Piston engine RPM overspeed
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Piston engine RPM overspeed
When a piston engine overspeeds, exactly what happens in terms of overstress/failure? Which components are most likely to fail and why?
I ask because I teach aerobatics in a fixed-pitch prop aircraft and as part of the ground school I want to be able to give more impact to the bit where I tell people to take care of the prop/engine RPM (specifically in this case a Lycoming 4 cylinder).
I ask because I teach aerobatics in a fixed-pitch prop aircraft and as part of the ground school I want to be able to give more impact to the bit where I tell people to take care of the prop/engine RPM (specifically in this case a Lycoming 4 cylinder).
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The reliability and service life of engines can be detrimentally affected if they are repeatedly operated at alternating high and low power applications which cause extreme changes in cylinder temperatures. Flight maneuvers which cause engine overspeed also contribute to abnormal wear characteristics that tend to shorten engine life. These factors must be considered to establish TBO of aerobatic engines; therefore it is the responsibility of the operator to determine the percentage of time the engine is used for aerobatics and establish his own TBO. The maximum recommended is the time specified in this
instruction.
Excerpt from Lycoming Service Instruction 1009AU.
And thats just for starts.
From Mandatory Service Bulletin 369J
For fixed wing aircraft, momentary overspeed is defined as an increase of no more than 10% of rated engine RPM for a period not exceeding 3 seconds
CAUTION
ENGINES MAY NOT BE CONTINUOUSLY OPERATED ABOVE SPECIFIED MAXIMUM CONTINUOUS RPM; TO DO SO WILL RESULT IN ABNORMAL WEAR ON BEARINGS, COUNTERWEIGHT ROLLERS AND OTHER ENGINE PARTS, CONCLUDING IN EVENTUAL ENGINE FAILURE.
CAUTION
IF OVERSPEED EXCEEDS 10% OF THE RPM VALUES IN THE COMPUTING-OVERSPEED COLUMN IN CHART I, IT IS RECOMMENDED THAT THE PROPELLER MANUFACTURER BE CONTACTED FOR POSSIBLE PROPELLER INSPECTION PROCEDURES.
Lets say you overspeed.....and its 5% or less.....Lycoming has recommendations in SB 369J:
Comply with Notes 1,2,5
1. Determine the cause for overspeed and correct it.
2. Drain the lubricating system.
a. Remove oil screens and filters and inspect for metal contamination.
b. Perform a differential pressure check on all cylinders to determine the sealing quality of the rings and valves. See latest revision to Service Instruction No. 1191 for procedure.
c. Using a borescope or equivalent instrument, examine the walls of each cylinder for scoring, which could be caused by stuck or broken piston rings.
d. Disassemble magnetos and inspect all components for damage; recondition or replace parts as required. Reassemble and test in accordance with the applicable magneto overhaul instruction manual. Also inspect condition of the magneto drive gears on the engine for looseness, which would indicate the supporting idler shafts are loose due to failure of safety attachments. If applicable, inspect condition of magneto bearing recess in crankcase for excessive wear. Repair as necessary in accordance with the latest revision of Service Instruction No. 1140 or Service Instruction No. 1197.
CAUTION
EARLIER SLICK MAGNETOS ARE NON-REPAIRABLE. CONSULT SLICK PUBLICATION.
5. Either repeated moments or short periods of operation in the overspeed region accelerate the rate of wear in the parts that comprise the valve train and consequently reduce the reliability of the engine. In addition to the checks normally performed on the engine during a 100-hour periodic maintenance inspection, also accomplish the following steps on page 4 and 5 before the aircraft is returned to service
a. Inspect all screens and filters in the lubrication system for metal contamination; if any unexplainable accumulation is discovered, the cause must be determined and corrected before the engine is returned to service.
b. By means of a borescope or equivalent illuminated magnifying optical device, determine the condition of the intake and exhaust valve faces and seat faces. Evidence of excessive wear, pounding, or grooving is reason for the valve and seat replacement.
c. Inspect external condition of valve keys, rockers, and exhaust valve guides for damage; particularly check valve springs for coil strikes or severe bottoming of the coils. If damage to springs is evident, remove them and check compression load as specified in Table of Limits; replace any that are not within limits.
d. Rotate the crankshaft by hand to determine if valve lift is uniform or equal for all cylinders; also note if valve rockers are free when the valves are closed. Unequal valve lift is an indication of bent push rods; and tight rockers, when valves are closed, indicate a tuliped valve or a damaged valve lifter. Repair any suspected damage before the engine is returned to service.
It gets worse.....Lets say you overspeed by more than 10%.
