An affordable twin???
 

Posting a reply on another thread about twins prompted my failing memory about this What - A Twin-Engine Kit Aircraft?. I hope Chris Heintz finishes the development of this before he finally retires!! I wonder if he chose the name Gemini to reflect that lovely, affordable (ish) twin built by Mr. Miles.

I think there could be a market for this amongst the safety concious who would prefer a twin for regular overwater / montain flying. Does anybody else agree or is the market completely catered for by the Vans family and their assorted high end kit built cousins?

See this thread on the CH601

http://www.pprune.org/private-flying...ht-banned.html

I think the problem is that the extra safety of a twin is illusory in practice in the GA context.

I have just 2 hours ME (against nearly 1k hours SE) but everybody knows there are all kinds of issues with twins. You need a lot of currency for it to be statistically safer, having two "cheap" engines (as in your URL) rather than one big old sturdy one is not necessarily going to be safer at all, there are complex issues with SE performance (not just at low speeds but also stuff like the SE ceiling which becomes pretty relevant over high terrain which is exactly where you may want the spare engine) and finally there is the extra drag which knocks the MPG by best part of a factor of two.

And finally there is the extra training, and the need for regular training because one does have to pass the 2 yearly checkride with a 'failed' engine. But cost is a personal decision! One would just hope to be getting something tangible for that extra money.

I'd rather have one PT6 than two knackered Lycomings.

Well yes but the cost is a bit of a problem, and cheap turbines are nowhere near being even on the horizon. And if you got one cheap you still have a much worse SFC than a piston engine. They are hugely reliable but make economic sense only at high altitude.

10540

It is a shame that cheap low powered turbines are not available.
I believe such a unit was/is being built for Mooney?

On the one hand we still have antiquated old engine designs powering our piston aircraft running on expensive and hard to get hold of Avgas.

Diesel units dont appear to be meeting their expectations.

Small low powered and low cost turbines should have been the future for GA


Pace

The last post on this development is about 10 years old :confused: Don't hold you breath. There is the Tecnam twin about to go on sale, which is probably a better bet if you have around Euro 350,000 to spend :hmm:

Just a note to say that revalidation or renewal on a twin is a yearly examiner exercise not a two yearly one.

I have a modest 70hrs on piston twins, I tend to agree with IO540 in supposing that theoretically you have double the chance of engine failure with a twin. If that is the case then a single would probably be preferred as most normally aspirated twins would struggle to hold 4,000' AMSL on one engine, probably more like 3,500; which is less than the MSA over just the hills of the UK. Add turbocharging and you're probably well to the right of relatively 'affordable' compared to a decent high performance single, even then don't expect much above 8,000'.

However, I wonder if you really do have double the chance of failure in a twin. I suspect that a significant number of engine problems/failures are due to maintenance, misuse, mismanagement or missing fuel. Suggesting that if one goes, the other may well follow soon suggesting that to most practical purposes the extra expense of a twin really probably is about peace of mind rather than any significant strides in terms of safety.

An ex-RAF chap at one of the clubs I'm with gave pretty much the same summary of the safety aspects of having a second engine. Apparently some twins cannot climb on a single engine, they can only maintain height. So if one engine fails at a point where you need to climb you are stuffed.

Should read "most knackered old training type twins...";) I agree the BE76 or Seminole have pretty crap SE performance.

I'd rather fly a decent twin any day though. With a SE if the engine fails you have one option, go down. Most light twins I've flown can fly quite happily on one engine, even when moderately loaded. Turbo's make a great difference though and even the likes of a Twin Star with 3 POB and overnight baggage and lots of fuel (probably at Max UUW) will happily climb on one engine to a good altitude.

Hello!

Our flying school Seminole (non-turbo!) easily maintains 6000ft on one engine. With the exception of the Alps, you can fly anywhere in central Europe at that altitude. And other than with some more complex twins like the C340 or C421B, all the fuel on board is available to the remaining engine, so longer overwater flights can safely be completed.

At this time of the year with poor weather and early darkness, I feel much safer in the lightest of light twins than in any piston single.

Greetings, Max

Thank you Englishal for fighting against the twin bashing.

