Reduced Performance from Piston Engines
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Reduced Performance from Piston Engines
We worry about turbine engines degrading in performance (particularly in helicopters), and we seem to think that piston engines either work properly and completely and are always able to put out 'baseline' power.
A recent discussion with a friend of mine who flies piston engine helicopters indicates that may not always be the case.
Which leads me to my question - how can we determine if a piston engine (with a manifold pressure gauge) is putting out the power it should be putting out?
Any thoughts? What does the fixed wing world do to determine if a cylinder doesn't have the proper compression, for example?
A recent discussion with a friend of mine who flies piston engine helicopters indicates that may not always be the case.
Which leads me to my question - how can we determine if a piston engine (with a manifold pressure gauge) is putting out the power it should be putting out?
Any thoughts? What does the fixed wing world do to determine if a cylinder doesn't have the proper compression, for example?
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Compression is easy to check on the ground. Also you could warm up the engine and sample the emissions. The real test would be to measure power output using a dynamometer, although difficult unless the installation intended the required instrumentation.
The easiest way for existing installations would be to measure power required to hover. Of course, you'd be subject to the accuracy and precision of many data sources (pressure altitude, temperature, manifold pressure, aircraft weight, wind, ...). Unfortunately, this would also detect changes in drivetrain losses, so not really what you're asking for, but still quite useful for the pilot.
Is this a question for which you already have a good answer?
The easiest way for existing installations would be to measure power required to hover. Of course, you'd be subject to the accuracy and precision of many data sources (pressure altitude, temperature, manifold pressure, aircraft weight, wind, ...). Unfortunately, this would also detect changes in drivetrain losses, so not really what you're asking for, but still quite useful for the pilot.
Is this a question for which you already have a good answer?
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Matthew:
thanks for the quick response.
I wish I had a ready answer to the question!
I think it's one that has been ignored as being too difficult for quite a while. I can see why they tried to correct that when turbine engines came out, but given that we've had engines lose compression since the dawn of the piston engine, why no-one has come up with an easy way to tell, on-board, if this is happening is an interesting question!
Your point about measuring power required to hover is a good one, but does require some discipline and knowledge of conditions.
Given that I just heard of an instructor who didn't even know where to look for the empty weight of his helicopter, I'm not hopeful we could ever get accurate weights as a starting point!
Still looking - I'm sure someone will come up with some good information.
thanks for the quick response.
I wish I had a ready answer to the question!
I think it's one that has been ignored as being too difficult for quite a while. I can see why they tried to correct that when turbine engines came out, but given that we've had engines lose compression since the dawn of the piston engine, why no-one has come up with an easy way to tell, on-board, if this is happening is an interesting question!
Your point about measuring power required to hover is a good one, but does require some discipline and knowledge of conditions.
Given that I just heard of an instructor who didn't even know where to look for the empty weight of his helicopter, I'm not hopeful we could ever get accurate weights as a starting point!
Still looking - I'm sure someone will come up with some good information.
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Shawn.
Some of the frustrations of power determination on piston engined helicopters:
Manifold guages whose reading does not correlate with the altimeter pressure reading.
Manifold guages that are not temperature compensated. Climb into cockpit when console has been cooking in the sun for hours - MAP over reading by more than one inch.
Same day with high humidity which is not scheduled in the flight manual.
Bugs on the blade leading edges in spite of cleaning just before wind up.
Weight schedule incorrectly recalculated from the "as weighed" figure and a lot of c..p under the seats etc, etc.
Some helicopters which in spite of engine/blade changes are always the dogs of the fleet.
Result: The only test is does that aircraft hover with the appropriate MAP margin on that day at that time and this may be widely different from another aircraft which is apparently in all respects similar.
This is all before the variations due to engine/blade deterioration is considered.
Some of the frustrations of power determination on piston engined helicopters:
Manifold guages whose reading does not correlate with the altimeter pressure reading.
Manifold guages that are not temperature compensated. Climb into cockpit when console has been cooking in the sun for hours - MAP over reading by more than one inch.
Same day with high humidity which is not scheduled in the flight manual.
Bugs on the blade leading edges in spite of cleaning just before wind up.
Weight schedule incorrectly recalculated from the "as weighed" figure and a lot of c..p under the seats etc, etc.
Some helicopters which in spite of engine/blade changes are always the dogs of the fleet.
Result: The only test is does that aircraft hover with the appropriate MAP margin on that day at that time and this may be widely different from another aircraft which is apparently in all respects similar.