You get to comply with Note 6:
6. Remove the engine from the aircraft; disassemble it and inspect the parts in accordance with the applicable overhaul manual. Replace any parts that are damaged or not within the service limits as shown in the Table of Limits. In engines equipped with dynamic counterweights, the bushings must be replaced in both counterweight and crankshaft.
Unless your student wants to pay for a complete teardown.....
Should put the fear of GOD into anyone....or at least give one pause.
instruction.
Excerpt from Lycoming Service Instruction 1009AU.
And thats just for starts.
From Mandatory Service Bulletin 369J
For fixed wing aircraft, momentary overspeed is defined as an increase of no more than 10% of rated engine RPM for a period not exceeding 3 seconds
CAUTION
ENGINES MAY NOT BE CONTINUOUSLY OPERATED ABOVE SPECIFIED MAXIMUM CONTINUOUS RPM; TO DO SO WILL RESULT IN ABNORMAL WEAR ON BEARINGS, COUNTERWEIGHT ROLLERS AND OTHER ENGINE PARTS, CONCLUDING IN EVENTUAL ENGINE FAILURE.
CAUTION
IF OVERSPEED EXCEEDS 10% OF THE RPM VALUES IN THE COMPUTING-OVERSPEED COLUMN IN CHART I, IT IS RECOMMENDED THAT THE PROPELLER MANUFACTURER BE CONTACTED FOR POSSIBLE PROPELLER INSPECTION PROCEDURES.
Lets say you overspeed.....and its 5% or less.....Lycoming has recommendations in SB 369J:
Comply with Notes 1,2,5
1. Determine the cause for overspeed and correct it.
2. Drain the lubricating system.
a. Remove oil screens and filters and inspect for metal contamination.
b. Perform a differential pressure check on all cylinders to determine the sealing quality of the rings and valves. See latest revision to Service Instruction No. 1191 for procedure.
c. Using a borescope or equivalent instrument, examine the walls of each cylinder for scoring, which could be caused by stuck or broken piston rings.
d. Disassemble magnetos and inspect all components for damage; recondition or replace parts as required. Reassemble and test in accordance with the applicable magneto overhaul instruction manual. Also inspect condition of the magneto drive gears on the engine for looseness, which would indicate the supporting idler shafts are loose due to failure of safety attachments. If applicable, inspect condition of magneto bearing recess in crankcase for excessive wear. Repair as necessary in accordance with the latest revision of Service Instruction No. 1140 or Service Instruction No. 1197.
CAUTION
EARLIER SLICK MAGNETOS ARE NON-REPAIRABLE. CONSULT SLICK PUBLICATION.
5. Either repeated moments or short periods of operation in the overspeed region accelerate the rate of wear in the parts that comprise the valve train and consequently reduce the reliability of the engine. In addition to the checks normally performed on the engine during a 100-hour periodic maintenance inspection, also accomplish the following steps on page 4 and 5 before the aircraft is returned to service
a. Inspect all screens and filters in the lubrication system for metal contamination; if any unexplainable accumulation is discovered, the cause must be determined and corrected before the engine is returned to service.
b. By means of a borescope or equivalent illuminated magnifying optical device, determine the condition of the intake and exhaust valve faces and seat faces. Evidence of excessive wear, pounding, or grooving is reason for the valve and seat replacement.
c. Inspect external condition of valve keys, rockers, and exhaust valve guides for damage; particularly check valve springs for coil strikes or severe bottoming of the coils. If damage to springs is evident, remove them and check compression load as specified in Table of Limits; replace any that are not within limits.
d. Rotate the crankshaft by hand to determine if valve lift is uniform or equal for all cylinders; also note if valve rockers are free when the valves are closed. Unequal valve lift is an indication of bent push rods; and tight rockers, when valves are closed, indicate a tuliped valve or a damaged valve lifter. Repair any suspected damage before the engine is returned to service.
It gets worse.....Lets say you overspeed by more than 10%.
You get to comply with Note 6:
6. Remove the engine from the aircraft; disassemble it and inspect the parts in accordance with the applicable overhaul manual. Replace any parts that are damaged or not within the service limits as shown in the Table of Limits. In engines equipped with dynamic counterweights, the bushings must be replaced in both counterweight and crankshaft.
Unless your student wants to pay for a complete teardown.....
Should put the fear of GOD into anyone....or at least give one pause.
Last edited by privateer01; 29th Mar 2010 at 05:38. Reason: Clarity syntax
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Which components are most likely to fail and why?
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For an alternative view - ask the aerobatic pilots and racers who regularly venture into that area - some are OK about it, some are not.
I know one guy who regularly takes his IO-360 above 3,000 rpm when doing aero's without any problems.