The higher up the flying ladder you go, the higher the requirements for currency, eye hand coordination and systematic handling of the craft.

Those who can only fly less than 25 hours per year should not fly twins or even complex singles, even less so if one wants to fly at altitude with oxygen or in marginal weather.

But those that can afford to fly regularly in a modern proven twin and who have the eye hand coordination, the urge to fly accurately and who by their very nature enjoy the renewal stress could do a lot worse than flying something like a good modern Seneca, a Baron or a DA42

Those people can afford to fly high over the Alps, Pyrenees, Carpats etc or at night over the sea with more confidence at speeds close to 200 knots true, often well above the weather and feel not too distant from the professional flyers.

I have found flying my twin mch more usefull than the PA28's i flew before, more enjoyable and indeed more people now fly with me.

Small "cheap-ish" turbines are available for the home built market..
Innodyn :: Aviation :: The Innodyn Turbines
Article on them here:
Aero-News Network: The Aviation and Aerospace World's Daily/Real-Time News and Information Service

Here is an RV with an Innodyne:

http://www.jeffsrv-7a.com/PLANES/innodynRV6.jpg

The biggest problem with these twin v. single debates is that they usually fail to separate the two very different things

1 - stuff which only a twin gives you

A spare engine
At the top piston end (e.g. a C421) loads of payload and speed

2 - stuff which a single can give you but is more common on twins

Full de-icing
Redundant electrical systems (see Cessna 400 for an example)
Good capability versus weather
Pressurisation

One would be a fool to be flying a twin to get a high operating ceiling for example, when any half decent SE IFR tourer can go to 20k and the turbo ones can do 25k.

If I recall right, the NTSB data shows SE turbines about 5x less likely to go down than piston twins. Now, this could be hiding a lot of factors, for example the population of SE turbines is likely to be younger and better maintained than the population of piston twins (just look around your airfield for ample evidence of this).

The extra cost of a piston twin is high. I reckon the total operating cost of a Meridian and even more so a Jetprop is going to be similar to the OC of a Seneca or something similar. Well, after the capital cost has been set aside :)

Bringing it back to the realms of affordable VFR day-only twins I can see why Chris Heintz isn't putting any effort into developing the Gemini. The extra cost of the 2nd engine isn't offset against the potentially increased risk of an engine failure. The reason I looked at this initially was that then I had the potential to be flying across to Ireland regularly and for that I would like two engines. I know the Rotax 912 or Jabiru engine that the Gemini would potentially use seem as reliable as your average car engine so the actual risk is relatively low, but half way across the cold Irish Sea I would feel greatly reassured looking past my 10-year old daughter at the spare "get out of jail free" card sitting on the wing!

the future of GA?
 

the future of GA?
On the bottom of the PDF it gives a table comparing the Tecnam with 2 rotax engines to its GA rivals, cessna 172,Cirrus,Piper,Da 40 ect

http://www.tecnam.com/PDF/schede/twin.pdf

Twins versus singles - now here is an old chestnut.

The first comment people usually make is that twins are less safe. That statement is wholly erroneous without qualification. Twins are, on the whole, used for quite different mission profiles. Far more poor weather sectors are flown in twins - because on the whole they can cope with worse weather.. The general statistics also pay regard to the experience over the last forty years. Particularly in the States it dawned many years ago that twins were susceptible to asymmetric accidents in particular, and accidents in general with pilots flying in conditions for which they had inadequate training. Twin training has improved enormously in that time and even more importantly the insurance companies had made it increasingly difficult for inadequately trained pilots to find cover. There is no evidence in the current environment that twins are less safe, and if you take account of the mission profiles flown they are more safe than singles.

There are many similarities with high performance singles. In the early years the accident rate in the Cirrus was far higher than you would have expected. Pilots with inadequate training were lulled into a false sense of security by the fancy avionics, performance and promise of a state girdling light single. Of course the Cirrus was still a single, and with its added performance less forgiving in poorly trained hands than a Cherokee. The training has improved enormously, and in the States at least the insurance companies have set the standard. The accident rate is probably now better than that for other singles. (Probably because statistically it is still early days).

It is true that the climb performance of many twins on a single engine is poor or almost non existent. It is equally true that for many twins the climb performance is acceptable. How many critical engine failures do you read about each year in the UK? Of these how many singles do well in terms of avoiding injuries?

