How Much More Reliable are Electric Propulsion systems than IC?

[djjones69]

Hey guys,

Over the last couple of weeks i've been trying to decide what backup system would be more reliable; an electric or internal combustion propulsion system.

I know this is a fairly ambiguous question that relies on a number of factors so i've given some basic details below;

The backup system is to be placed at the rear of a pylon mounted nacelle and is required to supply 5kW of power for 18 minutes in the event of the main propulsion system (IC at the front of the nacelle) failing.

Electric:

The electric system will likely use a Plettenburg motor with a high voltage ESC and roughly 22lb of lipo batteries.

IC:

The IC will likely use an overpowered, say 15Hp, engine. Due to issues with starting during flight the backup IC will be running continuously from the start at idle.

+++++++++++++++++++++

I've placed just three options on the poll for simplicity but if you have any reasonings for your decisions or suggestions please post them below.

Thanks a lot,


Dan

[portablevcb]

Your question got answered when you said the backup IC would be running at idle all the time. Not good for an engine.

But, you might consider a small turboprop. Extremely reliable startup. High power in light package. Even the smaller ones are computer controlled start. If you did that I'd vote for the IC.

For reference: At a school lab we had four Solar turbines (50hp, designed for ship firefighting pumps). Hand crank to start, no electronics. They would sit for 6 months with no maintenance at all. Then they would be fired up for about 15-30 min twice a day for a week. They were installed in ~1960 (?) and they were still being used in the 1990's. Never had a failure to start. Power levels measured on the Dyno were still over 46hp.

Heck, if you use a turboprop for the main IC you proabably don't need a backup :)

And, costs are dropping relatively fast because of small UAV production.

charlie

[djjones69]

cheers for the info.

I did consider a turboprop quite early on but due to the cost (although they are dropping as you mentioned) and in reality i'm not sure that their reliability will be that great. I'll definitely have another look into them though!

The issue with running the IC all the time would be the inflight start. I'm not sure how reliable IC engines are at inflight starting. Hence this post; the best place to find info is the place where they are used.

I've contacted several manufacturers and distributers and they will not give out any reliability data. So the main question is; what system can be made to be more reliable? and what are the costs (fixed, i.e. purchase, mass and variable, i.e. replacing batteries, fuel, maintenance costs etc.)?

I'm of the opinion that it is possible to make an extremely reliable electric system, albeit heavy and expensive. The issue that has been brought up by several people with the off-the-shelf IC engines appears to be their reliability, i.e. they break often and for different reasons. I've done some FMEA analysis and found the minimal cut sets and guessed at some reliability values for both the IC and electric systems but i feel that getting feedback from people such as yourself is far more important as you have real experience with these systems.

Cheers for the post. I would add turboprop to the poll but I feel it is unrealistic for me to use at this stage.

Cheers,

Dan

[treehog]

EDIT found original starter link

http://www.rcgroups.com/forums/showthread.php?t=822181

which changes the question completely
answer if you got mega buck and military funding do electric as its the most reliable
If its commercial application with normal budget constraints then throw gasoline at the problem as thats affordable and very reliable

END EDIT

For the question above no brainier a gasser even though electric would be the most reliable its toooo expensive and tooooo heavy

Pusher and tractor twin combos are the most best but pusher is most efficient and the tractor is switched off in flight
The pylon sounds strange but can be done

develop a in flight starter system similar to what ULM powered hang gliders whatever use using small battery to supply start up solutions
Bigger gassers have already got in flight start up solutions to be bought of the shelf

Gasoline or similar fuels will have a 1% the weight of lead acid and sub 5% the weight for lipos solutions cost fractionally the cost event though gasoline global efficiency is sub 20% it will be double the lipo global power solutions in those bigger dimensions


If you got mega buck then gas turbine solutions are real push button start solutions but use gobs off fuel

Ralf

[GordonTarling]

Dan - the weakest link in the electric system is probably the ESC. Like most electronic devices, they can fail early, or go on to last a long, long time if not abused. Today's brushless motors are very reliable, particularly if you are going to choose a Plettenberg or similar. Lipo batteries are generally fairly reliable these days, but, as has been said in the other thread, A123's are probably more reliable at the moment. You haven't mentioned what sort of altitude this backup system will need to operate from, but that can also affect the reliability.

[djjones69]

thanks for the comments treehog (p.s nice name!).........the initial electric system I have designed will cost around $5000USD but that could rise quickly if I feel that higher quality components are needed. The IC propulsion system is estimated at around $2000USD. Although the electric is nearly double the cost (and weight) it will be worth it if it provides more reliability.

