Backups- electric and jet for single engined airplane

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Posted (edited)

I thought I'd start a thread on this to avoid thread drift elsewhere

I've read a bit of people's experiments. I'll summerize what I wrote into an off topic post :

electric primary, electric backup, small turbines backup

Electric is coming. but what battery? here is a discussion

As a primary source of powered flight I think we are a long way off :

60kW (80HP) direct drive electric motors are easily available, approx 4HP / kg. they run on 350-450V DC with a specific driver.

To look at battery requirements, lets look at cruise at 75% = 45kW. 1 hour at 45kW = 45kWhour (energy requirement)

However batteries....LFP (the friendly ones) energy density is approx 0.16 kWh/kg. So 45kWh worth of batteries is 45 / 0.16 = 281 kg of batteries.

There have been some endeavours down having a lightweight ICE ;like Wankel/rotary driving a generator- IE take advantage of the high energy density of gasoline .

How about a electric motor as a backup ?- to extend glide distance. Let's call it 50% of full power- - or 30kW. - approx 10kg of motor-controller.

does the motor hang underneath with some sort of high blade count small diameter prop, or clutched onto the main prop shaft with some sort of ICE engine seize clutch release ? I dunno

If we need 30kW for 30 minutes for our backup thrust, now we are looking for 15kWh of batteries = 93kg of batteries. Hmm still too much ! That's a paying passenger (or at least the person buying lunch at the destination)

However given that the backup is a one shot- we do not need to use rechargable cells. there are primary batteries (non rechargable) that have VERY high energy density

One example - Aluminium fuel cells. I used these on project 20 years ago. 1.2kW/kg . So now our 15kWh requirement above is down to 12kg of batteries. Now we are talking.

They are CHEAP but they are one shot. The water gets poured into the empty plastic container of anhydrous potassium hydroxide (powder) and its ready to go, no matter how long it has been sitting around.

There are a few other limitations, like maximum power per kg, which likely limits minimum battery size to ~ 25kg. There are a few other twists, that's just an example of the use of primary cells. (because it doesn't need to be rechargable)

Onto small Jets !

Thrust for something like a J170 with 2200 PP and prop is about 128kg ( I think !) . There are small turbo jet engines, weighing 3kg with 31kg of thrust

That's 25% of your 100% thrust, but still very useful to keep you a little longer in the air. They consume about 1 litre per minute of Jet A, diesel or kero. so 30 minutes is about 23kg of liquid fuel

There are a few manufacturers, varies from 4000 to 6000 USD. all self contained, just add fuel.

That seems like a fairly simple solution hung underneath the engine between and fore of the main gear. I will leave the balance , thrust, heat and other problems to someone else. oh hmm noise wheel in the way ? minor details !

discuss.

Edited by RFguy
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I'll be the kill joy to the entire concept - not just the delivery options:

Any form of back-up power to cover failure of the primary motive power unit is at all time, when not in use, dead weight and comes straight off the useful load for people/baggage/fuel for the primary engine.

If you are looking at an RAAus registered airframe you are looking at up to 600kg MTOW only and generally your passenger/luggage/fuel load for the primary engine without extender is in the 200-250kg range for a two seater - around 140kg for a single seater.

No option you offer for delivery of an extender/backup power option will come in under the weight of an installed ballistic airframe recovery parachute - generally less than 20kg installed.

If I had to have 20kg lopped off my lifting capacity for the purpose of emergencies I'll take a full ariframe recovery system over a sumplimentary engine set up - more options to use in emergencies and less maintenance and system to go wrong at a critical time.

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agreed, about as good as it is going to get with the above examples is 25-30kg

The turbojet option is probably the most useful. The electric motor might be useful IF the prop and clutch and engine and motor were integrated. otherwise a motor + small ducted fan - prop is all extra drag unless it is contributing.

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20 kg is way to much for emergency power.

