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Homebuilt aircraft engine choice

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Homebuilt aircraft engine choice

Old 13th Aug 2002, 16:51
  #1 (permalink)  
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Homebuilt aircraft engine choice

A group of friends and a few engineers I know are seriously considering trying to make / build a homebuilt 2 seater aircraft.Discussing propulsion.

Thing i`m..wanting to get straight,is the engines availible at the moment..well to be honest I find them primative and darnright expensive.

Our idea..surely you could tune a Subaru turbocharged engine (280 BHP stock),remove the emission rubbish,tweak the boost / timing / compression ratio,ECU and have a more reliable and lighter engine at the end of it..is this allowed???The maintenance cost would be peanuts compared to the engines about today,and fuel efficiency should be astonishing.

Do any of you people know if you can modify a car engine,Or do you have to use an "approved" aviation engine??

Just got me thinking thats all.Please let me know what you think/know.

ETOPS773 is offline  
Old 13th Aug 2002, 17:15
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Modified Subaru engines are quite popular on homebuilts in some parts of the world, I've also seen Yamaha outboard engines, BMW motorcycle engines, etc. etc. used on a variety of homebuilts.

I don't think anybody is likely to try and stop you (even in the UK, but I would ask the relevant association what guidance they have to offer), but you'll find it a much bigger job than you expect. But...

- Nobody who has had much to do with the Rotax 912 or 914 engines would particularly recognise your rather jaundiced description of it's level of development (although no, they're not cheap).
- I would never, ever, contemplate what is in effect a new engine in a new airframe design. Have a new airframe, or a new engine, but not both - that's just asking for trouble.
- PFA have a limit on engine power that they are allowed to handle in homebuilts, worth checking what it is - I suspect around 200hp.
- Just cos it worked in a car / motorbike, don't expect it to work automatically in an aeroplane, too many things change.
- You'll need a gearbox, automotive engines rev too fast for a prop.

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Old 13th Aug 2002, 18:36
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PFA limit is 260 hp.

A quick search on the web will give you access to some American sites dedicated to converting just about any type of engine for use in a homebuilt, from V12s down. Bearing in mind their EAA rules are far less stringent then ours....

Designing a homebuilt from stratch is, a massive undertaking as you'll essentially need to work to JAR design codes etc etc, and PFA Engineering like to get involved soonest.

Far better to stick to a pre-approved PFA kit, think there is now nearly a couple of hundred to choose from.

I'm waiting to see if the RV-10 gets UK approval.... Unless of course your can design something with the performance of a Lancair IV-P !

Father Mulcahy
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Old 14th Aug 2002, 01:49
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ETOPS773, I have no knowledge of this area. I would expect though that some sort of certification would be needed.

Have a look at the Jabiru website and at the very least you''ll be contributing to Harpoon's inheritance.

Last edited by Capt Claret; 17th Aug 2002 at 14:10.
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Old 14th Aug 2002, 06:26
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Just for the record ETOPS, what country are you in? It makes a lot of difference when discussing amateur-design issues.

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Old 14th Aug 2002, 08:03
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We are all from the UK.

I did notice you mentioned earlier about the Rotax engines,I rate them very highly,its the old Lycoming / Continental engines i`m not such a big fan of.Didn`t mean to sound like a snob!

As for the 260 Hp limit,that would not really be an issue,as the subaru engine develops maximum power at 7300 RPM,with a T3 turbocharger. An idea is fit a smaller turbocharger,decrease the boost and limit the revs with an electronic limiter,and tune the engine for efficiency.The reason we want a comparitively mad amount of HP,considering C150s and other 2 seaters only have 100 or so,is the design criteria is speed,and modest range.The engine would help alot with both of these aspects.

As far as we know homebuilts in the UK can only fly within the UK on a "permit to fly",so as it would not be making any foreign trips,we could divert more attention to giving the aircraft a higher cruise speed and due to this reason,the size of the plane would best be described as a tight fit!! Not too dissimilar from the profile used in a glider (reason being the smaller an object is,the faster it moves through the air,and less drag created) ,and a rear mounted pusher prop will be doing the work.

Thankyou for the replies btw.
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Old 14th Aug 2002, 08:58
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If your wanting cheap running, have you thought about a Diesel ?

Lower fuel cost as it uses stock JETA1, plus about a quarter more range on the same mass of fuel compared to AVGAS. Simpler operation, no ignition system (less Electromagnetic Noise). Plenty of companies in Europe pushing this technology at the moment and with the FADEC stuff bolted on its almost leading edge. Well, 28% MAC maybe...

When thinking about auto-conversions you'll always come up the following argument.

The majority of aviation (reciprocating) engines are made to run for extended periods of time at a relatively low rpm (say 2700).

To get the best out of a automobile engine you'll need to rev the guts out of it, say (6000rpm or higher). Would you be prepared to sit behind a car engine running at that speed for a couple of hours at 3000'. Try it in your own car, just jack the wheels up

Moreover, the really clever bit for auto conversions is going to be the gearbox bolted to the front of the engine. You'll need to get that rpm down to a manageable prop speed. Another place where any problem will be mission critical.