Here are some figures comparing the two systems to a benchmark system with no backup;

Benchmark: No backup [IC]

Maximum TO: 190.24 lbs
OEW*: 118.90 lbs
Required Fuel Mass: 26.34 lbs
Payload: 45.00 lbs (minimum)

Wing Loading: 3.86 lbf/ft2
Wing Span: 17.6 ft
Wing Area: 49 ft2

Cost: $0 (as no extra backup system)

Backup 1: IC [IC-IC]

Maximum TO: 235 lbs
OEW*: 147.1 lbs
Required Fuel Mass: 36.7 lbs
Payload: 61.6 lbs (IC backup = 6.62 lbs)

Wing Loading: 3.86 lbf/ft2
Wing Span: 19.52 ft
Wing Area: 60.9 ft2

Cost: $1800 (as no extra backup system)

Backup 2: Electric [IC-ELEC]

Maximum TO: 293 lbs
OEW*: 183.6 lbs
Required Fuel Mass: 37.31 lbs
Payload: 72.87.00 lbs (ELEC system = 27.87 lbs)

Wing Loading: 3.86 lbf/ft2
Wing Span: 21.8 ft
Wing Area: 76 ft2

Cost: $4900


Clearly, the electric system 'costs' the most in terms of both financial cost and weight (mass) cost. But is the reliability it offers in terms of diversity worth this cost?

If I add the extra IC will it actually provide that much more reliability as it will have the same failure modes as the existing IC engine and has a number of non independant failures, i.e. if the fuel mixture causes failures both engines will fail, or if there is debris in the air both carbs will take in the debris and possibly fail.

So although the electric is much more 'expensive', it appears to add much more reliability. So i'm starting to sway towards the electric backup.

GordonTarling; really good points. The ESC was one of my real problem areas. I still haven't managed to find any that can handle continuously over 50V. This is pushing my current way up and therefore reducing my efficiency.
A123's would be really great due to their far superior reliability. However I can't seem to find a supplier and i'm not sure whether i'll be allowed to rip apart a load of dewalt battery packs. Also their capacitance seems to be relatively low.

The altitude will likely be around 2000ft AGL (approx. 3000 above sea level). Can you see any issues with this sort of altitude?

Cheers for the replies and everyone who has answered the poll,

Dan

[portablevcb]

I guess I would have to understand the overall project, including budget constraints and performance requirements before making any hard and fast rules. I still think a turboprop as main propulsion would give you a higher reliability than having backup systems. Backup systems are notorious for being less reliable than the main units.

Not enough money for turboprop, then design your UAV as a twin. Much more reliable than either of the other solutions and more efficient weight wise.

If the objective is to create a 'safety' system for someone else's products, then that is a different issue.

I guess I am of the KISS side. Make the main system simple and reliable, then an overly complex backup is not needed.

charlie

[djjones69]

Hey portablevcb,

I'd say that budget constraints aren't a massive issue and it is the whole aircraft that is being designed. I also attended the school of KISS; but one of the issues with unmanned vehicles is the lack of reliability; so keeping it simple isn't necessarily working.

What exactly do you mean by 'twin', i.e, twin IC engines always running?

cheers for the reply,

Dan

[portablevcb]

I guess I have not seen reliability issues with most UAV's. But my experience has been with the 'std' ones used by the military. The 'original' IAI UAV's were quite reliable with small 'off the shelf' piston engines. The issues I have seen have been when there is a failure to perform proper maintenance.

If you want a backup I would go with a std twin engine design. That way you have several advantages. Common parts being one of them. You can put in as much redundancy as you need, including a small reserve fuel tank and dual electrics. Use a push/pull design for best single engine performance. Probably the best return on power/wt compared to your other two options.

Having said that, there is already one out there. I don't remember the designation or mfg.

charlie

[djjones69]

I agree with the issue being mainly with maintenance. Using two IC will provide common parts but will also provide more ETOPs issues, i.e. different maintenance crews will have to be used for each different IC engine. Maintenance of the electric system can be made very simple. For example a BIT could be used to continuously monitor all systems on the electric and give a yes or no light. Maintenance and fault detection with an IC engine I would imagine can be fairly complex.

Also, the existing pusher/puller is the hunter, which has proved fairly effective.