As with most things mechanical, a lot depends on how often, how aggressively it is done, how regularly the oil is changed etc etc.
For the real pioneers - go to Reno and speak with the chaps taking their Continental O-200's over 4,000 rpm. Now they do admit that they don't reach TBO but they go reet fast
I know one guy who regularly takes his IO-360 above 3,000 rpm when doing aero's without any problems.
As with most things mechanical, a lot depends on how often, how aggressively it is done, how regularly the oil is changed etc etc.
For the real pioneers - go to Reno and speak with the chaps taking their Continental O-200's over 4,000 rpm. Now they do admit that they don't reach TBO but they go reet fast
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I would not be worried too much about overrevving a lycoming 360..
In the 25 years that I've been around aerobatics, starting with cap 10's in aeroclubs that have been used and often abused I've never seen one engine fail. They all made it to TBO.
I would worry more of a private airplane that collects dust in the hangar and is seldom used with great care by its owner.. A lot of private planes flying little over the years start to have problems way before they reach TBO.
An engine to be healthy has to run often. And for an amount of time that enables the oil temperature to get warm enough to allow for evaporation of the condensation it collects. Remeber a piston engine is a steam machine...
It is current practice to overspeed the engine past 2700 RPM, especially when learning aerobatics and practicing solo's. Too many things to concentrate upon...Easy to forget that poor prop starting to cavitate...
Time, training and experience tend to make things better.
One good tip we have and that makes things easy is the following:
Constant monitoring both tach and ASI is time consuming and distracting . Learn to watch the ASI only.
In the Cap 10 I used to fly years ago, the engine started to overrev past 2700 RPM when the ASI indicated 250 Km/h.
So depending on what type of prop you use, those figures may vary, but once you know that speed, keep it in mind and start easing the throttle when reaching it..That way you won't have to watch the tach all the time...
If you are concerned about your engine, there is worse:
Look at the oil pressure drop everytime you pass knife edge in a barrel roll, maintain knife edge for a while and the red low pressure light starts to blink!!
The Christen system works well once you are inverted or upright, that steel ball allows to switch from its inverted tank to the engine carter but does not seal at a 90 degrees angle !!!
And that terrible ever changing sound due to prop regulators in constant speed engines during aerobatics
Every change in pitch during manoeuvers entails a severe engine overspeed well beyond the red line, often above 3000 RPM. It takes a fraction of a second before the regulator overcomes the inertia and brings back the prop to the assigned RPM ( Red line in competitions.. believe me).
The engine not only has to cope with overspeed, and does it well, but much worse with enormous torsion forces which took their toll in the past when a few heavy metal props departed in flight with parts of the crankshaft.. Now with wood and composite props it does not seem to happen anymore.
Aircraft engines are big bore motors turning slowly, with a low HP/Liter ratio. Unless they are supercharged overrevving should not be a major concern, I can bet your engine will make it to the 1500 hours TBO + the extensions. Expecially if you take good care of it, keeping the RPM's low at start, waiting to have temp's in the green, carefully monitoring the oil temp especially in summer during aerobatic flights - temp may reach red line- reduce RPM's and wait one min or two...
Check compressions regularly, break open and inspect oil filters for particles, do an oil analysis even if you don't find anything...Those microscopic steel traces diluted in the oil if they are not found, chances are that they are where they belong, and those bearings and piston rings are still in a good shape...
In the 25 years that I've been around aerobatics, starting with cap 10's in aeroclubs that have been used and often abused I've never seen one engine fail. They all made it to TBO.
I would worry more of a private airplane that collects dust in the hangar and is seldom used with great care by its owner.. A lot of private planes flying little over the years start to have problems way before they reach TBO.
An engine to be healthy has to run often. And for an amount of time that enables the oil temperature to get warm enough to allow for evaporation of the condensation it collects. Remeber a piston engine is a steam machine...
It is current practice to overspeed the engine past 2700 RPM, especially when learning aerobatics and practicing solo's. Too many things to concentrate upon...Easy to forget that poor prop starting to cavitate...
Time, training and experience tend to make things better.
One good tip we have and that makes things easy is the following:
Constant monitoring both tach and ASI is time consuming and distracting . Learn to watch the ASI only.
In the Cap 10 I used to fly years ago, the engine started to overrev past 2700 RPM when the ASI indicated 250 Km/h.
So depending on what type of prop you use, those figures may vary, but once you know that speed, keep it in mind and start easing the throttle when reaching it..That way you won't have to watch the tach all the time...
If you are concerned about your engine, there is worse:
Look at the oil pressure drop everytime you pass knife edge in a barrel roll, maintain knife edge for a while and the red low pressure light starts to blink!!