This is all before the variations due to engine/blade deterioration is considered.
Well, if you don't know the weight of the aircraft (fixed or rotary), surely knowledge of relative engine power output is of little or no use anyway !
The standard test for piston-FW power is usually a performance climb, but parametric methods are more definitive. John Lowry's book Performance of Light Aircraft is a good baseline for parametric performance theory.
The approach taken there is generally however to look at the overall degredation in excess power - so poor compression, and dead flies on the leading edge, all bunch together.
G
The standard test for piston-FW power is usually a performance climb, but parametric methods are more definitive. John Lowry's book Performance of Light Aircraft is a good baseline for parametric performance theory.
The approach taken there is generally however to look at the overall degredation in excess power - so poor compression, and dead flies on the leading edge, all bunch together.
G
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In the spirit of trying to keep power required and power available separate, I'd like to know if there is a way to determine power available from a piston engine -
How do you determine if the MP gauge is not calibrated correctly? Can there be blockage from the engine to the gauge that might affect it?
Is there any way to measure compression at the cylinder directly? Has anyone made sensors that can be easily retrofitted for this? It seems to me that compression can go down pretty quickly, so periodic checks by maintenance might not be the answer.
Interesting situation, and I wonder why it's taken us so long to realize it (or perhaps, why I didn't understand this a lot earlier...)
How do you determine if the MP gauge is not calibrated correctly? Can there be blockage from the engine to the gauge that might affect it?
Is there any way to measure compression at the cylinder directly? Has anyone made sensors that can be easily retrofitted for this? It seems to me that compression can go down pretty quickly, so periodic checks by maintenance might not be the answer.
Interesting situation, and I wonder why it's taken us so long to realize it (or perhaps, why I didn't understand this a lot earlier...)
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Manifold gages can be checked against the altimeter set to zero but as I said before, the instruments are not temperature compensated or easily adjustable.
In fact, they are a pretty poor way of determining power margins where the difference between hover performance and takeoff capability in helicopters is relatively small.
The only answer is a mast torque indicator but I do not know of a piston helicopter that is fitted with one and I am at a loss to know why not. Probably for the same reason that carburettored fixed wings are generally not fitted with carb temperature gages - it would indicate a problem that the manufacturers would rather not have to deal with.
In fact, they are a pretty poor way of determining power margins where the difference between hover performance and takeoff capability in helicopters is relatively small.
The only answer is a mast torque indicator but I do not know of a piston helicopter that is fitted with one and I am at a loss to know why not. Probably for the same reason that carburettored fixed wings are generally not fitted with carb temperature gages - it would indicate a problem that the manufacturers would rather not have to deal with.
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Larger radials had torquemeters- usually measuring (via a hydraulic servo) the reaction of a stationary ring gear in the planetary set. In such an installation, you flew by torque reading, and the manifold pressure would gradually creep up as rings and valves deteriorated after many hours.
Modern turboshafts/turboprops also have torquemeters - in that case torque is the output, and TGT/EGT gradually rises with airfoil & seal deterioration.
Modern turboshafts/turboprops also have torquemeters - in that case torque is the output, and TGT/EGT gradually rises with airfoil & seal deterioration.
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Not sure if this is relevant or not. The Convair 440 had BMEP gauges which gave a remarkably accurate indication of power. The starboard engine had a compressor which furnished the air conditioning and extracted power from the engine to do so. On take off with equal manifold pressure (54"Hg I think) the left engine BMEP gauge would indicate 235 BMEP and the right engine indicated 225 BMEP reflecting the power loss. During the take off run and as the CHT increased for the same MP both BMEP needles would show a gradual decrease in power reflecting I presume the hotter charge. The BMEP readings by coincidence seemed to indicate a rough approximation of the horse power. Example being 235 BMEP is around 2350 HP.
If after an extensive run up the CHT was quite higher than usual and a take off commenced, there was a definate reduction in the BMEP readings.
This makes an interesting point concerning light training piston aircraft where one frequently sees students conducting high power "burning out" of fouled spark plugs which have given a big mag drop. Prolonged high power increases internal engine heat and in a twin engine aircraft if by misfortune an engine fails after lift off, the already "hot" other engine already degraded in power will maybe not permit a positive rate of climb. Morale being that prolonged high power on the ground might clear the plugs but exposes the aircraft to lower climb rates. All this theory from observing BMEP gauges! Maybe I am right or wrong in this speculation but it seems a logical assumption?
If after an extensive run up the CHT was quite higher than usual and a take off commenced, there was a definate reduction in the BMEP readings.