Therefore from a research point of view it is possible to argue that it will be more beneficial to investigate the possible advantages of an alternative configuration; IC-ELEC?

Dan

[treehog]

thanks for the comments treehog (p.s nice name!).........the initial electric system I have designed will cost around $5000USD but that could rise quickly ..............,
Dan


The name came about because even with this wonderful RC gear I seemed to spend more time climbing up tress to recover planes than even my free flight days something to do with the fast moving trees jumping in front of the model planes but lucky got them all back even if it took weeks


really the budgets outlined are typical hobby costs meant for hobby market with technical savy users and are way tooooo low for the harsh real world where baboon users that can break even unbreakable hammers will make guarantee claims an interesting exersize between expensive lawyers

There does exist a para glider back pack to replace the normal 15 BHP back pack motor and that was something like $7500 lipo pack for one hour on half power cant apparently run continuous so used in bursts and they already grilled a preditor motor and at generous 500 cycles for lipos that worked out at more than $50 a hour (mega bucks ) haven t found the link yet but this one has some info suspect the link is
in forum" life universe everthing "

some links
http://www.lightsportaircraft.ca/airventure-2006/electricpowered-poweredparaglider.html
http://www.marksparaglidingpages.com/forums/viewtopic.php?t=56
http://www.electricppg.com/Developments/LemakMotor/index.ht
http://www.electricppg.com/faqs.htm
http://www.nitrokillers.com/showthread.php?t=448

I wouldn't consider tackling a electric solution without a budget exceeding $50,000 :eek: and better still a bottomless pit like military to pay for it,
if its a product aimed to a off the shelf product like the typical run of the mill lawnmower product etc.

The big problem is you require HUMUNGUOS long motor run time and that a total different issue from the present early generation RC model electric solutions that exist today mostly operating sub 15 minutes at less~1/2 throttle and often are treated kindly with electric savy users

Present generation A123 and clones are way to bad in power density in this time to realistically consider for run times exceeding five minutes

Lipos are still fragile for dip stick non electric flying user public Joes to use without so many layers of safety design that it would be easy to paint yourself into a corner trying to out design the ability of the baboon factor to completely fry the electrics and send budgets orbital

Even in the real world airliner manufactures big outfits like Boing with real jets often have round electrical conduit junction boxes stuffed into square boxes from the 6 foot plus minimum wage Gorilla that works there and often requires several weeks of a hapless airlines time :eek: that bought the plane to rewire the plane so engine one is connected to engine one instead of engine three whatever (and those junction boxes were designed intentionally with super strong impossible to distort plastic and meant to be absolutely state of the art Gorilla/ idiot proof and absolutely impossible to wire the wrong way round and yet the Gorilla succeeds every time to defy :p
them and just try pulling it apart :eek: )
And thats from the so called trained assembly staff never mind the baboon public joe soap tinker with the stuff for fun factor like from the freebee bored college interns who when in dought hammer drive a screw driver through it just to verify ensure it really wont work ever again :D

IC is well known proven getting more reliable and getting more idiot proof, cheaper and nowadays probably with smaller motors nearly cheaper to replace motors with new ones than do the TBO total build overhauls

4 stroke options where only (petrol)Gasoline can be used without lubrication oil exist

I figure you will be gob smacked just how much it will cost to solve the problems using proven solutions like IC and wouldn't surprise me if it costs $50000 just to cure all IC issues never mind beating a path into the wilderness with electric

Even a three engine solution might work better
its not always necessary that all engines are the same HP

Twin IC motors of same HP and type running all the time are the norm work well on larger craft but might not work so well with smaller craft as engine size get too small to be reliable as single motor with extra solutions like twin magneto carburetor and running at lower revs and less stressed than two motors running at full tilt

Three engine combo of same HP and type running all the time are the norm work even better again on larger craft but might not work so well with smaller craft as engine size get to small to be reliable

Combinations like twins with push pull can use low power for pusher that just allow the plane to maintain height with bigger motor on front for take off or landing or reserve whatever to keep fuel use down but complex

Front engine can be left on tick over if the front engine is connected to clutch and prop is a folder type and motor on tick over will use very little fuel
but again complex
The same motor in the front can have two engine s driving the same prop with one on tick over and the third engine with electric start options to kick in when required so even more complex

Yes in the future and that might be as near as ~2 years but suspect its five years away the Electric will be possibly be the ideal affordable correct solution but without a near bottomless pit of cash to iron out the bugs and idiot proof it I seriously dought a realistic runner in this time period for power demands that exceed five minutes if you want a sell able proven product inside 6 months time frame

