ekimneirbo
Well-Known Member
To Auto engine or not to auto engine
Now that the thread "To aero engine or not to aero engine" has pretty well run its course,
I feel its only fair to give the other side a shot. Be forewarned that this will be a lengthy
discourse because I found with the previous thread that once discussion starts, its hard to
maintain the original theme of the thread. So I'm going to try to organize as much as I can
in the first post in order to keep the original idea to its conclusion.
When taking a finite math class many years ago, I was intrigued by two axioms. One
stated that if a coin was flipped, that every time it was flipped there was exactly a 50%
chance that a specific side would be facing up. Another axiom stated that if said coin
had been flipped 10 times and had come up HEADS all ten times....the odds were higher
than 50% that TAILS would come up on the next flip. I understand these two statements
yet they appear to be at odds with one another.
What does that have to do with auto engines in airplanes. Well, if you check certain
statistics, you will see that the odds are higher that if you adapt an auto engine to your
airplane you will crash. But then again, if you did everything correctly, the odds are that
you won't have a crash. The thing about statistics is that you can analyze them to pretty
much reach any conclusion that you wish.
Lets look at it from one of the many objective points of view that you can use.
Certified airplanes with certified engines that are professionally built, repetitively built,
professionally maintained, normally owned by experienced pilots, monitored by yearly
inspections, and subject to ADs generated by millions of hours flight time, are less prone
to crashing than an airplane built by a hobby builder, in his garage with limited resources
and virtually no previous experience. No big surprise there !....but many people will throw
out warnings that the homebuilt industry is fraught with peril...especially when you
choose an auto engine. So my opinion is that it comes as no big surprise that the accident
rate is higher in the Experimental Homebuilt world. If it was the other way around, there
wouldn't be any companies manufacturing airplane engines.
Couple that with the fact that data from the NTSB is usually used to try and make some sort of comparison.
One of the problems with that data is that once an engine is used in
a homebuilt, it is no longer a certified engine....even if it was certified up till then.
That can cause a problem when comparing the number of homebuilt crashes to the number
of certified airplanes that crash. Further complicating the process is that the FAA often
attributes the reason for a crash as being something like the pilots inability to fly the airplane....
not mentioning that the engine in the airplane had quit. Then when we actually discuss the fact
that an engine quit, did it quit because the engine failed...or did it quit because some other
component failed....such as lack of fuel caused by a clogged fuel line
or a plugged tank vent. There are often other contributing factors that aren't spelled out
when the statistics are compiled. If in the case of a clogged fuel line...would a certified
engine have fared any better than an automotive engine? So when you look at statistics
about the use of an auto engine, remember the axioms that I presented above, and that
a properly conceived and built auto engine and airplane has just as much chance of succeeding as the same airplane with a formerly certified engine does.
You have to consider a lot of things when trying to make comparisons. The statement
that usually are thrown at a builder are comments that an auto engine is not as reliable
as a certified aero engine..... This is pretty much untrue. In fact I'll stir the pot by saying
that most auto engines have superior reliability.
New Certifed engines are very reliable if well cared for, inspected regularly,flown regularly,
maintained regularly, and the ADs kept current. That is what built the rep-
utation flying millions of hours with very little catastropic failure.
New Auto engines run Billions of hours in every environment from brutal abuse to
Sunday drives with the wife and kids. Think of all the absolute worst conditions that
a gasoline engine can be exposed to. They are expected to start immediately from sub-
zero temperatures from Alaska to Iceland and perform equally well in the extreme heat
of the worlds deserts. They operate reliably and efficiently from Death Valley to the
thin air of Pikes Peak. Rain,snow,ice,hot cold,they start right up and get you where you
are going. What do you have to do to an aero engine to get it to restart on a hot or a very
cold day? Thats part of reliability....its not only about engine out problems.
Think about the engines that drive pumps endlessly...24 /7 365 and run for ten or fifteen
years with little or no maintainance in all kinds of weather. If aero engines were more
reliable, then thats what would be driving those pumps in Texas.
There are a myriad number of things that can and do affect aero engines. Some of those
things are catastropic but yet the engine sometimes continues to run and the worst hit you
take is to your wallet. The thing is, there are a lot of failure modes in aero engines that just
don't happen in auto engines.
There are also some possible failure modes in auto engines that aren't applicable to aero engines.
I just don't want someone to feel that choosing an aero engine will absolve any
concern about an engine failure....or that choosing an auto engine dooms you to failure.
Although there are some auto engines that are obviously poor choices because of inheirant
problems in their design, after a few years these problems always become documented and
and with todays internet......well known. On the other hand, it is easy to find the engines with
great reputations and proven reliability.So, proceeding on the premise that a person who is
smart enough to build a airplane should also be able to choose a good engine ....
there really isn't a reliability issue with auto engines.