This makes an interesting point concerning light training piston aircraft where one frequently sees students conducting high power "burning out" of fouled spark plugs which have given a big mag drop. Prolonged high power increases internal engine heat and in a twin engine aircraft if by misfortune an engine fails after lift off, the already "hot" other engine already degraded in power will maybe not permit a positive rate of climb. Morale being that prolonged high power on the ground might clear the plugs but exposes the aircraft to lower climb rates. All this theory from observing BMEP gauges! Maybe I am right or wrong in this speculation but it seems a logical assumption?
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If mx is checking the compression at every 50 & 100hr inspection (Im assuming used for hire and working on FAA regs, not sure what you're local regs are on inspections), I would think that would catch any drop on the entire engine well before it became a problem. If you're losing compression faster than that, theres got to be significant damage going on (likely uneven between cylinders too) and I would think it would manifest itself as roughness in addition to power loss.
How long are we considering prolonged, with respect to ground runs at high power? I was taught if the plugs don't clear noticeably in a 20-30 second "burn" that it needed looking at anyway.
How long are we considering prolonged, with respect to ground runs at high power? I was taught if the plugs don't clear noticeably in a 20-30 second "burn" that it needed looking at anyway.
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FlyVmo. You are quite right. However in the real world of general aviation it is often the case where charter pilots (and others who should know better)will stubbornly elect to try and clear fouled plugs by operating the offending engine at high power for as long as it takes to get the magneto drop back within limits. The fact the drop may be due to a defective magneto ( in which case high power run up's will not rectify the problem), rather than fouled plugs, escapes these people.
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recip engine power
A rough guide for power deficiency is:
power reduction due to MP . one inch of MP below 29 inches equals about a 3.5% hp.
Each 100 RPM below 2700(rated RPM) equals 2.5% in HP
Have a look at Kitplanes December 2006 pg 22
power reduction due to MP . one inch of MP below 29 inches equals about a 3.5% hp.
Each 100 RPM below 2700(rated RPM) equals 2.5% in HP
Have a look at Kitplanes December 2006 pg 22
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Shawn,
another angle on this is vibration. I have used the technique successfully on FW light aircraft, although as yet I have not had call to do it on a piston engined helo. Essentially as you are aware, the reduced compression will generate an uneven force on the crankshaft, thus inducing a signal on the vib sensors at 2 x Crankshaft speed. This is backed up by a strong vibration at piston firing frequency. Of course the strength of the "Strong" signal correlates to the severity of the compression loss. I must concede that on engines where I have detected loss of compression using this method, then the loss has been significant enough for the pilot to detect vibration in the cockpit, although without exception, and despite the higher frequency of the vibes, the Pilots always attribute the vibration to propeller out of balance. As to whether you could use this as a definitive method of detecting loss of compression is open to several questions, primarily one of cost, you need at least two localised sensors on a 4 cyl engine, one sensor for every two cylinders is a good rule of thumb. Add to that the cost of a spectrum analyser and we are suddenly talking about a couple of grand at least.
This method though provides an indicator, ultimately, we still conduct a standard compression check to confirm the cause, and identify the specific cylinder(s) at fault.
GW
another angle on this is vibration. I have used the technique successfully on FW light aircraft, although as yet I have not had call to do it on a piston engined helo. Essentially as you are aware, the reduced compression will generate an uneven force on the crankshaft, thus inducing a signal on the vib sensors at 2 x Crankshaft speed. This is backed up by a strong vibration at piston firing frequency. Of course the strength of the "Strong" signal correlates to the severity of the compression loss. I must concede that on engines where I have detected loss of compression using this method, then the loss has been significant enough for the pilot to detect vibration in the cockpit, although without exception, and despite the higher frequency of the vibes, the Pilots always attribute the vibration to propeller out of balance. As to whether you could use this as a definitive method of detecting loss of compression is open to several questions, primarily one of cost, you need at least two localised sensors on a 4 cyl engine, one sensor for every two cylinders is a good rule of thumb. Add to that the cost of a spectrum analyser and we are suddenly talking about a couple of grand at least.
This method though provides an indicator, ultimately, we still conduct a standard compression check to confirm the cause, and identify the specific cylinder(s) at fault.
GW
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One needn't pay for a purpose-built spectrum analyzer; a laptop with sound input and appropriate audio software ought to work fine. All you need is a pickup accelerometer on the crankcase. 
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a laptop with sound input and appropriate audio software ought to work fine. All you need is a pickup accelerometer on the crankcase.
