HONDA vtec engine
drives his diesel Citroens up to 350,000 miles before changing the car and even with those miles on the clock, the thing still sounds as good as new and he rarely has any engine problems. At an average speed of 50 mph (which is quite high average) that equates to 7000 hours
Airplane engines run at leats at 65% of their maximum.
That Citroen engine would be amazing if it would run 3500hrs at an average of 100MPH, but it won't.
With a small airplane a gearbox adds weight. weight is bad as it reduces usefull load, performance etc etc etc.
Light aircraft usually do not have an abundance of excess performance to begin with. So you are generally stuck with an engine that runs in the same RPM range as the propeller which is 2500-3000-ish.
Although there are some succesfull "gearbox" engines around:
DA40/42 w/ Thielert or Austro
Cessna 421, some models had geared engines.
Problem is aviation gearboxes do not have variable ratio.
Lets take a little example with the VTEC engine assuming we have a 2:1 gear box ratio;
VTEC 4000 RPM , propeller 2000 RPM
VTEC 8000 RPM , propeller 4000 RPM
And this is what happens when the propeller overspeeds
YouTube - Relentless mayday 9_14_2010_slow motion.mp4
Source: EAA News - Reno 2010: Relentless Loses Prop in Flight
Can't see why it should be a "no-no". Even if it is not ideal, what engine is? If the ideal engine existed, all others would disappear from the market. Aviation is all about compromises, and this engine might well be an acceptable compromise for some people in some planes in some applications.
I do think it is worth a try - if you've got a lot of time and a fair stock of mechanical ability and a well-equipped workshop available.
I do think it is worth a try - if you've got a lot of time and a fair stock of mechanical ability and a well-equipped workshop available.
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Originally Posted by IO540
I have flown about 1000 airborne hours, which is perhaps 1/2 of my total car / motorbike mileage, yet I have not had a single mechanical failure on the aircraft, whose engine sits there at 65% power all the time, and best part of 100% power for some minutes after each takeoff.
IO540 has your arithmetic deserted you or has mine? On the other hand it may be my lack of will power when her indoors is not present to govern the single malt intake, I know it's ealry, but when the cat's away!
There are two different threads going on here. One is about engines with a wide RPM range, and the other is gearboxes.
The advantage of a high revving engine is that it gets its power from its speed and not its capacity, so it can be smaller. If it has variable valve timing, of which V-TEC is one manufacturer's term for it; it can also idle comfortably without the wild valve overlaps that a 'stage 3' racing camshaft would have to get power at high RPM. To make this high RPM usable on an aeroplane propeller, which should work on the biggest volume of air as possible and keep the blade tip speed subsonic, a gearbox and, ideally, a variable pitch propeller, are required.
Now onto gearboxes. A gearbox works fine when it has a smooth constant torque coming into it and its not being asked to accelerate or decelerate. On a gas turbine, the torque is a continuous load so there isn't a problem.
On a car, between the engine and gearbox is a flywheel and a clutch or torque converter. On a direct drive aeroplane engine, the propeller IS the flywheel. The problems start when the engine and its flywheel are separated by a gearbox.
When the piston travels up the cylinder compressing the fuel mixture, this takes energy, so the engine slows down. When the mixture ignites, the engine accelerates. This results in torque reversal, which is why at a slow idle the engine rocks backwards and forwards, especially if it is on anti-vibration mounts, as the flywheel effect causes the crankshaft to try and go at constant speed, and the engine tries to rock in the opposite direction to accomodate the acceleration and deceleration.
If a gearbox is now added to the drivetrain it will try and accelerate and decellerate the propeller. The propeller being a good flywheel, takes no notice, so the changes in speed try and change the direction of the bit in between, AKA the gears. On a car if it is at a slow idle, the likelihood is the clutch is depressed, and the gearbox is disconnected from the engine.
In the aeroplane, if there is no clutch, the torque reversal is hammering away at the gears. This is worse at low RPMs as there is a longer duration and hence greater amplitude of movement (the hammer hits from further away). Some microlight engines use a centrifugal clutch, so the gearbox is only driven when the engine is above the damaging torque reversal RPM. The alternative as per Thielert is to put a torque limiter in, which is a clutch with a preset load on it. If the torque reversal gets too much, the clutch slips.
The advantage of a high revving engine is that it gets its power from its speed and not its capacity, so it can be smaller. If it has variable valve timing, of which V-TEC is one manufacturer's term for it; it can also idle comfortably without the wild valve overlaps that a 'stage 3' racing camshaft would have to get power at high RPM. To make this high RPM usable on an aeroplane propeller, which should work on the biggest volume of air as possible and keep the blade tip speed subsonic, a gearbox and, ideally, a variable pitch propeller, are required.