Now, 6 kg for effectively 150 hp for climb and takeoff sounds *superb* fun and value for weight. If the jet was just behind the propeller, could you get a cheaper and even lighter ram jet happening?

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What is the logic in having it? If the engine stops you glide down. If you extend the glide, how does that help. You can always find a situation where you needed another 300 m to reach the paddock. Its like saying if only I has another day of annual leave I could get much more done.

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The intention would be able increase the glide time/ distance . at 3000' AGL and 10:1 glide will be approx 5nm, approx 4.5 minutes.... I know it is not that simple because of changing density, temperature etc, not allowing for height and time lost to a lineup on the ground.

Quadrupling that time I think would be useful. just ideas....

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The intention would be able increase the glide time/ distance . at 3000' AGL and 10:1 glide will be approx 5nm, approx 4.5 minutes.... I know it is not that simple because of changing density, temperature etc, not allowing for height and time lost to a lineup on the ground.

Quadrupling that time I think would be useful. just ideas....

If only the engines would quite when you’re cruising around at height ... time to decide it’s needed + time to get it running = bad idea

If you have a bang handle it’s available on around 3seconds after you’ve pulled it.

Extender options are not viable - they are dead weight and complexity plus extra non-regular processes to get right in a stressful situation.

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Posted (edited)

Bang handles probably have the best overall utility. Maintain your engine as best, and for all the other reasons you cannot think of, there is the BRS....

There is a part of me that thinks BRS encourages moral hazard . But I think you will find if my wing gets cut off in a mid air collision, I probably wont think that !

This has been covered numerous times in other topics... I think I'd rather fly to a clear spot on engine fail than pull a chute... and I wont be flying ANYWHERE when I cannot glide to a clear spot. And I wont spin it. Of course famous last words... ha ha.

Edited by RFguy
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An engine for emergency use only is hard to justify. You'd be carrying it as a dead weight for an average of what 4+thousand hours to extend your glide range a bit. At least in a twin it's working for it's existence.. Planes do not plunge/fall out of the sky when an engine quits. Most engine failures would be on take off when they work hardest and you need them the most to keep running as the ground is close and you must react quickly.. Nev

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Looks promising, they finally got the LiS battery out the door.

90kWh battery .. 400Wh/kg IE 0.4 kWh/kg =

225kg for the battery

as for the range- yeah 2 hours at 45kW (2x45=90) with NO takeoff and NO reserve.

45kW = 60hp of course. IE 75% for 80hp ICE engine

a little bit of hype, stretching a bit

but good progress. They need to DOUBLE the energy weight density up to at least 800-1000Wh/kg. IE up around 1kWh/kg. Then for 45kW cruise, that is 45kg per hour of flying in cruise.

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Two-engine planes have the second engine for emergencies. Electric power makes sense to me for a second engine, there to assist take-offs and be there for emergencies. The main thing about safety following an engine failure though is altitude. The difference between being initially at 5,000 ft compared to 1,000 ft is enormous, just work out the difference in sq km of land to find a landing place. Don't forget that the last 300ft is only good for lining up the "landing". But ten minutes of electric power sure could help to find a better crash site, even starting from low.

I agree about weight being a problem. Personally, I would choose to have the extra power rather than a parachute on the plane. But that's just me.

In the meantime, I'm hoping that good maintenance makes the plane sufficiently reliable that it will not turn into a glider.

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Two-engine planes have the second engine for emergencies. Electric power makes sense to me for a second engine, there to assist take-offs and be there for emergencies. The main thing about safety following an engine failure though is altitude. The difference between being initially at 5,000 ft compared to 1,000 ft is enormous, just work out the difference in sq km of land to find a landing place. Don't forget that the last 300ft is only good for lining up the "landing". But ten minutes of electric power sure could help to find a better crash site, even starting from low.

I agree about weight being a problem. Personally, I would choose to have the extra power rather than a parachute on the plane. But that's just me.

In the meantime, I'm hoping that good maintenance makes the plane sufficiently reliable that it will not turn into a glider.

