cruising speed targets? I thought jets & ducted fans were more efficient above 400 kts or so, while (non ducted) propellers were more efficient below maybe 300 kts. But I'm mostly thinking in terms of turboprop vs. turbofan designs - not 100% sure if it applies the same way for electric types, although I assume it probably would.

jabl 2y ago

There are a couple companies working on 'cheap enough'(?) turbines in the GA size category. https://turb.aero/ https://turb.aero/ (latest news is from March 2023, not sure the company is still afloat?) https://www.turbotech-aero.com/ https://www.turbotech-aero.com/ Interestingly, the turbotech engines at least are recuperated engines, which is kind of unusual. But they claim it's necessary to get decent efficiency of such a small engine.

nostrademons 2y ago

Curious why the Wankel engine never took off in general aviation? A lot of the advantages of the Wankel engine seem like they'd be even more important in aviation: high power-to-weight ratio, can fit in small spaces, higher RPMs, no vibration, can use lower-octane fuel, reacts quickly to increased power demand, etc. The disadvantages - poor efficiency, poor emissions, and maintenance issues with the seals - are pretty big, but it seems like they'd be less of a problem with general aviation, where planes are used a fraction of the time that a family car would be and already have significant maintenance expectations.

potato3732842 2y ago
>and already have significant maintenance expectations. Exactly. It's bad enough with conservatively engineered piston engines. Adding apex seals is gonna make it a whole lot worse.
labcomputer 2y ago
They didn’t catch on for all the reasons you state: * High RPMs are bad in a aero engine. There are very few (no?) propellers on GA airplanes which operate above 3k RPM, so you need a reduction gearbox. That cuts into your weight savings and also reduces reliability. * Vibration isn’t a significant concern for GA airplanes. * Throttle response is not a significant concern in GA-sized reciprocating engines. * Poor efficiency is a major problem because 1 lb of extra fuel is 1 lb less payload. All airplanes are limited by takeoff weight. (The 3000 HP-class radials built by Wright and P&W at the end of WWII are some of the most efficient reciprocating engines ever built) * Maintenance is a huge concern for GA owners because labor costs $200/hr. Airplanes with 1200 TBO engines sell for a noticeable discount to airplanes with 2000 TBO engines.