Hey, no-one ever said it was going to be easy. Best of luck with your design. Pity you can't call it the ETOPS773 - Boeing might have something to say....

Father Mulcahy
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Old 14th Aug 2002, 09:57
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Permit to fly rules do not exclusively limit you to the UK, they do mean that you need permission to fly abroad. There's an ECAC (European Civil Aviation Conference) agreement quoted in CAA AN52 allowing you to fly in most European countries. You will be limited to day-VMC, but not necessarily to the UK (think of the couple of microlight pilots who have managed to fly around the world on permits to fly).

I don't know my way around the PFA system so well, but there's some useful odds an ends on the BMAA website at http://www.bmaa.org/tech2.htm that may help, I'd particularly point out their TILS 018 and 029, and form 041 - my experience is that the PFA and BMAA are generally happy accepting each other's paperwork anyway for design submission backup. If you are a PFA member, they do have a lot of information sheets available you can request on their website.

You certainly don't need an "approved" engine, but the less experience the PFA have of the engine you're intending to use, the more information they'll ask for. There's an excellent ground running schedule that goes a long way to proving the viability of an engine in JAR-22.

Almost universal in automotive conversion, in my experience anyway, people have design an adaptor mechanism and used the Rotax C-type gearbox, which seems to work very well with BMW engines, etc.

I agree about Lycontinentals, give me a 912 any day.

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Old 14th Aug 2002, 10:04
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The engine would be mounted behind the cockpit (such as in a mid engined car layout),and could probably get put some sound proofing around the bulkhead/firewall,cockpit side,but yes cockpit noise would be higher than a dedicated aviation engine we`d expect.

I agree the diesel engines are looking quite good on alot of fronts,but again the aviation ones cost mega £££,a person involved can get a brand new subaru unit for under £3k shipped in from asia,so thats why we`re looking at this one in particular with so much interest.

A gearbox will be needed,but the advantage of a turbocharged engine is they can be tuned to develop alot of power at quite low rpm compared to normally aspirated engines.So I think 4500 RPM would be "screaming" for what we need.The biggest noise from what I predict would be the incuction / exhaust note,as the modern day auto engines are very quiet.

A smaller turbocharger usually gives you immediate kick and will bounce you upto 4-5000 rpm in a car very quickly,then begin to tail off.this is favourable compared to the larger ones,have quite a bit of lag..definately a "dead" done upto 2500 rpm,then begin to feed in until you reach 3500-4000 rpm and its a non stop almighty surge of power upto the red line,usually 8000 rpm. That would be useless!!

Can just imagine,O-DEER trying to go around to avoid a rogue plane rolling on the runway,only to have to wait 5-6 seconds for the turbo to kick in whilst descending...then off like the starship enterprise when the turbo kicks in...geesh!!!

Last edited by ETOPS773; 14th Aug 2002 at 10:16.
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Old 15th Aug 2002, 08:23
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Have a look at the JABIRU range of engines, readily available in the UK, and the web site is not to hard to find.

Price wise they are really in the “converted Subaru” range, rather than the price range of overhauled aircraft engines, and come in just the range of horsepower you can use. And they are light.

They are being churned out as fast as they can make them, currently 50 to 60 a month, somebody likes them.

If you have a look at the cover of the AOPA Australia magazine
(www.aopa.com.au), you will see a pic. of a very nice Jabiru powered all metal scale Spitfire.

Tootle pip!!
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Old 15th Aug 2002, 13:20
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The RAF 2000 gyrocopter uses a Subaru engine mounted behind the pilot. They have a good deal of experience with operating this engine in an aviation environment, although it often criticized for being excessively noisy. They claim 130 horsepower for the engine, with a TBO of between 800-1000 hours.

The TBO figures for the Jabiru were aound 1000 hours last time I looked. I have heard comments that the Jabiru engines are a bit light on the claimed horsepower (including Genghis on PPRuNe).

You're right about the situation with Lycomings/Continentals, they need a heavy does of FADEC, Electronic Ignition and Fuel Injection to bring them into the 21st Century - but I think by the time they get it, there will be no 100LL AVGAS for them to drink. Better going with diesel. The UK has two good diesel engines being developed. See http://www.wilksch.com and http://www.dair.co.uk. The Diesel Air unit looks very light of what you are talking about, but the WAM160 may be suitable.

Have a look at the performacne figures for the VANS RV-9 to see what 125HP gets you, before deciding on 260HP.

The German Thielert engine is also worth a look at http://www.thielert.com/en/index.html .

If your planning to put the engine behind the pilot, will you be going for a pusher configuration?

Any other details available about your plans? Mission, speeds weights, dimensions.

Last edited by tacpot; 15th Aug 2002 at 13:27.
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Old 17th Aug 2002, 12:19
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Have a look at Subaruaircraft , may have some additional helpful info, they offer a firewall forward package popular in the RVs.
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