The Christen system works well once you are inverted or upright, that steel ball allows to switch from its inverted tank to the engine carter but does not seal at a 90 degrees angle !!!
And that terrible ever changing sound due to prop regulators in constant speed engines during aerobatics
Every change in pitch during manoeuvers entails a severe engine overspeed well beyond the red line, often above 3000 RPM. It takes a fraction of a second before the regulator overcomes the inertia and brings back the prop to the assigned RPM ( Red line in competitions.. believe me).
The engine not only has to cope with overspeed, and does it well, but much worse with enormous torsion forces which took their toll in the past when a few heavy metal props departed in flight with parts of the crankshaft.. Now with wood and composite props it does not seem to happen anymore.
Aircraft engines are big bore motors turning slowly, with a low HP/Liter ratio. Unless they are supercharged overrevving should not be a major concern, I can bet your engine will make it to the 1500 hours TBO + the extensions. Expecially if you take good care of it, keeping the RPM's low at start, waiting to have temp's in the green, carefully monitoring the oil temp especially in summer during aerobatic flights - temp may reach red line- reduce RPM's and wait one min or two...
Check compressions regularly, break open and inspect oil filters for particles, do an oil analysis even if you don't find anything...Those microscopic steel traces diluted in the oil if they are not found, chances are that they are where they belong, and those bearings and piston rings are still in a good shape...
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as part of the ground school I want to be able to give more impact to the bit where I tell people to take care of the prop/engine RPM (specifically in this case a Lycoming 4 cylinder).
Naturally I don't believe the engine will fly apart if done. As referenced above, many don't
More like Lycoming washing thier hands of liability.
However it is a Mandatory Service Bulletin.......
If the aircraft were experimental who cares.....
Certificated, Insured and used for instruction.....might be another matter.
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Worst overspeed I have ever seen was with a pilot doing aerobatics with a fixed fine pitch prop on a glider tug - valves bouncing and clattering,scared the hell out of me - waiting for the bang!
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Hi DB6,
All the reciprocating parts of your engine are subject to acceleration forces. The forces increase as the square of the revs. e.g. 40% increase in revs doubles the force on the components.
The engine parts will fail in time due to fatigue. The fatigue crack growth depends on the number of cycles, and the stress level above a certain threshold. (For steel components the threshold is about half the U.T.S. (ultimate tensile stress), for Aluminium it's about 1/3rd).
So if your valves don't foul the piston (due to the slow spring closure), then you've shortened the fatigue life of the highly stressed parts considerably by over-revving.
Edit. e.g. Suppose Max recommended RPM = 1,000 and at this speed the piston stress = 1/3 UTS (i.e. below the fatigue threshold). Then this piston would eventually wear away before it failed. If the engine was revved to 1,750 rpm, the stress on the piston is over 3 times as great (beyond the U.T.S.) and it would fail immediately. The life of this piston will shorten exponentially with RPM.
All the reciprocating parts of your engine are subject to acceleration forces. The forces increase as the square of the revs. e.g. 40% increase in revs doubles the force on the components.
The engine parts will fail in time due to fatigue. The fatigue crack growth depends on the number of cycles, and the stress level above a certain threshold. (For steel components the threshold is about half the U.T.S. (ultimate tensile stress), for Aluminium it's about 1/3rd).
So if your valves don't foul the piston (due to the slow spring closure), then you've shortened the fatigue life of the highly stressed parts considerably by over-revving.
Edit. e.g. Suppose Max recommended RPM = 1,000 and at this speed the piston stress = 1/3 UTS (i.e. below the fatigue threshold). Then this piston would eventually wear away before it failed. If the engine was revved to 1,750 rpm, the stress on the piston is over 3 times as great (beyond the U.T.S.) and it would fail immediately. The life of this piston will shorten exponentially with RPM.
Last edited by rudderrudderrat; 30th Mar 2010 at 13:02.
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Markkel, in knife edge you are correct that the valve possibly won't seal properly but I think it's really the oil pickup that is sucking air that creates the low pressure. The oil pickup is usually one one side of the sump so knife edge on one side may keep oil pressure but the other knife edge will not. Some exotic sumps have a swinging oil pickup which works like the flop tube in the fuel tank ! Good idea but how much knife edge do you want !?
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Fivegreen, you're right about the Christen valve.
If you have a Pitts, that chubby fuselage will generate enough lift to maintain knife edge, a real delight.
On my SU-29 it is possible to maintain knife edge for quite a while with enough speed, especially if you keep the nose 15-20 degrees over the horizon..