The whole idea of the 45kt stall assumes you are not going to crash, you, if you ace it are going to touch gound at 45 kts and that speed will quickly decay if you hit a rut or catch a wing, so you are then left with what solid object is in front of you and that won't be an impenetrable forest if you don't fly over pand you can't land on. When you think that through and apply the intentions of a forced landing it beats dropping a passenger to carry a spare engine or relying on a BRS.

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If you want belt and braces don't fly aircraft register-able with RAAus. In fact, don't bother flying at all. Plenty of twin engine crashes prove that.

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I never worry about engine failure in the glider. And after 20 years of worry about where I would go if the engine stops ( and it never has) then I'm getting less worried. It is still nice to have a glide to a paddock though and I get angry when prohibited unused airspace above me stops me flying at a safe height in the Jabiru. Why am I the voice in the wilderness about unsafe low altitudes forced on us without good reason?

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I never worry about engine failure in the glider. And after 20 years of worry about where I would go if the engine stops ( and it never has) then I'm getting less worried. It is still nice to have a glide to a paddock though and I get angry when prohibited unused airspace above me stops me flying at a safe height in the Jabiru. Why am I the voice in the wilderness about unsafe low altitudes forced on us without good reason?

Because we know the story. I’m the one in the Qantas/Jetstar and several times have come in so low I could tell if Popeye’s Captain had a moustache.

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I like this as a concept, but they should have made a new gearbox as a further develpoment incorporating the motor as a "built-in" component. This would save weight and bring the c of g back in a bit.

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Perhaps more suitable for another thread, but I've always thought electric turbo chargers for take off and climb could work on aircraft....

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Perhaps more suitable for another thread, but I've always thought electric turbo chargers for take off and climb could work on aircraft....

Downunder, Elaborate ?, like an electric motor driven supercharger ?

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Yes, electric supercharger is more accurate.

It's just a concept I think about occasionally.

In a fixed pitch prop at WOT, rpm reduces on take off and climb.

A supercharged boost in rpm, hence power, would allow shorter take of distance and faster climb out.

Either a standard climb angle at a faster speed or greater angle at published climb speed.

Batteries carried would allow X minutes of use and be recharged by the engine.

I wonder if an inline RC EDF motor/unit (or two) could be used? Being smaller and lighter than a conventional turbo type turbine and housing.

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Yes, electric supercharger is more accurate.

It's just a concept I think about occasionally.

In a fixcould be used? Being smaller and lighter than a conventional turbo type turbine and housing.

To start we need to understand the work to be done- as to figure out what are practical motor sizes for the compressor. Also, what sort of air charge or boost do you want ?

Who knows how to do these calcs off the top of their head ? IE work done at some flow rate at some altitude add some pressure downstream? Using a turbo housing and direct drive that with a motor might yield convenient mechanicals.

How are engines rated ? adding another 30% to an engine at max rating, how much margin you have, how much gas charge you can add without breaking something or overheating something would depend alot of the engine...and design assumptions and specs.

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Posted (edited)

For "electric assist ", I would think using a ducted fan and take advantage of the high electric motor RPM capability- because it is all weight -

-you can increase the thrust with higher rpm with no weight cost.

here is an EDF with 25kg of thrust . about 1/5 of TO thrust.

now we're talking ! this has legs. 15kW max input. let;s got 10% below as not to run everything at max stress..... 13.5kW. 13.5kW for 10 minutes = 2.25kWh. LFP at 0.16Whkg = 14kg battery. EDF = 12kg.

in practice, the battery will need some extra discharge capacity , depending on the type of cells.

Edited by RFguy
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actually it weights 3.5kg, not 12kg .

14kg battery + 3.5kg EDF = 17.5kg solution...

what has to be worked out now, what is the minimum thrust that is USEFUL for glide range extension.

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Downunder, Elaborate ?, like an electric motor driven supercharger ?

That's what he meant.

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