I have quite a few hours on 42s. An engine failure immediately after take off before you have cleaned up the undercarriage is an issue, but not unmanageable. An engine failure a few moments later is also an issue but far less of one. As long as you are on the ball the aircraft continues to climb quite nicely even at MAUTW - I have flown singles that barely do much better. An engine failure in the cruise is a complete non event. I had one last year. If I had to we would have flown on for as long as necessary. Personally I don’t subscribe to the school that a failure is anything other than an emergency and I don’t subscribe to flying home on one engine - but if there is no alternative one could and would. Had I been in a single which might easily have developed exactly the same fault I would have landed in a field. With luck I would not have been uninjured, with a whole lot more luck the aircraft might have survived without a reasonable amount of damage, but probably not.

In the last two weeks two Cirrus have sadly been lost due potentially to engine failures. In once case the pilot lost his life, in the other one crew member escaped without injury, the other was injured. The aircraft is a write off. In a twin with a current pilot if the accidents were caused by an engine failure neither the aircraft or crew would have suffered any consequence.

On the whole twins carry more weight. On the whole twins are better equipped to cope with weather. That does not mean to say singles cannot be so equipped but in reality few are. Twins will typically be certified for flight in known icing. Twins will typically have boots rather than fluid which is only “lifed” for as long as the fluid in the reservoir lasts. Twins on the whole have significantly higher cross wind limits and are more stable both on the approach and in flight. If you care to fly an Aztec in turbulence, and a single in the same conditions you may well wonder why the passengers are throwing up in the single. Twins on the whole have significantly more redundancy. Twins are on the whole are faster and perform significantly better - at least when both engines are working!

Some modern high performance singles attempt to emulate twins on some of these counts but none emulate twins on all counts.

In defence of twins you therefore get a great deal more than just an extra engine. On the whole you get better performance, greater comfort, more redundancy, improved all weather capability and, with a current and well trained pilot, more safety.

One last thing - you worry a whole lot less. I know engine failures are remarkably rare. I know the engine doesn’t know it is over the sea or the Alps or it is night or there is a 800 foot base. Never the less it still doesn’t stop me worrying in a single half way to the CI in the middle of February with a cold 5 foot swell beneath that an engine failure will definitely ruin my day. I would only fly twins if safety was the sole issue - as it is singles are good fun to fly for other reasons, and a whole lot less expensive.

So to answer your question Diamond were brave enough to try and prove there was a market for a modern twin - inspite of the ill informed bad publicity with which they had to contend - I think they have been very successful in that regard if let down by the choice of engine. Tecnam will hopefully apply the same formula to powering a mdern twin with a much cheaper Rotax already used in loads of non certified aircraft. They apparently have a pretty good order book. I hope others try.

I think the DA42 was a great formula which would have eventually been very successful.

Total flow rate is identical to my TB20, at the same cruise speed of 140kt (11GPH) but you get a spare motor in that.

I say "eventually" because Diamond p*ssed off so many people and that was before Thielert went bust. The reliability was so poor that one needed the second engine! I know of a number of owners and almost none of them dared to fly them anywhere too far. Defeats the point of a twin, completely.

I am sure 5-10 years from now it will be a well shaken down airframe with reasonably reliable engines. Shame it will have taken that long. That is longer than most private pilots' flying career.

IO540 - That isnt my experience, or the experience of owners I know, so perhaps the experience of owner / operators has been variable.

I have not had any failures on the 42 (other than the engine :)) which made be nervous about going far, and the failures that have occurred have been minor. That is the experience of others to whom I have spoken. I guess with any new aircraft (and I dont mean in the design sense) there is some inevitable beding in - I have had the coms fail for example, but I think in the early days you had similiar issues with your TB20. Whether or not these issues should arise at all is debatable - perhaps it is just poor quality control. Everyone is pi**ed over the issues with the engines but that is well rehearsed. 42s have had a lot of "heavy" use with the schools and have been used and abused as only a training aircraft can. As we have discussed how well they will really stand up to the test of time once they have been left out on the pan in all weathers remains to be seen - but that is another story.