Then why do auto engine powered airplanes have a worse record than aero engines?
First it goes back to the facts I mentioned above. The aero engine is a pretty good design
even though it is a very old design, but when the certified engine guys start talking about
the glowing record of aero engines, they include all the hours accumulated while these
engines were on their first TBO and in somewhat of an enclosed environment of being
professionally inspected and maintained. Thats a huge advantage for a comparison. It
pretty well skews any chance the automotive engine is going to have to prove itself in
an aero environment where every installation is a virtual prototype. It doesn't matter
if you build your airplane exactly like the plans say, its still a prototype and as such its going to
have more chance of failure than a professionally built $300,000 Cessna. So when you
get challenged by other builders throwing stats at you, just remember that its really an
unfair comparison and it probably will never be in the favor of the auto engine builder.
Now that I've talked about engine reliability, lets discuss the real problem with using an
auto engine......adapting it to the airplane and a propellor. The single biggest problem is
the propellor. A trend has grown to the point that many builders don't even think about
using an auto engine without a reduction drive of some kind. This is where a lot of problems
and eventual failures happen....thereby giving the auto engine an undeserved
bad reputation. The type of reduction drive used should be selected based on the amount
of power it will have to handle. One misconception here is that because an auto engine
will produce much more power than an aero engine if operated at higher rpm ranges, then
that must be the optimal way to adapt it. In some cases, large amounts of horsepower are
desirable. It you have a large heavy airplane and wish to travel long distances, a heavy
reduction drive may prove beneficial. If you have a smaller lighter airplane, the extra weight
of a redrive may render the airplane unflyable and excess horsepower could cause
the airplane rotate if applied too abruptly. You have to be realistic about how much horse
power your airplane can use.Are you better off with a smaller lighter engine with somewhat
less horsepower? Another consideration is tip speed. Most builders try to keep their prop at
2700 rpms and use as large a prop as possible. The large slow turning prop is the most efficient combination.
That is a true statement, but there are many builders out there who use somewhat smaller props
at a higher rpm. Now think about this for a minute. An aero engine is pretty much limited in its
rpm range by the fact that it uses humongus pistons and very beefy rods. They can actually turn
more than 2700 rpms but their reliability and longevity fall off dramatically. That being said, the
key thing to consider is that an aero engine will only gain a small amount of hp if spun at 3200 rpms.
A 320 cu in aero engine will gain maybe 15 hp. On the other hand, an auto engine is just beginning
to get into its power range and by running it at 3200 rpms, the engine could produce another 50 or more hp.
Also when considering auto engines you should consider that if you choose an engine that has several different
displacements available, choose the largest displacement available for that engine. There is virtually no weight
increase in many auto engines when using the largest displacement for that engine family. More displacement
means more available power for the same weight. My personal opinion is that if its possible to build a direct
drive combination that will work properly, then in smaller lighter airplanes that is the best course of action.
If you use the auto engine and it produces 50/75/100 more hp at 3200/3500 rpms, then even if your prop is
not as efficient, you can overcome and maybe exceed what a 2700 rpm prop will do with a lesser hp engine.
Another benefit of using an auto engine is that the smaller size of the pistons and the increased number of
them (V6 or V8) will produce a smoother running engine and be
less prone to vibration. Coupled with a properly designed direct drive and either a wooden or composite prop,
there should be less of a tendency toward any resonance problems. A DynaVipe or similar instrument should
be employed to verify the setup.
If using an adapter to bolt a prop directly to the crankshaft, its probably best to keep it as short as possible.
Personally I would probably use a splined adapter bolted to the crank, and then employ a splined shaft to drive the propellor.
A housing with some type of circulating oil supply would be used to support the shaft and employ a thrust surface in the housing or bearing.
I'm sure there are many more things to consider but this is enough for now. Going back to my original statements.....
Its like the coin flip comparison....You have two things which appear to be in conflict with one another.
Deciding to build an auto conversion means you will fall into a category that has a higher accident rate
but on the other hand, will it have any affect on you if you build your conversion properly? Will any fault
for failure be the engines failure or just the builders. Personally I think it is seldom the fault of the engine,
because they have proven reliability..........but each builders reliability may be different.
I recently looked up some information on a builder/pilot whose airplane was used for
a magazine story. The man was a retired high time commercial pilot and well respected.
He had installed an LT1 Chevy in an airplane of his own design. He used some methods
to build his airplane that were somewhat different. He flew the airplane for about ten
years until one day when he crashed. The engine conversion was still performing well
but a wing on the airplane became separated during flight. I have to wonder if this
was ultimately counted as another airplane with an auto engine that crashed? Like I
said, there are statistics, there are perceptions,and there are misconceptions.