Radials having a separate oil tank (Dry sump) do keep positive oil pressure all over the aerobatic flight envelope.
The separate SU-29 cylindrical tank bolted to the firewall has a swiveling arm in the form of a conrod rotating at 360 degress picking oil into the lines whatever you do. A bit like a model airplane with those plastic bottle reservoirs fitted with a steel tube attached to the flexible gasoline hose. Steel tube with a counterweight at its end moving freely around to gravity in the same direction as the fuel content.
Rightly so Rudderrudder !!! reciprocating parts are subject to considerable load, but while pushing an engine with max 1000 RPM to 1700 (70%) might send those pistons into free space and at best having the valves take a bite on pistons heads, Pushing a Lyco from 2700 to 3000 (10%) might not.
It is possible to overrev an engine fitted with a constant speed prop and adjusted governor to 2900-3000 RPM
In competition, ( and often in training) unlimited category aerobatics all engines are firewalled all the time at 2800-3000 RPM.. But those flat six lyco's don't usually make it to TBO. They endure too much wear, i believe not so much because of overspeeding but mostly due to lubrication wet sump issues..
Different story with the Russian radials, they are supercharged so no more than 100% power.. ( I use 82% continuous don't need more) Manuals say max continuous permissible is...82% for 5 min. Max 10% of TT.
But the folks who compete and I know quite a few of them use... 100% ALL THE TIME....To my knowledge there was never a problem.
The only major issues with Sukhois and Yaks as far as engines are concerned, are hydraulic locks failures, and believe me there have been a lot of them.
There is a pre engine start engine procedure to respect to the letter, you don't do it and the engine will bite you sooner or later. Installation of manifold drain kits and proper prop cycling help avoid that.
I remember driving my Honda bike 20 years ago, and sometimes while pushing the engine, I often missed a gear while shifting... winding the RPM needle .....another revolution..... Take that pistons and valves !!! Nevertheless as I always took good care of my engine by warming it up carefully, breaking it in when new in a religious way, despite what manuals say nowadays..
Never had any failures, never had to add a drop of oil in between oil changes...
If you have a Pitts, that chubby fuselage will generate enough lift to maintain knife edge, a real delight.
On my SU-29 it is possible to maintain knife edge for quite a while with enough speed, especially if you keep the nose 15-20 degrees over the horizon..
Radials having a separate oil tank (Dry sump) do keep positive oil pressure all over the aerobatic flight envelope.
The separate SU-29 cylindrical tank bolted to the firewall has a swiveling arm in the form of a conrod rotating at 360 degress picking oil into the lines whatever you do. A bit like a model airplane with those plastic bottle reservoirs fitted with a steel tube attached to the flexible gasoline hose. Steel tube with a counterweight at its end moving freely around to gravity in the same direction as the fuel content.
Rightly so Rudderrudder !!! reciprocating parts are subject to considerable load, but while pushing an engine with max 1000 RPM to 1700 (70%) might send those pistons into free space and at best having the valves take a bite on pistons heads, Pushing a Lyco from 2700 to 3000 (10%) might not.
It is possible to overrev an engine fitted with a constant speed prop and adjusted governor to 2900-3000 RPM
In competition, ( and often in training) unlimited category aerobatics all engines are firewalled all the time at 2800-3000 RPM.. But those flat six lyco's don't usually make it to TBO. They endure too much wear, i believe not so much because of overspeeding but mostly due to lubrication wet sump issues..
Different story with the Russian radials, they are supercharged so no more than 100% power.. ( I use 82% continuous don't need more) Manuals say max continuous permissible is...82% for 5 min. Max 10% of TT.
But the folks who compete and I know quite a few of them use... 100% ALL THE TIME....To my knowledge there was never a problem.
The only major issues with Sukhois and Yaks as far as engines are concerned, are hydraulic locks failures, and believe me there have been a lot of them.
There is a pre engine start engine procedure to respect to the letter, you don't do it and the engine will bite you sooner or later. Installation of manifold drain kits and proper prop cycling help avoid that.
I remember driving my Honda bike 20 years ago, and sometimes while pushing the engine, I often missed a gear while shifting... winding the RPM needle .....another revolution..... Take that pistons and valves !!! Nevertheless as I always took good care of my engine by warming it up carefully, breaking it in when new in a religious way, despite what manuals say nowadays..
Never had any failures, never had to add a drop of oil in between oil changes...
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Thanks for the info chaps, some good stuff to pass on. The engines are Lycoming AEIO-320s, RPM limit 2700 and they don't get much abuse but I feel it is always better when saying 'Don't Do It!' to say why as well. Sustained knife edge is not an issue in the Grob 115D2 I can assure you -wish it was!