Now onto gearboxes. A gearbox works fine when it has a smooth constant torque coming into it and its not being asked to accelerate or decelerate. On a gas turbine, the torque is a continuous load so there isn't a problem.
On a car, between the engine and gearbox is a flywheel and a clutch or torque converter. On a direct drive aeroplane engine, the propeller IS the flywheel. The problems start when the engine and its flywheel are separated by a gearbox.
When the piston travels up the cylinder compressing the fuel mixture, this takes energy, so the engine slows down. When the mixture ignites, the engine accelerates. This results in torque reversal, which is why at a slow idle the engine rocks backwards and forwards, especially if it is on anti-vibration mounts, as the flywheel effect causes the crankshaft to try and go at constant speed, and the engine tries to rock in the opposite direction to accomodate the acceleration and deceleration.
If a gearbox is now added to the drivetrain it will try and accelerate and decellerate the propeller. The propeller being a good flywheel, takes no notice, so the changes in speed try and change the direction of the bit in between, AKA the gears. On a car if it is at a slow idle, the likelihood is the clutch is depressed, and the gearbox is disconnected from the engine.
In the aeroplane, if there is no clutch, the torque reversal is hammering away at the gears. This is worse at low RPMs as there is a longer duration and hence greater amplitude of movement (the hammer hits from further away). Some microlight engines use a centrifugal clutch, so the gearbox is only driven when the engine is above the damaging torque reversal RPM. The alternative as per Thielert is to put a torque limiter in, which is a clutch with a preset load on it. If the torque reversal gets too much, the clutch slips.
Last edited by Mechta; 4th Feb 2011 at 17:44.
Things is a car only requires what 30-40(?)HP to run a 50 MPH which is about 20% of their maximum.
Airplane engines run at leats at 65% of their maximum.
That Citroen engine would be amazing if it would run 3500hrs at an average of 100MPH, but it won't.
Airplane engines run at leats at 65% of their maximum.
That Citroen engine would be amazing if it would run 3500hrs at an average of 100MPH, but it won't.
I owned a Citroen AX 1.4 diesel, and my memory of the weekly 100 mile journey to Filton, along the M4, was having my foot hard to the floor for most of the way. The car already had 150,000 miles on it when I bought it but never gave any trouble.
You may not get 3500 hours from the Citroen engine, but you could probably throw the engine away, buy another and still have change from the price of a Lycoming.
Here's the original article in French: Gaz'aile 2
Last edited by Mechta; 4th Feb 2011 at 18:27.
Mechta, many thanks for explaining about the need for damping - I had heard of it but never managed to understand. Have you any idea how this problem was tackled in the big engines like the RR Merlin? Very intensive maintenance ( if they ever made it to the specified time ) I should think?
As for the Citroen diesel: it still uses a reduction, as can be seen in one photograph in the article you mentioned. I suppose the belt(s?) take up the vibration in that installation? I remember seeing such a bird at Saint Yann last summer, that gold-anodised aluminium is not easy to forget. Was surprised to learn from the builder that the engine block as such had scarcely been modified.
And just to get the detail right: the French motorway speed limit is 130 km/h - my own country even has 120 km/h - but they are not very strict about it. As I've said before, the further South one goes, the greater the distance between legal and real.
As for the Citroen diesel: it still uses a reduction, as can be seen in one photograph in the article you mentioned. I suppose the belt(s?) take up the vibration in that installation? I remember seeing such a bird at Saint Yann last summer, that gold-anodised aluminium is not easy to forget. Was surprised to learn from the builder that the engine block as such had scarcely been modified.
And just to get the detail right: the French motorway speed limit is 130 km/h - my own country even has 120 km/h - but they are not very strict about it. As I've said before, the further South one goes, the greater the distance between legal and real.
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I take that to mean perhaps 150,000 miles flying the aircraft, and 300,000 total miles in cars/motorcycles. He doesn't drive much!
Actually I have probably driven 400k miles since 1977; 100k of that on 2 wheels.
Anti Chatter
When Pobjoy made his engine he relised that he would need revs to get the power but a slow prop speed for thrust and effiency.
Hence the reduction drive.However this drive also incorporated an "anti-chatter" device to help prevent the "chatter" that occured at low revs when the engine was warming up (less than 1100rpm).When power was increased to 1200 and over the engine suddenly will "go quiet" as the railway sleeper of a prop starts to help with the flywheel effect and the natural balance of the engine comes into effect.
There is no question with regard to the effect of a large dia prop on the smooth running of an engine plus the added efficient thrust it provides.
Of course being Pobjoy he also included an "oil slinger" centifugal strainer into the sysytem to help with the filtration.
With a double helical gear driving all this it was about as good as it gets from an engineering aspect, but no doubt expensive to produce.