Now that the thread "To aero engine or not to aero engine" has pretty well run its course,
I feel its only fair to give the other side a shot. Be forewarned that this will be a lengthy
discourse because I found with the previous thread that once discussion starts, its hard to
maintain the original theme of the thread. So I'm going to try to organize as much as I can
in the first post in order to keep the original idea to its conclusion.
When taking a finite math class many years ago, I was intrigued by two axioms. One
stated that if a coin was flipped, that every time it was flipped there was exactly a 50%
chance that a specific side would be facing up. Another axiom stated that if said coin
had been flipped 10 times and had come up HEADS all ten times....the odds were higher
than 50% that TAILS would come up on the next flip. I understand these two statements
yet they appear to be at odds with one another.
What does that have to do with auto engines in airplanes. Well, if you check certain
statistics, you will see that the odds are higher that if you adapt an auto engine to your
airplane you will crash. But then again, if you did everything correctly, the odds are that
you won't have a crash. The thing about statistics is that you can analyze them to pretty
much reach any conclusion that you wish.
Lets look at it from one of the many objective points of view that you can use.
Certified airplanes with certified engines that are professionally built, repetitively built,
professionally maintained, normally owned by experienced pilots, monitored by yearly
inspections, and subject to ADs generated by millions of hours flight time, are less prone
to crashing than an airplane built by a hobby builder, in his garage with limited resources
and virtually no previous experience. No big surprise there !....but many people will throw
out warnings that the homebuilt industry is fraught with peril...especially when you
choose an auto engine. So my opinion is that it comes as no big surprise that the accident
rate is higher in the Experimental Homebuilt world. If it was the other way around, there
wouldn't be any companies manufacturing airplane engines.
Couple that with the fact that data from the NTSB is usually used to try and make some sort of comparison.
One of the problems with that data is that once an engine is used in
a homebuilt, it is no longer a certified engine....even if it was certified up till then.
That can cause a problem when comparing the number of homebuilt crashes to the number
of certified airplanes that crash. Further complicating the process is that the FAA often
attributes the reason for a crash as being something like the pilots inability to fly the airplane....
not mentioning that the engine in the airplane had quit. Then when we actually discuss the fact
that an engine quit, did it quit because the engine failed...or did it quit because some other
component failed....such as lack of fuel caused by a clogged fuel line
or a plugged tank vent. There are often other contributing factors that aren't spelled out
when the statistics are compiled. If in the case of a clogged fuel line...would a certified
engine have fared any better than an automotive engine? So when you look at statistics
about the use of an auto engine, remember the axioms that I presented above, and that
a properly conceived and built auto engine and airplane has just as much chance of succeeding as the same airplane with a formerly certified engine does.
You have to consider a lot of things when trying to make comparisons. The statement
that usually are thrown at a builder are comments that an auto engine is not as reliable
as a certified aero engine..... This is pretty much untrue. In fact I'll stir the pot by saying
that most auto engines have superior reliability.
New Certifed engines are very reliable if well cared for, inspected regularly,flown regularly,
maintained regularly, and the ADs kept current. That is what built the rep-
utation flying millions of hours with very little catastropic failure.
New Auto engines run Billions of hours in every environment from brutal abuse to
Sunday drives with the wife and kids. Think of all the absolute worst conditions that
a gasoline engine can be exposed to. They are expected to start immediately from sub-
zero temperatures from Alaska to Iceland and perform equally well in the extreme heat
of the worlds deserts. They operate reliably and efficiently from Death Valley to the
thin air of Pikes Peak. Rain,snow,ice,hot cold,they start right up and get you where you
are going. What do you have to do to an aero engine to get it to restart on a hot or a very
cold day? Thats part of reliability....its not only about engine out problems.
Think about the engines that drive pumps endlessly...24 /7 365 and run for ten or fifteen
years with little or no maintainance in all kinds of weather. If aero engines were more
reliable, then thats what would be driving those pumps in Texas.
There are a myriad number of things that can and do affect aero engines. Some of those
things are catastropic but yet the engine sometimes continues to run and the worst hit you
take is to your wallet. The thing is, there are a lot of failure modes in aero engines that just
don't happen in auto engines.
There are also some possible failure modes in auto engines that aren't applicable to aero engines.
I just don't want someone to feel that choosing an aero engine will absolve any
concern about an engine failure....or that choosing an auto engine dooms you to failure.
Although there are some auto engines that are obviously poor choices because of inheirant
problems in their design, after a few years these problems always become documented and
and with todays internet......well known. On the other hand, it is easy to find the engines with
great reputations and proven reliability.So, proceeding on the premise that a person who is
smart enough to build a airplane should also be able to choose a good engine ....
there really isn't a reliability issue with auto engines.