I think the 42 with twin Lycomings as now been rolled off the production line could be a great twin fro anyone needing just 4 seats.

I would be interested to know what specific reliability issues people have had with 42s?

It is the engines, mostly.

Which is more than sufficient to deter serious usage.

Ah yes IO, you are right.

You dont really want to go anywhere in a twin if you are going to spend the whole time wondering when the engine is about to quit on you. I was at FL95 from Ireland having just got on top after a climb up through solid IMC from about 1,100 feet, and I recall thinking I would prefer to make my way down with the same number of engines as on the way up. :)

In answer to the original question, how about Gabriela Irimia?:8

how do aircraft engines compare to car engines for reliability generally?

That's really not the case. The truth is you have the same chance with everything, every time. Either it will, or it won't. Period. This doesn't increase with engines, with numbers of takeoffs, with anything. Either it will, or it won't. Either it will run, or it will fail.

One would imagine with four engines we'd be having failures right, left and center with odds and chances increasing in proporition to the number of engines...but suprisingly, no.

In a light piston twin, the second engine isn't there for safety. It's there for performance. Specifically, climb performance. Many light piston twins won't maintain altitude on one engine, let alone positive climb performance, and have very low single engine service ceilings. While this is often cited as a big disadvantage for the light twin...it's a given that singles don't have many options during a power loss, either.

One of the big advantages of a twin is redundancy of systems; additional hydraulic pump, additional vacum pump, additional generator, etc. Learning to fly a multi engine airplane, or any advanced or complex airplane for that matter, is learning to fly it with engines and systems failed...learning to handle it in less than ideal conditions.

Cheeky boy!:}

Yet another fallacy, we are on a roll.

Singles have been around that can match the performance of nearly every light twin for as long as I can remember.

Even today, you would have to go a long way to find a twin that can out perform a Mooney or even an SR22 in every practical sense. I can go faster in the SR22 than I can in either the 42 or an Aztec (two generations of twins in regular use), I can fly as high as makes no difference, and whilst I might get 200 feet more of climb performance out of the Aztec it as good as makes no difference.

I doubt the second engine was ever fitted for pure performance.

However passengers and regulatory authorities (and even some pilots) take comfort from the extra engine. The CAA has never approved single engine turbines (or anything else for that matter) under JAR-ops or before for IFR ops. Passengers are on the whole horrified when you let slip there is only the one fan.

Even the multi world started out with the 60 minute rule, that evolved into any transoceanic aircraft being required to have four engines. Whilst this has evolved again into ETOPS and will shortly evolve into LROPS the second engine is considered a prerequisite for safety, and even then, under ETOPS a second engine is still not quite considered as stacking the odds sufficiently in the passengers favour, hence the ETOPS requirements which are not, as many think, restricted to only transoceanic crossings even if in practice this is effectively the case.

No body wants to carry around an extra engine for performance, even if their was any truth in the fallacy, and everyone is desperate to see ETOPS evolve in LROPS but I don’t think even the commercial world will ever persuade the general public to get in an aircraft with one engine unless perhaps they think there is half a chance of landing it in a friendly field about the size of their back garden!!

I think the idea that you have twice the chance of a failure in a twin has grown from the conception that as you have two you have two engines to potentailly fail which doesnt mean twice as likely to fail.

One safety factor of a twin which is often missed is that the engines are on the wings rather than infront of you as in a single.

Aircraft design is attrocious regarding crash protection and a head on with a single on ground contact inevitably means that giant lump of metal in front of you joins you in the cockpit!

Normally aspirated twins do have a poor single engine ceiling but something like a Seneca Five turbo charged intercooled unit has a service ceiling single engine of 16500 feet which is pretty good.

Pace

Is this yet another fallacy? I thought we were on a roll.

If an engine fails once in every 1,000 hours, and never goes beyond a thousand hours, and the failure rate is linear, then there is twice the probability of one engine failing on twin in a given time frame because both engines eventually must fail on the twin whereas the single has only got one engine to fail.

In reality there are other factors involved that even where the same engine is concerned the probability of a failure in a twin will be different from in a single. Moreover engine usually dont fail before they become either time expired or have adequate work to enable them to continue to operate on extension where this is permitted.