I would sooner trust a single well proven ULM motor of 400cc type with twin magneto deicing carburetor whatever than all other options and live with the heavier weight and design around that even if the motor is only generating 1/2 its normal HP rating and bump up fuel tanks to suit if necessary as fuel is still fairly cheap
Using time served ULM mechanics who know their onions (* see PS edit ) with well proven ULM motors figure that will win hands down in projects from ~200lbs to ~600lbs and get customers insurance to cover the engine failures or SEP (somebody else problem ) it

For UAV sub 100lbs model plane motors are more suitable and above that its rapidly leaving the RC model world equipment behind so opinions would not be up to speed in these larger dimensions as few would have experience of ~200 LBS craft


2000 to 3000 feet is the worst possible place to fly a UAV as even in warm climates this region can easily breed icing conditions hail storms etc and down a plane which has no wing heating solutions much more often than any engine out issues (** see PS info)

Absolutely expect joe public to expect UAV to fly in sub zero arctic conditions no matter what the manual says ( as they don't read manuals and simply make guarantee claims which the large retail multiples simply honor as it gets returned to you and you get the debit against your account )

Anyway keep us posted and if your in need of a tree climber or if you want to hang out on a tree branch and chew on passing parkflys give us a call
who knows we might hit the mother load and get a juicy 200LBs UAV that try's to fly through our tree and gets all hung up to dry ready for us to eat for dinner :D
A Treehog's favorite side dish starter is ZAGI followed by....fat UAV ..washed down with high nitro :eek:

Ralf

PS edit

* I knew a crop duster pilot in suitable crop dusting plane with low wing and 200BHP plus motr mayby 500 hp cant remember all the details flying at 5000 feet midday good weather some cumulus clouds typical for tropics not much wind lightly laden and no heavy spray chemicals aboard as he going to the spray site over the Jamaican island and was hit by a mega down draft
he hit the power full gas hard as possible full flaps everything possible but just kept falling and falling until the plane hit the ground flat as pancake in less than a minute and plane exploded totally disintegrated leaving him sitting in his seat surrounded buy a broken up plane and in sever pain from a broken ankle (yes another miracle) so even very powerful aircraft can be swatted out of the sky at low heights
and I know umpteen icing events ffor ULM light aircraft with less good outcomes

**I knew a swiss ULM mechanic who inspected a brand new look alike fibre glass Cessna 150 with a ROTAX 90BHP motor
He informed owner that the carburetor was done in a new way which he could not be happy to use and recommended changing it back to conventional older proven method and could fairly guarantee the new solution would cause a engine outage fairly soon and was absolutely not to be trusted even though though the manual showed the new solution to be the correct and only solution to be done.
He implored proud new ULM owner to wait for clearance from manufacturer to modify the carburetor back to the other better solution without avail
The owner declined his advice the manufactures surely know better than plane mechanic and took off
He suffered a engine outage a few seconds after take off at ~500 feet circled back and landed at the shorter runway due to inability to glide to bigger runway ran out of runway as in when he hit the hanger square on broke both legs and was lucky to be extracted before plane ignited and caught fire

Moral know your onions and if you don't buy the solution from who does know his onions and listen to his advise if he says its fundamentally flawed and I think you need a good project manager to steer you clear of this cost underestimates for mega project where costs over runs will become orbital if there not priced out correctly

Ralf

[GordonTarling]

Dan - ESC's do exist that will handle over 50V - you just have to look harder! Same goes for A123 cells - available from many suppliers now. 2000 feet is no problem, it's when you get over 10 times that height that you experience problems!

[rroback]

I think a properly tuned gasoline engine would work nearly as well as electric. I'd make sure it's tuned well, and pack an extremely high powered on board starter. By using a redundant system, you'd be adding a lot of complexity, and I'm guessing end up with more problems. Don't expect to just mount a weedwacker engine.

[treehog]

http://www.electraflyer.com/tech.html

they have a full size man carrying hanglider and the big pack :eek: cost ~$7500 and the hangglider trike combo :eek: some ~$7000 and the fold up epoxy carbon prop :eek: ~ $600 and the list of extras makes a gas version hang glider trike ook very cheap even if fuel were to cost $50 a gallon

utube film flying at

http://youtube.com/watch?v=r22UkWbG5yM&NR=1

I wont be rush out to buy one next week got to save a few $$$ first


Ralf