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If you have an airplane with electric propellers for take off and landing and you have a turbofan/jet to reach higher speeds for cruise. For example, electric propellers can go to 250mph and turbofan to 550mph

  1. Do you switch off the electric propellers or leave them on ?
  2. Do they work in conjunction ?
  3. What is the effect on the propellers when the turbofan reaches 550mph ?
  4. Is the thrust added if both engines are working ?
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    $\begingroup$ imho, what could make sense is the opposite: jet for takeoff (high thrust) and electric for cruise (moderate thrust), but that electric power has to come from somewhere and with the current technology we are nowhere near to useful. $\endgroup$ – Federico Jun 2 '15 at 19:05
  • $\begingroup$ Have you actually checked if one set of props won't work for the whole flight? As was discussed by several people in your previous question on this subject, a well designed ducted fan might handle both sea-level take off and high speed cruise at an appropriate altitude. Modern turbofans handle both duties today, your only difference is electric power. $\endgroup$ – Andy Jun 3 '15 at 9:18
  • $\begingroup$ After thinking it over, based on all responses (thank you all), what i want is a normal turbofan with the front fan being able to start on electricity and once up in air switch on the noisy turbofan core combustion which then can drive the front fan as well. Again i understand it is counter productive not to use core thrust during take-off however my main objective is silent take-off and not noisy take-off. Some time economics have to suffer for comfort of people living near airfields. $\endgroup$ – Arturo Jun 11 '15 at 13:44
  • $\begingroup$ @Arturo First of all, electric motors aren't silent. Second, much of the noise is from air flow and spinning parts, not necessarily the combustion process itself. This would still happen with your proposed electric engine for the same thrust rating. Third, if you don't like the sound of jet engines, don't move next to an airport. That's a much more simple solution that spending billions of dollars in design, manufacturing, and maintenance costs while reducing safety. $\endgroup$ – reirab Sep 30 '15 at 21:12
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The B-36 Peacemaker had 6 radial engines, driving 6 propellers, AND 4 jet engines in 2 nacelles. The jet engines were used for takeoff and for "dash" over a target. The jets were turned off during cruise because they used considerably more fuel than the piston engines.

As such, they HAVE made propeller/jet hybrid aircraft in the past, though not electric.

To answer your questions:

  1. You probably want to feather the propellers and kill the electric motors. If the props can only drive you to 250 MPH, leaving them windmilling in the slipstream when the jets are running will add drag, which will only serve to hurt your cruise speed and/or increase your fuel consumption.

  2. They only work in conjunction as speed BELOW what your props can drive (eg 250 MPH, in your example). Beyond that, if it's speed you're after, you're better off without the props.

  3. At speeds above what the props can deliver, they're essentially small windmills. And not very efficient ones at that.

  4. See 2.

Now, what WOULD be helpful as a "hybrid" aircraft would be a gasoline engine, turning a prop, with an electric motor added between the engine and the prop. The electric motor could supply extra power during takeoff and function as a generator/dive brake on the prop during descent. In the event the gasoline engine failed, the prop/electric motor assembly could be declutched from the gasoline engine, providing you with temporary power to maneuver around and land safely. Gasoline engines in aircraft tend to be very reliable, but when they DO fail during take-off or landing, the most vulnerable parts of the flight, the results tend to be catastrophic. As such, the gasoline/electric hybrid would be of no use during cruise but it MIGHT improve the safety factor during the most vulnerable parts of the flight.

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  • $\begingroup$ How much drag produces a feathered propeller? Thanks. $\endgroup$ – Urquiola Jun 4 '15 at 9:35
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    $\begingroup$ Such a hybrid config would be an interesting option for the V22 Osprey - if one engine fails, switch the electric motor on the other side to generator mode. This eliminates the mechanical cross-connect which is currently needed to land after a single engine failure. $\endgroup$ – MSalters Jun 4 '15 at 11:24
  • $\begingroup$ @MSalters -- what would be even more interesting is if there were just electric motors on the wingtip nacelles and some kind of "inboard" motor somewhere in the fuselage. Something like that might make a smaller, lighter version of such an aircraft affordable by "mere mortals." Like me :-) $\endgroup$ – Meower68 Oct 6 '15 at 19:19
  • $\begingroup$ @Urquiola for an ATR-72 size/mission aircraft, I found that feathered, locked propellers (around 13-foot diameter) only added about 10-12 drag counts at Mach 0.45, 20 kft $\endgroup$ – costrom Jan 17 '17 at 20:28
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A hybrid just does not make sense on a plane for the same reason that a turboprops make sense on a plane and turboshafts do not for a car. Cars have rapidly varying load, planes do not. Also planes do not brake in the air (often), so regenerative brakes would be worthless. Overall the electric motor in a hybrid car is in effect used to smooth out the load on the internal combustion engine, and allow it to run at its most efficient rpm. In a turboprop, the variable pitch does the same thing, but much better.

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I guess there's no current experimental evidence to answer the question about an airplane with an hybrid engine and an added jet engine, but there are some publications about hybrid airplanes, such as: 'A simplified method to calculate the fuel burn of a hybrid electric airplane', R Jagannath et al, in 50th AIAA/ASME/SAE meeting, Propulsion and Energy forum, 2014

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