In reality, if you were to plot hours against failure rates (or at least problems identified requiring remedial action) would you end up with a bell curve? If so is that also why it is common to see twins with engine hours that are not the same?

However passengers and regulatory authorities (and even some pilots)


Hmmm let me think about this for a while......

.......I am flying over water at night far from land and the engine fails....

.......would I want to be in a SR22 or an Aztec? :confused:

Chuck

You have had enough thinking time .. .. ..

;)

The SR22?:ok:

The Safest Twin?
 

I have a friend with a Cessna 337 Skymaster. Under certain conditions, he does shut down the front engine and rides a good tailwind to his destination. These things are old, hard to work on, noisy, cramped,but really are quite slick in concept. Adam tried to build one and there are a few Rutan Defiants out there. Its a great idea that might be executed on a cheaper level if somebody had the time/money.

What about having the nose engine much smaller than the tail one? Use the front just for takeoff and loitering,but otherwise shut it off? Maybe make it water cooled so there is no thermal shock? Solid-fuel booster fired by a big red panic button for takeoff emergencies? What about a belly mounted micro-turbine with just enough power to maintain altitude?

Do I have too much time on my hands or what?:}

Could anyone enlighten me on the service ceiling of singles when their first engine fails?

IIRC
 

The performance calcs for the Duchess, IIRC, give about 15 or 16,000 as the maximum 'cruising' line. Can't recall what the 'service' or absolute figures are - would need to check the books - but it's not really relevant for this discussion.

What is important here is that mid channel when one donkey goes tilt - I've got another one! That's where the benefits are. A single may be faster, easier to manage, cheaper to operate but here's the real benefit - when my engine fails my DRIFTDOWN is significantly higher than yours!

Thats what I call a performance benefit. :ugh:


(And I'm not too sure I want to go long distance over water or over mountains in a Cirrus at the moment!)

This is not possible to compare.

The reason is that car engines spend most of their time at around 10% to 30% of max rated power. A typical 2 litre saloon, 130mph top speed, is running at about 30% power at 70mph.

Whereas an aircraft engine is running at 65% or maybe even 75% power continuously.

When one sees normal car engines used in real competitions e.g. rallying they tend to fall to bits really quickly. I gather the rally teams have a pile of spare engines.

Car engines have the big advantage of water cooling, which keeps temperatures under close control, but I think they are way less reliable than the old Lycos.

And they are not more efficient, either. They are much more efficient over a wide range of power settings, which is what one needs to sell cars, but if one compared constant operation at say 65%, the two would be very close.

You can't really make a comparison based on percent; the power percent for a horizontally opposed aircraft piston engine is based on propeller limitations, not what the engine can do. Aircraft engines typically operate in the 2000-2500 rpm range in cruise. So do cars, give or take. Aircraft engines are capable of far more power and far higher RPM's...the truth is that aircraft engines, while exposed to thermal stresses, generally operate far less harder than car engines.

Only seen a few so far, but... Head gasket problems seem fairly rare, unlike Thielert installations on other aircraft. Some aircraft have the odd ECU problem which seems to affect original and replacement ECUs on just one engine - possibly due to faulty injectors. Other problems are relatively minor stuff like worn autopilot gyros, faulty KAP140 autopilots, and the odd minor avionics issue, often related to the plugs & power supplies for the factory supplied Sennheiser headsets. The only other grounding fault I've seen recently was a couple of props either missing 1 of the 3 rubber protection strips, or 1 blade delaminating. I haven't got anywhere near the experience of many on here, but of the 50-odd DA42 hours I've done both in training and solo ferrying I haven't had a single heart stopping moment, only the odd "Hmmm, I wonder where this might going" query!

They are quite sensitive to decent maintenance, so can be a bit fragile if poorly maintained, but in general the work I've seen so far from a certain centre of excellence in the south west results in very few faults - and better still no new scratches on the gel coat.

Given the option of the simple and normal complex singles that I teach on, or the few light twins I've flown (DA42 and PA44) I would take the twin for pretty much every mission, for the confidence as much as anything else! The average school PA44 can be a bit limited in single engine performance at higher levels, but simultaneous twin engine failures are still reasonably rare...