What reduction gears DO NOT LIKE is the sort of rapid close rapid open operations that engines in training machines get, but then that can be incorporated in the "engine handling" training if required.
Being a radial cooling was never a problem unlike the "close cowled" flat 6 & 8 configuration of Mr Continental and Lycoming.
For years i flew Cessna 175 Skylark (GO 300e i think it was).This went like stink climbed like a homesick angel and was very smooth.However they had a poor reputation mainly due to being used as parachute planes and getting overheated.
If used for what they were designed for and the speed kept up on climbs they were fine.The rated power was at 2800 (with a large vp prop) so the TBO was only 1200 as opposed to 1500hrs but if operated by "thinking pilots" were a great machine out of a strip with a full load.
Hence the reduction drive.However this drive also incorporated an "anti-chatter" device to help prevent the "chatter" that occured at low revs when the engine was warming up (less than 1100rpm).When power was increased to 1200 and over the engine suddenly will "go quiet" as the railway sleeper of a prop starts to help with the flywheel effect and the natural balance of the engine comes into effect.
There is no question with regard to the effect of a large dia prop on the smooth running of an engine plus the added efficient thrust it provides.
Of course being Pobjoy he also included an "oil slinger" centifugal strainer into the sysytem to help with the filtration.
With a double helical gear driving all this it was about as good as it gets from an engineering aspect, but no doubt expensive to produce.
What reduction gears DO NOT LIKE is the sort of rapid close rapid open operations that engines in training machines get, but then that can be incorporated in the "engine handling" training if required.
Being a radial cooling was never a problem unlike the "close cowled" flat 6 & 8 configuration of Mr Continental and Lycoming.
For years i flew Cessna 175 Skylark (GO 300e i think it was).This went like stink climbed like a homesick angel and was very smooth.However they had a poor reputation mainly due to being used as parachute planes and getting overheated.
If used for what they were designed for and the speed kept up on climbs they were fine.The rated power was at 2800 (with a large vp prop) so the TBO was only 1200 as opposed to 1500hrs but if operated by "thinking pilots" were a great machine out of a strip with a full load.
Jan, The Merlin being a V12 had a power stroke every 1/6 of a revolution, whereas a four cylinder engine rotates 1/2 a revolution between power strokes, so there was a lot less opportunity for torque reversal with twelve cylinders. I'm not aware of any torque limiter or torsional damper on the Merlin to protect the gearbox, although I'm sure there will be a Ppruner who will know. I suspect the reduction in vibration was one of the reasons for going for so many cylinders, as well as to reduce the cross-sectional area.
The Merlin was also designed with a gearbox from the start, so the loads would have been taken into consideration in all parts at the design stage.
Thanks for the correction about the French autoroute speed limit. I knew it was high than UK motorway speed, but couldn't remember by how much.
As an aside, I've just found this description of the Dieselis, which if the figures are to believed would make anyone paying Lycoming running costs cry. DslGB001
The Merlin was also designed with a gearbox from the start, so the loads would have been taken into consideration in all parts at the design stage.
Thanks for the correction about the French autoroute speed limit. I knew it was high than UK motorway speed, but couldn't remember by how much.
As an aside, I've just found this description of the Dieselis, which if the figures are to believed would make anyone paying Lycoming running costs cry. DslGB001
Last edited by Mechta; 4th Feb 2011 at 22:49.
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The diesel aviation engines have a gearbox, and on the DA42 (the only diesel I have flown) they have a light composite prop and the engines are mounted on movable mounts. If you grab the spinner on the ground, you can wiggle the prop left and right and up and down. It is a bit disconcerting the first time you go into turbulence as the prop "wobbles" but it is very smooth in the aeroplane.
The engines themselves are variations on automotive engines, as the dipstick in the DA42 is the same as the one in my Audi
Engine mangement can be handled by FADEC which does a much better job of it than a human (as long as it doesn't stop doing that job!). Perhaps if someone came out with a relatively high power petrol engine/FADEC combination, it would be lighter and more economical than a Lycoming and less prone to damage from things like shock cooling / mishandling?
Actually that describes the Honda/TCM project exactly, I wonder if they are continuing with the project?
The engines themselves are variations on automotive engines, as the dipstick in the DA42 is the same as the one in my Audi
Engine mangement can be handled by FADEC which does a much better job of it than a human (as long as it doesn't stop doing that job!). Perhaps if someone came out with a relatively high power petrol engine/FADEC combination, it would be lighter and more economical than a Lycoming and less prone to damage from things like shock cooling / mishandling?Actually that describes the Honda/TCM project exactly, I wonder if they are continuing with the project?
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Perhaps if someone came out with a relatively high power petrol engine/FADEC combination, it would be lighter and more economical than a Lycoming and less prone to damage from things like shock cooling / mishandling?