Then why do auto engine powered airplanes have a worse record than aero engines?
First it goes back to the facts I mentioned above. The aero engine is a pretty good design
even though it is a very old design, but when the certified engine guys start talking about
the glowing record of aero engines, they include all the hours accumulated while these
engines were on their first TBO and in somewhat of an enclosed environment of being
professionally inspected and maintained. Thats a huge advantage for a comparison. It
pretty well skews any chance the automotive engine is going to have to prove itself in
an aero environment where every installation is a virtual prototype. It doesn't matter
if you build your airplane exactly like the plans say, its still a prototype and as such its going to
have more chance of failure than a professionally built $300,000 Cessna. So when you
get challenged by other builders throwing stats at you, just remember that its really an
unfair comparison and it probably will never be in the favor of the auto engine builder.
Now that I've talked about engine reliability, lets discuss the real problem with using an
auto engine......adapting it to the airplane and a propellor. The single biggest problem is
the propellor. A trend has grown to the point that many builders don't even think about
using an auto engine without a reduction drive of some kind. This is where a lot of problems
and eventual failures happen....thereby giving the auto engine an undeserved
bad reputation. The type of reduction drive used should be selected based on the amount
of power it will have to handle. One misconception here is that because an auto engine
will produce much more power than an aero engine if operated at higher rpm ranges, then
that must be the optimal way to adapt it. In some cases, large amounts of horsepower are
desirable. It you have a large heavy airplane and wish to travel long distances, a heavy
reduction drive may prove beneficial. If you have a smaller lighter airplane, the extra weight
of a redrive may render the airplane unflyable and excess horsepower could cause
the airplane rotate if applied too abruptly. You have to be realistic about how much horse
power your airplane can use.Are you better off with a smaller lighter engine with somewhat
less horsepower? Another consideration is tip speed. Most builders try to keep their prop at
2700 rpms and use as large a prop as possible. The large slow turning prop is the most efficient combination.
That is a true statement, but there are many builders out there who use somewhat smaller props
at a higher rpm. Now think about this for a minute. An aero engine is pretty much limited in its
rpm range by the fact that it uses humongus pistons and very beefy rods. They can actually turn
more than 2700 rpms but their reliability and longevity fall off dramatically. That being said, the
key thing to consider is that an aero engine will only gain a small amount of hp if spun at 3200 rpms.
A 320 cu in aero engine will gain maybe 15 hp. On the other hand, an auto engine is just beginning
to get into its power range and by running it at 3200 rpms, the engine could produce another 50 or more hp.
Also when considering auto engines you should consider that if you choose an engine that has several different
displacements available, choose the largest displacement available for that engine. There is virtually no weight
increase in many auto engines when using the largest displacement for that engine family. More displacement
means more available power for the same weight. My personal opinion is that if its possible to build a direct
drive combination that will work properly, then in smaller lighter airplanes that is the best course of action.
If you use the auto engine and it produces 50/75/100 more hp at 3200/3500 rpms, then even if your prop is
not as efficient, you can overcome and maybe exceed what a 2700 rpm prop will do with a lesser hp engine.
Another benefit of using an auto engine is that the smaller size of the pistons and the increased number of
them (V6 or V8) will produce a smoother running engine and be
less prone to vibration. Coupled with a properly designed direct drive and either a wooden or composite prop,
there should be less of a tendency toward any resonance problems. A DynaVipe or similar instrument should
be employed to verify the setup.
If using an adapter to bolt a prop directly to the crankshaft, its probably best to keep it as short as possible.
Personally I would probably use a splined adapter bolted to the crank, and then employ a splined shaft to drive the propellor.
A housing with some type of circulating oil supply would be used to support the shaft and employ a thrust surface in the housing or bearing.
I'm sure there are many more things to consider but this is enough for now. Going back to my original statements.....
Its like the coin flip comparison....You have two things which appear to be in conflict with one another.
Deciding to build an auto conversion means you will fall into a category that has a higher accident rate
but on the other hand, will it have any affect on you if you build your conversion properly? Will any fault
for failure be the engines failure or just the builders. Personally I think it is seldom the fault of the engine,
because they have proven reliability..........but each builders reliability may be different.
I recently looked up some information on a builder/pilot whose airplane was used for
a magazine story. The man was a retired high time commercial pilot and well respected.
He had installed an LT1 Chevy in an airplane of his own design. He used some methods
to build his airplane that were somewhat different. He flew the airplane for about ten
years until one day when he crashed. The engine conversion was still performing well
but a wing on the airplane became separated during flight. I have to wonder if this
was ultimately counted as another airplane with an auto engine that crashed? Like I
said, there are statistics, there are perceptions,and there are misconceptions.