Most car engines will make over 100,000 miles without any mechanical rectification. That is about 2,500 hours of operation, ignoring periods at idle, which adds to making a comparison complicated. Most aero engines will do well to get to 1,600 hours without a top overhaul, and a major overhaul by 2,000 hours. On that basis the car engine is vastly more reliable.

The engines are, as others have said, operating in very different environments.

However, my knowledge of performance cars would leave me to believe that lubrication and heat are the two most important factors in determining the life of a well built engine. While a road car may operate a great deal of its time at low rpm the aim of a race car is to operate continuously in its maximum power band. That will almost certainly mean between 5,000 and 8,000 rpm. In the case of a car add a few G in corners and it is all to easy for the engine to be starved of oil. For this reason we fit dry sumps, in some ways not dissimilar to the Christen systems on aircraft. Without these systems it is quite possible to destroy an engine in a single race. In a similar way light weight alloy car engines are hugely susceptible to excess heat. A loss of coolant will destroy a hot engine in a minute or two. We cringe when you watch novices starting the engine from cold and immediately applying throttle. Not only has all the oil found its way to the sump but it takes 5 seconds or so for oil pressure to have established even with a dry sump. In this 5 seconds a great deal of damage can be done.

The engines we use day to day in our cars are way short of the power the engine is capable of producing. The tolerances are large and the stress on the components is kept to a minimum. Despite what the advertising agencies may like to tell us the name of the game is to deliver an engine that is no where close to its peak performance capability, because by doing so it will be reliable. My 1600 engine in its usual road going guise develops less than 100 bhp, in some sportier cars that is pushed to 120 bhp. The same engine with hot cams, ported head, forged pistons and properly mapped will produce over 200 bhp - twice the power.

In some respects aero engines are no difference. On the whole the mapping is rudimentary and the engine is not designed to operate close to its potential. However, there is an equally big difference. I can easily apply +6-4g to my aero engine. I can easily chop the power, and without any thermal buffer from the water cooling system in my car engine expect the air flow to dissipate all the heat from beneath my tightly cowled engine. I can easily expect the engine to sit idle for two or three or four weeks, start the engine, and expect it to provide maximum power within 10 minutes of startup whilst also expecting it to reach its full operating temperature in the same time. In short even when we are kind to aero engines we give them a load of abuse. As to the original topic of the thread this is one of the reasons twin engine aircraft should be more kind to their engines - it is far easier to dissipate excess heat in a twin than in a single.

I reckon if you used your aero engine every day, always allowed at least 5 minutes to fully establish oil pressure, avoided shock cooling and any manoeuvers likely to restrict oil flow aero engines would do nearly as well as car engines.

The subject of "why we don't have car engines in a plane as they would obviously be so much more reliable" has been done to death on every pilot forum going :)

As Fuji suggests, one cannot do a direct comparison for a number of reasons.

But there are also others.

Car engines are designed to deliver their rated HP at high RPM, 5000-7000, while direct drive aero engines deliver a similar power at much lower RPM, say 2500 (my car and my plane are both 250HP, funnily enough) so the aero engine has to deliver an appropriately much higher torque (HP=rpm*torque). On an engine, everything incl crank stroke being equal, torque comes from piston surface area which is why my IO-540 is 8.8 litres whereas my car engine is only 3 litres.

But there is no way one could make the IO-540 rev at 7000rpm. It would shake itself to bits, I reckon. They have enough problems with dynamic stress on the crank at 2500rpm and have to add movable balance weights to reduce this.

So, the engine designs are very different.

I reckon every IO-540 would make TBO without any work, if it ran constantly at say 65% power, with good airflow. There are plenty that do make TBO in normal operation.

But I just don't believe that a car engine would run for 2000hrs at 65% power without something breaking. Not mechanically in the engine but on the ancillaries e.g. cooling hoses.

The basic mechanical reliability of some car engines is awesome. I gather Toyota spent USD 400M developing a certain 4 litre V8 engine for the U.S. market (as used in e.g. the V8 Lexus Soarer) and one just doesn't see any investment like that in aviation. But one cannot escape the fact that 99.9% of these engines will still spend 99% of their lives doing 10-30% of rated HP and that is bound to massively slant the figures.